Microstructure and mechanical properties of heat treated Al1.25CoCrCuFeNi high entropy alloys

The influence of cooling rates and heat treatments on the microstructure and mechanical properties of Al1.25CoCrCuFeNi high entropy alloys (HEA) were investigated using a variety of experimental techniques including DTA, XRD, SEM, TEM, microhardness, and compression tests. It was found that Al1.25CoCrCuFeNi solidifies dendritically with the majority of the dendrite core (DC) regions composed of modulated spinodal microstructure consisting of disordered Cr+Fe rich BCC nanosize lamellae, and Al+Ni-rich ordered B2 nanosize lamellae, along with elongated Cu-rich FCC precipitates at the interfaces. The ordered B2 phase solidified first and underwent spinodal decomposition to create the modulated structure. The interdendritic regions (ID) consisted of a bulk FCC Cu-rich phase with nanosize precipitates. Heat treatments at 650 and 850 °C caused the disordered BCC lamellae to transform to σ phase accompanied with soft FCC phases. Reheating to 1040 °C caused the σ phase to transform back to the disordered BCC as well as nucleation of another FCC phase. Depending on the heat treatment temperature, the DC regions transformed from the fine modulated microstructure to grains at 1040 °C and back to a coarse modulated microstructure after heat treatment at 1200 °C. The Cu-rich phases in the ID regions remained unchanged during heating up to 1040 °C however, there were several changes in the morphology of the nanosized precipitates in the Cu-rich phase. The interdendritic phase was found to dissolve at around 1200 °C. The influence of these changes on the mechanical properties are discussed.