Intermetallic formation and mechanical properties of Ni-Ti diffusion couples

Samples of Ni-Ti diffusion couples were prepared by hot pressing in a furnace with a vacuum of 10(-1) Pa. Scanning electron microscopy (SEM) investigations indicated that three intermetallic compounds, namely, NiTi2, NiTi, and Ni3Ti, were formed at the Ni-Ti interface at 650 degrees C. Ni3Ti first grew at the Ni substrate because of the diffusion of Ti into Ni, NiTi2 subsequently appeared, and NiTi finally developed between the NiTi2-Ni3Ti interface. As the thickness of the diffusion layers increased, a part of Ni3Ti was also consumed by Ti to form NiTi. The interfacial microstructure was changed by the current at 900 degrees C. A columnar crystal initially formed inside the Ni3Ti matrix, which was immediately adjacent to NiTi, and this crystal then elongated at a direction perpendicular to the plane of the joint. Ni3Ti vanished, and pure Ti was finally produced. The observed strength variations were discussed in relation to interfacial microstructures. Shear experiments showed that fractures were observed in Ni3Ti for the sample prepared at 650 degrees C. For the sample prepared at 900 degrees C, fractures were detected in NiTi2.