Density Functional Study of the β″ Phase in Al-Mg-Si Alloys

The β″ phase is the major hardening precipitate in Al-Mg-Si alloys. It was studied by atomistic calculations based on density functional theory (DFT), using an atomistic model where the precipitate was embedded in an Al matrix. This allowed quantifying and visualizing the coherency strain in the matrix and within the precipitates. The elastic strain was found to decrease exponentially in the matrix as a function of distance from the precipitate interface. The formation enthalpy of several different chemical compositions of β″ was calculated, and the most stable composition was found to be Mg₅Al₂Si₄. A study of the calculated valence charge density and electron localization function showed that the covalency network between Si-atoms in the precipitate structure is broken when the precipitate contains Al.