γ/γ′ interface-driven formation of imperfect twin boundaries and 9R phase in a medium-entropy superalloy

Medium-entropy superalloys (MESAs) show promise for extreme-temperature applications, yet their governing defect mechanisms remain unclear. Here, we reveal the hot-deformation behavior of a MESA at 1120 °C using atomic-scale characterization and molecular dynamics simulations. We observed that coherent twin boundaries (CTBs) become imperfect, forming incoherent twin boundaries (ITBs) as they propagate along γ/γ′ interfaces. These interface-driven ITBs subsequently serve as nucleation sites for a 9R phase transformation. This newly identified pathway, from interface to ITB to 9R phase, sheds new light on designing advanced superalloys with superior mechanical properties for extreme service.