| Intermetallics, which have high strength and hardness, good corrosion andhigh-temperature resistance, as well as special acoustics, optics and electricityproperties, can be potentially used as structural or functional materials.Brittleness at room remperature, however, affects tremendously theirapplications. Therefore researchers tried to find some approaches to improve theroom temperature brittleness and to further expand used fields of intermetallics.At present, research about intermetallics on home and abroad is focused onAl-containing and Si-containing series. The common secondary phase Mg17Al12in mircrostructures of AZ series Mg alloys is also a kind of Al-containgintermetallics. In recent years, properties of Mg-Al intermetallics in somerespects have been gained lots of attention. On on hand, Mg-Al intermetallicscan act as a protective coating, and their fabrication methods and corrosionresistance have been studied. On the other hand, the Mg-Al intermetallics can beused as a hydrogen storage material, whose hydrogen absorption/desorptionproperties are also investigated. In view of this deflciency, an alloying methodwas used and the effect of alloying elements on microstructures and mechanical properties of Mg17Al12intermetallics had been studied in this investigation.Moreover, some exploratory researches of their potential applications have beenconducted. In the experiments, the effects of Er, Y and B on the microstructures,phase composition, hardness, compression properties, corrosion resistance andheat stability of Mg17Al12intermetallics were studied in more detail. Consideringthe high hardness, but poor in ductility at room temperature of Mg17Al12, acomposite was fabricated by using open cell Al foam and Mg17Al12intermetallics. The energy absorption characteristic of the composited wasinvestigated. The effect of Y on the hydrogen storage properties of Mg17Al12wasstudied because Mg17Al12has a high ideal hydrogen storage capacity. Theprotective effect of Mg-Al-Er coating was also studied for Mg alloy. TheMg70Al30alloy with a near Mg-Mg17Al12eutectic compositon was fabricatd andits corrosion behaviour was investigated in acid solutions. At last, according tothe corrosion behaviour of eutectics, the Mg-Al alloy with a porous surface layerwas fabricated successfully. The main conclusions are as follows:(1) The microstructures of Mg-Al-Er alloys vary with the Er content. Whenthe Er content is less than1.0wt.%, the microstructure of the alloy only consistsof Mg17Al12matrix and Al3Er phase. When the Er content is more than3.0wt.%,not only the Al3Er phase appears in the matrix but also the Mg-Mg17Al12eutecticdoes. Compared with the Mg-44.3Al alloy (single phase Mg17Al12), the hardnessof all Er-containg alloys are improved, which is caused by a grain refinementeffect induced by Er addition. The experiment results are in good agreement with the computation results. Both the corrosion and wear resistance areimproved after adding Er. Among all alloys, the Mg-43.8Al-1.0Er alloy is thebest in corrosion resistance and supression ability for a cathode hydrogenevolution and the Mg-42Al-5.0Er alloy is the best in wear resistance.When the Y content is less than1.0wt.%, the Mg-Al-Y alloy is composedof Mg17Al12matrix and Al2Y phase. When the Y content is more than1.0wt.%,there is the Mg-Mg17Al12eutectic in the matrix. The alloys in hardness are veryclose to eachother,285~290HV, when the Y content is in a range of2.0wt.%~5.0wt.%.(2) A hot-press sintering method was used to fabricat the Mg-Al-Erintermetallic coating on AZ61Mg alloy surface. Electrochemistry and slidewear test results show that the coated specimens have better corrosion and wearresistance than the AZ61Mg matrix, and also better than the thermal diffusioncoating. That’s because the formed hard Al3Er secondary phase can improve thewear resistance of the coating. The element Er can suppress the hydrogenevolution at the same time.(3) During the30h ball-milling process, there is no phase transformationdetected in the Mg-xAl-yY alloy, but the diffraction intensity is decreased andthe diffraction peak is broadened. The Y addition can improve the hydrogenabsorption/desorption properties of Mg17Al12greatly, as shown in the improvedhydrogen absorption/desorption rate and hydrogen desorption plateau pressure.When the Y content is1.0wt.%, the alloy has the highest hydrogen absorption/desorption capacity and is3.39%and2.84%. Moreover, thehydrogen desorption plateau pressure is1.33MPa.(4) With the increasing of B content from0.1wt.%to1.35wt.%, themicrostructure of the Mg-Al-B alloy change from single phase dendrites toMg-Mg17Al12eutectics. When the B content is equal or lesser than1.25wt.%,the hardness of the alloys has a small change and is in the range of223HV~229HV. When the B content is1.35wt.%, the alloy has the lowesthardness,192HV. But the compression strain and ultimate compression strengthare improved25%and57%compared with that of the Mg-44.3Al alloy,respectively. With the increasing of B content, the melting point and corrosionpotential of the alloy are decreased monotonically.(5) The microstructure of the Mg70Al30alloy consists of α-Mg dendrites andMg-Mg17Al12eutectics. The corrosion behaviour of the alloy is different inH2SO4and acetic acid (HAc) solutions. The Mg70Al30alloy shows a generalcorrosion in H2SO4solution with a pH of1.0and a selective corrosioncharacteristic in HAc with a pH of1.0. The orientations of the grain in theeutectics have a significant effect on the corrosion behaviour of the Mg70Al30alloy in HAc solution. Point-like eutectics have a better resistance to peeling offinduced from corrosion. Grain orientations have little effect on corrosionbehaviour of the Mg70Al30in H2SO4solution. The eutectics have better corrosionresistance than the α-Mg dendrites in these two corrosive solutions. Thede-alloying can proceed as results of the Mg-Mg17Al12eutectic alloy having two phases with a great difference in corrosion potentials and the noble phase havinga good corrosion resistance in de-alloying electrolyte.(6) The composite, open-cell Al foam filled with Mg17Al12intermetallics,has a solid structure; however, it has the same compression properties with thatof the Al foam. The composite has a better compression property than thecounterpart Al foam. The plateau stress of the composite is1.5times that of theAl foam and the energy absorption capacity is improved obviously than Al foamunder all strain conditions. When the strain is less than0.02or in the range of0.2~0.6, the composite has a higher energy absorption efficiency than the Alfoam with the same diameter. However, the situation is reversed when the strainis in the range of0.02~0.2. |
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