MICROSTRUCTURE AND SHAPE-MEMORY-PROPERTIES OF THERMO-MECHANICAL TREATED Fe-Ni-Co-Ti-ALLOYS

Abstract
It has been shown that ausaged Fe-Ni-Co-Ti alloys exhibit shape-memory effects with a reversible martensitic transformation and a small temperature hysteresis between the forward and the reverse transformation. This is due to the precipitation of small coherent (Ni,Co,Fe)₃Ti particles which lead on the one hand to a very effective hardening of the austenite and on the other hand to high elastic back stresses after shearing together with the martensite into a metastable structure. A combined thermomechanical treatment with homogeneously distributed dislocations can favour the homogeneous distribution of the particles. In this case, the obtainable shape memory effect can reach a maximum. Mechanical and thermal properties were studied by means of hardness and dilatometric measurements. The microstructures before and after the martensitic transformation were investigated by optical microscopy. In order to proof the reversibility of the shape memory effect, springs of different dimensions were produced and their behaviour during the training and under different loads were investigated.