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 degrees
Celcius) 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‖(111)Al, [010]Sn‖[112]Al) and to that previously observed in the later stages of ageing of the ternary alloy Al–1.7 at.% Cu–0.01 at.% Sn.