Compound Growth due to Reactive Diffusion between Solid-Ni and Liquid-Zn

Abstract

To examine the kinetics of the reactive diffusion in the solid-Ni/liquid-Zn system, an isothermal bonding technique was used to prepare semi-infinite Ni/Zn diffusion couples. The diffusion couples were isothermally annealed in the temperature range of T = 773923 K for various periods up to t = 10.8 ks (3 h). In this temperature range, the ¢ 1 and £ phases are the stable intermetallic compounds in the binary NiZn system. During isothermal annealing, however, only the £ phase forms as a visible compound layer at the original Ni/Zn interface in the diffusion couple. The £ layer grows mainly towards the liquid-Zn and slightly into the solid-Ni. The mean thickness l of the £ layer is described as a power function of the annealing time t. According to the observation, the exponent of the power function is rather close to 0.5 independently of T. Thus, we may consider that the square of l is proportional to t at T = 773923 K: l 2 = Kt. This relationship is called a parabolic relationship. The temperature dependence of K was evaluated from the experimental results. The evaluation provides the activation enthalpy of 40 kJ/mol for the layer growth. [