Grain Refinement of CoNiCrMo Non-equiatomic Medium Entropy Alloy

다국어 초록 (Multilingual Abstract)

Using a thermal–mechanical process, the average grain size of Co35Ni34Cr24Mo7medium entropy alloy was reduced to 2 μm,resulted in 50% improvement of hardness. In this way, the kinetic of the recrystallization and grain growth were investigatedby annealing at the temperature range of 800–950 °C. Based on Differential Scanning Calorimetry analysis and microstructuralinvestigations, recrystallization began around 800 °C at prior grain boundaries, micro shear bands, and intersecteddeformation marking. Comparing Johnson–Mehl–Avrami–Kolmogorov and Austin-Ricket approaches for recrystallizationmodeling the, revealed that the latter is more accurate considering its basic theory, which involves reducing recrystallizationrate by impingement. Both recrystallization and grain growth activation energy were calculated around 380 kJ/mol, which isclose to the activation energy of self-diffusion of similar high/medium entropy alloys. The recrystallization and grain growthtime exponent were found to be less than 0.5, which indicates the heterogeneous deformation and recrystallization in structureand pining effect of Mo on the grain boundaries migration. It was observed that the annealed microstructure containsannealing twins, which their number and thickness are related to the recrystallized grain size. Moreover, it was revealed thatdue to the presence of annealing twins, the modified Hall–Petch equation with effective grain size is more appropriate toexplain the relation of grain size and hardness.