Shape memory alloys (SMAs) received considerable attention over the last decades. Due to their unique functional properties based on a fully reversible phase transformation from a high-temperature austenitic phase to a low-temperature martensitic phase, SMAs are very attractive for damping or actuation applications in various fields. However, conventional Ni-Ti SMAs suffer from high processing costs and their limited application range up to temperatures of about 100 °C. In order to widen the application range in terms of operating temperatures, high-temperature (HT-) SMAs featuring increased martensite start temperatures (Ms) have been introduced recently. Ternary Ni-Ti-X (X = Pt, Pd, Hf, Zr), Ti-Ta based alloys and Co-Ni-Ga were identified as attractive candidates for HT-SMA applications.
This study characterizes the mechanisms responsible for degradation of functional and structural properties in Co-Ni-Ga HT-SMAs. Detailed microstructure analyses using in-situ techniques and scanning electron microscopy were conducted allowing for correlation between the phase transformation and microstructural features. Due to highly anisotropic material behavior Co-Ni-Ga HT-SMA polycrystals suffer from intergranular constraints and defect generation. Grain boundary (GB) characteristics, i.e. GB misorientation, orientation of GB with respect to the loading axis and precipitation of secondary phases, are critical parameters to be evaluated for analysis of damage evolution.