The formation mechanism of Al6(Fe, Mn) in die-cast Al–Mg alloys

Abstract

The formation, 3D morphology and growth mechanism of the Al₆(Fe, Mn) phase were studied in Al–Mg–Mn–Fe alloys processed using high pressure die casting (HPDC). Thermodynamic calculations and the observed microstructures indicate that the Al₆(Fe, Mn) phase in the HPDC Al–Mg–Mn–Fe alloys is a dominant metastable phase in phase competition with the stable Al₁₃(Fe, Mn)₄ phase. The experimental results confirm that Mn addition in the alloy suppresses the transformation from the metastable Al₆(Fe, Mn) phase to the stable Al₁₃(Fe, Mn)₄ phase under non-equilibrium solidification conditions. Energy-dispersive spectroscopy (EDS) analysis of extracted particles reveals that the average Mn/Fe atomic ratio in the Al₆(Fe, Mn) phase decreases as the Mn/Fe atomic ratio in the melt decreases. It is also found that the Al₆(Fe, Mn) phase grows to form two elongated prism morphologies: rhombic prism (equilibrium morphology, bounded by {110} and {002}), and rectangular prism (growth morphology, bounded by {002}, {200} and {020}). The primary Al₆(Fe, Mn) phase shows a hollow structure and the eutectic phase is in the form of fine solid particles. The growth mechanism of the Al₆(Fe, Mn) phase is also elucidated according to the crystallographic rules and the morphological characteristics of the Al₆(Fe, Mn) phase.