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

Abstract

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.