| Magnesium alloys have many special advantages, such as low density, high specific strength, high specific rigidity, good casting capability, electromagnetic shielding performance and easy recycling etc. So they are known as 'the green project materials having development potential and best future in the 21st century. Magnesium alloys are extensively used in automobile, airplane, computer and communication equipments etc. AZ91 magnesium alloy which is widely used as the die-casting magnesium alloy, has many advantages, such as good casting property, high ambient temperature strength and low cost. However, because the coarseβ-Mg17Al12 phase precipitates along the grain boundary as the form of divorced eutectic when concreting. Because of the wide solidification temperature range, great solidification shrinkage, bad tendency of hot tearing, the molten technics of magnesium alloy become complex. The application and development of magnesium alloys is still confined. Therefore, it is important to develop some new magnesium alloys with high mechanical properties, high toughness, good figuration properties on the basis of AZ91 alloy.The practice proves that the alloys have fine equiaxed dendrite can improve the microstructure and mechanical properties. In this paper, Sr and B are added into the AZ91D alloy to carry through composite, refine microstructure, and then improve the intensity and toughness. Effects and mechanism of Sr and B addition on the microstructures and properties have been investigated using modern analysis and test facilities such as Optical Microscopy (OM), X-ray Diffraction(XRD) , Differential Scanning Calorimetric(DSC), Scanning Electron Microscopy(SEM) with Energy Dispersive Spectrometer (EDS) and whilst the test of their tensile test, fatigue test, fluidity, corrosion potential, polarization curves. It offers some experimental basement and theoretical reference for developing novel high-powered magnesium alloys.The results show that the grain size of AZ91D alloy was about 250μm, and the grain size AZ91D-0.8Sr alloy was 120μm, which was reduced by 60%.After added 0.03%B, the grain size was reduced from 125μm to 80μm, which was reduced by 36%. The main phases in the as cast AZ91D, AZ91D-0.2Sr, AZ91D-0.5Sr alloys areα-Mg andβ-Mg17Al12 phase. The new phase, Al4Sr was found in AZ91D-0.5Sr alloy with B addition. Because of the combine of Sr and Al, the content of Al around the Al4Sr was reduced. And then the content ofβ-Mg17Al12 was reduced, the figure changed from net-structure to spiccato and disper.The ultimate tensile strength and elongation of AZ91 alloy were 129MPa and 1.1%, respectively. After the addition of 0.8% Sr, the ultimate tensile strength reached 160MPa, which has increased 23.6%; The elongation of AZ91D-0.8Sr was 1.3%, which has increased 12%. The ultimate tensile strength and elongation of AZ91D-0.5Sr-0.09B alloy were 151MPa and 1.6%, respectively. The ultimate has increased 10.2%.Failure of AZ91D alloys was brittle through cleavage or quasi-cleavage fracture. The fracture of AZ91D with Sr and B additions obvious features the combination of quasi-cleavage and local gliding fracture. The toughness was improved.The liquidus temperature of AZ91D alloy decreased markedly with the addition of Sr, but the solidus temperature changed rearly. The solidus temperature fell from 596℃of the AZ91D alloy to 566℃after 0.5% Sr addition. The solidification range has decreased 32℃.The influence of 0.2%Sr and 0.5%Sr addition to AZ91D alloy at the process of solidification was decreased the solidus temperature and enriched on the solid/liquid interface. So it can restrain the growing up of the grain. Al and Sr form the high melting point phase Al4Sr with 0.8% Sr addition. The Al4Sr enriched on the solid/liquid interface and formed the super-cooling layer, restrained the growing up of the grain. So it can refine the microstructure.The results of fluidity showed that the fluidity is improved by adding Sr and B, then improved the die-casting properties. The length of fluidity sample of AZ91D was 330mm, and the length of fluidity sample of AZ91D-0.5Sr was 380mm, which has increased by 21%. The length of fluidity sample of AZ91D-0.5Sr-0.09B was 580mm, which has increased by 76%.The results of the corrosion potential of the alloys show that the corrosion potential of the alloys with Sr and B addition. The corrosion potential of alloy with 0.5% Sr and 0.09%B addition was about -1.55V, which was 0.04V higher than that of AZ91D alloy.The results of high cycle fatigue test show that the fatigue crack of the alloys initiates at gas hole and inclusions of alloy inside. The fatigue striation areas have been found in the fatigue fracture surface of specimens after 0.5Sr added. The fatigue fracture zone of the fatigue fracture surface in AZ91D alloy shows the mixed-rupture characteristics of quasi-cleavage and dimple. More dimples and lacerated ridges were found after the 0.5Sr addition. These dimple and lacerated ridges can decrease the velocity of the fatigue crack propagate, improve the fatigue properties.The mechanism of refining about Sr to AZ91D alloy is that, Sr enriched on the solid/liquid interface when concreting because it is the activity element on magnesium alloy. This will reduce the latent heat and increase the undercooling, and so increase the nucleation rate, thereby block the growth of the grain.The mechanism of complex refining about Sr and B to AZ91D alloy is that, B reacted to SrO and MgO in the molten metal. So the Mg and Sr oxid were released, and more Sr could be used to refine the grain or form the high melting point phase Al4Sr with Al.4 Furthermore, the reaction of B to MgO in the melting, has reduced the content of oxid, consequently cleanse the molten metal. So it can improve the compositive properties.
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