Hysteretic transformation behaviour of shape memory alloys

Abstract

After suitable thermomechanical treatment a series of Cu-based and Ni-Ti-alloys can show a reversible shape change during heating and/or cooling over a temperature region of about 40 degrees. Two important conditions for this effect are that a thermoelastic martensitic transformation occurs in this temperature range and that the total strain of its shape change does not exceed more than a few percent. At temperatures above the martensitic transformation temperatures, the material behaves pseudoelastic : by applying a strain, the martensitic transformation will start at a critical stress, dependent on the temperature. After complete transformation a considerable strain can be obtained which disappears during unloading. The stress strain curve, obtained at constant temperature, appears as a closed loop. Both reversible transformations, thermally and strain-induced, and so the related shape change, occur in a hysteretic manner. The origin of this hysteresis can be explained by thermodynamic considerations and the occurrence of transformation induced defects. The apparent plastic deformation is also related to preferential growth of selected martensitic variants. Mechanistically, the knowledge of the pseudoelastic loops is important to calculate the shape change during thermal cycling. It has also been shown that pseudoelastic loops for polycrystalline materials can be calculated using a rigourous mechanical model.