Abstract
Established theories of hydrogen embrittlement (HE) can partially describe this phenomenon by taking specific features of hydrogen-metal interactions into account. It is generally known that H atoms decrease the formation energies of defects such as free surfaces and dislocations, leading to enhanced decohesion and enhanced localized plasticity, respectively. In this talk, we will focus on another aspect of H-metal interactions. Since the H atoms occupy interstitial sites in the host metals, they increase the local stress field. This effect intensifies, if a large number of H atoms is attracted to defects such as grain boundaries (GBs) and is essential for the explanation of a certain feature of HE in polycrystalline nickel:
Extensive experimental observations indicate the presence of nano-voids and the increase of the free volume along the grain boundaries in hydrogen charged nickel. We explain this effect by molecular dynamics (MD) simulations and theoretical considerations. They show that the barrier for cross-slip of screw dislocations considerably decreases due to the H-induced stress field in the nanometer-sized regions around GBs. The enhanced cross slip of dislocations facilitates the formation of jogs at specific loading rates. These jogs can emit extra vacancies during the jog-drag process. The extra vacancies which are produced at nanometer distance from the GBs can diffuse toward them and form nano-voids at them.