Epitaxial NiTi Thin Films: A 3D Puzzle

NiTi films are widely used in micro applications due to their shape memory and superelasticity properties. When customizing the material for a specific miniature device, it is vital to understand the underlying martensitic microstructure, how it forms, and how it affects the shape memory effect. Up to now, most research on the martensitic microstructure in NiTi concentrates on NiTi bulk, but results derived from bulk materials are not always applicable for films as well. Even though polycrystalline NiTi films are widely available, these films contain grain boundaries, which hamper or even inhibit a scale bridging analysis of the martensitic microstructure. Therefore, the martensitic microstructure in NiTi films and its formation remains mostly unexplored. To improve NiTi for applications in miniature devices, it is thus helpful to study films without grain boundaries as model systems. In this study, the authors analyze single crystalline NiTi films grown by DC magnetron sputter deposition. These epitaxial films grow without large angle grain boundaries and make it possible to analyze the martensitic microstructure over several length scales. The work analyzed the martensitic microstructure and its nucleation with microscopy and X-ray methods and compared these measurements with orientation relationships calculated with the phenomenological theory of martensite. The results are the starting point to understand the formation of a hierarchical martensitic microstructure of NiTi in three dimensions.