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