Cyclic stability of the elastocaloric effect in heterophase [001]-oriented TiNi single crystals

The cyclic stability of the elastocaloric effect and the operating parameters (adiabatic cooling value ΔT_ad, coefficient of performance, operating temperature range) in Ni_50.8Ti_49.2 and Ni_51.5Ti_48.5 single crystals oriented along the ⟨001⟩-direction and containing dispersed Ti₃Ni₄ particles of different sizes were investigated, and the mechanisms of cyclic degradation of the elastocaloric effect were determined. Aged Ni_50.8Ti_49.2 single crystals containing nanosized Ti₃Ni₄ particles were shown to possess the optimal combination of operational properties for solid-state cooling. These crystals are characterized by high adiabatic temperature change ΔT_ad of 16.8–21.4 K over a wide temperature range above 160 K, the highest elastocaloric effect cyclic stability, and high coefficient of performance values up to 27.8. Ni_50.8Ti_49.2 single crystals with semi-coherent particles (400 nm in size) demonstrate maximum values of ΔT_ad = 25.3 K; but, they are not feasible for practical applications because of cyclic degradation caused by the formation of residual martensite and dislocations near large particles as well as low coefficient of performance up to 12.7. The use of Ni-rich Ni_51.5Ti_48.5 crystals can improve the operating characteristics of crystals with semi-coherent Ti₃Ni₄ particles and achieve high cyclic stability of the elastocaloric effect by strengthening the crystals through an increase in the volume fraction of particles and a decrease in the distance between them.