J. Frenzel, C. Haberland, Ruhr University Bochum, Bochum, Germany; K. Neuking, G. Eggeler, Ruhr-University Bochum, Bochum, Germany
The present work studies processing related aspects of thermomechanical treatments of Ni-rich NiTi shape memory alloys (SMAs). We show how the microstructure and the resulting phase transformation temperatures of the alloy respond to cold work and recrystallization. We provide a first experimental evidence how the presence of carbon (a typical melting related impurity in NiTi which results in the formation of TiC) affects the workability and the recrystallization of NiTi SMAs. It was found that increased carbon concentrations result in higher volume fractions of carbides and thus in a more brittle material behaviour. Carbon-rich NiTi alloys tend to form small cracks during cold rolling in case of high degrees of deformation. After cold work, the microstructure consists of austenite and stress induced martensite which is stabilized by dislocations. Furthermore, the phase transformations on cooling and heating are strongly impeded by dislocations. Subsequent recrystallization results in a fine equiaxial grain structure. As an interesting result, it was found that higher volume fractions of carbides in carbon-richer SMAs are associated with the formation of a finer grain structure as compared to the case of lower carbon concentrations. Carbides may well promote the nucleation of new grains during recrystallization und additionally impede the mobility of grain boundaries during grain growth.
Summary: The work investigates how carbides (a typical impurity related secondary phase in NiTi) affect the workability and the microstructural evolution during thermomechanical processing of Ni-rich NiTi shape memory alloys.
