Atomic-Scale In Situ Observations of Reversible Phase Transformation Assisted Twinning in a CrCoNi Medium-Entropy Alloy

Twinning is an important deformation mode of face-centered-cubic (FCC) medium- and high-entropy alloys, especially under extreme loading conditions. However, the twinning mechanism in these alloys that have a low or even negative stacking fault energy remains debated. Here, we report atomic-scale in situ observations of the deformation process of a prototypical CrCoNi medium-entropy alloy under tension. We found that the parent FCC phase first transforms into a hexagonal close-packed (HCP) phase through Shockley partial dislocations slipping on the alternate {111} planes. Subsequently, the HCP phase rapidly changes to an FCC twin band. Such reversible phase transformation assisted twinning is greatly promoted by external tensile loads, as elucidated by geometric phase analysis. These results indicate the previously underestimated role of the metastable HCP phase in nanotwin nucleation and early plastic deformations of CrCoNi alloys and shed light on microstructure regulation of medium-entropy alloys with enhanced mechanical properties.