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Solidification Of SiC_w/Al-18Si Composites Prepared By Squeeze Casting

Posted on:2010-04-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:H M WeiFull Text:PDF
GTID:1101360302965577Subject:Materials science
Abstract/Summary:PDF Full Text Request
The microstructure of metal matrix composites (MMCs), especially the interfacial condition, was affected by their solidification processing. Solidification processing of matrix alloys with the presence of reinforcement is crucial in determining the matrix microstructure during the solid state processing of MMCs. However, at present the solidification theory of MMCs is not consistent. More studies need to be carried out in the effect of matrix alloys, cooling conditions on the MMCs solidification. In this paper, the influence of the reinforcement, modifier, cooling rate and cooling pressure on the nucleation, growth and microstructure will be investigated based on the common Al-Si matrix alloys.The Al-18Si alloy was prepared by high purity aluminum and single crystal silicon and was modified by Al-10Sr. The SiC whisker reinforced Al-18Si alloy matrix composites (SiC_w/Al-18Si composites) were prepared by squeeze casting method. The alloys used are Al-Si alloy and Al-Si alloy with Sr addition, respectively. The volume fractions of SiC whisker (SiC_w) are 15%, 20% and 25%. The microstructure of the composite matrix alloys was studied by optical microscope, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive Spectrometer (EDS) etc. The nucleation and growth behavior were also analyzed.The microstructure observation and theoretical calculation show that the primary Si nucleates on the SiC_w surface. SiC_w is surrounded by Si phase. The interfacial bonding between Si phase and SiC_w is strong. Calculations show that the mismatch between (211)SiC and (110)Si is 6.18% and the mismatch between (110)SiC and (111)Si is 7.65%. SiC_w content, Sr modifier, remelting temperature, cooling rate and cooling pressure do not affect the nucleation site of primary Si.With the increase of SiC_w volume fraction, the size of primary Si and Al-Si eutectic of MMCs prepared by squeeze casting decreases, distribution of these phases change from non-uniform to uniform state. The amplitude of primary Si size is greater than that of Al-Si eutectic. The onset and peak temperatures of primary Si decrease with increasing of whisker content and the quantity of primary Si decreases as well. The change of the onset and peak temperatures of primary Si in the SiC_w/Al-18Si composites is greater than that of the SiC_w/Al-18Si-Sr composites.The morphology of primary Si in SiC_w/Al-18Si-Sr composites changes from snow-like to platelet, even to particle shape. The size of primary Si reduces to 1/10~1/6 of the original size. The morphology of Al-Si eutectic varies from rodlike to nodular, and distribution shifts from close-packed to diffusion. The size distribution of Al-Si eutectic reduces. Sr make the onset and peak temperature of primary Si decreases and quantity of primary Si reduces in the SiC_w/Al-18Si composites.Synergism of SiC_w and Sr refines the primary Si and Al-Si eutectic deeply. Sr addition weakens the restriction of SiC_w on the solidification. SiC_w weaken the effect of Sr on the solidification temperature, undercooling and distributing of primary Si.The primary Si of the composites is refined and distribute evenly with the increasing of cooling rate. The primary Si of the composites changes from tree-like to granular. When the cooling rate is 100~1000℃/min, some primary Si are granular, others are rodlike. With the increasing of cooling rate, the number granular Si reduced gradually and the aspect ratio of the rodlike Si increased. When the cooling rate is 3570℃/min, the aspect ratio of rodlike Si increases. The cooling rate has no effect on the Al-Si eutectic morphology. The undercooling of primary Si and Al-Si eutectic in SiC_w/Al-18Si composites increases with increasing of cooling rate. The latent heat increases with increasing of cooling rate. The latent heat of SiC_w/Al-18Si-Sr composites is greater than that of SiC_w/Al-18Si composites. The latent heat of composites with lower volume fraction of SiC_w is larger than that of composites with higher volume fraction SiC_w.The size of primary Si changes with cooling pressure. When the pressure is 0~30MPa, the size of primary Si becomes smaller with the increasing of pressure. When the pressure is 30~50MPa, the size of primary Si changes to larger with the increasing of pressure. The changes of Al-Si eutectic with pressure is the same as primary Si. With the increase of pressure, the effect of SiC_w on the size of Al-Si eutectic is weakened gradually, but the role of Sr does not change yet.
Keywords/Search Tags:metal matrix composites (MMCs), solidification, modification, Al-Si alloy, nucleation
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