Effect of Si on the evolution of plasticity mechanisms, grain refinement and hardness during high-pressure torsion of a non-equiatomic CoCrMnNi multi-principal element alloy

The present study unraveled the defining role of small silicon (Si) addition (5 atomic %) in dramatically altering the plasticity mechanisms, grain refinement and hardening response of a non-equiatomic CoCrMnNi multi-principal element alloy (MPEA) during high-pressure torsion (HPT) processing. Both the Si-free and the Si-added MPEAs had a face-centered cubic (FCC) structure and were subjected to a quasi-constrained HPT processing at 6 GPa pressure to different number of turns (0.5 and 5). Microstructure evolution was studied at the center and edge of the HPT-processed discs using X-ray diffraction line profile analysis (XLPA) and transmission electron microscopy (TEM). Si addition altered the predominant plasticity mechanism from micro-band formation to extensive occurrence of nano-twinning at the early stage of HPT processing. At later stages of HPT processing, both alloys exhibited deformation twinning but its propensity was considerably higher for the Si-added MPEA, as revealed by-50% higher twin fault probability. Additionally, the Si-added MPEA showed-30% higher dislocation density at any given stage of HPT processing compared to the Si-free MPEA. A significantly accelerated nano-structuring coupled with a finer saturation grain size was observed in the Si-added MPEA (34 nm for Si-free versus 23 nm for Si-added). These effects can be explained by the influence of Si addition on lowering the stacking fault energy (SFE) (from-40 mJ/m2 in Si-free to-20 mJ/m2 in Si-added MPEA) and increasing the solute pinning effect of Si on lattice defects. The plasticity mechanisms at nano-scale were also influenced by the presence of Si as confirmed by the for-mation of nano-twins and stacking faults inside the nano-grains for the Si-added and Si-free MPEAs, respectively. The differences in plasticity mechanisms and microstructure evolution resulted in enhanced hardness in the early stages of HPT processing for the Si-added MPEA, but the difference in hardness between the two alloys tended to be reduced at higher strains.