Molecular Dynamics Investigation of Hydrogen Embrittlement in Steel

Abstract

Hydrogen embrittlement, the detrimental effect of hydrogen on the mechanical properties (strength, ductility, toughness, etc.) of metals and alloys, has been an important engineering challenge. Despite the great effort devoted to this subject in the past, the mechanisms by which hydrogen embrittles most metallic materials remain unclear. In this work, molecular dynamics (MD) studies have been carried out in order to investigate hydrogen embrittlement in steel. Tensile test simulations of pure iron and ferrite have initially been performed. Noticeable differences in the strength and the deformation behaviour of pure iron and the solid solution were found. These differences become sufficiently large at strain rates below 109 s −1 in the [1 1 1] and [1 1 0] directions, confirming the strengthening role of carbon atoms. MD simulations using a periodic array of dislocations were performed to investigate the effects of hydrogen on the dislocation mobility in the presence and absence of a C interstitial under applied shear stress. Due to the high tensile stresses, hydrogen diffuses to and accumulates at the tensile side of the dislocation core, forming a Cottrell cloud. Hydrogen solutes decrease dislocation mobility by several orders of magnitude. The presence of hydrogen in a dislocation pinned by a carbon interstitial increases the stress needed to unpin the dislocation. These results suggest that hydrogen is unlikely to facilitate dislocation glide or to aid dislocations overcoming C interstitials. The effect of hydrogen on the fracture process was also studied. Crack propagation was studied on the (1 1 1)[1 1 2], the (1 1 1)[1 1 0] and the (1 0 0)[0 1 0] orientations. The results show a range of mechanisms leading to inferior fracture toughness as the hydrogen concentration is increased. For the chosen simulation conditions, the operation of these mechanisms depends mostly on the crystal orientation and the hydrogen concentration. On the (1 1 1)[1 1 2] orientation at low hydrogen concentrations, fracture resembles that of the Adsorption Induced Dislocation Emission (AIDE) mechanism while at higher concentrations the nucleation of dislocations is prevented and fracture occurs via a cleavage-like mechanism. This trend was also repeated on the (1 1 1)[1 1 0] orientation. Furthermore, at higher concentrations, increased plasticity caused by intense dislocation activity leading to nano-void formation was also seen on this orientation. These features are consistent with the Hydrogen Enhanced Localised Plasticity (HELP) model. On the (1 0 0)[0 1 0] orientation, hydrogen reduces the lattice cohesion and fracture occurred at lower values of stress intensity, which is in agreement with the Hydrogen Enhanced Decohesion (HEDE) model. Based on these simulations, it has been concluded that there exists a spectrum of operational hydrogen embrittlement mechanisms in the nanoscale fracture process of Fe. The dominance of HEDE, HELP, AIDE or other failure mechanism depends on the crack-tip conditions, such as the local hydrogen concentration, crack orientation and crack velocity