Fe、Si元素在7xxx系铝合金熔体中的成簇特性研究

The coarse Fe rich and Si rich phases in 7xxx-series Aluminum Alloy are damaging to the fracture toughness and corrosion resistance. They are difficult to be dissolved by the subsequent homogenization annealing, thermal deformation, solid solution and aging treatment. Even when the concentration of Fe, Si is very low, they are still can be found in the solidification structure. The molten alloy is not without structure, but with short or medium range orders. As the parent state of solid state, its structure must influence its solidification process. The clustering nature of solute element may also influence its segregation behavior..The synchrotron X ray diffraction, X ray small angle scattering, ab initio molecular dynamics simulation and classical molecular dynamics simulation will be employed to study the clustering natures of trace Fe and Si in molten Al in the present proposal. The effect of alloying elements Zn, Mg and Cu on their clustering natures will also be studied carefully. As a result, the clustering nature and mechanism for Fe and Si in molten 7xxx-series Aluminum Alloy are made clear. Next, the migration mechanism of Fe and Si near the solid / liquid interface will be studied by simulating the crystal growth. The segregation mechanism for Fe and Si elements will be elucidate at atomic level..All the efforts in the present proposal can promote the improvement of theory of segregation and provide some necessary theory to control the size of Fe and Si rich phases in the solidification structure of 7xxx-series Aluminum Alloy.

7xxx系铝合金中粗大的富Fe、富Si相是导致断裂韧性、抗腐蚀性降低的重要因素。它们经过后续的均匀化退火、热加工变形、固溶等热处理后都难以溶解。即便Fe、Si含量极低，凝固组织中还是存在粗大的富Fe、富Si相。合金熔体并非是无结构的，而是短程有序或中程有序。作为固态的母态，熔体的结构必然影响其凝固的过程。溶质元素的成簇特性也必然与其偏析特性存在关联。本项目将利用同步辐射X射线衍射、X射线小角散射、从头算分子动力学模拟以及经典分子动力学模拟相结合来研究微量Fe、Si元素在纯铝熔体中的成簇特性，探索Zn、Mg、Cu元素对其成簇特性的影响规律，阐明Fe、Si元素在7xxx系铝合金熔体中的成簇特性和机制；通过模拟晶体生长，探明Fe、Si元素在固/液界面附近的迁移机制，从原子层次阐明Fe、Si元素偏析的动力学机制,为调控7xxx系铝合金凝固组织中有害的富Fe和富Si相尺寸提供必要的理论基础。

7xxx系铝合金中粗大的富Fe、富Si相是导致断裂韧性、抗腐蚀性降低的重要因素。粗大富Fe、富Si相是凝固时严重的偏析导致的。本项目通过从头算分子动力学（AIMD）模拟和同步辐射X射线小角散射实验（SAXS）研究了一系列二元、三元体系Al合金熔体，探明了Fe、Si在铝/7xxx系铝合金熔体中的成簇特性，发现了熔体结构与平衡分配系数之间的关联性，初步阐释了Fe/Si偏析的动力学机制。.主要结论如下：.(1)强相互作用熔体中，溶质元素为中心多面体倾向相互缔结，形成中程有序结构，如Al-Fe/Al-Co/Al-Ni, Ba-Bi体系；.(2)即使Fe含量很低，中Fe原子为中心的多面体依然会通过共用顶点、棱、面等方式缔结；.(3)三元体系Al-Mn-Si、Al-Fe-Si、Al-Fe-Cu体系溶质为中心多面体会相互缔结；.(4)富Si的铝熔体存在高密度和低密度两种状态；0Pa压强下，Al0.15Si0.85 和Al0.175Si0.825熔体的高密度态和低密度态共存；多态性导致亚稳态的熔体中Si聚集以及不均匀分布；.(5)Al-Zn-X体系中，当X（第四周期过渡族金属）元素从周期表从左向右变换，X与Zn之间的相互作用大体上逐渐减小；Sc与Zn之间相互作用最强。稀土金属La、Ce与Zn元素存在强的相互作用；Mg、Ca、Sr和Zn之间存在强的相互作用；.(6)对于Al-M体系，Fe\Co\Ni为中心多面体为类晶结构，相互缔结形成中程有序，对应k0 < 1，Ti\Cr\V为中心多面体为类二十面体结构，无明显化学序，中程无序，对应k0 > 1；.(7)对于Ni1-xMx体系{M= [Ta、 W、 Re、 Os]， [Mo、Nb、 Ru]，[ Ti 和 Cr]}，当溶质元素为中心的球壳比Ni为中心的球壳致密，k0 < 1，反之依然；.(8)单个Fe为中心类晶多面体使Al晶体生长变慢，相互缔结在一起的Fe为中心多面体阻碍晶体生长；Si为中心多面体使Al晶体生长变慢。