| In this paper, direct melt reaction technique combined with blade intensive shearing process was applied to prepare sub-micron in-situ particle reinforced Al-Mg-Si-O composites by the chemical reaction between sub-micron SiO2 particles and Al-Mg-Si alloy. The process and products of this in-situ reaction were concretely analyzed, and then the effect of SiO2 addition, T6 heat treatment and multiple forging on the microstructure and tensile property of composites were also investigated, and then the tensile testing at elevated temperature was employed to study on the superplasticity of composites.Sub-micron MgAl2O4 reinforcement particles were generated by the reaction between sub-micron SiO2 and molten Al-Mg-Si alloy, and the matrix was composed of α-Al, Mg2Si and Si phases. The effect of SiO2 addition on the refinement of α-Al dendrites was found. It was demonstrated that the SiO2 addition with low volume fraction (<1 vol.%) refined the a-Al dendrites obviously and MgAl2O4 agglomeration was merely found on the grain boundaries. With the further increase of SiO2 addition, MgAl2O4 agglomeration at grain boundaries grew in number and the subsequent refinement of a-Al dendrites was not found. The composite obtained best morphology with 1 vol.% SiO2 addition. The morphology of Mg2Si particles was significantly improved by T6 heat treatment. After T6 heat treatment, reticular Mg2Si at a-Al grain boundary turned into fine spherical or ellipsoidal particles, the maximum size of which was 8μm. A uniform distribution of Mg2Si particles in the matrix was facilitated by multiple forging, while the MgAl2O4 particle agglomeration was also reduced to certain level.With the increase of SiO2 addition, the tensile strength improved while the elongation decreased. Compared with Al-2.53Mg-1.47Si alloy, the tensile strength of composites with 0.5,1, 2 and 3 vol.% SiO2 addition respectively increased by 9.2%,18.4%,23.7% and 26.1%, while the elongation decreased by 6.5%,12.4%,32.8% and 59.5%, respectively. The elongation of the composite with 1 vol.% SiO2 addition after T6 heat treatment and multiple forging was 19.83%, which increased by 313.9% compared with that of its as-cast state.The tensile tesing at elevated temperature under different conditions were conducted on the T6+MF state composites. The results showed that the maximum true stress value decreased while the elongation increased with increasing temperature and decreasing strain rate. The 126% elongation of the composites were achieved under the condition of 560 ℃,10-4 s-1. The values of m were all less than 0.3 at the test temperature range, and increased with the increase of temperature. The m value was likely to continue to increase at a further increasing temperature. The value of Q was much higher than the self diffusion activation energy of Al, by which a grain boundary sliding caused by dynamic recrystallization mechanism could be concluded.Tensile fracture at elevate temperature of the composites was mainly composed of dimples, showed a ductile fracture characteristic. The dimple size and morphology became smaller and shallower with the decrease of temperature and the increase of strain rate. The process of fracture was composed of cavity nucleation, growth and connection. Cavities most likely formed in irregular grain boundary. With the ongoing deformation, the cavity size continually increased, the mechanism was gradually transformed from diffusion controlling mechanism into strain controlling mechanism. |
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