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, the majority of 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 as miniature devices, it is thus helpful to study films without grain boundaries as model systems.

Here, we 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. We combine results from local scanning electron microscopy and global x-ray diffraction to understand how different types of twin boundaries occur and finally form the complete martensitic microstructure. Using in-situ measurements, we additionally examine the nucleation and growth processes of the martensite. Our results are the starting point to understand the formation of a hierarchical martensitic microstructure of NiTi in three dimensions.