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  • Effects of Copper Content on Microstructure and Mechanical Properties of Powder-Forged Rod Fe-C-Cu Alloys Manufactured at Elevated Temperature
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-17
    S. Wang, Q. Wang, H.L. Wang, F.P. Liu, W.J. Yao, F. Jiang, J. Sun, F.Y. Wang

    The microstructure and mechanical properties of powder-forged Fe-C-Cu alloys manufactured at an elevated temperature were evaluated. It was found that the mechanical properties of the alloys were no worse than those in the literature, and the contribution of copper to the strength was quantified: the major strengthening mechanism of copper is grain-refinement strengthening rather than precipitation strengthening. Then, to ensure the safety of the elevated-temperature manufacturing, the hot-deformation behaviour of the Fe-C-2.5Cu alloy was investigated via construction of the constitutive equation and the hot-processing map, which proved that the high-temperature manufacturing was not only safe but also efficient. Finally, fatigue tests were conducted on the powder-forged connecting rods and 5154 wrought steel connecting rods for the same engine application, and the results showed that the powder-forged connecting rods had better fatigue strength and higher reliability.

    更新日期:2018-11-17
  • Grain Boundary Precipitation of Tantalum and NiAl in Superelastic FeNiCoAlTaB Alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-17
    Cheng Zhang, Chaoyi Zhu, Sumin Shin, Lee Casalena, Kenneth Vecchio

    The precipitation behavior of tantalum (Ta) and NiAl along grain boundaries has been investigated in polycrystalline Fe-28.5Ni-17.5Co-11.5Al-2.5Ta-0.05B (NCATB) (at.%) alloy in order to understand their role in limiting its superelastic behavior. It is found that Ta precipitates at grain boundary triple-junctions first, then along grain boundaries, and acts as nuclei for β-NiAl phase formation. Small amounts of boron is also found distributed adjacent to the Ta precipitates. Consequently, the martensitic transformation associated with the superelastic response is promoted along grain boundaries by the Ta and β-NiAl precipitated there. Furthermore, increasing aging temperature facilitates the precipitation of Ta and β-NiAl along grain boundaries. The mean cell width of precipitates along grain boundaries significantly affects the fracture mode and ductility of NCATB specimens. Both the fraction of low-angle boundaries and the intensity of recrystallization texture are significantly increased with increasing reduction in thickness of cold-rolled specimens over 95%, which is effective for the suppression of Ta and β-NiAl formation as their formation occurs primarily on high angle grain boundaries. As a result, the superelastic behavior of the FeNiCoAlTaB alloy is closely linked to the reduction in thickness, which increases the ratio of low angle to high angle boundaries, and hence the propensity for grain boundary failure associated with the grain boundary precipitates and grain boundary initiated martensitic transformation.

    更新日期:2018-11-17
  • High temperature nano-indentation of tungsten: modelling and experimental validation
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-17
    Xiazi Xiao, D. Terentyev, A. Ruiz, A. Zinovev, A. Bakaev, E.E. Zhurkin

    It is very well known that tungsten is intrinsically brittle at room temperature, and the characterization of its ductile properties by conventional mechanical tests is possible only above the ductile-to-brittle transition temperature (DBTT), i.e. above 500–700 K. However, the design of tungsten-based components often requires the knowledge of constitutive laws below the DBTT. Here, we carried out instrumented hardness measurements in the temperature range of 300–691 K by nano-indentation. The obtained results are used to extend a set of constitutive laws for the plastic deformation of tungsten, developed earlier on the basis of tensile data, which now covers the temperature range of 300–1273 K. The validation of the constitutive laws was realized by the crystal plasticity finite element method (CPFEM) model, which was applied to simulate the nano-indentation loading curves. The distribution of stress and strain under the indenter was also studied by the CPFEM to bring an insight on the extension of the plastic zone in the process of the indentation, which is of crucial importance when nano-indentation is used to resolve the microstructural features generated by e.g. irradiation by energetic particles, plasma exposure or thermo-mechanical treatment.

    更新日期:2018-11-17
  • Effects of nanosized TiCp on the microstructure evolution and tensile properties of an Al-Mg-Si alloy during cold rolling
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-17
    Run Geng, Feng Qiu, Qing-Long Zhao, Yu-Yang Gao, Qi-Chuan Jiang

    The effects of a small addition of nanosized TiC particles (TiCp) on the microstructure evolution of the Al-Mg-Si (6061) alloy throughout the entire fabrication process including casting, cold rolling and heat treatments were investigated. TiCp impeded the recrystallization and grain growth, refined the grain structure, and evidently increased the tensile strength without obvious decrease on ductility. The ultimate tensile strength and yield strength of the 1.0 wt.% TiCp/6061 composite were 330 MPa and 275 MPa in T6 state and 438 MPa and 426 MPa in the as-rolled state, an increase of 42 MPa and 38 MPa, 41 MPa and 52 MPa, respectively, compared to the matrix 6061 alloy. The strength increase is attributed to strengthening effect of TiCp, dislocation accumulation and precipitate strengthening.

    更新日期:2018-11-17
  • Analysis of flow stress and microstructure during hot compression of 6099 aluminum alloy (AA6099)
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-17
    Ahmad Chamanfa, Mohammed T. Alamoudi, Nicholas E. Nanninga, Wojciech Z. Misiolek

    The hot deformation behavior of a newly developed AA6099 alloy was investigated after homogenization. To this end, Gleeble® isothermal hot compression tests were performed at temperatures of 350, 400, 450, and 500 °C, and strain rates of 0.01, 0.1, and 1 s−1 up to a true strain of 0.8. The dependency of the flow stress on the deformation temperature, strain, and strain rate was analyzed. In addition, the constitutive equations relating the flow stress to the deformation temperature and strain rate were derived using a power-law empirical model. Also, the microstructures and dynamic softening mechanisms of the alloy under various deformation conditions were examined using light optical microscopy (LOM) and electron back scattered diffraction (EBSD) techniques. The correlation between the flow behavior, different microstructures, and dynamic softening mechanisms at various hot compression conditions was determined. The kinetic analysis and microstructural evolution indicated that the main flow softening mechanism for homogenized AA6099 was dynamic recovery (DRV). Partial dynamic recrystallization (DRX) enhanced the flow softening especially at low deformation temperatures and high strain rates, i.e., at high Zener-Hollomon parameter (Z) values.

    更新日期:2018-11-17
  • Synergistic effect of mechanical and environmental damages of 316LN stainless steel under different fatigue strain amplitudes in high-temperature pressurized water
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-17
    Ziyu Zhang, Jibo Tan, Xinqiang Wu, En-Hou Han, Wei Ke

    Corrosion fatigue (CF) behavior of 316LN stainless steel under different strain amplitudes (0.3%-0.8%) in 325 °C water was investigated at a strain rate of 0.04%s−1. It was found that the fatigue life and strain amplitude has a linear relation in double logarithmic coordinate. The environmental damage increased with decreasing strain amplitudes. The synergistic effect of mechanical and environmental damages during the CF process in high-temperature pressurized water is discussed, and a calculation equation for environmental fatigue correction factor Fen is proposed to calculate the strain amplitude effect.

    更新日期:2018-11-17
  • Yield Stress in Titanium Nickelide-Based Alloys with Thermoelastic Martensitic Transformations
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-17
    Aleksander Lotkov, Victor Grishkov, Victor Timkin, Anatoly Baturin, Dorzhima Zhapova

    It is shown that in alloys with thermoelastic martensitic transformations, shape memory and superelasticity effects, the real yield stress (τ0.3) can be determined only by a joint analysis of "stress-total strain” and "plastic deformation-total strain” dependences. We suggest to determine of the dependence of the "plastic deformation – total deformation" from isothermal cycles of "loading at successively increasing values of the applied external stress – unloading" and the subsequent heating of unloaded specimens up to finish of the return of inelastic martensitic deformation. The residual deformation equals to the plastic deformation in this case. Using Ti49.2Ni50.8 (at.%) binary alloy specimens whose structure was changed from coarse-grained to micro- and submicrocrystalline by means of warm rolling, the yield point τ0.3 in torsion of specimens was demonstrated to be localized on "τ-γ" dependence at the end of pseudo- yield plateau or at the beginning of the subsequent linear section of deformation development under loading. The development of plastic deformation at τ > τ0.3 was also studied.

    更新日期:2018-11-17
  • Microstructural changes during creep and aging of a heat resistant MARBN steel and their effect on the electrochemical behaviour
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-17
    K.G. Abstoss, S. Schmigalla, S. Schultze, P. Mayr

    The high creep strength of advanced boron alloyed 9 wt.-% Cr steels results from a precipitate stabilised martensitic microstructure. Grain boundaries and sub-boundaries are effectively pinned by boron stabilised precipitates. However, during creep exposure several degradation mechanisms with regards to precipitates are active such as, particle coarsening, phase transformations, dissolution of precipitates and precipitation of new phases. This diffusion driven phenomena affect the type, size and distribution of precipitates and lead to a deterioration of creep strength with increasing time of exposure. Within this work, an experimental melt of a 9Cr3Co3WVNbBN steel grade was investigated in terms of microstructural evolution during creep and aging and electrochemical behaviour. Creep specimens, tested up to 24,606 h were analysed to quantify the microstructural and electrochemical behaviour of the grip and the necked portion of the creep exposed specimens by z-phase indication of the main precipitates and Electrochemical potentiodynamic reactivation (EPR)-test. The change in diameter, phase fraction, number density and current density was quantified. The results of microstructural changes during creep exposure are compared with the electrochemical response of the specimens in the EPR-test. By understanding the correlation between microstructural changes and the results of the EPR-test, a possibility for a nearly non-desctructive on-site evaluation of the creep state of power plant components might be possible in future.

