SOLIDIFICATION AND STRUCTURAL CHARACTERISTICS OF ALPHA(AL)-MG2SI EUTECTIC

Solidification and structural characteristics of alpha(Al)-Mg2Si eutec tic in the Al-Mg-Si system were investigated by thermoanalysis and dir ectional solidification techniques. The two-phase eutectic line of L d ouble left right arrow alpha(Al) + Mg2Si, the temperature-change secti on of a pseudobinary alpha(Al)-Mg2Si system and the eutectic coupled z one within this section have been measured accurately. It was demonstr ated that the coupled zone in the pseudobinary alpha(Al)-Mg2Si system could be described as C = 1.35 x 10(3)V + C-O, where C is the molar pe rcentage of Mg2Si, V is the solidification velocity in m/s, C-O is the molar percentage of Mg2Si when the eutectic structure is obtained thr ough a near-equilibrium solidification, and 3.65 mol% < C-O < 5.0 mol% . In this system, Mg2Si is the nucleating and leading phase during eut ectic growth, and the primary crystals exhibits a morphology of regula r hexahedron with {100} faces. In the eutectic alloy, the morphology o f phase Mg2Si transforms from lamellae to rods when the solidification velocity is higher than 1.1 x 10(-5) m/s. The relationship between th e lamellar or interrod spacing, lambda of eutectic Mg2Si and the solid ification velocity, V, fits the equation lambda(1.93)V = 7.1 x 10(-17) G(-0.414) where G is the temperature gradient ahead of the solid-liqui d interface in the melt, which is in K/m, lambda is in m and V is in m /s.