Abstract
The morphology and crystal structure of a previously unreported type of precipitate in aluminium–zinc alloys, designated Y phase, has been studied by means of transmission electron microscopy. Y phase precipitates were formed only in samples that were rapidly quenched before ageing, and then only at ageing temperatures within a narrow range on either side of the limiting temperature for GP zone formation. The precipitates were found to have an octahedral shape, with the faces of each octahedron parallel to {l11} planes of the matrix. Evidence obtained indicated that the Y phase has a face-centred-cubic unit cell with a lattice parameter of 0.377 ± 0.001 nm. During isothermal ageing the Y phase precipitates transformed to the equilibrium phase by the epitaxial growth of zinc platelets at the {111} interfaces between Y phase and matrix.
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