Thermodynamic calculated and TEM observed microstructure of Al–Li–Mg–Si alloys

The microstructures of two Al–Li–Mg–Si alloys have been studied by thermodynamic calculation and TEM observation in this paper. The results show that the addition of lithium changes the precipitation behavior of Al–Mg–Si alloys. The calculated Al–Li–Mg–Si phase diagrams are used to investigate the equilibrium phase distribution as a function of temperature for the Al–1.8Li–1.8Mg–0.8Si alloy and Al–1.8Li–2.8Mg–0.8Si alloy. According to calculation, δ′ phase (Al3Li) becomes the mayor precipitate in Al–1.8Li–1.8Mg–0.8Si alloy while a few amount of β′(Mg2Si) phase and AlLiSi phase precipitate. TEM observation show that δ′ phase and AlLiSi phase are precipitated in Al–1.8Li–1.8Mg–0.8Si and heterogeneous rod Mg2Si phase is also observed. Calculation on Al–1.8Li–2.8Mg–0.8Si show that both δ′ phase and Mg2Si phase precipitate on aging temperature. Experimental results are in agreement with calculation.