Cyclic Degradation of Co49Ni21Ga30 High-Temperature Shape Memory Alloy: On the Roles of Dislocation Activity and Chemical Order

Abstract

Conventional shape memory alloys (SMAs), such as binary Ni–Ti, are typically limited to service temperatures below 100 °C. Recent studies on Co–Ni–Ga high-temperature SMAs revealed the potential that these alloys can be used up to temperatures of about 400 °C. Analysis of the cyclic functional properties showed that degradation in these alloys is mainly triggered by intensive dislocation motion. However, data on the cyclic stress–strain response and the mechanisms leading to functional degradation of Co–Ni–Ga above 300 °C were missing in open literature. Current results reveal that above 300 °C diffusion-controlled mechanisms, e.g., precipitation of secondary phases and changes in the chemical degree of order, seem to dictate cyclic instability. Detailed neutron and transmission electron microscopy analyses following superelastic cycling in a temperature range of 200–400 °C were employed to characterize the changes in degradation behavior above 300 °C.