Phase stability and structural features of matrix-embedded hardening precipitates in Al–Mg–Si alloys in the early stages of evolution

The strength of Al–Mg–Si aluminium alloys depends critically on nanometre-size Mg_xSi_yAl_z-type precipitates that have a face-centered cubic-based structure. In this work, a large number of early structures are investigated by means of first-principles calculations. Both platelet-type and needle-type precipitates are considered. Calculations show that for alloys with an Mg:Si ratio smaller than one, needle-type precipitates with Si pillars extending in the needle direction are energetically favoured. The formation of Si pillars and the low density cylinder is described. For alloys with an Mg:Si ratio larger than one, platelet-type precipitates consisting of stacked layers of Mg, Si and Al atoms are energetically favoured. Using both the information on the formation enthalpies and the calculated lattice mismatch with the Al matrix, it is discussed which structures are likely to be formed. The earliest, most favourable structures with high Al content are the needle-type initial-β″ Mg₂Si₃Al₆ structure and the platelet-type structures {MgSi}₂Al₁₀, {MgAl}₁Al₁₀, Mg₃Si₂Al₅ and Mg₂Si₁Al₃.