Thermal fatigue of Fe–Ni–Co–Ti shape-memory-alloys

Thermal fatigue was investigated for the shape-memory-alloy system Fe–Ni–Co–Ti. Due to the lack of experimental data special interest was laid on the microstructures and transformation properties before and after thermal fatigue. Specimens were aged in the austenitic state in order to get maximum austenite-hardness and, thereby, a thermoelastic transformation behaviour. After this thermal cycling was carried out in the temperature range −196–+300°C. Dilatometric measurements as well as detailed microscopic investigations gave information about the transformation behaviour, change of transformation temperatures and the corresponding microstructures as well as the hardness of the austenite. It can be shown that the fatigue behavior corresponds in a characteristic way with the degree of the Ms-temperature as well as the temperature hysteresis between complete forward and re-transformation ΔTH. Also, the microstructures which can be obtained after a different number of cycles show a distinct dependence on these parameters. The results give, for the first time, very useful information about the optimal conditions for the application of these new shape-memory-alloys and, thereby, for the maximum expected operating lifetime during fatigue.