Neutron Diffraction of Co-Ni-Ga High-Temperature Shape-Memory Single Crystals

Superelastic Co-Ni-Ga ferromagnetic high-temperature shape memory alloys (HT-SMA) are characterized by a wide range of tunable transformation temperatures and lower processing costs compared to Ni-Ti-X HT-SMAs which renders them useful for high-temperature applications in automotive and aerospace industries. The material shows fully recoverable transformation strains up to 4.5% under compressive and up to 8% under tensile loads in superelastic single-cycle tests up to 400°C test temperature. However, cyclic testing at elevated temperatures leads to functional fatigue, i.e. intensive accumulation of permanent strain and decrease of critical stress for phase transformation, related to the formation of residual martensite mainly induced by an increased dislocation activity, as we found by neutron-diffraction and TEM analysis.

In neutron diffraction experiments we characterized Co₄₉Ni₂₁Ga₃₀ single crystals that had been subjected to isothermal mechanical cycling experiments up to 400°C. We can show that upon isothermal uniaxial cyclic testing a single martensite variant is likely to be stabilized. Significant peak broadening of both the martensitic and austenitic phases allows to determine the extent of dislocation activity along specific crystallographic directions at different fatigue levels. Further in-situ temperature- and loading-experiments will reveal the transformation behaviour and formation of martensite variants under stress as opposed to the stress-free state.