Development Of Al-Si-Fe Master Alloy And Its Underlying Effect In Enhancing Silicon Phase Nucleation In Al-Si Alloys

The formation of solid crystals from solution is a first class phase transformation, which is commenced by nucleation. The initial nucleation determines the finial morphology and size distribution of solid crystals, and further determines the corresponding performances and proprieties. For example, in Al-Si alloys, the nucleation of aluminum phase (primary α-Al and eutectic Al) and silicon phase (primary silicon and eutectic silicon) determine the corresponding grains’size and morphology, and then control many kinds of properties, e.g., mechanical properties and corrosion resistance. Hence, the nucleation process of Al-Si melts are usually carefully controlled in industry, in order to control the produced castings’s properties. Based on the classical nucleation theory, there are two kinds of nucleation methods, i.e., homogeneous nucleation and heterogeneous nucleation. Based on heterogeneous nucleation, a master alloy (grain refiner or inoculant) is often introduced into melts to enhance heterogeneous nucleation and refine grains. However, this thesis reports a kind of nucleation mechanism beyond classical nucleation framework, cluster agglomeration based nucleation mechanism, which is based on systematically experimental results and can be used to control solidification structure.First, through systematically investigation of Al-Si-Fe solidification behavior, the author proposes a method to instigate cluster agglomeration based nucleation. Iron is an inevitable impurity in Al-Si alloys, which is always viewed as a detrimental element. However, β-Al5FeSi containing Al, Fe and Si elements has be identified as a nucleating substrate of silicon phase, and this may offer a way to utilize the superfluous iron in Al-Si alloys. Combing calculated phase diagram, and XRD and SEM measurements, a kind of Al-Si-Fe master alloy is successfully synthesized, which can be used to inoculate the silicon phase in Al-Si melts. In with the in-situ XRD patterns measured during the Al-10Si-2Fe solidification process, prepeak represents a kind of (AlFeSi) clusters appears and its position coincides with α-Al8Fe2Si but differs largely from β-Al5FeSi. Therefore, the master alloy is proposed to undergo the following structural evolution:β-Al5FeSiâ†'α-Al8Fe2Siâ†'(AlFeSi) cluster, when added into Al-Si melts.Second, the inoculating efficiency of the Al-Si-Fe master alloy on primary silicon in Al-20Si melts and its dependence on melting temperature and holding time are carefully investigated. In Al-20Si melts, the primary silicon size increases with time (60min,120min and180min, held at780℃) and returns to nature level finally. When held at800℃and850℃for60min, primary silicon is refined in the former one while unrefined in the later one. In another case, the inoculated melts are held at700℃,750℃,800℃for15min, and the primary silicon size increases marginally with temperature but is much smaller than that in uninoculated alloy. In the inoculated silicon particles, a high density of a kind of undermined nanoscale particles are identified. Apparently, the (AlFeSi) clusters evolved from the Al-Si-Fe master alloy play a key role in enhancing the nucleation of silicon phase and finally evolve into these iron-impoverished nanoscale particles.Third, the inoculating efficiency of Al-Si-Fe master alloy on eutectic cells in Sr-modified Al-10Si melt is investigated. Further combined with Sr-modification mechanism, a cluster agglomeration based nucleation mechanism is proposed, which is responsible for the function of the Al-Si-Fe master alloy. After further inoculation, eutectic cells size decreases from1450μm in exclusively modified sample to350μm. However, in the investigated period, eutectic cannot recover its normal level (＜100μm) in unmodified sample before modification effect disappears completely, and eutectic cell size decreases with decreasing Sr level. In the modified and further inoculated silicon, a high density of a kind of nanosacle particles(～10nm) appear, but the normal twins that should appear in (111)Si disappear anomalously. These nanoscale particles are consistent with those identified in the inoculated Al-20Si alloy, which evolve from (AlFeSi) clusters. According to a Sr-modification mechanism, impurity induced twins, Sr-modification is always accompanied by a high density of twins in (111)Si. Therefore, the abnormal disappearance of twins is attributed to altered nucleation and growth method imposed by inoculation, i.e., nucleation and growth no longer depend on individual atom accretion. Considering all the above, the nucleation mechanism of silicon after inoculation is described as:the (AlFeSi) clusters evolve from Al-Si-Fe master alloy stimulate the segregation of silicon atoms about them, and induce the migration of iron atoms into melt, which finally forms the precursor. The nucleation and growth of silicon are achieved by agglomeration and accretion of the precursors and residual individual atoms. Since Sr alters the atomic liquid structure of Al-Si melt and delays the segregation of silicon atoms around (AlFeSi) clusters, the temperature at which (AlFeSi) function is consequently delayed. In this circumstance, inoculation has little effect of the nucleation temperature of eutectic cell but can dramatically increase the minimum temperature before recalescence. For partially modified melt, the dependence of eutectic cells size on Sr level is explained by a two-step nucleation process. When the Sr level is insufficient (e.g.,<80ppm), the initial nucleation of eutectic cell is not affected by Sr, but the second-step nucleation is completely affected by Sr. As a result, the second-step nucleated eutectic cell usually may have a bigger diameter, which is dependent on the Sr level. The higher the initial Sr level, the bigger the second-step nucleated eutectic cell.