    更新日期:2018-11-17
  • The effect of continuous confined strip shearing deformation on the mechanical properties of AZ31 magnesium alloys
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-16
    Jinhua Peng, Zhen Zhang, Pengju Yang, Yaozu Li, Peng Guo, Wei Zhou, Yucheng Wu

    Continuous confined strip shearing deformation (CCSS) was conducted on AZ31 magnesium alloys. The microstructure and texture were modified and special inclined basal texture was obtained through different routes. The newly formed basal texture was favor for the basal slip which brought about lower micro-hardness and yield strength but higher elongation. The strong tensile-compression asymmetry in the base material was weakened because basal slip was the dominating deformation mechanism both in tensile and compression tests. Meanwhile, {10–12} twinning was suppressed because of easier activation of basal slip. The strain hardening behavior was also changed and the strain hardening rate almost decreased along the whole deformation process; and only a slight increase was found at the stage2 of route A in tensile test. The strain hardening exponents were scattered from different stages due to the specific deformation mechanism at each stage.

    更新日期:2018-11-16
  • Effect of TiN addition on the microstructure and mechanical properties of TiB2-FeNi based cermets
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-16
    Ning Wu, Fengdan Xue, Jianying Wang, Hailin Yang, Fenghua Luo, Jianming Ruan

    The TiB2-x vol% TiN-20 vol% (Fe-Ni) system cermets with various volume fraction of TiN were successfully fabricated through gas pressure sintering. The effects of TiN content on the mechanical properties and microstructure of TiB2-based cermets was explored in detail. The microstructure and compositional analyses of TiB2-based cermets were investigated by SEM, EPMA, TEM, and X-ray diffraction. The results shown that only TiB2, TiN and Fe0.64Ni0.36 phases in the TiB2-based cermets, not any other brittle phases such as M2B, M23B6 could be found. The TiB2 grains formed a typical core-rim structure, but the TiN particles didn't. The dominant fracture mode of TiB2-based cermets transformed from intergranular fracture to transgranular fracture as the TiN content increasing from 0 to 25 vol%. Toughening mechanisms of TiB2-based cermets transformed from debonding, crack deflection to crack bridging and crack deflection as the TiN content increasing. The properties of TiB2-based cermets have been greatly enhanced as the TiN content varying from 0 to 25 vol%. The transversal rupture strength, indentation fracture toughness, Vickers hardness, and relative density of TiB2–20 vol% TiN-20 vol% (Fe-Ni) cermet reached up to 1191.12 ± 40 MPa, 18.77 ± 0.40 MPa·m1/2, 14.40 ± 0.20 GPa, and 99.74 ± 0.20%, respectively.

    更新日期:2018-11-16
  • Simple method for tailoring the optimum microstructures of high-strength low-alloyed steels by the use of constitutive equation
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-16
    Young Min Kim, Sung Kyu Kim, Nack J. Kim

    A simple approach for tailoring the microstructure to meet the property requirements of low carbon steels is studied with the material constants of the Swift equation. Experimental data of the specimens subjected to various chemical compositions and rolling conditions were used to elucidate the relationship among the microstructures, the tensile properties, and the material constants of the Swift equation. This relationship reveals that the material constants of the Swift equation are closely related to the microstructural features of low carbon steels, and in particular constant b can be expressed as a function of the volume fraction of the constituent phases. The approach proposed in this study includes (i) the determination of appropriate range of constant b to meet the required properties and (ii) the selection of the optimum microstructures corresponding to the determined b values. Since this simple approach can provide a guidance in how to control the microstructure to acquire target properties and link microstructural features with macroscopic constitutive behavior, it is very useful for the development of new advanced steels and to save time and production cost.

    更新日期:2018-11-16
  • Creep in bulk metallic glasses. Transition from linear to non linear regime
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-16
    J.C. Qiao, J.M. Pelletier, Y. Yao

    Tensile creep is investigated in a La60Al25Ni15 (at.%) bulk metallic glass. Temperature and stress are chosen in a large range and tests are performed during long times (up to 105s). The compliance of the samples, i.e. the ratio between the induced strain and the applied stress, is deduced from the resulting strain. A transition from a linear regime to a non-linear regime is progressively observed when either temperature or stress level is increased. This transition indicates a modification of the physical mechanism involved in the deformation. The physical model initially proposed by Perez et al is used to describe this transition. When a stress is applied shear micro-domains (SMDs) are progressively formed in appropriate shear planes and when the stress or temperature is high enough a coalescence of these SMDs occurs progressively. The phenomenon becomes irreversible and then the viscoplasticity takes place. The transition is therefore linked to the onset of viscoplasticity.

    更新日期:2018-11-16
  • Effect of Sm on Thermal and Mechanical Properties of Cu-Zr-Al Bulk Metallic Glasses
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-16
    F. Sıkan, S.E. Atabay, A. Motallebzadeh, S. Ozerinc, I. Kalay, Y.E. Kalay

    The effect of rare-earth (Sm) microalloying on the thermal stability and phase selection along with the effect of nanocrystallization on the mechanical properties of amorphous melt-spun ribbons of Zr50Cu40Al10, Zr49Cu39.2Al9.8Sm2 and Zr48Cu38.4Al9.6Sm4 alloys were investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), Vickers and nanoindentation hardness tests and micropillar compression analysis. XRD and TEM analyses showed that all samples were fully amorphous in as-spun state; however, crystallization sequences for the Sm-free and the Sm micro-alloyed samples were different during devitrification. Combined study of XRD, DSC and TEM on melt-spun ribbons show that Zr48Cu38.4Al9.6Sm4 have nanocrystallization of Cu2Sm phase with an average diameter of 10 nm, which was absent in Zr50Cu40Al10, prior to crystallization of Cu10Zr7 phase. The nanoindentation and micropillar compression tests revealed Cu2Sm nanocrystals embedded in Zr48Cu38.4Al9.6Sm4 alloy improves strength and hardness. On the other hand, presence of these nanocrystals deteriorate shear band stability and thus result in a catastrophic brittle fracture through a single shear band burst.

    更新日期:2018-11-16
  • INTO THE QUENCHING & PARTITIONING OF A 0.2 C STEEL: AN IN-SITU SYNCHROTRON STUDY
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-16
    Pierre Huyghe, Matteo Caruso, Jean-Louis Collet, Sylvain Dépinoy, Stéphane Godet

    Quenching and partitioning (Q&P) is an effective way for retaining austenite at room temperature. The actual mechanisms responsible for austenite stabilization are still under debate, since it is impossible to track carbon diffusion with the conventional metallographic tools. The present work depicts several Q&P heat treatments performed on a 0.2 C commercial grade steel by in-situ High Energy X-Ray Diffraction (HEXRD). More specifically, the effect of three different initial quenching temperatures on the microstructural evolution occurring during Q&P was scrutinized in details.It was shown that about 50% of the initial carbon partitions effectively to austenite. A carbon enrichment up to 0.8 wt.% is sufficient to retain austenite at room temperature as grain size refinement contributes to further stabilize austenite. The origin of carbon enrichment in retained austenite depends on the initial quench temperature (QT). For low QT, corresponding to an initial martensite fraction larger than 0.75, austenite carbon enrichment is ensured by carbon partitioning from supersaturated martensite. For higher QT, austenite carbon enrichment results from both carbon partitioning from martensite and carbon rejected during the bainite transformation. The former mechanism proceeds rapidly and already starts during the reheating stage to the partitioning temperature. The latter is slower as the carbon enrichment is coupled to bainite formation.

    更新日期:2018-11-16
  • Effects of Nucleation Site and Morphology of Carbide-Free Bainite on Microstructures and Properties of Bainite/Martensite Multi-phase Steels
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-16
    Jingli Zhao, Xianming Zhao, Xiaoyu Zhao, Chunyu Dong, Shouxu Kang

    The microstructures and mechanical properties of bainite/martensite (B/M) multiphase steels treated with high-temperature duration and continuous cooling were studied. The steels treated at 623 K and cooling rate of 0.5 K/s exhibited a multiphase microstructure containing carbide-free bainite, martensite and film-like retained austenite. Observation in-situ showed the carbide-free bainite was nucleated and dispersed intragranularly with excellent tensile strength of 1920 MPa and total elongation of 14.6%.

    更新日期:2018-11-16
  • Thermomechanical behavior of amorphous alloys based on titanium at the temperature range of the glass transition and crystallization
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-16
    A. Sypien, G. Garzel, T. Czeppe

    Knowledge and systematic research on the mechanical properties of amorphous materials in the transition temperature range from amorphous to crystalline is a fundamental scientific interest. Planning the use of these materials must be based on thermomechanical properties information. TiZrCuPd amorphous alloys belong to the potentially lead-free solders of relatively large glass forming ability. The paper presents results of the differential scanning calorimetry, thermomechanical, microstructural and phase analysis of the TiZrCuPd amorphous alloys containing 14 at.% Pd and 2 and 3 at.% Sn additions at the expenses of Cu content. The samples were investigated in the tensile-load modulated and dilatometric modes in the temperature range of the glass transition and crystallization. The relation between two steps of crystallization process, plastic flow and resulting ability for the deformation was analyzed and related to the microstructure and phase analysis. The determined temperature range of glass transition and primary crystallization show enhanced ductility in the dependence on the Sn additions and homogeneous deformation in this range. The engineering strain of alloys reveals its dependence on the temperature, especially visible in the sample for the 2 at.% Sn additive.

