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 stressstrain 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.

Energy-Efficient Elastocaloric Cooling by Flexibly and Reversibly Transferring Interface in Magnetic Shape-Memory Alloys

“…In our previous study, the elastocaloric effect of Ni 50 Fe 19 Ga 27 Co 4 single crystal apparently decreases from 10 to 7.2 K after 3000 cycles . Also, Krooß et al reported that the superelastic behavior of Co–Ni–Ga single crystal undergoes a dramatic degradation during 1000 cyclic deformation . In comparison, the present Ni 50.0 Fe 19.0 Ga 27.1 Co 3.9 single crystal exhibits outstanding mechanical and functional stabilities.…”

“…It is well-known that the dislocation motion governs functional degradation and superelastic stability of SMAs. Compared with Co–Ni–Ga and Ni 50 Fe 19 Ga 27 Co 4 single crystals which represent nonmodulated L 1 0 martensite, in this work, the modulated martensitic structure with a high density of micro-twins normally possesses the low twin boundary energy, and also a relatively small lattice misfit between austenite and martensite, thus it is prone to reduce the critical transformation stress and prolong the fatigue life. Therefore, the Ni 50.0 Fe 19.0 Ga 27.1 Co 3.9 single crystal has superior mechanical and functional stabilities.…”