The roles of solidification cooling rate and (Mn,Cr) alloying elements in the modification of β-AlFeSi and hardness evolvements in near-eutectic Al-Si alloys

High recycling rates worldwide underline the economic and environmental value of aluminum scrap. However, several alloying elements in recycled Al negatively affect the final properties of castings, among which Fe has the most permissive effect on the mechanical properties. The extremely restricted solid solubility of Fe in Al leads to the formation of brittle Fe-containing intermetallic compounds (IMCs), and a possible strategy is to use modifiers of harmful IMCs by alloying. The present work aims to investigate the substitutional characteristic of Fe, Mn, Cr in the α-IMC taking the Al-12 %Si alloy as a reference, i.e., by analyzing the directional solidification (DS) of binary (Al-12 %Si), ternary (Al-12 %Si-1 %Fe) and quaternary (Al-12 %Si-1 %Fe-1 %Mn and Al-12 %Si-1 %Fe-0.6 %Cr) alloys. For all DS alloys castings, the macrostructure is shown to be characterized by columnar grains associated with solidification cooling rates (Ṫ) from 0.4 to 42.3 °C/s. The microstructure of the four alloys examined is shown to be typified by an α-Al dendritic matrix with interdendritic regions formed by α-Al, Si and different IMCs, which have been characterized and associated with the local Ṫ. The evolvement of iron-containing IMCs morphologies according to the addition of alloying elements in Al-Si alloys, is as follows: plate-like for Fe, plate-like and Chinese script for Mn, and fishbone and trefoil/blocky for Cr. Experimental power laws equations are determined relating the primary (λ1), secondary (λ2) and tertiary dendritic arm spacings (λ3) to Ṫ, for any experimentally examined DS alloy casting. The microhardness (HV) of the alloys is correlated with λ3 and Hall-Petch type equations are derived relating HV to both λ3 and Ṫ. The Al-12 %Si-1 %Fe-0.6 %Cr alloy achieved the highest HV among all the alloys examined.