Significant annealing-induced hardening effect in nanolaminated-nanotwinned (CrCoNi)_97.4Al_0.8Ti_1.8 medium-entropy alloy by severe cold rolling

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)_97.4Al_0.8Ti_1.8 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)_97.4Al_0.8Ti_1.8 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.