Quantification of microsegregation during rapid solidification of Al-Cu powders
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AL solidification processes of binary alloys follow a Scheil-Gulliver solidification path for the distribution of solute. The proportion of the primary proeutectic and eutectic that form during solidification, as well as the solute content of these phases, can be determined using the equilibrium phase diagram. However, during rapid solidification, the undercooling and/or high cooling rates attained by the liquid can result in a metastable situation for both the pro-eutectic phase and the eutectic solidification. Consequently, different microstructures can form that are not expected from the equilibrium diagram or the relative proportions of phases and their solute content will vary from the equilibrium state. Such deviations from equilibrium and formation of new microstructures are the motivation for rapid solidification processing.[1] Reduced microsegregation and metastable extension of solute solubility are attractive features of rapid solidification processing. Microsegregation of the solute between the pro-eutectic phase and the liquid in rapid solidification processing can be studied in terms of the eutectic structures. The type or amount of eutectic is one measure of the extent of microsegregation under rapid solidification conditions. Wei et al. have shown that coupled growth and divorced growth occurred in Co-Sn[2] and Cu-Sb[3] systems, both produced from drop-tube experiments. Their results show that the nucleation undercooling during rapid solidification can affect the formation of eutectic. Sarreal et al.[4] have shown a change in the amount of eutectic formed as cooling ARVIND PRASAD, Graduate Student, and HANI NENEIN, Professor, are with the Department of Chemical & Materials Engineering, University of Alberta, Edmonton, AB, T6G 2G6, Canada. Contact e-mail: hani.henein @ualberta.ca KELLY CONLON, Research Associate, is with the Fuel Development Branch, Atomic Energy of Canada, Ltd., Stn. 63, Chalk River, ON, K0J 1P9, Canada. Manuscript submitted August 18, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS
rates are varied in the Al-Cu system. They studied Al 4.9 wt pct Cu alloy with cooling rates ranging from 10"1 to 104 K/s. There are few experimental data on the amounts of individual phases that constitute the eutectic under rapid solidification conditions. Such information can provide additional important information on solute distribution in rapid solidification processing such as strip casting. Some of the work done in this area has been limited to correlating eutectic morphology with the equilibrium amounts of individual phases constituting the eutectic.[5–8] For the Al-Cu system, crystallographic growth characteristics of the eutectic phases,[9,10] faceting behavior of CuAl2,[11] and morphologic characterization[12] have been reported. In this work, we report a new technique to quantify the average solute solubility in the primary phase. The paper describes the average solute solubility studied as a function of cooling rate and alloy composition. Figure 1 shows the schematic of a cooling curve for a droplet soli
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