Diffusion phenomena in shape memory NiTi alloys subjected to severe plastic deformation

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Among commercial shape memory alloys the intermetallic Ni-Ti alloys are the most exploited ones because of their superior shape memory effect and superelasiticity, mechanical properties, corrosion resistance and biocompatibility. However, still little is known about the diffusivities of the components. Diffusion of Ti in bulk and along short-circuit diffusion paths (dislocations and grain boundaries) in this material is not investigated at all, although it probably determines e.g. the hightemperature creep rates. The diffusion phenomena are especially important in nanostructured NiTi alloys, which offer significant advantages concerning biophysical properties and technical performance. The current project aims first to study bulk and grain boundary diffusion of Ti and Ni in the stoichiometric and Ni-rich NiTi alloys in the B2 phase field. Reliable data on mobility of both components will be measured in an extended temperature interval. For the first time the direct data about the diffusion rate of the hypothetically slowest component, Ti, will directly be obtained. Nanostructured and ultrafine-grained material will be produced by severe plastic deformation. Shortcircuit diffusion will carefully be investigated as a function of grain size, elastic and superelastic deformation and thermal treatment. The results could be used for the prediction of e.g. creep rates and fatigue life performance under mechanical and thermal load and for the ductility improvement of ultrafme grained NiTi alloys.