GOALI: Atomic Scale Modeling and Experimental Characterization of Non-Basal Deformation Modes in Mg Alloys

技术摘要：本项目将为通过选择性合金化设计强度和延展性良好结合的镁合金提供定量和系统方法提供科学依据。该方法基于非基础变形模式（包括c+a滑移和孪晶）的电子结构计算，包括精确计算合金化引起的化学效应，并结合实验观察和计算预测的验证。将进行从头算和原子模拟来研究(i)促进孪核和孪核生长的张力孪核机制和合金；（ii） Mg中c+a位错的核心结构和迁移率；（iii）合金对c+a位错结构和集体行为的影响。利用传统的透射电镜技术，包括明场和弱光束技术，对变形的Mg和二元Mg合金单晶进行了研究，以了解化学对特定加载形式下基于c+a位错的变形响应的影响。这将提供与计算的连接。此外，位错核心结构的像差校正高分辨率电子显微镜将使与计算结果更直接的联系。实验和计算的结合将提供对镁合金非基底变形的全面理解，化学对其相对容易程度的影响，并为微结构指导和科学的合金设计提供途径。摘要：镁和镁基合金的高强度重量比使其成为交通运输部门的优秀候选人，特别是汽车行业，重点是生产重量更轻，更省油的车辆。然而，有限的室温成形性（例如，通过在模具中成形产生有用形状所需的特征）阻碍了镁合金的广泛使用。在高温下成形会增加成本，降低材料的竞争力。成形性与塑性变形的难易程度有关，这种现象是由称为滑移和/或孪生的原子水平过程促进的。从本质上讲，这些原子级机制是材料对施加外力的反应，这些机制使材料的宏观形状发生永久性变化，称为塑性变形。塑性变形的困难会导致另一种不良反应，即过早破坏/断裂。镁合金的有限成形性与各向异性塑性变形有关。这意味着塑性变形在正在成形的板材的某些方向上很容易，而在其他方向上则不容易。这种各向异性的原因在于触发某些形式的塑性变形所需的应力（或力）的强烈差异，而不是其他形式，这是该合金系统的一个特征。该项目的重点是通过计算和实验相结合的方法来确定合金元素，从而降低这些变形模式之间的临界应力差，促进各向同性塑性变形，从而提高室温成形性。

Technical Abstract: This project will provide a scientific basis for a quantitative and systematic approach to design Magnesium alloys with favorable combinations of strength and ductility through selective alloying. The approach is based on electronic-structure calculations of non-basal deformation modes including c+a slip and twinning including accurately accounting for chemistry effects due to alloying coupled with experimental observation and verification of computational predictions. Ab initio and atomistic simulations will be done to investigate (i) Mechanisms for tension twin nucleation and alloys that promote nucleation and growth of twins (ii) the core structures and mobility of c+a dislocations in Mg (iii) effects of alloying on the structure and collective behavior of c+a dislocations. Conventional transmission electron microscopy including bright field, and weak beam techniques will be conducted on deformed single crystal of Mg and binary Mg alloys to understand the effect of chemistry on c+a dislocations -based deformation response for specific forms of loading. This will provide connections to computations. Furthermore, aberration-corrected high resolution electron microscopy of dislocation core structures will enable a more direct connection with results from computations. The experiments and computations together will provide a comprehensive understanding of non-basal deformation in Mg alloys, the effect of chemistry on its relative ease, and a pathway to microstructurally-guided and scientifically-informed alloy design.Non-technical Abstract: The high strength-to-weight ratio of magnesium and magnesium-based alloys makes them excellent candidates for the transportation sector and in particular, the automotive industry that is focused on producing lighter-weight, more fuel-efficient vehicles. However, limited room temperature formability (a feature required to produce useful shapes by forming in a die for example) has prohibited the widespread use of Mg alloys. Forming at elevated temperatures adds cost and makes the material less competitive. Formability is related to the ease of plastic deformation, a phenomenon that is facilitated by atomic level processes called slip and/or twinning. In essence, these atomic level mechanisms are the material's response to application of external forces and these mechanisms enable permanent macroscopic shape change in a material, referred to as plastic deformation. Difficulty in plastic deformation encourages an alternate undesirable response which is premature failure/fracture. In the case of magnesium alloys, the limited formability is related to anisotropic plastic deformation. This means plastic deformation is easy in some directions of the sheet that is being formed but not in others. The cause of this anisotropy lies in the strong differentials in stress (or force) needed to trigger some forms of plastic deformation as opposed to others, a characteristic of this alloy system. This project is focused on identifying alloying elements using a combination of computations and experiments that can reduce the critical stress differential between these deformation modes to facilitate isotropic plastic deformation and thereby improve room temperature formability.