Atomic scale investigation of the crystal structure and interfaces of the B′ precipitate in Al-Mg-Si alloys

B′ is a common type of metastable precipitate in over-aged Al-Mg-Si alloys, which is long regarded as a variation of the Q precipitate in Al-Mg-Si-Cu alloys due to the similarity between their lattice parameters. Atomic-resolution high angle annular dark field scanning transmission electron microscopy and energy dispersive X-ray elemental mapping at low beam damage conditions, as well as first-principles calculations were used to explore the atomistic structure of B′. It has a hexagonal unit cell with a space group P6¯ and lattice parameters a= 10.3(1) Å, c = 4.05 Å. The M sites in B′, which are analogous to Cu sites in the Q structure, were inferred with 50% Si atoms and 50% vacancies. The chemical nature of other sites agrees well with the model predicted by Ravi et al. The determined model Al3Mg9Si8 has the lowest formation enthalpy and the smallest lattice misfit with the Al matrix along the [0001] growth direction. Step-like boundaries with alternately arranged Mg-Si-Mg-Si atoms were observed at the coherent interfaces (101¯0)B′ // (5¯10)Al. A layer of defect structures sandwiched in the B′ precipitate was found geometrically necessary to allow both (5¯10)Al interfaces to arrange coherently in 3 dimensions and thus to relieve the strain of the surrounding matrix. The transformation from U1 → U2 → B′ was evidenced and the reverse transformation B′ → U2 nucleating at the incoherent B′/Al interfaces (150)Al is also likely. These results will provide new insight into the aging precipitation and compositional design of Al-Mg-Si(-Cu) alloys.