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Abstract

Titanium aluminides are among the favorite candidates for structural intermetallics for high-temperature applications because of their high strength and low density. However, the ductility and fracture strength of these materials still has to be improved. In two-phase (γ and α2) lamellar structures, which consist of various interfaces and lamellae on the nano- to micrometer-scale, one focus is on the role of interfaces in the deformation of the microstructure.

For a better interpretation and understanding of experimental observations based on the knowledge of nano-scale deformation mechanisms, such as lamellar boundary sliding and/or dislocation nucleation, an atomistic modeling approach is required. In this study, we have carried out ab-initio density functional theory (DFT) calculations and molecular dynamics (MD) simulations. Special emphasis has been put on both tensile and shear properties of different variants of the lamellar boundaries.

We carried out ab-initio uni-axial mechanical tests to obtain the tensile strength, work of separation and cohesive parameters. Surprisingly, the normal strengths of all different interfaces as well as the single crystalline phases are in the same range, while they have different interfacial energies and works of separation. From the generalized stacking-fault energy surfaces we obtained the theoretical shear strength along various directions. The results showed the occurrence of easy shearing directions and certain invariant properties for different interface variants.

From MD simulations using an embedded-atom method type potential we found out that shear loading of a true twin type γ/γ interface can result in either grain boundary migration, or grain boundary sliding and stacking fault creation, depending on the loading direction. This behaviour can be interpreted based on the ab-initio stacking fault energies. These initial simulations already show that the interplay between interface geometry and loading conditions has an impact on the deformation mechanism.