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This study investigates the precipitation of Fe containing intermetallic compounds in A413 aluminium alloy in the Cr- and Co-modified conditions. The Fe content varied from 0.5 to 2.5 wt-%. For each amount of Fe, different levels of Cr and Co were added to identify the optimum Fe:Cr and Fe:Co ratios to eliminate the beta-phase and obtain microstructures containing well distributed fine intermetallic alpha-phase in dendritic or Chinese script morphology. It was found that the addition of Cr and Co changes the morphology of Fe rich intermetallic compounds from the harmful plate like beta-phase to a less damaging morphology. The critical ratios were Fe:Cr = 3.0 and Fe:Co = 1.0 approximately. In addition, it was noted that the addition of Cr or Co promotes the formation of primary Si. The results suggest that the presence and the form of all precipitates (both Fe rich and primary Si) seem to be strongly related to the presence of bifilms (doubled over oxide films) suspended in the liquid alloy. (c) 2005 W. S. Maney & Son Ltd.
Mahta et al. (Tue,) studied this question.