Martensitic phase transformations and twinning in epitaxially grown Nickel Titanium films

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In this research project we study epitaxially grown NiTi shape memory alloy films. These thin films (with thicknesses ranging from about 100 nm to several microns) are used as a model system for highly precise, fundamental investigations on martensitic transformations, for instance to analyze on several length scales the self-similar martensitic microstructures in NiTi. Most importantly, we will consider the key question whether the morphology of martensitic microstructures in NiTi is determined by thermodynamic equilibrium processes or by the most easily accessible transformation paths. The planned experimental work considers epitaxial growth of NiTi films on different MgO and Al2O3 substrates. Moreover, we use X-ray diffraction, atomic force microscopy and electron microscopy methods to analyze the twinned microstructures and the corresponding textures of NiTi martensites. In in-situ investigations, the martensitic phase transition is documented by atomic force microscopy measurements during cooling, with a particular focus on nucleation mechanisms. The stress-induced transformation is triggered using nanoindentation and small-scale tensile testing of free-standing thin films; the activated twinning systems are analyzed by means of micromechanical and continuum mechanical calculations considering elastic anisotropy of both high and low temperature phases. The experimental and theoretical results of our investigations on the well-defined model system will allow to describe formation of martensitic microstructures in NiTi across several relevant length-scales. The effect of different twin boundary energies on the formation of hierarchically structured martensites will be considered, and a comparison with the multi-step nucleation model, where the kinetics of the phase transition is determined by the pathway with the lowest energy barrier, will be performed.