Understanding PGE geochemistry in low abundance (i.e. sub ppb level) geological samples has been hampered by the absence of an accurate, low blank, preconcentration technique. Ni- S fire assay, the traditional preconcentration method, has been critically assessed and combined with isotope dilution to enable reproducible analysis of standards in the 1-l0ppb concentration range. In addition, a new anion exchange chromatography, low blank, isotope dilution method has been developed that allows analysis of Re, Os, Ir, Ru, Ft and Pd, at pg levels, from the same sample aliquot by ICP-MS. This method enables PGE abundances and Os isotopic ratios to be determined on the same sample dissolution, permitting geochronological studies. Samples are digested in Carius tubes and total procedural blanks are routinely less than l0pg/g for all elements, except Pt (25pg/g). Reproducibility is sufficient at the lOppt level to confidently identify inter-element PGE fractionations. Using this anion exchange preconcentration technique, PGEs have been characterised in a suite of plume-generated picrites from West Greenland, and a suite of subduction-related lavas, including fractionation series from Grenada (Lesser Antilles arc) and from Izu-Bonin. PGE concentrations are higher within the picrites of West Greenland, than in the picrites of Grenada, or boninites of Izu-Bonin. This is attributed to higher degrees of melting and less fractionation during West Greenland magma genesis. There is similarity in the PGE-pattems between all environments suggesting that relative PGE behaviour is not greatly altered during mantle melting in a subduction zone vs a plume. Subtle differences occur in inter-PGE ratios between the different environments and within the fractionation suites. Os and Ir do not behave in an analogous manner during fractionation indicating "compatibility" with different crystallising phases. The PGEs are associated with olivine fractionation in all of the primitive rock types, but, may also be compatible with other phases (e.g. magnetite/chromite/amphibole) during high-level fractionation in arc magma chambers. The PGE signatures in the evolved arc rocks are controlled by multi-stage fractionation. PGE concentrations decrease as fractionation progresses, except for Re and Ru. PGE compatibility during arc-lava fractionation decreases in the order Os>Ir>Pt>Pd>Ru>Re. There is not a strong sulfide control on PGE fractionation in any of the rock suites analysed, probably because the primary melts were generally S- undersaturated. This places important constraints on the nature of the mantle in the different tectonic environments. Os-isotopic studies indicate the presence of a radiogenic component in both the West Greenland picrite plume source region (recycled crust or outer core material) and the Grenada picrite source region (slab-derived fluids).

A research dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science in Engineering, 2016 / The Fe-Al alloys have been viewed as attractive for applications in the energy sector, for example boilers and pipes in power plants. As such, improvement of the properties of an Fe-40 at.% Al alloy were attempted by adding small amounts of precious metal to the alloy. An Fe-40 at.% Al alloy produced by melting and casting in a button arc furnace, showed very large grains in the microstructure and had a hardness of ~400HV. Precious metals (Ag, Ru, Pt, Pd) were added in amounts of either 0.2 at.% or 0.5 at.%. Additions of Ag and Ru were found to decrease the hardness, but the addition of Pt and Pd did not affect the hardness significantly. Oxidation testing of the alloys showed that they were very resistant to oxidation, although there was some small internal oxides in the sample containing Pt. Corrosion testing of the samples showed that the precious metals helped to greatly decrease the corrosion rate of the FeAl alloy in H2SO4. The mechanical alloying and sintering parameters were first optimised using the plain FeAl alloy and the sintered plain FeAl had significantly smaller grains than the as-cast material under all conditions, and that the hardness had increased as a result. The mechanical alloying also produced a carbide phase in the material that was not able to be removed. After mechanical alloying and sintering of the precious metal containing alloys, the microstructure had been refined slightly more than plain FeAl in all the alloys, except that containing Ru, which was the same as the plain alloy. The presence of Pt and Pd increased the hardness of the alloy slightly, while Ru and Ag produced no change. Oxidation testing showed that the sintered compacts had significantly less grain growth than the as-cast samples, and that generally the oxidation resistance was good. The exception to this was the sample containing Pd, which showed sub-surface oxidation. Corrosion testing of the sintered compacts showed that the Ru and Pt containing alloys performed better than the as-cast alloys. Both the Pd and Ag containing samples suffered from pitting, and the Pd sample was corroded away completely during the test in 3.8M H2SO4 solution. It was concluded that there was a significant effect on the microstructure of the FeAl alloy due to the processing route used, and that this also produced a significant effect on the hardness and corrosion resistance, with both improving when non-equilibrium processing was used. The presence of precious metals in small amounts increased the corrosion resistance of the alloy, and overall, only produced small changes in the hardness, and oxidation properties of the alloy. / GR2016

