UBC Theses and Dissertations
Characterization of creep and microstructure of novel high-temperature magnesium alloys Fletcher, Michelle Lynn
The desire for fuel efficient, yet high performance, vehicles in the automotive industry has resulted in a high demand for light weight structural materials. Magnesium alloys are one of the lightest structural materials available to engineering designers. Wrought magnesium alloy bars, sections and tubes have been used in the aerospace, electronics and automotive industries, where component weight is of concern. The operating temperature of these components is typically limited to below 100°C, since creep resistance begins to deteriorate above this temperature. Creep deformation in magnesium alloys has been generally contributed to grain boundary sliding and plastic deformation leading to inter-granular failure. This research investigated the creep resistance of five wrought magnesium alloys (AE42, AJ32, AX30, EZ33 and ZE10) developed for elevated temperature automotive applications. Non-conventional techniques were utilized to study the creep resistance of these alloys on the micro and macro scale at temperatures ranging from 25°C to 175°C. Neutron diffraction techniques were utilized to measure alloy texture, total strain and elastic creep strain. Metallographic techniques were subsequently used to analyze microstructural constituents in each alloy. The alloy microstructure was then correlated to the alloy’s creep resistance. The results indicate that the aluminum free magnesium alloys (i.e., EZ33 and ZE10) had higher creep resistance compared to aluminum containing alloys (i.e., AE42, AJ32 and AX30). For the aluminum containing alloys, twinning and formation of a large amount of the Mg₁₇Al₁₂ intermetallic compound likely contributed to a decreased creep resistance. Strontium and calcium were both seen to limit Mg₁₇Al₁₂ formation, thus improving creep resistance of the AJ32 and AX30 alloys with respect to the AE42 alloy, respectively. Both the EZ33 and ZE10 alloys contained nanoprecipitates uniformly dispersed throughout the matrix, possibly contributing to dispersion strengthening and improved creep resistance. The results of neutron diffraction studies suggest that the aluminum containing alloys have experienced unique lattice structure changes on different crystallographic planes. In contrast, the aluminum free alloys had very stable crystallographic lattice strains throughout the duration of creep testing.
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