Mechanical Properties of Fe-Ni-Co-Ti-Shape Memory Alloys

Abstract

It has been shown that Fe-Ni-Co—Ti alloys exhibit the shape memory effect (1-3). The ef­ fect can only be observed after an ausageing procedure, which leads to a two-phase microstructure consisting of austenite and coherent and ordered y'-(Ni,Co)3Ti--precipitates. Since the size of the particles is below a critical value d c, the precipitation of y' leads to a consider­ able decrease of the temperature hysteresis. The small particles shear into a metastable, tetragonal distorted structure a ' during the transformation (4,5). a' is less stable than y', so that more energy is required for the martensitic transformation. According to that the reverse transformation is supported by regaining the invested energy. The small hysteresis (AT = 40°C) can not be found in the homogeneous state.