Place: Materials Science and Engineering MSE, Darmstadt, Germany
To improve the ductility and fracture strength of TiAl alloys, one of the crucial tasks is the correct understanding of the deformation mechanisms on different scales. While the gamma-single crystal with L10 structure shows less ductility than expected, the two phase TiAl-Ti3Al lamellar microstructures including various types of interfaces show an improved ductility. The small tetragonality of the gamma-phase (c/a = 1.02), affects the dislocation based plasticity mechanisms on the one hand, and misorientation at interfaces on the other hand, resulting in two important parameters influencing the deformation mechanisms in this microstructure. In this study, we have carried out ab-initio density functional theory (DFT) calculations and molecular dynamics (MD) simulations using an embedded-atom method type potential on the TiAl lamellar interfaces structure (including the tetragonality of the gamma-phase) to get insight into the atomistic processes during deformation of such microstructures. From the quantitative ab-initio generalized stacking-fault energy surfaces we obtained the theoretical shear strength along various directions. Furthermore, according to the energy landscape at each interface the corresponding dislocation dissociation reactions were analyzed and the probability of their occurrence was evaluated. In the next step, MD simulations were carried out on bicrystal samples as well as multi-lamellar boxes under different loading conditions to study the dynamic behavior of the various interfaces and then the complex microstructure. The results showed that shear loading of the most frequent interface (i.e. the true twin type gamma/gamma) can result in either grain boundary migration, or grain boundary sliding and stacking fault creation, depending on the loading direction. The MD results were interpreted based on the ab-initio gamma-surfaces for each interface. There was a very good consistency between DFT and MD results. These simulations show that the interplay between interface geometry and atomistic structure and loading conditions has an impact on the deformability of the lamellar microstructures.