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  • Mechanical and thermoelectric properties of environment friendly higher manganese silicide fabricated using water atomization and spark plasma sintering
    Intermetallics (IF 3.353) Pub Date : 2020-01-24
    Peyala Dharmaiah; Chul-hee Lee; Gian Song; Soon-Jik Hong

    Here, we report on an economical, viable, one-step method for fabrication of Higher Manganese Silicide MnSi1.75 (HMS). Fabrication of the HMS was followed by spark plasma sintering (SPS) at different temperatures (850, 900, 950 and 1000 °C), after which the thermoelectric and mechanical properties were investigated. It was found that the electrical conductivity significantly increased with increasing SPS temperature. This was attributed to the simultaneous increase of carrier concentration and mobility, while a reduction in the Seebeck coefficient was attributed to the formation of secondary phase in the HMS samples. A low thermal conductivity of 1.86 W/mK at 723 K was obtained for the sample sintered at 850 °C, which was attributed to increased scattering of heat-carrying phonons due to pores or voids, and grain boundaries. In addition, a maximum ZT of 0.27 was obtained at 723 K for the HMS-1000 sample, which is comparable with other previously reported data for undoped HMS material. Nevertheless, the HMS-1000 sample exhibited compressive strength of 1208 MPa and fracture toughness of 1.99 MPa m1/2, which is significantly higher than other state-of-the-art thermoelectric materials. The proposed fabrication method could be adopted for mass market applications requiring superior mechanical and thermoelectric properties.

    更新日期:2020-01-24
  • Structure and magnetic properties of Alnico8 alloy thermo-magnetically treated under a 10 T magnetic field
    Intermetallics (IF 3.353) Pub Date : 2020-01-23
    X.Y. Sun; C.L. Chen; M.Y. Ma; L. Yang; L.X. Lv; S. Atroshenko; W.Z. Shao; L. Zhen

    The structure and compositions of spinodal phases in Alnico8 alloy isothermally treated at 830 °C under external magnetic field (Hext) up to 10 T were investigated by transmission electron microscopy (TEM). Under ultra-high magnetic field (10 T), the spinodal structure of Alnico8 alloy presents interesting diversity for the shapes of α1 phase particles, namely, Π-shape, H-shape and Ш-shape apart from regular rod-shape. Composition analysis shows that the Fe was mainly distributed in the α1 phase. The Co content in the α1 phase was lower than that in the α2 phase as evidenced by the point analysis results, and Co content inα1 phase of the alloy treated without magnetic field was higher than that of the alloy treated with a magnetic field. In contrast, the Ni, Al, Cu and Ti were concentrated in the α2 phase. The average hyperfine field of the alloy treated without magnetic field was about 301.3 kOe. While the alloy treated under magnetic field, it was slightly decreased, which was about 292.0 and 296.1 kOe under 0.7 and 10 T, respectively. Magnetic properties of the alloy treated under a 10 T magnetic field were the best, which were about 1.09T, 396Oe and 9.5 kJ/m3 for Br, Hc and (BH)max respectively. Especially for the coercivity, which was mainly contributed by the anisotropy of the α1 phase induced by the high magnetic field.

    更新日期:2020-01-23
  • Burst-like shape recovery and caloric effects in Ni–Fe–Ga–Co single crystalline shape memory alloys
    Intermetallics (IF 3.353) Pub Date : 2020-01-23
    V.I. Nikolaev; S.I. Stepanov; P.N. Yakushev; V.M. Krymov; S.B. Kustov

    Shape recovery in conventional shape memory alloys is typically a relatively slow process showing low heating rate sensitivity. In contrast to that, some shape memory alloys such as Ni–Fe–Ga–Co, Cu–Al–Ni, Cu–Al–Fe–Mn can exhibit the effect of burst-like shape recovery when the original shape is restored almost instantaneously in a very narrow temperature range. Here we report on an abnormal stress-strain behavior of Ni49Fe18Ga27Co6 single crystals upon uniaxial compression along the [110] axis and caloric effects during burst-like recovery of shape memory strain in the samples. Differential scanning calorimetry (DSC) study revealed that DSC peak shifts to lower temperatures with increasing heating rate.

    更新日期:2020-01-23
  • Oxidation behaviour of a novel refractory high entropy alloy at elevated temperatures
    Intermetallics (IF 3.353) Pub Date : 2020-01-21
    Kai-Chi Lo; Hideyuki Murakami; Jien-Wei Yeh; An-Chou Yeh

    The refractory high entropy alloy (RHEA) has shown great potentials for high temperature applications beyond modern Ni-based superalloy. However, its oxidation behaviours are rarely reported and understood. In this work, the oxidation behaviours of a novel RHEA “NV1”, Cr-17.6Al-20.3Mo-15.2Nb-2.9Si-13.4Ta-5.4Ti (in at%), were examined at 1200, 1300, and 1400 °C up to 100 h. At 1200 °C, the oxidation kinetics curve yielded toward parabolic behaviour owing to the formation of a rutile-type complex oxide layer with Al2O3 and Cr2O3 dispersions; breakaway oxidation contributed by Cr2O3 evaporation occurred at 1300 °C; a single power-law behaviour governed the oxidation kinetics curve at 1400 °C, and mullite was identified within the oxide layer. This work provides guidelines for understanding the oxidation mechanisms and improving oxidation resistance of RHEA at elevated temperature.

    更新日期:2020-01-22
  • Quasicrystalline phase and crystalline approximant in Ni–Mn–In Heusler alloy system
    Intermetallics (IF 3.353) Pub Date : 2020-01-20
    X.-Z. Li; W.-Y. Zhang; D.J. Sellmyer

    Intermetallic compounds and micro-structures in Ni–Mn–In alloy system were investigated with transmission electron microscopy, selected-area and nano-beam electron diffraction, and energy-dispersive x-ray spectroscopy. Together with two types of Heusler phases, a decagonal quasicrystalline phase and a structurally related crystallite were found in the Ni–Mn–In system. As is well-known, most of the decagonal quasicrystals are aluminum-based intermetallic phases. It is interesting to investigate the aluminum-free decagonal quasicrystalline phase as a new member of the quasicrystal family. Structural characterization was carried out on the decagonal quasicrystalline phase and the crystalline approximant in the Ni–Mn–In alloy system.

    更新日期:2020-01-21
  • Effect of Al substitution on the magnetocaloric properties of Ni-Co-Mn-Sn multifunctional alloys
    Intermetallics (IF 3.353) Pub Date : 2020-01-21
    L. Huang; D.Y. Cong; Y. Ren; K.X. Wei; Y.D. Wang

    Ni-Mn-based metamagnetic shape memory alloys have been extensively studied due to their multifunctional properties including magnetocaloric effect (MCE), magnetoresistance effect, and magnetic-field-induced strain. However, the large intrinsic thermal hysteresis of these alloys sets a limit on further development for practical applications. Here, with a few atomic percent Al substitution, we greatly reduced the thermal hysteresis and transformation temperature interval while maintaining the large magnetization difference (ΔM) between austenite and martensite. The simultaneous achievement of a large ΔM and low thermal hysteresis leads to a remarkable enhancement in MCE upon Al substitution, and a maximum magnetic entropy change of 31.6 J/kg K is achieved under a field of 5 T in Ni39Co11Mn40Sn8Al2. Furthermore, our in situ high-energy X-ray diffraction experiments show that the Ni39Co11Mn40Sn8Al2 has good geometric compatibility between austenite and martensite, which accounts for the reduced thermal hysteresis in this alloy. This study is instructive for the development of high-performance magnetocaloric materials in the metamagnetic shape memory alloys.

    更新日期:2020-01-21
  • Differences in texture evolution from low-entropy to high-entropy face-centered cubic alloys during tension test
    Intermetallics (IF 3.353) Pub Date : 2020-01-10
    Kuan-Hao Lin; Shou-Yi Chang; Yu-Chieh Lo; Chun-Chieh Wang; Su-Jien Lin; Jien-Wei Yeh
    更新日期:2020-01-11
  • Microstructures and room temperature tensile properties of as-cast and directionally solidified AlCoCrFeNi2.1 eutectic high-entropy alloy
    Intermetallics (IF 3.353) Pub Date : 2020-01-09
    Lei Wang; Chengli Yao; Jun Shen; Yunpeng Zhang; Tao Wang; Yuhui Ge; Luhan Gao; Guojun Zhang

    The microstructures and room temperature tensile properties of as-cast and directionally solidified AlCoCrFeNi2.1 eutectic high-entropy alloy were investigated using OM, SEM, EDS, TEM and tensile tests. The microstructure of as-cast alloy, including the well-aligned lamellar structures, the radial emanating lamellar structure and mesh-like structure, consists of NiAl-rich (B2) and CoCrFeNi-rich (L12) phases. From the longitudinal section of directionally solidified alloy, when the withdrawal rate is 6 μm/s, the solid-liquid interface morphology is planar, and the well-aligned lamellar structure is observed. The three-dimensional morphologies of well-aligned lamellar structures are well characterized by deep etching. With increasing the withdrawal rate to 15 μm/s, the solid-liquid interface morphology change from planar to cellular, and the microstructure changes from planar eutectic to cellular eutectic, even the dendritic eutectic at local region occurs at 60 and 120 μm/s. From the transverse section, the well-aligned lamellar structures occur in the cell interior, and the interlamellar spacing decreases gradually with increasing the withdrawal rates. The TEM result indicates that NiAl-rich phase has an orientation relationship with CoCrFeNi-rich phase of [1¯11] NiAl-rich//[1¯22]CoCrFeNi-rich and (211)NiAl-rich//(022¯)CoCrFeNi-rich. The nano-precipitates occur in NiAl-rich (B2) and CoCrFeNi-rich (L12) phases. The directionally solidified alloy possesses a good combination of strength and ductility at 60 μm/s. In particular, using the directional solidification technology is a good way to improve the ductility of the alloy (∼37%). The strengthening and plasticizing mechanisms are discussed by analyzing the fracture and side surfaces.