    更新日期:2018-11-16
  • A kinetic Monte Carlo study of vacancy diffusion in non-dilute Ni-Re alloys
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-16
    Kamal Nayan Goswami, Alessandro Mottura

    The beneficial effect of Re on the creep strengthening properties in single crystal Ni-based superalloys is well known, albeit understanding the underlying mechanism is still an ongoing area of investigation. The microstructure in these alloys comprises of cuboids of the hard precipitate phase embedded in a softer matrix phase. At high temperatures, the glide of creep dislocations is restricted to the matrix only, and dislocation climb is required to get around a precipitate. Vacancy diffusion is an essential component of dislocation climb and elements like Re which are slow-diffusing in Ni are expected to affect this phenomenon. In the present work, we aim to study this by calculating the effect of Re composition on the rate of vacancy diffusion in Ni using kinetic Monte Carlo simulations. First principles electronic structure calculations based on density functional theory have been used to calculate the thermodynamic and kinetic parameters in both dilute as well as non-dilute alloys. Results suggest appreciable modification of the vacancy diffusion coefficients, indicating that the beneficial role of Re in Ni-based superalloys can be largely explained by its effect on vacancy diffusion.

    更新日期:2018-11-16
  • The preparation and performance of grain size gradient TWIP steel fabricated by laser heat treatment
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-16
    Kun Wang, Aiping Wei, Zimu Shi, Xizhang Chen, Jixing Lin, Xian Tong, Zixiong Tao, Xiangjian Chen

    The grain size gradient structure is a promising solution to resolve the problem of strength-ductility trade-off. In this work, a gradient micron-structure (GMS) sample was fabricated by applying a laser heat treatment on the surface of a cold-rolled TWIP steel plate. The recrystallized microstructure showed a linear gradient in grain size from ~25.5 μm to ~13.3 μm throughout the cross section. The yield and ultimate strengths of the GMS sample were 306.3 MPa and 674.1 MPa, respectively, which are lower than those of a sample annealed at 850 °C for 60 min but higher than those of the one annealed at 950 °C for 60 min. It suggests the strengths of the GMS sample obey the rule of mixtures as the coarse grains in the upper surface can be considered as the soft part and the fine grains as the hard ones. However, the GMS sample exhibited a similar tensile elongation (60.7%) as the annealed (850 °C, 1 h) sample does (61.0%). When the strong strain concentration is increased, a large number of small cracks in the lower surface may connect and propagate to the upper side rapidly, resulting in a premature failure.

    更新日期:2018-11-16
  • Effect of thermomechanical processing at α+γ two-phase temperatures on microstructure and mechanical property of 5Mn-0.1C-2Si medium-manganese steel
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-15
    Yu Bai, Yoichiro Matsui, Akinobu Shibata, Nobuhiro Tsuji

    In the present study, thermomechanical processes at ferrite + austenite two-phase temperatures were applied to a 5Mn-0.1C-2Si (wt %) medium-manganese steel having a martensite starting microstructure to investigate the effect of high-temperature processing on microstructure evolution and mechanical properties. Microstructural observations clarified that not only hot compression but also isothermal holding after the hot compression greatly increased the amount of retained austenite at room temperature compared with the statically annealed specimen. The specimens thermomechanically-processed at various temperatures showed complicated changes in mechanical properties at room temperature. Even when the specimens included similar amounts of retained austenite, their mechanical responses varied widely, which was considered to be attributed to the chemical compositions of retained austenite controlled by partitioning of alloying elements during processing in two-phase temperatures.

    更新日期:2018-11-15
  • Microstructural evolution and its influence on toughness in simulated inter-critical heat affected zone of large thickness bainitic steel
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-15
    Xinjie Di, Min Tong, Chengning Li, Chen Zhao, Dongpo Wang

    Simulated inter-critical heat affected zones (ICHAZ) of a large thickness bainitic steel were prepared with three different peak temperatures (Tp) of 750 °C, 780 °C and 800 °C. The microstructure and crystallographic feature were investigated. Toughness of simulated ICHAZ specimens was assessed using instrumental Charpy impact test at −40 °C. It was found that the volume fraction of newly formed bainite increased and the effective grain size decreased with the increase in inter-critical Tp. The toughness decreased slightly with the Tp of 750 °C compared with the base metal (BM), while the toughness improved sharply when the Tp exceeded 780 °C. The network-like martensite was primarily responsible for the low crack initiation energy with the Tp of 750 °C. In contrast, the fine bainite formed with the Tp above 780 °C was effective in increasing the crack nucleation resistance, resulting in the high initiation energy. The matrix softening as well as the grain refinement played important roles in improving the crack propagation energy. Furthermore, it was confirmed that the presence of a multi-phase microstructure can lead to the scattering of toughness.

    更新日期:2018-11-15
  • Influence of composition and grain size on the damage evolution in MAX phases investigated by acoustic emission
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-15
    Karolina Kozak, Mirosław M. Bućko, Leszek Chlubny, Jerzy Lis, Guy Antou, Thierry Chotard

    In this study, the influence of the grain size and phase composition on damage mechanisms of Ti3SiC2-based materials are investigated. Commercially available and self-propagating high-temperature synthesized powders were sintered via spark plasma sintering and hot pressing methods. Materials with different amounts of Ti3SiC2 (from 52 to 72 wt. %) and various mean grain sizes (from 8 to 20 µm) were characterized by performing bending tests coupled with acoustic emission measurements. It permits to distinguish the involved damage mechanisms and their chronology in MAX phase-based materials. With the increase of the applied stress, damage begins with the onset of delaminations within MAX phase grains, then friction processes occur within previously formed microcracks, and finally (before rupture) new microcracks are generated due to the elasticity mismatch between phases. Increasing Ti3SiC2 content and grain size emphasizes the two first damage stages, and finally leads to a more pronounced nonlinear behavior.

    更新日期:2018-11-15
  • Ni-Al2O3 nacre-like composites through hot-pressing of freeze-cast foams
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-14
    Maxime Garnier, David Dunand

    A multi-step process is developed to create a nacre-like ceramic/metal composite. First, a slurry of Ni-coated alumina platelets and NiO nano-particles is directionally freeze-cast and sublimed, creating a foam with aligned lamellar walls containing the two solid phases. Second, the foam is sintered under hydrogen to reduce the NiO phases to metallic Ni micro-particles. Third, hot pressing densifies both the channels between the aligned walls and some of the porosity within the walls, resulting in a brick-and-mortar composite with partially-aligned alumina platelets and 30 vol.% Ni. Evidence of various toughening mechanisms - crack deflection, branching and bridging, as well metal plasticity - is observed during three-point bending tests of double-edge notched beams.

    更新日期:2018-11-14
  • Microstructural evolution and mechanical properties of a linear friction welded two-phase Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy joint
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-14
    Xinyu Wang, Wenya Li, Tiejun Ma, Xiawei Yang, Achilles Vairis

    The microstructure, microtexture evolution and mechanical properties of a linear friction welded two-phase Ti-6.5Al-3.5Mo-1.5Zr-0.3Si titanium alloy joint were investigated. Results show clear microstructural differences at the joint center and edge. A martensitic and acicular α microstructure was found in the weld center zone (WCZ) at the joint center and a Widmanstatten microstructure can be identified in the WCZ at the joint edge. Further away from the WCZ, the microstructural differences smoothed out. The cross-weld crystallographic texture was assessed using electron back scattered diffraction. There are different α texture components in WCZ and thermomechanically-affected zone, but the β textures are similar in these two areas. The microstructural differences at the joint center and edge are the cause of the differences in microhardness distribution and tensile properties along the welding direction. Following post weld heat treatment, the inhomogeneity in both microstructure and mechanical properties were reduced.

    更新日期:2018-11-14
  • Mechanical properties of Zr-based bulk metallic glass parts fabricated by laser-foil-printing additive manufacturing
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-14
    Yingqi Li, Yiyu Shen, Ming C. Leu, Hai-Lung Tsai

    The application of bulk metallic glasses (BMGs) has been traditionally limited to parts with small dimensions and simple geometries, due to the requirement of fast cooling during the conventional process of casting. This research exemplifies a promising additive manufacturing method, i.e., laser-foil-printing (LFP), to fabricate high-quality BMG parts with large dimensions and complex geometries. In this study, Zr52.5Ti5Al10Ni14.6Cu17.9 BMG parts were fabricated by LFP technology in which MG foils are laser welded layer-by-layer upon a substrate. The mechanical properties of the fabricated BMG parts were measured using micro-indentation, tensile test and four-point bending test, and compared to as-cast BMG parts. Through LFP, as rapid cooling rates can be achieved, fully amorphous and nearly fully dense BMG parts without cracking have been successfully made. The glass transition temperature, crystallization temperature, and melting temperature of the fabricated parts are nearly the same as those of the as-cast parts. Additionally, the fabricated BMG parts exhibit mechanical properties, including micro-hardness, tensile strength, and flexural strength, comparable to the as-cast BMG parts.

