Orientation dependence of elastocaloric effect in Ni-Mn-Ti single crystals

The single-crystalline elastocaloric alloy demonstrates unique advantages in the field of solid-state refrigeration due to its strong anisotropy and lack of grain boundaries. The directional solidification technique was utilized to fabricate three types of single crystal Ni₅₀Mn_31.6Ti_18.4 alloys with specific crystallographic orientations: [100]A, [103]A, and [111]A. During the stress release process, the elastocaloric effect of this alloy exhibits significant orientation dependence, as evidenced by their distinct adiabatic temperature changes for alloys with orientations of [100]A, [103]A, and [111]A are 18.8 K, 20.9 K, and 27.3 K, respectively. This orientation-dependent behavior can be attributed to the differences in the magnitude of the complete martensitic transformation strain among various orientations under the same strain loading. Specifically, orientations with lower complete martensitic transformation strains exhibit more complete transformations from austenite to martensite during the process, thus enabling a larger elastocaloric effect. The differences in complete martensitic transformation strain mainly originate from the crystallographic relationship between the stress direction and the crystallographic systems involved in the transformation from the parent phase to martensite. In order to predict the theoretical transformation strain levels as a function of crystallographic orientation under compression, we utilized a framework developed based on the “energy minimization theory” to calculate the complete martensitic phase transformation strain for different orientations, and the calculation results are in high agreement with the experimental outcomes. Investigating the phase transformation behavior and elastocaloric effect of single crystals with different orientations offers crucial insights for material selection and performance enhancement in solid-state refrigeration applications.