A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering. Johannesburg 1988 / Fatigue crack closure characteristics were studied in a 70i7 aluminium alloy in the as received (AR) and heat treated (HT) conditions with the view of establishing the general closure trends and how well the modified closure parameter AKeff (- K^ax ' K0_) was capable of characterising fatigue crack growth. The results of this work indicated that a significant amount of strain intensification occurred below K0p which implied that AKe££ generally underestimated the stress intensity range experienced at the crack tip during cyclic loading. The major objective of this dissertation was to determin' whether a relationship exists between the distance over which closure develops from zero to a steady state jlcsure value, and the distance over which short crack growth behaviour occurs. Two techniques were used to eliminate closure for a fatigue crack in a compact temion specimen. Closure development was then measured as a function crack length and a steady state closure value was approached tier about 0.19 mm in the AR material and 0.?8 mm in the HT material irrespective of AK. This distance compared well with that distance over which short cracks exhibited "anomalous" behaviour. This implies that anomalous short ctack behaviour is dependent on the distance over which closure develops in this alloy, although microstructural influences may play a rola.

No description available.

Thesis ( MTech(Mechanical Engineering))--Cape Peninsula University of Technology, 2011 / This study deals with the development of aero morphing reinforced composite materials embedded with NiTi alloys. It is shown that the composite materials can be manufactured using resin infusion process to produce better mechanical properties such as tensile strength and material stiffness. These composite materials are modelled experimentally using temperature and time parameters. The object of the modelling is to determine the effect of process temperature on the smart material alloy (SMA). As a result, a composite structural designer would now possess an added dimension in optimising material design. In addition, the study is conducted to analyse the structural behaviour of composite materials when embedded and when not embedded with NiTi alloys. The analysis is constrained to the evaluation of material tensile strength and stiffness upon performance of composite structures. A macro mechanical approach is employed to perform the analysis in specimens with different fibre orientation [0°, 45° and 90°]. The estimation of tensile strength and stiffness parameters is based on the characteristics obtained from a macro mechanical approach. The orientation which posses the best material properties is selected to embed NiTi alloys. The experimental results of unembedded specimens are validated with the application of micromechanics equations and an Ansys software finite element modelling tool. There is fair agreement between the finite element simulation of macro mechanical test of the specimens and the measured experimental results. Although the macro mechanical approach is found to be successful, it is imperative to characterise the material interface strength of embedded specimens using a pull out test. The pullout test showed to some great extent the properties of reinforced composite embedded with NiTi alloys.

