Stacking fault driven phase transformation in CrCoNi medium entropy alloy

He, H.*; Naeem, M.*; Zhang, F.*; Zhao, Y.*; Harjo, S.   ; Kawasaki, Takuro   ; Wang, B.*; Wu, X.*; Lan, S.*; Wu, Z.*; Yin, W.*; Wu, Y.*; Lu, Z.*; Kai, J.-J.*; Liu, C.-T.*; Wang, X.-L.*

In CrCoNi, a so-called medium-entropy alloy, an fcc-to-hcp phase transformation has long been anticipated. Here, we report an in situ loading study with neutron diffraction, which revealed a bulk fcc-to-hcp phase transformation in CrCoNi at 15 K under tensile loading. By correlating deformation characteristics of the fcc phase with the development of the hcp phase, it is shown that the nucleation of the hcp phase was triggered by intrinsic stacking faults. The confirmation of a bulk phase transformation adds to the myriads of deformation mechanisms available in CrCoNi, which together underpin the unusually large ductility at low temperatures.