稀土合金化高Si镁合金中耐热相的共析出行为、相互作用及其组织调控机制

It is very important to effectively control the morphology, size and distribution of the primary Mg2Si and Al-RE phases in the magnesium alloy with high content Si and RE (Rare Earth) elements in order to strengthen and improve the heat-resistant ability, because they are very easy to be coarsing under the conventional casting condition. .In this project, profound and systematic study on the dispersing and co-growth phenomenon of Al-RE and Mg2Si phases which was observed in our earlier research will be performed. The interfacial microstructure, orientation relationship and three-dimensional morphology of the co-growth phases will be quantitatively analyzed by using EBSD (Electron Back Scattering Diffraction), HRTEM (High-Resolution Transmission Electron Microscopy) and reconstruction technique based on the serial-sections method. Meanwhile, the first-principles calculation on atomic-scale will be employed to explore the structure and properties of Al-RE/Mg2Si interface and Mg2Si surface absorbed with RE atoms. The effect of two phases on the growth behavior of each other under special elemental content and in-situ reaction temperature, as well as the effect of RE atoms on the growth of Mg2Si phase will be analyzed by DSC (Differential Scanning Calorimetry) and thermodynamic methods. .The aim of this project is to build the growth model of co-growth phases, and to discover the energic mechanism of heterogeneous nucleation and the dynamic mechanism of primary Mg2Si phases’s growth. This study would be expected to propose a novel modifying method based on adjusting the content of RE elements and the reaction temperature, thus to provide scientific basis for developing high performance, low-cost heat-resistant magnesium alloy and the solidification controlling technique.

稀土合金化高Si镁合金中的初生Mg2Si与Al-RE相在常规铸造条件下容易粗化，对其形态、尺寸与分布的有效控制是综合利用两种耐热相的强化作用和进一步提高镁合金耐热性的关键。本项目针对前期实验观察到Al-RE/Mg2Si相的弥散化、共析出现象展开深入研究，利用EBSD、HRTEM手段和三维重构技术对共析出相的结构、界面取向关系和形貌特征进行显微分析和定量表征，同时采用第一性原理计算，从原子尺度研究共析出相界面、稀土原子吸附Mg2Si表面的结构与特性，结合差热分析和热力学分析，阐明特定成分与温度条件下两种耐热相对各自析出行为的影响规律，以及稀土溶质原子对Mg2Si长大的影响规律，从而建立共析出相的晶体生长模型，揭示镁合金凝固过程中耐热相形核与长大的能量机制和结晶动力学机制。通过本研究，将有望提出基于成分与温度控制的多相合金组织调控方法，为开发高性能低成本耐热镁合金及其凝固控制技术提供科学依据。

合金化是提高金属材料室温和高温强度的重要手段，采用稀土元素对高Si耐热镁合金进行合金化处理，探析和充分利用不同合金相的力学性质和结晶行为是提高稀土相的强化效果和镁合金高温性能的关键。本项目针对镁合金中各种耐热相的共析出机理和协同强化机制开展理论计算和实验研究。.首先，基于相图分析法进行成分设计，通过快速凝固、高压压铸、重力铸造分别制备了具有特定成分配比的Mg-Al-Si-RE合金，综合利用显微分析、相图分析、热力学分析等手段，研究了Mg2Si、CeSi2、Al11Ce3（Al4Ce）、Al4Sr、REMg2Si2等耐热相的形态、尺寸、分布及其对铸态、压铸态和快速凝固试样力学性能的影响规律。其次，采用高分辨显微分析、晶面取向分析手段对合金样品中共析出相的微观形貌、成分、晶体结构，以及共析出相界面的显微结构、晶面取向关系、立体形貌进行分析与表征；同时，采用基于密度泛函理论的第一原理计算方法，对Mg-Al-Si-RE多元合金中CeMg2Si2、Mg2Si、Al-RE、RE-Si、Ca-Si、CaMgSi等主要合金相的结构稳定性、力学和热力学性质等体相性质，Al-Re相、CeMg2Si2、Al4Sr相等与Mg2Si相的界面性质，微量稀土原子(La、Ce、Nd和Sm)、碱土金属原子(Ca、Sr)在Mg2Si(100)和(111)表面的吸附和和掺杂特性进行计算。最后，基于实验和计算数据综合分析和探讨了稀土和碱土元素对高Si镁合金组织和性能的影响机制，发现微量合金元素既能通过形成多种弥散分布的合金相来强化合金基体，又能通过异质形核作用和液固界面吸附毒化来细化Mg2Si相，从而实现协同强化。.本项目研究提出了一种利用多种合金相协同增强机制开发低成本耐高温镁合金的方法，并通过调节镁合金中稀土元素与其他合金元素成分配比、凝固速度、原位反应温度等来调控铸态组织以获得最优综合性能。研究成果为完善稀土耐热镁合金中合金相形态与尺寸的调控提供了科学基础，丰富了金属凝固组织控制理论。