Unraveling dual phase transformations in a CrCoNi medium-entropy alloy

The emergence of multi-principal element alloys (MPEAs) holds great promise for the development of high performance metallic materials. However, it remains unclear whether MPEAs can provide previously unknown deformation mechanisms to drastically enhance their mechanical performance. Here we report a new deformation mechanism of mechanically-induced dual phase transformations from the facecentered cubic (FCC) to hexagonal close- packed (HCP) phase and then back to the FCC phase with nanotwins in a CrCoNi medium-entropy alloy (MEA). During the two sequential steps of phase transformation, continued shear occurs in the same ⟨110⟩_FCC ⟨112¯0⟩_HCP direction along different {111}_FCC (0001)_HCP planes, producing a total shear transformation strain up to 70%. The dual phase transformations stem from a unique capability of facile slip in between the close-packed {111}_FCC || (0001)_HCP atomic layers in both FCC and HCP phases, leading to flexible stacking sequences of those close-packed layers with low stacking fault energies. Our work demonstrates that MPEAs can offer unconventional deformation mechanisms such as dual phase transformations in the CrCoNi MEA, thereby opening opportunities for enhancing the mechanical properties of advanced alloys.