The structure of the metastable precipitates formed during ageing of an Al-Mg-Si alloy

Abstract The morphology and crystal structure of the metastable precipitates formed during ageing of an A1-1·2%Mg2Si alloy were studied by means of transmission electron microscopy. The first precipitates formed were needleshaped G.P. zones, aligned along ⟨100⟩ directions of the matrix, which were coherent with the matrix along their major axes. In directions normal to the needle-axis the zones assumed an atomic arrangement different from that of the matrix and this gave rise to a cylindrical elastic strain field in the matrix surrounding each zone. During ageing, these zones developed into rodshaped precipitates with a hexagonal unit cell (α = 0·705±0·005 nm; c = 0·405±0·001 nm). Some rods had clearly evolved from zones which had been nucleated heterogeneously on matrix dislocations. When the rods had grown to about 1 μ in length, they began to lose coherency with the matreix along their major axes. Rods which had lost coherency appeared to have a hexagonal unit cell which was slightly different from that of rods which had not lost coherency. Finally, the rods transformed into platelets of the equilibrium phase (mg2Si).