Intermetallic formation and mechanical properties of Ni-Ti diffusion couples

Abstract 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, NiTi 2 , NiTi, and Ni 3 Ti, were formed at the Ni-Ti interface at 650 °C. Ni 3 Ti first grew at the Ni substrate because of the diffusion of Ti into Ni, NiTi 2 subsequently appeared, and NiTi finally developed between the NiTi 2 -Ni 3 Ti interface. As the thickness of the diffusion layers increased, a part of Ni 3 Ti was also consumed by Ti to form NiTi. The interfacial microstructure was changed by the current at 900 °C. A columnar crystal initially formed inside the Ni 3 Ti matrix, which was immediately adjacent to NiTi, and this crystal then elongated at a direction perpendicular to the plane of the joint. Ni 3 Ti 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 Ni 3 Ti for the sample prepared at 650 °C. For the sample prepared at 900 °C, fractures were detected in NiTi 2 .