    更新日期:2020-01-09
  • Crystallization and corrosion resistance of Zr–Ti–Y–Al–Cu–Ni–Co–Fe complex multi-component bulk metallic glasses
    Intermetallics (IF 3.353) Pub Date : 2020-01-08
    Chen Chen; Hang Zhang; Yanzhou Fan; Weiwei Zhang; Ran Wei; Shaokang Guan; Tan Wang; Bin Kong; Tao Zhang; Fushan Li

    A series of Zr–Ti–Y–Al–Cu–Ni–Co–Fe complex multi-component bulk metallic glasses (CMBMGs) were designed using multi-substitution strategy in Zr55Cu35Al10, Zr60Cu30Al10 and Zr60Cu28Al12 BMGs, and their crystallization and corrosion behaviors were systematically investigated. It is found that, CMBMGs have higher complexity in crystallization compared to their ternary counterparts, and well-dispersed nano-crystals with diameters of 5–30 nm could be easily formed in CMBMGs in a wide range of annealing temperature, reflecting sluggish crystallization kinetics. Moreover, these CMBMGs exhibit considerable higher corrosion resistance than their ternary counterparts. Besides, both the glass transition temperature (Tg) and onset crystallization temperature (Tx) decrease compared to their ternary counterparts. In addition, it is interesting that the crystallization behavior and corrosion resistance of these Zr–Ti–Y–Al–Cu–Ni–Co–Fe CMBMGs are mainly influenced by the elements substituted in Zr–Cu–Al alloys, which provides a novel route for compositional optimization of CMBMGs. This work is beneficial to better understand the phase formation and properties of BMGs with complex compositions.

    更新日期:2020-01-08
  • Enhanced tensile strength and fracture toughness of a Ti-TiAl metal-intermetallic laminate (MIL) composite
    Intermetallics (IF 3.353) Pub Date : 2020-01-07
    Wei Sun; Fenghai You; Fantao Kong; Xiaopeng Wang; Yuyong Chen

    High temperature titanium alloy/Ti-43Al-9V metal-intermetallic laminate composite was successfully fabricated by hot-pack rolling. The titanium alloy exhibits a lamellar structure and the TiAl alloy consists of B2 phase and elongated γ phase. Due to the low rolling and annealing temperature, the TiAl alloy shows a low extent of dynamic recrystallization (69.3%) and a high dislocations density (17%). The composite shows an excellent combination of the enhanced tensile strength and fracture toughness. The tensile strength of composite at 25 °C and 700 °C are 845 MPa and 730 MPa respectively. The fracture toughness of composite are 29.4 MPa∙m1/2 and 26.6 MPa∙m1/2 in arrester orientation and divider orientation respectively, which are much higher than monolithic Ti-43Al-9V alloy. The strengthening of composite is mainly attributed to high density dislocations and sub-structures stored in composite and the toughening mechanisms of composite are crack deflection, secondary cracks and crack tip convolution.

    更新日期:2020-01-07
  • Fast rejuvenation in bulk metallic glass induced by ultrasonic vibration precompression
    Intermetallics (IF 3.353) Pub Date : 2020-01-07
    Yan Lou; Xiao Liu; Xiaole Yang; Yang Ge; Dandan Zhao; Hao Wang; Lai-Chang Zhang; Zhiyuan Liu

    Tuning energy state of metastable bulk metallic glasses (BMGs) is well known significant for understanding their glass nature and controlling their performances. Using a newly developed ultrasonic vibration precompression (UVPC) method, fast rejuvenation is achieved in a Zr-based BMG in seconds. The rejuvenated BMG has more heterogeneous structure and largely improved plasticity. The underlying mechanism is revealed. During UVPC treatment, high frequency strain energy is converted into thermal/internal energy due to heterogeneous atomic packing structure of BMG. The combined effect, resulting from external applied elastic stress, internal converted heat and ultrasonic resonance of atoms, drives the loosely packed atoms to a higher energy basin and simultaneously introduces more free volumes in the structure of the BMG. Therefore, the UVPCed BMG has more stored energy and more flow units to initiate shear bands. The findings in this work provide an effective method to modulate the energy state of BMGs and shed insights into comprehensive understanding of energy-structure-properties relationship of amorphous materials.

    更新日期:2020-01-07
  • Magnetocaloric effect of the Gd3-xTbxCo system
    Intermetallics (IF 3.353) Pub Date : 2020-01-02
    A. Guzik; E. Talik; P. Zajdel

    Gd3-xTbxCo compounds with x = 0, 0.25, 0.5, 1, 1.5, 2, 2.5 and 3 were investigated using structural and magnetic methods. The lattice parameters, and the location of 4c and 8d Wyckoff sites occupied by Gd, Tb, and Co atoms, in an Fe3C–orthorhombic type of structure, were refined by the Rietveld method. The magnetic ordering temperature changes linearly with Tb content. The magnetocaloric effect (MCE) in Tb3Co at 77 K and a magnetic field of 7 T is equal to −28.8 J/kgK. An inverse MCE observed in GdTb2Co reaches its maximum value of 6.8 J/kgK at 47 K and a magnetic field of 6 T.

    更新日期:2020-01-02
  • Simultaneously enhancing strength and toughness of Zr-based bulk metallic glasses via minor Hf addition
    Intermetallics (IF 3.353) Pub Date : 2019-12-30
    Y.H. Zhu; Z.W. Zhu; S. Chen; H.M. Fu; H.W. Zhang; H. Li; A.M. Wang; H.F. Zhang

    Zr65.5-xHfxNb3Cu13Ni11Al7.5 (x = 0.0, 0.5, 1.0, 1.5 and 2.0 at.%) bulk metallic glasses (BMGs) were successfully prepared by copper mold casting. The effects of minor Hf addition on the mechanical properties and fracture mechanisms were investigated. The quasi-static compressive mechanical properties of Zr65.5-xHfxNb3Cu13Ni11Al7.5 alloys were significantly improved by Hf addition. The optimum compressive fracture strength (σf) and fracture strain (εf) are 1516 MPa and 2.6%, respectively, for the Zr64.5Hf1.0Nb3Cu13Ni11Al7.5 alloy. The impact toughness of the Zr64Hf1.5Nb3Cu13Ni11Al7.5 alloy (Ak = 127 kJ/m2) is nearly four times than that of the Zr65.5Nb3Cu13Ni11Al7.5 alloy (Ak = 33 kJ/m2). The increase of the fracture surface area and the ratio of vein-like patterns regions caused by Hf addition play an important role in fracture energy dissipation under dynamic loading. The dependence of mechanical properties on the Hf content is closely related to the variation of free volume in the current alloys. The relationship between the Hf addition and the free volume change is discussed in detail. These results may provide new insights into simultaneously enhancing the plasticity and toughness of BMGs with minor alloying.

    更新日期:2019-12-30
  • The effect of bcc/fcc phase preference on magnetic properties of AlxCrFeCoNi high entropy alloys
    Intermetallics (IF 3.353) Pub Date : 2019-12-24
    J. Cieslak; J. Tobola; M. Reissner

    AlxFeNiCrCo (0≤x≤1.5) high entropy alloys (HEA) have been studied both experimentally and theoretically, considering the multi-phase character of investigated samples. Magnetic properties of these alloys (ordering temperatures and average magnetic moments) were determined from magnetization measurements (VSM), being found strongly dependent on Al content. Decomposition of this HEA system into phases with different stoichiometries and different crystal structures was analyzed using electronic structure calculations by the KKR and KKR-CPA methods, and discussed in terms of total energy balance. Additional energy lowering was achieved by admitting atoms separation in Al- and Ni-rich bcc-phase within B2-like crystal structure, and in consequence ordering of some elements constituting HEA, has been determined. Accounting for all experimentally detected phases in AlxFeCrNiCo samples, it was found that calculated total and site-decomposed magnetic moments strongly depended on stoichiometries, more particularly on crystal structures. Comparison of KKR calculations based on ordered supercell models and KKR-CPA results performed for fully disordered model of HEA systems, clearly indicates strong influence of local atoms neighborhood on magnetic moments. It is observed that in spite of increase of nominal Al content in AlxFeNiCrCo HEA, the actual concentration of Al in the detected fcc and bcc phases only slightly changes. Hence the strong measured variation of saturated magnetization with x is essentially driven by relative phase contributions. On the whole, much better agreement between measured and theoretical magnetic moments was obtained when averaging computational results obtained for experimentally detected phases and their contributions, than KKR-CPA results achieved for single-phase HEAs with nominal compositions.

