In the current project almost all goals were achieved. However, several unexpected results were obtained, too. Among such results were the first precise determination of the diffusional grain boundary width in severely deformed materials and the unambiguous determination of the excess free volume accommodation at grain boundaries during severe deformation. These achievements allowed the first analysis of the underlying diffusion mechanisms of atomic transport in deformation-modified grain boundaries that in fact was provided by a combination of diffusion measurements with substitutionally and interstitionally diffusing solutes. The effect of precipitation on diffusion phenomena in NiTi was investigated, too, and for the first time fundamentally different contributions of coherent and non-coherent particles on grain boundary diffusion kinetics has been established. The influence of severe deformation on microstructure, kinetic and thermodynamic properties of NiTi was examined in detail and the effect of solid-state amorphization during annealing was established and precisely analyzed. The entire amount of results on grain refinement under severe deformation contributed to the development of a new model of the smallest grain size which can be produced by such treatment.