Dislocations at ductile/plastic crack tips: in-situ TEM observations

In-situ observations of dislocation structures ahead of crack tips in TEM metal foils are reviewed. Two cases are compared in particular: Structure development during in-situ straining to failure of (i) electron-transparent foils ahead of the tip of a growing crack that spreads from the thinnest regions or perforations and (ii) initially non-transparent thick foils. In the latter case cracks formed only after substantial in-situ straining, and they propagated along dislocation cell walls via repeated stimulated crack nucleation ahead of the tip. This behavior was shown to adequately simulate bulk behavior and such cracks do not exhibit dislocation-free zones at their tips. By contrast, dislocation-free regions along ligaments formed by crack propagation and observed in thin (e.g. about 100 nm or less) TEM foils an found to be artifacts due to strong dislocation image forces. These image forces at the same time limit mutual dislocation interactions to the thickness of the foil, and rotate the dislocations to be normal to the foil plane, meanwhile straightening them. This behavior has no correspondence to conditions at real cracks in bulk materials. Theoretical expressions are derived for the dislocation densities ahead of crack tips that give rise to long-range and shorter range stress fields in mode I crack tip configurations, respectively.