Microstructural evolution and superelastic properties of ultrafine-grained NiTi-based shape memory alloy via sintering of amorphous ribbon precursor

NiTi-based shape memory alloys (SMAs) are considered as cutting-edge intelligent functional materials. However, it remains a great challenge to obtain ultrafine-grained (UFGed) bulk materials with mm-scale size as well as outstanding superelastic properties. Here, UFGed bulk Ti35Zr15Ni35Cu15 NiTi-based SMA is successfully prepared via spark plasma sintering of amorphous ribbon precursor at different sintering temperatures, and microstructural evolution and superelastic properties are symmetrically investigated. It is found that its grain size ranges from UFG to micro-grain with increased sintering temperature regardless of the predominant B2 matrix in all bulk samples. Interestingly, the orientation relationships between B2 matrix and nano-scale fcc (Ti,Zr)2Ni precipitate evolve from coherent to incoherent. Consequently, the UFGed samples exhibit perfect superelasticity with the high recoverable strain of ∼5.8%, the stable recovery rate above 99%, and the great critical stress inducing martensitic transformation higher than 1 GPa, far superior to the corresponding ones of suction-cast micro-grained TiZrNiCu SMAs. Fundamentally, the perfect superelasticity is attributed to the good resistance to dislocation slip or grain boundary slip by residual nano-scale amorphous phase or secondary phase of coherent and semi-coherent fcc (Ti,Zr)2Ni precipitate. In addition, the gentle superelastic plateau is associated to the favorable transfer stress and the strong ability to accommodate dislocation movement, which is generated by the coherent interface between nano-scale fcc (Ti,Zr)2Ni and UFGed B2 matrix. These results suggest that spark plasma sintering of amorphous alloy precursor is a feasible route to obtaining excellent superelasticity in NiTi-based SMAs.