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Eutectic modification of Al-Si casting alloys
The change of microstructure of eutectic silicon from plate- to coral-like in Al-Si casting alloys is well known for enhancing their ductility. This is achieved by adding low concentrations of a modifying agent. Amongst the elements proposed as modifiers, only strontium, sodium and europium induce a plate-to-coral transition, while others such as ytterbium, only refine the silicon plates. The exact mechanism for the remarkable plate-to-coral change, and the reason why certain elements only refine the structure, is still not completely understood. In this investigation, atom probe tomography and transmission electron microscopy were used to analyze and compare the crystal structure and the distribution of solute atoms in silicon at the atomic level. An unmodified alloy and alloys modified by strontium, sodium, europium and ytterbium were studied. Elements inducing silicon plate-to-coral transition were found to contain nanometer sized clusters at the defects in silicon with stoichiometries corresponding to compounds formed at the ternary eutectic reaction of each system. In contrast ytterbium, that only refines the silicon plates, is unable to form clusters in silicon. It is propose that the formation of clusters of AlSiNa, Al2Si2Sr and Al2Si2Eu at the silicon/liquid interface during solidification restrict silicon growth and increase growth direction diversity. The incorporation of clusters explains the high density of crystallographic defects and the structural modification. ; Die Zugabe geringer Konzentrationen eines Veredelungselementes in Al-Si-Gusslegierungen führt zu einer für die Duktilität günstigen Morphologieänderung des eutektischen Silizium. Dabei bewirken die Veredelungselemente Strontium, Natrium und Europium einen Übergang von einer platten- zu einer korallenartigen Morphologie, während andere wie Ytterbium nur das Eutektikum verfeinern. Der zu Grunde liegende Mechanismus ist noch nicht vollständig verstanden. In dieser Arbeit wurden mit Hilfe der Atomsondentomographie und der ...
Eutectic modification of Al-Si casting alloys
The change of microstructure of eutectic silicon from plate- to coral-like in Al-Si casting alloys is well known for enhancing their ductility. This is achieved by adding low concentrations of a modifying agent. Amongst the elements proposed as modifiers, only strontium, sodium and europium induce a plate-to-coral transition, while others such as ytterbium, only refine the silicon plates. The exact mechanism for the remarkable plate-to-coral change, and the reason why certain elements only refine the structure, is still not completely understood. In this investigation, atom probe tomography and transmission electron microscopy were used to analyze and compare the crystal structure and the distribution of solute atoms in silicon at the atomic level. An unmodified alloy and alloys modified by strontium, sodium, europium and ytterbium were studied. Elements inducing silicon plate-to-coral transition were found to contain nanometer sized clusters at the defects in silicon with stoichiometries corresponding to compounds formed at the ternary eutectic reaction of each system. In contrast ytterbium, that only refines the silicon plates, is unable to form clusters in silicon. It is propose that the formation of clusters of AlSiNa, Al2Si2Sr and Al2Si2Eu at the silicon/liquid interface during solidification restrict silicon growth and increase growth direction diversity. The incorporation of clusters explains the high density of crystallographic defects and the structural modification. ; Die Zugabe geringer Konzentrationen eines Veredelungselementes in Al-Si-Gusslegierungen führt zu einer für die Duktilität günstigen Morphologieänderung des eutektischen Silizium. Dabei bewirken die Veredelungselemente Strontium, Natrium und Europium einen Übergang von einer platten- zu einer korallenartigen Morphologie, während andere wie Ytterbium nur das Eutektikum verfeinern. Der zu Grunde liegende Mechanismus ist noch nicht vollständig verstanden. In dieser Arbeit wurden mit Hilfe der Atomsondentomographie und der ...
Eutectic modification of Al-Si casting alloys
Barrirero, Jenifer (author) / Mücklich, Frank
2019-01-01
hdl:20.500.11880/28316
Theses
Electronic Resource
English
Eutectic modification of Al-Si casting alloys
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