Analysis of damage-tolerance of TRIP-assisted V10Cr10Fe45Co30Ni5 high-entropy alloy at room and cryogenic temperatures

https://doi.org/10.1016/j.jallcom.2020.156090Get rights and content

Highlights

  • The retention in fracture toughness with decreasing temperature for the TRIP-assisted metastable HEA is achieved.

  • Exceptional fracture toughness properties with decreasing temperature are not limited to TWIP-assisted High-Entropy Alloy.

  • Steady strain hardening by TRIP and ductile fracture by BCC martensite contribute to the superior cryogenic damage-tolerance.

  • The SFE decreases in the order of the V10Cr10Fe45Co20Ni15, V10Cr10Fe45Co30Ni5, CrMnFeCoNi, CrCoNi, and Fe50Mn30Co10Cr10.

  • Driving force for martensitic transformation is largest in the V10Cr10Fe45Co30Ni5 alloy, causing the pronounced TRIP effect.

Abstract

A single-phase face-centered-cubic (FCC) high- or medium-entropy alloys (HEAs or MEAs) have attracted great attentions due to their novel damage-tolerance properties (strength, ductility, and fracture toughness) by generating nano-twins at cryogenic temperature. The fracture toughness assessment is essential for evaluating the reliability of high-performance materials for cryogenic applications; however, fracture studies on single-phase FCC HEAs showing transformation-induced plasticity (TRIP) have been hardly conducted. In this study, thus, damage-tolerance mechanisms of a V10Cr10Fe45Co30Ni5 HEA showing the FCC to body-centered-cubic (BCC) TRIP were investigated at room and cryogenic temperatures. At room temperature (298 K), the alloy shows the tensile strength of 731 MPa, elongation of 40%, and fracture toughness (KJIc) of 230 MPa m1/2. At cryogenic temperature (77 K), the strength and elongation improve to 1.2 GPa and 66%, respectively, while the KJIc remains almost constant at 237 MPa m1/2. Dislocation-mediated plasticity prevails at 298 K; however, the TRIP from FCC to BCC occurs at 77 K. Deformation and fracture mechanisms are analyzed by stacking fault energies and differences in Gibbs free energies between phases calculated by ab-initio methods, and are compared to those of CrMnFeCoNi, CrCoNi, Fe50Mn30Co10Cr10, and V10Cr10Fe45Co20Ni15 alloys. Despite the presence of a considerable amount of BCC which is intrinsically brittle at low temperature, the transformed BCC martensite shows ductile fracture after the fracture toughness test even in cryogenic environments. These results demonstrate that the FCC to BCC TRIP can be an attractive route in a field of HEA design to overcome the strength and toughness trade-off at cryogenic temperature.

Introduction

Superior strength and toughness are generally required as essential properties in engineering structural materials, although they conflict with each other, like strength and ductility. The toughness often deteriorates at cryogenic temperature by an inherent crystallographic problem or a reduction in capacity of damage tolerance [[1], [2], [3]]. Recently developed high and medium entropy alloys (HEAs and MEAs, respectively), e.g., equi-atomic CoCrFeMnNi or CrCoNi alloys, show excellent tensile properties at cryogenic temperature by a mechanism of twinning-induced plasticity (TWIP) [4,5]. Their fracture toughness is higher at both room and cryogenic temperatures than 200 MPa m1/2, and tends to enhance with decreasing temperature.

However, the superior fracture toughness is not an inherent feature of all multi-principal element alloys (MPEAs, representative terminology of both HEAs and MEAs). Li et al. [6] reported that (FCC + BCC)- or BCC-based MPEAs exhibited relatively worse fracture toughness than the FCC-based ones. In particular, the BCC phase of large volume fraction or grain size often resulted in the ductile-to-brittle transition at cryogenic temperature [[7], [8], [9]]. Jo et al. [10] reported that brittle σ phases precipitated at FCC grain boundaries deteriorated the fracture toughness in the V20Cr15Fe20Ni45 HEA. Thus, a single FCC phase is basically recommended in the design of fracture-resistant alloys for cryogenic applications. Equi-atomic MPEAs such as CrMnFeCoNi, CrMnCoNi, CrFeCoNi, CrCoNi, and FeCoNi having a single FCC phase are mainly developed on the basis of the representative CrMnFeCoNi alloy system [11,12]. Other equi-atomic MPEAs are also developed by adding V, Cu, Mo, Al, or Ti, but often include toughness-deteriorating phases of σ, FCC1+FCC2, μ, B2, or Laves phases [[13], [14], [15], [16], [17], [18]], respectively. In order to obtain a single FCC phase, therefore, non-equi-atomic HEAs which are deviated from the original design criterion on component composition of 5–35 at.% have been proposed, and show excellent tensile properties along with a stable single FCC structure [19,20].

