An equiatomic CrCoNi medium-entropy alloy was subjected to high-pressure torsion (HPT) in order refine the microstructure to a grain size of about 50 nm, which is leading to a strong increase in the materials hardness. Subsequently the thermodynamic stability of the medium entropy alloy was evaluated by isothermal and isochronal heat treatments. Annealed samples were investigated by scanning and transmission electron microscopy as well as X-ray diffraction and were subjected to tensile tests to establish microstructure-property relationships.
A minority phase embedded in the face-centered cubic matrix of the CrCoNi alloy could be observed in multiple annealed states, as-well as the as-received and HPT-processed material. For 200h of annealing at 500°C the phase it was determined that the minority phase has a hexagonal-closed-packed crystal structure. The most likely explanation for the formation of the phase is a preferential segregation of Cobalt atoms to stacking faults.
Additionally, since the nanocrystalline CrCoNi alloy is prone to abnormal grain growth in a certain temperature range, the possibility of tailoring a bi-modal grain size distribution was explored in order to optimize mechanical properties. This way a three-fold increase in ductility with only a small decrease in strength could be achieved compared to the nanocrystalline material.