INTERNAL HYDROGEN EFFECTS ON THRESHOLDS FOR CRACK-GROWTH IN THE IRON-BASED SUPERALLOY IN903

This study of internal hydrogen-induced crack growth in the iron-based superalloy IN903 shows that slow crack growth thresholds are significantly lower than fracture toughness values at the same precharged hydrogen concentrations. However, failure in all precharged samples occurred by slip band fracture which differed only in the extent of local surface plasticity. Quantitative fractography of these surface fracture features indicates that the crack tip hydrogen concentrations at threshold were higher than in fracture toughness samples. These higher concentrations are due to crack tip stress enhancement when sufficient time exists for hydrogen redistribution. In addition, continuum models based on mechanisms of failure demonstrate that the matrix carbides control crack growth susceptibility in slow crack growth and fracture toughness samples by establishing the characteristic distance that the crack tip stresses and strains must span to initiate fracture.