Abstract
Metastability-engineering strategy is an important topic for high entropy alloys (HEAs), owing to the transformation-induced plasticity effect (TRIP). In this work, TRIP effects of Co-Cr-Fe-Ni HEAs are investigated. Results indicate the tensile deformation-induced martensitic transformation occurs in Co35Cr25Fe40−xNix (x = 0–15 at %) HEAs. The excellent combination of tensile strength (760 MPa–1000 MPa) and elongation (65–35%) owe to solid solution strengthening of Co and Cr, and the TRIP effect. In non-equal molar Co-Cr-Fe-Ni systems, with the decrease of Ni content, the values of stacking fault energy (SFE) decrease; thus, TRIP phenomena occurs. Based on the experimental investigation in three different regions of the Co-Cr-Fe-Ni multicomponent phase diagram, the face-centered cubic structured Co-Cr-Fe-Ni HEAs with VEC of ~8.0 is more metastable, and TRIP phenomena are more likely to occur.
Highlights
High entropy alloys (HEAs), or multi-principal-component alloys [1] show remarkable properties, mainly for their twinning induced plasticity (TWIP) effect [2], or transformation induced plasticity (TRIP) effect [3], due to overcoming the strength-ductility trade-off in the composition design of alloys.Generally, low stacking fault energy (SFE) high entropy alloys (HEAs) with face-centered cubic (FCC) single phases are more likely to deform by twinning, with increased strain hardening rates [2,4,5,6]
The results indicate that a ~3 vol % thermally induced hexagonal close packed (HCP) phase exists in the annealed Ni-10 sample, with valence electron concentration (VEC) of 8.05, which is not captured in the XRD results
The results show that all elements are uniformly distributed in Co35 Cr25 Fe40 with FCC and HCP dual-phase
Summary
Low stacking fault energy (SFE) HEAs with face-centered cubic (FCC) single phases are more likely to deform by twinning, with increased strain hardening rates [2,4,5,6]. The very low SFE values at cryogenic conditions suggest that the possible presence of the TRIP effect via ab initio calculations [7]. CoCrNi alloy exhibits an excellent combination of strength and ductility—greater than that of CoCrFeMnNi alloy at both cryogenic and room temperatures [5]. This indicates that stronger alloys are Metals 2018, 8, 369; doi:10.3390/met8050369 www.mdpi.com/journal/metals
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