The volume and grain boundary diffusion of Cr in Ni-16Cr-7wt%Fe alloys containing 0.004, 0.015 or 0.07wt%C were measured by using 51Cr radioactive tracers at 858 to 1424K, and serial radio-frequency sputter micro-sectioning techniques. It was found that, whereas the bulk diffusion coefficients were largely unaffected by the presence of C, an increase in C content markedly reduced the mobility of Cr atoms along the grain boundaries. As a result, the difference between the activation energy for lattice and grain boundary diffusion decreased with increasing C content. For 0.07at%C, the activation energies for volume and grain boundary diffusion were almost the same. For all 3 alloys, the difference between the volume diffusion coefficient, Dv, and δDgb decreased gradually in a regular manner with increasing temperature. The bulk diffusivity of 51Cr could be described by:

0.07wt%C:     D (m2/s) = 1.8 x 10-2 exp[-340(kJ/mol)/RT]

0.015wt%C:     D (m2/s) = 1.0 x 10-3 exp[-300(kJ/mol)/RT]

0.004wt%C:     D (m2/s) = 5.1 x 10-4 exp[-286(kJ/mol)/RT]

The grain-boundary diffusivity of 51Cr could be described by:

0.07wt%C:     δD (m3/s) = 8.2 x 10-8 exp[-335(kJ/mol)/RT]

0.015wt%C:     δD (m3/s) = 1.9 x 10-9 exp[-227(kJ/mol)/RT]

0.004wt%C:     δD (m3/s) = 4.8 x 10-12 exp[-203(kJ/mol)/RT]

The grain boundary energy was reduced by the addition of C, and increased with temperature. At higher temperatures, there was no difference between the grain boundary energies of the 3 alloys.

T.F.Chen, G.P.Tiwari, Y.Iijima, K.Yamauchi: Materials Transactions, 2003, 44[1], 40-6