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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. [ABSTRACT FROM AUTHOR]

Very recently 63 Cu  NMR has been shown to be extremely sensitive in detecting and differentiating between the precipitate phases that form in Al–Cu alloys during heat treatment. This technique is now used to quantify the effectiveness of small additions of Sn to the alloy Al–1.7  at.%  Cu in promoting the rapid nucleation and growth of the θ′-phase precipitate. Two parallel series of 63 Cu  NMR spectra were recorded for Al–1.7  at.%  Cu and Al–1.7  at.%  Cu–0.01  at.%  Sn: (i) aged at 130° C to observe the comparative rate of phase evolution and (ii) aged at 200° C to observe the rate of growth of θ′-phase and to compare with the Vickers hardness of the alloys aged at 200° C for similar periods. Evidence is presented that a metastable precursor phase to θ′ (labelled ) is formed in Al–Cu–Sn which transforms to θ′ on further aging. [ABSTRACT FROM AUTHOR]

The quasicrystal structure of primary intermetallic particles found in the ascast microstructure of a permanent mould-cast Mg-8wt%Zn-4wt%Al alloy has been characterized using transmission electron microscopy. Most primary intermetallic particles have an average composition of Mg[sub 9±2]Zn[sub 4±1]Al[sub 3±1] and a primitive icosahedral structure, with a quasilattice parameter of 0.515nm and a point group of m35. The metastable quasicrystalline phase persists for extended periods at elevated temperatures; while the volume fraction decreases with thermal exposure at 325°C, the decrease is small up to 120h and a fraction persists after 720h. There is no evidence of transformation of or within the quasicrystalline phase during this period. Instead, the metastable quasicrystalline phase is gradually replaced by the equilibrium Φ phase (Mg[sub 21](Al,Zn)[sub 17]; space group, Pbcm). The relationship between the quasicrystalline phase, its cubic approximant T phase (Mg[sub 32](Al,Zn)[sub 49]; space group, Im3) and the Φ phase is discussed. [ABSTRACT FROM AUTHOR]