Recently, we developed non-equi-atomic VCrFeCoNi alloys of stable single FCC structure based on a thermodynamic calculation [21,22]. A transition from the TWIP to another powerful deformation mechanism of transformation-induced plasticity (TRIP) occurred with increasing Co content, which led to a large strength improvement [22]. Particularly, the transformation from FCC to BCC, i.e., BCC-TRIP, occurring in the V10Cr10Fe45Co30Ni5 HEA led to novel cryogenic and dynamic tensile properties [23]. However, its fracture toughness studies required for reliable cryogenic applications of such high-performance alloys were not investigated yet. Thus, we conducted the fracture toughness evaluation of the V10Cr10Fe45Co30Ni5 HEA at room and cryogenic temperatures, and the resultant data were compared with those of the previously reported CrMnFeCoNi, CrCoNi, and V10Cr10Fe45Co20Ni15 alloys. Deformation and fracture mechanisms occurring at crack-tip areas during the fracture toughness test as well as microstructural evolutions occurring during the tensile test were correlated in detail. Stacking fault energies (SFEs) and differences in Gibbs free energy between phases were calculated by ab-initio methods to understand the deformation behavior of various alloys. Our results demonstrate that the BCC-TRIP can be favorably utilized for achieving the excellent fracture toughness at cryogenic temperature.

Section snippets

HEA fabrication

The V10Cr10Fe45Co30Ni5 (at.%) alloy was cast in a vacuum-induction melting instrument under an Ar atmosphere to produce an ingot (58 × 80 × 108 mm3). The ingot was hot-rolled (reduction ratio of ∼60%) at 1373-1173 K, homogenized at 1473 K for 2 h, water-quenched, sand-blasted for removing surface oxide scales, cold-rolled (reduction ratio of ∼50%) to a thickness of 10 mm, annealed at 1123 K for 1 h, and water-quenched. The composition of the annealed plate was analyzed by an

Microstructures

An SEM-BSE image of the as-annealed V10Cr10Fe45Co30Ni5 alloy is shown in Fig. 1(a). The alloy is composed of fully recrystallized grains of about 6 μm in size. A few black-colored particles are randomly distributed throughout the specimen. According to the EDS data of the particles in Table 1, they are identified as V-rich oxides having large amounts of V and O. These oxides often form during the casting of alloys containing reactive elements (such as V, Cr, and Mn), and they remain even after

Calculated stacking fault energy and Gibbs free energy difference

As the temperature decreases, mechanical properties are enhanced in equi-atomic CrMnFeCoNi and CrCoNi alloys and non-equi-atomic V10Cr10Fe45Co20Ni15 alloy [4,5,25]. All these three alloys produce a large amount of deformation twins with decreasing temperature. The resulting high strain-hardening exponents (>0.35) promote the delay of local necking, thereby leading to the good damage-tolerance as shown in the strength, elongation, and toughness data in Table 2. In the present V10Cr10Fe45Co30Ni5

Conclusions

In this work, a damage-tolerance mechanism of the V10Cr10Fe45Co30Ni5 HEA was investigated at both room and cryogenic temperatures. Tensile deformation mechanisms were revealed in detail and correlated with SFE and difference in Gibbs free energy between BCC and FCC obtained from ab-initio calculations. Room- and cryogenic-toughening mechanisms were discussed by examining the crack tip areas after fracture toughness tests. The main conclusions are as follows:

  • (1)

    The as-annealed V10Cr10Fe45Co30Ni5

CRediT authorship contribution statement

Yong Hee Jo: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Visualization. Junha Yang: Investigation. Kyung-Yeon Doh: Software, Formal analysis, Investigation, Writing - original draft. Woojin An: Investigation. Dae Woong Kim: Investigation. Hyokyung Sung: Validation, Resources, Data curation. Donghwa Lee: Validation, Resources, Data curation. Hyoung Seop Kim: Resources, Funding acquisition. Seok Su Sohn: Writing - review & editing,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Korea University Grant for Prof. S.S. Sohn, by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (2016M3D1A1023384), and by the Brain Korea 21 PLUS Project for Center for Creative Industrial Materials. The authors would like to gratefully acknowledge the kind support of Dr. W.-M. Choi and Professor B.-J. Lee at the POSTECH.

