AZ80N與LZ101鎂合金之熱機處理與表面鍍膜

Abstract

本研究針對AZ80N (AZ80+2wt%Li)和LZ101 (Mg-10Li-0.5Zn)鎂合金探討其微結構、析出行為、與機械性質等相關特性，同時分別利用原子沉積技術沉積LiAlO2薄膜和磁控濺鍍技術與原子沉積技術沉積Al/Al2O3薄膜於LZ101基材，將針對其化學成份、微結構、抗腐蝕、磨耗和坑穴沖蝕進行研究。 實驗結果顯示，擠型材AZ80N鎂合金相對於AZ80有較佳的延展性與韌性。AZ80N合金經400°C固溶處理後，於170°C-100小時與250°C-8小時時效處理，分別析出AlLi+Mg17Al12和Mg17Al12析出物，且皆呈現明顯的析出強化現象，其強度分別達370MPa和350MPa。同時，擠型材AZ80N鎂合金於170°C-48小時與250°C-3小時時效處理，也皆呈現明顯的析出強化效果，其強度分別達350MPa和340MPa。另外，AZ80N合金經400°C固溶處理後，藉由不同輥壓量與擠製溫度變化，與固溶材比較鎂合金基地有再結晶發生，呈現晶粒細化之效果，60%輥壓量強度可達375MPa。然而AZ80N-T6和AZ80N輥壓材有較差的延展性乃由於大量析出物導致差排與雙晶不易滑移，使之延展性較差。而AZ80N-T5有較佳的延展性，此結果將可有效提昇工程應用。 此外，鎂鋰合金有良好的成形性與較低密度，但是強度較低，不利工程應用。因此本研究探討Mg-10Li-0.5Zn (LZ101)鎂鋰合金施以熱處理，針對其析出行為深入探討。然而，由於鎂鋰合金有極差之抗腐蝕、磨耗及坑穴沖蝕限制其應用。因此本研究亦利用原子層沈積技術(ALD)於鎂鋰合金上鍍上65-200nm 的Al2O3 薄膜，由於鋰原子的擴散，將使原子沉積LiAlO2薄膜於鎂鋰合金，而非Al2O3薄膜。實驗結果顯示，LiAlO2薄膜為非晶質結構，且原子比為Li:Al:O = 1:1:2。LiAlO2薄膜有好的抗腐蝕性，較低的磨擦係數，高的H/E比，強的附著性，平整度。然而，由於鋰原子擴散，使之不易瞭解原子沉積Al2O3薄膜對鎂鋰合金之抗腐蝕等特性，因此本研究亦藉由R.F.濺鍍技術於鎂鋰合金先鍍上一層Al薄膜，之後也藉由原子層積技術鍍上一層Al2O3薄膜，結果顯示相對於單層，雙層亦有更佳的抗腐蝕特性，也更能提升Mg-10Li-0.5Zn 鎂鋰合金之工業上發展與應用。

In the present study, we study microstructures, precipitation behaviors and mechanical properties of as-extruded AZ80N (AZ80+2wt.%Li) and LZ101 (Mg-10Li-0.5Zn) Mg alloy. Meanwhile, a monolayer of LiAlO2 and a dual-layer of Al/Al2O3 films were deposited on the LZ101 substrates using atomic layer deposition (ALD) and both techniques of magnetron sputtering and atomic layer deposition (ALD), respectively. Their chemical compositions, microstructures, corrosion, wear and cavitation-erosion behaviors will be carefully examined. Experimental results show adding 2wt.% Li to AZ80 alloy can obviously increase the ductility and impact toughness. The 400°C solution-treated AZ80N specimens produce the AlLi+Mg17Al12 and Mg17Al12 precipitates after aging at 170°C and 250°C, respectively. The T6-170°C and T6-250°C aged specimens have maximum tensile strengths of 370MPa and 350MPa and with corresponding aging times of 100 hours and 8 hours, respectively. Besides, the as-extruded AZ80N specimens also produce the AlLi+Mg17Al12 and Mg17Al12 precipitates after aging at 170°C and 250°C, respectively. The 170°C and 250°C aged AZ80N specimens have maximum tensile strengths of 350MPa and 340MPa and with corresponding aging times of 48 hours and 3 hours, respectively. On the other hand, the 400°C solution-treated AZ80N specimens with subsequent hot rolling at 300°C exhibit a fine grain structure with a high density of twinning defects. The 60% hot rolled AZ80N specimen has a tensile strength of 375MPa. However, the T6-treated and hot-rolled AZ80N specimens exhibit poor ductility (elongation~2-4%) due to the high amounts of particle-type and lamellar precipitates, which hinder the movement of dislocations and twins. On the contrary, T5-treated AZ80N alloy could exhibit excellent elongation, and can be applied for the engineering components. In addition, the Mg-Li alloy has excellent formability, as well as their extra-low density. But, these alloys exhibit a low mechanical strength and are not very useful for engineering application. Hence, the Mg-10Li-0.5Zn (LZ101) alloy is prepared in the present study, and its crystal structure, mechanical property and aging behavior are systematically investigated. However, the Mg-Li alloys have disadvantage of poor corrosion, wear, cavitation-erosion resistances. The poor corrosion, wear, cavitation-erosion resistances limits the application of Mg-Li alloys and needs to be resolved effectively. Therefore, the ALD technique was employed to deposit protective Al2O3 films on the Mg-Li alloys. It is believed that the Li atoms within the Mg-Li substrate will diffuse out to react with Al and/or O atoms during the ALD process. Hence, the LiAlO2 films, instead of Al2O3 films, are deposited on the Mg-Li substrates. The ALD-deposited LiAlO2 films exhibit an amorphous structure and have an atomic ratios of Li:Al:O = 1:1:2. The ALD-deposited LiAlO2 films show properties of good corrosion resistance, low friction coefficient, high hardness/elastic (H/E) ratio, strong adhesion, smooth surface roughness and conformable coverage. These excellent properties of ALD-deposited LiAlO2 films can significantly improve the corrosion, wear performance and cavitation–erosion resistance of LZ101 alloy. Besides, to obstruct diffusion of Li atom, and then understand effects of Al2O3 films on corrosion resistance of Mg-Li substrate, an interlayer deposited onto the Mg-Li alloy plays an important role. In this study, Al interlayer with a thickness of 200nm was pre-sputtered on the LZ101 substrates. Afterwards, Al2O3 films were deposited by the atomic layer deposition (ALD) technique on these LZ101 substrates with pre-sputtered Al interlayer. The potentio-dynamic polarization measurement shows that the LZ101 specimen with Al/Al2O3 dual films exhibits a better corrosion resistance than those specimens with a single film of sputtered Al or ALD-deposited Al2O3. The much thinner multilayer can provide a good instance for reducing weight and cost of thin protective films as shown in this study.