Thesis (Ph. D. (Physics)) -- University of Limpopo, 2013. / A combination of ball milling experiments and ab initio calculations in this study successfully yielded results that shed light into understanding the fundamental basis for immiscibility and the concept of mechanical alloying in Ti-Mg system. In addition, the conditions for achieving extended solid solubility in elements that usually do not dissolve in each other under thermodynamic equilibrium conditions have been predicted using ultrasoft (US) and norm-conserving (NC) pseudopotentials. Hydostatic pressures required to stabilize ordered phases were determined. Our new systematic representation of martensitic transformation (MT) paths as a result of dislocation necessary to induce α→FCC, α→BCC and α→ω phase transitions led to, for the first time, a direct determination of CRSS and tensile strength for Ti and Mg HCP metals. Furthermore, a new ω phase which is less stable than α phase at 0 GPa is proposed. Based on this phase, α→ω deformation path which yielded the onset of uniaxial transition pressure of 4.167 GPa is reported. Attempts of synthesizing Ti-Mg solid solutions by means of Simoloyer high energy ball mill were not successful; however, nanocrystalline Mg-TiH2-x composites were instead formed. These results were attributed to quick formation of metastable Ti hydrides or cold welding at early stages of BM prior to alloying, thus serving as possible obstacles to forming such solid solutions. The deformed Ti crystals adsorbed H+ from the stearic acid leading to formation of metastable orthorhombic TiH2-x phase which later transformed to a tetragonal TiH2-x or even cubic TiH2 when stoichiometric amount of H2 had been adsorbed. Although the yield was significantly lower, the product of milling a mixture of coarse Mg and fine Ti particles was comprised of Ti particles adhering around ductile Mg particles in a core shell manner. The adhesion of the fine hard titanium particles on the surface of the large ductile magnesium particles impeded the further plastic deformation of the titanium particles, thus suppressing the formation of the faults necessary for mechanical alloying. Nanocrystalline Ti powder of about 40 nm was produced by 30h ball milling. During BM of Ti powder, solid-state transformation from HCP to FCC occurred in the presence of PCA with lattice parameters of 4.242 and 4.240 Å after 24 and 30 h, respectively, v due to protonation. When Ti powder was milled in the absence of PCA, no phase transformation was observed for both uninterrupted and interrupted milling cycles. In addition, nanocrystalline Mg powder with crystallite size varying between 60 and below 40 nm was produced by ball milling. However, no solid-state transformation took place even if the powder was milled for 90 h. Therefore, we evidently report for the first time that the interstitial H+ is the driving force for α → FCC phase transformation in ball milled Ti powder. Our theoretical results predicted the ω phase to be the ground-state structure of Ti at 0K and P=0 GPa, in support of other previously reported calculations. We noticed that the stability of the α phase was surpassed by that of the FCC lattice at ~ 100 GPa, corresponding with sudden sharp rise in c/a ratio, hence attributed to α → FCC phase transition. Similar results were obtained for Mg at 50 GPa, although in this case the crossing of lattice energies coincided with minimum c/a. However, using our proposed HCP→BCC MT path mechanism for Mg, it is evident that the minimum c/a at 50 GPa corresponds to a change in the preferred deformation slip from basal (below 10 GPa) to prismatic rather than phase transition. Nonetheless, the proposed MT model predicts that both elemental Ti and Mg prefer to deform via prismatic slip as indicated by lower shear stress as well as CRSS values compared to those calculated for basal slip. Theoretical findings from ab initio calculations on hypothetical ordered Ti-Mg phases indicated absence of intermetallic phases at equilibrium conditions, in agreement with experimental data. However, the formation becomes possible at 80 GPa and above with respect to c/a ratio but requires at least 200 GPa with respect to stable lattices. Using calculated heats of formation, elasticity and DOS, it has been possible to show that L12 TiMg3 could not form even at high pressure as 250 GPa. Nonetheless, both approaches indicate that forming an intermetallic compound between Ti and Mg requires a crystal structure change, α→FCC for Ti and HCP→BCC for Mg. Proposed DFT-based solid solution model for predicting phase stability and elastic properties of binary random alloys, with Mg-Li system serving as a test case, successfully yielded reliable results comparable to experimental data. This method was successfully applied to study an immiscible Ti-Mg system and the solubility limit vi was for the first time theoretically established. Based on formation energy of Ti-Mg solid solutions, our calculations predicted for the first time that the solubility of up to 60 and 100 at.% Mg into Ti with the use of USP and NCP, respectively, to be thermodynamically favourable with necessary lattice kinetics being the main challenge. Nonetheless, NCP proved to be reliable in predicting structural and elastic properties of disordered alloys.

