Effect of Mn and Sr on intermetallics in Fe-rich eutectic Al-Si alloy

Abstract

The effect of Mn and Sr on the precipitation of iron intermetallics from A413 aluminium alloy melts has been studied. A high concentration of iron (2.5% Fe) was selected to facilitate the identification of the intermetallic compounds. As usual for the iron-rich intermetallic particles, many were observed to be cracked, often precisely down their central axis.Mn additions above 0.9 per cent, in the absence of strontium, refines the beta-phase from large plates (needles in 2-D section) to a more numerous, compact, and polyhedral form. This phenomenon, often known as the “fragmentation” of the alphaphase intermetallics, is identified as a conversion from plate to dendritic morphology, plus an effect deriving from the non-straightening of double oxide films (bifilms) on which the intermetallics grow.Sr addition at 0.03 per cent produced the expected modification of eutectic silicon, but in the presence of Mn, Sr had negligible influence on altering the plate-like form of the primary beta-phase. However, the use of higher levels of Sr (more than 0.1%) was found to be a powerful factor in changing the morphology of the intermetallics from plate to star-like (i.e. dendritic). Sr was also observed to expand the central bifilms in intermetallics, converting the central cracks into open porosity. This was interpreted as the result of an increased hydrogen content opening bifilms.