Solidification of Iron-Rich Intermetallic Phases in Al-4.5Cu-0.3Fe Cast Alloy

The iron-rich intermetallics in A206 type cast aluminum alloy (Al-4.5Cu-0.3Fe) were investigated using thermal analysis, interrupted quenching testing, and differential scanning calorimetry (DSC). An optical microscope with image analysis and scanning electron microscopy were used to identify and quantify the iron-rich intermetallics. Basically, two kinds of iron-rich intermetallics, Chinese script Al₁₅(FeMn)₃(SiCu)₂ (α-Fe) and platelet Al₇Cu₂(FeMn) or Al₇Cu₂Fe (β-Fe), are found in the final microstructures of the A206 cast alloys. All the possible solidification reactions, precipitation temperatures, and nucleation mechanisms for the iron-rich phases were investigated systematically. It is found that both α-Fe and β-Fe phases can precipitate on the oxide films. The α-Fe can also nucleate on Al₆(FeMnCu) and Al₃Ti particles. In addition, the previously formed α-Fe phase is also favorable for the nucleation of the β-Fe phase formed subsequently. These nucleation events were not only observed metallographically but also supported by the calculated planar disregistries. A possible nucleation hierarchy has been suggested.