EFFECT OF β-AL5FESI AND π-AL8MG3FESI6 PHASES ON THE IMPACT TOUGHNESS AND FRACTOGRAPHY OF AL-SI-MG-BASED ALLOYS

This study investigated the decomposition of pi-AlMgFeSi into beta-Al5FeSi phase needles during extended periods of solution heat treatment and its effects on the mechanical properties of Al-7Si-0.55Mg-0.1Fe alloys. An analysis of the results obtained from the Charpy impact test using unnotched samples shows that the highest initiation and propagation energies are obtained for the as-cast and heat-treated alloys when these alloys are solidified at a high cooling rate and modified with strontium. An increase in the solution treatment time improves the impact properties of the alloys compared to the as-cast condition. In accordance with this finding, the recommended solution treatment time at which the maximum initiation and propagation energy values can be obtained is 20 h for all alloys studied. The results also show that the impact properties are more sensitive to the changes occurring in the microstructure which result from solution heat treatment and Sr modification, namely, the eutectic Si and pi-phase morphologies, rather than those related to the tensile properties, i.e., to the Mg content in the matrix. Fracture analysis was carried out using a scanning electron microscope equipped with an EDX system for element analysis. The results obtained show that the tensile and impact fracture behavior of the Al-7Si-0.55Mg-0.1Fe alloys is controlled mainly by the morphology of the eutectic silicon. The pi-phase iron intermetallics act as crack initiation sites and provide an easy path for crack propagation in both non-modified and Sr-modified alloys. The fracture analysis of the 80-h solution-treated sample shows the presence of newly formed beta-phase needles which provide an additional source for crack initiation, and thus all the 80-h solution-treated alloys show the lowest energy values.