Ultra-small-angle neutron scattering study on temperature-dependent precipitate evolution in CoCrFeNiMo_0.3 high entropy alloy

Phase evolution of CoCrFeNiMo_0.3 high entropy alloy (HEA) under different annealing temperatures was investigated by means of ultra-small-angle neutron scattering (USANS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). For the as-cast CoCrFeNiMo_0.3 alloy, a dendritic structure consisting of mainly face-centered cubic (FCC) solid solution matrix and small amount of micro-sized σ precipitates was observed. Upon isothermal annealing, secondary phase precipitation occurred and precipitates with two size ranges (i.e., microscale and nanoscale) generated in the alloy. These precipitates were identified as newly formed μ and σ phases. Specifically, the size distribution of the nanoscale μ precipitates was determined by USANS measurements. It was found that the size of the nanoscale precipitates increased with the annealing temperature, jumping from 72 nm at 750 °C to 258 nm at 800 °C, which contributed to the sharp drop in hardness of the alloy. The present alloy maintained high hardness within a wide temperature range of 550–750 °C, which makes it a promising material for structural applications at intermediate temperatures.