| Summary: | 博士 === 國立中央大學 === 機械工程學系 === 103 === The main precipitation strengthening phases of heat treatable Al-4.6Cu-Mg-Ag alloys are Ω and θ’. The thermal stable Ω phase precipitates on the {111}α planes, the primary slip planes of the aluminum alloy and possess excellent mechanical strength under 200℃. Al-4.6Cu-Mg-Ag alloys are widely applied for moderate-temperature and high-strength applications in the aviation and military industries because of their excellent strength and thermal stability. The Mg-Ag clusters and the dislocations are the nucleation sites of Ω and θ’. The Cu/Mg ratio affects the relative distribution of Ω, S’ and θ’ phases, and influences the thermal stability of Al-4.6Cu-Mg-Ag alloys. The concentration of magnesium is an important factor for precipitation of Ω phase. The cold working introduces numerous dislocations and increases the heterogeneous nucleating sites of θ’ phase. Moreover, the precipitation characteristics of Ω and θ’ phases are also affected by solid solubility of the alloying elements in the matrix.
The Mg-Ag clusters form during natural aging and encourage the precipitation of Ω phase. The cold work introduces lots of dislocations and benefits for the precipitation of θ’ phase, but depresses the formation of Mg-Ag clusters. In order to interpret how the Mg-Ag clusters, dislocations and alloying elements influence the strengthening phases of Al-4.6Cu-Mg-0.5Ag alloy, hence, present works attempt to examine the effects of heat treatment and cold working on the microstructures and mechanical properties of different Cu/Mg ratio alloys, and start with various natural aging period and cold working percentages, subsequently heat treated to T7 and T8. Further, the other purpose of this investigation is to examine the effects of rare earth (scandium, lanthanum, cerium) and transition elements (zirconium) on the microstructures, grain sizes, precipitation specifics and mechanical properties of Al-4.6Cu-
0.3Mg-0.6Ag (A201) after T7 heat treatment. To completely explain the relationship among the process, structure and property, then the features of this high strength aluminum alloy can been understood completely.
The relative microstructure variations were elucidated by the observations of optical microscope (OM), Electron probe X-ray microanalysis (EPMA), differential scanning calorimeter (DSC), electrical conductivity meter (%IACS), transmission electron microscopy (TEM). Mechanical properties were correlated with Rockwell hardness and tensile testing. The results showed that natural aging treatment has little noticeable benefit on the quantity of precipitation strengthening phases and mechanical properties, but it increases the precipitation strengthening rate at the initial stage of artificial aging. Cold working brings more lattice defects and suppresses the precipitation of Ω phase but encourages the θ’ phase. The above-mentioned precipitation phenomena are more obvious in high degrees of cold working and high Cu/Mg ratio alloy. More θ’ phases precipitate and increase the strength, but decrease the ductility. Adding the alloying elements of Sc, Zr, La and Ce to A201 alloys, the Al3Sc, Al3(ScxZr1-x), W(Al8.5-4Cu6.6-4Sc), La-rich and Ce-rich intermetallic compounds form, and the W, La-rich and Ce-rich phase can’t dissolve to matrix, reducing effective quantity of Cu and Mg atoms in α-matrix, disadvantaging the precipitation of Ω and θ’ phases. Although the grain refinement is obtained by the addition of alloying elements, the decrease of strengthening phases lowers the strength but promotes the ductility of A201 alloys.
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