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In this study, voids commonly associated with tin particles in two aluminium alloys containing microalloying additions (0.01 at.%) of tin have been observed by transmission electron microscopy. The voids were generated by quenching the alloys at moderate rates (102-103 K s-1) from a temperature (718 K) in excess of the melting temperature (501 K) of elemental tin in tin-microalloyed aluminium. Estimates of the void volume as a function of the volume of the associated tin particle reveal a linear relationship consistent with the excess volume resulting from the solidification of the tin particle. The formation and stabilisation of shrinkage voids in metallic alloys are suggested to arise from a combination of high vacancy supersaturation, the large volumetric misfit strain of the solidifying tin particle and a reduction in void surface energy associated with segregation of alloying elements. [ABSTRACT FROM AUTHOR]

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 structure of carbon (C) and boron nitride (BN) columnar particles produced alongside by heat-treating a turbostratic B-C-N compound was investigated by analytical transmission electron microscopy. The particles were shown to be helical cones with a well-defined apex, and a helical pitch equal to usually one and at most two layers. The cone apex angle distribution was found to be centred on different values for C and BN. These results strongly suggested that BN and C cones nucleated from different ring defects in the hexagonal networks: square-like ring defects in BN and pentagon-like ring defects in C. These observations were consistent with the supposition that the creation of non B-N bonds and hence odd-membered rings is avoided in BN. Such restriction in BN could also allow one to understand the observed morphological differences between C and BN cones, and in particular the relative difficulty for BN cone edges to join via curved sheets. [ABSTRACT FROM AUTHOR]