WEB Understanding the martensitic microstructure in NiTi shape memory alloys with epitaxial thin filmsWednesday (23.09.2020) 15:55 - 16:10 F: Functional Materials, Surfaces, and Devices 2 Part of:
The shape memory alloy NiTi has been widely used for actuators in medical devices, consumer electronics and automotive applications. The superelastic and shape memory properties in NiTi are the result of a martensitic transformation from a cubic (B2) to a monoclinic (B19’) structure, which gives rise to a complex martensitic microstructure. Compared to bulk materials, the microstructure can differ for thin films, which are needed for miniaturized actuators. To customize the material for a specific purpose, a thorough understanding of this microstructure is helpful. In polycrystalline materials, however, the microstructure is influenced by grain boundaries, which complicates an analysis substantially.
Here, we use epitaxial NiTi films as a model system to examine the martensitic transformation and microstructure. These single-crystalline films are grown with DC magnetron sputter deposition on different substrates and in different thicknesses and are analyzed with a combination of microscopy and texture measurements. Due to the well-defined orientation relation between substrate and film, we can identify different features of the martensitic transformation such as twin boundaries and habit planes and their orientation. Additionally, we examine the nucleation and growth processes of the B19’ martensite with in-situ measurements. Our analysis reveals that twins on different length scales form a hierarchically twinned martensitic microstructure, which is necessary for a complete transformation of the material.