| Due to join ceramic particles in, Magnesium matrix composites not only keep lightweight characteristics of the magnesium alloy, but also had high strength, high modulus, excellent damping properties and low thermal expansion coefficient. Magnesium matrix composites were becoming lightweight, high-performance structural materials after following the Aluminum matrix composites. There were great potential in the aerospace, defense, automotive and electronic fields of application, has become one of hot spots in the field of the international lightweight structural materialsThe SiCp/Mg composites with the AZ91D magnesium alloy which is reinforced by SiC particles in different sizes, respectively were 11, 14, 21, 33, 47μm, had been fabricated by vacuum pressure infiltration. The flame-retardant and protection mechanism of SiC porous ceramic layer on the melt magnesium were studied under the conditions of Vacuum and atmospheric pressure. The pattern and average of the liquid infiltration process were analyzed by means of electrode contact test. The microstructure and interface structure of high volume fraction SiCp / AZ91D composite were investigated. The main results are as follows:1 The SiC ceramics porous layer suppressed effectively the volatility and combustion of magnesium under the conditions of Vacuum and atmospheric pressure, played protective function of the being flame-resistant, and has realized the non-pollution flame-resistant protection. At the same time, the method eliminated effectively flaws such as mixture, blowhole and so on which existed easily in the traditional infiltration preparation of Magnesium matrix composites. Moreover, but also discovered that the characteristic parameter of the ceramic porous layer (the aperture equivalent and the height of porous layer) have the important influence to the effect of protection: The aperture equivalent of porous layer is smaller, the height of porous layer is bigger, its effect of protection is better. When infiltration-temperature is 973K,infiltration- time is 120min and infiltration-pressure is 300KPa,the aperture equivalent was 5.28μm, the height was 20 mm, the burning rate of magnesium was 11.5%. Compares with the RJ-2, it were lower and had solved the difficult problem of resolver mixture.2 The main factors of the infiltration rate were the size of the aperture equivalent of SiC performs, and the infiltration rate was proportional to the aperture equivalent. The size of equivalent aperture de=8.30μm is a threshold of the infiltration rate, When the size of the aperture equivalent is less than 8.30μm, the increase trends of the average rate of infiltration have remained relatively flat . When the size of the aperture equivalent is greater than 8.30μm, the average rate of infiltration increased sharply. The process of magnesium melt infiltrate SiC performs was small Reynolds number flow. When infiltration-temperature is 973K , infiltration-time is 120min and infiltration-pressure is 300KPa, the Reynolds number were as follows: 9,18,48,83,748. When the Reynolds number was 9, the pattern of infiltration was laminar flow state, the rest belong to the turbulent state. The way of infiltration is two layers infiltration. Firstly, magnesium melt occured the effect of frozen to the wall. Performs was coated basically by the liquid magnesium alloy, and then the liquid magnesium alloy would infiltrate rapidly relatively to the center through the poor compaction of the parts. Then, the no infiltration area of performs was divided into upper and lower layers. Finally, each layer was infiltrated in accordance with the way of infiltration of center from around, and ultimately completed the full infiltration of performs.3 The observation on microstructure and phase structure analysis showed that in SiCp/Mg composites,the reinforcement particles distributed uniformly, the morphology of particle was integrity, no obvious casting defects such as voids and inclusions existed; Under transmission electron microscope found that the interface is straight, there is no chemical reaction between particles and magnesium alloy , but there were some sizes precipitates for Mg2Zn11, MgO. The phase relationship between Mg2Zn11 and SiC was (0 12 )SiC /(/021)Mg2Zn11,[ 221]SiC //[124]Mg2Zn11;The phase relationship between Mg2Zn11 and Mg was (1 1 1)Mg /(/520)Mg2Zn11,[ 781]Mg //[258]Mg2Zn11; The phase relationship between MgO and Mg was ( 2 06) Mg /(/044)MgO, [ 040]Mg //[011]MgO.At the same time, there were high density of dislocations and dislocation tangles existed on the interface of SiC / Mg composites.
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