Stacking Fault Driven Phase Transformation in CrCoNi Medium Entropy Alloy.

Phase transformation is an effective means to increase the ductility of a material. However, even for a commonly observed face-centered-cubic to hexagonal-close-packed (-to-) phase transformation, the underlying mechanisms are far from being settled. In fact, different transformation pathways have been proposed, especially with regard to nucleation of the phase at the nanoscale. In CrCoNi, a so-called medium-entropy alloy, an -to- phase transformation has long been anticipated. Here, we report an loading study with neutron diffraction, which revealed a bulk -to- phase transformation in CrCoNi at 15 K under tensile loading. By correlating deformation characteristics of the phase with the development of the phase, it is shown that the nucleation of the 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.