Pathways Towards Grain Boundary Engineering for Improved Structural Performance in Polycrystalline Co–Ni–Ga Shape Memory Alloys

In recent years, Co–Ni–Ga high-temperature shape memory alloys (HT-SMAs) attracted a lot of scientific attention due to their superior functional material properties. In the single-crystalline state, Co–Ni–Ga HT-SMAs feature a good pseudoelastic response up to 500 °C. However, in the polycrystalline condition Co–Ni–Ga suffers significant grain constraints and premature fracture at grain boundaries. In this regard, crystallographic orientations of the grains being involved as well as morphology and geometrical orientation of the grain boundaries with respect to the loading direction under pseudoelastic deformation are expected to be of crucial importance. Therefore, this study addresses the structural integrity of engineered grain boundaries, i.e., specifically selected grain boundaries in terms of orientation, grain boundary morphology, and crystallographic grain orientations of adjacent grains. Mechanical tests combined with in situ methods and post-mortem scanning electron microscopy investigations are used to shed light on the prevailing microstructural features resulting in any kind of structural degradation.