    更新日期:2019-12-25
  • Synergistic regulation of electrical-thermal effect leading to an optimized thermoelectric performance in Co doping n-type Bi2(Te0.97Se0.03)3
    Intermetallics (IF 3.353) Pub Date : 2019-12-23
    Tu Lv; Zhenming Li; Quanxin Yang; Allen Benton; Haotian Zheng; Guiying Xu

    Herein the effect of Co element doping on the thermoelectric properties of n-type Bi2(Te0.97Se0.03)3 alloys fabricated by a melting synthesis technique followed by spark plasma sintering was reported. The micro-morphology and composition of the specimens was characterized by means of X-ray diffraction and scanning electron microscopy. The electrical and thermal properties of (Bi1-xCox)2(Te0.97Se0.03)3 with x = 0%, 2%, 4% and 8% were measured from 298K to 473K. We found that (i) the micro-scale secondary phase CoTe2 was embedded into the matrix due to the low solubility of Co element; (ii) Co doping generally improved the effective mass due to the scattering by local magnetic moments carried by Co ion and practically retained the carrier concentration, thereby yielding the higher Seebeck coefficient; (iii)both the electronic and lattice thermal conductivity decreased simultaneously by introducing the hierarchical defects (point defects and the micro-scale defects). Specifically, the x = 8% sample obtained a minimum thermal conductivity ~1.26 Wm−1K−1 at 398K. This results in a maximum ZT value of ~0.77, approximately ~20% higher than that of pristine Bi2(Te0.97Se0.03)3.

    更新日期:2019-12-23
  • Effect of hot rolling temperature on microstructure evolution, deformation texture and nanoindentation properties of an intermetallic Ti-43Al-9V-0.2Y alloy
    Intermetallics (IF 3.353) Pub Date : 2019-12-19
    Q.B. Wang; S.Z. Zhang; C.J. Zhang; Z.W. Song; D.D. Zhu; D. Dong; S.L. Zhang

    In this paper, microstructure evolution, deformation texture and nanoindentation properties of an intermetallic Ti-43Al-9V-0.2Y (at. %) alloy have been investigated after hot rolled in α+β+γ (R#1) and β (R#2) phase field regions. The primary components of sample R#1 are (β+γ) mixture and (α2+γ) lamellar structure. With the temperature increases to 1250 °C, the microstructure of R#2 shows a new characteristic of martensitic α2 and acicular γ phase. And γ phase has different precipitation mechanisms by hot rolling at 1200 °C and 1250 °C. In R#1, γ phase is obtained through three provenances, which are the precipitating from β to form (β+γ) mixture, the precipitating from α phase to form (α2+γ) lamellar structure, and the remnant γ phase during cooling from α+β+γ phase field. However, for R#2, γ phase always nucleates and grows within β matrix. Meanwhile, microstructure evolution of R#2 is discussed. In addition, the increase in rolling temperature also results in a significant change of the fiber texture. Comparing with γ phase, β phase has the stronger texture, which is due to the formation of γ grain along the K-S orientation relationship in β phase. In R#1 state, β phase exhibits a typical {121} <1–11> texture, while γ phase shows {031} <−5-13> and {112} <1–10> texture. Aiming at R#2, β phase displays {010} <100> cube texture, and γ phase has {-215} <5–53> and Goss {110} <001> texture. The hardness (H) and reduced elastic modulus (Er) are investigated by nanoindentation. The (β+γ) mixture structure in R#2 has the highest H (12.63 GPa), whereas the Er is related to the microstructure composition.

    更新日期:2019-12-19
  • Speromagnetism and asperomagnetism as the ground states of the Tb-Dy-Ho-Er-Tm “ideal” high-entropy alloy
    Intermetallics (IF 3.353) Pub Date : 2019-12-18
    M. Krnel; S. Vrtnik; A. Jelen; P. Koželj; Z. Jagličić; A. Meden; M. Feuerbacher; J. Dolinšek

    We address the nature of the collective magnetic state in an ideal high-entropy alloy (HEA), representing a magnetically concentrated system with all lattice sites occupied by localized magnetic moments and containing randomness and frustration due to chemical disorder. Being a “metallic glass on a topologically ordered lattice”, HEAs possess simultaneously the properties of an ordered crystal and an amorphous glass. The influence of this crystal-glass duality on the collective magnetic state was studied experimentally on a hexagonal Tb-Dy-Ho-Er-Tm (TDHET) HEA, composed of rare-earth (RE) elements with zero pair mixing enthalpies that assure completely random mixing of the elements and very similar atomic radii that minimize lattice distortions, representing a prototype of an ideal HEA. The TDHET HEA is characterized by probability distributions of the atomic moments P(μ), the exchange interactions P(J), the magnetocrystalline anisotropy P(D), and the dipolar interactions P(Hd). Based on the measurements of the static and dynamic magnetization, the magnetization M(H) curves, the thermoremanent magnetization, the specific heat and the magnetoresistance, we found that the collective magnetic state of the TDHET is temperature-dependent, forming a speromagnetic (SPM) state in the temperature range between about 140 and 30 K and an asperomagnetic (ASPM) state below 20 K. In the intermediate temperature range between 30 and 20 K, a spin glass (SG) state is formed, representing a transition state between the speromagnetic and asperomagnetic states. The observed temperature evolution of the magnetic ground state in the TDHET HEA upon cooling in the sequence SPM→SG→ASPM is a result of temperature-dependent, competing magnetic interactions. The distribution of the exchange interactions P(J) shifts continuously on the J axis from the high-temperature SPM-type with the average interaction biased towards a net negative value, J‾<0, through the SG-type with J‾=0, to the low-temperature ASPM-type with J‾>0. This shift is a band-structure effect, closely linked with the crystallinity of the spin system, which the TDHET HEA shares with the topologically ordered crystals. The probability distributions P(μ), P(J), P(D) and P(Hd) are, on the other hand, a consequence of chemical disorder, a property that the TDHET HEA shares with the amorphous magnets. Both features, the topologically ordered lattice and the amorphous-type chemical disorder essentially determine the magnetic state of an ideal, RE-based HEA.

    更新日期:2019-12-19
  • Effect of alloying elements on the γ’ antiphase boundary energy in Ni-base superalloys
    Intermetallics (IF 3.353) Pub Date : 2019-12-16
    M. Dodaran; A. Hemmasian Ettefagh; S.M. Guo; M.M. Khonsari; W.J. Meng; N. Shamsaei; S. Shao
    更新日期:2019-12-17
  • Oxide dispersion strengthened FeCoNi concentrated solid-solution alloys synthesized by mechanical alloying
    Intermetallics (IF 3.353) Pub Date : 2019-12-09
    Yuanhang Guo; Mingyang Li; Cunguang Chen; Pei Li; Wuming Li; Qingzhu Ji; Yanwen Zhang; Yongqin Chang
    更新日期:2019-12-17
  • Effect of Cr content on interdiffusion and Kirkendall pore formation during homogenization of pack-aluminized Ni and Ni–Cr wires
    Intermetallics (IF 3.353) Pub Date : 2019-12-10
    A.E. Paz y Puente; D.C. Dunand

    Using both ex situ metallographic imaging and in situ X-ray tomographic microscopy, we investigate the kinetics of Al- and Ni-interdiffusion during homogenization at 825–1100 °C for surface-aluminized, 50 μm diameter Ni wires with 0, 10 or 20 wt%Cr. Kirkendall pores, which are created due to imbalanced diffusion of atomic species, are not observed at any of the homogenization temperatures in the Cr-free Ni–Al wires, which equilibrate to Ni-rich β-NiAl. By contrast, during homogenization of the aluminized Ni–10Cr and Ni–20Cr wires to β-NiAl(Cr) at 1000 and 1100 °C, numerous Kirkendall pores are created within the wire volume, indicating that the addition of Cr significantly increases the imbalance between the Ni and Al diffusivities. These pores eventually coalesce into a single cavity, with crescent-shape cross-sections and high aspect ratio aligned with the axis of the wires, so that a tubular β-NiAl(Cr) structure is formed. Tomography shows that the Ni-rich β-NiAl(Cr) reaction layer grows radially within the Ni–10Cr and Ni–20Cr wires annealed in situ at 825, 900, and 1000 °C according to a parabolic law. The growth kinetics of this layer increase slightly with increasing Cr content and obey an Arrhenius relationship, from which an activation energy of ~200 kJ/mol is calculated, in good agreement with literature values for interdiffusion in binary NiAl. The two methods, ex situ metallography and in situ X-ray tomography, are complementary. While tomography very rapidly acquires numerous cross-sectional images showing phase contrast on a single wire, thus replacing interrupted annealing and destructive imaging of multiple samples, metallography has higher spatial resolution and can identify additional phases. In the present case, metallography revealed that α-Cr precipitates form during homogenization of the aluminized Ni–Cr wires due to the limited solubility of Cr in β-NiAl; upon full homogenization, these precipitates re-dissolved in the aluminized Ni–10Cr wires, but remained stable in the Ni–20Cr wires.

    更新日期:2019-12-17
  • Metadynamic recrystallization behavior of β-solidified TiAl alloy during post-annealing after hot deformation
    Intermetallics (IF 3.353) Pub Date : 2019-12-13
    Bin Tang; William Yi Wang; Lin Xiang; Yan Liu; Lei Zhu; Shengli Ji; Jian He; Jinshan Li
    更新日期:2019-12-17
  • Tuning magnetic properties based on FeCoNiSi0.4Al0.4 with dual-phase nano-crystal and nano-amorphous microstructure
    Intermetallics (IF 3.353) Pub Date : 2019-12-13
    Bin Zhang; Yuping Duan; Xuan Yang; Guojia Ma; Tongmin Wang; Xinglong Dong; yuansong Zeng

    The idea of adjusting the crystallinity of powders via controlling the phase composition of milling precursor (ribbons) has been used to prepare FeCoNiSi0.4Al0.4 high entropy alloy powders (HEAPs) with dual-phase nano-crystal (NC) and nano-amorphous (NA) microstructure. The crystallinity of HEAPs are related to the FCC phase proportion of milling precursor: within 60%, positive correlation; beyond 60%, negative correlation. Different phase structure for ribbons can be obtained by taking advantage of the difference in phase stability, as an extension of annealing time induces the increment of FCC phase content. The introduction of nano-scale phase separation in HEA ribbons provides a prerequisite. Multiphase NCs distribute randomly in matrix, while NCs with FCC phase are aggregated into about 50 nm regions embedded in matrix. Three kinds of phase transition are co-existed during the dry-milling process, generating the coexistence of NCs and NAs. Due to the unique microstructure, in especial NCs doped by NAs, and the skillful preparation technology, a small Hc, varying from 12.97 Oe (C0) to 28.33 Oe (C50), and a large complex permeability (μ′), varying from 2.60 (C0) to 2.30 (C50), can be obtained.