    更新日期:2018-11-14
  • Dependency of rate sensitive DRX behavior on interstitial content of a Fe-Cr-Ni-Mo alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-14
    Santosh Kumar, Dipti Samantaray, B. Aashranth, Nachiket Keskar, M. Arvinth Davinci, Utpal Borah, Dinesh Srivastava, A.K. Bhaduri

    In the present work the commonly stated but not explicitly demonstrated association between the chemical composition of a material, its stacking fault energy (SFE) and dynamic recrystallization (DRX) behaviour, is established using thermo-mechanical testing, microstructural characterization and thermodynamic calculations on a Fe-Cr-Ni-Mo model alloy with varying interstitial content (0.1 < (N+C) < 0.25 wt.%). The thermodynamic calculation shows that SFE decreases from 27 mJ/m2 to 17 mJ/m2 with increase in interstitial content at room temperature. The values of SFE rise upto 205 mJ/m2 to 200 mJ/m2 at 1323 K. The alloy with minimum interstitial content and maximum SFE exhibits a sluggish DRX behaviour in the strain rate domain 0.1–1 s−1. The sluggishness gradually diminishes as the interstitial content is increased and SFE is decreased. The rate-dependency of nucleation is explained on the basis of interstitial content, which changes the SFE. The present investigation also elucidates the rate- dependent grain growth for the model alloy.

    更新日期:2018-11-14
  • On understanding the microstructure of SiC/SiC Ceramic Matrix Composites (CMCs) after a material removal process
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-14
    O. Gavalda Diaz, D.A. Axinte, P. Butler-Smith, D. Novovic

    The unique material nature (e.g. hard, brittle, heterogeneous and orthotropic) of SiC-based Ceramic Matrix Composites (CMCs) highly affects the outcomes of machining process by inducing high thermo-mechanical loads during material removal. This can result in severe material damage which in turn causes a reduction of the in-service life of critical structural ceramic components (such as in aero-engines or nuclear reactors). In this study, the phenomenon by which the material removal mechanism during drilling influences the CMC surface integrity are discussed by characterising the fracture and deformation phenomena on the CMC's constituents - i.e. SiC and Si materials. Moreover, the strain induced to the surface, together with the changes in chemical composition are characterised via micro Raman spectroscopy and related to the principles of residual stresses upon cutting. This results in a novel understanding of the material removal process that governs cutting of SiC-based CMCs while emphasising how the different microstructure, morphology and nature of ceramics behave under the same cutting conditions. This study has therefore led to a comprehension of how the microstructure of complex hierarchical ceramic materials such as SiC/SiC CMCs is affected by a mechanical cutting process and opens avenues to understand the structure damage under other machining operations (e.g. milling, grinding).

    更新日期:2018-11-14
  • Novel NiAl-strengthened high entropy alloys with balanced tensile strength and ductility
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-13
    Haoyan Diao, Dong Ma, Rui Feng, Tingkun Liu, Chao Pu, Chuan Zhang, Wei Guo, Jonathan D. Poplawsky, Yanfei Gao, Peter K. Liaw

    A single phase, face-centered-cubic (FCC) Al0.3CoCrFeNi high entropy alloy usually has low yield strength. Here, a precipitate-strengthened Al0.3CoCrFeNi has been developed, exhibiting enhanced yield strength while retaining good ductility, which is attributed to a novel microstructure comprising a finely distributed, needle-like B2 phase within the grains of the FCC matrix and a granular σ phase along the grain boundaries. Such a microstructure was obtained by a two-step heat treatment of an as-cast Al0.3CoCrFeNi, whose parameters were determined by integrating CALPHAD-based thermodynamic calculations with microstructural characterization by atom probe tomography. In situ neutron diffraction, in conjunction with crystal-plasticity finite-element simulations, has revealed the strengthening effect owing to the load partitioning between the constituent phases. This work has important implications for understanding phase stability and deformation mechanisms in multi-principal component alloys, and paves the way for developing novel microstructures in complex alloys using correlative techniques.

    更新日期:2018-11-13
  • The effect of pre-twinning on the mechanical behavior of free-end torsion for an extruded AZ31 magnesium alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-13
    Chong Yang, Hongmin Liu, Baocheng Yang, Baodong Shi, Yan Peng, Fusheng Pan, Lu Wu

    In this paper, the effect of pre-twinning on the mechanical behavior of free-end torsion (FET) for an extruded AZ31 Mg alloy was investigated via pre-compression along extruded direction with 1.5%, 3.5%, 5.5% and 6.5% plastic strain levels. After pre-compression, torsion was performed at room temperature. The yield strength was enhanced remarkably during FET test after 3.5% pre-compression. However, the change of yield strength is small during FET with pre-compression (PRC) greater than 3.5%. Refinement hardening induced by twinning contributes to the enhancement of yield strength for PRC 3.5 specimen. With pre-strain increasing, the change of yield strength is ascribed to dislocation hardening. The stage II and stage III of strain hardening were observed during pure torsion and FET test after 1.5% pre-compression. In contrast, the stage II is suppressed and the stage III only occurs during FETs after 3.5%, 5.5% and 6.5%. The pre-twinning favor the activation of prismatic slip under torsion, which enhances multiple slip and dynamic recovery. The transition of deformation mechanisms from basal slip to multiple slip leads to the change of strain hardening during FET after different pre-strain levels.

    更新日期:2018-11-13
  • Thermal Activation Based Constitutive Model for High-Temperature Dynamic Deformation of AZ31B Magnesium Alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-13
    Qijun Xie, Zhiwu Zhu, Guozheng Kang

    Based on the existing experimental results of dynamic deformation at room temperature [1], a high-temperature dynamic test and a few supplementary microstructure observations by using an optical microscope were performed. Based on these experimental results, a thermal-activation-based dynamic constitutive model was proposed to describe the temperature-dependent dynamic deformation of an extruded AZ31B magnesium alloy. In the proposed model, both dislocation slipping and twinning were taken as shear movements with fixed shear planes and directions but with different thermodynamic features. Then, the total flow stress was divided into two parts, i.e., non-thermally and thermally activated ones. In addition, the effects of applied strain, strain rate, and temperature on the dynamic deformation of the extruded AZ31B magnesium alloy were incorporated into the proposed constitutive model. It is demonstrated that the flow stresses predicted by the proposed model show good agreement with the experimental results.

    更新日期:2018-11-13
  • Microstructure analysis and low-cycle fatigue behavior of spray-formed Al–Li alloy 2195 extruded plate
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-13
    Qingyou Zhang, Cunsheng Zhang, Jun Lin, Guoqun Zhao, Liang Chen, Hao Zhang

    Microstructure and low-cycle fatigue behavior of spray-formed Al–Li alloy 2195 extruded plate were investigated in this work. The spray-formed alloy after hot extrusion experiment was treated with solid solution treatment and artificial aging. Microstructure analysis indicated the aged plate was dominated by elongated unrecrystallized grains, and had a rolling-type texture along extrusion direction with the highest intensity at Brass component. The existence of T1 phase strengthened the alloy crucially, but δ′ phase was basically absent. Then, the fully-reversed strain-controlled low-cycle fatigue tests were conducted at total strain amplitudes ranging from 0.4% to 1.0% for samples along two orthogonal directions. The stress-strain hysteresis loops were acquired, and the cyclic stress response curves were derived. At low strain amplitudes (0.4–0.5%), the initial cyclic hardening was slight and followed by a cyclic stability, while at higher strain amplitudes (0.6–1.0%), the alloy merely presented a continuously increasing cycle hardening behavior. Moreover, the fatigue life model based on the total strain energy was built and found to be suitable to predict life. Finally, the fatigue fractography observation showed that the fatigue source is relatively concentrated and the fracture surface had typical fatigue striations at 0.5% strain amplitude, while multiple cracks originated on the sample surface and the final fracture zone showed a ductile characteristic at 1.0%. The deformed microstructure near fracture surfaces were observed, and it was found that the cyclic hardening and stability were closely associated with the interaction between moving dislocations and obstacles including (sub)grain boundaries and secondary phase particles against them.

    更新日期:2018-11-13
  • Effect of post-bond heat treatment on microstructural evolution and mechanical properties of brazed ultrathin-walled structure
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-13
    W.P. Han, M. Wan, R. Zhao, H. Kang, C. Cheng, C.J. Wang

    A nickel-based superalloy Inconel 718 ultrathin-walled structure was manufactured by high-temperature brazing using Ni-Cr-Si filler metal. The microstructure in the brazed fillet was analysed, and the solidification behaviour and the formation of the eutectic island in the brazed fillet were investigated. The effects of post-bond heat treatment (PBHT) with different temperatures and durations on the microstructural evolution and mechanical properties of the brazed ultrathin-walled structure were investigated in detail. The results indicate that PBHT at a relatively high temperature significantly affects the mechanical properties of the ultrathin-walled structure, which is attributed to the formation of the precipitates at a deeper location in the base metal. In addition, the mechanical performance is enhanced by the generation of δ- and γ''-phases under relatively low temperatures (980 ℃ and 740 ℃). On the other hand, the decrease in mechanical performance is caused by the formation of network-like precipitates and the Ni-Si-Nb phase at the grain boundary over extended ageing times. By analysing the fracture mode evolution and the propagation of cracks in the brazing fillet, the effect of PBHT on fracture morphology evolution is determined primarily by the proportion of brittle fractures and the formation of the strengthening phase in the base metal.

