The self-diffusion coefficients of 59Fe in an Fe-50.8%Co alloy were determined, at 877 to 1238K, by using sputter micro-sectioning techniques. It was found that, above the order-disorder transition temperature (1003K), the Arrhenius plots of the self-diffusion coefficients exhibited a slight deviation from linearity. This was attributed to magnetic spin ordering of the alloy. At temperatures below 1003K, the Arrhenius plots curved markedly downwards due to B2-type atomic ordering. The temperature dependence of the self-diffusion coefficients across the phase transition temperature (table 53) could be described by:

D(m2/s) = 0.0007 exp[-241(kJ/mol){1+0.16s2+0.21u2}/RT]

where s was the ratio of the spontaneous magnetization at a given temperature, to that at absolute zero, and u was the long-range order parameter. Both magnetic spin ordering and atomic ordering led to an increase in the activation energy for self-diffusion.

Y.Iijima, C.G.Lee: Acta Metallurgica et Materialia, 1995, 43[3], 1183-8

Table 52

Self-Diffusion of 57Co in Equiatomic Fe-Co