Thermodynamics and kinetics of interdiffusion in Ni//NiAl diffusion couples

The growth behavior of Ni3Al after nucleation at the Ni/NiAl interface was investigated by using macroscopic diffusion couples at low annealing temperatures of 973-1073 K. The thickness and grain size of the Ni3Al phase at different annealing temperatures were statistically analyzed by scanning electron microscopy to evaluate the growth kinetics of the Ni3Al layer. According to the growth rate exponent, the growth of Ni3Al layer to NiAl is almost completely controlled by volume diffusion (VD) when it is above 1023 K, and by boundary diffusion (BD) when it is below 1023 K. In contrast, the growth of Ni3Al into Ni is controlled by VD almost whether at 1023 K or 1073 K. Remarkably, a continuous Al-rich Ni grain layer was formed at the Ni3Al/Ni interface by diffusioninduced recrystallization (DIR), and the experimental results for DIR region of composition and growth behavior were numerically analyzed using the thermodynamic and kinetic models, respectively. The analyses suggest that the composition of the DIR region in the Ni(Al) binary system can be determined by the thermodynamic conditions of the chemical driving force model (CDF model). Additionally, kinetic analysis using the new extended model (NE model) indicates that under current annealing conditions, interface reaction and BD control growth at the moving boundary of the DIR region. Furthermore, the temperature stability range of Ni5Al3 in the Ni//NiAl system was subjected to systematic analysis.