THE KINETICS AND MICROMECHANICS OF HYDROGEN-ASSISTED CRACKING IN FE-3 PCT SI SINGLE-CRYSTALS

The kinetic and micromechanical behavior of hydrogen-assisted cracking (HAC) was evaluated on Fe-3 wt pct Si single crystals with {001} <010> and {001} <110> orientations. This was accomplished by sustained load tests in gaseous hydrogen under various temperatures and in humid air at ambient temperature. Severe plastic deformation was observed to accompany the hydrogen-assisted sustained load slow crack growth. The crack growth proceeded by a tunneling behavior in both orientations, indicating that plane strain conditions promoted the kinetic process. Microscopically, the crack propagated discontinuously with a 1-mu-m size instability along <110> directions, as revealed by fractographic and acoustic emission (AE) results. The crack growth rate was found to exhibit a plateau region with regard to the applied stress intensity. In this stage II regime, the growth rate increased with temperature in the lower temperature Arrhenius rate regime, with an apparent activation energy of 25 kj/mol. After reaching its maximum at about 100-degrees-C, the growth rate dropped down rapidly, and no slow crack growth was observed above 160-degrees-C. The experimental observations were analyzed to show that the kinetics of HAC were controlled by hydrogen availability rather than by plasticity. Based on this, a transient kinetic model was applied to fit the data. The present interpretation of the crack tip stress field and the HAC nucleation site strongly implies that long-range diffusion of hydrogen over distances ranging from one to several microns is required. The kinetic aspects of this study are presented here with the underlying micromechanics being considered in terms of a decohesion mechanism.