Deformation-Driven Bidirectional Transformation Promotes Bulk Nanostructure Formation in a Metastable Interstitial High Entropy Alloy

Abstract

We investigate the mechanisms of deformation-driven forward and reverse (bidirectional) martensitic transformation and the associated nanostructure formation in a metastable carbon-doped high entropy alloy (HEA) upon cold rolling. At thickness reductions below 14%, forward hexagonal-close packed (HCP) martensitic transformation prevails in the single face-centered cubic (FCC) matrix. Surprisingly, at the intersections of two crossing HCP lamellae, deformation-induced reverse transformation from the HCP martensite back to the FCC phase occurs. At higher thickness reductions around 26%-34%, multiple deformation kink bands develop, mainly on the pyramidal habit planes of the HCP martensite, among which reverted FCC phase is also observed resulting in a dual-phase nano-laminated structure. The deformation-induced reverted FCC phase regions exhibit a twin stacking sequence relative to the prior FCC matrix, which is related to the underlying dislocation reactions and rearrangement of the partial dislocations. At 67% thickness reduction, the deformation bands develop further into micro-shear bands consisting of nanosized (sub)grains. For rendering the dual-phase nanostructure back to single-phase FCC, 400 °C/10 min tempering is applied on a 34% cold-rolled specimen. The resulting nanostructure is characterized by nano-(sub)grains and nano-twins. It exhibits an excellent strength-ductility synergy (ultimate tensile strength 1.05 GPa at 35% total elongation) due to the improved work hardening enabled by both, FCC-HCP martensitic transformation in confined regions and mechanical twinning. With this we show that bulk nanostructured alloys with bidirectional transformation can be designed by tuning the materials' phase stability to their thermodynamic limits with the aim to trigger sequential athermal forward and reverse transformation under load.