SELF-DIFFUSION IN BCC AND ORDERED PHASES OF AN EQUIATOMIC IRON COBALT ALLOY

Self-diffusion coefficients of Fe-59 and Co-57 in an Fe-50.8 at.% Co alloy have been determined in the temperature range between 877 and 1238 K by use of the sputter-microsectioning technique. Above the order-disorder transformation temperature (1003 K), the Arrhenius plots of the self-diffusion coefficients of both components show a slight deviation from linearity due to the magnetic spin ordering of the alloy. Below 1003 K the Arrhenius plots become curved downward remarkably owing to the B2 type of atomic ordering. The temperature dependence of the self-diffusion coefficients D-Fe and D-Co across the phase transformation temperature can be expressed by D-Fe = 7.0 x 10(-4) exp{-(241 kJ mol(-1))[1 + 0.16s(2) + 0.21u(2)]/RT}m(2) s(-1) and D-Co = 0.31 x 10(-4) exp{-(201 kJ mol(-1))[1 + 0.29s(2) + 0.27u(2)]/RT}m(2) s(-1) where s is the ratio of the spontaneous magnetization at T to that at 0 K, and u is the long range order parameter. Both the magnetic spin ordering and the atomic ordering of the alloy yield an increase in the activation energies for self-diffusion.