Morphology evolution of R phase in a NiTiFeNb alloy: insights from in situ experiments and phase field modeling

Martensite, thermoelastic or non-thermoelastic, typically prefers a plate-like morphology along habit planes to minimize elastic strain energy. A particle-like morphology is thereby rarely seen and its underlying mechanism remains unclear. This study investigates the morphology evolution of martensitic R phase in a well-crystallized NiTiFeNb alloy using in situ experimental and phase field modeling methods. It is found that the particle-like morphology of R phase is probably related to the minimal elastic constants upon transformation. Furthermore, numerous anti-phase boundaries (APBs) are observed within the R phase plates. These APBs arise due to the opposite shuffling of atoms within a single R variant. These findings advance the current understanding in the martensitic transformation physics of R-containing NiTi-based alloys.