Effect Of Impurity Phase And Microstructure And Properties On The Second Relative Magnesium Alloy

Magnesium alloys are widely used in the fields of automotive, aerospace, medical, electronics because of its low density, high specific strength and high specific stiffness properties.So magnesium alloys are regarded as the most promising metals in21th century. However, the impurities and second-phase particles can affect the mechanical properties of magnesium alloys. Therefore, it is meaningful to improve the performance of magnesium alloys by controlling the shape, size, distribution of the impurities. A large number of studies are focused on the melt purification at present but the effect of impurities on the microstructure and mechanical properties of magnesium alloys is less studied. In this paper, the type, content, distribution of impurities and the senond phase in magnesium and its alloys processed by directional solidification and conventional solidification were studied.The following results have been achieved:(1) Impurity phases in pure magnesium are MgO, Mg2Si, Mg2Ca and mixtures with containning impurity elements, such as Fe, Cl, etc. The main impurity phases are MgO,70%content of which are below1μm and9%are over3μm in mean diameter. The most content of MgO in single pure Mg is at the bottom area, but the size is small. The content of MgO at the top area in single caystal is less, but the size is bigger.(2) There appeared segregation in Mg-Al alloys processed by directional solidification and formed three different morphologies area:dendrite area at the bottom, isometric in central area and columnar crystal at the top area. Mg-Al alloys and pure Mg have the same type of impurity phases. The morphology of MgO in Mg-Al alloys primarily transform into spherical form from multiple forms in pure Mg. The mean diameter of MgO on the top part of the single crystal increased with increasing Al content. Eutectic β-Mg17Al12were easily damaged due to stress concentration, however, the lamellate precipitated phase-(3-Mg17Al12nucleated at grain boundary and grown into grain interior has not obvious influence on the mechanical properties of Mg-Al alloys.(3) Impurity phase and the second phase have been greatly affected by solution treatment. The size and area fraction content of impurity phase in Mg-2%A1and Mg-4%Al alloy prepared by conventional casting increased after solution treatment. Large second phases along the grain boundaries in Mg-6%AI alloy have been gradually dissolved into Mg marix. Due to the limited solid solubility of Al element in magnesium matrix, the second phases were not completely dissolved in Mg matrix, and the undissolved phases distributed discontinuously along the grain boundaries, leading to the reduction in strength and plasticity.