The C diffusivity was investigated over the entire phase field, and at temperatures ranging from 1179 to 2413K, by using diffusion couples. Concentration versus depth C profiles were obtained by means of sputtered neutral mass spectrometry and electron microprobe analysis. The diffusion coefficients which were obtained by using the 2 techniques, over different temperature ranges, agreed closely. At temperatures above 1468K, they exhibited a marked concentration dependence and could be described by:

ln[D (m2/s) = 26.3 - 148.5x + 150.3x2 - 310(kJ/mol)/RT

where x was the C concentration. A thermodynamic factor, which was deduced from a thermodynamic analysis of the system, was used to describe the concentration dependence. The tracer diffusion coefficients which were calculated from the chemical diffusion coefficients, with the help of the thermodynamic factor, were in accord with published data. At temperatures below 1468K, concentration-independent diffusion coefficients were obtained which indicated a lower activation energy of:

ln[D (m2/s) = 21.2 - 174(kJ/mol)/RT

This behavior was attributed to the predominance of grain-boundary diffusion over volume diffusion at low temperatures.

K.Albertsen, H.J.Schaller: Berichte der Bunsengesellschaft für Physikalische Chemie, 1994, 98[10], 1224-30