Grain Refinement of CoNiCrMo Non-equiatomic Medium Entropy Alloy

Using a thermal-mechanical process, the average grain size of Co35Ni34Cr24Mo7 medium entropy alloy was reduced to 2 mu m, resulted in 50% improvement of hardness. In this way, the kinetic of the recrystallization and grain growth were investigated by annealing at the temperature range of 800-950 degrees C. Based on Differential Scanning Calorimetry analysis and microstructural investigations, recrystallization began around 800 degrees C at prior grain boundaries, micro shear bands, and intersected deformation marking. Comparing Johnson-Mehl-Avrami-Kolmogorov and Austin-Ricket approaches for recrystallization modeling the, revealed that the latter is more accurate considering its basic theory, which involves reducing recrystallization rate by impingement. Both recrystallization and grain growth activation energy were calculated around 380 kJ/mol, which is close to the activation energy of self-diffusion of similar high/medium entropy alloys. The recrystallization and grain growth time exponent were found to be less than 0.5, which indicates the heterogeneous deformation and recrystallization in structure and pining effect of Mo on the grain boundaries migration. It was observed that the annealed microstructure contains annealing twins, which their number and thickness are related to the recrystallized grain size. Moreover, it was revealed that due to the presence of annealing twins, the modified Hall-Petch equation with effective grain size is more appropriate to explain the relation of grain size and hardness.

Automated summary

Through a thermal-mechanical process, the average grain size of Co35Ni34Cr24Mo7 medium entropy alloy was reduced to 2 μm, resulting in a 50% improvement in hardness. The kinetics of recrystallization and grain growth were investigated through annealing experiments. It was found that the Austin-Ricket approach for recrystallization modeling was more accurate, and the presence of annealing twins affected the relation between grain size and hardness.