Investigation of nano-scale precipitates in the CoCrFeMnNi high-entropy alloy processed by high-pressure torsion

Abstract

The influence of high-pressure torsion (HPT) at room and elevated temperatures on the structure of the high-entropy alloy CoCrFeMnNi has been investigated. Dynamic processes of phase transformation, including solid solution decomposition and dispersion particle precipitation, have been identified. The microstructure of CoCrFeMnNi was studied using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction analysis (XRD). The kinetics of phase transformations was investigated by differential scanning calorimetry (DSC). As a result of deformation twinning processes in the HPT process at room temperature, there is a significant grain refinement of the structure of the studied HEA to nanocrystalline size of 50 nm. In turn, during the HPT process at 300 °C, the dynamic recovery process with the decomposition of the solid solution begins. As a result, it was found that the increasing temperature of HPT leads to the formation of UFG structure with a grain size of ~400 nm. At the same time, nano-scale precipitates consisting of the following phases are formed in individual grains: Cr-rich BCC phase and tetragonal σ-phase with variable phase composition. The size of the precipitates reaches up to 10 nm with a volume fraction of less than 3%.