PHENOMENOLOGICAL CONSIDERATION ON COMPOSITIONAL DEVIATION FROM EQUILIBRIUM AT BCC-FCC INTERFACE IN FE-CR DIFFUSION COUPLES

The phase boundary compositions at the alpha / gamma interface in gamma Fe:Cr diffusion couples were examined by electron microprobe. The results show that the boundary compositions deviate from equilibrium values. The compositional deviations are analyzed in terms of atomic mobility on the basis of the phenomenological theory of material transport. It is found that the atomic mobility in the transfer across the alpha / gamma interface is lower than in lattice diffusion in the alpha or gamma phase by a factor of 10** minus **2 to 10** minus **4. The reason for this low mobility in the transfer across the interface is considered to be due to a large negative value for the entropy of activation in the transfer process involved.