Microstructural evolution after thermomechanical processing in an equiatomic, single-phase CoCrFeMnNi high-entropy alloy with special focus on twin boundaries

Abstract

The FCC-structured equiatomic CoCrFeMnNi high-entropy alloy was produced by arc melting and drop casting. After homogenization, the drop-cast ingots were cold rolled to sheets with six different final thicknesses (thickness reductions of 21, 41, 61, 84, 92 and 96%). Samples were cut from the rolled sheets and annealed for 1 h at temperatures between 400 and 1000 °C. The recrystallization temperature was then determined as a function of cold work by means of scanning electron microscopy and electron backscatter diffraction measurements. Additionally, Vickers indentation was performed on these samples. It was found that the microhardness first tends to increase slightly upon annealing below the recrystallization temperature but then drops steeply for higher annealing temperatures due to the onset of recrystallization. To study grain growth kinetics, samples that underwent 96% cold rolling were first recrystallized for 1 h at 800 °C, which is the lowest temperature at which complete recrystallization occurs, and then annealed at temperatures between 800 and 1150 °C for various times. The grain growth exponent was determined to be approximately n = 3, and the activation energy Q = 325 kJ/mol, both of which agree well with published values for this alloy. EBSD measurements were made in the as-recrystallized and grain growth samples to analyze the annealing twins. The density of annealing twins in the grain growth samples was found to depend only on grain size, i.e., it was independent of annealing temperature and time. No such correlation could be found for the as-recrystallized samples. These observations are discussed in the framework of existing theories for the formation of annealing twins.