The initiation and propagation of nm-scale cracks was investigated by means of dislocation modelling and in situ transmission electron microscopic observations of this intermetallic compound under mode-I loading. A discrete dislocation model was proposed in order to assess quasi-static equilibrium, the emission of dislocations from the crack tip and the shielding of near-tip dislocations. The equilibrium location and number of dislocations were determined by applying a minimum-energy requirement. The in situ transmission electron microscopy revealed that, when cracks propagated directly from the thin edge of a double-jet hole, no dislocation was emitted from the crack tip. However, in thicker regions of the foils, large numbers of dislocations were emitted from the crack tip and a nanosized crack formed ahead of the crack tip region; but not at the crack tip. A finite-element discrete-dislocation calculation provided some insight into how dislocation shielding led to nanocrack nucleation. It also indicated the appearance of a tensile stress peak ahead of the crack tip as the dislocations piled up in front of the crack tip. Transmission electron microscopic observations showed that the distance between the discontinuous nanocracks and the main crack-tip ranged from 4 to 100nm. This was dependent upon the applied load, such that the distances increased with increasing applied stress-intensity factor. A giant super-dislocation and a mini-dislocation array were used to simulate the effect of grain boundaries or interfaces upon the peak stress at the crack tip. It was found that the grain boundary or interface controlled the magnitude of the stress peak at the crack tip.

Nucleation of Nanocracks by a Quasicleavage Process in a Dislocation-Free Zone. S.X.Mao, X.P.Li: Philosophical Magazine A, 1999, 79[8], 1817-37