Evolution of quasi-brittle hydrogen-assisted damages in a dual-phase steel

We studied the statistical quantitative analysis of the hydrogen-assisted damage evolution behavior from nano- to micro-scale by combining positron annihilation spectroscopy（PAS）and scanning electron microscopy（SEM）-based damage characterization in a dual-phase steel with a tensile strength of 960 MPa. The total elongation was markedly decreased by hydrogen pre-charging（0.32 mass ppm H）from 17％ to 4％. We divided the damage evolution behavior into three stages: damage incubation; arrest; growth, and evaluated the effects of hydrogen pre-charging on each stage. The damage nucleation was caused by martensite fracture and enhanced by hydrogen pre-charging. However, PAS showed no enhancement of vacancy formation by hydrogen. The statistical damage quantitative analysis indicated in the damage arrest stage that the critical damage size corresponding to the blunt limit of the damage tip was decreased from 〜1 μm² in the uncharged specimen to 〜0.5 μm² in the hydrogen pre-charged specimen. The damage growth in the third stage was accelerated by hydrogen pre- charging owing to quasi-brittle damage propagation through the ferrite cleavage plane or ferrite/martensite interface. SEM observation showed that the cleavage propagation in ferrite was accompanied by the local plastic deformation. To explain this fracture acceleration, we proposed cooperative contribution of the enhancement of the local plastic deformation through adsorption-induced dislocation emission （AIDE）mechanism and the cleavage fracture through hydrogen enhanced decohesion（HEDE）mechanism.