Double-humped strain hardening in a metastable ferrous medium-entropy alloy by cryogenic pre-straining and subsequent heat treatment

Significant benefits of either heterogeneous microstructures or deformation-induced martensitic transformation (DIMT) in metallic materials stem from their superior strain hardening and tensile properties. Herein, we present an unprecedented strain hardening behavior at 77 K in a ferrous medium-entropy alloy comprising a metastable face-centered cubic (FCC) bimodal microstructure with different grain sizes. Pre-straining yields fine body-centered cubic (BCC) martensites, and subsequent heat treatment causes reverse martensitic transformation from BCC to FCC, affording the bimodal FCC microstructure. Furthermore, the yield strength is enhanced due to the presence of submicron FCC grains and geometrically necessary dislocations (GNDs) that are generated during the pre-straining. Profuse GNDs in reversely transformed FCC promote DIMT. Moreover, when true strain exceeds 0.2, widespread DIMT in the interior of the remaining FCC drastically increases the strain hardening rate and consequently delays necking. The outstanding tensile properties derived from this thermomechanical process are because of the DIMT and hetero-deformation-induced strengthening. (c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

The strain hardening behavior of a ferrous medium-entropy alloy at 77 K is presented. It shows a bimodal microstructure with different grain sizes, resulting from pre-straining and subsequent heat treatment. The presence of submicron FCC grains and GNDs enhances the yield strength, and the DIMT and hetero-deformation-induced strengthening delay necking and improve strain hardening rate.