The effect of lattice misfit on the interface structure of equilibrated γ-Ag2Al precipitates

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Abstract
The effect of lattice misfit on precipitate morphology and interface dislocation structure is investigated using the γ-Ag2AI phase of an A1–4.2 at.% Ag alloy as the reference system. Control of the lattice misfit has been achieved by ternary additions of Mg and Cu. The hexagonal plate morphology of the γ phase in the binary system becomes rounded with 0.51 at.% Mg addition and forms perfect hexagons with 0.51 at.% Cu addition. The progressive change in plate morphology could be correlated with change in the misfit which increased from 0.76% to 1.11% with the Mg addition and decreased to 0.42% with the Cu addition. Electron probe microanalysis showed that the Mg atoms preferentially partition to the γ phase, whereas Cu atoms partition equally between the precipitate and matrix phases. Direct transmission electron microscopy observations made on the interface structure showed the ternary Mg alloy to consist of a hexagonal network of (a/6)〈112〉 Shockley partial dislocations having an equilibrium spacing. A single array of (a/2)〈110〉 dislocations was observed in the binary and ternary Cu systems. X-ray diffractometer determination of lattice spacings showed the linear array spacing to be non-equilibrium. The failure to attain equilibrium interfacial structures and the role of lattice misfit on the overall microstructure are discussed.