| Al-Mg2Si in situ composites, as a new class of lightweight particulate reinforced aluminum matrix composites, have great applications in aerospace, automobile and shipbuilding and other fields due to their low density, high specific strength, wear resistance and high-temperature resistance, and the excellent castability. However, practical engineering applications of these composites as structural components have been greatly limited owing to their insufficient strength, low ductility, and poor processability and formability. The low strength and ductility are mainly attributed to the coarse primary Mg2Si particles, the brittle eutectic matrix and the inhomogeneous distribution of Mg2Si in the hypereutectic Al-Mg2Si composites produced by ingot metallurgy (I/M) process, which are largely controlled by the alloy composition and the adopted processing technique. Therefore, microstructural refinement and homogenization by the reasonable composition design and choice of processing techniques are critical for the improvement of strength, ductility and toughness of Al-Mg2Si in situ composites.In this paper, ECAP (Equal Channel Angular Pressing) was used to refine and homogenize the microstructures of Al-Mg2Si composites. With the design of new alloy composition and appropriate choice of ECAP routes based on the analysis of their shear deformation behaviors, a new-type hypoeutectic Al-Mg2Si composite with high strength and ductility was obtained. Microstructures were characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer (EDS) and transmission electron microscope (TEM), and tensile mechanical properties were measured by computer controlled electronic universal testing machine at room temperature and at the elevated temperature of 150℃.The results show that Sb has an obvious refinement effect on eutectic Mg2Si in the composite but has slight effect on the modification of Mg2Si morphology. Sb promotes the nucleation of Mg2Si by formation of Mg3Sb2 heterogeneous nuclei, which results in a significant increase in the amount of eutectic Mg2Si and a decrease in eutectic cell size and especially inα-Al grain size. Also, Sb induces the curling and branching of eutectic Mg2Si lamellae and leads to a pronounced reduction in lamellar spacing. As a result, the mechanical properties of the composite after Sb modification are significantly improved, the yield strength increases from 153 MPa to 184 MPa, the tensile strength increases from 167 MPa to 282MPa, and the elongation increases from 0.43% to 3.9%, respectively.ECAP significantly refines the microstructures of Al-Mg2Si composites. After multi-pass ECAP processing, ultrafine-grained structures are achieved in the hypoeutectic Al-Mg2Si composites and the mechanical properties of these composites are enhanced greatly.After ECAP processing through 4 passes in route Bc, coarseα-Al dendrites in the as-cast Al-10Mg-4Si composite are refined to～0.9μm and the initial large lamellar eutectic Mg2Si cells are entirely transformed into short fibers of～1μm in length and -0.4μm in diameter. After 8-pass ECAP processing, the microstructure of the composite is further refined and the average grain size of α-Al grains becomes -0.3μm, and Mg2Si fibers are further transformed into～0.4μm equiaxed particles. The tensile strength and the elongation of the Al-10Mg-4Si composite increase from 167 MPa and 0.43% in the as-cast state to 308 MPa and 13.7% after 4 passes of ECAP, and to 297 MPa and 15.15% after 8 passes, respectively. Compared with 4-pass ECAP processing, the elongation of the composite after 8 passes of ECAP processing further increases while the tensile strength slightly decreases which may be related to the texture formation in the material after multi-pass processing.The strength and the ductility of the ECAP processed composite were greatly enhanced by combination of ECAP and Sb modification.The mechanical properties of the processed composite are further improved when the Sb-modified composite after ECAP processing is aged artificially at 170℃for 3 h. The yield strength, the tensile strength and the elongation of the aged composite are 246 MPa,404 MPa and 14.2%, respectively, increased by 13%,3% and 67% compared to those of the processed sample before aging.The combined routes 2A+4BA+2A,4A+4BA exhibit a remarkable particle redistribution effectiveness in comparison with the conventional route Bc. Mg2Si particles are distributed dispersedly in the matrix and the ductility and the toughness of the composite are greatly enhanced.Transformation matrix and pole figure were employed to describe the shearing deformation behaviors of the samples during ECAP processing. On this basis, the efficiencies of ECAP on microstructural refinement and particle redistribution in different processing routes were analyzed and predicted, the redundant strain nature in route BC was clarified and new-type ECAP routes BC-UD2 and BC-UD3 were designed and their effectiveness was predicted.After 8-pass ECAP processing, route Bc exhibits the highest particle refinement efficiency and the lowest particle redistribution efficiency. In contrast, route BC-UD2 presents the highest particle redistribution efficiency and the lowest particle refinement efficiency. While route BC-UD3 is between route BC and BC-UD2 in the particle refinement and particle redistribution efficiency. Consequently, the composite processed by route BC-UD3 obtains the best comprehensive mechanical properties and the highest strength at the elevated temperature of 150℃.
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