eutectic according to this reaction: Liquid → Al(solid) + Si(solid) + β At slower solidification rates,
however, primary β should form. At higher iron contents (e.g., 0.6%) pri- mary β forms before any Al-Si eutec- tic, regardless of the freezing rate. This alloy was studied by the
Fig. 4. The ternary phase diagram for the Al-Si-Fe system is shown.
magnesium, but this is ignored in the calculation.) To simplify pre- sentation of the results, only the phase boundaries are shown. Also a detailed section from Fig. 5 is taken. The result is shown in Fig. 6 for alloy iron contents of 0.3 and 0.6%. Two segregation curves are given for each case. The lower (red) curve
is for a solidification time of 10 seconds. The upper (blue curve) is for a longer freezing time of 1,000 seconds. From this result, it can be seen that 0.3% iron is a borderline case for this alloy. In rapidly solidified parts of a casting there should be no primary β phase, only a ternary
Center for Advanced Solidification Technologies (CAST) researchers in Australia. They found that casting defects were associated with iron contents that produced primary β. However, when they switched to a higher silicon version of the same alloy, the defects went away. The reason for this behavior may be seen by considering Fig. 7. Similar calculations are made for the same two iron contents. In this alloy the higher iron content (0.6%) becomes the borderline case. Therefore, 9% silicon alloy can tolerate twice the iron content of the 5% silicon alloy. In exactly this manner, the CAST
June 2014 MODERN CASTING | 37
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