静电场辅助时效对镁铝系镁合金的析出行为调控改性及其作用机制研究

Ageing precipitation strengthening is a very significant way to improve the mechanical properties of cast Mg alloy. However, part of Mg alloys, such as the commonly used low-cost Mg-Al alloy, the discontinuous precipitation mode exists in the ageing process, which is a major problem that greatly reduces the ageing strengthening effect. The traditional micro-alloying method to modify this problem comes to the bottleneck, and thus it is extremely urgent to develop the new technology and theory. The present project for the first time employs the new “electric field aiding ageing” method developed in Al alloy to explore the application into typical Mg-Al alloy. Firstly, the ageing parameters will be optimized to obtain the best strengthening effect, and the ageing microstructures will be characterized using OM/SEM/TEM. The relationship between parameters, microstructures and mechanical properties will be established. Secondly, the effects of electric field on the ageing precipitation kinetics will be revealed by the measurement of electrical conductivity of aged samples. Meanwhile, the effects of electric field on the change in vacancy concentration during ageing process will be revealed by the measurement of positron annihilation lifetime spectra. Finally, comprehensive analysis based on all the data will be conducted to clearly clarify the mechanism by which the electric field influences the ageing precipitation process. The new technology and theory for precipitation modification of Mg-Al alloy not only bases on the application background, but also is of great scientific significance for the development of solid-state phase transformation technology and theory of Mg alloys.

时效析出强化是提高铸造镁合金力学性能的重要手段；但对于以常用Mg-Al系为代表的、在时效过程中存在非连续析出模式的部分镁合金系而言，该析出模式是严重削弱时效强化效果的重大顽疾。目前传统合金化法对此顽疾解决的效果已达瓶颈，发展其调控改性新技术及理论迫在眉睫。本项目首次将铝合金中新发展的静电场辅助时效技术，应用在典型镁铝合金的时效析出调控改性中进行探索研究。首先，优化静电场辅助时效的工艺参数以获得最佳强化效果，以OM/SEM/TEM表征时效组织演变规律，并建立工艺-组织-性能之间的关系；其次，以时效过程样品的电导率测试揭示静电场对析出动力学的影响规律，以正电子湮没寿命谱测试揭示静电场对空位浓度的影响规律；最后，综合分析，从本质上阐明静电场对时效行为的调控改性作用机制。项目获得的镁合金时效析出行为的调控改性新工艺及机理，不仅具有迫切的应用背景，而且对于提升镁合金固态相变技术与理论具有重要的意义。

时效析出强化是提高铸造镁合金力学性能的重要手段；但对于以常用Mg-Al系为代表的、在时效过程中存在非连续析出(discontinuous precipitate, DP)的部分镁合金系而言，该析出模式是严重削弱时效强化效果的重大顽疾。目前传统合金化法对此顽疾解决的效果已达瓶颈，发展其调控改性新技术及理论迫在眉睫。本项目首次将铝合金中新发展的静电场辅助时效技术，应用于典型Mg-Al合金的时效析出调控改性研究。首先，研究了合金化元素Sm、Cu对Mg-Al系合金强韧化行为的影响，其中主要是试图借助Cu元素进行改性DP-Mg17Al12析出行为；其次，重点研究了AZ91镁合金在静电场辅助时效作用下的时效析出行为，优化工艺参数以获得最佳强化效果，建立工艺-组织-性能之间的关系，并基于时效过程样品的电导率变化规律揭示空位浓度演化行为，阐明静电场对Mg-Al系合金时效行为的调控改性作用机制。. 研究结果表明：微量合金化元素Cu对AZ91合金时效析出行为具有一定改性的作用，推测其主要原因是Cu提升了空位浓度以及随后的连续析出相(continuous precipitate, CP)之形核率，从而抑制DP相的生成；然而，含Cu合金出现了无法固溶的大块状Al7Cu3Mg相，对力学性能不利。在AZ91镁合金200°C早期时效过程，施加静电场E=4kV/cm，能提升晶内连续析出相CP的数量密度、且在一定程度上细化DP相，表明能调控改性析出行为，使得峰时效力学强度得以一定程度的提升。基于对时效过程样品的电导率变化进行理论分析，表明可能的作用机制是，静电场有效提升了空位浓度、延缓了空位寿命，因此有利于提升CP相的形核率、抑制或细化DP相。更深入的分析工作尚在进行。. 项目获得的Mg-Al合金时效析出行为的调控改性新工艺及理论，对于提升镁合金热处理技术水平、丰富其固态相变理论、以及扩大镁合金的工业应用具有重要的参考意义。