| Magnesium alloys, which are called the21century green engineering material, are the lightest metal structural materials of practical application. But when the using temperature is more than120℃, magnesium alloys are prone to creep, heat-resistant and the mechanical properties of high temperature become poor, it limits the wide application of magnesium alloys greatly. For example, it’s difficult to used for power systems that has the great significance for car lightweight. The rare-earth element Y is fruitful in resources, the atom and the ionic radius are similar with magnesium, the solid solubility to be big in the magnesium so that can engender solid solution strengthening to magnesium alloys; on the other hand, it also the effective element of ageing strengthening and dispersion strengthen. Therefore, using Y to enhance the high temperature performance of magnesium alloys has become the focus in the realm.In this study, the effects of Y on the grain size, morphology, organization microstructure, room and high temperature mechanical properties of Mg-9A1-2.25Sr alloys were investigated by OM, SEM, EDS, XRD and tensile testing which were made in the pit magnesium alloy furnace under CO2+SF6protection with Ar top-blown mixing refining. It offers theory basis for enhancing high temperature performance and enlarging application scope of magnesium alloy and rare earth element.The following conclusions were reached in this paper:(1) Through the cold simulation experiment phenomenon and the hot state contrast test and the income data, it showed that the flow of Ar top-blown was in300Ncm3/s~500Ncm3/s range and the best point was450Ncm3/s under the conditions of this subject test.(2) The gas flow of top-blown can effectively control the speed of circulation mixing and the area size of the spout eye in the melt flow field, the losses was reduce by increasing the liquid-liquid phase reaction rate and preventing the reaction between magnesium melt and oxygen. Compared with artificial mixing refining, the loss rate of Mg, base earth elements and rare earth elements were reduced by8.3%,10.1%,32.5%respectively. Meanwhile, the total loss rate was reduced by11%. It improved the utilization rate of elements significantly.(3) Along with the increase of the content of Y element, the grain size of Mg-9Al-2.25Sr alloys was a parabolic trend changes, first increases, then decreases. The magnesium alloy’s grain size was the smallest and evenly distributed when the content of Y was0.35%; But when the content of Y was more than0.35%, a mass of A12Y phase gathered and blunted its dispersion distribution effect in the alloy matrix. At the same time, because of the increased consumption of Al element, the number of Al atoms decrease and low melting point compounds were formed. Among them, abundant of cellular network of Al7Sr8phase appeared and the alloy solidification organization deteriorated, the grain degrees grew up again.(4) Along with the increase of the content of Y element, the mechanical properties of Mg-9Al-2.25Sr alloys at room and high temperature (200℃) was a parabolic trend changes, first increases, then decreases. The turning point of room temperature and high temperature all appear at Y element content was0.35%. Compared with the alloy without Y, the tensile strength(σb), the yield strength(σ0.2) and the elongation ratio (δ) of the alloy with0.35%Y at room temperature srespectively achieves207.65MPa,159.77MPa and9.75%, were increased by67.9%,61.5%and89.7%respectively, the hardness achieves95.5HBS. Meanwhile, the tensile strength (σb), the yield strength (σ0.2) and the elongation ratio (δ) of the alloy with0.35%Y at high temperature srespectively achieves192.52MPa,135.83MPa and9.89%, were increased by72.4%,65.6%and66.2%respectively.(5) The fracture morphology of Mg-9Al-2.25Sr-xY alloys presented the mixed characteristics of quasi-cleavage and part-ductile fracture. Along with the increase of the content of Y element, the number of tear ridge increased and a small amount of toughness nest appeard, it’s brittle and toughness composite fracture. Because of a mass of Al2Y phase gathered and abundant of cellular network of Al7Sr8phase appeared, it made the size of the second phase which at the grain boundaries more bigger so that stress concentration was emerged under external force; Moreover, the A12Y phase as brittle phase itself, all of these lead to fracture and appeard a large number of cleavage steps, it’s brittle fracture characteristics. |
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