Synergistic strengthening mechanisms of dual-phase CoCrFeMnNi high-entropy alloys with grain nano-gradient structure

Abstract

The "inverted" relationship between strength and plasticity of metal structural materials has limited its application at a higher level and in a wider range, and has become the main bottleneck restricting the development and application of metal materials. It is also difficult for high entropy alloys (HEAs) with ordered structure and disordered composition to escape this constraint. This study investigates the synergistic effect of grain nano-gradient (GNG) structure and phase transformation-induced plasticity (TRIP) on the strengthening and toughening of HEAs through molecular dynamics (MD) simulations. When the volume fraction of the thermally induced HCP phase (V_HCP) is 21 %, the best TRIP effect is observed (i.e., macroscopic strain of 15 %, the volume fraction of HCP phase increased by 14.41 %), and significant plastic behavior is observed for the first time in disappeared transverse stacking fault (SF) areas. It is found that the increase in dislocation density (e.g., V_HCP = 0 %, dislocation density increased from 1.94 × 10¹⁷ m² to 3.04 × 10¹⁷ m²) and the accumulation at the coarse-fine grain interface contribute to the heterogeneous deformation induced (HDI) strengthening and hardening mechanism. Strengthening mechanism can be divided into three stages: (a) In the early stage, it is enhanced by dislocations and SFs. (b) In the middle stage, it mainly relies on HDI enhancement. (c) In the late stage, FCC to HCP phase transformation occurs.