Electron microscopy has been used throughout the years to visualize structural changes in materials undergoing phase transformations. However, there are always details about what happens right before a transformation and the repeatability of a transformation that are difficult to track. In this paper we demonstrated that electron microscopy coupled with high-speed direct electron detectors can be used to characterize the forward and reverse martensitic transformations exhibited in NiTi, with nanoscale precision, and at large fields of view. Our method enables direct observation and characterization of 3 unique B19′ martensite variants that are differentiated by the planes on which they appear. Moreover, we track their formation while cooling past the martensitic transformation temperature. The B19′ variant phases and associated strains are mapped at different temperature steps and are directly compared via intermittent 4D-STEM scans to study the transformation. The B2 austenite pre-transitional microstructure is compared to the martensite phase transformation after multiple temperature cycles in order to improve our understanding of cyclic evolution of the martensite lath structure. Our results demonstrated how 4D-STEM can improve our understanding of complex transformation mechanisms that are of particular importance for engineering materials such as shape memory alloys.
