Microstructure evolution and Al₃(Sc_1−xZr_x) precipitates’ kinetics in Al-Zn-Mg alloy during homogenization

Abstract

The microstructure evolutions of two Al-Zn-Mg alloys, one of which was alloyed with Sc and Zr, and the kinetics of Al₃(Sc_1−xZr_x) precipitates in the Al-Zn-Mg alloy during homogenization were investigated. Both alloys under as-cast condition with supersaturated, non-equilibrium T(Mg₃₂(Al, Zn)₄₉) phase and impurities phase were displayed. When the homogenization temperatures are below 350 °C, Zn and Mg atoms precipitate from matrix; however, when the temperatures are above 400 °C, T phase dissolves into matrix, enhancing solid-solution strengthening. Kinetics of Al₃(Sc_1−xZr_x) precipitates was studied based on Jmat Pro software calculation and the difference values between the hardness of the two alloys in each homogenization condition. The calculations predict that the Sc and Zr solubilities in α-Al decline with the presence of Mg and Zn. Investigation of the difference values reveals that when the temperature is between 300 °C and 350 °C, the nucleation rate of Al₃(Sc_1−xZr_x) precipitates is the highest and the strengthening effect from Al₃(Sc_1−xZr_x) precipitates is the best. After homogenization at 470 °C for 12 h, non-equilibrium T phase disappears, while impurity phase remains. The mean diameter of Al₃(Sc_1−xZr_x) precipitates is around 18 nm. Ideas about better fulfilling the potentials of Sc and Zr were proposed at last.

Keywords

Al-Zn-Mg-Sc-Zr alloy / homogenization / Al₃(Sc_1−xZr_x) precipitates / kinetics / microstructures

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