The Thermo-Mechanical Treatments and Surface Coatings of AZ80N and LZ101 Mg Alloys
Date Issued
2011
Date
2011
Author(s)
Wang, Po-Cheng
Abstract
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.
Subjects
Magnesium alloy
thermo-mechanical treatment
atomic layer deposition
Type
thesis
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