DISLOCATION MODELING AND ACOUSTIC-EMISSION OBSERVATION OF ALTERNATING DUCTILE/BRITTLE EVENTS IN FE-3WT-PERCENT SI CRYSTALS

It has been established that hydrogen assisted subcritical crack growth in Fe-3wt% Si single crystals is discontinuous while accompanied by substantial plasticity. Proposed micromechanisms of this process are addressed via observed fine-scale {100} cleavage features, acoustic emission tracking and computer simulation analysis. The near crack tip stress distribution in an elastic-plastic analysis enabled insight into how dislocation shielding led to mirocrack nucleation. Such discretized computational models include: the stress due to the dislocation self field, the stress from crack-dislocation interactions, the stress from the crack-external stress interaction and the external applied stress. Stress distributions in both macroscopic and microscopic scales of interest were also examined, appropriate to different slip systems. It was found that the stress at the crack tip became slightly compressive while the position of the maximum stress, approaching the theoretical strength shifts to about 2-30 nm from the crack tip. The high stress region may be correlated with the observed 1-mu-m discontinuous crack instabilities as detected by acoustic emission. The mutual feedback from experimental findings related to acoustic emission and crystallographical habits vis-a-vis theoretical aspects are analyzed. This approach results in a micromechanical model which has implications to both hydrogen embrittlement thresholds and the ductile-brittle transition.