基于孪生调控时效硬化型Mg-Gd(-Ag/Zn)合金沉淀相析出行为的研究

Lightweight high-performance Mg alloys are in great demand for reducing the weight of equipment. Material design and microstructural regulation of high-performance Mg alloys have always been key research topics. The recent reports indicate that twinning can promote the precipitation of Mg-Al and Mg-Zn based alloys, however, the effect of twinning on the precipitation behavior of coherent precipitates in Mg-rare earth alloys remains unclear. In the present application, representatively age-hardenable Mg-Gd(-Ag/Zn) alloys are selected as the research objects. Firstly, the evolution and transformation of precipitates inside the twins, at the twin boundaries or in the Mg matrix during ageing will be systematically investigated, as to reveal the role of twinning on the precipitation behavior of Mg-Gd(-Ag/Zn) alloys. Then, the crystal structures and compositions of the precipitate nucleation sites at the twin boundaries will be studied at the atomic level, aiming to disclose the heterogenous nucleation mechanisms of the precipitates at the twin boundaries in the Mg-Gd(-Ag/Zn) alloys. Finally, the mechanical properties of Mg-Gd(-Ag/Zn) alloys with different pre-twin structures will be tested and the related microstructural features, deformation/fracture modes will be comprehensively analyzed, in order to find out the strengthening and toughening mechanism of twin-precipitate structure in the Mg-Gd(-Ag/Zn) alloys. The results obtained from this project contribute to a deeper understanding of phase transformation in Mg alloys and providing guidelines for tailoring microstructures and mechanical properties of Mg alloys via twinning.

轻质高性能镁合金在轻量化装备上有巨大需求，高性能镁合金的材料设计与组织调控一直是镁合金领域研究的重点。最近研究报道表明，孪生能促进Mg-Al和Mg-Zn系合金沉淀相析出，然而孪生如何调控以共格沉淀相为主的镁稀土合金析出行为尚不清楚。本申请以最具代表的时效硬化型Mg-Gd(-Ag/Zn)合金为研究对象，系统研究预变形后沉淀相时效过程中在孪晶内、孪晶界和基体中的演化与转变，探明孪生调控Mg-Gd(-Ag/Zn)合金沉淀相析出行为的规律；通过原子尺度结构和成分分析，揭示Mg-Gd(-Ag/Zn)合金沉淀相在孪晶界非均匀形核的微观机理；通过调控时效前预孪晶组织，深入分析材料的组织特征、力学性能、变形和断裂模式，阐明含孪生-沉淀相结构Mg-Gd(-Ag/Zn)合金的强韧化规律。通过本项目的研究可提高镁合金析出相变的理论认识，为利用孪生调控镁合金组织和力学性能提供重要的理论指导。

近年来，孪生影响非共格沉淀相相变行为的作用已有多处报道，然而孪生如何影响共格沉淀相的析出及相变机理并不清楚。本项目以具有代表性共格沉淀相的Mg-Gd(-Ag/Zn)合金为研究对象，系统研究了预变形对Mg-Gd(-Ag/Zn）合金孪晶组织的影响，孪晶对Mg-Gd(-Ag/Zn)合金沉淀相析出行为的影响以及孪晶内/孪晶界上沉淀相非均匀形核的微观机理。研究发现，采用5%小应变的交叉压缩可在具有随机取向的Mg-Gd(-Ag/Zn)合金中形成12%-18%的拉伸孪晶组织；含孪晶样品的时效硬化速率显著提升，峰时效态孪晶内部的沉淀相类型发生转变，Mg-Gd-Ag合金由单一γ''沉淀相转变为γ''、γ和β'三相组织，Mg-Gd-Zn合金由γ''转变为γ'相，同时孪晶内部沉淀相数密度较基体沉淀相显著提高；原子尺度分析表明 a位错及c+a位错分解形成的I1和I2层错可诱导特定沉淀相非均匀形核并促进其生长，从而可通过孪生来调控Mg-Gd(-Ag/Zn)合金的析出行为及力学性能。通过本项目的研究，进一步提升了孪生调控析出相变的认识，为进一步利用孪晶获得强韧兼备的镁合金提供了新的思路。