Volume and grain boundary diffusion of14C in α-Fe

Volume diffusion of 14C in high purity α-Fe single crystals is measured in the wide temperature range of 1168 to 596 K applying the radiotracer technique and serial sectioning of the samples. In the temperature range in which the present tracer and previous Snoek-effect measurements overlap a good agreement is found between the directly and indirectly measured diffusion coefficients. From this it is concluded that in the whole temperature range C-atoms diffuse interstitially only via octahedral sites in α-Fe. The data reveal a dependence on temperature which is typical for the influence of the magnetic phase transition on diffusion. The interstitial C diffusion responds more strongly to the stiffening of the lattice, which is related to the magnetic ordering, than the vacancy mediated self-diffusion. Grain boundary diffusion of C in α-Fe is observed below 740 K. The evaluation of the diffusion profiles results in Pc = sδD which is 8 to 9 orders of magnitude larger than PFe = δD measured for grain boundary self-diffusion in the same Fe-material. Taking into account the segregation coefficient s, the still large difference between D and D leads to the conclusion that C diffuses interstitially also in α-Fe grain boundaries.