Effects of Mg, Fe, Be additions and solution heat treatment on the π-AlMgFeSi iron intermetallic phase in Al-7Si-Mg alloys

The presence of magnesium in Be-free Al-7Si-Mg alloys results in the formation of an undesirable iron-intermetallic known as the pi-AlMgFeSi phase. The effect of Mg, Fe, and Be on the formation of this phase in both unmodified and Sr-modified Al-7Si-xMg-yFe alloys containing 0.4-0.8-wt% Mg and 0.1-0.8-wt% Fe has been investigated at a dendrite arm spacing of 65 mu m. A qualitative microstructural examination was carried out to study the effect of solution heat treatment (540 A degrees C/8 h) on the decomposition of the pi-AlMgFeSi phase ("pi-phase") in Al-7Si-xMg-0.1Fe alloys containing 0.4-1.0-wt% Mg. The results indicate that increasing the Mg and Fe content increases the amount of the pi-AlMgFeSi phase formed. Quantitative measurements revealed a reduction in the surface fraction of the pi-phase after solution heat treatment. Different levels of decomposition of the pi-phase into needles of beta-Al5FeSi iron intermetallic phase ("beta-phase") were observed at 0.4-, 0.6-, and 0.8-wt% Mg, after solution heat treatment.