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

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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.