Grain boundary migration of substitutional and interstitial atoms in α-iron

The effect of grain boundaries (GBs) on the diffusion of interstitial and substitutional atoms in α-iron was studied experimentally and theoretically. Autoradiography and the sectioning method, applied to different grain size polycrystals, prove that a higher GB area leads to a reduction of the migration of the 14C isotope. An opposite effect is observed for the 60Co isotope. Theoretical calculations of hydrogen and carbon migration in selected GBs having different disorientation angles show that, for both interstitial elements, the activation enthalpy for migration is higher in the GBs than in the bulk. Both experimental and theoretical results lead to the conclusion that GBs in α-iron act as traps for interstitial atoms and retard their diffusion. To put these results into perspective, hydrogen-induced intergranular cracking is discussed in relation to the specificity of hydrogen diffusivity along the GBs.