Keizo Nishida1, Tsuyoshi Yamamoto2 and Takuo Nagata3
1Metals Research Institute, Faculty of Engineering, Hokkaido University, Sapporo
Iron test piece was diffusion-annealed in an evacuated capsule with the powdered Fe-Al alloy ( α+ζ ) as an Al vapor source at 800° ∼ 1100°C and each diffusion-penetration curve was analysed by means of a Balluffi's method to obtain the chemical diffusion coefficients ( D˜) with the concentration of Al. From the values of coefficient the activation energies for diffusion ( Q˜) with the composition of the alloy were obtained. D˜ at each temperature was strongly dependent upon Al concentration of α -range in an Fe-Al system and each showed a peak value at some composition of Al and these peaks fitted the order-disorder boundary of this alloy ( α ) very well. The dependency of D˜ with temperature in the Arrhenius plot showed a good lineality from which the activation energy for diffusion was calculated. It was found to decrease steeply with Al concentration down to 43 kcal/mol in a disorder range and rose up in an ordered range in general. But it showed a high value of activation energy (69 kcal/mol) at the threshold of an ordered phase. This seemingly abnormal behavior of the initial high value in the ordered phase was discussed. Fine alumina markers placed at first on the test piece were found in the ordered phase after annealing at each temperature, localized in higher Al concentration with higher temperature, and the intrinsic diffusion coefficient of Al showed the value of about 1.4 to 2.0 times higher than that of iron. 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).
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