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

Nanocrystalline NiTi microwires exhibit stable superelastic effect and are highly potential as elastocaloric materials. Here, nanocrystalline Ni50.5Ti49.5 microwires of diameter 109 mu m 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 (Delta S = similar to 65 J/kg.K) within a wider operation temperature window (Delta T-win = similar to 90 K) compared to B2 <-> R transition (Delta S = similar to 18 J/kg.K with Delta Twin = 12 K). However, the hysteresis for B2 <-> R transition (Delta T-hys < 10 K) was much smaller than that for B2 <-> B19' transition (Delta T-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. (C) 2019 Elsevier B.V. All rights reserved.