Influence of Chemical Composition and Melting Process on Hot Rolling of NiTiHf Shape Memory Alloy

In NiTi shape memory alloys an important aim is to elevate the martensitic transformation temperature in order to satisfy the high temperature applications and addition of Pt, Au, Zr or Hf to NiTi can increase the transformation temperatures above 100°C. Among these high temperature shape memory alloys, NiTiHf alloys seem to be more practical for engineering applications, because of their high transformation temperatures, good thermal stability and lower price. But their recovery strain in comparison with binary NiTi alloys is less. One of methods for improvement of recovery strain is thermomechanical treatment. In this research, the as-cast ingot of Ni₅₀Ti₄₀Hf₁₀ shape memory alloy was prepared by vacuum arc melting and then specimens hot rolled for a 10-30% thickness reduction. Result obtained by bending test showed that with increasing applied strain, the amount of recovery strain increased. In all hot rolled samples until 3% strain, recovery is complete but in above 3% strain due to occurrence of slip deformation, full recovery not happened. In hot rolled specimens, due to increasing of strength and also critical stress for slip, recovery strain increased.