| Summary: | 碩士 === 國立東華大學 === 材料科學與工程學系 === 101 === In order to extend the application of magnesium alloys in the automotive field, the room and elevated temperature strengths of rare-earth element added magnesium alloys were investigated. In this research, the Mg97Zn1(Mm-La)2 and Mg97Zn1(Mm-Ce)2 alloy billets were extruded into planes. According to the tensile results, both as-extruded alloys possess high tensile yield strength over 300 MPa. Besides, the high temperature tensile test at 473 K, result shows that both alloys still keep high yield strength of 200 MPa. According to the nano-indentation test result, the second phase has much higher hardness than matrix, the strengthening mechanisms were composite strengthening. In XRD and TEM results, both alloys comprise two phases of h.c.p.-Mg and (La,Ce)(Zn,Mg)12. Form the diffraction pattern and high resolution lattice image, the secondary orientation of phase was separated from matrix by anti-phase boundaries, and the anti-phase boundaries are parallel to the (001) plane of secondary phase.
The hot rolling with 30 % reduction at different temperatures of 573 K, 673 K and 773 K were performed. The tensile test results show that the yield strength decreases with increasing hot rolling temperature. The reason is due to the recovery and recrystallization of matrix during high temperature rolling. From the microstructure, the second phases were cracked and refined after hot rolling. The finer dispersion of second phase can be obtained at higher temperature rolling of 773 K. According to the high temperature tensile results, both alloys 80 % hot rolled at 773 K have better elevated temperature strengths than as-extruded. That means optimum high temperature mechanical properties will be achieved after higher rolling reduction.
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