Atomistic mechanisms of hydrogen embrittlement

The detrimental effects of the H on the mechanical properties of the metals are known for more than a century. One of the most important degradation mechanisms is H embrittlement (HE). In this thesis, we examined a few famous proposed mechanisms in the field by performing careful atomistic simulations. Moreover, novel mechanisms which can be responsible for HE process in metals are demonstrated in this work. First, we used atomistic simulations to investigate the effects of segregated H on the behavior of cracks along various symmetric tilt grain boundaries in fcc Nickel. Mode I fracture behavior is then studied, examining the influence of H in altering the competition between dislocation emission (âductileâ behavior) and cleavage fracture (âbrittleâ behavior) for intergranular cracks. Simulations revealed that the embrittling effects of H atoms are limited. We examined the effect of H atoms on the nucleation of intergranular cracks in Ni. The theoretical strengths are $\sim$ 25 GPa and the yield strengths are $\sim$ 10 GPa, so that (i) the theoretical strength is always well above the yield strength, with or without H, and (ii) both strengths are far above the bulk plastic flow stress, $\sigma_y^B$ of Ni and Ni alloys. So H does not significantly facilitate nucleation of intergranular cracks. We performed simulations of the interactions between dislocations, H atoms, and vacancies to assess the viability of a recently-proposed mechanism for the formation of nanoscale voids in Fe-based steels in the presence of H. The effectiveness of annihilation/reduction processes is not reduced by the presence of H in the vacancy clusters because typical V-H cluster binding energies are much lower than the vacancy formation energy, except at very high H content in the cluster. Experimental observations of nanovoids on the fracture surfaces of steels must be due to as-yet undetermined processes. The possible strengthening effects of H atoms in metals at low temperature is examined via the solute strengthening (SS) theory. The results of the SS theory can explain recent experimental observations of strengthening of H-charged polycrystalline nickel at low temperature. Moreover, the possible softening/hardening effects of H atoms due to their interaction with the pre-existing with solutes are demonstrated and for the first time, a softening process in nickel alloys is shown. The effect of the H atoms in increasing the precipitate hardening in $\alpha$-Iron is also shown in this thesis. The direct molecular dynamics simulations of the bow out of an edge dislocation in H-free and H-charged samples reveals that the presence of H atoms decreases the magnitude of the bow out of the dislocation. The hardening effect of H on the interaction of dislocations and grain boundaries in nickel is also investigated in this thesis. To this end, we simulated the interaction of mixed and screw dislocations with the grain boundaries that have access to the slip planes in nickel. The presence of H atoms along the grain boundaries induces stress in the neighborhood of the grain boundary. These stress fields can repel/attract mixed dislocations while the screw dislocations are not interacting with them. The simulation of the interaction of the mixed dislocations with the H-free and H-charged GBs shows hardening due to the presence of H atoms. The simulations of the screw dislocations do not show significant hardening due to the presence of this stress f

Effect of hydrogen on phase stabilities in steels

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Hydrogen enhances cross-slip of dislocations in the vicinity of grain boundaries

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Atomistic study of hydrogen embrittlement of grain boundaries in nickel: II. Decohesion

Atomistic simulations of bicrystal samples containing a grain boundary are used to examine the effect of hydrogen atoms on the nucleation of intergranular cracks in Ni. Specifically, the theoretical strength is obtained by rigid separation of the two crystals above and below the GB and the yield strength (point of dislocation emission) is obtained by standard tension testing normal to the GB. These strengths are computed in pure Ni and Ni with H segregated to the grain boundaries under conditions typical of H embrittlement in Ni, and in artificially highly-H-saturated states. In all GBs studied here, the theoretical strength sigma(y) is not significantly reduced by the presence of the hydrogen atoms. Similarly, with the exception of the Ni Sigma 27(115) boundary, the yield strength sigma(y) is not significantly altered by the presence of segregated H atoms. In all cases, the theoretical strengths are similar to 25 GPa and the yield strengths are similar to 10 GPa, so that (i) the theoretical strength is always well above the yield strength, with or without H, and (ii) both strengths are far above the bulk plastic flow stress, sigma(B)(y) of Ni and Ni alloys. Significant reductions in fracture energy (25%-45%) are only achieved for some of the artificially high-H-segregation cases and then only when all the H around the GB is allow to diffuse locally to the fracture surface, which corresponds to unlikely out-of-equilibrium segregation plus local kinetics. Complementing recent work showing that H does not change the ability of GB cracks to emit dislocations and blunt, the present work indicates that equilibrium segregation of hydrogen atoms to GBs has little effect on lowering the GB strength and energy, and so does not significantly facilitate nucleation of intergranular cracks

Defect-hydrogen interaction in Al alloys: Challenges and benefits revealed by ab initio calculations

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Modeling Human Visual Search Performance on Realistic Webpages Using Analytical and Deep Learning Methods

Modeling visual search not only offers an opportunity to predict the usability of an interface before actually testing it on real users, but also advances scientific understanding about human behavior. In this work, we first conduct a set of analyses on a large-scale dataset of visual search tasks on realistic webpages. We then present a deep neural network that learns to predict the scannability of webpage content, i.e., how easy it is for a user to find a specific target. Our model leverages both heuristic-based features such as target size and unstructured features such as raw image pixels. This approach allows us to model complex interactions that might be involved in a realistic visual search task, which can not be easily achieved by traditional analytical models. We analyze the model behavior to offer our insights into how the salience map learned by the model aligns with human intuition and how the learned semantic representation of each target type relates to its visual search performance.Comment: the 2020 CHI Conference on Human Factors in Computing System

Spermatic cord metastasis as early manifestation of small bowel adenocarcinoma

Malignant tumors of the spermatic cord are rare. There are a few case reports on spermatic cord metastasis from colonic, gastric, pancreas, and prostatic cancer. Here, we report a 36-year-old man with brucellosis presenting with spermatic cord metastasis as early manifestation of small bowel adenocarcinoma