| Magnesium alloys have been considered as the kind of compellent metallic structural materials in the field of automobile, electron industry and so on, by right of their low density, high specific strength and stiffness, good machining properties etc. However, there are always much conflicts and obstacles on developping and using high-strength and heat-resistent magnesium alloys. So, reducing costs and improving heat resistence will be the key points in developping magnesium alloys.In the present work, the effects and mechanisms of the alkaline-earth elements(xCa and y1-y4Sr) on microstructure, properties and ageing behavior of AM80 of the Mg-Al series alloys have been investigated by optical microscope, scanning electron microscope, X-ray difractometer, diferential thermal analysis, electronic universal testing machine, etc. In addition, the author has also optimized the casting process parameters of AM80-Ca-Sr alloy.The results showed that the AM80-Ca-Sr alloy had minimum shrinkage and oxide inclusion defects as well as small grain size, showing amendatory tensile properties when taking the bottom-filling foundry way, the pouring temperature and mold preheating temperature were 740oСand 250oС, respectively.xCa additions refined the microstructure of AZ80 alloy, making the grain size decrese from 109.6μm to 64.8μm. With y1-y4Sr additions on the AM80-xCa alloy, y1Sr coarsened the grain; with increases in Sr content from y2 to y4, the grain size decreased to 65.8μm again. xCa additions bonded Al atoms, generating bone-shaped high-melt Al2Ca phase and depressing the precipitation of the low-meltβ-Mg17Al12 phase. With y1-y3Sr additions on the AM80-xCa alloy, Al2Ca phase became small-sized or banding, dispersing at the grain boundaries. When the Sr content increased to y4, the secondary phases became continuous net-worked over again, but more refiner than AM80-xCa alloy. After y1-y4Sr additions, the solid solubility of Al on theα-Mg matrices increased gradually, furtherly depressing the precipitation of theβphase.At room temperature and elevated temperature, xCa decareased the total tensile properties of AM80 alloy. When the Sr content added to the AM80-xCa alloy increased from y1 to y4, the total tensile properties firstly deteriorated, and then improved, finally deteriorated again. And, the AM80-xCa-y2Sr alloy had the better properties, whose tensile strength were 173MPa and 160MPa, respectively and whose elongation were 5.2% and 8.2%, respectively. At the temperature of 200oСand under the loading of 56MPa, xCa improved the heat-resistent property of AM80 alloy. With increases in Sr content from y1 to y4, the heat-resistent property of the AM80-xCa alloy increased by and large. AM80-xCa-y4Sr alloy had the optimal creep-resistant property, whose minimum creep rate and 100h creep elongation were 1.7×10-9S-1 and 0.8%, respectively. But the creep-resistant property of AM80-xCa-y3Sr alloy abruptly deteriorated. It was supposed the deterioration was relative to the long-banding secondary phase on the grain boundary which could weaken the strength of the grain boundary and generate serious stress concentration.Ca and Sr blocked the discontinuous precipitation of theβphase, depressed the ageing process of AM80 alloy. Despite they lowed the ageing strengthening, they could improve the thermal stability of the microstructure.
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