The morphology of precipitates in an AlGe alloy-II. Analysis using symmetry

G. R. Hugo, B. C. Muddle

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The range of morphologies observed for Ge precipitates in quenched and aged AlGe alloys (4.0 wt% Ge) has been analysed in terms of the symmetry of precipitate and matrix phases and the orientation relationship between them. For precipitates in the form of (i) laths parallel to 〈100〉Al, (ii) rectangular plates on {100}Al, (iii) hexagonal plates on {111}Al, and (iv) rods parallel to 〈110〉Al, each precipitate shape is found to be consistent with the symmetry of the intersection point group defined by point symmetry elements common to precipitate and matrix structures in the observed orientation relationship. For those precipitates which exhibit an identity or "cube-cube" relationship with the matrix phase, there is a correspondence between translational elements of symmetry in the two structures and a complete analysis requires the determination of the full space group defined by the intersection of the individual space groups of the precipitate (Ge) and matrix (Al) structures. Those precipitates in the form of regular tetrahedra with {111}Al facets are found to be consistent with the intersection space group (F43m)_describing the lattice obtained by superposition of the Ge and Al lattices in an identity relationship, with a Ge atom site somewhere coincident with an Al atom site. However, it remains impossible to account for those Ge precipitates occurring as triangular plates on {111}Al in terms of this analysis. It seems likely that such precipitates nucleate heterogeneously and that it will be necessary to incorporate the symmetry of the nucleating defect in a complete symmetry analysis. One possible analysis is outlined briefly.

Original languageEnglish
Pages (from-to)365-374
Number of pages10
JournalActa Metallurgica Et Materialia
Issue number2
Publication statusPublished - 1 Jan 1990

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