Acceleration of strain-driven ferritic grains formation in a martensitic Fe-based medium-entropy alloy

Although martensite is thermodynamically metastable, it often exhibits strong mechanical stability under deformation and usually undergoes only refinement of its needle-like microstructure (i.e., blocks and/or laths), even under extremely large equivalent strain up to ε = 15. Here, we report a strain-induced transition from martensite to equiaxed ferritic grains in a Fe-based medium-entropy alloy processed by high-pressure torsion at room temperature. Remarkably, this transition occurs already at a relatively low strain (~ ε = 6.14), distinguishing it from the conventional strain-driven fragmentation process typically observed in low-alloy steels. Experimental analyses reveal a progressive structural evolution in which the martensitic blocks are first refined and then transformed into equiaxed ferritic grains, with the characteristic size reduced from 7.2 to 0.8 μm, accompanied by texture weakening, a pronounced increase in the fraction of high-angle grain boundaries, a non-monotonic change in dislocation density, and hardness enhancement. The transition is attributed to the activation of dynamic recrystallization, which is promoted under limited strain by severe lattice distortion and compositional complexity that hinder dislocation glide and accelerate dislocation accumulation. Within the thermo-kinetic framework of generalized stability, this microstructure evolution reflects the combined principle of a large driving force and a high generalized stability. These findings demonstrate that the deformation resistance of martensitic structures can be overcome through strain-driven recrystallization mechanisms in complex body-centered cubic alloys, offering a new pathway for microstructural transformation in metastable high-strength materials.