References (69)

  • W.H. Liu et al.

    Ductile CoCrFeNiMox high entropy alloys strengthened by hard intermetallic phases

    Acta Mater.

    (2016)
  • G.A. Salishchev et al.

    Effect of Mn and V on structure and mechanical properties of high-entropy alloys based on CoCrFeNi system

    J. Alloys Compd.

    (2014)
  • M.J. Yao et al.

    A novel, single phase, non-equiatomic FeMnNiCoCr high-entropy alloy with exceptional phase stability and tensile ductility

    Scripta Mater.

    (2014)
  • K.G. Pradeep et al.

    Non-equiatomic high entropy alloys: approach towards rapid alloy screening and property-oriented design

    Mater. Sci. Eng., A

    (2015)
  • W.-M. Choi et al.

    A thermodynamic description of the Co-Cr-Fe-Ni-V system for high-entropy alloy design

    Calphad

    (2019)
  • Y.H. Jo et al.

    Effects of deformation–induced BCC martensitic transformation and twinning on impact toughness and dynamic tensile response in metastable VCrFeCoNi high–entropy alloy

    J. Alloys Compd.

    (2019)
  • Y.H. Jo et al.

    Cryogenic-temperature fracture toughness analysis of non-equi-atomic V10Cr10Fe45Co20Ni15 high-entropy alloy

    J. Alloys Compd.

    (2019)
  • L. Vitos et al.

    Application of the exact muffin-tin orbitals theory: the spherical cell approximation

    Comput. Mater. Sci.

    (2000)
  • J. Staunton et al.

    The “disordered local moment” picture of itinerant magnetism at finite temperatures

    J. Magn. Magn Mater.

    (1984)
  • N. Choi et al.

    Characterization of non-metallic inclusions and their influence on the mechanical properties of a FCC single-phase high-entropy alloy

    J. Alloys Compd.

    (2018)
  • G. Laplanche et al.

    Reasons for the superior mechanical properties of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi

    Acta Mater.

    (2017)
  • G. Laplanche et al.

    Microstructure evolution and critical stress for twinning in the CrMnFeCoNi high–entropy alloy

    Acta Mater.

    (2016)
  • F. Otto et al.

    The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy

    Acta Mater.

    (2013)
  • M. Zhang et al.

    Evolution in microstructures and mechanical properties of pure copper subjected to severe plastic deformation

    Met. Mater. Int.

    (2019)
  • A. Dumay et al.

    Influence of addition elements on the stacking-fault energy and mechanical properties of an austenitic Fe–Mn–C steel

    Mater. Sci. Eng., A

    (2008)
  • J. Su et al.

    Hierarchical microstructure design to tune the mechanical behavior of an interstitial TRIP-TWIP high-entropy alloy

    Acta Mater.

    (2019)
  • S. Allain et al.

    Correlations between the calculated stacking fault energy and the plasticity mechanisms in Fe–Mn–C alloys

    Mater. Sci. Eng., A

    (2004)
  • Z. Wang et al.

    On the mechanism of extraordinary strain hardening in an interstitial high-entropy alloy under cryogenic conditions

    J. Alloys Compd.

    (2019)
  • J. Miao et al.

    The evolution of the deformation substructure in a Ni–Co–Cr equiatomic solid solution alloy

    Acta Mater.

    (2017)
  • S.S. Sohn et al.

    Effects of Mn and Al contents on cryogenic–temperature tensile and Charpy impact properties in four austenitic high–Mn steels

    Acta Mater.

    (2015)
  • B. Sundman et al.

    The thermo-calc databank system

    Calphad

    (1985)
  • K.-G. Chin et al.

    Thermodynamic calculation on the stability of (Fe, Mn) 3AlC carbide in high aluminum steels

    J. Alloys Compd.

    (2010)
  • S.J. Sun et al.

    Transition of twinning behavior in CoCrFeMnNi high entropy alloy with grain refinement

    Mater. Sci. Eng., A

    (2018)
  • O.N. Senkov et al.

    Mechanical properties of Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 refractory high entropy alloys

    Intermetallics

    (2011)
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