A Study Of The Microstructure And Properties Of Mesophase Pitch-basedcarbon Fiber Reinforced Aluminum Compoites

In this work, seven kinds of unidirectional mesophase pitch-based Cf/Al composites were fabricated by pressure infiltration method. Optical microscopy(OM), scanning electronic microscopy(SEM), transmission electronic microscopy as well as energy dispersive spectrometer(EDS) were employed to analyse the microstructure of the composites and the carhon ifbers. Thermal diffusions(TD), thermal conductivities and the coefficient of thermal expansions(CTE) of the composites were mesured both along and perpenticular to the direciton of carbon fibers. Meanwhile, the scale stabilities, elastic modulus and the bending strength of different composites along the direction of carbon fibers were also tested. And then, the effects of different carbon fibers(K1100, P120 and P100) and matrixs(1199. 6063 and 5A06) on the microstructure and the properties of the composites have been characterized.Though the characterization of carbon fibers, following conclusions could be drawn: if the carbon fiber have higher degree of graphitization, bigger size of nonagrain and more intergrated graphite plane, the carbon fiber have higher thermal conductivity along the fiber axis. Therefore, P100, P120 and K1100 carbon fibers used as reforcement in this work have higer thermal conductivity than Pan-based carbon fibers.With the help of the equipment stated above, the microstructure of the composites were clearly oberved and some conclusions could be drawn as follows:(i) the interface between carbon fibers and the Al matrix were fine and no carbide(e.g. Al4C3) formed on the interface;(ii) when 6063 alloy was used as the matrix, the Mg and Si element were prone to be soluted in the Al and formed small disc-like G.P. Zones;(iii) when the addition of Mg element reseached to 6.5wt%(5A06), most Mg element(about 4wt.%) soluted in Al and the others segregated on the surface of the carbon fiber to form Al3Mg2 precipitation. Due to the soluted Mg element in the matrix, K1100/5A06 composite showed a lower thermal conductivity.The composites used 1199 alloy and 6063 alloy as matrix showed a property of low thermal expension along fiber axis ranging from-1×10-6/K to 1×10-6/K, while the composite used 5A06 alloy as matrix had a higher coefficient of thermal expension along fiber axis owing to the high coefficient of thermal expension of 5A06 alloy. The coefficients of thermal expension perpenticular to fiber axis were comparable to the coefficients of thermal expension of coresponding matrix changing from 16×10-6/K to 26×10-6/K, which were accord with the kural-Min model. The experiment of thermal cyclings showed following rules: open loop was observed during the first cycle due to the relaxation of residual stress in the composties formed during the fabrication process; attributed to the plastical deformation of matrix, stress hyteresis loops were formed during the heating and cooling cycling.Owing to the effect of Si and Mg element in 6063 alloy, the composite with 6063 as matrix had better mechanical propertites(both elastic modulus and bending strength) than those with 1199 as matrix. More specifically, the elatic modulus and the bending strength of P120/6063 composites could reach to 496.4GPa and 703 MPa, respectively. Pulling out of carbon fibers was the main failure mechanism of the Cf/Al composites in this work from the fracture morphology. However, the number and length of pull-out fibers could be decreased by the means of alloying.