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Is hydrogen-enhanced decohesion at grain boundaries in ferritic steels a consequence of co-segregation? - Insights from ab-initio calculations
- Date: 13.06.2019
- Time: 4:30 p.m.
- Place: ALEMI workshop Vancouver, Canada
Free atomic hydrogen severly limits the structural integrity of many metals and alloys used in engineering, such as iron, steel, and nickel alloys. At the same time, it is a candidate for an environmentally friendly, carbon free fuel. So the different interactions of H with metal and alloy structures, commonly referred to as hydrogen embrittlement (HE), are an active field of research. Hydrogen embrittlement in high-strength ferritic steels is very sensitive to the amount of mobile hydrogen in the microstructure. Thus, reducing the amount of diffusing H is generally assumed to be beneficial w.r.t. preventing HE. Grain boundaries are some of the strongest traps - however, they can also act as crack nucleation sites and/or crack paths. Even more so, since H, distributed along a grain boundary plane, weakens the cohesion across this plane. Thus, an understanding and predictive modeling of the dual role of grain boundaries in hydrogen enhanced decohesion (HEDE) in ferritic microstructures is essential. A thermodynamics/continuum mechanics framework to equilibrate the H chemical potential in a microstructure and to derive the resulting local H concentration at and cohesive strength of crystallographic bulk planes already exists. The relevant parameters for this model can be determined from ab-initio density functional theory calculations, providing a quantitative and consistent approach. Details will be introduced in the presentation. Furthermore, it will be discussed to what extent this approach can be transferred to grain boundaries of different structures. Last not least, steel is a multicomponent alloy, and HEDE is not only sensitive to the local structure, but also the local chemistry. For instance, calculations of the segregation energies of mixed compositions indicate a competition of H and C for segregation sites at grain boundaries, and the HEDE mechanism could thus be understood as a reduction of the cohesion enhancing effect of C on the grain boundaries. The possibility to influence the segregation behaviour by additional alloying elements such as Cr, V, and Mn, will also be evaluated in the talk.