The effect of combined recrystallization and precipitation processes on the functional and structural properties in NiTi alloys

To obtain an optimum shape memory component, functional and structural properties have to be connected during fabrication. To fulfil these requirements thermomechanical treatments are used to generate favourable microstructures by ''combined reactions''. Combined reactions can be composed of recrystallization, precipitation and phase transformation. All of these solid-state reactions are able to influence each other and subsequently the reversible phase transformations. To distinguish between the effects of exclusive precipitation, exclusive recrystallization and combined reactions, two different compositions of NiTi-alloys were chosen (with low tendency for precipitation 50.2 at% Ni and with higher tendency 50.8 at% Ni). The alloys have been modified systematically by aging, plastic deformation and combinations of both. For characterization light microscopy (LM) and transmission electron microscopy (TEM) are combined with hardness tests and DSC-measurements. The results of investigation are plotted in time-temperature-reaction diagrams (t-T-r).