Temperature- and strain-dependent thermally-activated deformation mechanism of a ferrous medium-entropy alloy
Introduction
During the last decade, a great deal of attention has been focused on multi-component high-entropy alloys (HEAs) and medium-entropy alloys (MEAs) with single-phase solid solution for their outstanding mechanical properties according to large configurational entropy [[1], [2], [3], [4], [5], [6], [7]]. Recently, the studies on these alloy groups with dual-phase [[8], [9], [10]], metastability-engineering of near-single phase [11,12], or utilization of precipitation phases [[13], [14], [15], [16]] have been spotlighted for extending the scope of alloy design strategies. Meanwhile, superior tensile strength and ductility with outstanding strain hardening rate of HEAs/MEAs have been achieved through martensitic transformation from face-centered cubic (FCC) to hexagonal close-packed (HCP) and/or body-centered cubic (BCC) at cryogenic temperature (77 K) [11,[17], [18], [19], [20]]. Notably, recent studies on MEAs dealing with the phase transformation of single or near-single FCC phase [11,17] exhibit an exceptional combination of strength and ductility at cryogenic temperatures.
In particular, a ferrous medium-entropy alloy (FeMEA) corresponding to near-single FCC-structured Fe60Co15Ni15Cr10 (at%) alloy is highly expected to possess superior mechanical properties at 77 K [11]. In designing FeCoNiCr-based alloy system, low Cr concentration of 10 at% can suppress the formation of intermetallic compounds, such as Cr-rich σ phase, which degrade the mechanical properties of the alloy [11]. Besides, the FeMEA shows beneficial effects on both cost-effectiveness and tunable FCC phase stability for metastability-engineering with an increase in Fe and decreases in Co and Ni concentrations [11,17]. As shown in the previous work [11], the FeMEA exhibits excellent tensile properties at cryogenic temperatures due to the sequential operation of BCC martensitic transformation along the grain boundaries and the shear bands within FCC grains.
The analyses on thermally-activated deformation have figured out the rate-controlling mechanisms fairly related to dislocation behaviors of HEAs [[21], [22], [23], [24], [25], [26]]. An equiatomic CoCrFeMnNi HEA with a single FCC phase was discussed on the thermally-activated deformation behavior elaborating that planar dislocation slip is the rate-controlling mechanism, while Peierls lattice friction stress stands for BCC alloys [21]. By investigating the plastic strain dependence of strain rate sensitivity of CoCrFeMnNi HEA [22], nanoscale heterogeneities were revealed. Basu et al. [23] investigated the strain rate sensitivity of transformation-induced plasticity (TRIP)-assisted dual-phase HEA at room temperature, demonstrating that FCC to HCP martensitic transformation accompanied with dislocation glide is a primary deformation mechanism. Despite a few reports, there are still insufficient attempts to figure out the thermally-activated mechanism based on the on-going deformation of multi-component alloys that exhibit metastability-engineering.
In this work, we investigated the temperature and strain dependence of deformation behavior of an FeMEA based on the thermally-activated deformation mechanism. Since the primary rate-controlling deformation mechanism considerably depends on the crystal structure [21], the present TRIP-assisted alloy with a change in the crystal structure through quasi-static deformation at 77 K has a coupling effect on the rate-controlling mechanism due to multi-phase microstructure. Thus, revealing the rate-controlling mechanism leads to a deeper understanding of temperature and strain rate dependence of the present alloy. In particular, predominant rate-controlling mechanisms with respect to plastic strain were elucidated through strain rate jump tests and the corresponding microstructural analyses at both 298 K and 77 K.
Section snippets
Experimental procedure
An ingot of Fe60Co15Ni15Cr10 (at%) FeMEA was cast in a dimension of 70 × 35 × 7 mm3 using vacuum induction melting equipment (MC100V, Indutherm, Walzbachtal-Wossingen, Germany) under an argon atmosphere. Purity of the elemental metals was at least 99.95%. The rectangular ingot was homogenized at 1373 K for 6 h, followed by water quenching. The homogenized ingot was cold-rolled at 298 K from 7 to 1.5 mm with a thickness reduction ratio of 78.6%. After that, the tensile specimens were obtained
Tensile properties and microstructures
Fig. 1 shows true and engineering stress-strain curves and strain hardening rate (SHR) of the alloy at 298 K and 77 K. At 77 K, tensile properties exhibit exceptional yield strength (YS) of 383 ± 36 MPa, ultimate tensile strength (UTS) of 1357 ± 30 MPa, and elongation of 63.5 ± 0.55% with substantial strain hardening compared to those at 298 K.
Fig. 2, Fig. 3 reveal the microstructural evolution of the present alloy during deformation at 298 K and 77 K, respectively. In Fig. 2(a), EBSD inverse
Conclusion
The thermally-activated deformation behavior of the Fe60Co15Ni15Cr10 (at%) ferrous MEA was explored using strain rate jump tests. Higher strain rate sensitivity of the present alloy at 77 K than that at 298 K was rationalized by the thermally-activated deformation mechanism. The core findings disclosed in this study are described below:
- (1)
At 298 K, dynamic recovery associated with the cross-slip mechanism is predominant at early deformation. As the true plastic strain exceeds 0.1, dislocation
CRediT authorship contribution statement
Jungwan Lee: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing – original draft. Jongun Moon: Conceptualization, Formal analysis, Investigation, Methodology, Validation, Writing – review & editing, Supervision. Jae Wung Bae: Methodology, Investigation, Resources. Jeong Min Park: Conceptualization, Methodology, Validation. Hyeonseok Kwon: Methodology, Investigation. Hidemi Kato: Validation, Investigation. Hyoung Seop Kim: Funding acquisition,
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 Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF–2016M3D1A1023384). J.M. acknowledges support from Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education [2020R1A6A3A03037509].
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2022, Materials Science and Engineering: ACitation Excerpt :Owing to the deformation-induced fcc-to-bcc phase transformation, these alloys showed a good balance of strength (up to approximately 1.5 GPa) and ductility (up to 87%) at 77 K. A more detailed study of the Fe60Co15Ni15Cr10 alloy suggests that unusual mechanical properties during cryogenic deformation can be associated with a relatively high strain rate sensitivity and low activation volume [5]. Interstitial elements such as C can also strongly affect the fcc phase stability; in addition, carbon promotes additional strengthening mechanisms such as interstitial solid solution strengthening and/or precipitation hardening [2,11–13].