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A study is made of the temperature dependence of the lattice scattering mobility of electrons in germanium. Previous work on this subject has been restricted to a range of temperatures from 100°K to 300°K. In this range it is possible to use specimens in which the only scattering the electron suffers is that due to the lattice vibrations; the lattice mobility can then be deduced in a straightforward manner from measurements of the Hall constant and conductivity of the material. It was found that over this restricted temperature range the temperature dependence of the lattice mobility could be represented approximately by the form μαΤ⁻¹‧⁶⁶. It has, however, been predicted, on theoretical grounds, that such a simple power law dependence is insufficient, especially when the temperature range is greater. The present work carries out an extension of the measurements to higher temperatures and studies more carefully the approximation of a simple power law dependence. It is found that if the lattice mobility is expressed in the form μαΤ⁻a, then a has to be considered as increasing from about 1.7 to 1.9 between 200 and 400°K. These results are in qualitative agreement with the theoretical predictions. In order to extend the temperature range, strongly n-type specimens of germanium were required. Several basic and permanent crystal preparation facilities, including a crystal grower and wire-saw cutter, were designed and constructed. The conductivities and Hall coefficients of several specimens, prepared with different concentrations, were measured over the appropriate temperature range. The lattice mobility in these specimens cannot be deduced directly from such measurements as the electrons also suffer scattering from the ionized impurities present. An analysis is given which enables the lattice effects to be separated from the impurity effects. This analysis is based on an assumed power law dependence of the lattice relaxation time on temperature and of the impurity scattering relaxation time on temperature and impurity concentration. The separation of these two scattering effects is performed in a way almost independent of the other factors on which they depend. Some information was also obtained on the impurity scattering mobility. This slightly favours a screened rather than a cut-off Coulomb scattering potential. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Natural convection in liquid metals has been studied by direct observation of the fluid flow, using radioactive tracer techniques. The study is of importance in understanding the solidification of metals since fluid flow strongly influences the heat and mass transfer in the system which in turn strongly influences the structure, homogeneity, and mechanical properties of the solid metal produced. The system examined in this investigation was a rectangular liquid cell of variable thickness, positioned on edge. A small driving force for natural convection was imposed across the liquid cell and when steady state conditions were reached, a small amount of the same material containing a radioactive isotope was added to the top of the cell. The tracer material was picked up by the flow and after a given time interval the liquid was quenched to fix the tracer position. The resultant solid block was autoradiographed to determine the distribution of the added radioactive material. Thermal convection was observed in liquid tin and liquid lead using radioactive Sn¹¹³ and radioactive TI²º⁴ respectively. The results show that the flow rates increase with increasing temperature difference across the liquid cell, increasing average temperature, and increasing liquid cell thickness. Flow rates with Grashof numbers from 10⁶ to 10⁸ were experimentally observed. A finite difference numerical solution for the problem of thermal convection is presented for Prandtl numbers of 10.0, 1.0, 0.1, and 0.0127 with Grashof numbers from 2 x 10³ to 2 x 10⁷. The experimental results for liquid tin (Pr = 0.0127) are found to approach the theoretical analysis for large cell thicknesses and large temperature differences. The flow behavior of various types of fluids is compared with liquid metals to show that non-metallic analogies to .metallic flow problems have very limited value. Solute convection is experimentally considered from three different viewpoints; a) independent solute convection, b) the influence of solute convection on thermal convection, and c) the thermal and solute conditions for complete liquid mixing. It was found that there must be a horizontal density inversion across the whole liquid cell for complete mixing to occur throughout the liquid zone. Interdendritic liquid flow resulting from the natural convection in the residual liquid pool was observed in lead-tin alloys. The flow penetrated into the solid-liquid zone to a point of approximately 12 - 22 % solid for primary dendrite spacings of from 700 to 1000 microns. Several experimental models are presented for interdendritic flow. A three-dimensional wire mesh model predicts that the finer the dendrite structure, the greater the flow penetration into the solid-liquid zone. The experimental results for the lead-tin alloys compared favorably with the model. As an extension of the fluid flow considerations, an investigation was carried out to determine macrosegregation in castings which have imposed fluid flow patterns. The macrosegregation present in stationary, rotated, and oscillated castings of Al - 3 wt. % Ag was determined by measuring the distribution of radioactive silver added to the melt. It was found that, no significant macrosegregation was present in the stationary and rotated castings. Extensive macro-segregation was detected in the oscillated casting. For the oscillated case the macrosegregation can be accounted for on the basis of the long range movement of dendrite fragments which break and/or melt off in the solid-liquid interface region. This movement is a direct result of turbulent waves associated with the oscillation. The maximum silver concentration is shown to be related to the columnar-to-equiaxed transition. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Electron beam hearth remelting is extensively used in refining of superalloys, titanium alloys and the recycling of these materials. The removal of impurities and exhogenous particles during the hearth melting operation depends primarily on the time at temperature relationship developed within a pool of molten metal. In the past hearth melters have acted largely on empirical evidence to specify such parameters as melt rates, power levels and skull sizes. This work describes a mathematical model which could be used to predict certain parameters (such as pool volume or alloy element evaporation rates) when given skull geometry, power input and melt rate. A three dimensional steady state heat transfer model of both the skull and water cooled copper mould during electron beam hearth remelting has been developed. The model has been used to investigate the effects of surface temperature, liquid motion, power input, skull geometry, presence of the hearth mould and melt rate on parameters such as pool volume during skull melting. In general the choice of any combination of operating parameters depends on a balance between the refining capacity of the process (i.e. liquid volume) and the loss of alloy elements by evaporation. In the case of melting pure materials (e.g. CP titanium) the balance is between refining capacity and efficient energy use. It was found that forced convection is significantly more effective in increasing the volume of the liquid pool than any other single parameter. Increasing the power input to the skull, increasing the skull width and removing the water cooled copper mould from around the skull also increase the pool volume. The evaporation rates of alloy elements within the skull were most effected by changes in the power distribution and the degree of liquid motion. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

