By using density functional theory, a study was made of the segregation and diffusion of C interstitials, at a Σ3 (11¯2) tilt grain boundary, as a function of C coverage and Ni magnetic state. It was found that segregation was most favorable for a so-called chequerboard configuration, at a coverage of ½ monolayer, in both the non-magnetic and ferromagnetic states; with ferromagnetic (non-magnetic) heats of segregation that approached -0.5 (-0.3)eV per C atom. The segregation was 0.1 to 0.2eV stronger in the ferromagnetic state, because C interstitials suppressed Ni magnetism. This suppression was minimized when the C resided at the boundary. Grain-boundary C diffusion was predicted to be highly anisotropic. The calculated energy barriers for migration parallel to the [1¯10] tilt axis were significantly smaller (0.44 to 0.77eV) than that for lattice diffusion (1.62eV). The barriers perpendicular to [110] were comparable to those for lattice diffusion. The changes in barrier height due to Ni magnetism were relatively small, and were evident only when the diffusion pathway bisected highly compressed grain boundary sites.

D.J.Siegel, J.C.Hamilton: Acta Materialia, 2005, 53[1], 87-96