Transforming microstructures and mechanical properties of (CoCrNi)93-xAl7Ndx medium entropy alloy films by annealing

Abstract

In this context, we aim to investigate the effects of Nd content and annealing on the microstructures and mechanical properties of a series of (CoCrNi)93-xAl7Ndx (x = 0, 0.22, 0.56, 1.05, 1.73, 2.55, 3.53) medium entropy alloy films. The results showed that doping with Nd resulted in grain refinement and triggered a transition from a single FCC solid solution to nanocrystalline and amorphous structures. The mechanical properties, evaluated using nanoindentation and micropillar compression tests, showed that 0.56 at.% Nd doping yielded the optimum outcome due to the formation of a solid solution and grain refinement. However, the mechanical properties dropped sharply with further increases in Nd content due to the inverse Hall-Petch effect and the transition from an FCC to an amorphous matrix. The annealing treatment (550 °C for 10 min) was found to enhance atomic diffusion and migration, facilitating lattice rearrangement and the formation of a large number of precipitates, including BCC Cr-rich, B2 NiAl, L12 Ni3Al, and HCP NdNi5 phases. In addition, we also found that annealing resulted in an increase in strength and a decrease in ductility, which was contrary to the behavior of conventional metal materials.