Deformation behaviour and two-way shape memory effect of NiTi alloys

The influence of different heat treatments on a NiTi shape memory alloy with a nominal composition of 50.3 at.% Ti was studied with respect to mechanical and shape memory properties. The deformation behaviour both of martensitic condition B19' at room temperature and high temperature phase B2 at 170 °C was investigated by tensile tests. The range of near-constant stress during deformation, the socalled martensite plateau, is more pronounced in the annealed samples. The yield strength of the high temperature phase strongly depends on the heat treatment condition. Annealing at 550 °C reduces it to about 50 %. A two-way shape memory effect was induced by repeatedly heating and cooling under constrained conditions. Measurements of the transformation temperatures demonstrate that in specific cases the austenization during heating may occur in two steps.