Title: Solidification of Al-based Ternary and Quaternary Eutectic Alloys
Other Titles: Stolling van Al-gebaseerde ternaire en quaternaire eutectische legeringen
Authors: Zhou, Bin
Issue Date: 24-Sep-2014
Abstract: With the increasing demand for stronger and better-performance materials in response to the limited energy supply and the finite natural resources, it is important to understand the microstructure formation in a multicomponent and multiphase alloys. This knowledge can help to tackle the challenges in designing new materials that are typically multicomponent. Eutectic alloys are one of the interesting materials offering superior casting and advantageous mechanical properties. However, the current knowledge is only limited to binary and some ternary systems. Thus the aim of this study is to expand the eutectic growth theory to higher order system, including further investigation of a special divorced eutectic growth in a ternary alloy and the expansion of the knowledge of the coupled eutectic growth to a model quaternary system.Both unconstrained and constrained growth processing techniques are used in this study. The former is realized in a differential scanning calorimeter (DSC) which offers accurate heating and cooling control and thus near isothermal conditions. The latter is achieved either in the advanced solidification processing facility (ASPF) of Department of Materials Engineering, KU Leuven or in the European Space Agency Solidification and Quench Furnace (ESA SQF) installed in the Materials Science Lab (MSL) on the International Space Station (ISS). Two types of three-dimensional (3D) characterization techniques, namely computer tomography (CT) and serial sectioning technique using the focused ion beam technique (FIB), are applied to characterize the intermetallic phase particles in the Al-Mn-Si alloy and the quaternary eutectic structure in the model Al-Cu-Ag-Mg quaternary alloy.Banded structure caused by the symbiotic growth mechanism is successfully achieved in the Al-Mn-Si sample processed in the ESA SQF. The 3D analysis shows that intermetallic particles formed in the Al-Mn-Si eutectic alloy have different sizes, which contradicts the single value predicted by the conventional particle pushing and engulfment transition model (PET). A new mechanism including the effect of TiB2 particles which was neglected in previous studies is proposed and shows qualitative agreement with the experimental observations.Various three-phase and four-phase eutectics are determined in the model Al-Cu-Ag-Mg alloy processed in the DSC. The univariant three-phase eutectics in the quaternary alloy show similar morphologies as their counterparts in the ternary eutectics. The four-phase eutectics in the quaternary alloy are categorized to two groups based on their different constituent eutectic phases and show primarily fibrous morphology. 3D reconstruction of the directionally solidified quaternary eutectic confirms the fibrous structure and unveils the fiber merging and splitting behaviors that are different from the 2D observations. The formation of the complex four-phase eutectic structure is a result of the complex solute distribution process in multicomponent systems and is in agreement with the prediction of Cooksey and Hellawel.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Sustainable Metals Processing and Recycling

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