固溶熱處理ZA85鎂合金經等通道轉角擠製後顯微組織與機械性質改善之研究

Abstract

本研究之ZA85鎂合金(Mg-8wt.%Zn-5wt.%Al)是以重力澆鑄法製成，並使用EDS確認合金成分以及XRD檢定晶出相為τ相，其組成為Mg32(Al,Zn)49，鑄造材之平均晶粒尺寸為150μm。將合金錠切成適當尺寸的試棒後進行固溶熱處理，其參數為335oC持溫48小時，再經ECAE製程，之後以OM、SEM觀察顯微組織與拉伸試片的破斷面，並探討其硬度、室溫與高溫拉伸性質。 ZA85鎂合金晶粒尺寸隨製程變化為從經過固溶熱處理後之平均晶粒尺寸為170μm，ECAE一道次之平均晶粒尺寸為13.6μm，ECAE二道次之平均晶粒尺寸為10μm，ECAE四道次之平均晶粒尺寸為6.8μm，最終ECAE六道次之平均晶粒尺寸為4.3μm。 ZA85鎂合金經過固溶熱處理後的硬度、室溫與高溫拉伸強度皆會隨ECAE製程增加至四道次達最大值，室溫下其硬度為HRB 46，最大抗拉強度與降伏強度 在室溫、100oC與200oC下分別為415MPa / 284MPa、361MPa / 206MPa及261MPa / 173MPa，而ZA85鎂合金經過固溶熱處理後但未進行ECAE製程，其室溫下之硬度為HRB 19，最大抗拉強度與降伏強度在室溫、100oC與200oC下分別為243MPa / 152MPa、206MPa / 125及157MPa / 101MPa，皆比有進行ECAE製程差。 伸長量部分固溶熱處理後經ECAE六道次為最佳，室溫下其伸長量為9%，拉伸試片的破斷面密布細小之酒窩狀組織，而100oC及200oC下其伸長量分別為23.3%與60%，拉伸試片的破斷面為大型酒窩狀組織，而固溶熱處理後但未進行ECAE室溫下其伸長量為4.2%，100oC下其伸長量為6.6%，200oC下其伸長量為7.4%，由拉伸試片破斷面主要為脆性斷裂。

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.