On the Interdiffusion in α-Solid Solution of the Fe–Al System in Al Vapor

Abstract

Iron test piece was diffusion-annealed in an evacuated capsule with the powdered Fe–Al alloy (α+ζ) as an aluminum vapor source at 800°C to 1100°C and each diffusion-penetration curve was analyzed by means of Balluffi’s method to determine the dependence of interdiffusion coefficients (\ ildeD) on the concentration of Al. From the values of coefficients the activation energies for interdiffusion (\ ildeQ) were obtained as the function of Al concentration in the alloy. \ ildeD at each temperature was strongly dependent upon the Al concentration of the α-range in an Fe–Al system and each \ ildeD showed a peak value for a certain concentration of Al and these peaks corresponded to the order-disorder boundary of this alloy (α) very well. The dependence of \ ildeD upon temperature in the Arrhenius plot showed a good linearity, from which the activation energy for interdiffusion was calculated. The energy decreased steeply with Al concentration down to 43 kcal/mol in a disorder range and increased in an ordered range in general. But the activation energy showed a high value (69 kcal/mol) at the initial stage of the ordered phase. Fine alumina markers placed initially on the test piece were found in the ordered phase after annealing at each temperature to shift to higher Al concentration with increasing temperature and the intrinsic diffusion coefficient of Al was about 1.4 to 2.0 times higher than that of Fe. The mean activation energies in the range of composition of 30.5 to 42.8 at% Al of both constituents were about the same (60 kcal/mol).