No description available.

M.Ing. / This dissertation investigates the fatigue and monotonic tensile properties of cast aluminium alloy wheels in various post cast conditions. It was found that monotonic tensile properties could be used in the original universal slopes method of Manson to predict the fatigue properties as a conservative first approximation for A356 cast aluminium alloy wheels. Using finite element analysis and the predicted fatigue properties the fatigue life of A356 aluminium alloy wheels could be determined. Further work is required on the surface effect of paint on the wheel and residual stress on the surface of the wheel.

M.Ing. / Grade 4 titanium is a commercially pure grade titanium alloy extensively used in various industries including the chemical industry and more recently in the biomedical industry. Grade 4 has found a niche as a biomedical material for production of components such as orthopaedic and dental implants. Its physical properties such as high corrosion resistance, low thermal conductivity and high strength make it suitable for these applications. These properties also make it hard-to-machine similar to the other grades of titanium alloys and other metals such as nickel based alloys. During machining of titanium, elevated temperatures are generated at the tool-workpiece interface due to its low thermal conductivity. Its high strength is also maintained at these high temperatures. These tend to impair the cutting tool affecting its machinability. Various investigations on other grades of titanium and other hard-to-machine materials have shown that machining at high cutting speeds may improve certain aspects of their machinability. High speed machining (HSM) is used to improve productivity in the machining process and to therefore lower manufacturing costs. HSM may, however, change the surface integrity of the machined material. Surface integrity refers to the properties of the surface and sub-surface of a machined component which may be quite different from the substrate. The properties of the surface and sub-surface of a component may have a marked effect on the functional behaviour of a machined component. Fatigue life and wear are examples of properties that may be significantly influenced by a change in the surface integrity. Surface integrity may include the topography, the metallurgy and various other mechanical properties. It is evaluated by examination of surface integrity indicators. In this investigation the three main surface integrity indicators are examined. These are surface roughness, sub-surface hardness and residual stress. White layer thickness and chip morphology were also observed as results of the machining process used. The effect of HSM on the surface integrity of grade 4 is largely unknown. This investigation therefore aims to address this limitation by conducting an experimental investigation on the effect of HSM on selected surface integrity indicators for grade 4. Two forged bars of grade 4 alloy were machined using a CNC lathe at two depths of cut, 0.2mm and 1mm. Each bar was machined at varying cutting speeds ranging from 70m/min to 290m/min at intervals of approximately 20m/min. Machined samples were prepared from these cutting speeds and depths of cut. The three surface integrity indicators were then evaluated with respect to the cutting speed and depth of cut (DoC). iv Results show that a combination of intermediate cutting speeds and low DoC may have desirable effects on the surface integrity of grade 4. Highest compressive stresses were obtained when machining with these conditions. High compressive stresses are favourable in cases where the fatigue life of a material is an important factor in the functionality of a component. Subsurface hardening was noticed at 0.2mm DoC, with no subsurface softening at all cutting speeds. Surface hardness higher than the bulk hardness tends to improve the wear resistance of the machined material. Though surface roughness values for all depths of cut were below the standard fine finish of 1.6μm, roughness values of samples machined at 0.2mm DoC continued to decrease with increase in cutting speed. Low surface roughness values may also influence the improvement of fatigue life of the machined components. These machining conditions, (intermediate cutting speeds and low DoC), seem to have promoted mechanically dominated deformation during machining rather than thermal dominated deformation. Thermal dominated deformation was prominent on titanium machined at DoC of 1mm.

M.Eng. (Mechanical Engineering) / High speed machining (HSM) has the potential to greatly increase productivity and to lower manufacturing costs if workpiece surface integrity can be controlled. The surface fmish of a biomaterial is vitally important for proper implant functioning, and is the focus of this study. Grade 4 titanium was turned on a lathe with cutting speeds increasing from the conventional to the high speed range. The surface finish was assessed using profilometry, atomic force microscopy, and contact angle measurement. The ability of the material to bond directly with bone was predicted by cell adhesion studies. Results indicate that there is a general relationship between cutting speed, surface roughness, contact angle, and cell adhesion. Turning grade 4 titanium at cutting speeds between 150m/min and 200m/min may provide an optimal surface for osseointegration.

