| Summary: | 碩士 === 國立交通大學 === 材料科學與工程學系所 === 101 === The effect of equal channel angular extrusion (ECAE) on the microstructures and mechanical properties of the solution heat treated ZA85 alloy (Mg-8wt.%Zn-5wt.%Al) was investigated in this study. EDS was used for chemical composition analysis and XRD was used for the second phase identification. The second phase is τ phase and its chemical formula is Mg32(Al,Zn)49. The average grain size of as-cast ZA85 was 150μm. The microstructures and the fracture of tensile test sample cross-section were examined by OM and SEM. Rockwell indenter was used for hardness test and Instron 8501 was used for tensile test at room temperature, 100oC and 200oC.
ECAE process was a kind of grain refinement process. In this study, the average grain size reduced from 170μm which was after solution heat treatment to 13.6μm by one pass. The average grain size reduced to 10μm by two passes. The average grain size reduced to 6.8μm by four passes further. Finally, The average grain size reduced to 4.3μm by six passes.
The ultra tensile strength (UTS) and the yield strength (YS) of solution heat treated ZA85 alloy were 243MPa / 152MPa, 206MPa / 125 and 157MPa / 101MPa at room temperature (RT), 100oC and 200oC, respectively. After ECAE process, the UTS and YS increased with ECAE pass until four passes. The UTS and YS were 415MPa /284MPa、361MPa / 206MPa and 261MPa / 173MPa at RT, 100oC and 200oC, respectively. It was obviously that ECAE process could strengthen the solution heated
ZA85 alloy.
The elongation of solution heat treated ZA85 alloy was 4.2%, 6.6% and 7.4% at RT, 100oC and 200oC, respectively. All the fractography were brittle fracture mainly. The maximum elongation at RT, 100oC and 200oC were ECAE six passes. The elongation was 9% and the fractography was very small dimple separated widely at RT. When the temperature was up to 100oC and 200oC, the big dimple occurred. Besides, the elongation was 23.3% and 60% respectively. ECAE process was good at improving the elongation of solution heated ZA85 alloy.
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