    更新日期:2018-11-13
  • Compositional design of strong and ductile (tensile) Ti-Zr-Nb-Ta medium entropy alloys (MEAs) using the atomic mismatch approach
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-13
    V.T. Nguyen, M. Qian, Z. Shi, T. Song, L. Huang, J. Zou

    New non-equiatomic Ti(25+x)-Zr25-Nb25-Ta(25-x) (x = 0, 5, 10, 15, 20, in at. %) medium entropy alloys (MEAs) have been designed using the atomic mismatch approach and fabricated through a conventional arc-melting process. These novel MEAs were derived from a recently developed equiatomic Ti-Zr-Nb-Ta MEA by gradually replacing its Ta content with Ti. Each non-equiatomic MEA solidified as a single solid-solution phase, which was characterized in detail and compared with PandatTM simulation and empirical rules. Systematic tensile mechanical property data revealed the existence of a brittle-to-ductile transition for Ti-Zr-Nb-Ta MEAs, i.e., when 15 at.% of Ta in the equiatomic Ti25-Zr25-Nb25-Ta25 MEA was replaced by Ti to become a Ti40-Zr25-Nb25-Ta10 MEA. The transition occurs corresponding to a small reduction in atomic mismatch from 4.72% to 4.65% but a signficant drop in nanoindentation hardness from 4.2 GPa to 3.5 GPa. In particular, both the as-cast Ti40-Zr25-Nb25-Ta10 and Ti45-Zr25-Nb25-Ta5 MEAs exhibited excellent tensile strain to fracture (>18%) and tensile strength (>900 MPa) with much reduced density compared to the brittle Ti25-Zr25-Nb25-Ta25 MEA. They are both among a very small number of strong and ductile (tensile strain >15%) HEAs reported to date. Their tensile mechanical properties can be further tuned by adjusting the atomic mismatch of the resulting single solid-solution phase in conjunction with the improved understanding of the microstructures of these MEAs.

    更新日期:2018-11-13
  • Microstructural characteristics and mechanical properties of extruded Al-4Cu-1Li-0.4Mg-0.1Zr-xZn alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-12
    Jiangwei Sun, Liang Zhang, Guohua Wu, Xiaolong Zhang, Mian Rong, Cunlong Wang

    The microstructures and mechanical properties of cast and extruded Al-4Cu-1Li-0.4Mg-0.1Zr-xZn alloys were investigated and the amount of Zn additions was up to 1.1 wt.%. The results showed that Zn addition could bring additional MgZn2 phases at the grain boundary and slightly refine the grain size in the as-cast alloy. The mechanical properties of extruded Al-4Cu-1Li-0.4Mg-0.1Zr-xZn alloy increased gradually with the Zn content under different conditions. The yield strength (YS) and ultimate tensile strength (UTS) of T8 treated (stretched by 4% and aged at 150 °C for 32 h) 1.1 Zn alloy could reach 639 MPa and 666 MPa, while elongation (EL) remained at 10.9%. For alloys peak-aged at 175 °C for 32 h (T6 treated), Zn could effectively promote the precipitation of T1 phases (Al2CuLi) but decrease the volume fraction of θ´ phases (Al2Cu) in the matrix. The coarsening of T1 phases could be obviously suppressed by the addition of Zn under overaged condition. However, the precipitate of T1 phases promoted by Zn under the T8 condition is not as obvious as the T6 condition.

    更新日期:2018-11-12
  • Effects of carbon and nitrogen on austenite stability and tensile deformation behavior of 15Cr-15Mn-4Ni based austenitic stainless steels
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-12
    Nithi Saenarjhan, Jee-Hyun Kang, Sung-Joon Kim

    The austenite stability and deformation behavior of 15Cr-15Mn-4Ni based austenitic stainless steels alloyed with C or N are studied. Stability of austenite against martensitic transformation was enhanced with increasing C or N content, while N was a more effective element at an equivalent concentration. With the addition of interstitial elements, both yield and tensile strengths increased. The more pronounced effect of N than C on the yield strength was likely to be due to the formation of short range ordering (SRO) and its higher binding energy with dislocations. The strain hardening behavior was described by classifying the deformation regime into two parts. At low strain, short-range obstacle was a main governing factor, while strain-induced martensitic transformation and dislocation interaction controlled the strain hardening at high strain. The difference between C and N on strain hardening at low strain arose from the existence of SRO which induced the formation of planar slip in N containing alloys. On the other hand, the difference in strain hardening behavior at high strain was attributed to the difficulty of SRO recovery in N containing alloys and a larger increase of SFE by N addition in comparison with C.

    更新日期:2018-11-12
  • Effects of Initial Microstructures on the Microstructural Evolution and Corresponding Mechanical Property of K424 Superalloy after Overheating Exposure
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-12
    X.F. Yuan, J.T. Wu, J.T. Li, W. Li, J.C. Zhao, P. Yan

    Service safety and maintenance costs of turbine components after overheating exposure are closely related to the initial microstructures and subsequent microstructural evolutions after overheating. However, comparative investigation about microstructural evolutions and corresponding mechanical properties of cast polycrystalline superalloys with different initial microstructures after overheating exposure has been rarely reported. In this paper, stress rupture tests at 975 °C /196 MPa, tensile tests at room temperature and microhardness measurements were conducted in both as-cast and standard heat-treated K424 alloy after overheating at the temperature range from 1000 °C to 1200 °C for 2 h. The microstructural evolutions and fracture features were then investigated. The results showed that the mechanical properties didn’t decrease for standard heat treated specimens and even increased gradually for as-cast specimens with increasing the overheating temperature. Microstructural characterization indicated that the grain structures, the morphologies of MC carbides and γ/γ′ eutectics showed no significant change after overheating at different temperatures. The mechanical properties were improved by the more cuboidal γ′ precipitates in the dendritic and interdendritic region, the sufficient re-precipitation of ultra-fine γ′ particles as well as the dissolution of γ′ particles along grain boundary. The initial microstructures had a significant influence on the microstructural evolution and corresponding mechanical properties after overheating exposure. In addition, the high temperature and room temperature fracture behaviors of overheated K424 alloy were also discussed.

    更新日期:2018-11-12
  • Deformation mechanisms of 304L stainless steel with heterogeneous lamella structure
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-10
    Jiansheng Li, Chao Fang, Yanfang Liu, Zhaowen Huang, Shuaizhuo Wang, Qingzhong Mao, Yusheng Li

    Deformation behavior of 304 L stainless steel with heterogeneous lamella structure mainly contains three stages: yield drop stage (~0.2–2%), involving accumulation of stacking faults/geometrically necessary dislocations and initiation of strain-induced martensitic transformation (SIMT); high-speed work-hardening stage (~2–15%), related to dislocation slipping and SIMT; steady work-hardening stage (~15–32%), dominated by SIMT.

    更新日期:2018-11-10
  • Effects of Pre-deformation on Precipitation Behaviors and Properties in Cu-Ni-Si-Cr Alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-10
    Yake Wu, Ya Li, Junyong Lu, Sai Tan, Feng Jiang, Jun Sun

    The effects of pre-deformation on the precipitation behaviors and properties in Cu-Ni-Si-Cr alloy were investigated. The results showed the electrical conductivity and the strength of the alloys after peak aging increased with the pre-deformation amount. Quantitative analyses demonstrated the deformation-induced defects improved the precipitation and the number density of precipitates but reduced their average radii. As a result, the conductivity improved due to the slight promotion of precipitation while the strength increased mostly from the enhanced precipitate strengthening and then from the unremoved work hardening. At elevated temperatures, the effects of pre-deformation were greatly weakened and then the difference among the alloys after different treatments disappeared.

    更新日期:2018-11-10
  • Effects of cooling path and resulting microstructure on the impact toughness of a hot stamping martensitic stainless steel
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-10
    Hélène Godin, Jean-Denis Mithieux, Coralie Parrens, Guillaume Badinier, Mohamed Sennour, Anne-Françoise Gourgues-Lorenzon

    The present study examined the effect of microstructural characteristics on the toughness properties of a hot stamping martensitic stainless steel. Moderately slow cooling during the martensitic transformation leads to the auto-tempering of the martensite laths and the stabilization of thin austenite films. The amounts of retained austenite and cementite precipitates were quantified for various cooling conditions. Charpy impact toughness tests were performed over a large range of temperatures to characterize the ductile-to-brittle transition. Decreasing the cooling rate from 300°C/s down to 3°C/s increased the retained austenite fraction from 0.6% up to 2.6% and decreased the ductile-to-brittle transition temperature by 140°C. The critical cleavage fracture stress was determined to be around 2400 MPa whatever the cooling rate, by applying the local approach to fracture. However, it has been demonstrated that a higher retained austenite fraction modifies incipient plasticity and decreases the yield stress by 60 MPa. As a result, retained austenite delays cleavage fracture by increasing the strain necessary to reach the critical cleavage fracture stress required to trigger cleavage initiation in the ductile-to-brittle transition domain. In this way, retained austenite plays a determining role to decrease the ductile-to-brittle transition temperature. It is thus beneficial to design cooling rates in order to increase the retained austenite fraction and to improve impact toughness at low temperatures.

    更新日期:2018-11-10
  • Isothermal and thermomechanical fatigue behavior of Inconel 718 superalloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-10
    Wenkai Deng, Jinghao Xu, Yunming Hu, Zaiwang Huang, Liang Jiang

    The fatigue behavior and damage mechanisms of Inconel 718 are investigated and discussed under thermomechanical fatigue (TMF) and isothermal fatigue (IF) test conditions. The experimental results show that cyclic temperature accelerates fatigue degradation compared to isothermal equivalents, and the fatigue life under in-phase (IP) TMF mode is shorter than that under out-of-phase (OP) mode. The S-N data can be fitted by Coffin-Manson equation under LCF condition, and TMF lifetime can be predicted by the model based on hysteresis loop energy with a comprehensive consideration of mechanical and thermal strains.

