Stacking Fault Driven Phase Transformation in CrCoNi Medium Entropy Alloy

Abstract

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 (<i>fcc</i>-to-<i>hcp</i>) 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 <i>hcp</i> phase at the nanoscale. In CrCoNi, a so-called medium-entropy alloy, an <i>fcc</i>-to-<i>hcp</i> phase transformation has long been anticipated. Here, we report an <i>in situ</i> loading study with neutron diffraction, which revealed a bulk <i>fcc</i>-to-<i>hcp</i> phase transformation in CrCoNi at 15 K under tensile loading. By correlating deformation characteristics of the <i>fcc</i> phase with the development of the <i>hcp</i> phase, it is shown that the nucleation of the <i>hcp</i> 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.