Quantitative analysis of diffusion kinetics of intermetallic formation in Ni–Ti system

Abstract

The diffusion couple method is an appreciated experimental approach in taking in phase relations in multi-component systems. To investigate the growth mechanism of the phases and the diffusion of the components, Ni–Ti diffusion couple experiments are carried out at four different temperatures of 750, 800, 850, and 900 °C up to 24 h. Microstructural observations using scanning electron microscopy equipped with energy dispersive spectroscopy at the interface of the nickel-titanium indicates formation of three major intermetallic compounds by the sequence of NiTi2, Ni3Ti and NiTi. Using typical marker tests with incremental diffusion couples, data about mobilities of elemental type intermetallic phase media can be attained, and tracer diffusion coefficients can be presumed. By using of cerium oxide particles as an inert marker, it is shown that the diffusion of nickel atoms toward titanium is faster than the diffusion of titanium atoms through the compound layers. The marker plane is detected at the interface of Ni/Ni3Ti.The ratio of intrinsic diffusivities indicates that Ni is the predominant diffusing element in this system. The activation energies for the formation and growth of NiTi2, NiTi, Ni3Ti, and the total interfacial layers are found to be respectively 204, 151, 38, and 154 kJ/mol. In addition, the integrated diffusion coefficients are calculated by use of Wagner method. The activation energies for the integrated diffusion coefficient of NiTi2, NiTi and Ni3Ti are calculated as 168,149 and 78 kJ/mol, respectively.