Fabrication of a Functionally Graded TiNi Shape Memory Alloy Wire by Powder Metallurgy and Plastic Working (1st Report, Fabrication of a TiNi Shape Memory Alloy Billet and Its Phase Transformation Properties)

Abstract

The martensitic and the reverse transformation temperatures are directly related to the bending rigidity of the TiNi shape memory alloy (SMA) wire. In this paper, aiming at obtaining a functionally graded TiNi shape memory alloy wire that varies in bending rigidity from high to low along the wire axis, a new fabrication process of combined powder metallurgy and plastic working is proposed. First, a multi-layered TiNi green compact, where Ti-Ni compositions varied layer by layer, was sintered by means of pulse current pressure sintering technique, and then it was hot-extruded and cold drawn into a wire. In this paper, as a first step of fabrication of a wire, we investigated transformation properties and texture morphologies of sintered and solution treated compacts. From differential scanning calorimetry and X-ray diffraction measurements, it was found that in this process the solution treatment of the billet plays an important role to induce the transformation temperature gradient in the sintered compact, since it greatly activates the interdiffusion between Ti and Ni particles.