Hierarchically activated deformation mechanisms to form ultra-fine grain microstructure in carbon containing FeMnCoCr twinning induced plasticity high entropy alloy

Abstract

Nanotructural evolution and grain refinement leading to heterogeneous bimodal structure were investigated during thermomechanical processing in carbon-containing FeMnCoCr twinning induced plasticity (TWIP) high-entropy alloy (HEA). The homogenized single-phase face-centered cubic alloy with the stacking fault energy of 19.4 ± 2 mJ m−2 was cold rolled up to thickness reduction of 84 % and annealed at 850 °C. Planar slip with profuse nanoscale deformation twinning was the dominant deformation feature at low rolling reduction (32 %). The heterogeneous structure could be obtained through subdivision of microstructure, continuous dynamic recrystallization and static recrystallization described as follows: (i) Hierarchical mechanical twinning, (ii) Interaction of twin-matrix (T/M) lamellae with shear-bands and (iii) Continuous dynamic recrystallization (CDRX) within the strain-induced boundaries (SIBs), (iv) Subsequent static recrystallization of the region with twin-matrix (T/M) lamellae and shear-bands. The strength and ductility enhancement during deformation was attributed to the hierarchy of microstructural evolution consisting of TWIP and microband-induced plasticity (MBIP). The heterogeneous bimodal structure composed of ultra-fine grains and larger grains with an average grain size of 0.5 μm and 3 μm respectively was achieved by post-rolling (84 %) annealing at 850 °C. Favorable strength-ductility combination with the ultimate tensile strength of 840 MPa and elongation of ~88 % was achieved by formation of heterogeneous bimodal microstructure through thermomechanical processing.