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

现代结构材料通常表现出层次结构，因此在不同长度尺度上相互依赖的变形机制决定了宏观行为。使用具有代表性体积单元的计算材料模型或合适的均质化方法，我们可以确定主导这种变形行为的相关微观结构参数。然而，通常在最小长度尺度上对缺陷-微观结构相互作用的理解是缺失的。这种理解对于有目的地改进连续介质模型和系统地优化微观结构至关重要。在这种情况下，最重要的是确定不同过程的意义，如位错运动和孪生以及它们发生的条件。这些条件可以在原子模拟中系统地变化，并且可以分析结果过程。本文采用分子动力学模拟方法研究了两相TiAl合金层状组织的变形和断裂。通过建立层状组织可以优化TiAl合金的强度和变形率。除了晶粒尺寸外，它们还可以通过其他微观结构参数来表征，例如晶粒内α -2片层的间距、α -2片层之间的γ -片层厚度以及某些γ - γ界面（伪孪晶、旋转边界、真孪晶）的顺序和频率。所有这些参数都有助于限制位错运动，分别形成孪晶，其方式仍未解决。实验中观察到hall - petch型强化行为。在这个项目中，它的有效性范围和个人微观结构参数对它的贡献方式将被澄清。在多相纳米结构合金中，哪个长度尺度主导强度和韧性，哪个临界值控制孪晶形成和位错运动，这些问题对于建立分层微观组织中变形和断裂的定量多尺度模型具有普遍的重要性。

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.