Abstract
Density functional theory (DFT) calculations are carried out to investigate the role of grain boundaries on the energetics related to the oxygen and hydrogen segregation-induced embrittlement in polycrystalline Ni systems. Four model grain boundary (GB) systems for nickel are chosen to investigate this effect. These model GBs are the Σ5 (012) GB, the Σ5 (013) GB, the Σ11 (113) GB, and the Σ3 (111) coherent twin boundary (CTB). The chosen GBs enable the investigation of the role of the CTB in the embrittlement and decohesion mechanisms in comparison with the other GBs. The embrittling mechanism considered here is based on the investigation of the energetics related to (a) the segregation of atoms of embrittling species (oxygen, hydrogen) at the GB; (b) the formation of vacancies due to the segregation of embrittling species at the GB; and (c) the energetics related to decohesion at the GB as a function of concentration/accumulation of the embrittling species at the GB. DFT calculations suggest that the segregation of the embrittling species and the embrittling effect are closely related to the local atomic structure of the GB and the associated excess free volume. In particular, it is found that the Σ3 (111) CTB is less prone to segregation of oxygen and hydrogen based on the binding energetics of the embrittling species. However, among all the GBs considered, the Σ3 (111) CTB is found to be most susceptible to GB decohesion and crack formation in the presence of small amounts of segregated oxygen atoms. This dual behavior of the Σ3 (111) CTB is also confirmed for the case of hydrogen as the embrittling species using DFT simulations. Thus, the segregation-resistant Σ3 (111) CTB is observed to be the most susceptible to crack formation in the presence of small amounts of segregated embrittling atoms. The energetics of segregation of the embrittling species and the effect of segregation on the vacancy formation energies and GB decohesion are discussed.
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