Abstract
We introduce a novel transformation-induced plasticity mechanism, i.e., a martensitic transformation from fcc phase to bcc phase, in medium-entropy alloys (MEAs). A VCrFeCoNi MEA system is designed by thermodynamic calculations in consideration of phase stability between bcc and fcc phases. The resultantly formed bcc martensite favorably contributes to the transformation-induced plasticity, thereby leading to a significant enhancement in both strength and ductility as well as strain hardening. We reveal the microstructural evolutions according to the Co-Ni balance and their contributions to a mechanical response. The Co-Ni balance plays a leading role in phase stability and consequently tunes the cryogenic-temperature strength-ductility balance. The main difference from recently-reported metastable high-entropy dual-phase alloys is the formation of bcc martensite as a daughter phase, which shows significant effects on strain hardening. The hcp phase in the present MEA mostly acts as a nucleation site for the bcc martensite. Our findings demonstrate that the fcc to bcc transformation can be an attractive route to a new MEA design strategy for improving cryogenic strength-ductility.
Highlights
Development, transportation, and preservation of resources through recent scientific and technological advances are increasingly demanding novel metals to be applied in extreme and risky environments
It is well known that a key parameter for designing metastable medium-entropy alloys (MEAs) as well as conventional structural alloys which show the TRansformation-Induced Plasticity (TRIP) behavior, i.e., the fcc to bcc transformation, is the stacking fault energy (SFE) of fcc
Though the SFE contains a term expressed by a difference in Gibbs free energies between hcp and fcc phases[6,23,24], the fcc to bcc transformation is analyzed by the SFE
Summary
Development, transportation, and preservation of resources through recent scientific and technological advances are increasingly demanding novel metals to be applied in extreme and risky environments. Unceasing efforts have been made to improve both strength and ductility[1,2,3,4], but cryogenic environments often cause a severe deterioration in ductility because of low damage-tolerance capacities and crystallographic problems[5,6] In this respect, recently developed face-centered-cubic (fcc) high-entropy alloys (HEAs) present excellent tensile properties and exceptional fracture toughness by forming mechanical nano-twins at cryogenic temperature[7,8]. The deformation-induced martensitic transformation, which has been widely employed in numerous high-strength structural alloys, is introduced into HEAs. For example, (1) fcc to hexagonal-close-packed (hcp) transformation in FeMnCoCr, CoCrFeMnNi and CrCoNi alloys[1,16,17,18], (2) body-centered-cubic (bcc) to orthorhombic transformation in Al0.6CoCrFeNi alloy[19], and (3) bcc to hcp transformation in TaHfZrTi alloy[20] were reported. The resultantly formed bcc martensite favorably contributes to the TRansformation-Induced Plasticity (TRIP), thereby leading to a significant enhancement in both strength and ductility as well as strain hardening
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