Transformation behavior of precipitates during artificial aging at 170 °C in Al–Mg–Si–Cu alloys with and without Zn addition

Abstract

The transformation behavior of precipitates in Al–Mg–Si–Cu alloys with and without Zn addition during artificial aging at 170 °C was investigated in this study. For Al–Mg–Si–Cu alloys with and without Zn addition, two types of precipitates, needle-like β″ and lath-like L, are formed with the aging time increasing from 8 to 34 h. The needle-like β″ precipitate exhibits a Mg-to-Si atomic ratio of 1.31 which is in agreement with the theoretical ratio of 1.25 for β″ (Mg5Al2Si4). In addition, the β″ precipitate is supposed to contain Al and Cu. With further partitioning of Mg, Si and Cu and release of Al, the needle-like β″ precipitate can transform to lath-like L phase. The L phase has significantly higher concentrations of Mg, Si and Cu and lower concentration of Al, with a higher Mg-to-Si atomic ratio (1.41) deviated from theoretical ratio of β″. However, the major of Zn has a random distribution in Al matrix and exhibits no significant partitioning into precipitates. Zn remained in matrix can enhance the transformation of β″ precipitates to L precipitates.