ZA85鎂合金經等通道轉角擠型後顯微組織與機械性質改善之研究

Abstract

本實驗係以重力澆鑄法製成ZA85 (Mg-8wt%Zn-5wt%Al) 鎂合金鑄錠，獲得平均晶粒大小約為150μm。使用EDS進行成份分析，主要析出相經由XRD證實為τ-Mg32(Al,Zn)49，並無β-Mg17Al12相析出。接著將實驗合金經ECAE製程後，以OM、SEM觀察顯微組織的變化，並以洛氏硬度計以及拉伸試驗機探討其常溫和高溫機械性質。 研究結果顯示，高溫下對ZA85鎂合金進行ECAE，使晶粒細化的機制為動態再結晶。在較高溫的250℃下進行ECAE，經過兩次擠型後平均晶粒尺寸可細化至13.5μm，然而，進一步的擠型至四次後，晶粒因為溫度效應而成長至19.7μm；在較低溫的180℃進行ECAE，則不會有晶粒成長的現象，經過六次擠型後可將晶粒細化至4.2μm。 ZA85鎂合金的室溫、高溫強度以及延性皆會隨著ECAE次數的增加而明顯提升，本實驗在180℃進行六次ECAE後，可獲得最佳的性質：硬度值由N=0的HRB 19增加至HRB 46。常溫拉伸性質的部分，最大抗拉強度由175MPa增加至402MPa；降伏強度由131MPa增加至281MPa；伸長率由2.3%增加至6.4%。高溫拉伸性質的部分，100℃的最大抗拉強度由139MPa增加至318MPa；降伏強度由94MPa增加至197MPa；伸長率由3.9%增加至14.6%，200℃的最大抗拉強度由105MPa增加至249MPa；降伏強度由74MPa增加至162MPa；伸長率由5.1%增加至28.5%。

In this research, the effect of equal channel angular extrusion (ECAE) on microstructures and mechanical properties of as-cast magnesium alloy ZA85 (Mg-8wt%Zn-5wt%Al) with the initial grain size of about 150μm was investigated. Energy dispersive system (EDS) was used for chemical composition analysis. The second phase of as-cast ZA85 alloy confirmed by x-ray diffraction pattern (XRD) was mainly τ-phase (Mg32(Al,Zn)49), instead of β-phase (Mg17Al12). The microstructures were examined by optical microscopy (OM) and scanning electron microscopy (SEM). Rockwell Indenter and Instron 8501 were used for mechanical tests. During ECAE process, the mechanism of grain refinement was dynamic recrystallization. At higher ECAE temperature such as 250℃, the average grain size was reduced to 13.5μm by two-passes. However, with the further passes up to four, the average grain size was increased to 19.7μm due to the grain growth of high temperature effect. On the other hand, at lower ECAE temperature such as 180℃, the grains were refined significantly from 150μm to 4.2μm by six-passes. Both the strength and ductility of ZA85 alloy could be remarkably improved by ECAE process. The best parameter of this research was ECAE at 180℃ through six-passes. The hardness of as cast alloy was promoted from HRB 19 up to HRB 46. The UTS and YS at room temperature were increased from 175MPa and 131MPa to 402MPa and 281MPa, respectively. The elongation was also increased from 2.3% to 6.4%. At high temperature tensile tests, the UTS, YS and elongation at 100℃ were increased from 139MPa, 94MPa and 3.9% to 318MPa, 197MPa and 14.6%, respectively. The UTS, YS and elongation at 200℃ were increased from 105MPa, 74MPa and 5.1% to 249MPa, 162MPa and 28.5%, respectively.