Phase stability and structural relations of nanometer-sized, matrix-embedded precipitate phases in Al–Mg–Si alloys in the late stages of evolution

AlMgSi alloys are commonly used in the automotive industry. In the bake-hardening process of AA6xxx alloys, nanometer-size MgxSiyAlz-type precipitates play a crucial role. A large number of structures and compositions occur between the initial supersaturated solid solution and the stable Mg2Si phase. The transition from pre-β″ to β″ phase marks the transition from face-centered cubic (fcc)-type structures to non-fcc-type structures. A new pre-β″ phase is identified with the composition Mg4Si7 that is energetically very favorable. Although the late precipitate phases (β″, β′, U1, U2 and U3) are very different in composition and crystallography, their mutual correspondence is made clear by an analysis of the orientation relationships and substructures. It is shown that they share the same substructures consisting of Mg hexagons which enclose a parallelogram of four atoms. Phase transitions are characterized by columnar and planar shifts of these substructures, and not by a total rearrangement of all the atoms.