    更新日期:2019-12-17
  • Designing novel lightweight, high-strength and high-plasticity Tix(AlCrNb)100-x medium-entropy alloys
    Intermetallics (IF 3.353) Pub Date : 2019-12-09
    Y.C. Liao, T.H. Li, P.H. Tsai, J.S.C. Jang, K.C. Hsieh, C.Y. Chen, J.C. Huang, H.J. Wu, Y.C. Lo, C.W. Huang, I.Y. Tsao

    Due to the demand for reduced energy consumption by transportation vehicles, lighter weight high-entropy alloys (with density comparable to that of commercial Ti alloys around 4.5–5 g/cm3) are attracting more attention in terms of alloy design and application as structural materials. In this study, a nonequiatomic quaternary alloy system, Tix(AlCrNb)100-x, was designed through the calculation of phase diagrams. Simulation results reveal that a single body-centered cubic (BCC) phase can be formed and can be stable at temperatures above 950 °C. Accordingly, a series of Tix(AlCrNb)100-x (x = 45–80) alloys containing a BCC structure were prepared through vacuum arc melting and rapid cooling. The designed alloys can exhibit desirable mechanical properties with high compression yield strength about 1500 MPa, high compression fracture strength about 1800 MPa and high compression plasticity more than 30% at room temperature. Moreover, the Ti65 alloy can demonstrate a tensile strength of 1200 MPa with a tensile elongation of 32%, after a homogenization treatment for 24 h. The specific compression and tensile strength can reach 0.36 and 0.24 GPa cm3/g.

    更新日期:2019-12-09
  • High-temperature mechanical behavior of B2-ordered Ti–Mo–Al alloys
    Intermetallics (IF 3.353) Pub Date : 2019-12-07
    Y. Lu, J. Yamada, R. Miyata, H. Kato, K. Yoshimi

    Compressive stress–strain responses of stoichiometric and off-stoichiometric B2-ordered Ti–Mo–Al alloys over the temperature range 973–1373 K were examined to investigate the effects of composition and temperature on the mechanical behavior. The Ti–Mo–Al alloys exhibited impressive yield strengths at elevated temperatures, e.g., 818 MPa for Ti50Mo35Al15 at 1073 K and 490 MPa for Ti50Mo25Al25 at 1273 K. Positive temperature dependence of yield strength was observed in Ti50Mo25Al25 below 1273 K but it had not been seen in either Ti60Mo25Al15 or Ti50Mo35Al15 throughout the test temperatures. Dislocation analyses of the compressed specimens indicated that the Ti–Mo–Al alloys primarily deformed via <111>slip. The mechanisms responsible for the variations of mechanical properties with composition and temperature are discussed by correlating with the antisite defects.

    更新日期:2019-12-07
  • Formation and magnetic properties of hexagonal DO19 phase in Fe2MnGa: An experimental and theoretical investigation
    Intermetallics (IF 3.353) Pub Date : 2019-12-06
    Qingshuai Li, Kaichen Sun, Shuang Liu, Hongzhi Luo, Fanbin Meng

    The formation and magnetic properties of Fe2MnGa hexagonal DO19 phase were investigated in detail. Hexagonal Fe2MnGa can be stabilized by furnace cooling, while the FCC phase can be obtained by quenching from high temperature. Strong ferromagnetic character is identified in Fe2MnGa DO19 phase. Its saturation magnetic moment Ms at 5 K is as large as 4.40 μB/f.u. and Curie temperature TC is 563 K. First-principles calculations indicate that Fe and Mn atoms tend to occupy the 6h sites randomly in the DO19 lattice. The DO19 type Fe2MnGa shows a normal ferromagnetic metal character in its electronic structure. Similar to the experimental results, the calculated total magnetic moment is also quite large and mainly comes from the contributions of Fe and Mn spin moments. Both of them are larger than 1.8 μB and are parallel coupled. These properties are quite different from the antiferromagnetic character in FCC Fe2MnGa.

    更新日期:2019-12-06
  • Excellent magnetic properties of (Fe0.7Co0.3)83.7Si4B8P3.6Cu0.7 ribbons and microwires
    Intermetallics (IF 3.353) Pub Date : 2019-11-30
    V. Zhukova, M. Ipatov, P. Corte-Leon, J.M. Blanco, E. Zanaeva, A.I. Bazlov, J. Jiang, D.V. Louzguine-Luzgin, J. Olivera, A. Zhukov
    更新日期:2019-11-30
  • A new Zr-rich intermetallic phase in an Al-14Si-3Cu-4.5Ni casting alloy with trace additions of Zr
    Intermetallics (IF 3.353) Pub Date : 2019-11-28
    Min-Su Jo, Young-Hee Cho, Jung-Moo Lee, Su-Hyeon Kim, Jun-Yun Kang, Jae-Gil Jung, Soo-Bae Kim, Jae-il Jang

    A trace addition of Zr to a multicomponent Al-14Si-3Cu-4.5Ni casting alloy has often been found to form a coarse plate-like Zr-rich intermetallic compound with a size of a few tens of μm at the beginning of solidification. Evaluation of the phase composition has confirmed that the Zr-rich intermetallic phase consists of multicomponent constituent elements, mainly Al, Si, Ni and Fe, and is present in a form of (Al,Si)3(Zr,Ni,Fe). The existence of such a complex (Al,Si)3(Zr,Ni,Fe) phase has not yet been predicted by phase equilibria calculation of the multicomponent Al-14Si-CuNiMg systems. In this paper, transmission electron microscopy studies combined with electron backscattered diffraction pattern analysis reveals the crystallography information of the (Al,Si)3(Zr,Ni,Fe) phase for the first time and identifies it as a modified Al3Zr-D023 crystal (tetragonal, I4/mmm(139)) with slightly changed lattice parameters of a = b = 3.275 Å and c = 15.475 Å. The primary (Al,Si)3(Zr,Ni,Fe) phase is found to nucleate directly onto AlP particles. The mechanical properties of the Zr-rich intermetallics were also investigated using nanoindentation at both room and elevated temperatures and were compared to those of the Si phase.

    更新日期:2019-11-29
  • Determination of heat treatment parameters by experiments and CALPHAD for precipitate hardening of Cr-Alloys with Si, Ge and Mo
    Intermetallics (IF 3.353) Pub Date : 2019-11-18
    Petra Pfizenmaier, Anke S. Ulrich, Mathias C. Galetz, Uwe Glatzel

    Binary Cr–Si alloys as well as ternary Cr–Si-Ge and Cr–Si–Mo-systems on the Cr rich side of the phase diagram are examined at 1050 °C and 1350 °C regarding the maximum solubility in the bcc solid solution matrix and the A15-phase volume fraction. SEM, EDS and image analysis measurements showed, that the addition of 2 at.% Ge has the same effect on the A15-phase formation as Si but leads to a coarser microstructure. The addition of 2 at.% Mo leads to a decreased A15-phase volume fraction but a small-sized precipitate distribution. The maximum solubility of Si in the binary Cr–Si system is still not clear and has to be verified in further examinations. Thermo-Calc calculations showed, that a reduction of Si content and the addition of Ge and Mo is beneficial for an increase in the process window for solution heat treatment and a decrease of the solvus temperature.

    更新日期:2019-11-18
  • Investigation of the mechanical and microstructural evolution of a Cu based bulk metallic glass during ion irradiation
    Intermetallics (IF 3.353) Pub Date : 2019-11-16
    J. Brechtl, S. Agarwal, M.L. Crespillo, J. Salasin, T. Yang, H. Bei, S.J. Zinkle

    Ion irradiation and annealing experiments were performed on Cu60Zr20Hf10Ti10 bulk metallic glass (BMG) specimens to investigate their irradiation- and temperature-induced microstructural and mechanical property evolution. For the ion irradiations, samples were exposed to 9 MeV Ni3+ ions to a midrange (~1.2 μm depth) dose of 10 displacements per atom (dpa) at temperatures ranging from room temperature to 360 °C (the corresponding peak dose at ~2.8 μm depth was ~25 dpa). Bulk X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterization revealed that the alloy did not crystallize during irradiation up to 290 °C but did partially crystallize at 360 °C. XRD analysis revealed that the crystallization which occurred in the sample irradiated at 360 °C was caused by thermal effects instead of irradiation displacement damage. Subsequent Rietveld refinement analysis of the XRD measurements revealed the presence of two distinct crystal phases, namely a CuTiZr hexagonal structure belonging to the P63/mmc space group and a CuTi tetragonal structure belonging to the P4/mmm space group. Nanoindentation experiments revealed that no pronounced hardness changes occurred in the specimens irradiated at room temperature and 290°C, although significant hardening was observed in the sample irradiated at 360 °C. The significant increase in the hardness at 360°C was ascribed to thermally induced partial crystallization of the alloy instead of the ion irradiation. In general, the results of the nanoindentation experiments and XRD characterization suggest that although the Cu BMG exhibits good stability during irradiation at temperatures up to 290 °C it is not suitable for irradiation environments where the temperature is 360 °C for extended periods of time. The Lam and Chong extrapolation method, which has been used to study the indentation size effect (ISE) in amorphous alloys, was employed to quantify how irradiation and temperature affect this type of behavior in the BMG. However, the poor linear fitting of the indentation hardness data by this model indicate that a new ISE model is likely needed to quantify indentation hardening in BMGs.

