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Register a new userMechanisms of Ti3Al precipitation in hcp alpha-Ti
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Nucleation and growth of Ti$_3$Al textalpha{}$_2$ ordered domains in textalpha{}-Ti--Al--X alloys were characterised using a combination of transmission electron microscopy, atom probe tomography and small angle X-ray scattering. Model alloys based on Ti--7Al~(wt.%) and containing O, V and Mo were aged at SI{550}{celsius} for times up to SI{120}{day} and the resulting precipitate dispersions were observed at intermediate points. Precipitates grew to around SI{30}{ anometre} in size, with a volume fraction of 6--10% depending on tertiary solutes. Interstitial O was found to increase the equilibrium volume fraction of textalpha{}$_2$, while V and Mo showed relatively little influence. Addition of any of the solutes in this study, but most prominently Mo, was found to increase nucleation density and decrease precipitate size and possibly coarsening rate. Coarsening can be described by the Lifshitz-Slyozov-Wagner model, suggesting a matrix diffusion-controlled coarsening mechanism (rather than control by interfacial coherency). Solutionising temperature was found to affect nucleation number density with an activation energy of $E_{mathrm{f}} = 1.5pm{}0.4$~eV, supporting the hypothesis that vacancy concentration affects textalpha{}$_2$ nucleation. The observation that all solutes increase nucleation number density is also consistent with a vacancy-controlled nucleation mechanism.
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MgB2 samples prepared by solid-state reaction were investigated using high-resolution transmission electron microscopy (HREM), X-ray energy-dispersive spectroscopy (EDX), electron energy-loss spectroscopy (EELS), and energy-filtered imaging. Large amounts of coherent precipitates with a size range from about 5 nm up to about 100 nm were found in the MgB2 crystallite matrices. The precipitates are of different shapes including sphere, ellipsoid, and faceted polyhedron depending on the size of the precipitates. EDX and EELS analyses confirm that smaller precipitates contain magnesium, boron and oxygen while larger faceted precipitates contain mainly magnesium and oxygen, implying that the oxygen content increases with precipitate size. HREM and electron diffraction investigations found that the precipitates have the same crystal lattice structure as that of MgB2 but with various composition modulations depending on the composition of the precipitates. The precipitates transform to the MgO phase after long exposure to residual oxygen in flowing Ar gas at high temperatures. The effect of the precipitates in different size ranges on flux pinning is discussed.
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