In-situ Characterization Of Functional Properties In Polycrystalline Co-Ni-Ga High-Temperature Shape Memory Alloys

In the last decades shape memory alloys (SMAs) have received considerable attention due to their unique properties. However, conventional SMAs, such as Ni-Ti, suffer from high processing costs and their limited application range up to temperatures of about 100 °C. To overcome current limitations, high-temperature SMAs, i.e. Ni-Ti-X (X= Zr, Hf, Pd, Pt), Ti-Ta-Al and Co-Ni-Ga, have been introduced recently.

This study highlights the mechanisms responsible for degradation of functional and structural properties in Co-Ni-Ga HT-SMAs. Detailed microstructure analyses using in-situ techniques and 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 second phase precipitations, play a pivotal role regarding damage evolution. Particularly grain boundary triple junctions promote functional degradation, whereas bamboo-like microstructures result in excellent performance.