Effect of Heat Treatment on the Mechanical Behavior and Fracture of TiNi Alloy

Abstract

The mechanical properties and fracture of a TiNi alloy (Ti-55.8 wt % Ni) have been investigated under vacuum heat treatment at 700–1200°C. Three-point bending and low cycle fatigue tests were conducted on heat treated wire samples under extreme loading conditions (large strains and alternating bending loads) to determine the effect of the annealing temperature on the superelastic behavior of the alloy. It was found that an increase in the heat treatment temperature leads to grain coarsening in the alloy, but the coarsening effect on its superelastic behavior is insignificant at low bending strains (4.0–4.5%). With heat treatment temperature variation from 700 to 1200°C, the shape of the alloy stress-strain curves remains almost unchanged for all bended samples, but with increasing heat treatment temperature the martensitic shear stress and residual strain slightly increase. In low cycle bending tests, the alloy ductility reduces significantly after heat treatment above 1100°C. Fractographic analysis of the tested alloy samples revealed different fracture surface structures depending on the heat treatment conditions, but the same fracture mechanism. In all cases, fracture occurs by quasi-cleavage, and the microcrack nucleus is associated with Ti₂Ni/Ti₄Ni₂O inclusion particles or surface defects. The general results indicate the possibility of diffusion welding of TiNi alloys at a temperature of 1000–1100°C, without pronounced changes in their mechanical properties and ductility.