Achieving excellent strength-ductility synergy via high density dislocation and nano-twinning in cryo-rolled FeCoCrNiMo0.2 high-entropy alloy

Abstract

High-density dislocations and nano-twinning are known to significantly influence the work hardening ability of materials, presenting a potent approach for enhancing their overall strength. Here, a superior tensile strength-ductility synergy in a FeCoCrNiMo0.2 high entropy alloy (HEA) with high density dislocation and nano-twinning was achieved by employing cryo-rolling followed by subsequent annealing. Through electron microscope observation, the effects of geometrically necessary dislocation density and nano-twinning on tensile strength and plasticity are elucidated. The microstructure analysis reveals a substantial presence of dislocations and nano-twinning within the deformed grains following short-time annealing at 600 °C and 800 °C. The strategic presence of nano-twins serves as effective impediments to dislocation slip, resulting in the storage of dislocations within the grains, which contributes to the alloy's superior strength-ductility synergy and remarkable work hardening ability. Impressively, the obtained tensile strength-ductility matching exhibits yield strength and ductility values of 992.1 MPa/0.19 and 719 MPa/0.39, respectively. Upon increasing the annealing temperature to 1000 °C, high-density annealing twins emerge in the recrystallized grains, accompanied by a concomitant reduction in the geometrically necessary dislocation density due to dislocation rearrangement and annihilation. Furthermore, a small amount of topological close-packed (TCP) phase and L12 phase precipitation during annealing are observed, contributing to increased work hardening ability of the alloy. The findings underscore the importance of microstructural engineering as a valuable strategy for optimizing the mechanical properties of HEAs with a face-centered cubic structure.