| (Al2O3+Al3Zr)p/A356 composites are successfully fabricated from A356-15wt.% Zr(CO3)2 system by in-situ melt reaction technique combined with micro-alloying technique. Effects of micro-alloying on the microstructure, solidification behaviors, reaction mechanism and mechanical properties of the as-prepared composites are investigated by using Optical microscopy (OM), scanning electron microscopy (SEM) and X-ray Diffractometer (XRD) modern analysis methods.The results show that micro-alloying can promote the chemical reaction speed and reduce the reaction time. The reaction time is determined by the time consumed for the reaction taking place and for the resultants diffusing in the matrix alloy. The additions of RE (rare earth) elements can reduce the surface free-energy of aluminum melt and improve the wettability between aluminum melt and ZrO2 particles. Thus the chemical reactions are promoted. In the process of diffusing, Mn and Cr elements can accelerate the nucleating rate for Al3Zr particles, and the concentration of Zr atomics in the aluminum melt is reduced. These contribute to the increase of the diffusing speed for Zr atomics and the decrease of the the reaction time.The solidification microstructure analysis of (Al3Zr+Al2O3)p/A356 composites shows that Mn and Cr elements can nucleate Al3Zr particles, and contribute to the insitu particulates refining. In additional, the Al3Zr particles can be the foundation base forα-Al nucleating due to the crystalographic orientation relationships, which meet with the corresponding crystal lattice structure between Al3Zr particles andα-Al. These contribute to the refined microstructure of the matrix.The depressing effects of theα-Al on the growth of the eutectic silicon are strengthened because of the the acceleration of the growth ofα-Al. These are helpful to the morphologies of Si in the composites.The surface free-energies of aluminum melt are reduced, and the reinforcement particles distributed more uniformly in the matrix because of the addition of the RE elements assisted with the manganese and chromium elements. Theα-Al phases and the morphologies of silicon are obviously modified by adding the RE elements in to the melt.The effects of the alloying elements added into the melt on the morphologies of ferrum phase in composites are that the manganese and chromium elements can transform bulky needle-shape iron phase into Chinese-character shape or short rod shape, and a small amount of bulky needle-shape iron phase is still observed in the matrix. The morphologies of iron phase in composites are changed into little blocky shape or granular shape with adding the RE element assisted with the manganese and chromium elements into the melt.The effects of the alloying elements added into the melt on the metallurgical qualities are that the surface oxide films of the melt are compacted. These contribute to stopping the oxygen reacting with aluminum and decreasing the free [H] content and effect on impurity removal decrease heterogeneous nucleation site of bubble. Thus, the metallurgical qualities of the composites are improved by alloying elements.The tests of the mechanical properties of the composites show that adding Mn,Cr and RE elements can improve mechanical properties of (Al3Zr+Al2O3)p/A356 composites. When 0.2wt% Mn and 0.2wt% Cr are added into the melt, the tensile strengthσb and elongation of the compositesδreach to 326.1 MPa and 4.6%, which increase by 18.02% and 24.32%, comparing with the composites without adding Mn and Cr, respectively. When 0.3wt% RE is added to the composites assisted with 0.2wt% Mn and 0.2wt% Cr, the tensile strengthσb and elongation of the compositesδreach to 378.8 MPa and 7.5%, which are increased by 16.16% and 50%, respectively, than that of the composites micro-alloyed with 0.2wt% Mn and 0.2wt% Cr, increase by 37.1% and 102.7% than that of the composites without adding alloy elements.The analysis results of the tensile fracture morphology and mechanism of crack initiation and crack growth indicate that the strengthening mechanisms of A356 matrix composites reinforced by in-situ Al3Zr and Al2O3 particles include dislocation strengthening, solution strengthening, grain-refined strengthening.
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