Functional Fatigue and Tension–Compression Asymmetry in [001]-Oriented Co49Ni21Ga30 High-Temperature Shape Memory Alloy Single Crystals

Abstract

Conventional shape memory alloys cannot be employed for applications in the elevated temperature regime due to rapid functional degradation. Co–Ni–Ga has shown the potential to be used up to temperatures of about 400 °C due to a fully reversible superelastic stress–strain response. However, available results only highlight the superelastic response for single cycle tests. So far, no data addressing cyclic loading and functional fatigue are available. In order to close this gap, the current study reports on the cyclic degradation behavior and tension–compression asymmetry in [001]-oriented Co49Ni21Ga30 single crystals at elevated temperatures. The cyclic stress-strain response of the material under displacement controlled superelastic loading conditions was found to be dictated by the number of active martensite variants and different resulting stabilization effects. Co–Ni–Ga shows a large superelastic temperature window of about 400 °C under tension and compression, but a linear Clausius–Clapeyron relationship could only be observed up to a temperature of 200 °C. In the present experiments, the samples were subjected to 1000 cycles at different temperatures. Degradation mechanisms were characterized by neutron diffraction and transmission electron microscopy. The results in this study confirm the potential of these alloys for damping applications at elevated temperatures.