Fracture in nanolamellar materials: continuum and atomistic models with application to titanium aluminides
Molecular statics simulations of crack growth in fully lamellar Ti-Al are performed to elucidate the role of lamellar structure in determining deformation and fracture toughness in nanoscale structures. The lamellar boundaries are highly effective in inhibiting dislocation transfer from one phase into the other. indicating that interfacial dislocation pinning influences the competition between dislocation emission and cleavage. A continuum model for dislocation emission and cleavage fracture of blunted cracks is thus extended to account for dislocation shielding and crack blunting in nanolamellar materials, leading to material classifications of brittle (cleavage with no dislocation emission), ductile (dislocation emission with no cleavage) and quasiductile (dislocation emission followed by cleavage). In the quasiductile regime, the material toughness is predicted to scale with the square root of the lamellar thickness, that is thicker lamellae are tougher, and the number of emitted dislocations at cleavage scales linearly with the lamellar thickness. Simulations of crack growth in nanoscale gamma-TiAl surrounded by alpha(2)-Ti3Al show quasiductile behaviour with the fracture toughness and number of emitted dislocations scaling as predicted by the model. Simulations of crack growth in alpha(2) surrounded by gamma layers show no evidence of cleavage fracture. and hence this phase is ductile. Cracks at the gamma-alpha(2) interface are found to blunt and deflect into the gamma phase, showing that this interface is not a low-toughness boundary. The fracture toughnesses computed for the gamma-TiAl are comparable with those measured experimentally on oriented polysynthetically twinned crystals of Ti-Al. These results indicate that, (i) nanoscale material toughness may scale with grain size owing to the inhibition of dislocation propagation by grain boundaries or interfaces, (ii) the fracture toughness in fully lamellar Ti-Al microstructures is controlled by thin layers of TiAl sandwiched between Ti3Al layers and, (iii) the microcracking observed in these materials may be caused by the spatial variations in TiAl lamellar thickness intrinsic to these microstructures.