    更新日期:2019-11-18
  • Controlling intermetallic compounds formation during laser welding of NiTi to 316L stainless steel
    Intermetallics (IF 3.353) Pub Date : 2019-11-15
    A. Shamsolhodaei, J.P. Oliveira, N. Schell, E. Maawad, B. Panton, Y.N. Zhou
    更新日期:2019-11-15
  • Study on γ′ precipitation behavior in a nickel-based PM superalloy during interrupted continuous cooling
    Intermetallics (IF 3.353) Pub Date : 2019-11-15
    Hailiang Huang, Hongfei Zhang, Benfu Hu, Hao Wang, Guoquan Liu

    The multi-stage nucleation phenomenon and the subsequent evolution of γ′ precipitation during cooling process of super-solvus heat treatment for a nickel-based powder metallurgy (PM) superalloy are studied by means of Gleeble thermal simulation, field emission scanning electron microscope (FE-SEM), energy dispersive spectrometer (EDS) and three-dimensional atom probe tomography (3D-APT). The results show that the secondary γ′ phases precipitated preferentially at a relatively high temperature during the cooling process. As the temperature decreased with a slow rate, the secondary γ′ phases grew up and changed shape gradually and parts of the tertiary γ′ phases precipitated, resulting in a multimodal size distribution of γ′ phases. While, when the cooling rate is relatively fast, the γ′ phase exhibits a monomodal size distribution. The results of EDS and 3D-APT show that there is a depleted zone of Al and Ti elements near the interface between secondary γ′ and the γ matrix, while Al and Ti elements is enriched in γ matrix away from the secondary γ′ precipitates. The distribution of alloying elements in the γ′ phases and γ matrix change with the decrease of the temperature, and the phase interface is a favorable place for the dynamic exchange of alloying elements.

    更新日期:2019-11-15
  • The effect of the parameters of the powder bed fusion process on the microstructure and mechanical properties of CrFeCoNi medium-entropy alloys
    Intermetallics (IF 3.353) Pub Date : 2019-11-14
    Y. Kuzminova, D. Firsov, A. Dudin, S. Sergeev, A. Zhilyaev, A. Dyakov, A. Chupeeva, A. Alekseev, D. Martynov, I. Akhatov, S. Evlashin
    更新日期:2019-11-14
  • Thermodynamic modelling of the Ni–Zr system
    Intermetallics (IF 3.353) Pub Date : 2019-11-13
    Asmita Jana, Soumya Sridar, Suzana G. Fries, Thomas Hammerschmidt, K.C. Hari Kumar

    In this work, we report the thermodynamic modelling of the Ni–Zr system using the Calphad method combined with ab initio calculations. Density functional theory (DFT) is employed to calculate the enthalpy of formation of the intermediate phases. The calculated enthalpies of formation are in close agreement with the experimental data. An approach based on special quasirandom structures (SQS) was used for calculating the enthalpy of mixing of the fcc solid solution. The vibrational contribution to the heat capacities of NiZr, NiZr2, Ni3Zr and Ni7Zr2 phases were calculated using the quasiharmonic approximation (QHA) and the corresponding electronic contribution was obtained using an approach based on Mermin statistics. The total heat capacities for these phases were fitted to appropriate expressions and integrated to obtain the Gibbs energy functions valid down to 0 K. The calculated thermochemical properties along with critically selected experimental constitutional and thermochemical data served as input for the thermodynamic optimisation of the system. The calculated phase equilibria and the thermodynamic properties using the optimised Gibbs energy functions are in good agreement with the input data. The calculated congruent melting points of NiZr and NiZr2 phases are close to the recent experimental data. The Ni10Z7 phase forms by a peritectic reaction, which is also in agreement with the experimental data.

    更新日期:2019-11-13
  • Highly porous Ni-free Ti-based scaffolds with large recoverable strain for biomedical applications
    Intermetallics (IF 3.353) Pub Date : 2019-11-13
    Shuanglei Li, Yeon-wook Kim, Mi-seon Choi, Tae-hyun Nam

    Highly porous Ti–18Zr-12.5Nb–2Sn (at.%) scaffolds with porosities of 70%, 75% and 80% were prepared by sintering rapidly solidified alloy fibers. Superelastic behavior of alloy fiber was investigated by the dynamic mechanical analyzer (DMA). Phase identifications of alloy fiber and scaffold were studied by X-ray diffraction (XRD). Precipitates in alloy fiber were investigated by transmission electron microscope (TEM). Porous structure of alloy scaffold was observed by scanning electron microscope (SEM). Mechanical properties and the superelasticity of alloy scaffold were investigated using compressive test. The superelasticity of alloy fibers was improved after aged at 673 K for 3.6 ks due to α precipitates. The sintered scaffolds showed three-dimensional networks with fiber-fiber sintering joints. The compressive yield stress and elastic modulus of the alloy scaffolds aged at 673 K for 3.6 ks were in the range of 5.0–16.7 MPa and 0.33–1.05 GPa, respectively. These values were a close match to those for cancellous bone. The recoverable strain increased from 3.2% to 3.7% with the increase in porosity from 70% to 80% in aged Ti–Zr–Nb–Sn scaffolds when tested at human body temperature (310 K).

    更新日期:2019-11-13
  • Approaching the melting temperature: There regimes in the non-isothermal crystallization of Ce68Al10Cu20Co2 bulk metallic glass revealed by nanocalorimetry
    Intermetallics (IF 3.353) Pub Date : 2019-11-13
    Bingge Zhao, Alisson M. Rodrigues, Kai Ding, Hailan Ma, Guanzhi Wu, Qijie Zhai, Yulai Gao

    Nanocalorimetry is an ideal candidate for revealing the crystallization of metallic glass in a wide temperature range attributing to its ultrafast heating rate and ultrahigh sensitivity. In this study, in situ preparation of Ce68Al10Cu20Co2 (at.%) metallic glass was realized by the nanocalorimetry. By the subsequent reheating at the rates from 500 to 40,000 K/s, the crystallization in a wide temperature range was demonstrated. The crystallization activation energy and Avrami exponent are calculated with the aid of the Kissinger and Johnson-Mehl-Avrami (JMA) equations to reveal the temperature-dependent nucleation and crystal growth behavior in the undercooled liquid, where three crystallization regimes are identified. At low temperature, the crystallization occurs with constant activation energy. In an intermediate temperature range, the crystallization is controlled by crystal growth with a reduction of activation energy. At temperatures approaching Tm, an increased activation energy resulting from the thermodynamic driving force dominates the nucleation-controlled crystallization. This study provides a novel strategy to reveal the crystallization behavior in the undercooled liquid from Tg to Tm and to demonstrate the transition from growth-controlled crystallization to nucleation-controlled one with the decrease of undercooling.

    更新日期:2019-11-13
  • Microstructure, microhardness and oxidation behavior of Mo-Si-B alloys in the Moss+Mo2B+Mo5SiB2 three phase region
    Intermetallics (IF 3.353) Pub Date : 2019-11-08
    L. Liu, C. Shi, C. Zhang, P.M. Voyles, J.H. Fournelle, J.H. Perepezko

    A series of Mo-Si-B alloys in the Moss+T2 (Mo5SiB2)+Mo2B three phase region were designed to examine the effect of the lower Si solubility limit in the Moss phase on the microstructure, hardness and oxidation behavior. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Vickers hardness tests and Electron Probe Microanalysis (EPMA) were carried out for the evaluation. The XRD results show an unusual lattice constant expansion for the Moss that is consistent with B as an interstitial solute. SEM images establish the microstructure evolution during the annealing. The analysis of the indentations produced by the Vickers hardness tests demonstrate that the samples in the Moss + Mo2B + T2 three phase region have about a 50% higher fracture toughness than comparable samples in Moss + Mo3Si + T2 three phase region due to the lower Si solubility in the Moss phase. The thermogravimetric analysis (TGA) results show that the oxidation resistance is directly related to T2 phase fraction in the microstructure. Alloys with a Moss + Mo2B + T2 microstructure exhibit a similar oxidation behavior to those with a Moss + Mo3Si + T2 microstructure when the T2 phase fraction in the two alloys is similar. Mo-Si-B alloys in the Moss + Mo2B + T2 three phase region could achieve better mechanical properties than alloys in the Moss + Mo3Si + T2 three phase region and are attractive candidates for high temperature applications.

    更新日期:2019-11-11
  • Gum-like mechanical behavior of a partially ordered Al5Nb24Ti40V5Zr26 high entropy alloy
    Intermetallics (IF 3.353) Pub Date : 2019-11-08
    S. Zherebtsov, N. Yurchenko, E. Panina, M. Tikhonovsky, N. Stepanov

    Microstructure and mechanical behavior of a new bcc Al5Nb24Ti40V5Zr26 high-entropy alloy in the as-cast, cold-rolled and recrystallized conditions were studied. In the as-cast condition, the Al5Nb24Ti40V5Zr26 alloy had a single bcc phase coarse-grained (~80 μm) structure. After cold rolling to a total thickness strain 80% the alloy demonstrated typical of deformed metals tensile behavior with a short hardening stage and early necking. Annealing at 800 °C resulted in recrystallization of the matrix and formation of a small fraction of C14 type Laves phase particles. After annealing at 1100 °C, a partial B2 ordering of the matrix phase was found. In the annealed condition, the alloy showed some typical of gum metals features associated with low work hardening and large elongation. The manifestation of a gum-like behavior can be ascribed to the formation of dislocation channels due to the local disordering of the bcc matrix in shear planes. In addition, an unusual (inverse) dependence of yield strength on the recrystallized grain size was observed, most possibly due to a different degree of ordering of the matrix phase.

