Research On The Thermal Physical Properties Of SiCp/Mg Composites Based On The Vacuum Low Pressure Infiltration

Silicon carbide particle reinforced magnesium metal matrix composite （SiCp/Mgcomposite） are strongly competitive in military, aerospace, space and other advancedtechnology fields because of their advantages in many aspects, such as low density,good thermal performance, excellent heat conducting performance and so on. Based onthe AZ91D magnesium alloy and pure magnesium as matrix, and respectively on theSiC particles of39μm,50μm,69μm,98μm and115μm as the reinforcement, the articleintroduces the preparation of SiCp/Mg composite of47%volume fraction with themethod of vacuum low-pressure infiltration under the condition of the porous ceramiccoating protecting the magnesium melt; the observation and analysis of microstructureof these composites by OM, XRD and TEM and the determination of their thermalexpansion coefficient and thermal conductivity by Thermal expansion instrument andlaser thermal conductivity meter; the key discussion about the sizes of SiC particles,alloy and the influence of heat treatment on composite material thermal expansioncoefficient and thermal conductivity, and and its mechanism. The main results obtainedare as follows:The pure magnesium of different sizes and the composites based on AZ91D, whichare prepared by vacuum low-pressure infiltration have the characteristics of uniformparticle distribution, a slight reaction interface reactant ten to hundreds of nanometers inand near the interfaces and dense structure. The density of composites with increase inparticle sizes shows an overall decreasing trend, while T4heat treatment significantlyraises the density of composite materials, the density of the composites beingapproximately in the range of2.38-2.4g/cm³.With the the influence of the particles size, alloy and heat treatment on compositematerial thermal expansion coefficient, the thermal expansion coefficient of compositematerials measured values between the Kerner model and Turner model theory of value.In fact, the thermal expansion coefficient of SiCp/AZ91D composite shows a risingtrend in fluctuation after the first lower with particles size increasing, the value being inthe12.5-13.5×10^-6/K range. The main reason is that with the increase of particles size,the dispersion and distribution of follow-up effect trends to be gradually weakened inthe matrix alloy. Then the interfacial area is another key factor affecting the thermal expansion coefficient of composite materials. With the increase of the particles size andthe decrease of surface area, the reduction of the degree of restraint on the matrix resultsin the thermal expansion coefficient of composite materials showing a rising trend influctuation. The thermal expansion coefficient of SiCp/AZ91D composite is less thanSiCp/pure magnesium composite about1×10^-6/K, increasing about8%, and the rate ofincrease is not great. After the T4treatment, the thermal expansion coefficient curve ofSiCp/AZ91D is consistent with as-cast SiCp/AZ91D composite, and the difference isvery small. The main reason is that the relief of the thermal residual stress, the reductionof dislocation density and a significant increase in Mg2Si phase promote the thermalexpansion properties, but the density to improve after heat treatment inhibits the thermalexpansion properties. That’s to say, it is the result of both of them.At the same time, the study also gains the rule of what influence SiC particles,alloy and Heat treatment have on the thermal conductivity of composite materials andfinds they also have significant influence on Magnesium matrix composites. Thethermal conductivity of SiCp/AZ91D composite shows a slowly rising trend after thefirst lower with particles size increasing, and the overall presents a "V"-shaped trend,the value being in the85-110W/m K and the particle size of69μ m was the lowestvalue; and the thermal conductivity of SiCp/Mg composite has the same trend. Thereason for that is that with particles size increasing, the diffusion particles in the alloymatrix will reduce, which is not good for the electronic movement. The interfacethermal resistance, as another key factor affecting the thermal conductivity of compositematerials, will reduce with particles size increasing, whose obstacle to the heatconduction of the composite leads to the thermal conductivity increasing after.By the study found of different matrix alloy, we can find the thermal conductivityof pure magnesium matrix composite rate reached140-150W/m K, which is higherthan AZ91D composite about50%. After the comparison, we can conclude that thethermal conductivity of AZ91D composite is far greater than the rate of the AZ91Dalloy matrix, which maybe do with the SiC larger particle size—greater than the criticalparticle size, and the particle thermal conductivity—greater than the thermalconductivity of matrix. The original SiC particles with high purity, infiltration ofcomposites’ density and microstructure of nearly defect-free help to improve theirthermal performance. Thermal conductivity of pure magnesium matrix composite rate isless than the rate of pure magnesium maybe in that the introduction of a large number of interface in composite materials has an effect of scattering on the electron and phononmotion, which will impede heat conduct. However, the larger particles weaken thenegative influence of interfacial thermal resistance on heat conduction to some extent.Thermal conductivity of SiC/AZ91D composite compared to cast increased by30%after T4heat treatment. Because of composite had a mass of dislocations、residualthermal stresses etc. interface defect when the alloy is solid; of which, dislocations hadscattered with the thermal motion of free electrons scattering, and its free path haddropped, so result in thermal conductivity of the composite material to reduced. Butcomposite density after T4heat treatment had enhanced, meanwhile, heat transmissionchannel of composite increased, and the dislocation density of the composite materialafter heat treatment had reduced, matrix thermal residual stress somewhat better.Ultimately contributed to the improvement of thermal conductivity.SiCp/Mg composite exhibit excellent thermal conductivity properties. Bycomparative analysis of Maxwell model and experimental results, it is found that thethermal conductivity rate of the composite material comply with Maxwell model, and itis related to the thermal conductivity of the matrix alloy. This conclusion has theoreticalsignificance for composite material selection when λ_p> λ_m,λ_c> λ_m,whenλ_p<λ_m, λ_c <λ_m.