Crystallization and dissolution studies of iron intermetallics in Al-Si alloys

The crystallization behaviour of iron intermetallics during solidification and the dissolution behaviour of iron intermetallics during heat treatment and their subsequent effect on mechanical properties have been studied.Addition of iron decreases DAS and affects eutectic silicon particle size as nucleation and growth of silicon crystals takes place on the $ beta$-AlFeSi platelets, thus minimizing isothermal dendrite arm coarsening at the eutectic temperature. Increase of iron also increases the size and volume percent of the iron bearing intermetallics.In the absence of manganese, the iron intermetallics crystallize in the $ beta$-phase, at all cooling rates ranging from 0.1 to 20$ sp circ$C/s when cast from a normal casting temperature of 750$ sp circ$C. In the presence of manganese, the iron intermetallic crystallizes in $ alpha$-phase at low cooling rates and in both the $ alpha$- and $ beta$-phases at high cooling rates. This reverse crystallization behaviour is explained based on the segregation effect displayed by the phase diagram.When the melt is superheated to a high temperature (about 200 to 300 degrees above the liquidus temperature), the iron intermetallic crystallizes in the $ alpha$-phase at high cooling rates. This behaviour is attributed to the fact that $ gamma$-alumina which forms at low melt temperatures ($ le$750$ sp circ$C) acts as a nucleus for crystallization of $ beta$-phase. When the melt is superheated to a high temperature ($ ge$850$ sp circ$C), the $ gamma$-alumina transforms to $ alpha$-alumina. The $ alpha$-alumina is found to be a poor nucleus for the $ beta$-phase crystallization, and as a result the $ alpha$-phase forms. The importance of nucleation and growth undercooling for the crystallization of iron intermetallics is highlighted.Investigation of the dissolution behaviour of the iron intermetallics on non-equilibrium heat treatment indicates that the $ beta$-phase platelets dissolve slowly through concurrent fragmentation and then dissolution at the plate tips. Addition of manganese hinders the dissolution of iron intermetallics. The amount of liquid phase formed during non-equilibrium heat treatment increases dramatically once a critical temperature is exceeded. This critical temperature is estimated to be 520 and 515$ sp circ$C for samples initially solidified at 10 and 15$ sp circ$C/s respectively.The 0.15% Fe alloy exhibits the highest tensile strength and percent elongation compared to 1.0% Fe and 1.0% Fe + 0.5% Mn alloys under as-cast and equilibrium heat treated conditions. However, under non-equilibrium heat treatment conditions, ie., 30 degrees above the T4 equilibrium solution temperature, the strength properties of 1.0% Fe alloy exceed, or at least equal, that of the equilibrium heat treated 0.15% Fe alloy.An attempt is made to correlate the iron intermetallics present in the microstructure and mechanical properties with the associated fracture mode in this alloy