Role of strain rate in phase stability and deformation mechanism of non-equiatomic Fe38-xMn30Co15Cr15Ni2Gdx high-entropy alloy

Abstract

Excellent combination of strength and ductility makes metastable high entropy alloys (HEAs) stand out from multi-principal elements alloys. In this study, micro-alloying Gd element is effectively solid solutionized in metastable single FCC phase FeMnCoCrNi HEA system, to bring the FCC phase stability decreased that causes a pronounced FCC → HCP phase transformation during compression. In order to decouple the temperature effect from the influence of dynamic strain rate on phase transformation, a wide strain rate range is adopted from 10 −3 –10 −1 s −1 to 1000–5000 s −1 . The results show that the increased strain rate and adiabatic temperature rise both can restrain the phase transformation. Under low strain rates (10 −3 –10 −1 s −1 ), phase transformation is activated in all tested samples and gradually suppressed with the increase of strain rates, because the rate-induced shear bands inhibit the growth of the HCP phase. The reduction of strain hardening during phase transformation is related to the increase of local thermal activation volume due to disentangling of dislocations. Under dynamic strain rates (1000–5000 s −1 ), the phase transformation is completely substituted by deformation twinning due to the significant adiabatic temperature rise. The present work provides insights into the influence of strain rate on phase stability and deformation mechanism in a wide strain rate range. • Solid solution of Gd decreases the FCC phase stability bringing pronounced FCC → HCP phase transformation during quasi-state compression. • Phase transformation related dislocation dissociation enlarges the thermally activated volume causing the decrease in strain hardening rate. • Under low strain rates (10 −3 –10 −1 s −1 ), the increased strain rates activate shear bands earlier that hinder the growth of the HCP phase, it is a rate-related effect on phase transformation. • Under high strain rates (1000–5000 s −1 ), adiabatic temperature rise enhances the phase stability which is a thermo-related effect, co-works with the rate-related effect, making the phase transformation completely suppressed.