Micro-mechanisms of shear deformation during high-pressure torsion of equiatomic FeMnNi medium entropy alloy

Abstract

A FeMnNi equiatomic medium entropy alloy was subjected to high-pressure torsion up to 5 turns under 2 GPa pressure at room temperature to achieve wide range of microstructures comprising of heterogeneous sub-microcrystalline structures to uniform nanocrystalline grains as a function of strain. Heterogeneous intragranular deformation caused by the operation of octahedral and partial slip systems causes rapid grain fragmentation without significant change in aspect ratio. The grain size was reduced from nearly 26 μm, as measured by EBSD, to approximately 59 nm after five turns, as determined by TEM, accompanied by a significant increase in hardness after the first turn, with saturation observed up to five turns. Nanoindentation experiments indicated that the improvement in strength was accompanied with substantial plasticity indicated by the plasticity index W_p/W_t during nanoindentation unloading cycle. This is followed by a decrease in strain rate sensitivity and marginal decrease in activation volume till a shear strain of about ε_vm ≈ 29, after which both remain almost constant indicating a steady state of microstructural refinement. Microstructural analyses, including EBSD and TEM, confirmed that grain subdivision occurs through dislocation rearrangement, leading to the formation of low-angle grain boundaries that progressively transform to high-angle grain boundaries contributing to grain size refinement by grain subdivision with the increase in strain. The process of grain refinement does not involve nucleation of dislocation free grains as observed for classical discontinuous recrystallization and is attributed to continuous dynamic recrystallization (CDRX) or extended dynamic recovery. Thus, the FeMnNi medium entropy alloy exhibits scope for microstructural engineering in the ultrafine and nanocrystalline regime to achieve optimum combination of strength and ductility.

Graphical abstract