A research report submitted in fulfilment of the requirements for the degree of Master of Science in Engineering (50/50) to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, 2018 / Metal dusting (MD) is a severe form of corrosion in which iron, steels, and nickel (Ni) and cobalt (Co) based alloys disintegrate into a metal or carbide powder with a coke deposit when exposed to strongly carburising gases (carbon activity, ac>1) at elevated temperatures (400800°C). Temperature affects both the driving force and rate of the reaction, represented by gas phase supersaturation with carbon, and the rates of the various processes involved in converting that energy difference into the dusting process. Therefore, process streams such as reformer gas can be benign when hot, but becomes aggressive below critical temperatures. There are different views in literature about the effect of temperature on metal dusting of different materials and alloys. Alloy 800 experiences metal dusting (MD) at 525°C, which is the temperature of the tube sheet of reformers in petrochemical industry. This alloy is specifically used for tube ferrules in the reformers. The reformer trains can reach a critical (highest) internal temperature of 650°C. Therefore, these two temperatures were compared. The effect of temperature and exposure time on the metal dusting of Alloy 800 were investigated in terms of the form of attack and the degradation mechanism. From the results obtained, it was observed that the longer exposure periods result in more carbon deposition and the carbon filaments in the coke become finer as compared to the nanotubes obtained after shorter exposure periods. The alloy suffered metal dusting attack after a relatively short exposure period of three days (72 hours) at both temperatures of 525°C and 650°C, with very little coking. / TL2019

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfilment of the requirements for the degree of Master of Science in Engineering 15 October 2016 / The alloys Ti-52.5Al-10.0Ni (at.%) and Ti-52.5Al-10.0Ni-0.2Ru (at.%) were made by mixing, and melting their powders in a button arc furnace under an argon atmosphere. The high temperature oxidation and room temperature corrosion of behaviour of the alloys was investigated. Isothermal oxidation in air at 950°C for 120 hours and 720 hours was done. Cyclic oxidation behaviour of the alloys was also investigated in air and in a hot salt (Na2SO4) environment. The corrosion tests were conducted in 5 wt% and 25 wt% HCl. All the samples were characterised using scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray diffraction and hardness measurements. On solidification, the Ti-52.5Al-10.0Ni (at.%) alloy formed dendrites of γ-TiAl (~55 at.% Al) surrounded by a eutectic of γ-TiAl + Ti2NiAl3 (τ3) phases. Most of the nickel was found in the Ti2NiAl3 (τ3) phase (~12 at.%) with trace amounts in the dendrites (~0.5 at.%). The Ti-52.5Al-10.0Ni-0.2Ru (at.%) alloy formed dendrites of γ-TiAl (~53 at.% Al) surrounded by a eutectic of γ-TiAl + Ti2NiAl3 (τ3). Most of the nickel (~15 at.%) and ruthenium (~0.3 at.%) were in solid solution in the Ti2NiAl3 (τ3) phase, although small amounts of both metals were present in the dendrites (~1 at.% Ni and 0.1 at.% Ru). Under isothermal oxidation conditions, both alloys showed good oxidation resistance with a low mass gain (< 2%). The alloys formed a continuous scale of TiO2 and Al2O3 with good adherence to the substrate, but as exposure time increased, the scale was severely degraded and exfoliated from the surface. Cyclic oxidation conditions were more aggressive for both alloys. The Ti-52.5Al-10.0Ni-0.2Ru (at.%) alloy was more resistant and formed a nickel-rich sub-surface zone between the substrate and intermixed oxide layer. Both alloys had a fairly good corrosion resistance in HCl due to the presence of nickel. They formed a thin and non-continuous Al2O3 oxide scale on the surface of the γ-TiAl dendrites, with Ti3NiAl2O on the γ-TiAl + Ti2NiAl3 (τ3) eutectic regions. The acid mainly corroded the τ3 phase, thus attacking the eutectic and leaving the γ-TiAl dendrites exposed. / MT2017

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