Solidification and Structural Characteristics of α(Al)–Mg₂Si Eutectic

Abstract

Solidification and structural characteristics of α(Al)–Mg₂Si eutectic in the Al–Mg–Si system were investigated by thermoanalysis and directional solidification techniques. The two-phase eutectic line of L⇔α(Al)+Mg₂Si, the temperature-change section of a pseudobinary α(Al)–Mg₂Si system and the eutectic coupled zone within this section have been measured accurately. It was demonstrated that the coupled zone in the pseudobinary α(Al)–Mg₂Si system could be described as C=1.35×10³V+C₀, where C is the molar percentage of Mg₂Si, V is the solidification velocity in m/s, C₀ is the molar percentage of Mg₂Si when the eutectic structure is obtained through a near-equilibrium solidification, and 3.65 mol%<C₀<5.0 mol%. In this system, Mg₂Si is the nucleating and leading phase during eutectic growth, and the primary crystals exhibits a morphology of regular hexahedron with {100} faces. In the eutectic alloy, the morphology of phase Mg₂Si transforms from lamellae to rods when the solidification velocity is higher than 1.1×10⁻⁵ m/s. The relationship between the lamellar or interrod spacing, λ of eutectic Mg₂Si and the solidification velocity, V, fits the equation
λ^1.93V=7.1×10⁻¹⁷G^−0.414
where G is the temperature gradient ahead of the solid-liquid interface in the melt, which is in K/m, λ is in m and V is in m/s.