    更新日期:2018-11-10
  • An Investigation of Microstructural Evolution in Electron Beam Welded RAFM steel and 316LN SS dissimilar joint under creep loading conditions
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-10
    V.D. Vijayanand, J. Vanaja, C.R. Das, K. Mariappan, Abhishek Thakur, Shamima Hussain, G.V. Prasad Reddy, G. Sasikala, S.K. Albert

    Microstructural evolution in creep tested electron beam welded Reduced Activation Ferritic Martensitic (RAFM) steel and 316LN stainless steel dissimilar weld joints has been studied at 823 K under different stress levels. The rupture life of the weld joints spanned between 17596 to 190 hours with variation in applied stress from 160 to 220 MPa. Failure of the weld joints occurred in the RAFM steel side at all stress levels. Microstructural examination showed that the failure location shifted from the unaffected base metal region of the RAFM steel to the ‘soft region’ within the heat affected zone (HAZ) in the RAFM steel side with decrease in applied stress. The strength mismatch between various regions within the HAZ of RAFM steel and the microstructural instability of ‘soft region’ in the RAFM steel are the two important factors which dictate the failure location and rupture life of the joints.

    更新日期:2018-11-10
  • Optimizing mechanical properties of gradient-structured low-carbon steel by manipulating grain size distribution
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-10
    Lina Wang, Bin Li, Yindong Shi, Guangwei Huang, Wenpeng Song, Shaoyuan Li

    In this paper, various gradient structures (i.e., the ferrite grain size increases continuously with the depth) with different grain size distributions are produced in low-carbon steel by using pre-torsion deformation and thermal annealing at ~550 °C to 700 °C for 0.5 h. Results indicate that the grain size distribution possesses a crucial influence on mechanical properties of gradient-structured samples. The yield strength decreases steady accompanied by a gradual increase in uniform ductility and static toughness with increasing the ferrite grain size. The optimal synergy of high yield strength (σy~506.0±3.9 MPa), promising uniform ductility (εu~9.6±0.5%) and static toughness (Ur~108.8±4.1 MJ/m3) is achieved by a specific gradient structure with the grain size distribution from ~7.2 μm at the surface to ~16.5 μm at the core, which is much better than that (σy~318.2±1.6 MPa, εu~13.4±0.4% and Ur~102.5±4.8 MJ/m3) of its coarse-grained (CG) counterpart. Related strengthening and toughening mechanisms are also discussed.

    更新日期:2018-11-10
  • Experiment on Properties Differentiation in Tailor Rolled Blank of Dual Phase Steel
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-09
    Diwen Ke, Xianghua Liu, Ying Zhi, Xianlei Hu, Lizhong Liu

    The mechanical properties in tailor rolled blank of dual phase steel (DP-TRB) at different thickness zones present significant differences after annealing. In this paper, the differences in mechanical property of the tailor rolled blank were investigated. The work hardening behavior is analyzed by C-J analysis method. The results indicate that the microstructure of DP-TRB is composed of ferrite and martensite at 800–820 ℃ . It is interesting that the martensite islands gradually chang to equiaxed morphology from rod-like shape at annealing temperature of 800 ℃ . In addition, the differences in mechanical properties between different thicknesses zones become less with the increased intercritical temperature from 760 ℃ to 820 ℃ . It indicates that the difference in yield strength decreases from 230 MPa to 60 MPa with the intercritical temperature changing from 760 ℃ to 820 ℃ . Ferrite recrystallization or not is the main reason for the differential properties. The critical thickness for recrystallization is same ~ 1.6 mm, and the 1.6 mm zones is experimentally proved to possess the largest strength at 760 ℃ , 800 ℃ , and 820 ℃ . The samples in thinner zones are softened because of the recrystallization, while the samples in the larger thickness zone are not strong enough as 1.6 mm with less work-hardening. After the yield point, there is an extra work hardening stage in 1.6 mm and 1.8 mm samples relative to 1.0–1.4 mm samples, which can eliminate nonuniform deformation and prepare for the followed uniform deformation. Consequently, turning of this feature of properties differentiation rationally would play an important role in application of DP-TRB.

    更新日期:2018-11-09
  • Effects of Zn content on microstructures and mechanical properties of in-situ TiB2/Al-Zn-Mg-Cu composites subjected to hot extrusion
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-09
    Ming Wang, Yan Wang, Jun Liu, Zhe Chen, Han Chen, Yi Wu, Fengguo Zhang, Haowei Wang

    Microstructures and mechanical properties of in-situ TiB2 reinforced Al-xZn-2.5Mg-2.4Cu-0.15Zr composites with different Zn content (i.e. x=6.9, 9 and 12 wt.%) were studied in this work. Detailed microstructural investigations have been carried out via scanning electron microscopy and associated orientation imaging microscopy. The results indicate that as Zn content increased from 6.9 to 12 wt.%, the severe clusters of TiB2 particles were noticeably dispersed owing to the decreased interfacial energy between TiB2 particles and Al matrix. The average grain size of extruded samples decreases and the grain shapes tend to be more equiaxed with the increase of Zn content, thanks to the formation of LAGBs and its transformation to HAGBs surrounding dispersed TiB2 particles in the grain. In addition, the yield strength of extruded samples increases from 597 MPa to 707 MPa with the increase of Zn content from 6.9 to 12 wt.%, via grain-boundary strengthening mechanism and reinforcing particles strengthening mechanism.

    更新日期:2018-11-09
  • Improved Cold-Rollability of Duplex Lightweight Steels Utilizing Deformation-Induced Ferritic Transformation
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-09
    Yongmoon Lee, Jae Nam Kim, Geonhyeong Kim, Taekyung Lee, Chong Soo Lee

    The Fe–Mn–Al–C duplex lightweight steels have received considerable attention from the automotive industry because of their low density and good mechanical performance. However, their poor cold-rollability limits their applications. This study proposes a thermomechanical treatment that enhances the cold-rollability of duplex lightweight steels via deformation-induced ferritic transformation (DIFT). DIFT activated by warm rolling destroys the initial band structure and causes a partial grain refinement, thereby causing mechanical strengthening without the loss of ductility. Subsequent annealing enabled cold rolling, while the microstructure formed by DIFT was observed to remain. The steels fabricated using the proposed method exhibited more cold-rollability compared with that exhibited by the conventionally hot-rolled samples. Such an improvement can be attributed to the increased austenitic stability and the resultant inhibition of strain-induced martensitic transformation via DIFT.

    更新日期:2018-11-09
  • In-situ SEM Study of Slip-controlled Short-crack Growth in Single-crystal Nickel Superalloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-09
    L Zhang, LG Zhao, A Roy, VV Silberschmidt, G McColvin

    Initiation and growth of short cracks in a nickel-based single crystal were studied by carrying out in-situ fatigue experiments within a scanning electron microscope (SEM). Specimens with two different crystallographic orientations, i.e., [001] and [111], were tested under load-controlled tension fatigue in vacuum. Slip-caused crack initiation was identified at room temperature while initiation of a mode-I crack was observed at 650 °C. Slip traces continuously developed ahead of the crack tip once initiated and acted as nuclei for early-stage crack growth at both room and high temperature (650 °C). These slip traces were caused by accumulated shear deformation of activated octahedral slip systems, which were specifically identified by analysing the surface slip traces and crack-propagation planes. The crack-growth rates were evaluated against stress intensity factor range, revealing the anomaly of slip-controlled short-crack growth. The effects of crystallographic orientations and temperature on fatigue crack growth were subsequently analysed and discussed, including the influence of microstructural features such as carbides and pores.

    更新日期:2018-11-09
  • Design of a low density Fe-Mn-Al-C steel with high strength-high ductility combination involving TRIP effect and dynamic carbon partitioning
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-09
    J. Kang, Y.J. Li, X.H. Wang, H.S. Wang, G. Yuan, R.D.K Misra, G.D. Wang

    A novel process involving hot rolling and air cooling followed by dynamic carbon partitioning is proposed to design a low density Fe-Mn-Al-C steel with high strength-high ductility combination. The low density alloy 0.25C-3Mn-2Al (wt%) steel was designed to control the phase transformation and achieve dynamic carbon partitioning, thereby obtaining bainite/martensite matrix embedded with nano-sized retained austenite (RA). The effect of different air-cooling finish temperatures on the microstructure and mechanical properties is elucidated in the study described here. Multi-phase microstructures of ferrite, martensite/bainite and RA were obtained during air-cooling in the temperature range of 360–510 ℃. It was interesting that bainite matrix was obtained at finish temperature of 400 ℃, while the martensite matrix including lath and twin martensite was obtained on air cooling temperature to 510 ℃. The twin martensite resulted in higher tensile strength of ~1096 MPa in sample air cooled to 510 ℃. The RA in samples subjected to dynamic partitioning was high, significantly approaching 27.3%. Additionally, RA was characterized into two types, film and blocky. A large amount of blocky RA in sample air cooled to 510 ℃ leaded to 21.4% transformed RA during uniform deformation. Consequently, excellent combination of high tensile strength of ~1096 MPa and uniform elongation of ~16% was attained in sample air cooled to 510 ℃. The study simplifies the existing processes and breaks the constraint for quenching and partitioning treatment limited by quenching temperature below Ms. It has important implications for developing the new generation hot rolled high strength steels.

