J. Laeng, H. Yang, Y. Liu, University of Western Australia, Crawley, Australia
This paper reports on a study of solid–state phase formation of Ni-Ti via diffusion reactions during sintering. Samples of different Ni-Ti compositions were synthesized from elemental powders and sintered in vacuum. Phase formation analysis by means of scanning electron microscopy, x-ray energy dispersive spectroscopy and x-ray diffraction revealed that the sintered samples consisted of Ti(Ni), Ni(Ti), NiTi2, NiTi and Ni3Ti in co-existence at different stages of sintering. Such co-existence is not expected from the equilibrium phase diagram. Post-sintering heat treatment at excessively high temperatures for prolonged times did not seem to be able to the desired single B2 phase. Thermodynamic analysis indicates that formation of NiTi is not fovoured in direct reactions between Ni and Ti, but can be formed via secondary reactions involving primary reaction products NiTi2 and Ni3Ti. Such reactions are difficult in solid state due to the difficulties of long-distance diffusion required. Such findings cast doubts of the feasibility of solid state synthesis of binary NiTi, for example porous NiTi.
