Microstructure and martensitic transformation in sintered NiTiCu alloys

The main aim of this work was to characterise microstructure and phase transformation, which occurs in Ni (50-X) Ti50CuX shape memory alloys (where X = 2, 3, 5, 10, 15, 20 and 25%) produced by powder technology. The blends were sintered applying various conditions, considering temperature and time: temperature varied from 850 degrees C to 1100 degrees C whereas time from 5 to 50 hr. In order to receive homogeneous alloys, which reveal reversible martensitic transformation, the sintering condition were optimised. It was found, that for alloys containing lower copper addition (less than 10%), the best sintering temperature was 940 degrees C and time 7 hr. For higher Cu content (10-25%) lowering of sintering temperature to 850 degrees C and extension of sintering time up to 20 hr were needed. Depending on copper content, X-ray diffraction studies revealed two different martensitic structures: monoclinic - B19' and orthorhombic - B19.