The Sondheimer theory was tested by looking for oscillatory phenomena in a group of single crystals representing a range in dimensions from matchbox geometry to thin-film geometry. The single crystals were identical with respect to impurity content, strain, orientation, surface condition, and probe placement.

m alloys used in automotive, aerospace and military applications. Superplasticity requires fine grains with high-angle boundaries and resistance to failure by cavitation. OIM permits grain-specific orientation determination and quantitative assessment of the grain-to-grain disorientation distribution as well as grain size measurement in materials. The current work offers significant new insights into the development and response of superplastic microstructures; in particular, OIM data may be employed to delineate the transition from slip to grain boundary sliding in superplastic 5083 materials. / US Navy (USN) author

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 fulfillment of the requirements for the degree of Master of Science in Engineering. Johannesburg, 1998. / Experiments have been conducted at around 17000C to determine the whether carbothermic reduction of alumina is possible at these temperatures. Total pressure of the system was reduced to around 30 kPa and various metallic solvents such as copper, nickel, iron and tin were used to dissolve the metallic aluminium produced. The use of a solvent (and hence decreasing the activity of metallic aluminium) and a lower pressure are thermodynamic requirements to increase the extent of reduction under a given set of conditions. This enables the use of lower temperatures than are required under atmospheric conditions. The highest recovery of aluminium was achieved with the nickel solvent decreasing in order from iron, copper and tin. This ranking was also in accord with the extent of deviation from ideality in the respective binary solutions of these solvents with aluminiur, The nickel-aluminium system displays the largest negative deviation from ideality whereas the till, aluminium system showed a positive deviation. The rate and extent of the reduction was found to be highly dependent on temperature and pressure. The pseudo first order reaction rate was found to be the primary order for the reduction of aluminium in all the solvents used. It is also suggested that the reduction rate was controlled primarily by chemical reaction rate father than by transport processes. This is due to the extreme sensitivity of the rate and extent of the reaction to temperature. / AC2017

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.

Summary in Chinese. / Thesis (M.B.A.)--Chinese University of Hong Kong. / Bibliography: leaves 189-192.

For additive manufacturing, research has shown that the chemistry and microstructural properties of the feedstock powder can significantly affect the properties of the consolidated material. Thermal treatment and recycling parameters for powders used in both solid and liquid state processes can further affect the microstructure and properties of the consolidated parts. Understanding the powder microstructure and effects of powder pre-treatment can aid in optimizing the properties of the final consolidated part. This research proposes a method for the characterization and optimization of powder pre-processing thermal parameters using aluminum alloy powder as examples. Light microscopy, electron microscopy, and hardness were used to evaluate each condition.

"During the solidification process in casting, hot tearing may occur. It is a severe defect that normally involves the formation of a macroscopic tear, which generates cracks either on the surface or inside the casting. Over the past decades, many strategies have been developed to evaluate the hot tearing tendency. Unfortunately, most of the tests can only provide qualitative information. Therefore, a reliable and quantitative test to evaluate hot tearing in aluminum alloys is highly desirable. To address this issue, WPI and CANMET MTL (both members of the Light Metal Alliance) jointly developed a quantitative hot tearing test and established a specific methodology. Using a constrained rod mold, the hot tearing formation can be quantitatively studied by measuring the contraction force, time and temperature during solidification for a restrained casting or linear contraction, time and temperature for a relaxed casting. This study investigated cast aluminum alloys A380.1 and A390 and wrought aluminum alloys 6061 and 7075. The results show that wrought aluminum alloys have a much stronger hot tearing tendency than cast aluminum alloys based on a quantitative analysis. Also, the study involves the effects of adding strontium and oxides respectively into the cast aluminum alloy A380.1. Compared with the pure A380.1 alloy, the introduction of strontium decreases the hot tearing tendency, while the inclusion of oxide greatly increases the hot tearing. The information obtained through these tests provides a database of hot tearing phenomenon and establishes a new hot tearing index."

Aluminium composites reinforced by particles have received considerable attention because of their superior mechanical properties over monolithic aluminum matrix. Over the last ten years, nanocomposites with nano-sized reinforcements have become a revolutionary progress for composites because they have different strengthening mechanisms as compared to that in composites with micro-sized reinforcements. Consequently novel properties can be expected from the nanometric particulate reinforced composites. The aim of this project was to fabricate SiC (50nm)/7075 aluminium composites via a modified powder metallurgy and extrusion route. Ageing treatment was used to increase the strength of the composites and mechanical tests, including tensile test and abrasive wear test, were performed. The effects of nanometric silicon carbide particulates to the ageing behaviours and mechanical properties of the composites have been studied by optical metallography, scanning electron microscopy and transmission electron microscopy. It was found that the dispersion of nanometric silicon carbide was not homogeneous, but tended to disperse along grain boundaries. Clustering of these nano-reinforcements was also found within the grains. This was particular true when the amount of nano-reinforcement increased to 5%. Compared with the monolithic 7075 alloy, the 1 vol.% SiC (50nm)/7075 aluminium had a higher strength because of effective dislocation pinnings by the reinforcements, while 5% SiC (50nm)/7075 had a much lower strength and ductility because of severe aggregation of nanometric particulates. Nanometric silicon carbide was not as effective as the micro ones in improving abrasive wear resistance of aluminium, this was because of micro-cracking in the aggregation and relatively large abrasive grit. In summary, the addition of a small amount of SiC nanoreinforcements has a high potential to further strengthen 7xxx aluminium alloy. However, the clustering of reinforcements in the matrix will detrimentally affect the strength and ductility of the alloy. The wear resistance of nanometric particulate reinforced composites was inferior to those with micrometric reinforcements. It is suggested that by improving the dispersion of nanometric reinforcements, as well as putting in reinforcememts with different sizes, the mechanical properties and wear resistance can both be increased.