Elastocaloric effects related to B2↔R and B2↔B19′ martensite transformations in nanocrystalline Ni50.5Ti49.5 microwires

Abstract

Nanocrystalline NiTi microwires exhibit stable superelastic effect and are highly potential as elastocaloric materials. Here, nanocrystalline Ni50.5Ti49.5 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 (ΔTwin = ∼90 K) compared to B2↔R transition (ΔS = ∼18 J/kg·K with ΔTwin = 12 K). However, the hysteresis for B2↔R transition (ΔThys <10 K) was much smaller than that for B2↔B19′ transition (ΔThys >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.