Molecular dynamics simulations to identify atomistic deformationmechanisms in two-phase lamellar TiAl alloys

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Modern structural materials usually display a hierarchical microstructure, and hence interdependent deformation mechanisms on different length-scales, which determine the macroscopic behaviour. Using computational material models with representative volume elements or suitable homogenisation methods, we can identify the relevant microstructural parameters that dominate this deformation behaviour. However, very often the understanding of defect-microstructure interactions on the smallest length scale is missing. This understanding is crucial for a purposeful improvement of continuum models and a systematic optimisation of the microstructure. Of highest importance in this context is to determine the significance of different processes such as dislocation motion and twinning and the conditions under which they occur. These conditions can be varied systematically within atomistic simulations, and the resulting processes can be analysed.In the project at hand deformation and fracture of lamellar microstructures in two-phase TiAl alloys is investigated using molecular dynamics simulations. The ratio ofstrength and deformability of TiAl alloys can be optimized by creating lamellar microstructures. They are characterised by additional microstructural parameters besides the grain size, such as the spacing of alpha-2 lamellae within the grains, the thickness of the gamma-lamellae between the alpha-2 lamellae, and the sequence and frequency of certain gamma-gamma interfaces (pseudo-twin, rotational boundary, true twin). All these parameters contribute to the confinement of dislocation motion, respectively twin formation, in a manner which is still unresolved. Experimentally, a Hall-Petch-type strengthening behaviour is observed. In this project its range of validity and the way in which the individual microstructural parameters contribute to it, will be clarified. The questions, which length- scale in a multiphase, nano-structured alloy dominates the strength and toughness, and which critical values control twin formation and dislocation motion, are of general importance for formulating quantitative multiscale models of deformation and fracture in hierarchical microstructures.