| Summary: | 碩士 === 國立中央大學 === 機械工程研究所 === 94 === Abstract
Modification of the Al-Si eutectic from a flake-like to a fine fibrous silicon structure can be achieved in two ways ; by addition of certain elements(chemical modification) or with a rapid cooling rate(quench modification). Several elements are known to cause chemical modification. The most common elements used in industry today are Sr and Na, which change the silicon morphology from coarse and plate-like to fine and fibrous, and Sb which causes a refinement in the plate-like silicon structure. The aim of the present paper was to investigate the individual effects of rare earth element La and Ce on the eutectic microstructure and mechanical properties in A356 (Al-7Si-0.35Mg) alloys.
In this study, eutectic microstructures and mechanical properties in samples modified by minor rare earth La and Ce in an A356 alloy have been determined by optical microscopy(OM), scanning electron microscopy(SEM),thermal analysis, image analysis, hardness test and tensile test.
The fine fibrous eutectic structure that was obtained with La additions is typical of that obtained with conventional modifiers such as Sr and Na. Modification on the eutectic microstructure and mechanical properties improves with increasing additions of La until a fully modified eutectic structure is obtained at 1.0 wt.% La.
Large additions of Ce did not modify the eutectic silicon and only resulted in a small refinement of the plate-like morphology. The behavior of the majority of rare earth element Ce is therefore more similar to that of Sb than Sr or Na.
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