| 摘要: | 在全球性輕量化及重環保之潮流下，鎂合金已成為新時代材料的寵兒，鎂及鎂合金已廣泛地作為結構及非結構材料。鎂合金之比剛性，為目前結構用金屬最高者，鎂合金具備吸收電磁波防電磁干擾、散熱快及吸震耐摔等性能，因而大受3C產業之青睞。鎂合金添加鋰元素，除可降低鎂合金之密度，符合輕量化之要求，亦可大幅改善鎂合金之加工性質。 近幾年來，一種嶄新的晶粒細化技術，即等通道彎角擠製（ECAE）技術，被廣泛的研究應用於許多鎂合金材料上，但是對於Mg-Li-Zn合金的研究卻未多見。本研究針對五種Mg-Li-Zn合金添加不同合金元素，分析其微結構、耐蝕性及機械性質之不同。施以ECAE實驗，以探討各種ECAE製程參數對擠製後微結構及機械性質之影響。 Mg-Li-Zn合金之α相主要提供強度及硬度，而β相主要提供延展性。Mg-Li-Zn合金添加Mn，雖會增加耐蝕性，但也造成強度降低及伸長率增加。Mg-Li-Zn合金添加Al量，不但會增進大氣中之耐蝕性，而且會使強度增加，而伸長率卻會減少。Mg-Li-Zn合金經ECAE擠製4道次，以900-100℃-Bc4擠製Mg-9Li-1Zn合金之抗拉強度增加最多(＋41.8MPa)，而伸長量減少量卻最少(-25﹪)。Mg-Li-Zn合金經ECAE加工，若採用同樣擠製方位C及擠製4道次之條件下，LZ111合金之α相微硬度增加幅度22.4%為五種Mg-Li-Zn合金中最大，LZM910合金之β相微硬度之增加幅度28%為最大。實驗材料五種Mg-Li-Zn合金β相晶粒之細化效果，均相當不錯，甚至可達次微米之晶粒。因實驗材料主要透過ECAE加工，將其α相予以分斷及細化於β相中，以阻止β相中之差排移動，而達到分散強化之目的。於高溫拉伸測試時，細化之α相亦可阻止β相晶粒之成長，可增進材料之超塑性行為。利用TEM及EDS分析確認LZ111合金經ECAE加工試棒中有MgLiZn化合物(FCC結構)及其上之退火雙晶，其雙晶面為(111)面；LZ91合金經ECAE加工試棒中有MgZn2化合物(HCP結構)；LAZM9310合金經ECAE加工試棒中有ZnO化合物(HCP結構)。LZ91合金經900-100℃-Bc8擠製試棒，於250℃及1×10-4s-1條件下，進行高溫超塑性拉伸測試，可獲得350%之最高伸長率。LZ91鎂合金於100℃時效10小時有硬度頂峰值，以XRD檢測發現α(0002)主頂峰之額外隆起部份，推測為半穩定之 (MgLi2Zn)相。本研究發現LAZM9310合金於100℃時效10小時之β相有硬度頂峰值，推測為時效強化相AlLi析出物。 Magnesium and magnesium alloys are used in a wide variety of structural and nonstructural applications. It is commonly recognized that magnesium possesses poor formability because of its hexagonal close-packed structure. To make up for this shortcoming and further reduce weight, alloying magnesium with lithium of extremely low density, 0.534 g/cm3, can achieve both goals. In recent years, the equal channel angular extrusion (ECAE) process is an innovative method to refine grain structure on many magnesium alloys. However, Mg-Li-Zn alloys were not much involved such that the relevant literatures are just two. Five Mg-Li-Zn alloys namely Mg-11%Li-1%Zn, Mg-9%Li-1%Zn, Mg-9%Li-1%Zn-0.2%Mn, Mg-9%Li-1%Zn-1%Al-0.2%Mn, and Mg-9%Li-3%Al-1%Zn-0.2%Mn were prepared. These alloys had been processed by equal channel angular extrusion (ECAE), and the subsequent mechanical properties and microstructures were studied. After ECAE process, the room temperature strength was significantly enhanced at a modest cost of elongation reduction. The α phase mainly provides the strength and hardness on Mg-Li-Zn alloys and the β phase provides the ductility. In each of the Mg-Li-Zn alloy, the increase in Mn content raises the saltwater corrosion resistance and elongation but reduces the strength. The increase in Al content raises the air corrosion resistance and strength but reduces the elongation. From the results of these five Mg-Li-Zn alloys processed by ECAE, Mg-9Li-1Zn alloy processed under condition, 900-100℃-Bc4, shows the greatest increase in tensile strength about 41.8 MPa and the least decrease in elongation about 25%. With the same ECAE process, Mg-11%Li-1%Zn alloy shows the greatest increase in Micro-Vickers hardness of α phase about 22.4% and Mg-9%Li-1%Zn-0.2%Mn alloy shows the greatest increase in Micro-Vickers hardness of β phase about 28%. The microstructures of these five Mg-Li-Zn alloys were found out equiaxed subgrain structure and sub-micrometer grain size. The fine particles in Mg-11Li-1Zn alloy specimens processed under condition, 900-100℃-Bc4, had been identified by the TEM and EDS as the MgLiZn, which had been found out the annealing twins and (111) is twin plane. The fine particles in Mg-9Li-1Zn alloy specimens processed under condition, 900-100℃-Bc4, had been identified by the TEM and EDS as the MgZn2. The fine particles in LAZM9310 alloy specimens processed under condition, 900-175℃-C4, had been identified by the TEM and EDS as the ZnO. The grains in both α and β phases are recrystallized due to high strain that accumulated during ECAE processing and the fine recrystallized grains increases the grain boundary area, thereby enhancing grain boundary sliding and superplasticity. The specimens of LZ91 alloy processed under condition, 900-100℃-Bc8, were expected to show superplastic elongation and indeed the highest elongation was 350%, which was tested at 250℃ and initial strain rate of 1×10-4 s-1. All aged specimens of LZ91 alloy were checked by XRD and onlyα and β phases were obviously detectable showing peaks. The specimens treated at 50 ℃/100 hours and 100 ℃/10 hours had shown extra bump adjacent to the main peak ofα(0002). This is speculated to be the metastable (MgLi2Zn) phase. The micro hardness tests were done in all aged specimens of LAZM9310 alloy. The specimens treated at 100 ℃/10 hours had shown higher hardness of β phase. This is speculated to be the AlLi phase. |
