Superelastic stability of stress induced R→B19′ transformation in nanocrystalline NiTi memory alloy
A nanocrystalline NiTi memory alloy wire sample with an average grain size of 15 nm was obtained by large deformation cold drawing and low temperature annealing of Ti49.8 Ni50.2(at%)memory alloy wire.Then,20 tensile cycles experiments with a strain of 6%were used to study the superelastic stability of the nanocrystalline NiTi alloy during stress-induced R→B19′ transformation.The results show that after 20 cycles of tensile deformation,the superelastic stress of the nanocrystalline NiTi alloy is 734 MPa,the reduction rate of superelastic stress is 16%,and the residual strain is 0.61%.This comprehensive superelastic performance is superior to previous coarse-grained NiTi memory alloys.This is mainly due to the significant increase in yield strength of the nanocrystalline NiTi memory alloy due to fine grain strengthening,thus reducing plastic deformation during martensitic transformation of the alloy and enabling the nanocrystalline NiTi memory alloy to exhibit high superelastic stability.
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