Effects of Cu on the microstructural and mechanical properties of sputter deposited Ni-Ti thin films

The microstructure of sputter deposited Ti-rich Ni-Ti thin films doped with Cu in the range 0-20.4 at.% and annealed for 1 h at 500 and 600 °C has been investigated and correlated with the mechanical properties of the films measured by depth-sensing nanoindentation. X-ray diffraction analysis showed the microstructural evolution of Ni-Ti thin films when doped with Cu and annealed at different temperatures. Heat treatments promoted the nucleation and growth of Ti2Ni precipitates in Ti-rich Ni-Ti thin films, which affected the stability of austenitic and martensitic phases at ambient temperature. Doping with Cu caused the formation of Ti(Ni, Cu)2 plate precipitates, which became more finely and densely dispersed in the grains with increasing Cu content. TEM analysis showed a columnar grain morphology extended through the whole films thickness, while with increasing Cu content a noticeable lateral grain refinement was induced by segregation of a (Ni, Cu)-rich phase to grain boundaries. The nano-hardness increased almost linearly with increasing Cu content owing to this grain refinement, though differences between samples annealed at different temperatures were found which could be related to the evolution of Ti(Ni, Cu)2 plate precipitates with annealing temperature and Cu content. The Young's modulus exhibited a similar dependence on Cu content as nano-hardness, though no significant differences were observed with increasing annealing temperatures.