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Abstract
The effect of iron, manganese, and chromium concentration on the morphology of complex intermetallics has been studied in an Al-12.7% Si alloy containing 0.1 % Cr with three levels of iron (0.4, 0.8, 1.2%), and five different levels of manganese ranging from 0.0 to 0.5%. The intermetallic compounds form at high iron contents or at high concentrations of manganese and chromium with low iron. Star-like intermetallics form when the sludge factor is greater than 1.35. 0.1% chromium with more than 0.2% manganese are needed to convert all platelet phases to small star-like intermetallics. It was found that the temperature of sludge formation increases when the content of iron in the alloy is increased.
When the cooling rate is slow, very large intermetallics of Chinese script morphology form in a coarse microstructure. The amount and the volume percentage of the compounds increase significantly with decreasing cooling rate. Polyhedral (sludge) particles form at a slow cooling rate and high iron concentration. The morphology and volume fraction of all types of intermetallics are dependent on the cooling rate. Fast cooling rate and manganese addition can change the brittle iron-platelet phases which are harmful to the mechanical properties of the alloy into the very small star-like compounds. When the total volume percent of intermetallics increases with decreasing cooling rate, the number of particles of each phase per unit area decreases and the size of particles of each morphology increases. The volume percentage of intermetallics decreases significantly with increasing cooling rate; a relatively large amount of impurity elements can be dissolved in the microstructure when the cooling rate is high. An exponential equation can express the effect of cooling rate on the total volume percentage of intermetallic compounds with a good correlation between the experimental data and the equation.