Martensitic Phase Transformation in CrCoNi Medium- and CrMnFeCoNi High-Entropy Alloy

The phase transformation from face-centered cubic (fcc) to hexagonal close-packed (hcp) in the equiatomic CrCoNi medium- and CrMnFeCoNi high-entropy alloy has been investigated with diffraction of high-energy synchrotron radiation. The transformation has been induced by high pressure torsion at room and liquid nitrogen temperature by applying different hydrostatic pressures and shear strains. The hcp phase fraction after pressure release and heating-up to room temperature as a function of these parameters has been determined by Rietveld analysis. It increases with pressure and decreasing temperature. Depending on temperature a certain pressure is necessary to induce the phase transformation. The onset pressure depends on hydrostaticity; it is lowered by shear stresses. It is also lower for the medium-entropy alloy due to a lower stacking fault energy. The reverse transformation develops over a long period of time due to destabilization of the hcp phase. Moreover, the effect of the phase transformation on the microhardness of the HEA at room temperature will be demonstrated and discussed.