Temperature dependence of incomplete martensitic transformation and elastocaloric properties of superelastic NiTi: Experiment and phase-field simulation

Abstract

Partial phase transformation in NiTi-based refrigerants usually enables efficient and durable elastocaloric cooling, but its thermomechanical behavior with varying temperatures remains unclear. Keeping this in view, the elastocaloric effect of NiTi under incomplete transformation across 15–100°C is investigated and a superelastic deformation window between 25–85°C is identified. Synchronous infrared thermography and digital image correlation, and an innovative macro-micro phase-field model are employed to examine martensitic transformation and elastocaloric properties of NiTi within the superelastic window. Experimental and simulated results consistently reveal that the spatiotemporal thermal profiles correlate with Lüders strain band evolution. As superelastic deformation temperature increases, strain localization intensifies, with Lüders bands favoring an inward strain growth over an outward expansion, resulting in a smaller yet more deformed martensitic transformation zone. The aggravated strain inhomogeneity makes the local endothermic undercooling tested at 85°C up to about twice (−30.05°C) that at 25°C (−15.32°C), boosting the global cooling capacity by 65%, despite constant strain. The seeming contradiction between the larger elastocaloric effect and the narrower apparent martensitic transformation zone is elucidated by recourse to the simulations. It is found that the martensitic transformation within the Lüders bands is incomplete, proceeding in a macroscopically uniform but microscopically heterogeneous manner. Elevated temperatures within the superelastic window increase the transformed volume fraction and enhance martensitic transformation, thereby strengthening the global caloric effect. The work sheds light on the interplay between partial martensitic transformation and thermal behavior in NiTi under varying superelastic deformation temperatures, providing insights for advanced elastocaloric cooling applications.