Papers by Author: Timothy J. Bastow

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Authors: Timothy J. Bastow, Anita J. Hill
Abstract: Nuclear magnetic resonance (NMR) is shown to be a sensitive metallurgical characterization technique for metastable phase development during early stage aging of the high purity experimental alloy Al(1.05Cu 1.7Mg) (at.%). It is shown, using 63Cu NMR and positron annihilation lifetime spectroscopy (PALS), how room temperature structural evolution proceeds in Al(1.05Cu 1.7Mg) prepared in the form of supersaturated solid solution. The combination of NMR and PALS allows identification of solute aggregate chemistry and defect kinetics during natural aging. Guinier-Preston-Bagaryatsky (GPB) zone formation is detected by NMR within 0.5 h of quench, and the percentage of Cu atoms in GPB zones increases to ~80% within 50 h at room temperature, with the residual 20% of Cu atoms remaining in solid solution. The formation of GPB zones corresponds with the Vickers hardness, in fact the hardness is shown to directly depend on the amount of total Cu partitioned to GPB zones. The vacancy kinetics, as measured by PALS, show an exponential decay in time following the quench with the majority of decay complete within 50 h supporting the notion that early stage hardening by GPB zone formation is controlled by defect concentration and availability.
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Authors: Timothy J. Bastow, Anita J. Hill
Abstract: GP and GPB zone formation in Al-Cu-Mg alloys proceeds rapidly at room temperature immediately subsequent to STQ. This structure evolution is well known for GP zones but not for GPB zones. In many age-hardenable Al-Cu-Mg alloys this vacancy assisted diffusion of Cu from solid solution to form zones is essentially complete within 50 hours with only a small residual quantity of Cu remaining in solid solution. The alloy then remains in this metastable state. This zone formation is observed here using 63Cu NMR for the alloys AA2014 and AA2124 which lie in the α-θ (GP) and α-S (GPB) phase fields respectively. However these zones which form so readily at room temperature are unstable on aging at higher temperatures. Rapid dissolution of the zones, and their reversion back into solid solution at elevated temperatures is explicitly demonstrated by 63Cu nuclear magnetic resonance (NMR). At this stage the Cu is shown to remain stably in solid solution at room temperature. Further aging at the same elevated temperature is then shown to reform the zones with further continuous evolution to either the θ-phases (AA2014) or S-phase (AA2124).
1385
Authors: James Mardel, K.R. Chynoweth, M.E. Smith, C.H.J. Johnson, Timothy J. Bastow, Anita J. Hill
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Authors: Kate M. Nairn, Brian M. Gable, Rüdiger Stark, Natalie Ciccosillo, Anita J. Hill, Barry C. Muddle, Timothy J. Bastow
Abstract: Nuclear magnetic resonance (NMR) was used to systematically monitor the amount of copper in the matrix of various age hardenable Al-Cu alloys as a function of ageing treatment. These NMR results were used to compare the measured amount of copper residing in the matrix during microstructural evolution with that suggested from the equilibrium phase diagrams. Our results indicate that the matrix copper composition for under- and peak-aged microstructures is generally far from equilibrium, with much more copper in solution than expected based on equilibrium phase diagrams. NMR is shown to be a sensitive and efficient metallurgical characterization technique to monitor the partitioning of solute in both non-equilibrium and equilibrium phases.
591
Authors: Terence Kratzer, Brian M. Gable, Kate M. Nairn, Timothy J. Bastow, Barry C. Muddle, Anita J. Hill
Abstract: The effect of Si additions on vacancy behavior following solution treatment and quenching was studied for an Al-Cu-Mg-Ag alloy system using positron annihilation lifetime spectroscopy (PALS). Both the initial positron lifetime and steady state positron lifetime increase as the Si concentration increases. This behavior is interpreted in terms of Si interacting with vacancies, leading to their retention in the alloy. The ability of PALS to monitor vacancy behavior after solution treatment should allow the improved prediction of phase transformation kinetics and hence the tailoring of ageing treatments.
597
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