Alloy B is relatively new precipitation hardening superalloy. It´s applications are in the hot sections of the aero engines, rocket nozzles, gas turbines and in the chemical and petro-leum applications. The alloy is characterized by keeping high strength at elevated tempera-tures and high creep resistance. It´s excellent mechanical properties and corrosion resis-tance are due to the balanced amount of the coherent γ' matrix, combined with other alloy-ing elements and carbides. There are three types of carbides which can be found in nickel-based superalloys: MC, M 23C6 and M6C. Primary MC carbides act as source of carbon for the secondary carbides, which precipitate at the grain boundaries. They can have strengthening effect by hindering the movement of dislocations. In this work both simulation and experimental analysis are conducted in order to investi-gate the behaviour of the secondary carbides. JMatPro simulation is used to predict the behaviour of the material. Heat treatments are conducted at soak temperatures ranging from 920 °C to 1130 °C, with steps of 30 °C, and dwell times of 0.5, 1, 2 and 24 hours. Experimental methods included analysis at LOM, SEM, EDS, manual point counting and hardness tests. Main results show chromium rich M 23C6 carbides are stable at lower temperature compared to molybdenum rich M6C. Both appear as fine and discrete particles at the grain boundaries at 1070 °C. This morphology is believed to be beneficial for the mechanical properties of the alloy. The volume fraction varies between 0.6 and 1.3%. Hardness values are relevant in the range of 920-1010 °C. Above this range there is sudden drop of the hardness

A study of the reduction of black cobalt oxide, using hydrogen as the reductant, showed that the temperature and time used for reduction exerted a profound influence on the proportions of face centred cubic (a) and hexagonal close packed (e) cobalt allotropes obtained in the reduced metal powder. Reduction under certain conditions yielded a faulted hexagonal cobalt powder product which contained stacking faults. The influence of oxygen and titanium on cobalt materials showed that these impurities did not stabilise the proportions of either the cubic or hexagonal phases under the conditions, which were used. The grinding of cobalt powders from different sources showed different tendencies towards agglomeration, when acetome was used as the milling fluid. It was found that powders which contained an initially higher proportion of the hexagonal phase -had a strong tendency towards agglomeration during the initial stages of grinding. The importance of the different phases of cobalt present in the metal powders, and their different agglomeration tendencies on grinding, on the milling required for satisfactory densification of cobalt - tungsten carbide compacts has been shown; it was (iv) found that hard metal alloys prepared using powders with an initially higher proportion of the hexagonal phase required less milling to achieve a given sintered density, than did mixtures prepared with other cobalt powder*;. A correlation analysis between the properties of the milled hard metal powders and the properties of the sintered compacts was carried out; it was found that the most important correlation was between the specific surface area of the milled hard metal powder and the coercivity of the sintered product.

No description available.

Metal dusting corrosion has been known for more than a 100 years as an industrial problem. As a result of extensive research over the last five decades several mechanisms have been evolved involving ferritic materials. However, a complete understanding is yet lacking. One of the most referred models, developed by Hochman-Grabke, suggests that formation of metastable cementite and its subsequent decomposition is the central aspect of the process. To verify this hypothesis, an Fe-Si model was designed based on silicon's ability to retard cementite formation. However, this strategy was unsuccessful because silicon oxidized and amount of silicon remaining after silica formation was not sufficient to suppress cementite. On the other hand, germanium does not form a stable oxide in the conditions employed. A alloying with germanium did prevent Fe3C formation, but not dusting, which resulted from an alternative mechanism. Dusted particles were confirmed to be cementite for pure iron specimens (where cementite scale formed) and ferrite for alloys that did not form cementite. These observations are inconsistent with the prior model. In addition, the general features of metal dusting corrosion have been characterized. Kinetics of coking and metal wastage for ferritic materials (Fe, Fe-Si, Fe-Ge and Fe-Ge-Ni) were found to be linear in nature, though respective rates may vary due to the differences in alloy catalytic activity and reaction morphologies. The carbon diffusion coefficient in cementite was evaluated from Fe3C scaling rates. Crystallographic orientations of different forms of cementite were established. Internal cementite precipitates in pure iron accounted for by a very high degree of supersaturation with respect to carbon, indicating a non-equilibrium situation. Coking and dusting rates were found to be strongly correlated and their gas composition dependence indicate the contribution of the Boudouard reaction. Reactions with fixed carbon activity gases demonstrated that kinetics rather than thermodynamics control the reaction rates. However, at a particular temperature, these rates increase with carbon activity. Activation energies for coking and dusting are equal for a given alloy, meaning that the same process controls them. For Fe-lOGe alloy, in the early stages of reaction, grains with near (001) surfaces were more susceptible to graphitization than grains having near (110) surfaces, but the underlying cause has not been revealed.

