Phase Transformation Induced by High Pressure Torsion in the High-Entropy Alloy CrMnFeCoNi

The forward and reverse phase transformation from face-centered cubic (fcc) to hexagonal
close-packed (hcp) in the equiatomic high-entropy alloy (HEA) CrMnFeCoNi has been investigated
with diffraction of high-energy synchrotron radiation. The forward transformation has been induced
by high pressure torsion at room and liquid nitrogen temperature by applying different hydrostatic
pressures and large shear strains. The volume fraction of hcp phase has been determined by Rietveld
analysis after pressure release and heating-up to room temperature as a function of hydrostatic
pressure. It increases with pressure and decreasing temperature. Depending on temperature, a
certain pressure is necessary to induce the phase transformation. In addition, the onset pressure
depends on hydrostaticity; it is lowered by shear stresses. The reverse transformation evolves over
a long period of time at ambient conditions due to the destabilization of the hcp phase. The effect
of the phase transformation on the microstructure and texture development and corresponding
microhardness of the HEA at room temperature is demonstrated. The phase transformation leads
to an inhomogeneous microstructure, weakening of the shear texture, and a surprising hardness
anomaly. Reasons for the hardness anomaly are discussed in detail.