界面-孔洞交互作用下多晶钛铝基合金微观塑性变形行为的原子模拟

Microstructures, particularly some defects like dislocations, interfaces and voids et al., have decisive effect on the mechanical properties and deformation mechanisms of metallic materials, which is one of the most important areas of investigations in material science. Material computation contributes to penetration insight into the deformation mechanisms, especially for the processes that incapable or hardly reachable experimentally. In the meanwhile, it exhibits a combination of high efficiency and convenience. Lamellar structure is a typical microstructure in TiAl-based alloys, in which different grain boundaries and inter-phase interfaces with complex orientations exist. The co-existence of these complex interfaces has duplex effects on the mechanical properties. On the other hand, the fracture process of metallic materials affected by void growth and coalescence directly, and this is one of the most important reasons for material damage. Moreover, void nucleation easily initiates on or in the vicinity of interfaces. Employing molecular dynamics method, the present study discusses two important issues, the effect of interfaces and voids on the plastic deformation mechanisms and the mechanical properties of TiAl-based alloys. By building a series of polycrystalline simulation boxes, the dynamic evolutionary process of dislocation configurations during the plastic deformation was investigated. The interface mechanical behaviors of the polycrystalline TiAl-based alloys are analyzed with the condition of applied stress, such as emission and propagation of dislocations from interfaces and rotation of interfaces. Then it is established that the constitutive relationship between the intrinsic interface structure parameters including the interface misorientations and mechanical properties. And it is also analyzed that the effect of void distribution and interaction with interfaces on the microscopic deformation mechanisms of polycrystalline TiAl-based alloys. The study aims to explore the dynamic coupling response between intefaces, voids and material properties in service, which can provide perspective guidline to optimizing design of the microstrucutres for improvement of material mechanical properties.

计算机辅助的材料设计有助于从微观层次深入理解显微组织结构，特别是位错、界面、孔洞等缺陷结构，对金属材料力学性能和塑性变形行为的影响机制；尤其是原子尺度的模拟可以再现某些实验难以观测到的过程，且高效便捷。片层组织是TiAl基合金的典型微观组织结构之一，片层组织中存在着的复杂界面对其塑性变形影响很大。孔洞是影响金属结构材料服役性能的另一种典型缺陷，且易于在界面及其附近萌生。本研究采用分子动力学方法模拟含微孔洞的TiAl基多晶体系的塑性变形，研究变形中的位错组态及其演化动力学，分析应力载荷作用下多晶界面处的位错形核扩展、界面转动（迁移）等力学行为；明确界面取向等结构参数与力学性能间的本构关系；揭示孔洞位置及其与界面的间距对体系位错组态及力学行为的影响规律；探索服役条件下界面、孔洞与材料性能的动态耦合响应机制，为改善力学性能的微观组织优化设计提供方向性指导。

TiAl基合金是一种在航空航天等领域具有广泛应用前景的优质轻型结构材料。界面与孔洞是影响TiAl基合金塑性变形及服役性能的典型缺陷组织，本研究采用分子动力学方法针对TiAl基合金中复杂的界面及其与孔洞的交互作用展开探索。. 项目基于重位点阵理论构建含有对称倾斜晶界和孔洞的γ-TiAl双晶模型，基于 Voronoi算法构建包含孔洞的γ-TiAl多晶模型，并对双晶和多晶的拉伸变形进行一系列分子动力学模拟。通过观察拉伸变形中的微观原子构型演变和分析断裂过程，发现晶粒的相对取向和晶界原子结构是影响位错形核临界应力的主要因素。位错优先在取向差角度较大的晶界处形核、扩展，同时裂纹也易于在晶界处形成并沿晶界面扩展。多晶的晶界在拉伸变形中发生显著的扭曲和转动，表明位错协调的界面滑动是其主要微观塑性变形机制。孔洞在晶界中心及附近的存在使得晶界发射位错的临界应力减小，体系更容易通过位错形核、滑移来协调变形，提高了材料的断裂韧性。晶界与孔洞的交互显著影响体系的力学性能及微观变形机制；主要表现为随着孔洞与晶界的相对间距减小，位错不易从晶界处形核，而是优先在晶内或孔洞表面形核；反之，随着间距增大，孔洞与晶界的相互阻碍作用减弱，位错首先在晶界形核。. 本项目构建了含微孔洞的TiAl基多晶分子动力学模型，获得了应力载荷作用下体系拉伸变形中的界面/孔洞原子构型及位错组态的演化规律，从原子尺度阐明界面与孔洞交互作用下TiAl基合金的微观塑性变形机制，有助于进一步理解含微孔洞的TiAl基合金多晶体系的塑性变形行为。