| 其他摘要 | Magnesium alloys have attracted increasing interest from automotive and aeronautic industries due to their low density, high strength/stiffness to weight ratio, good damping capacity, diecastability and recycling potential. Compared with aluminum alloy, the applications of magnesium alloy are limited because of their poor ductility, workability and corrosion resistance. Refining the microstructure by means of rapid solidification (RS) process is one of the highly potential approaches to improve the mechanical properties. In this dissertation, copper-mould casting process was utilized to prepare RS samples. Microstructure was characterized by x-ray diffraction (XRD), optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). Compression tests and hardness test were carried out to measure mechanical properties of these alloys. Present research includes the following aspects:
RS AZ91HP samples with promoted mechanical properties were prepared. RS and conventional casting (CC) alloy display yield strength of 183MPa and 136MPa, as well as a plastic strain of 3.6% and 3.1%, respectively. The improvement of mechanical properties resulted from the evolution of microstucture, such as grain refinement, the secondary dendrite arm spacing (SDAS) decreasing and reduction of microporosity. Comparied with CC alloy, RS AZ91HP alloy has good mechanical properties.
The corrosion properties of CC alloy and RS alloy in different NaCl solutions are studied by static weight loss test and polarization curve measurement. The results show that RS alloy has better corrosion resistance. This is attributed to (1) a higher volume fraction of β-Mg17Al12 phase, (2) chemical homogeneity of -Mg phase, and (3) less porosity. This study shows that evalution of microstucture resulted from rapid solidification can influence the corrosion performance significantly.
Because of the need for better mechanical properties, heat treatment was applied to RS alloy. During the solid solution process, brittle β-Mg17Al12 phase dissolved in the matrix, which made the strength decreased and the ductility increased. During artificial aging process, the morphology and the amount of non-continuous β-Mg17Al12 phase changed with aging time. β-Mg17Al12 phase precipitated in the forms of cell and lamellar along grain boundaries. Continuous β-Mg17Al12 phase precipitated within the grains with the increase of aging time. After artificial aging, the strength was increased and the ductility was decreased. The morphology and the amount of β-Mg17Al12 phase were proved to have a significant effect on the mechanical properties of the alloys.
The RS Mg-Zn-Ce alloy was synthesized. The experiment results showed that RS alloy exhibited compressive yield strength, fracture strength and elongation as high as 245MPa, 430MPa and 10%, respectively. Improvement of mechanical properties should be attributed to the evolution of the microstructure, i.e. grain size was refined, homogeneously distributed eutectic phase was obtained, dendrite arm spacing decreased and volume fraction of divorced eutectic increased. Grain refinement can be used to explain the mechanical properties difference between rapid solidification and conventional casting.
To further improve the mechanical properties of RS Mg-Zn-Ce alloy, minor Ag was added. The improvement of compressive yield strength can be attributed to the solid solution of Ag in the matrix. Moreover, the decrease of SDAS resulted in secondary phase strengthening. RS alloys contain a considerable amount of mircoporosity, which may lead to the variability of mechanical properties. |
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