Microstructure evolution and superelasticity behavior of Ti-Ni-Hf shape memory alloy composite with multi-scale and heterogeneous reinforcements

In the present study, the in-situ TiB whisker was introduced into the Ti-Ni-Hf shape memory alloy composite by the in-situ reaction of the Ti-Ni-Hf alloy powder and TiB₂ powders. The (Ti,Hf)₂Ni phase also precipitated, accompanied with the formation of TiB phase. Moreover, the residual TiB₂ particles can be observed, as the TiB₂ content was higher than 0.7 wt%. Thereinto, the larger scale reinforcements constituted the quasi-continuous network structure. The smaller scale reinforcements distributed in the interior of the network structure. The two-scale reinforcements showed the uniform distribution at macroscopic level and inhomogeneous distribution at microscopic level. The single stage B19′↔B2 martensitic transformation occurred in the Ti-Ni-Hf composites. In addition, the martensitic transformation temperatures continuously decreased with the increased TiB₂ content owing to the compositional and mechanical effect. The moderate TiB₂ addition not noly enhanced the matrix strength, but also significantly improved the superelasticity. The excellent superelaticity with the completely recoverable strain of 4 % can be obtained in the Ti-Ni-Hf composite containing 0.7 wt% TiB₂.