Micro-Structure Of Liquid Al-Si Alloys And Evolution Of Si Clusters

This paper presents the variation of the micro-structure of liquid Al-Si alloys , on the atomic scale, with the temperature, adding elements , etc. The liquid structure of pure Al, Al melt with solid particles , and Al-Si alloys are studied by 9-9 X-ray diffraction analysis, and the evolution of the liquid structure was analyzed when we were adding modifying agent Sr and Sb to the melt and using DC on it ; Thermal parameters of solidification were measured in cooling process by means of DSC. More attention has been paid on the Si clusters in the Al-Si alloys and on the relationship between the solid and liquid structure of Al-16%Si alloy.In the range of 670-1420 ℃, structure parameters of liquid Al were measured It is found that atomic density of pure Al melt is 0.0486-0.0524 unit/(A|°)~3, its nearest coordination number is 11.236-11.247 , and its nearest neighbor atomic distance is 2.76-2.85(A|°). The slight decrease of the nearest neighbor atomic distance with rising of temperature testifies thermal expansion of melt realized by formation of the free volume but not by the increase of the atomic distance uniformly.Referred to the structure in the mixture of Al melt with adding solid particles, the atomic density of Al melt and the mixture was for the first time compared with increasing temperature. It showed that the decreasing rate of atomic densities of the mixture is lower than that of Al melt, meaning particles can weaken the thermal expansion of the mixture with temperature and implying that part of free volume in the mixture was occupied by particles.Structure parameters of liquid Al-Si alloys in range of 0-20%Si were measured in this paper. It is found that atomic densities of hypo-eutectic alloy increased and coordination number decreased with Si% rising , and there was a maximum of atomic density at eutectic composition. This is because the free volume was consumed up by the particles since the concentration of free volume in the Al melt is limited.According to the properties of Gaussian function , radial distribution function (RDF) of the liquid metal was for the first time demonstrated to be separated by Gaussian peaks, in the zone of 0-6 A, based on the separation of distribution function 4πr ρ(r) with Gaussian functions that is supported by Einstein model of liquid model. The difference between the separation result by these two method is less than 1.5%, and the difference between result of separation of RDF and some literature is less than 0.8%.With rising of Si concentration in the alloy melt, small Si-Si peaks were found in Gaussian separation of RDF of the melts , implying Si-Si clusters with covalence bond in the melt, whose area firstly reduce in low temperature range (below 875 ℃) and secondly increase in high temperature (over 875 ℃). Al-Si clusters exist in the melt, which could be detected by our equipment, interacting with Al-Al and Si- Siclusters when conditions of melt changed.Modifying agent Sr can weaken the covalence bond of Si-Si clusters in the Al-Si melts, reducing the size and amount of Si-Si clusters, and restrain the segregation of Si atoms in melt and strengthen the nucleation of Al phase.The first peak of S(Q) of eutectic Al-Si alloy lowered with adding Sb element, at the same time, there is a prepeak on the S(Q), which disappeared when temperature is high enough. Prepeak means the existing of medium-range order or chemical order in the melt. Prepeaks on the structure factor, powder X-ray diffraction, SEM, and direct current(DC) processing on the alloy melt showed together Sb atoms formed compound in the melt. The Gaussian separation of RDF of melt with and without Sb addition indicates Sb can strengthen the Si clusters in the eutectic alloy. DSC analysis showed the supercooling temperature became less with Sb addition. These make it clear that Sb can wider the order range and promote the nucleation of Si phase in cooling process.The superheating at high temperature has no obvious influence on the size and shape of eutectic silicon of hypo-eutectic Al-Si alloy in solid state, but can increase the size and amount of primary silicon phase while decrease the size and amount of eutectic silicon phase of the hyper-eutectic Al-Si alloys. The primary and eutectic solidification temperature of Al-16%Si increase with superheating temperature, so does its latent heat of the solidification .The temperature of primary solidification , temperature and latent heat of secondly solidification lowered with raising temperature, while the latent of primary solidification kept constant , with superheating temperature below 875 °C; The temperature and latent heat of primary solidification and secondly solidification increased with raising temperature with superheating temperature over 875 °C. This testified that more Si clusters formed with increasing superheating temperature.Above the liquidus, there is a small crystalline peak on the diffraction intensity curve of Al-16%Si alloy after superheated at 1250, this means the Si-Si clusters are promoted by superheating, resulting in stronger nucleation of Si phase in cooling process, which is different viewpoint that the liquid structure of the alloy become more and more uniform with increasing temperature.Combining the morphology of the Al-16%Si , solidification parameters with variable superheating temperature, and the result of X-ray diffraction on liquid structure, conclusion can be gotten that evolution of Si clusters is the clue of the relationship between the solid and liquid structure of Al-16%Si alloy.