Interdiffusion in Cobalt-Nickel Alloys

Interdiffusion in Co-Ni alloys was studied in the temperature range between 880° and 1300°C, using diffusion couples of pure cobalt and nickel. Matano’s method was employed to determine the interdiffusion coefficient, \ ildeD, as a function of concentration.It was found that above 1190°C, \ ildeD behaved as in an ideal solid solution. Below 1145°C, the dependence of \ ildeD on the concentration showed a maximum value at 75 at%Ni, which might be attributed to the short range order. Below 1000°C, interdiffusion was enhanced by some short-circuiting diffusion. The effect of magnetic transformation on the interdiffusion was clearly detected.The calculated values of \ ildeD based on Darken’s equation were compared with the experimental values for 1127°C, and the latter was shown to be smaller than the former in the whole range of composition. From the Arrhenius relationship, the activation energy \ ildeQ and the frequency factor \ ildeD0 for the interdiffusion were calculated. \ ildeQ shows a minimum at about 70 at%Ni supposedly corresponding to the maximum of \ ildeD at 75 at%Ni.The Kirkendall voids were not formed and the Kirkendall markers were found to be almost immobile in this system, suggesting that the intrinsic diffusion coefficient of both elements in this system is nearly the same.