Microstructure study of cold rolled Al0.32CoCrFeMnNi high-entropy alloy: Interactions between recrystallization and precipitation

The equimolar CoCrFeMnNi alloy is widely studied for its excellent mechanical properties at cryogenic temperatures. Interestingly, it presents a unique combination of strength and ductility mainly due to the occurrence of twinning at these low temperatures. Motivated by the desire to improve its room temperature strength, this study focuses on the optimization of the microstructure through alloying and thermomechanical treatments. More specifically, the effect of alloying with aluminium is studied with the aim of triggering precipitation strengthening. The composition Al0.32CoCrFeMnNi is chosen on the basis of thermodynamic data and submitted to various processing routes including cold rolling and annealing steps. The annealing temperature is shown to be the key parameter determining the governing microstructure transformations. At annealing temperatures below 900 degrees C, precipitation is the main phenomenon taking place. Above 900 degrees C, recrystallization is governing the evolution of the microstructure. At 900 degrees C, both mechanisms occur simultaneously. Recrystallization occurs first on grain boundaries and shear bands while precipitation is activated both in recrystallized and unrecrystallized regions with distinctive morphologies. It is shown to impede further recrystallization. Thermodynamic calculations are performed to explain the precipitation sequence observed in both regions. By increasing the rolling level from 60% to 80%, recrystallization kinetics is accelerated and complete recrystallization reached. A wide range of microstructure is achieved through the variety of thermomechanical treatments explored in this study. These various microstructures in turn translate into a wide range of hardness levels and tensile properties.