含铝镁合金中针状稀土相球化机制及其控制

Rare earth alloying has already become an important measure of improving magnesium alloy properties. However, the acicular morphology, of rare earth-enriched phases in magnesium alloy containing aluminum, is very harmful to the alloy's toughness and ductility. So, spheroidization of the acicular phases is very urgent. Liquid quenching techniques will be used for preparation of diverse specimen containing rare earth-enriched phases during the phases precipitating, nucleating and growing stages. Relation between character of modification elements and nucleation incubation period, nucleation order, distribution, amount and dissolution, diffusion, distribution of solute and structure, grow orientation, mechanism of crystal nucleus among diverse specimen will be studied, with TEM, SEM, electron probe analyzer and phase structure identification, morphology observation, quantitative metallographic analysis. Nucleation thermodynamic of the rare earth-enriched phases will also be investigated using Miedema Moldle. Through these thermodynamic and kinetic analyses, critical factors of influencing rare earth-enriched phase spheroidization will be discovered and spheroidization mechanism of acicular rare earth-enriched phases will also be revealed. Then, optimized study on chemical composition of the nodularizer will be carried out and shape controlling of rare earth-enriched phases will be achieved. Finally, Theoretical basis, for composition design and preparation of high-performance magnesium alloy containing aluminum, will be provided and secondary phase spheroidization theory in metal materials will be expanded.

稀土合金化已成为提高镁合金性能的重要途径。但含铝镁合金中稀土相的针状形态严重制约着合金机械性能特别是塑性和强韧性的有效提高。如何将针状相球化，是当前亟待解决的问题。本项目采用液淬技术获得稀土相析出、形核、长大各阶段的试样；利用TEM、高分辨SEM、电子探针等手段，以及相结构鉴定、形貌观察、定量金相等方法，分析针状稀土相与前期工作得到的球状稀土相之间及其它不同形态稀土相之间形核孕育期、形核次序、分布与数量，溶质溶解、扩散与分布，晶核结构、生长取向，长大机制等不同特性及其与变质元素性质之间的本质联系；借助Miedema等热力学模型分析稀土相形核热力学。通过以上热力学、动力学分析，确定决定针状相球化的关键因素，揭示针状稀土相的球化机制。在此基础上，优化稀土相球化剂成分，对稀土相的形态实现有效控制，为高性能球状稀土相含铝镁合金的成分设计与制备提供理论依据，并丰富金属材料第二相球化理论。

本项目对AZ31+1%RE合金中针状稀土相形成机理、针状相的球化及其机理、球化剂的优化进行了系统研究，主要结果如下：（1）热力学计算结果表明合金体系中Al-RE混合焓最小， Al11RE3作为领先相在976.81K时优先析出；晶面错配度的计算结果显示Al11RE3不能作为基体相的异质形核核心，故其沿晶界分布；针状稀土相由Al-Al11RE3共晶反应生成，晶型为正交点阵，优先生长方向是[01-1]。（2）AZ31+1%RE合金中加入1wt%的Sb后，长度为20～50μm的针状相基本被直径5μm左右的球状RE-Sb相取代。热力学计算和液淬试验结果表明RE-Sb相优先于Mg-Sb相和Al-RE相形核长大，Sb对针状稀土相形貌的改善是相选择的结果；晶面错配度的计算结果表明CeSb相可作为镁基体形核衬底，故分布于晶粒内部； {111}、{100}、{110}晶面生长为近球状多面体CeSb相的显露面。（3）针状稀土相的球化剂优化为无毒性的Al-P中间合金，最佳加入量为2.5wt%，合金屈服强度、延伸率与原来相比分别提高9.8%和23.7%。 AlP相的{111}面和Al11Ce3相的{112}面晶格错配度δ=0.78%，AlP相可作为Al11Ce3 相的异质形核核心，改变针状相的形貌。综上所述，本项目确定了针状稀土相球化的关键因素，实现了对针状稀土相的有效控制，并对球化剂成功进行了优化，可为高性能含铝镁合金的设计与开发提供理论指导。