The effects of microstructure and intermetallic phases of directionally solidified Al–Fe alloys on microhardness

Iron is the dominant impurity in Al of commercial grade purity and is also added deliberately in some Al alloys compositions to improve the high temperature strength. Al–Fe castings show the formation of different metastable intermetallic phases (AlmFe, Al6Fe, etc.), which may affect decisively the mechanical properties. In the present study, upward transient directional solidification experiments were carried out with hypoeutectic Al-1.0 and 1.5 wt% Fe alloys. The microstructure evolution and Vickers microhardness (HV) profiles were determined for both alloys, and characterization of Al–Fe intermetallics was carried out. The cell spacing, λc, has been correlated with the local HV microindentation and Hall–Petch type equations are proposed. Comparisons were performed with hardness results available for hypoeutectic Al–Ni alloys. It was found that the microhardness is affected by both the iron content of the alloy and the cellular spacing.

► Non-equilibrium/equilibrium microstructures of Al–Fe alloys have been characterized.
► The correlation microstructure/cooling rate is fundamental.
► The effect of cell spacing and type of intermetallics on hardness was investigated.
► Hall–Petch type equations are proposed relating hardness to the cell spacing.