Papers by Author: Anita J. Hill

Abstract: Self-assembled amphiphile systems are utilized in a wide variety of applications including drug delivery and energy storage. Nano-scale physical and chemical interactions govern the packing of self-assembled amphiphilic molecules, resulting in thermodynamically stable phases of defined geometries. Possible phases include micellar, hexagonal, cubic, lamellar and sponge phases. The internal nano-structure of the amphiphile self-assembly materials plays an important role in the properties of these systems and their application. To date small angle x-ray scattering (SAXS) has been the most common technique used to characterise their structure. We explore positron annihilation lifetime spectroscopy (PALS) as an alternative and/or complementary technique for this purpose, using the phytantriol/water system. While PALS is a well established technique for characterising many materials, the coexistence of aqueous and hydrophobic regions in a soft self-assembled amphiphile material poses a challenge to the analysis and interpretation of the results. In order to alleviate these difficulties we developed a computer program for general-purpose PALS data analysis called PAScual. Amongst the most salient features of this new code are the possibility to perform bounded fits and the option of using advanced algorithms to provide a more robust and unbiased fit: on the one hand, it incorporates a global nonlinear optimisation routine based on the Simulated Annealing algorithm and, on the other hand it gives information on the reliability of the results by means of a Markov Chain Monte-Carlo Bayesian Inference method. In this work we present the newly developed PALS data analysis techniques as well as the results for the phytantriol/water system, comparing them with additional data obtained from complementary techniques.

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.

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.

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).

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.