Hierarchical precipitates, sequential deformation-induced phase transformation, and enhanced back stress strengthening of the micro-alloyed high entropy alloy

Abstract

We report the annealing time-dependent microstructures and deformation mechanisms of the novel face-centered cubic Fe49.5Mn30Co10Cr10C0.2Ti0.1V0.1 Mou HEA. Three types of precipitates, sigma-phase, Cr-rich MC-type carbides, and nano-scale (Ti, V, Mo)C, are present after cold-rolling and annealing at 600 degrees C. Such hierarchical precipitates could lead to sluggish recrystallization and grain growth upon annealing. The partially recrystallized microstructures and hierarchical precipitates could lead to a high yield strength even for prolonged annealing conditions. Deformation mechanisms change with annealing time. The materials annealed for short times (< 2 h) are deformed by dislocation glide, deformation twinning, and deformation-induced epsilon phase. A longer annealing time (> 10 h) triggers a multi-variant epsilon phase, reverse transformation from epsilon to gamma, and the multi-step sequential transformation, gamma -> epsilon -> reverse transformed gamma from epsilon -> epsilon transformed from the reverse transformed gamma. Further, materials annealed for longer times shows a higher contribution of back stress strengthening, which could be attributed to the increase in gamma/epsilon and gamma/sigma interfaces. The activation of various deformation mechanisms and high back stress strengthening could lead to a superior strain hardening capacity and strength-ductility combination (YS: 699 MPa, UTS: 1041 MPa, TE: 45%) of the material annealed for 10 h. The present work provides the novel microstructure design solution of the metastable high entropy alloys with exceptional mechanical properties, utilizing hierarchical precipitates, sequential deformation-induced phase transformation, and enhanced back stress strengthening. (C) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.