Nucleation of nanocracks by a quasicleavage process in a dislocation-free zone

The initiation and propagation of nanometre-scale cracks has been investiga ted in detail using dislocation modelling and ill situ transmission electro n microscopy (TEM) observations. for the intermetallic compound Fe3Al under mode I loading. A discrete dislocation model is proposed to assess quasist atic equilibrium, emission of dislocations from the crack tip and shielding of near-tip dislocations. The equilibrium location and number of dislocati ons are determined by a minimum-energy requirement. The in-situ TEM test re vealed the following response. When cracks propagate directly from the thin edge of a double-jet hole, no dislocation is emitted from the crack tip. H owever: in thicker regions of the foils, a large number of dislocations are emitted from the crack tip, and a nano-sized crack is formed in front of t he crack tip region but not at the crack tip. A finite-element method-discr ete dislocation calculation provides insight into how dislocation shielding leads to nanocrack nucleation. It also indicates the emergence of a tensil e stress peak ahead of the crack tip, as the dislocations pile up in the fr ont of crack tip. From TEM observation, the distances between discontinuous nanocracks and the main crack tip were in the range 4-100 nm, which is dep endent on the applied loading, such that the distances increase with increa sing applied stress intensity factor. A gigantic superdislocation and a min idislocation array are used to simulate the effect of grain boundary (or in terface) on the peak stress at crack tip. It was found that grain boundary (or interface) controls the magnitude of the stress peak at the crack tip.