    更新日期:2019-11-11
  • Corrosion behaviors of FeCoNiCrx (x = 0, 0.5, 1.0) multi-principal element alloys: Role of Cr-induced segregation
    Intermetallics (IF 3.353) Pub Date : 2019-11-08
    Wenke Chai, Tao Lu, Ye Pan

    The as-cast microstructures and corrosion behaviors of the FeCoNiCrx (x = 0, 0.5, 1.0) multi-principal element alloys in both 0.5 M sulfuric acid and 3.5% sodium chloride solutions were systematically investigated. With the increased content of Cr, the as-cast microstructure shows a cellular-dendrite transformation due to the Cr-induced segregation. FeCoNiCr0.5 alloy exhibits an outstanding anticorrosion performance in both 0.5 M sulfuric acid and 3.5% sodium chloride solutions, while the excess addition of Cr element leads to the noticeable localized corrosion in FeCoNiCr alloy. The results indicate that the distribution of Cr element plays a non-ignorable role in the anticorrosion performance for FeCoNiCr-based multi-principal element alloy.

    更新日期:2019-11-08
  • The transformation characteristics of the NiTi–V composite with dual-scale bcc-V fibers
    Intermetallics (IF 3.353) Pub Date : 2019-11-05
    Hongliang Yang, Jian Chen, Yiqun Wu, Xiaobin Shi, Yongtao Li, Jiang Jiang
    更新日期:2019-11-06
  • Temperature rises during strain-rate dependent avalanches in bulk metallic glasses
    Intermetallics (IF 3.353) Pub Date : 2019-11-02
    J.J. Li, J.F. Fan, Z. Wang, Y.C. Wu, K.A. Dahmen, J.W. Qiao

    Localized heating during slip avalanches is relevant to the understanding of the transition from shear banding to cracking in bulk metallic glasses (BMGs). Here we focus on the released elastic energies (incorporating the influence of machine stiffness) and avalanche durations for regularly appearing large avalanches to study the maximum temperature increases of a Zr-based BMG compressed at three strain rates of 3 × 10−5, 3 × 10−4, and 3 × 10−3 s−1. The maximum released elastic energies during slip avalanches decrease with the increase of strain rates. The durations of the avalanches are on the scale of tens of milliseconds for each strain rate. As strain rate increases, the maximum temperature rises due to shear-band slips are decreased from 2.6, 2.3 to 1.9 K. Moderate heating during the large slip avalanches for three strain rate implies that thermal softening has only a small effect on the shear-banding instability.

    更新日期:2019-11-04
  • Nanocrystalline MnAlV rare-earth-free Permanent Magnetic Alloys with Improved Magnetization and Thermal Stability
    Intermetallics (IF 3.353) Pub Date : 2019-11-01
    Zhen Xiang, Baiwen·Deng, Xiang Zhang, Xiao Wang, Erbiao Cui, Lunzhou Yu, Yiming Song, Wei Lu

    Ferromagnetic τ-MnAl alloy with attractive magnetic properties has been considered as one of the most promising rare-earth-free permanent magnetic materials. However, there are difficulties in obtaining the high-purity τ-MnAl alloy. In this study, the crystallographic structure, phase transformation and magnetic properties of nanocrystalline MnAlV alloys prepared by the melt-spinning technique were systematically investigated. The results showed that the phase composition, microstructure and magnetic properties were tailored by V doping in the MnAl alloys. V doping was effective in promoting the formation of the τ phase in the as-spun MnAl alloy during the melt-spinning process. In addition, V doping improved the thermal stability of ε phase and τ phase in MnAl alloys, which was derived from the different atomic radii and negative mixing enthalpy. As a result, the high-purity τ phase with a saturation magnetization of 120.5 emu/g and coercivity of 1030 Oe was achieved in the Mn53.35Al43.65V3 alloy. Therefore, this work contributed to the design and preparation of nanocrystalline τ-MnAl rare-earth-free permanent magnetic materials.

    更新日期:2019-11-01
  • Effects of multiaxial forging on microstructure and high temperature mechanical properties of powder metallurgy Ti-45Al-7Nb-0.3W alloy
    Intermetallics (IF 3.353) Pub Date : 2019-10-31
    Huizhong Li, Yu Long, Xiaopeng Liang, Yixuan Che, Zhenqi Liu, Yong Liu, Hao Xu, Li Wang

    Uniaxial forging (UAF) and multiaxial forging (MAF) were conducted on a Ti-45Al-7Nb-0.3W (at. %) alloy produced by powder metallurgy (PM) technique, and the deformed microstructures and deformation mechanism were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results show that the near-gamma microstructure (NG) transformed into a fine and uniform duplex microstructure (DP) after MAF, and the mean grain size of the multiaxial forged alloy was 5 μm. The microstructure of the uniaxial forged alloy was inhomogeneous and anisotropy, while the microstructure of multiaxial forged alloy was very uniform and a higher amount of the β phase was distributed at the grain boundaries. During forging, deformation induced microstructure changes include dislocation movements, mechanical twinning, layer decomposition and dynamic recrystallization (DRX). The DRX occurred repeatedly and more severely during MAF than UAF process. Correspondingly, the multiaxial forged TiAl alloy exhibited excellent high temperature mechanical properties compared to the uniaxial forged alloy. At a temperature of 750 °C, the yield strength, ultimate tensile strength and elongation of the multiaxial forged alloy were 623 MPa, 697 MPa and 4.5%, respectively. It was noteworthy that the multiaxial forged alloy achieved highly plastic deformation and the elongation reached 173% at 900 °C with a strain rate of 1 × 10−3 s−1.

    更新日期:2019-11-01
  • Thermodynamic analysis of the topologically close packed σ phase in the Co-Cr system.
    Intermetallics (IF 3.353) Pub Date : 2019-10-04
    Peisheng Wang,Matthew C Peters,Ursula R Kattner,Kamal Choudhary,Gregory B Olson

    Density functional theory (DFT) calculations show that it is essential to consider the magnetic contribution to the total energy for the end-members of the σ phase. A more straightforward method to use the DFT results in a CALPHAD (Calculation of phase diagrams) description has been applied in the present work. It was found that only the results from DFT calculations considering spin-polarization are necessary to obtain a reliable description of the σ phase. The benefits of this method are: the DFT calculation work can be reduced and the CALPHAD description of the magnetic contribution is more reliable. A revised thermodynamic description of the Co-Cr system is presented which gives improved agreement with experimental phase boundary data for the σ phase.

    更新日期:2019-11-01
  • The Ni-rich part of the Al-Ge-Ni phase diagram.
    Intermetallics (IF 3.353) Pub Date : 2013-01-01
    Isabella Jandl,Thomas L Reichmann,Klaus W Richter

    The Ni-rich part of the ternary system Al-Ge-Ni (xNi > 50 at.%) was investigated by means of optical microscopy, powder X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM). The two isothermal sections at 550 °C and 700 °C were determined. Within these two sections a new ternary phase, designated as τ4, AlyGe9-yNi13±x (hP66, Ga3Ge6Ni13-type) was detected and investigated by single crystal X-ray diffraction. Another ternary low temperature phase, τ5, was found only in the isothermal section at 550 °C around the composition AlGeNi4. This compound was found to crystallise in the Co2Si type structure (oP12, Pnma). The structure was identified by Rietveld refinement of powder data. The NiAs type (B8) phase based on binary Ge3Ni5 revealed an extended solid solubility of Al and the two isotypic compounds AlNi3 and GeNi3 form a complete solid solution. Based on DTA results, six vertical sections at 55, 60, 70, 75 and 80 at.% Ni and at a constant Al:Ni ratio of 1:3 were constructed. Furthermore, the liquidus surface projection and the reaction scheme (Scheil diagram) were completed by combining our results with previous results from the Ni-poor part of the phase diagram. Six invariant ternary reactions were identified in the Ni-rich part of the system.

    更新日期:2019-11-01
  • Characterization of the Fe-rich corner of Al-Fe-Si-Ti.
    Intermetallics (IF 3.353) Pub Date : 2013-08-01
    Martin C J Marker,Liliana I Duarte,Christian Leinenbach,Klaus W Richter

    The quaternary system Al-Fe-Si-Ti was studied in the iron-rich corner for sections at 50, 60 and 70 at.% Fe at 900 °C. Isothermal phase equilibria were investigated by a combination of optical microscopy, X-ray powder diffraction (XRD) followed by Rietveld refinement and Electron Probe Microanalysis (EPMA). Phase boundaries of the phases, in particular of the Laves phase (Fe2Ti) and of the extended phase field of A2/B2/D03, were investigated. Selected samples containing the Laves phase and the B2 phase were characterized by microhardness measurements at different compositions throughout the quaternary homogeneity range of the phases.

