Elastocaloric Effect in Heterophase TiNi Single Crystals

The paper presents studies of the elastocaloric effect during the stress-induced B2–(R)–B19′ martensitic transformation depending on the microstructure and test temperature in Ni_50.6Ti_49.4 and Ni_50.8Ti_49.2 (at.%) single crystals oriented along the [001]_B2 direction. The aging of TiNi single crystals at 573 and 823 K for 1–1.5 h improves the characteristics of superelasticity and elastocaloric effect. Precipitating large Ti₃Ni₄ particles with the size of ~ 400 nm aged at 823 K leads to an increase in the temperature range of elastocaloric effect and in the maximum adiabatic cooling ΔT_ad up to 24.2–25.3 K compared with quenched single crystals (ΔT_ad = 14.3 K). TiNi single crystals containing nanosized Ti₃Ni₄ particles smaller than 10 nm (aging at 573 K) have a distinguishing feature: two-stage reverse B19′–R–B2 martensitic transformation leads to staging on the elastocaloric effect temperature dependence. The maximum ΔT_ad in these single crystals is lower compared with single crystals aged at 823 K. It is equal to 16.8 K and 21.3 K in Ni_50.6Ti_49.4 and Ni_50.8Ti_49.2 alloys, respectively. However they demonstrate record coefficient of performance up to 27.8 in the Ni_50.6Ti_49.4, which characterizes them as promising for further use in solid-state cooling devices.