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In order to modify in-situ synthesized Mg2Si particles in Mg2Si/Mg-4Si composite, the modification effect of calcium-magnesia phosphate fertilizer on primary Mg2Si phase in Mg2Si/Mg-4Si composite was investigated by means of X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis. The results indicate that the morphology of the primary Mg2Si phase apparently changes from coarse dendrites to fine dispersive polygonal particles, and the mean size is decreased from 277 µm to 17 µm. With the addition of 4.0wt.% calcium-magnesia phosphate fertilizer as a modifier, the ultimate tensile strength and elongation of the Mg2Si/Mg-4Si composite are increased from 78.7 MPa and 2.1% to 105.2 MPa and 2.6%, as compared to those of the base composite, which is probably attributed to the formation of the phosphorous compound and the cluster of Ca compounds that acted as the heterogeneous nucleation substrates of the primary Mg2Si particles, resulting in a refined distribution of these precipitates. [ABSTRACT FROM AUTHOR]

The modification effects of ytterbium (Yb), Na3PO4 and Yb + Na3PO4 on primary Mg2Si phase in Mg- 4Si alloys were investigated by means of X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis in this work. The results indicate that the morphology of the primary Mg2Si phase apparently changes from coarse dendrites to fine dispersive polygonal particles and the mean size decreases from 276.6 µm to 7.1 µm, with combined modification of 0.8wt.% Yb and 2.64wt.% Na3PO4. Such a morphological evolution results in improvement in the ultimate tensile strength and elongation of the alloys as compared to the base alloy. This may be attributed to the formation of the YbP particles that acted as the heterogeneous nucleation substrates for the primary Mg2Si particles, resulting in a refined distribution of these precipitates. The results of XRD examination show that there was no reaction between Si and Yb or Na3PO4. Solo addition of Yb or Na3PO4 into the melt has no real modification effect on the microstructure, but the primary Mg2Si particles and α-Mg phases become coarser than that in the unmodified alloy. [ABSTRACT FROM AUTHOR]