Solid oxide fuel cells operate at elevated temperature, oxidising fuel gases to generate electricity. The fuel gas streams in the fuel cell systems are rich in carbon and have very low oxygen potential. Under these conditions, alloys can undergo metal dusting, which causes pitting or general thinning of the alloys. This process is not yet fully understood. It is, hence, not possible to accurately predict the susceptibility of a particular alloy in the atmospheres relevant to SOFC. Model Fe-Cr and Fe-Ni-Cr alloys were exposed to test the hypothesis that cementite formation and its decomposition is necessary for metal dusting to occur. A series of ferritic and austenitic engineering alloys were also exposed to compare their dusting rates. Two specimens of each alloy were studied, one was etched in a H3PO4-15%H2SO4-21%H2O solution and the other was ground to a 600-grit finish. The alloys were exposed to a CO-26%H2-6%H2O gas mixture at 680oC under thermal cyclic conditions. The hot gas composition corresponded to ac = 2.9 and an oxygen potential high enough to oxidise chromium, but not iron or nickel. All the alloys were shown to undergo internal carburisation, metal dusting and coking once the protective chromium oxide scale was damaged. Fe-25Cr was less resistant than Fe-60Cr because of its lower chromium content. However, ferritic Fe-25Cr-based steels are more resistant to dusting than austenitic Fe-25Cr-25Ni. The present findings are consistent with the earlier conclusions that cementite formation is essential for dusting on ferritic steels and that dusting of austenitic alloys does not involve the prior formation of cementite and its decomposition. The onset of metal dusting was more accelerated for most austenitic engineering alloys (Alloy 800, Inconel 601, 690, 693 and Alloy 602CA) than for engineering ferritic steels (Fe-27Cr-0.001Y). However, the alloy with the best performance was austenitic Inconel 625, which was still protected by its Cr2O3 scale after 500 one-hour cycles. In both ferritic and austenitic chromia-formers alloys, the surface ground specimens were more resistant to metal dusting than the electropolished specimens. In contrast, ferritic alumina-formers with electropolished surfaces did not dust during the entire experimental periods of 1200 one-hour cycle, but the alloys with ground surfaces slowly underwent dusting attack. The coke deposits formed consisted largely of graphite nanotubes, containing small particles at the tube tips. These particles were identified as single crystal cementite, in the case of ferritic steels, and austenite, for the austenitic alloys. This is not in agreement with the currently accepted dusting model for ferritic steels that cementite decomposition yields iron particles, which catalyse coke deposition. EDX analysis of the cementite particles, showed that the only metal detected was iron, thus differing in chemistry from the (Fe,Cr)3C surface layer. Similarly, the austenite particles contained only nickel and iron, differing in chemistry from the disintegrated alloy surfaces. These results suggested that the particles were formed in the coke in the carbon-supersaturated gas, rather than disintegration of the alloy surface layer. Strong orientation relationships were determined between the graphite and cementite particles; however, no clear crystallographic relationship was deduced between the graphite and austenite. Relative alloy performance appears consistent using the present multiple one-hour cycle and the results of others using a smaller number of lengthy cycles. Hourly thermal cycling was shown to accelerate the dusting onset for both electropolished chromia-formers and surface ground alumina-formers. Protective oxide scales spall at a critical thickness and carbon attack results when the alloy surfaces are depleted of scale-forming elements and healing becomes impossible. On this basis, analytical models were developed and used to predict the incubation periods for oxide failure and the subsequent carbon attack. Upon testing, these models were, however, found to be not qualitatively meaningful in predicting the onset of dusting observed in the present study. Gross oversimplifications involved in the model and the absence of reliable data for many parameters required for the computations prevented even an approximate quantitative prediction.

This thesis describes the stress corrosion cracking of a series of aluminium bronze alloys and attempts to characterize the cracking in relation to the various mechanisms proposed. The theories of cracking applicable to the aluminium bronze/ammonia system include; the anodic dissolution, the film rupture and the embrittlement theories, which are described in the literature review. The copper aluminium alloy system was selected for investigation as it exhibits a martensite transformation, making the cracking of these alloys in ammonia solutions a usefulful comparison with the cracking of stainless steels in chloride solutions. The study used 'U-bend' specimens stressed in ammonia solution preconcentrated with copper, while the oxidation of the alloys under the same conditions was examined using unstressed specimens. Oxidation in this thesis is taken to mean an anodic dissolution process with or without the formation of a metallic oxide layer. The tests showed three binary alloys to be susceptible to intergranular and transgranular cracking, while two commercial alloys only cracked transgranularly and only in pH = 11 ammonia solution. Stress corrosion conditions gave rise to sharp, narrow cracks traversing all the phases in the alloy with matching features on opposing fracture surfaces, whereas oxidation gave rise to less distinct, more rounded features with a definite phase dependence. Further experiments showed the limited penetration and reactivity of a corrosive solution along narrow cracks. It is concluded that stress corrosion cracking and oxidation occur by different mechanisms and that the mechanism of stress corrosion cracking is unlikely to involve oxidation as a necessary step to cracking. The results of the present study support an embrittlement mechanism of stress corrosion cracking in aluminium bronze alloys.

