Assisted nucleation of θ′ phase in Al–Cu–Sn: the modified crystallography of tin precipitates

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Abstract
The formation of particles of elemental tin in association with the nucleation of the precipitate phase θ′ in an Al–1.7 at.% Cu–0.01 at.% Sn alloy has been investigated by high resolution transmission electron microscopy. Analysis of lattice images has demonstrated that these tin particles associated with θ′ platelets formed during short-term ageing (typically 3 min at 200°C) exhibit a crystallographic form that is distinctly different from that previously reported in such ternary alloys and also from that observed in the binary alloy Al–0.01 at.% Sn. Those tin particles that could be clearly identified as β-Sn phase shared a crystallographic relationship of the form (001)Sn‖(001)Al‖(001)θ′, [110]Sn‖[100]Al ‖ [100]θ′ with the α-Al and θ′ phases with which they were simultaneously in contact. The observations imply that, while tin may assist nucleation of platelets of θ′, the formation of θ′ equally influences the crystallography of β-Sn that forms in association with θ′. Following prolonged ageing (typically 50 h at 200°C), the β-Sn particles in contact with θ′ plates had coarsened and diminished in number, and most now exhibited an orientation relationship with α-Al matrix similar to that observed in the binary alloy Al–0.01 at.% Sn (i.e. (100)Sn ) and to that previously observed in the later stages of ageing of the ternary alloy Al–1.7 at.% Cu–0.01 at.% Sn.