Molecular statics simulations were made of crack growth in fully lamellar alloys in order to determine the role played by lamellar structures in determining the deformation and fracture toughness behaviours of nanoscale structures. The lamellar boundaries were very effective in inhibiting dislocation transfer from one phase into the other; thus indicating that interfacial dislocation pinning influenced the competition between dislocation emission and cleavage. A continuum model for the dislocation emission and cleavage fracture of blunted cracks was extended so as to account for dislocation shielding and crack-blunting in nano-lamellar materials. This led to their classification into brittle (cleavage with no dislocation emission), ductile (dislocation emission with no cleavage) or quasi-ductile (dislocation emission followed by cleavage). In the quasi-ductile regime, the toughness was predicted to depend upon the square root of the lamellar thickness, and the number of emitted dislocations at cleavage scaled linearly with the lamellar thickness. Simulations of crack growth in nanoscale γ-TiAl, surrounded by α2-Ti3Al, revealed a quasi-ductile behaviour; with the fracture toughness and number of emitted dislocations scaling as predicted by the model. The overall results indicated that nanoscale toughness could scale with the grain size, due to the inhibition of dislocation propagation by grain boundaries or interfaces.

Fracture in Nanolamellar Materials - Continuum and Atomistic Models with Application to Titanium Aluminides. A.Ramasubramaniam, W.A.Curtin, D.Farkas: Philosophical Magazine A, 2002, 82[12], 2397-417