PSI - Issue 13

Eunan J. McEniry et al. / Procedia Structural Integrity 13 (2018) 1099–1104 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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Figure 2: Images of the Σ9 { 1 2 11} [11̅ 0] (left image), Σ13 { 1 3 11} [11̅ 0] (centre) and Σ17 { 1 4 11} [11̅ 0] (right) tilt GBs considered in the simulations. The green circle denotes the “void” -like hole, which is found to be most favourable for segregation for both hydrogen and carbon. 3. Cosegregation of C and H at grain boundaries in Fe The next step is to consider co-segregation effects. In this case, we define the co-segregation energy as − = {( + + ) + ( + + ) − + + }, where + + denotes the energy of a cell containing the grain boundary and both solute atoms. As with the definition of , a positive value of indicates that it is favourable for the two atoms A and B to segregate together at the boundary. We consider the three different scenarios; where two H atoms attempt to cosegregate, where two C atoms co-segregate and where one C and one H atom segregate together. In the case of the twist boundaries, we find no particular tendency toward segregation when solute atoms are placed at neighbouring “trap” sites. In the case of Σ3, the interaction between H located at the neighbouring trap sites is slightly repulsive, while for C repulsive is significant, indicating that full C coverage on this boundary is unlikely. However, there is a slight tendency towards C-H co- segregation at the Σ3 boundary, with the presence of C (and the slight increase of the in-plane lattice constant) increasing the segregation energy of H. For the other twist boundaries, the distances between adjacent trap sites are much larger, and hence the effects of co-segregation are considerably weaker. Within the precision of the current calculations, the tendency of segregated atoms to repel or attract is difficult to ascertain; it is however apparent that there is no large driving force toward co-segregation for these boundaries. For the tilt boundaries, the results are more interesting. In the case of H atoms located at adjacent void sites along the edge dislocation, the interaction is repulsive for the nearest neighbour, and negligible for the 2nd nearest neighbour. Hence, H accumulation along the edge (into the plane of the image on Figure 2) can be relatively large, but will not saturate with full coverage. H-H interactions between the voids (parallel to the GB plane) are smaller due to the larger distance between void sites in this direction. For C, the repulsive character is much larger, and is significant even for larger distances between the binding sites. However, when co-segregation effects between C and H are considered, we have the interesting finding that C present at a particular void site may attract H to an adjacent site. Due to an increase in the local strain induced by the presence of a carbon atom, the displacement between the two grains is very slightly increased. As a result, at the adjacent void, the available (Voronoi) volume for H is increased by ~5%, with a subsequent increase in the solubility of H. Hence, for this class of tilt grain boundaries, the presence of C atoms at the boundary may enhance the H concentration at nearby vacant interstitial sites. Moreover, initial calculations indicate that the presence of C atoms in the voids also increases the segregation energies of H at additional sites in the boundary plane, which, in the absence of C, would be unfavourable for H. Further calculations in this direction are left for future work.