Significant annealing-induced hardening effect in nanolaminated-nanotwinned (CrCoNi)97.4Al0.8Ti1.8 medium-entropy alloy by severe cold rolling

Abstract

Due to the easy coarsening caused by poor thermal stability, the verified annealing-induced hardening in nanograined metals can only maintain at a relatively low-temperature range. In this study, a nanolaminated (CrCoNi)97.4Al0.8Ti1.8 medium-entropy alloy with an average lamellae thickness of ∼ 20 nm embedded by thinner nanotwins was fabricated by severe cold rolling to achieve superior thermal stability. Compared with the conventional nanotwinned CrCoNi with nanotwins inside ultra-fined grains, the hierarchical nanolaminated-nanotwinned (CrCoNi)97.4Al0.8Ti1.8 exhibits a significant annealing-induced hardening effect, i.e., hardness increasing from ∼ 250 HV in the original specimen to ∼ 500 HV in the cold-rolled status and finally ∼ 630 HV after annealing at 600 °C for 1 h. Detailed microstructure characterizations reveal that the reduced dislocation density and formation of L12 ordered domain are mainly responsible for such hardening effect, which is facilitated by the effectively suppressed coarsening with annealing temperature, i.e., slow detwinning process and well-retained low-angle nanolamellar structure. The coarsening mechanisms from the cold-rolled nanolamellae to the fully recrystallized micro-equiaxed structures under the annealing temperatures ranging from 400 to 800 °C were also elucidated by atomic observations.