Evolution of Fe-Rich Intermetallics in Al-Si-Cu 319 Cast Alloy with Various Fe, Mo, and Mn Contents

In the present work, the precipitation and preferential selection of Fe-rich intermetallics in Al-Si-Cu 319 cast alloys with two Fe contents (0.3 and 0.7wtpct) and various Mo contents (0 to 0.4wtpct) were investigated. The results showed that two types of platelet beta-Fe (defined as eutectic beta-Fe and pre-eutectic beta-Fe) and alpha-Fe can precipitate depending on the alloy compositions, such as the Fe and Mo contents. Generally, the addition of Mo promotes the formation of alpha-Fe instead of beta-Fe. However, its effect on the phase competition between beta-Fe and alpha-Fe is greatly related to the Fe content. In alloys with low Fe content (0.3pct), an addition of 0.37pct Mo can promote the complete precipitation of alpha-Fe and suppress the formation of eutectic beta-Fe. However, in alloys with high Fe content (0.7pct), only the pre-eutectic beta-Fe amount decreases with increasing Mo addition, leaving the eutectic beta-Fe almost unchanged, while both pre-eutectic beta-Fe and eutectic beta-Fe can be fully suppressed with a combined addition of Mn (0.24pct) and Mo (0.4pct). Compared with Mn, Mo exhibits a stronger effect on the promotion of alpha-Fe. The combined addition of Mn and Mo can achieve better modifications of both pre-eutectic and eutectic beta-Fe in 319 alloys with high Fe content.