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Abstract

Excellent combination of high specific strength, low density, and good structural stability makes titanium aluminides (TiAl alloys) promising candidates for aerospace structural materials even at high temperatures. Particularly, the lamellar two-phase TiAl alloys, consisting of fine lamellae of the tetragonal $\gamma$-TiAl and hexagonal $\alpha_2$-Ti$_3$Al phase, seem to provide the best combination of strength and deformability. However, this microstructure of TiAl alloys exhibits low fracture toughness at low temperatures. The $\gamma$-TiAl phase is found to be more susceptible to brittle fracture, while $\alpha_2$-Ti$_3$Al exhibits a somewhat higher resistance to crack growth. The objective of this work is to understand the atomistic details of crack – microstructure interactions which are essential for developing new strategies for improving the effective toughness of the microstructure against crack growth. In this work, we study brittle versus ductile failure mechanisms in $\gamma$-TiAl and $\alpha_2$-Ti$_3$Al, as well as in bicrystals which contain $\gamma$/$\gamma$ and $\alpha_2$/$\gamma$ interfaces. We employ large-scale atomistic simulations and compare our findings to the predictions of anisotropic linear elastic fracture mechanics based Griffith and Rice criteria. We observe significant directional dependence of crack tip mechanisms both in the single crystal or two-phase lamellar microstructures. A change in orientation, or the interaction with a second phase at the internal interfaces of a microstructure, affects both, the mechanism and the resistance to crack propagation. Nevertheless, we observe that anisotropic Griffith and Rice criteria are reliable for determining the direction-dependent crack tip mechanisms if all the available dislocation slip systems and their dependency are well understood and taken into account. Yet, atomistic simulations are necessary to understand crack blunting or shear instabilities other than dislocation emission or cleavage.