Effect Of Calcium Magnesium Phosphate Fertilizer, Yb And Na 3 PO 4 Modification Treatment And Heat Treatment On Mg 2 Si Reinforced Phase In Magnesium/Aluminum Matrix Composites

Recently,metal matrix composites（MMCs）have attracted extensive attention in automobile industry,in order to meet the demands of weight reduction and fuel economization.As compared to the adscititious reinforced composites,the in-situ synthesized composites that the reinforcement phase is generateed in the matrix,have many advantages such as small dimension,clean interface and pollution-free,good thermal stability,good compatibility with the matrix and low cost,which has become an important development direction of metal matrix composites.Mg₂ Si intermetallic exhibits a low density of 1.99×103 kg·m-3,a high elastic modulus of 120 Gpa,a high melting temperature of 1,085 °C,a high hardness of 4.5×109 N·m-2 and a low thermal expansion coefficient of 7.5×10-6 K-1,hence,it has high potential as the reinforcing particulate of the particulate metal matrix composites（PMMCs）.However,as in-situ Mg₂ Si reinforced PMMCs,Mg₂Si/（Mg/Al）do not have satisfactory mechanical properties in general,due to the exhibition of coarse dendritic primary Mg₂ Si particles and the brittle Chinese script shape eutectic Mg₂ Si phases.It is proposed that a modification process seems to be the most hopeful route to improve the mechanical properties of in-situ composite prepared by simple casting.The modification mechanisms of the Mg₂ Si particles by adding various elements,such as B,Ba,Ca,P and RE,have been discussed in previous studies.Based on the above statement,the modification effect of calcium-magnesia phosphate fertilizer on the microstructure and mechanical properties of Mg₂Si/Mg-4Si composite and the modification effect of Yb and Na₃PO₄ on primary Mg₂ Si crystals in Mg-2Si composite;and the modification effect of Yb,Na₃PO₄ and solution t on primary Mg₂ Si crystals in Mg₂Si/Al-20Mg₂Si-3Si composite were investigated by means of X-ray diffraction（XRD）,optical microscopy（OM）,scanning electron microscopy（SEM）and energy dispersive spectroscopy（EDS）analysis in this article.And the purpose of this preliminary experimental is to find an effective and simplified casting process route to produce a relatively fine and dispersive Mg₂ Si particle reinforced magnesium metal matrix composite.The major study results are as follows:（1）The primary Mg₂ Si particles in the Mg₂Si/Mg-4Si composite can be significantly modified by the addition of calcium-magnesia phosphate fertilizer with 3% to 6%.The morphology of the primary Mg₂ Si is changed from coarse dendrite to fine dispersive polygon,and the mean size is decreased obviously from 277μm to 17μm.（2）The refinement mechanism may be attributed to the appearance of phosphorous compound and the cluster of Ca compounds during solidification which acts as heterogeneous nucleation cores for the primary Mg₂ Si particles and results in the modification of primary Mg₂ Si particles.（3）With 4.0 wt.% calcium-magnesia phosphate fertilizer modifier introduced,ultimate tensile strength and elongation of the Mg₂Si/Mg-4Si composite is increased from 78.7 MPa and 2.1% to 105.2 MPa and 2.6%,as compared to that of the base composite.（4）The primary Mg₂ Si particles in the Mg₂Si/Mg-2Si composite can be significantly modified by co-modified of Yb and Na₃PO₄.When the content of Na₃PO₄ is stability at 2.64 wt.% the average grain sizes of the primary Mg₂ Si particles and α-Mg halos are decreased initially and then slowly increased with the increasing Yb content.With 0.8wt.% Yb and 2.64 wt.% Na₃PO₄ introduced,the primary Mg₂ Si particles have the smallest mean size of 9.40μm,Which is probably attributed to the formation of the Yb compound that acted as the heterogeneous nucleation substrates of the primary Mg₂ Si particles,resulting in a refined distribution of these precipitates.（5）The primary Mg₂ Si particles in the Mg₂Si/Al-20Mg₂Si-3Si composite can be significantly modified by the addition of 2.64 wt.%Na₃PO₄.The morphology of the primary Mg₂ Si particles is changed from coarse dendrite to fine dispersive hexahedron.The refinement mechanism may be attributed to the appearance of AlP compound during solidification which acts as heterogeneous nucleation cores for the primary Mg₂ Si particles and results in the modification of primary Mg₂ Si particles.（6）The primary Mg₂ Si particles in the Mg₂Si/Al-20Mg₂Si-3Si composite can be significantly modified by the combined addition of 2.64 wt.%Na₃PO₄ and 0.8wt%Yb.The morphology of the primary Mg₂ Si particles is changed from coarse dendrite to fine dispersive hexahedron.The refinement mechanism may be attributed to the appearance of AlP compound in the melt which acts as heterogeneous nucleation cores for the primary Mg₂ Si particles and the poison effect of Yb.（7）After 520℃ solid solution treatment,the Chinese script shaped Mg₂ Si phases are changed from Chinese script shape to short pole or round shape,and more evident with Yb added,thus the mechanical properties are improved.This can be attribrted to the different concentration at different radius of curvature.