含Mn的Mg-Zn系合金中先析α-Mn相晶体学特征与棒状β'1相析出形核的关联性研究

Mg–Zn alloys, the high strengths of which are mainly determined by the β'1 rod-shaped precipitates, show great potential for the development of low-cost and high-strength precipitation-hardened wrought magnesium alloys. Some studies indicate that the addition of Mn element can result in a significant enhancement in age-hardening and strengths. Mn element distributes dispersively in the Mg matrix mainly as pure α-Mn phases which can act as heterogeneous nucleation sites for the β'1 rod-shaped precipitates during the aging treatment, resulting in a finer distribution of precipitates. Although it has been demonstrated that addition of Mn can enhance the age-hardening response of Mg-Zn alloys, it remains to understand the mechanism how the α-Mn phases can apparently enhance the nucleation rates and, thus, the volume fraction of β'1 rod-shaped precipitates. It is unclear why various forms of α-Mn phases coexist, as well as various orientation relationships coexist between α-Mn and Mg matrix. In this project, the advanced analysis methods such as high-angle annular dark-field scanning transmission electron microscopy (HAADF) and three-dimensional (3D) tomography are used to investigate the microstructure of Mg-Zn-Sn(-Mn) alloy. The crystallographic nature of the coexistence of various forms of the α-Mn phases and various orientation relationships between the α-Mn and the matrix can be clear by the accurate and thorough characterization of the crystal structure, forms and orientation relationship of these α-Mn phases. The mechanism that the α-Mn phases facilitate the nucleation rate of the β'1 rod-shaped precipitates can be revealed via the quantitative characterization of the orientation relationship and interfacial structure among the α-Mn phase, β'1 rods and Mg matrix. This project will enrich the theory of manipulating precipitate shape in magnesium alloys, and provide a foundation on the design of high performance precipitation-hardened magnesium alloys by alloying.

Mg-Zn系合金是一种极具研发前景的低成本、高性能析出强化镁合金体系，其主要强化相为棒状β'1相。添加Mn可以显著增强Mg-Zn系合金的析出强化效果和强度，主要是由于先析出的α-Mn弥散相在时效过程中会成为β'1相的优先形核位置，从而大幅提高了β'1相的形核率和体积分数。然而，对于α-Mn相的形态和晶体学特征及其对β'1相析出形核和长大的影响机制还缺乏充分的认识。基于此，本项目以Mg-Zn-Sn（-Mn）合金为研究对象，通过对α-Mn相形貌和位向关系的准确表征，揭示α-Mn相同时存在多种形貌和多种位向关系的晶体学本质；通过对不同时效阶段α-Mn相与基体、析出相之间位向关系及界面结构原子尺度的定量表征，结合相变晶体学理论，阐述α-Mn相的形貌和界面特性对β'1相析出形核和形貌改变的微观机制。本项目可以丰富镁合金中的析出相调控理论，为通过合金化提高镁合金的性能及开发新合金提供理论依据。

Mn元素添加对于析出相（尤其是棒状β'1相）的形核及形貌调控起到重要作用，然而对于Mn元素的存在形式及其对棒状β'1相形核长大的作用机制还缺乏充分的认识。基于此，本项目以Mg-6Zn-4Sn（-1Mn）高性能变形镁合金为研究对象，主要开展了如下研究：（1）Mg-6Zn-4Sn-1Mn合金中α-Mn相的形态和特征，研究了从铸态经均匀化、热挤压直到固溶处理的过程中Mn元素存在形式的演变过程；（2）研究了Mg-6Zn-4Sn-1Mn合金时效过程中析出相（尤其是棒状β'1相）的组织演变，确定了棒状β'1相与Mg基体、α-Mn相三者之间的位向关系；（3）结合α-Mn相形态和位向关系结果，探讨不同形貌α-Mn析出相的形成和生长机制，分析α-Mn相对棒状β'1相析出形核的作用机制。获得主要结果：（1）Mn主要以单质相存在，对原有物相组成没有影响;（2）Mn能细化铸态枝晶，弥散分布在基体和第二相内；均匀化处理后，残留共晶化合物中有Mn富集，基体内部析出了α-Mn相，呈现杆状和多边形状；挤压后，α-Mn析出相能阻碍晶界迁移，抑制动态再结晶，具有明显细晶作用；固溶后，含Mn合金晶粒尺寸远小于未含Mn合金，先前析出的α-Mn相会发生长大;（3）不论是挤压态还是时效态，含Mn合金力学性能都远大于未含Mn合金。对于挤压态而言，Mn起细化晶粒和弥散强化作用；对于时效态而言，α-Mn颗粒相可以起到阻碍固溶处理态晶粒的长大，时效处理过程中还可以作为β''1杆状相的异质形核核心，促进β'1杆状相的析出;（4）Mn在试验合金中的作用主要包括：改善铸造性能、细化晶粒、异质形核核心和提高室温力学性能等，采用高分辨电子显微技术分析了α-Mn和β'1（MgZn2）杆状相的位向关系，证实了两者呈共格关系，因此α-Mn颗粒在时效过程中可以作为β''1杆状相的异质形核核心。本项目的研究为（微）合金化元素在析出相形核及形貌调控方面的作用有更为深刻的理解，为通过微合金化提高镁合金的性能及开发新型Mg-Zn系高性能镁合金提供理论依据。

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