Elastocaloric effects related to B2↔R and B2↔B19′ martensite transformations in nanocrystalline Ni_50.5Ti_49.5 microwires

Nanocrystalline NiTi microwires exhibit stable superelastic effect and are highly potential as elastocaloric materials. Here, nanocrystalline Ni_50.5Ti_49.5 microwires of diameter 109 μm were prepared by multi-step cold-drawn and the elastocaloric effect (eCE) by taking the advantage of B2↔R and B2↔B19′ transformations have been investigated. Both transitions in the nanocrystalline microwires displayed comparable or even superior caloric effect compared to other elastocaloric and magnetocaloric materials. The B2↔B19′ transformation exhibited larger stress-induced entropy change (ΔS = ∼65 J/kg·K) within a wider operation temperature window (ΔT_win = ∼90 K) compared to B2↔R transition (ΔS = ∼18 J/kg·K with ΔT_win = 12 K). However, the hysteresis for B2↔R transition (ΔT_hys <10 K) was much smaller than that for B2↔B19′ transition (ΔT_hys >50 K) thanks to the smaller transition strain in the latter case. As a result, eCE produced by B2↔R tranistion possessed a more stable cyclic stability. Therefore, both B2↔R and B2↔B19′ transitions in the present microwires are promising candidates in the micro-sized refrigeration field.