Microstructures And Mechanical And Corrosion Behavior Of Silicon Carbide Particle Reinforced Aluminum Metal Matrix Composites

The effects of size and volume fraction on the mechanical properties and microstructures of particle reinforced pure aluminum metal matrix composites(Al MMCs) were studied. The Al MMCs reinforced with 25nm, 150nm and 3.5μm SiC particles and the Al without particles for comparison were fabricated with a powder metallurgical process. The results show that the yield strength, ultimate tensile strength, Young's modulus and macrohardness all increase with increasing the volume fraction and decreasing the size of the particles, whereas the elongation and reduction in area decrease. The nanometric powders have markedly strengthening effect on pure Al when the volume fraction of SiC is very low. However, when the volume fraction increases, the strength increase is affected by the particle agglomeration to some extent. The particle agglomeration may also result in lower plasticity of the MMCs.The relationship between strength and hardness was investigated. The results show that the ratio of ultimate tensile strength (σ_b) to Brinell's hardness (H_B) for the fine particle(25nm and 150nm) reinforced MMCs is independent of volume fraction of SiC particles, and it can be expressed as σ_b/H_B≈1/3. However, for more larger SiC(3.5μm) reinforced MMCs, σ_b/H_B≈1/3 is satisfied well only when the volume fraction is higher. When the volume fraction is lower (≤10 pct), the result obtained from the above relation will give an error about 9 pct, i.e. a non-linear relationship exists between the σ_b and H_B. The experimental results also show that the relation σ_b/H_B≈1/3 is satisfied very well when the volume fraction of SiC is higher than 10 pct, and it is independent of SiC size.During the MMCs fracture, the fracture path tends to avoid SiC particles. However, as the volume fraction increases, the fracture tendency of the nano or sub-micrometric SiC agglomeration increases;and for the bigger particle(3.5μm SiC), the particle fracture tends to increase.It is well known that the presence of particles can affect the texture developmentduring deformation, but this effect has not yet been studied quantitatively as a function of the concentration and size of the hard phase carefully. The texture development is investigated in SiCp/pure Al MMCs for a wide range of particle volume fractions and sizes.The results show that the extrusion texture in pure Al is composed of fibre. When SiC is introduced into pure aluminum, the main component of texture is not modified, but the intensity of the component evolves with the volume fraction and average size of SiC particles.For the MMCs reinforced with 3.5um SiC particles, when the volume fraction of SiC is low, the intensity of the Al texture is a little higher than in the pure Al, whereas for a SiC concentration higher than 5%, the texture becomes more isotropic and is lower than in the pure Al. For the MMCs reinforced with 150nm SiC, the intensity of the Al texture is higher than in the matrix, and increases with increasing the volume fraction of SiC particles. And for 25nm SiC reinforced pure Al MMCs, the texture intensity is much higher than in the Al, and increases quickly as the volume fraction of SiC particles increases. And it is found that nanometric particles may introduce some new component into the deformation texture. For all the MMCs, when the volume fraction of SiC particles is constant, the texture intensity increases as the size of SiC particles decreases.Research devoted to corrosion behaviour of MMCs has been spare in comparison with research on fabrication and mechanical behaviour, and some results about the corrosion behaviour of SiCp/Al MMCs are often contradictory, particularly about that of SiCp/2024Al MMCs. The influences of volume fraction and average size of SiC paniculate reinforcements on the corrosion characteristics of SiCp/2024 Al MMCs in aerated 3.5% NaCl solution were investigated. The electrochemical behavior was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy, the general corrosion behavior of these composites was studied further by immersion test. The results show that pitting susceptibility is about the same for the composites and their alloy. The corrosion potentials are also independent of SiC phase. The corrosion resistance for the composites decreases as the volume fraction increased or particle size was reduced.Pit morphology was observed with SEM. During anodic polarisation in 3.5% NaCl, pits on the MMC is more numerous and non-uniform in size compared with pits on the 2024A1, and severe crevice corrosion is only found at the bottom of pits on the MMC. The biggest pits on the long term immersion MMC are deeper than that on the 2024A1, and crevice corrosion is only found also. And crevice corrosion may make pitting corrosion resistance degrade. Pit morphology is also affected by the volume fraction and average size of SiC particles. For the MMCs, the pits are more numerous and slightly smaller in size as the volume fraction increased or particle size was reduced.The corrosion mechanism of the MMCs is galvanic corrosion between the SiC and the matrix and between the phase rich in Cu and the matrix.In order to improve the corrosion resistance of the SiCp/2024Al MMC, it was anodized in sulfuric acid. The corrosion protection of the anodized coating on the MMC was evaluated by both potentiodynamic polarization and electrochemical impedance spectroscopy and compared with that of sulfuric acid anodized 2024Al. The results show that the anodized coating on 2024A1 provides excellent corrosion resistance to 3.5% NaCl solution. The anodized coating on the SiCp/2024Al MMC provides satisfactory corrosion protection, and the corrosion resistance of the film sealed in dichromate solution is better than that in hot water. The anodized film on SiCp/2024Al MMC is not as effective as on 2024A1 because the structure of the anodized layer is affected by the SiC particulates.The investigation regarding the mechanism of film formation, performed on a scanning electron microscope, reveals that the thickness of the anodic film is non-uniform;however, the film-solution interface is relatively uniform compared with film-composite interface. This is believed to be due to the non-uniform distribution of SiC particles in the matrix. The thickness of the film increases with increasing the current density, and the film formed on 2024A1 is much thicker than that on the MMC in the same condition. The surface morphology of the film on MMC formed at low current density exhibits more smoothly than that at higher current density. This is believed to be the main reason of high corrosion resistance provided by film formed on the MMC at lower current density.Observation in the transmission electron microscope indicates that the pores in the anodic film which was formed, appears round when they grow perpendicular to the surface and reveals elongated when they develop at an angle on a rough surface or are shielded by SiC particles. The distribution of pores in the anodized MMC is not uniform. The pores, which develop near SiC particles, are more in quantity and bigger in average size than that away from SiC. The biggest size of pore is about 50nm. And the distribution of pores in the anodized 2024A1 is fairly uniform, and the pores exhibit a narrow size distribution also, the sizes are approximately 10-30nm. This shows that the film on anodized MMC possesses the microstructural features of lower corrosion resistance.