    更新日期:2019-11-01
  • Re-investigation of phase equilibria in the system Al-Cu and structural analysis of the high-temperature phase η1-Al1-δCu.
    Intermetallics (IF 3.353) Pub Date : 2011-11-01
    Norbert Ponweiser,Christian L Lengauer,Klaus W Richter

    The phase equilibria and reaction temperatures in the system Al-Cu were re-investigated by a combination of optical microscopy, powder X-ray diffraction (XRD) at ambient and elevated temperature, differential thermal analysis (DTA) and scanning electron microscopy (SEM). A full description of the phase diagram is given. The phase equilibria and invariant reactions in the Cu-poor part of the phase diagram could be confirmed. The Cu-rich part shows some differences in phase equilibria and invariant reactions compared to the known phase diagram. A two phase field was found between the high temperature phase η1 and the low temperature phase η2 thus indicating a first order transition. In the ζ1/ζ2 region of the phase diagram recent findings on the thermal stability could be widely confirmed. Contrary to previous results, the two phase field between δ and γ1 is very narrow. The results of the current work indicate the absence of the high temperature β0 phase as well as the absence of a two phase field between γ1 and γ0 suggesting a higher order transition between γ1 and γ0. The structure of γ0 (I-43m, Cu5Zn8-type) was confirmed by means of high-temperature XRD. Powder XRD was also used to determine the structure of the high temperature phase η1-Al1-δCu. The phase is orthorhombic (space group Cmmm) and the lattice parameters are a = 4.1450(1) Å, b = 12.3004(4) Å and c = 8.720(1) Å; atomic coordinates are given.

    更新日期:2019-11-01
  • Phase equilibria in the ternary In-Ni-Sn system at 700 °C.
    Intermetallics (IF 3.353) Pub Date : 2013-04-01
    C Schmetterer,A Zemanova,H Flandorfer,A Kroupa,H Ipser

    The phase equilibria of the ternary system In-Ni-Sn were investigated experimentally at 700 °C using X-ray diffraction (XRD) and scanning electron microscopy (SEM) including electron micro probe analysis (EMPA) and energy dispersive X-ray spectroscopy (EDX). A corresponding isothermal section was established based on these results. This particular temperature was chosen because it allowed obtaining reliable results within reasonable time. The existence of the ternary phase InNi6Sn5 was confirmed whereas the ternary compound In2NiSn, reported earlier in literature, was found to be part of a large solid solution field based on binary InNi. The ternary solubility of the binary phases was established, and continuous solid solutions were found between the isostructural phases Ni3Sn LT and InNi3 as well as between Ni3Sn2 HT and InNi2. In addition, this isothermal section could be well reproduced by CALPHAD modelling. The resulting calculated isotherm at 700 °C is presented, too, and compared with the experimental results.

    更新日期:2019-11-01
  • The Cu-Sn phase diagram, Part I: New experimental results.
    Intermetallics (IF 3.353) Pub Date : 2013-03-01
    S Fürtauer,D Li,D Cupid,H Flandorfer

    Phase diagram investigation of the Cu-Sn system was carried out on twenty Cu-rich samples by thermal analysis (DTA), metallographic methods (EPMA/SEM-EDX) and crystallographic analysis (powder XRD, high temperature powder XRD). One main issue in this work was to investigate the high temperature phases beta (W-type) and gamma (BiF3-type) and to check the phase relations between them. In the high temperature powder XRD experiments the presence of the two-phase-field between the beta- and the gamma-phase could not be confirmed. Detailed study of primary literature together with our experimental results leads to a new phase diagram version with a higher order transformation between these two high temperature phases. The present work is designated as part I of our joint publication. The new findings described here have been included into a completely new thermodynamic assessment of the Cu-Sn phase diagram which is presented in part II.

    更新日期:2019-11-01
  • Phase equilibria and structural investigations in the Ni-poor part of the system Al-Ge-Ni.
    Intermetallics (IF 3.353) Pub Date : 2012-09-01
    Thomas L Reichmann,Liliana I Duarte,Herta S Effenberger,Christian Leinenbach,Klaus W Richter

    The ternary phase diagram Al-Ge-Ni was investigated between 0 and 50 at.% Ni by a combination of differential thermal analysis (DTA), powder- and single-crystal X-ray diffraction (XRD), metallography and electron probe microanalysis (EPMA). Ternary phase equilibria and accurate phase compositions of the equilibrium phases were determined within two partial isothermal sections at 400 and 700 °C, respectively. The two binary intermediate phases AlNi and Al3Ni2 were found to form extended solid solutions with Ge in the ternary. Three new ternary phases were found to exist in the Ni-poor part of the phase diagram which were designated as τ1 (oC24, CoGe2-type), τ2 (at approximately Al67.5Ge18.0Ni14.5) and τ3 (cF12, CaF2-type). The ternary phases show only small homogeneity ranges. While τ1 was investigated by single crystal X-ray diffraction, τ2 and τ3 were identified from their powder diffraction pattern. Ternary phase reactions and melting behaviour were studied by means of DTA. A total number of eleven invariant reactions could be derived from these data, which are one ternary eutectic reaction, six transition reactions, three ternary peritectic reactions and one maximum. Based on the measured DTA values three vertical sections at 10, 20 and 35 at.% Ni were constructed. Additionally, all experimental results were combined to a ternary reaction scheme (Scheil diagram) and a liquidus surface projection.

    更新日期:2019-11-01
  • Enthalpies of mixing of liquid systems for lead free soldering: Co-Sb-Sn.
    Intermetallics (IF 3.353) Pub Date : 2012-04-01
    A Elmahfoudi,A Sabbar,H Flandorfer

    The partial and integral enthalpy of mixing of molten ternary Co-Sb-Sn alloys was determined performing high temperature drop calorimetry in a large compositional range at 1273 K. Measurements have been done along five sections, xSb/xSn ≈ 1:1, xSb/xSn ≈ 1:3, xSb/xSn ≈ 3:1, xCo/xSn ≈ 1:4, and xCo/xSb ≈ 1:5. Additionally, binary alloys of the constituent systems Co-Sb and Co-Sn were investigated at the same temperature. All the binary data were evaluated by means of a standard Redlich-Kister polynomial fit whereas ternary data were fitted on the basis of an extended Redlich-Kister-Muggianu model for substitutional solutions. An iso-enthalpy plot of the ternary system was constructed. In addition, the extrapolation Model of Toop was applied and compared to our data.

    更新日期:2019-11-01
  • Phase equilibria and structural investigations in the system Al-Fe-Si.
    Intermetallics (IF 3.353) Pub Date : 2011-12-01
    Martin C J Marker,Barbara Skolyszewska-Kühberger,Herta S Effenberger,Clemens Schmetterer,Klaus W Richter

    The Al-Fe-Si system was studied for an isothermal section at 800 °C in the Al-rich part and at 900 °C in the Fe-rich part, and for half a dozen vertical sections at 27, 35, 40, 50 and 60 at.% Fe and 5 at.% Al. Optical microscopy and powder X-ray diffraction (XRD) was used for initial sample characterization, and Electron Probe Microanalysis (EPMA) and Scanning Electron Microscopy (SEM) of the annealed samples was used to determine the exact phase compositions. Thermal reactions were studied by Differential Thermal Analysis (DTA). Our experimental results are generally in good agreement with the most recent phase diagram versions of the system Al-Fe-Si. A new ternary high-temperature phase τ12 (cF96, NiTi2-type) with the composition Al48Fe36Si16 was discovered and was structurally characterized by means of single-crystal and powder XRD. The variation of the lattice parameters of the triclinic phase τ1 with the composition Al2+x Fe3Si3-x (-0.3 < x < 1.3) was studied in detail. For the binary phase FeSi2 only small solubility of Al was found in the low-temperature modification LT-FeSi2 (ζβ ) but significant solubility in the high-temperature modification HT-FeSi2 (ζα ) (8.5 at.% Al). It was found that the high-temperature modification of FeSi2 is stabilized down to much lower temperature in the ternary, confirming earlier literature suggestions on this issue. DTA results in four selected vertical sections were compared with calculated sections based on a recent CALPHAD assessment. The deviations of liquidus values are significant suggesting the need for improvement of the thermodynamic models.

    更新日期:2019-11-01
  • The high-temperature phase equilibria of the Ni-Sn-Zn system: Isothermal sections.
    Intermetallics (IF 3.353) Pub Date : 2011-10-01
    Clemens Schmetterer,Divakar Rajamohan,Herbert Ipser,Hans Flandorfer

    In this work three complete isothermal sections of the Ni-Sn-Zn system at 700, 800 and 900 °C are presented. They were constructed based on experimental investigation of more than 60 alloy samples. Powder XRD, single crystal XRD, EPMA, and DTA measurements on selected samples were carried out. Two new ternary compounds, designated as τ2 (Ni5Sn4Zn) and τ3 (Ni7Sn9Zn5), were identified and their homogeneity ranges and crystal structures could be described. Whereas τ3 is only present at 700 °C, the τ2-phase was found at both 700 and 800 °C. No truly ternary compound could be found in the isothermal section at 900 °C. A seemingly ternary compound at 20 at% Sn in the Ni-rich part of Ni-Sn-Zn was found at 800 and 900 °C. Our XRD results, however, indicate that this phase is a ternary solid solution of Ni3Sn-HT from constituent binary Ni-Sn. It is stabilized to lower temperatures by additions of Zn. These new experimental results will provide valuable information to the thermodynamic description of alloy systems relevant for high-temperature lead-free soldering.

