Micro/Nano-structure,Mechanical Properties And Thermal Properties Of SiC-Al Powder Mixture Compacted By Equal Channel Angular And Torsion

As a kind of structural materials, SiCp/Al composites not only have lower density than traditional steel materials, but also exhibit excellently mechanical properties: stiffness, strength, dimensional stability and so on. Meanwhile, remarkable abrasion resistance, corrosion resistance, thermal properties and electrical properties make themselves a reasonable choice for functional materials in some circumstances. Traditional methods for fabricating such materials have some disadvantages, such as the necessity of secondary operation or the high fabrication cost. This study developed a new method for fabricating such materials. The powder mixture of Al and oxidized SiC was pre-compacted and wrapped by a copper tube.The tube was placed in the well-designed die of equal channel angular pressing and torsion (ECAP-T) and pressed at a certain temperature, leading to the consolidation of the powder mixture. This method can supply sufficiently hydrostatic pressure which homogenizes the distribution of SiC particles, refines Al grains, diminishes the pores and inhibits the emergence of cracks. Since the powder can be consolidated into dense bulk by ECAP-T without secondary operation, the fabrication efficiency is higher than those of traditional methods. Moreover, the fabrication temperature was relatively low, which is beneficial for saving energy.As a modified sever plastic deformation (SPD) process, ECAP-T processing which combine equal channel angular pressing (ECAP) with twist extrusion (TE) can not only supply a large amount of deformation, but also improve uniformity coefficient of deformation. Recently, the feasibility of compacting SiC-Al powder mixture by ECAP-T was confirmed and the metallographic structure of the deformed sample was preliminarily observed and a few mechanical properties were studied. However, SiC-Al interface, Al crystallographic structure, some mechanical and thermal properties haven’t been studied.ECAP-T process was numerically simulated by finite element method and the flownet and the velocity map of the powder wrapped in a tube were obtained. The deformation degree of the powder at different locations and the flowing characteristics of the powder at different times were analyzed. Based on the guidance of the numerical simulation, the powder samples with different SiC contents (10 wt.%,20 wt.% and 40 wt.%) were processed by different passes (1 pass,2 passes and 4 passes) ECAP-T at different temperatures (150℃,250℃ and 350℃). The densities of the deformed samples were examined and the optical microscopy (OM) images were observed. The results show that increasing deformation passes or decreasing of SiC contents can cause the density growth of the sample, the amount reduction of SiC clusters and the homogenization of SiC distribution. The deformation temperature (above 250 ℃) has little effect on the density and the SiC distribution. When the deformation temperature is too low (150 ℃), the pores and SiC clusters can be obviously observed.The interfaces of the composites fabricated by 1 pass ECAP-T at 250 ℃ were observed and characterized via transmission electron microscope (TEM), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The results show that the element interdiffusion exists between the matrix Al and the reinforcement SiC. It leads to the start-up of the protective reaction between the matrix Al and the oxidation layer SiO2 of the reinforcement. The reaction generates the new phase Al2O3 and avoid the emergence of the harmful phase Al4C3. Increasing the deformation passes can improve the interdiffusion degree and the reaction degree, therefore the consolidation of the interface was strengthened.The X-ray diffraction (XRD) profiles of the samples with 10 wt.% SiC fabricated by ECAP-T at 250℃ were analyzed and the relationship between deformation degree and Al crystallographic structure was obtained. The results show that increasing ECAP-T passes can enlarge the average size of Al subgrains, decrease the microstrain inside Al matrix and elevate the dislocation densities of Al grains. However, the variation amplitudes of both Al subgrain size and the microstrain are relatively weak, compared with pure Al powder processed by ECAP-T. The dislocation densities of Al grains inside the composites are also lower than those of pure Al.The mechanical and thermal properties of the deformed samples under different experiments conditions were examined by controlling variable method. The results show that increasing ECAP-T passes or SiC content can improve the strength, the stiffness and the hardness of the sample, diminish the thermal conductivity of the sample and make the sample can more easily obtain low coefficient of thermal expansion (CTE). However, the fabrication temperature has little effect on the mechanical and the thermal properties. It is noteworthy that too low fabrication temperature (150℃) should be avoided, or else the samples will obtain poor properties, due to the emergence of large pores. The mechanisms of three experiment parameters (ECAP-T passes, fabrication temperature and SiC content) influencing the properties of the samples were analyzed from several aspects (the SiC particles distribution, SiC-Al interface and Al crystallographic structure).