Grain boundary segregation in magnesium alloys and its role in controlling the microstructure and mechanical properties

添加稀土的镁锌合金表现出较强的冷成形性能，这主要是由于在再结晶过程中形成的具有商业吸引力的弱织构。然而，这些合金体系中的织构改性作用主要是由单一合金元素引起的还是由它们的组合引起的，目前还不清楚。由于锌和稀土的复杂相互作用，其确切的潜在机制仍然是一个突出的问题。假设变形过程中对动态再结晶的抑制和静态再结晶过程中溶质偏析效应对晶界运动的延缓是理解稀土对织构影响的关键。基于大原子尺寸失配与镁和高体积溶解度相结合的选择标准，本项目旨在研究几种潜在的稀土元素（Gd、Dy和Er）和非稀土元素（如Ca）的GB偏析特征，这些元素具有相当于稀土金属的大原子尺寸，从而有望显示出稀土织构变化效应。稀土元素相对昂贵，而且在全球范围内不易获得，这一事实使得将Ca作为稀土替代品的研究在技术上具有吸引力。将特别关注溶质阻力对改变需要边界迁移的常见再结晶和生长机制的影响，以及这如何影响退火过程中织构和微观结构的发展。除了单一合金元素对二元合金的影响外，还将研究稀土和锌在三元合金中的协同效应和不同浓度比的影响。该工作计划将利用强大的结构和化学表征技术，如3D原子探针断层扫描，以及先进的各向异性晶粒生长水平集建模，以了解选择性晶粒生长过程及其对织构形成的影响。该项目的研究成果将为一种新的合金设计理念提供良好的物理基础，该理念利用GB偏析来调整镁合金在热机械加工过程中的微观结构，以获得提高强度和延展性的独特组合。

Magnesium-zinc alloys containing rare earth (RE) additions exhibit enhanced cold formability owing mostly to commercially attractive weak textures developed upon recrystallization. However, it is still unclear whether the texture modification effect in these alloy systems arises primarily from single alloying elements or from their combination. Due to the complex interplay of Zn and RE the exact underlying mechanisms remain an outstanding issue. It is hypothesized that suppression of dynamic recrystallization during deformation and retardation of grain boundary motion during static recrystallization due to solute segregation effects play a key role in understanding the rare earth effect on texture. Based on a selection criterion combining large atomic size misfit with magnesium and high bulk solubility, the proposed project aims to investigate the GB segregation characteristics of several potential RE (Gd, Dy and Er) and non-RE elements, such as Ca that has a large atomic size equivalent to RE metals, and is thus expected to show the RE texture change effect. The fact that RE elements are relatively expensive and not readily available across the globe renders the investigation of Ca as a substitute for RE technologically attractive. Special attention will be given to resulting solute drag effects on altering common recrystallization and growth mechanisms requiring boundary migration, and how this affects the development of texture and microstructure during annealing. Beside the effect of single alloying elements in binary alloys, synergy effects and effects of varying the concentration ratios of RE and Zn in ternary alloys will be investigated. The work program will utilize powerful structural and chemical characterization techniques, such as 3D atom probe tomography, as well as advanced level-set modeling of anisotropic grain growth to understand the process of selective grain growth and its effect on texture formation. It is highly anticipated that the research findings of the project will lead to sound physical basis for a new alloy design concept exploiting GB segregation for tailoring the microstructure of Mg alloys during thermomechanical processing to obtain a unique combination of improved strength and ductility.