Microstructure evolution and enhanced mechanical properties of eutectic Al–Si die cast alloy by combined alloying Mg and La

In this paper, an efficient route that combined addition of excessive Mg (3.6wt.%) and a trace of La (0.5wt.%) as well as subsequent T6 heat treatment has been first proposed to modify the detrimental platelet-like β-Al5FeSi Fe-rich intermetallic to valuable granular-like π-Al8Mg3FeSi6 phase in eutectic Al–Si die alloys. The results of the DSC analysis revealed that majority of Mg play the leading role for precipitation of large script-like π-phase while rare earth La has the significant effect on modifying the eutectic Si phase simultaneously. A new phenomenon has been found that the morphology of the π-phase transformed from the script-like to fine granular-like by two main stages, including fragmentation and growth during solution treatment. By calculation, the coarsening rate coefficient of the segmented π-phase was about 2.9×10−19m3/h during growth stage. The tensile testing measurements showed that the ultimate tensile strength (UTS), yield strength (YS), elongation to fracture (EI) and the quality index (Q) of T6 treated eutectic Al–Si die alloys alloying with 3.6wt.%Mg and 0.5wt.% La can reach 312MPa, 270MPa, 4.28% and 406MPa respectively, and the enhanced mechanical properties corresponding to the evolution of the microstructure. [Display omitted] •Designed a high performance eutectic Al–Si die alloy by alloying Mg and La•All β-phases have changed to script-like π-phases by adding 3.6% Mg and 0.5% La.•Script-like π-phase changed to granular-like π-phase during solution treatment•The fragmentation mechanism of the script-like π-phase has been proposed.•Effects of Fe-rich intermetallics on the fracture behavior have been analyzed.