    更新日期:2018-11-09
  • Deformation behaviors of a hot rolled near-β Ti-5Al-5Mo-5V-1Cr-1Fe alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-09
    Yanghuanzi Li, Xiaoqin Ou, Song Ni, Min Song

    A near-β Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy was hot rolled at 700 °C with a total thickness reduction of 20%, 40%, 60%, and 80%, respectively. The microstructural evolution was analyzed using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The mechanical properties, including the strength and ductility, were also examined. The microstructural evolution of the hot-rolled alloy can be described as a serial “rotation-bending-fragmenting-spheroidization” process of the lamellar α phase and the dynamic recrystallization of the matrix β phase. The microstructures, including the orientation relationship, the distribution of dislocations, and the formation of { 10 1 ̅ 1 } twins and other substructures, were systematically discussed. According to the results of the tensile tests, the strength increased while the ductility decreased with increasing thickness reduction, due to the evolution of dislocation substructures. In addition, the alloy was dominated by a mixed fracture mechanism of micro-void coalescence and intergranular fracture after hot rolling with a small thickness reduction, but exhibited a quasi-cleavage fracture mechanism after hot rolling with a large thickness reduction.

    更新日期:2018-11-09
  • On the dynamic behavior and relationship to mechanical properties of cold-rolled Fe-0.2C-15Mn-3Al steel at intermediate strain rate
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-09
    Z.Y. Tang, J.N. Huang, H. Ding, Z.H. Cai, R.D.K. Misra

    The dynamic tensile behavior of Fe-15Mn-3Al-0.23 C TRIP/TWIP steel was studied at intermediate strain rates in the range of 7 to 700 s−1. The results showed that transformation induced plasticity (TRIP) effect, twinning induced plasticity (TWIP) effect, dislocation glide and adiabatic temperature rise effect coexisted during dynamic deformation. With the increase of strain rate from 7 to 70 s−1, the number of deformation twins increased and some intersecting twins appeared. However, when the strain rate approached 700 s−1, the amount of deformation twins were decreased and intersecting twins disappeared. During deformation at strain rates from 7 to 700 s−1, the strain rate had no significant effect on TRIP effect, and deformation twinning was the main deformation mechanism. The ultimate tensile strength (UTS) showed a positive strain rate sensitivity with increased strain rate, and the experimental steel exhibited remarkable mechanical properties with the product of ultimate tensile strength and total elongation (PSE) of 61.5 GPa% at the strain rate of 70 s−1.

    更新日期:2018-11-09
  • Effects of nanosized particles on microstructure and mechanical properties of an aged in-situ TiB2/Al-Cu-Li composite
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-09
    Bowen Zhao, Qing Yang, Liang Wu, Xianfeng Li, Mingliang Wang, Haowei Wang

    The effects of nanosized TiB2 particles on microstructural and mechanical properties of the hot extruded in-situ TiB2/Al-Cu-Li composite after T6 heat treatment are quantitatively evaluated by using scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction technique and X-ray diffraction. Most TiB2 particles are aggregated together to form the particle bands. The composite develops the typical fiber textures, <111> and paralleling to extruded direction, while the matrix alloy only develops weak <111> and <113> textures. The intensity and volume fraction of major texture components can increase significantly with the addition of TiB2 particles. Due to the introduction of TiB2 particles, T1 and S phases not only have higher number density, but also are smaller. In addition, the T1 phases can connect with each other to form a continuous network in the composite. The θ’ phase, identified in matrix alloy, is rarely observed in the composite. The yield strength and ultimate tensile strength of composite have increased by 248 MPa and 152 MPa compared with matrix alloy, respectively.

    更新日期:2018-11-09
  • Damage mechanism and life prediction based on tensile-stress- and compressive-stress-dominated low-cycle fatigue of a directionally solidified Ni-Based superalloy DZ445
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-09
    Biao Ding, Weili Ren, Jianchao Peng, Tianxiang Zheng, Long Hou, Jianbo Yu, Zhongmin Ren, Yunbo Zhong

    Based on the tensile stress-dominated (strain ratio, R = ε min / ε max = 0 ) and compressive stress-dominated (R=∞) low-cycle fatigue (LCF) of a first-generation directionally solidified Ni-based superalloy DZ445 at 900 °C, the paper investigated the contribution of tensile stress and compressive stress to the damage during LCF and creep-fatigue deformation. The results demonstrated that the fatigue life in the case of tensile stress-dominated LCF is about half of that in compressive stress-dominated LCF. Under the case of tensile stress-dominated LCF, the maximum tensile stress was about twice the maximum compressive stress, thus, the tensile mean stress of about 100 MPa occurs. The hysteresis loop showed a very small amount of plastic strain. The transgranular mode and single-source fatigue characteristics were shown in the fracture surface. There were some characteristics of slip bands and cross-slip in the dislocation structures. The stress response in compressive stress-dominated LCF was opposite to that in tensile stress-dominated LCF. The fractograph exhibited mixed transgranular and intergranular modes with plastic characteristics such as dimples and multi-source fatigue characteristics. In addition to the slip bands and cross-slip characteristics, there were also some stacking faults and dislocation networks. The change in fatigue life and stress response could be explained by these microstructure features at both cases well. Based on the above two LCF damage mechanisms, energy-based life prediction model was modified. The data of LCF and creep-fatigue tests at 900 °C was used to verify the prediction accuracy of the modified life prediction model, which was found to have a good prediction accuracy.

    更新日期:2018-11-09
  • Microstructure anisotropy and its implication in mechanical properties of biomedical titanium alloy processed by electron beam melting
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-09
    M. Wang, H.Q. Li, D.J. Lou, C.X. Qin, J. Jiang, X.Y. Fang, Y.B. Guo

    Titanium alloy Ti-6Al-4V processed by electron beam melting (EBM) has a great potential for orthopedic and aerospace applications. However, the process induced porosity and microstructure anisotropy will have a significant impact on the material properties. This work has found that spherical and elongated pores with strong size effect are common characteristics for the as-EBM samples made with horizontal, diagonal, and vertical orientations w.r.t. the substrate. Furthermore, the major axis of the elongated pores is perpendicular to build direction for samples with different build orientations. The microstructure consists of columnar prior β grains delineated by grain boundary α and transformed α/β structures with α’ marteniste and basket weave morphology. Of note is that a high fraction of twin boundaries are prevalent in α (α’) phase. The configuration of the applied load w.r.t. the major axis of the elongated pores is the most significant influencing factor to mechanical properties, while the columnar prior β grain structure is secondary. Fractography reveals that microcracks tend to originate from elongated pores for cleavage fracture. In addition, the co-existing local terrace-like and shallow dimples are attributed to the intergranular crack propagation from the lamella α grain boundaries. Thus, the anisotropy of porosity and microstructure is of significance to enhance mechanical properties in process development.

    更新日期:2018-11-09
  • Effect of graphene nanoplatelets content on the microstructural and mechanical properties of AZ80 magnesium alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-08
    M. Torabi Parizi, G.R. Ebrahimi, H.R. Ezatpour

    In general, graphene nanoplatelets (GNPs) did not entirely imparted their extraordinary properties to the particulate magnesium matrix composites (PMgMCs) due to the poor wettability between GNPs and Mg matrix alloys as well as presence of agglomerated GNPs. Therefore, in this study, microstructure and mechanical characterizations were used to investigate effect of GNPs content on the microstructural and mechanical properties of AZ80 magnesium alloy. Current study explored the potential and mechanism of GNPs in improvement of mechanical properties of PMgMCs with insight of microstructure. Therefore, AZ80 reinforced by low contents of GNPs (0.1 and 0.6 wt. %) were fabricated by rheo casting followed by hot extrusion. Simultaneously enhancement of tensile properties, the strengthening and the fracture strain efficiencies were achieved in AZ80/0.1GNPs composite by cost effectiveness and simple adaptability of manufacture method which results fairly uniform distribution of GNPs. The addition of 0.1 wt.% GNPs led to grain refinement ( 10%), dynamic recrystallization, stronger basal texture ( 750%), more dissolution of β-eutectic phase ( 66%) which results more dynamic precipitates ( 70%) and reduction of yield asymmetry ( 23%). Compared to the AZ80 alloy, the tensile and compressive yield strengths of AZ80/0.1GNPs composite were enhanced by 40% and 15%, respectively. In both tensile and compressive testes, the effective load transfer was the most important strengthening mechanism. Also, the tensile and compressive failure strains of AZ80/0.1GNPs nanocomposite were enhanced by 50% and 37%, respectively. The uniform dispersion of GNPs, increase of non-basal slip, grain refinement, lower eutectic content and smaller discontinuous intergranular precipitates increased failure strain.

