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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4. Effect of co- segregation on mechanical properties of Σ3{110} Following examination of the energetics of co-segregation of C and H, the next step is to examine the effect that such co-segregation has on the mechanical stability of such grain boundaries. As an initial test case, we have taken the Σ3{110} twist boundary and performed simulated tensile tests on a bicrystal containing such a boundary. These tests are performed at 0K, where the strain is applied incrementally, with full relaxation of the atomic positions. Hence, the present simulations correspond to the low-temperature limit, or the limit of very high strain rates, where thermally activated processes such as diffusion are suppressed, The various scenarios considered (as illustrated in Figure 3) are the defect-free case, a situation where both favourable grain boundary interstitial sites (in the primitive plane of the grain boundary) are occupied by hydrogen, one where both sites are occupied by carbon, and the final scenario, where both carbon and hydrogen are present. As observed in our previous work (McEniry et al. 2017), for this particular boundary, the presence of hydrogen increases the elongation to fracture. We also find that carbon increases both the elongation to fracture, and the maximum obtained stress. However, in the scenario where both carbon and hydrogen are present, the stress to fracture is significantly decreased, also in comparison to the defect - free case. Therefore, we conclude, for this boundary, that understanding the co-segregation of carbon and hydrogen is essential in order to study the effects of hydrogen embrittlement in carbon-containing high-strength steels.

Figure 3. Stress- strain curves of the Σ3{110} twist GB in the absence and presence of segregating carbon and hydrogen.

5. Conclusions

In the current work, an atomistic modelling scheme for the Fe-C-H ternary system has been developed and applied to the study of segregation and co-segregation effects of C and H at a number of grain boundaries in α -Fe. Our main findings are that the segregation energies of C are generally larger than those of H; however, the interactions between C atoms located at adjacent binding sites are typically repulsive. As a result, full saturation of these boundaries may not be possible, and, particularly in the case of the tilt grain boundaries considered, H may occupy some of the available interstitial sites. In one particular case, namely that of a Σ3{110} twist boundary, the co-segregation of C and H has a large deleterious effect on the mechanical stability of the grain boundary. The preliminary results obtained here open up a number of interesting avenues for future research. The segregation and co-segregation energies obtained here can be implemented in a Monte Carlo framework in order to investigate the full segregation profile as a function of C and H concentration and temperature. It is also intended to