Fig. 1. The phase diagram of Al-Si shows the eutectic point.
Fig. 2. This phase diagram shows the shift of eutectic point due to solid solution of Ba in Si as imposed in the Al-Si phase diagram.
Two major components coexist in the microstructure of hypoeutectic Al-Si alloys: the primary, aluminum rich phase and the eutectic micro- structure. The primary phase con- tains about 1.67% silicon as a solid solution and is in dendrite form. The eutectic structure, consisting of
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an aluminum-rich solid solution and virtually pure silicon, exists between the arms of the primary aluminum dendrites. Refinements of silicon by adding trace amount of impurities such as sodium and strontium can improve mechanical properties of resulting castings.
However, current impurity-con-
taining hypoeutectic Al-Si cast alloys have yielded only modest improve- ments in ultimate tensile stress (UTS) (not in excess of 180 MPa) and ductil- ity (roughly 10%). Two reasons explain these moderate increases: The silicon phase in these cast
alloys is not sufficiently refined to offer a high UTS value. The eutectic point permits the
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proportion of primary aluminum and eutectic structure to promote a ductility less than 5%. T e potential exists to alter the
primary aluminum to eutectic struc- ture ratio and refi ne silicon morphol- ogy of Al-Si alloys with the addition of barium to improve strength and ductility. Recent work on the solidi- fi cation of hypereutectic Al-Si alloys (having between 15-20% silicon) has focused on the solubility of barium in the silicon phase. T is research has established primary silicon-free hypereutectic alloys with up to 17wt% silicon can be produced by directional solidifi cations. A shift of the normal eutectic point (shown in Figure 2) from 12.7wt% to 17.0wt% silicon caused by the addition of barium into the melt and related impurity modifi - cation mechanisms may help develop these alloys.
The same concept, which alters
the ratio of the primary aluminum to eutectic phase and refines the morphology of eutectic silicon, has now been used to develop high strength, highly ductile hypoeutectic Al-Si alloys by conventional casting.
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