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Atomistic Investigation of the Role of Grain Boundary Structure on Hydrogen Segregation and Embrittlement in alpha Fe

Kiran Solanki (Arizona State University), Mark Tschopp (ORAU/Army Research Laboratory), Mehul Bhatia ()

Materials for Extreme Environments: Multiscale Experiments and Simulations

Mon 10:45 - 12:15

Salomon 203

Material strengthening and embrittlement are controlled by complex intrinsic interactions between dislocations and hydrogen-induced defect structures that strongly alter the observed deformation mechanisms in materials. In this work, we reported molecular statics simulations at zero temperature for pure alpha Fe with a single H atom at an interstitial and vacancy site, and two H atoms at an interstitial and vacancy site for the various symmetric tilt grain boundary (STGB) systems. Simulation results show that the grain boundary system has a smaller effect than the type of H defect configuration (interstitial H, H vacancy, interstitial 2H, and 2H vacancy). For example, the segregation energy of hydrogen configurations as a function of distance is comparable between symmetric tilt grain boundary systems. However, the segregation energy of the [100] STGB system with H at an interstitial site is 23 percentage of the segregation energy of 2H at a similar interstitial site. This implies that there is a large binding energy associated with two interstitial H atoms in the grain boundary. Thus, the energy gained by this H H reaction is 54 percentage of the segregation energy of 2H in an interstitial site, creating a large driving force for H atoms to bind to each other within the grain boundary. Moreover, the cohesive energy of 125 STGBs was calculated for various local H concentrations. We found that as the grain boundary energy approaches zero, the energy gained by trapping more hydrogen atoms is negligible and the grain boundary can fail via cleavage. These results also show that there is a strong correlation between the grain boundary character and the trapping limit (saturation limit) for hydrogen. Finally, we developed an atomistic modeling framework to address the probabilistic nature of H segregation and the consequent embrittlement of the grain boundary.