Place: Hydrogen in metals – current understanding and future needs Oxford, United Kingdom
The identification of the underlying mechanisms of hydrogen embrittlement is being complicated by the fact that there is a strong dependency on the composition and microstructure of the material. Among the main mechanisms that have been identified so far, hydrogen enhanced decohesion (HEDE) is expected to be most relevant at grain boundaries and interfaces, where H is trapped, but distributed along a crystallographic plane. Across these planes, H is expected to weaken the interatomic bonds due to charge transfer from the host metal to the H impurity atom. Indeed, our ab-initio density functional theory study of H segregation at special grain boundaries in Fe confirm the latter assumption. However, calculations of the work of separation and strength show only a weak, often negligible effect of H on the cohesive properties. This contradiction to experimental evidence for ferritic steel alloys is resolved, if the co-segregation of C to the grain boundaries is taken into account. Calculations of the segregation energies of mixed compositions indicate a competition of H and C for segregation sites, and the HEDE mechanism can thus be understood as a reduction of the cohesion enhancing effect of C on the grain boundaries.The additional influence of grain boundary structure, and the possibility to influence the segregation behaviour by additional alloying elements such as Cr, V, and Mn, will be discussed.