    更新日期:2018-11-08
  • Influence of cyclic loading on the structure and double-stage structure relaxation behavior of a Zr-Cu-Fe-Al metallic glass
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-08
    D.V. Louzguine-Luzgin, M.Yu. Zadorozhnyy, S.V. Ketov, J. Jiang, I.S. Golovin, A.S. Aronin

    The thermal and mechanical relaxation spectra of a Zr-Cu-Fe-Al bulk metallic glass were obtained before and after room-temperature cyclic loading up to 0.4% of elastic strain. The loading was performed using a dynamic mechanical analyzer at an oscillation frequency of 3 Hz to induce the structural changes within the glassy phase. A double-stage structure relaxation behavior was monitored using the same dynamic mechanical analyzer at a lower strain. These structure changes within the glassy phase were studied with X-ray diffractometry and high-resolution transmission electron microscopy. The cycling loading at room temperature lead to drastic changes in the atomic structure of the bulk metallic glassy samples. In particular, it caused partial nanocrystallization after 10000 elastic cycles, which in turn influenced the relaxation behavior. The structural relaxation process of the Zr-Cu-Fe-Al bulk metallic glass monitored by thermal measurements is different from that observed by the dynamic mechanical analysis.

    更新日期:2018-11-08
  • Rotary swaged laminated Cu-Al composites: effect of structure on residual stress and mechanical and electric properties
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-06
    Radim Kocich, Lenka Kunčická, Karel Dvořák, Adéla Macháčková

    This study focuses on the investigation of uniquely sequenced Cu-Al clad composite wires, which are considered as perspective materials for applications within a wide range of industrial and commercial branches. The wires were produced by rotary swaging under variable temperature (at 20 °C and 250 °C). The required diameter of the final wire was 5 mm, however, detailed analyses were performed on 10 mm, 7.5 mm, 6 mm and 5 mm samples. The swaged composite wires were studied from the viewpoints of deformation behaviour, structure and mutual interfaces, and electric and mechanical properties via scanning and transmission electron microscopy and neutron diffraction, testing of electrical resistivity, and mechanical testing with implemented acoustic emission detection. The results showed non-negligible effects of both the variables/influencing factors (swaging degree and temperature) on the investigated parameters. The development of intermetallics was observed at the interfaces of samples swaged with high swaging degrees, but only at 250 °C. These intermetallics provoked signal activity during acoustic emission detection and deteriorated the electric conductivity. On the other hand, substantial work hardening occurring during swaging at 20 °C also decreased the conductivity. By these reasons, the electric properties were the poorest for work hardened samples with occurring intermetallics. Both the final 5 mm composites exhibited satisfactory bonding of both the components and recrystallized structures with ultra-fine grains ensuring the ultimate tensile strength higher than 200 MPa.

    更新日期:2018-11-06
  • Study of precipitation-assisted stress relaxation and creep behavior during the ageing of a nickel-iron superalloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-06
    Hailong Qin, Zhongnan Bi, Dongfeng Li, Ruiyao Zhang, Tung Lik Lee, Guang Feng, Hongbiao Dong, Jinhui Du, Ji Zhang

    To study the stress relaxation and creep behavior of a nickel-iron base superalloy during the ageing process, solution annealed microstructures were subjected to loading experiments at various temperatures. Neutron diffraction (ND) measurements were carried out during the loading process to record the elastic lattice strain response of the γ, γ″ and γ′ phases. The results show that over time, increased length contraction occurred at all three temperatures and the sample tested at 720 °C experienced the most significant shrinkage. During the precipitation of the γ″ phase, the chemical composition of the γ matrix will change as the niobium atoms migrate to the form γ″ precipitates, which directly contributes to a change in the lattice parameter of the matrix and governs the samples' volume shrinkage. The creep strain at 720 °C was positive, while negative creep could be seen at 500 °C and 620 °C. The observed negative creep at 500 °C and 620 °C was caused by the samples' shrinkage, and the creep strain at 720 °C was positive because creep plastic strain is relatively large and thus dominates the measured creep deformation. The stress relaxation results exhibited a significant increase with the increase in test duration at 500 °C, 620 °C and experienced a normal decrease at 720 °C. Stress increase during the relaxation tests is a manifestation of the phenomenon of “negative creep”. The precipitation induced volume shrinkage was found to be the key reason for the observed negative creep and abnormal stress relaxation.

    更新日期:2018-11-06
  • Effect of Supercritical CO2 on the Performance of 740 H Fusion Welds
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-06
    Andrew Meyer Brittan, Jacob Mahaffey, Mark Anderson, Kumar Sridharan

    The supercritical carbon dioxide Brayton cycle is of significant interest due to the potential for high efficiency and low capital cost compared to Rankine power cycles. Maximum temperature and pressure conditions produce higher efficiencies, leading to the desire for materials with high strength at elevated temperatures, such as INCONEL 740 H. This alloy has been found to have good corrosion properties, but the mechanical and weld performance in carbon dioxide has not been studied. This study has attempted to address this issue by testing the mechanical properties in both base material and transverse gas tungsten arc welded samples before and after exposure to supercritical carbon dioxide at 750 °C, 20 MPa for 1,000 hours. After exposure to CO2, base material showed an increase in strength with a moderate drop in ductility, while welded samples exhibited a decrease in both yield strength and elongation. This matched well with samples which were thermally aged in a neutral environment. Further analysis indicated the CO2 environment had little effect beyond a depth of 5μm of the surface, and no effect beyond 10–15μm. The evolution of mechanical properties of 740 H in this environment, therefore, is dominated by the effect of thermal aging, with only trace impact by the CO2 environment. The thermal aging effect was shown to be mechanically detrimental within the fusion zone of the weld due to micro-segregation during the welding process.

    更新日期:2018-11-06
  • Molecular dynamics and experimental studies of nanoindentation on nanoporous silica aerogels
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-06
    Sandeep P. Patil, Vinayak G. Parale, Hyung-Ho Park, Bernd Markert

    The nanomechanics during the indentation test on low-density nanoporous silica aerogels remains one of the least understood and explored areas of mechanics. In the present work, we performed nanoindentation using a spherical indenter on silica aerogels to investigate the mechanical properties, such as elastic modulus and hardness, and also, the deformation behaviour. Using all-atom simulations on large samples, the elastic modulus is computed from the elastic part of force–depth curves that can be fitted to the Hertz law, which shows that it increases with density. We proposed a novel approach to calculate the projected true contact area in nanoindentation and to estimate an accurate hardness of silica aerogel, which has a highly complex and randomly arranged network of atoms structure. The experimental studies of nanoindentation are performed on silica aerogel, which reveals that the measured elastic modulus is in good agreement with the simulations. However, the measured hardness values are nearly close to the projected contact area method. It suggests that in all-atom simulations the computed high hardness values using the proposed true area method are the actual local contact pressure. This new understanding may help to expand the use of computer simulations to explore the nanoindentation processes at the molecular level and to advance the macroscopic hardness calculation.

    更新日期:2018-11-06
  • Effect of Powder Oxidation on Interparticle Boundaries and Mechanical Properties of Bulk Al Prepared by Spark Plasma Sintering of Al Powder
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-06
    Lei Cao, Wei Zeng, Yuehuang Xie, Jiamiao Liang, Deliang Zhang

    Oxidation of an as-gas atomized Al powder prevents the transformation of interparticle boundaries (IPBs) into γ-Al2O3 nanoparticle-decorated grain boundaries in the bulk Al prepared by spark plasma sintering, and leads to formation of continuous γ-Al2O3 layers with nanopores at the IPBs, causing a dramatic decrease of the elongation to fracture.

    更新日期:2018-11-06
  • Simultaneously improving the strength and ductility of extruded bimodal size SiCp/AZ61 composites: synergistic effect of micron/nano SiCp and submicron Mg17Al12 precipitates
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-05
    Xiaofeng Niu, Gang Li, Zengyao Zhang, Pengwen Zhou, Hongxia Wang, Shaoxiong Zhang, Weili Cheng

    AZ61 matrix reinforced by micro- and nano-scaled SiC particles fabricated via semisolid stirring assisted ultrasonic vibration was subjected to extruded at different speeds (0.1, 0.5 and 2.5 mm/s) with a constant temperature of 280 ℃. The effect of extrusion speed on the microstructure and tensile properties of the extruded composite was investigated. Grain size, precipitates fraction and texture were found to be greatly affected by the extrusion speed, resulting in tensile properties showing superior strength and ductility as the extrusion speed being of 0.1 mm/s. The improved strength and ductility were ascribed to the synergistic effect of micron/nano SiCp and submicron Mg17Al12 precipitates. The strengthening mechanism of extruded composites was also discussed.

    更新日期:2018-11-05
  • Reducing arc heat input and obtaining equiaxed grains by hot-wire method during arc additive manufacturing titanium alloy
    Mater. Sci. Eng. A (IF 3.414) Pub Date : 2018-11-05
    Zixiang Li, Changmeng Liu, Tianqiu Xu, Lei Ji, Donghai Wang, Jiping Lu, Shuyuan Ma, Hongli Fan

    Arc additive manufacturing technology has high deposition efficiencies and material utilization rates. However, the large heat input and high temperature gradient during arc additive manufacturing process lead to the formation of the coarse columnar grains. Due to the presence of the coarse columnar grains, the mechanical properties of the part have great anisotropy which limits the application of it. In this experiment, a method named hot-wire arc additive manufacturing was adopted to reduce the arc heat input and refine the columnar grains, and four thin-walled samples with the material of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si were manufactured. It is interesting to find that the coarse columnar grains have been greatly refined, and finally a part consisting of equiaxed grains and short columnar grains was obtained. At the same time, the width of the α-lath has also been refined. The mechanical properties are in accordance with the grain changes and the anisotropy almost disappeared. And it seems that a part with comprehensive mechanical properties can be obtained by the hot-wire arc additive manufacturing.

    更新日期:2018-11-05
Some contents have been Reproduced with permission of the American Chemical Society.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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