Hydrogen-enhanced local plasticity at dilute bulk H concentrations: The role of HaH interactions and the formation of local hydrides

• Published in: Acta materialia 2011-05, Vol.59 (8), p.2969-2980
• Main Authors: von Pezold, Johann, Lymperakis, Liverios, Neugebeauer, JAPGrg
• Format: Article
• Language: English
• Subjects: Dilution; Dislocations; Edge dislocations; Embrittlement; Hydrides; Mathematical models; Plasticity; Stress concentration; Stress shielding
• ISSN: 1359-6454
• DOI: 10.1016/j.actamat.2011.01.037

The effect of H on the stress-mediated dislocationadislocation interaction in face-centered cubic metals is studied by a hierarchical approach combining ab initio electronic structure calculations, semi-empirical embedded atom method potentials and a lattice-gas Hamiltonian, using the NiH system as a model system. The H distribution around edge dislocations is found to depend critically on HaH interactions within the metal matrix. In the absence of these interactions a dilute H distribution is obtained at realistic bulk H concentrations. In contrast, our calculations show that even a weakly attractive interaction between interstitial H atoms in the host matrix dramatically increases the local H concentration in the tensile strain field of the dislocation, resulting in the formation of a local hydride phase along the dislocation line. While in the absence of any HaH interactions huge bulk H concentrations of the order of 10at.% are required to induce the stress shielding effect underlying the hydrogen-enhanced local plasticity (HELP) mechanism, the presence of even weak HaH interactions dramatically reduces the onset H concentration of HELP. As a consequence bulk H concentrations as low as 1.6A10a2 at.% (160ppm) may result in embrittlement. The formation of a local hydride induces a strong, short-range shielding effect along the slip plane of the dislocation, while normal to the glide plane the shear stress is predominantly increased. The stress shielding effect is correlated with a reduced dislocation separation at dislocation pile-up tips, which may result in the nucleation of micro-cracks and hence the onset of embrittlement.