Part I discusses the development of an aluminum-magnesium-silicon alloy that may combine strength, extrudability, favorable corrosion resistance with low cost and scrap compatibility. The first part of the study determined the effects of small composition, heat treatment and mechanical processing changes on the ambient temperature tensile properties of the alloy. A combination of magnesium and silicon of about 2%, 1% copper, 0.2% chromium and 0.1% vanadium can produce a T6 alloy with significant higher strength, fatigue and corrosion fatigue properties for both ingot and extrusion than those of 6061 but with only a modest increase in cost. The new alloy has been designated as AA6069. The second part of the study determined the T6 properties of 6069 alloy. The tensile test results of cold and hot extrusions of hollow, solid bars, and high pressure cylinders indicate that the T6 properties ranged from 55-70 ksi (380-490 MPa) UTS, 50-65 ksi (345-450 MPa) yield strength, and 10-18% elongation. It also appears that the fracture toughness and general corrosion resistance in saline environment are comparable or better than those of 6061 T6. Part II attempted to evaluate the formation, formability, thermal and mechanical properties of semi-solid A356, A357 and modified aluminum silicon semi-solid alloys. The semi-solid alloy microstructure was produced in this study by purely thermal treatment rather than conventional and expensive electromagnetic or mechanical stirring. Three heat-up stages in semi-solid treatment were evaluated. Stage I is related to the heating of the alloy in the solid state. Stage II is related to the eutectic reaction. Stage III is related to the heating of the semi-solid slurry. Stage II requires the longest time of the three heat-up stages due to the endothermic reaction on heating. An increase of furnace temperature can greatly reduce the time of stage II. The atmosphere (vacuum, air, argon) of the semi-solid treatment does not appear to greatly affect the T6 properties of semi-solid alloys. The microstructure and T6 properties of semi-solid A356 do not appear sensitive to the homogenization treatments before semi-solid treatment. The porosity of semi-solid ingots and pressed parts increases as the cooling rate decreases in unformed and subsequent-to-moderate pressure forming. The T6 properties basically appear sensitive to voids, with a degradation of properties as the void concentration increases. The formability of A357 may be improved as the spheroidal particle size decreases. Hence, formability may improve with decreasing ingot grain size. The mechanism of coarsening of the solid phase at isothermal temperatures is related to Ostwald ripening and/or "merging" of particles. The mechanical properties of die-casting parts show that the method of thermal treatment to produce a spheroidal microstructure is an effective method for industrial production of semi-solid aluminum-silicon alloys. / Graduation date: 1997

It is well established that the extended range V(I) transition behavior in filamentary superconducting composites depends strongly on the processing conditions of the composite. Based on previous work, a model proposed by Warnes et al predicts that the second derivative of the V(I) transition is related to a more fundamental property the critical current distribution in the composite. The present work consolidates the said model by showing that in spite of the V(I) transition being dependent on the matrix resistance, the second derivative is relatively independent. Also, the second derivative V(I) curve has been correlated to the critical current distributions and area distributions in monofilament and multifilamentary superconductors, using image analysis techniques. The work provides a better understanding of the critical current distributions in composite superconductors and can be an effective tool for measuring conductor quality more precisely. In addition to this, image analysis techniques have been used to study the effect of processing on the structure of wires - and have been related to the predictions of a model of inplane stresses in composites. / Graduation date: 1996

Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, March 1990. / Thesis Advisor(s): Fox, A.G. "March 1990." Description based on signature page as viewed on August 26, 2009. Author(s) subject terms: Aluminum-Lithium Alloys, Microstructural development. Includes bibliographical references (p. 43-47). Also available online.