Influences Of Bi, Sb And P Modification On Microstructure And Mechanical Properties Of In-situ Mg₂Si/AZ91D Composites

Mg₂Si becomes the ideal reinforcement of the Mg matrix composite because of its high melting point and hardness. But under the conventional casting condition, in-situ Mg₂ Si is dendritic and in Chinese script type that seriously dissevers the matrix and reduces the mechanical properties of the composite limiting the widespread use of it. The effects of Bi, Sb and P modification on the structure and properties of the in-situ Mg₂Si/AZ91 D composite are studied and modification mechanism is explored in this paper.The experimental results show that:Bi, Sb and P modification all can change the morphology and size of the primary Mg₂ Si, eutectic Mg₂ Si and β-Mg17Al12 in in-situ Mg₂Si/AZ91 D composite. But the modification effects of them are different and the modification effect of P modification is the best. The three kinds of modification all can make massive dendritic primary Mg₂ Si turn into polygonal or granular primary Mg₂ Si, change the morphology of the eutectic Mg₂ Si from Chinese script to short rod, vermicule and graininess and the morphology of the β-Mg17Al12 phase changes from intermittent reticular to slight lath-shaped or graininess. The best adding amounts of Bi, Sb and P modifiers are 0.8%, 0.8% and 0.2% respectively.When the 0.8% Bi modifier is used, the average grain size of the primary Mg₂ Si is 13μm that decreases by 81% compared to it without Bi modification. The room temperature tensile strength reaches to 290 MPa and the elongation reaches to 6.7% that increase by 26% and 109% respectively compared to them without Bi modification. The high temperature（150℃） tensile strength is 203 MPa and the elongation is 11.2% that increase by 53% and 138% compared to them without modification. The size of the primary Mg₂ Si is 13.8μm after the 0.8% Sb modification that decreases by 80% compared to it without the modification. The room temperature tensile strength reaches to 290 MPa and the elongation reaches to 6.3% after the 0.8% Sb modification that increase by 25.5% and 97% respectively compared to them without Sb modification. The high temperature（150℃） tensile strength is 199 MPa and the elongation is 11.1% that increase by 50% and 136% compared to them without modification. The average grain size of Mg₂ Si is 7.2μm after 0.2% P modification. The room temperature tensile strength is 280 MPa and the elongation is 92% after the 0.2% P modification that increase by 21.2% and 188% respectively compared to them without P modification. The high temperature（150℃） tensile strength is 200 MPa and the elongation is 13% after 0.2% P modification that increase by 50% and 177% compared to them without P modification.Bi reacts with Mg to form the eutectic Mg3Bi2. It can not be used as the crystallization nuclei of the primary Mg₂ Si. But the element Bi gathers at the Mg₂ Si interface front hindering the growth of the primary Mg₂ Si. The element Bi can enter the Mg₂ Si lattice causing the lattice distortion and changing the interfical energy that changes the growth pattern of Mg₂ Si.Sb reacts with Mg to form the eutectic Mg3Sb2. It can be used as the crystallization nuclei of the primary Mg₂ Si and increases the nucleation rate refining the Mg₂ Si. The enrichment of Sb in the Mg₂ Si interface front reduces the preferredl growth speed along [100] direction that makes massive dendritic Mg₂ Si turn into polygonal or granular Mg₂ Si. The AlP in the melt can be used as the crystallization nuclei of Mg₂ Si. The dissolution and re-generation of AlP decrease the size of AlP and increase the amount of AlP that cause the increasement of nucleation rate and refine Mg₂ Si. The solidification setup temperature of the primary and eutectic Mg₂ Si dropped, the nucleation undercooling increases and the critical grain radius of Mg₂ Si in the melt decreased after P modification. At the same time, the solidification setup temperature drops making the primary Mg₂ Si and eutectic Mg₂ Si have no time to turn into the dendritic and chinese script type.