Tracer diffusion in pure and boron-doped Ni₃Al

Diffusion of ⁶³Ni has been measured in pure and boron-doped polycrystalline Ni₃Al intermetallic compounds as a function of temperature (692-1352°C), A1 concentration (24-26 at.% Al), and boron content (0-500 wt ppm). Volume and grain-boundary diffusion of ⁶⁰Co and ⁶⁸Ge have also been measured in pure Ni₃, Al. Both conventional grinding and ion beam sputtering techniques have been used for the determination of the concentration profiles. The diffusivity of Ni, D_Ni^*, is independent of Al content above 1000° C, indicating that antisite defects are prevailing on both sides of stoichiometry. However, D_Ni^* shows a minimum at the stoichiometric composition below 1000°C, and this trend becomes clearer with decreasing temperature. The diffusivities of⁶⁰Co and⁶⁸Ge are also independent of Al concentration in the temperature range between 880 and 1200°C, but their grain-boundary diffusion depends on Al concentration. The addition of boron linearly increases D_Ni^*, above 1000°C, but at lower temperatures,D_Ni^*, in off-stoichiometric compositions decreases to the value for stoichiometric Ni₃Al on the addition of 100 wt ppm boron, and then increases as above with the further additions of boron to ~500 wt ppm. The present diffusion data suggest that a small concentration of vacancies, independent of temperature, is present on both sides of, and at, stoichiometry at low temperature.