Production and Characterization of FE-NI-CO-TI Shape Memory Springs with One- and Two-Way Effect

Abstract

AbstractIt has been shown that ausaged Fe-Ni-Co-Ti alloys exhibit a reversible martensitic transformation with 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. On the one hand they lead to a very effective precipitation hardening of the austenite and on the other hand to high elastic back stresses after shearing together with the martensite into a metastable structure. The martensitic morphology in such states is typically lathlike (thin plate martensite).While yield stress and tensile strength increase during ausaging, the toughness, especially the elongation at fracture changes for the worse. The results of this work show that this can be prevented by a combined thermo-mechanical treatment. At the same time a remarkable twoway shape memory effect can be observed.For practical applications the use of springs seems the best way for this type of Fe-based alloys because the recovery strain is low in comparison to Ni-Ti alloys. On the other side, due to the special transformation mechanism in Fe-Ni-Co-Ti alloys it can be assumed that the possible power in the two-way-effect should nearly reach the value of that obtained in binary Ni-Ti alloys.Mechanical and thermal properties of different springs made from Fe-Ni-Co-Ti shape memory alloys were studied by means of special deformation tests. Thereby, the maximum obtainable power in the reversible shape-memory effect of the investigated springs was of special interest. The martensite morphologies were investigated by optical-microscopy.