Diffusion ofCo60in Some Ni-Al Alloys Containing Excess Vacancies

The aluminum-rich compositions in the $\delta$ phase of Ni-Al exhibit vacancy densities considerably greater than the normal equilibrium concentrations. Measurements were made of the diffusion of ${\mathrm{Co}}^{60}$ in six compositions in the $\delta$ phase of Ni-Al at 1050°C, 1150°C, 1250°C, and 1350°C. The diffusion coefficient had a minimum value at the stoichiometric composition. The activation energies had a maximum value at this composition and remained essentially constant at a lower value for the aluminum-rich alloys. An interpretation of this behavior in terms of a vacancy mechanism is discussed.