Thesis (Ph.D.)--University of Delaware, 2007. / Principal faculty advisor: Jingguang G. Chen, Dept. of Chemical Engineering. Includes bibliographical references.

No description available.

In this work the chromium solubility in MC, and M in Cr3C2 and Cr7C3 carbides in the Ti-Cr-C and Ta-Cr-C system have been examined experimentally. Special attention is given to the cubic MC phase due to its frequent use in industrial cemented carbides. A sample series was made where half of the samples were arc-melted and all samples were heat-treated at different temperatures. By arc-melting some of the samples it was possible to compare the arc-melted and non arc-melted samples to confirm equilibrium. Three phases were expected in each sample. The microstructure was examined by LOM and SEM. The phases were identified by XRD and the amount of Cr in each phase was measured by WDS in FEG-SEM or by microprobe analysis. A higher temperature for the heat-treatment allows more Cr to dissolve in the cubic carbide. Arc-melted samples allow more Cr to dissolve than the same system which has not been arc-melted. The Cr solubility in the cubic carbide in non arc-melted samples at 1400 degree Celcius is 8,1±0,4 at% in (Ti, Cr)C and 7,6±0,3 at% in (Ta, Cr)C. According to the samples the phase diagrams based on thermodynamic calculations are different to experimental data. Therefore, more experimental data should be made to update existing ternary diagrams.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the academic requirements for the Degree of Master of Science. March 2016. / Tungsten carbide (WC) is characterized by its high strength, toughness, hardness, its high resistance to wear and can also be employed at high temperatures. It is used mainly in the form of cemented tungsten carbides which are produced by combining grains of tungsten carbide into a binder matrix element, for example cobalt (Co). Tungsten carbide is commonly used in industrial machinery as cutting tools and abrasives. The primary aim of this project is to investigate the effects of iron (Fe) as an alternative/additional binder in a tungsten carbide system. Therefore, two samples WC-10wt%Co-6wt%TiC and WC-10wt%Co-6wt%TiC-20wt%Fe alloys were prepared by milling and followed by sintering. Several studies have been undertaken in this project to ascertain the effect of Fe on the structural, electronic, magnetic and physical properties of the as-milled and as-sintered samples. A number of different experimental methods were applied to give such information. Transmission Mössbauer spectroscopy and conversion electron Mössbauer spectroscopy were employed as the main techniques to determine the charge states of Fe, Fe phases and other complex phases in the WC-10wt%Co-6wt%TiC-20wt%Fe alloy from the hyperfine interaction parameters. In addition, applied Vickers hardness test, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and strain analysis were utilized as complementary characterization techniques. The Vickers hardness value of 1358±70 HV was measured for the WC-Co-TiC sample whilst a lower value of 820±41 HV was found for the WC-Co-TiC-Fe sample. The microstructure analysis (SEM/EDS) confirmed the presence of WC in the matrix, and Co, Ti, and Fe as the binder elements. XRD results show the formation of the FeCo alloy in the as-milled powder and as-sintered samples. The strain analysis was performed on the as-milled powder, and the as-sintered samples by adopting the method used in accordance with the Topas description, defined in the Topas manual. The WC-Co-TiC sample showed that the tungsten carbide phase appears to have little strain whilst the titanium carbide phase appeared to have no strain. In the WC-Co-TiC-Fe sample, all phases show no strain. The Mössbauer spectrum at room temperature acquired from transmission Mössbauer spectroscopy was fitted with one sextet S1 attributed to -Fe. Best fits to the data obtained from conversion electron Mössbauer spectroscopy required four spectral components: two sextets S1 and S2 assigned to FeCo, one doublet D1 assigned to FeWC and one single SL1 assigned to FeTi alloy. / LG2017

MSc., Faculty of Science, University of the Witwatersrand, 2011 / This dissertation discusses the synthesis method known as laser pyrolysis. The theory on laser pyrolysis has been inferred since 1975, but it is insufficient in predicting the products that can be formed. This is due to the use of a laser, which leads to indecisive reaction pathways from precursor to product. In this work, the laser wavelength and power are varied to initiate a starting point in understanding the complex nature of the laser–precursor interaction, in addition to studying the resulting nanomaterial that is formed by the corresponding laser pyrolysis parameters. The results are justified based on linear and nonlinear optical processes, as well as photophysical and photochemical processes. Experiments to produce tungsten trioxide nanowires were conducted, but similar products could not be achieved, due to the difficulty in emulating ‘sensitive’ variables such as gas pressure and flow rates. However, it was discovered for the first time using this method that six-sided tungsten oxide “stars” can be grown.

No description available.