Hydrogen effects on dislocation structures and interactions

Abstract

Hydrogen embrittlement (HE) is a complex process, in which the interactions of H atoms, vacancies, and dislocations lead to a macroscopic loss of ductility. Although this phenomenon is commonly observed, its microscopic origins remain unclear. In this thesis we study the process of HE, starting from the microscale, using atomistic simulations and modeling to connect with higher length scales. Passing information from physically realistic atomic scale simulations allows for improved understanding of the underlying mechanisms of H embrittlement and specifically how effects attributed to H contribute at the meso and macroscales. We study these contributions in three parts. First, a method for computing the distribution of H around an edge dislocation is presented and compared to an alternative approach. The presented method is then exercised in an example, studying the effect of H on the stacking fault width (SFW) of an extended edge dislocation. It is shown that H acts to decrease the SFW. Further, only the H very locally around and inside the dislocation cores and stacking fault contribute significantly to the observed decrease in the SFW. We then turn our focus to the role of H on the stabilization and clustering of vacancies. A new model is developed for the production of excess vacancies by plastic deformation. This model is integrated into an existing macroscale computational framework. Lastly, the role of H on the formation energy of vacancy clusters is studied using a hybrid molecular statics / Monte Carlo method. These calculations show that H tends to decrease the formation energy of vacancy clusters which leads to a decrease in the critical cluster size for void nucleation.