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The ASED-MO theory was used to study the electronic effects of S and the S-Mn couple upon the chemical embrittlement of Fe grain boundaries. The results obtained for S alone in a model of grain boundary (GB) are consistent with its observed behavior as a chemical embrittling agent. It was found that the total energy of the cluster decreases when the S atom is located at the GB. When S segregate at the Fe GB containing Mn, the embrittlement process was modified. The crystal orbital overlap population (COOP) curves gives a measure of Fe-Fe bond weakening due to the segregated atoms at the GB. Our calculations show that Mn behaves as a weak embrittler on the Fe GB. The Fe-Mn bonds were strengthened, while Fe-Fe bonds of the capped trigonal prism of the GB (CTP) were weakened. On the other hand, when S segregate at the Mn/Fe cluster, some metallic bonds were resistant to chemical embrittlement. © 2007 Elsevier B.V. All rights reserved. Fil: Gesari, Susana Beatriz. Universidad Nacional del Sur. Departamento de Física; Argentina Fil: Pronsato, Maria Estela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; Argentina Fil: Juan, Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; Argentina

The H–Fe interaction at a grain boundary (GB) in BCC Fe was studied using qualitative electronic structure calculations in the framework of the atom superposition and electron delocalization molecular orbital (ASED-MO) theory. Calculations were performed using an Fe 196 cluster to simulate the 36.9° [1 0 0] {0 1 3} symmetrical tilt GB structure. The most stable positions for one H atom and two H atoms at the GB core were determined. The total energy of the cluster decreases when the H atoms are at that location, making it a possible site for H accumulation. An analysis of the orbital interaction reveals that H–Fe bonding involves mainly the Fe 4s and H 1s orbitals. In general, H drain charge from the first neighbor Fe atoms. The crystal orbital overlap population (COOP) curves gives a measure of Fe–Fe bond weakening due to H segregate at GB. Some Fe–Fe bonds in the GB plane shows a 60% decrease in the overlap population when H is present. H–H interaction was also analyzed. Although some H–H association is reveled no bond is formed between the impurity atoms.

The H–Fe interaction in a dislocated bcc structure was studied using qualitative electronic structure calculations in the framework of the atom superposition and electron delocalisation molecular orbital (ASED-MO) theory. The effect on the electronic structure after introducing an a /2 [1 1 1] mixed dislocation was analyzed and compared with the effect of introducing an a [0 1 0] edge dislocation. Calculations were performed using Fe 85 clusters to simulate a dislocated bcc structure. The cluster geometry and atomic parameters were optimized to make a better approximation to the repulsive energy terms. The most stable positions for two H atoms inside the cluster were determined. The total energy of the cluster decreases when the H atoms are located near the dislocation void, making it a possible region for H-accumulation. The hydrogen atoms bond to their closest neighbors weakening the bond between iron atoms. The H–H interaction was also analyzed. In general, the behavior of H and its influence on the Fe atoms first neighbors result to have similarities for both dislocations analyzed.

The bonding of HFe is analyzed using qualitative electronic calculations in the framework of the atom superposition and electron delocalization orbital cluster method (ASED-MO). The changes in the electronic structure of bcc Fe upon introduction, a screw dislocation are compared with Fe surfaces. A comparison is drawn with H adsorption at Fe ( 1 1 2) surface and H absorption in the bulk Fe with a dislocation. The results suggest that the detrimental effect of H on the FeFe bonds in a screw dislocation is almost half-severe than in other defects, such as an edge dislocation or vacancy and it is limited to its first neighbor. An analysis of the orbital interaction reveals that FeH bonding involves mainly the Fe 4s and H 1s orbitals. When H is in the bulk, the region near screw dislocation core is much more repulsive than in a normal Fe lattice. These results allow us consider that H accumulation could be less favorable near a screw dislocation than an edge dislocation.