    更新日期:2019-11-01
  • Experimental investigation of the Cu-Si phase diagram at x(Cu)>0.72.
    Intermetallics (IF 3.353) Pub Date : 2011-10-01
    Katarzyna Sufryd,Norbert Ponweiser,Paola Riani,Klaus W Richter,Gabriele Cacciamani

    Cu-Si phase equilibria have been investigated at compositions greater than 72 at.% Cu by X-ray diffraction, optical and electronic microscopy, electron probe microanalysis and differential thermal analysis. The general aspects of the phase equilibria already reported in literature have been substantially confirmed, but selected composition ranges and the nature of a few invariant equilibria have been modified. In particular stability ranges of the β, δ and η phases have been slightly modified as well as temperature and nature of the invariant equilibria related to the γ ⇄ δ transformation. Stability of the ɛ-(Cu15Si4) phase has been especially investigated concluding that it is thermodynamically stable but kinetically inhibited by nucleation difficulties which become especially effective when samples are synthesized in very high purity conditions. Crystal structure and composition ranges of the high temperature β and δ phases, despite difficulties by the non-quenchability of these phases, have been investigated by different methods including high temperature XRD.

    更新日期:2019-11-01
  • Study of reorientation processes in L10-ordered FePt thin films.
    Intermetallics (IF 3.353) Pub Date : 2010-11-01
    M Rennhofer,M Kozlowski,B Laenens,B Sepiol,R Kozubski,D Smeets,A Vantomme

    We report on the development of structural and magnetic order in epitaxially grown L10 FePt thin films. Upon annealing, the easy axis of magnetization changes from the out-of-plain into the in-plain direction. We found that the overall fraction of reoriented domains first increases but after certain time decreases before achieving a saturated state. The results are based on conversion electron Mössbauer spectroscopy studies and confirm Monte Carlo simulations in nano-layered FePt. We present a modified version of the Johnson-Mehl-Avrami (JMA) model adequately describing the experimental findings. Two dynamical processes, the first being a 2D-growth, dominate the initial state of sample annealing and the second being a 3D-growth, dominate the late stage close to saturation. From an Arrhenius plots of JMA coefficients for both processes we extracted the activation energies of the underlying dynamics which are 1.5(1) eV for disordering and 0.8(2) eV for ordering.

    更新日期:2019-11-01
  • Microstructure and mechanical properties of novel Zr–Al–V alloys processed by hot rolling
    Intermetallics (IF 3.353) Pub Date : 2019-10-28
    X. Zhang, B. Zhang, S.G. Liu, X.Y. Zhang, M.Z. Ma, R.P. Liu

    In this study, the effects of vanadium (V) addition on the microstructures and mechanical properties of hot-rolled Zr–9Al–χV alloys (χ = 0, 2 and 4 at. %) were systematically investigated. Microscopic analyses indicated that the microstructures of hot-rolled Zr–9Al–χV alloys were sensitive to the V content. The hot-rolled Zr–9Al alloy was completely dynamically recrystallized and consisted of the α phase only, whereas the hot-rolled Zr–9Al–2V alloy consisted of partially recrystallized α and transformed β (α′ + β). The tensile tests showed that the difference between the ultimate and yield strengths of the hot-rolled Zr–9Al alloy approximated 20 MPa, whereas that of the hot-rolled Zr–9Al–2V alloy was about 200 MPa. The evident strain hardening of the hot-rolled Zr–9Al–2V alloy was caused by the mechanical contrast between the soft and hard components of the microstructure. The hot-rolled Zr–9Al–4V alloy consisted of large elongated β grains, in which equiaxed α and acicular α" grains were distributed. The engineering stress–strain curve of the hot-rolled Zr–9Al–4V alloy exhibited a double-yielding behaviour, which was caused by stress-induced martensite phase transformation: β→α".

    更新日期:2019-10-28
  • Pattern formation mechanism of directionally-solidified MoSi2/Mo5Si3 eutectic by phase-field simulation
    Intermetallics (IF 3.353) Pub Date : 2019-10-25
    Chuanqi Zhu, Yuichiro Koizumi, Akihiko Chiba, Koretaka Yuge, Kyosuke Kishida, Haruyuki Inui

    A phase-field study has been conducted to obtain an understanding of the formation mechanism of the script lamellar pattern of MoSi2/Mo5Si3 eutectic composite, which is a promising candidate for high-temperature structural application. The spacing of the lamellar pattern in the simulation results shows good agreement with that of experimental observations and analytical solutions under three growth velocities: 10 mm h−1, 50 mm h−1, and 100 mm h−1. The discontinuity of Mo5Si3 rods, in contrast to the regular eutectic with a continuous pattern, is claimed to be caused by the instability of the solid-liquid interface. In this study, the implementation of Mo5Si3 nucleation over the solid-liquid interface has been proposed and successfully reproduced the characteristic of discontinuity. A highly random and intersected lamellar pattern similar to that observed in the ternary MoSi2/Mo5Si3 eutectic alloyed with 0.1 at% Co has been obtained in simulation owing to the increase in the frequency of nucleation. In addition, it has been demonstrated that the inclination of the Mo5Si3 rod can be reproduced by taking into account the strong relaxation of lattice strain energy, which is generally considered to be negligible in eutectic reaction, as the result of the formation of ledge-terrace structure.

    更新日期:2019-10-25
  • Investigation of phase selection hierarchy in Mn–Al alloys
    Intermetallics (IF 3.353) Pub Date : 2019-09-30
    Ayse Merve Genc, Ozgun Acar, Servet Turan, Ilkay Kalay, Umut Savacı, Yunus Eren Kalay

    Primarily attributed to the formation of the ferromagnetic τ-phase, near equiatomic composition of Mn–Al have recently received much attention in permanent magnet industry. Several mechanisms have been proposed in literature for the τ-phase formation but controversy still arises regarding the dominating mode. In the current work, MnAl-based alloys having different compositions in a range between Mn50.5Al49.5 and Mn57Al43 have been studied by means of in-situ high energy X-ray diffraction, differential scanning calorimetry (DSC) and magnetic measurements. Synchrotron and DSC experiments showed the dependence of the τ-MnAl on the intermediate disordered ε′-phase. Alloys having 53.4 at% or less Mn (S1, S2) followed a transformation route of ε+ε'→τ→β+γ2 upon annealing. Alloys having more than 53.4 at% of Mn had only ε-phase. High energy X-ray diffraction patterns showed that ε-phase directly transformed into stable phases in the absence of ε′-phase. It is observed that ε′ not only promoted the ferromagnetic τ-phase but also stabilized it by delaying the nucleation of stable phases.

    更新日期:2019-10-24
  • The tremendous influence of hydrogenation on magnetism of NdMnGe
    Intermetallics (IF 3.353) Pub Date : 2019-10-01
    I.A. Ovchenkova, I.S. Tereshina, A.E. Bogdanov, E.A. Tereshina-Chitrova, M.A. Paukov, D.I. Gorbunov, S.A. Nikitin

    The hydride NdMnGeH with the tetragonal ZrSiCuAs-type of structure (P4/nmm, N129, tP8) was obtained by hydrogen absorption at 523 K and 1 MPa from the NdMnGe intermetallic compound with a the tetragonal CeFeSi crystal structure (P4/nmm, N129, tP6). Measurements of magnetization in high magnetic fields up to 60 T and heat capacity measurements reveal pronounced changes in the magnetic properties of NdMnGe after hydrogenation. The Nd sublattice changes its ordering type from ferromagnetic to the antiferromagnetic one with a more than twofold decrease of and its magnetic ordering temperature (from 199 to 84 K). We explain the observed effects by the altered exchange interactions within the Nd sublattice resulting from the changed Nd–Nd interplane distances by interstitial atoms. The results are compared with data obtained previously for the NdMn1-xTixGe compounds, where the Ti substitution also changes significantly the magnetic properties.

    更新日期:2019-10-24
  • Spin-glass freezing in single-crystalline Pr2NiSi3
    Intermetallics (IF 3.353) Pub Date : 2019-10-23
    Maria Szlawska

    A single crystal of Pr2NiSi3 was examined by means of x-ray diffraction, magnetization, ac susceptibility, heat capacity and electrical resistivity measurements performed in wide ranges of the temperature and external magnetic fields. The compound crystallizes with simple hexagonal AlB2-type unit cell with crystallographic disorder in the non-magnetic atom sublattice. The results indicate formation at low temperatures of a spin-glass state with characteristic freezing temperature Tf=3.2K, which most likely originates from intrinsic crystallographic disorder in the AlB2-type unit cell.

    更新日期:2019-10-24
  • Effects of NiAl precipitate microstructure, loading axis and temperature on deformation behavior of Fe–Al–Ni single crystals
    Intermetallics (IF 3.353) Pub Date : 2019-10-10
    Hiroyuki Y. Yasuda, Taisuke Edahiro, Naoki Takeoka, Takashi Yoshimoto, Masataka Mizuno, Ken Cho

    Effects of size and volume fraction of the B2-type NiAl precipitates on the deformation behavior of Fe–Al–Ni single crystals were investigated. In the crystals with different Ni contents, the fine NiAl phase was precipitated in the bcc matrix with small misfit strain. Strong precipitation hardening occurred due to the difference in primary slip system between the bcc matrix and the NiAl precipitates. Moreover, not only <111> slip but also <001> slip occurred depending on the loading axis, the precipitate morphology and the deformation temperature, though the occurrence of <001> slip is generally impossible in bcc metals. The dependence of the critical resolved shear stress for <111> slip on the precipitate size was consistent with the modified precipitation hardening theory. To understand the precipitation hardening, an antiphase boundary energy of the NiAl precipitates was evaluated by first-principles calculations. Moreover, higher volume fraction of the NiAl precipitates was favorable for the activation of <001> slip. Furthermore, the activated slip system varied with deformation temperature, and therefore, the temperature dependence of the yield stress was closely related to the slip system.

    更新日期:2019-10-24
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