Thesis (Ph. D. (Physics)) -- University of Limpopo, 2013 / A combination of ball milling experiments and ab initio calculations in this study successfully yielded results that shed light into understanding the fundamental basis for immiscibility and the concept of mechanical alloying in Ti-Mg system. In addition, the conditions for achieving extended solid solubility in elements that usually do not dissolve in each other under thermodynamic equilibrium conditions have been predicted using ultrasoft (US) and norm-conserving (NC) pseudopotentials. Hydostatic pressures required to stabilize ordered phases were determined. Our new systematic representation of martensitic transformation (MT) paths as a result of dislocation necessary to induce α→FCC, α→BCC and α→ω phase transitions led to, for the first time, a direct determination of CRSS and tensile strength for Ti and Mg HCP metals. Furthermore, a new ω phase which is less stable than α phase at 0 GPa is proposed. Based on this phase, α→ω deformation path which yielded the onset of uniaxial transition pressure of 4.167 GPa is reported. Attempts of synthesizing Ti-Mg solid solutions by means of Simoloyer high energy ball mill were not successful; however, nanocrystalline Mg-TiH2-x composites were instead formed. These results were attributed to quick formation of metastable Ti hydrides or cold welding at early stages of BM prior to alloying, thus serving as possible obstacles to forming such solid solutions. The deformed Ti crystals adsorbed H+ from the stearic acid leading to formation of metastable orthorhombic TiH2-x phase which later transformed to a tetragonal TiH2-x or even cubic TiH2 when stoichiometric amount of H2 had been adsorbed. Although the yield was significantly lower, the product of milling a mixture of coarse Mg and fine Ti particles was comprised of Ti particles adhering around ductile Mg particles in a core shell manner. The adhesion of the fine hard titanium particles on the surface of the large ductile magnesium particles impeded the further plastic deformation of the titanium particles, thus suppressing the formation of the faults necessary for mechanical alloying. Nanocrystalline Ti powder of about 40 nm was produced by 30h ball milling. During BM of Ti powder, solid-state transformation from HCP to FCC occurred in the presence of PCA with lattice parameters of 4.242 and 4.240 Å after 24 and 30 h, respectively, v due to protonation. When Ti powder was milled in the absence of PCA, no phase transformation was observed for both uninterrupted and interrupted milling cycles. In addition, nanocrystalline Mg powder with crystallite size varying between 60 and below 40 nm was produced by ball milling. However, no solid-state transformation took place even if the powder was milled for 90 h. Therefore, we evidently report for the first time that the interstitial H+ is the driving force for α → FCC phase transformation in ball milled Ti powder. Our theoretical results predicted the ω phase to be the ground-state structure of Ti at 0K and P=0 GPa, in support of other previously reported calculations. We noticed that the stability of the α phase was surpassed by that of the FCC lattice at ~ 100 GPa, corresponding with sudden sharp rise in c/a ratio, hence attributed to α → FCC phase transition. Similar results were obtained for Mg at 50 GPa, although in this case the crossing of lattice energies coincided with minimum c/a. However, using our proposed HCP→BCC MT path mechanism for Mg, it is evident that the minimum c/a at 50 GPa corresponds to a change in the preferred deformation slip from basal (below 10 GPa) to prismatic rather than phase transition. Nonetheless, the proposed MT model predicts that both elemental Ti and Mg prefer to deform via prismatic slip as indicated by lower shear stress as well as CRSS values compared to those calculated for basal slip. Theoretical findings from ab initio calculations on hypothetical ordered Ti-Mg phases indicated absence of intermetallic phases at equilibrium conditions, in agreement with experimental data. However, the formation becomes possible at 80 GPa and above with respect to c/a ratio but requires at least 200 GPa with respect to stable lattices. Using calculated heats of formation, elasticity and DOS, it has been possible to show that L12 TiMg3 could not form even at high pressure as 250 GPa. Nonetheless, both approaches indicate that forming an intermetallic compound between Ti and Mg requires a crystal structure change, α→FCC for Ti and HCP→BCC for Mg. Proposed DFT-based solid solution model for predicting phase stability and elastic properties of binary random alloys, with Mg-Li system serving as a test case, successfully yielded reliable results comparable to experimental data. This method was successfully applied to study an immiscible Ti-Mg system and the solubility limit vi was for the first time theoretically established. Based on formation energy of Ti-Mg solid solutions, our calculations predicted for the first time that the solubility of up to 60 and 100 at.% Mg into Ti with the use of USP and NCP, respectively, to be thermodynamically favourable with necessary lattice kinetics being the main challenge. Nonetheless, NCP proved to be reliable in predicting structural and elastic properties of disordered alloys.

Stress-corrosion cracking of austenitic alloys in an oxygen-free carbon dioxide environment containing chloride ions was studied under static conditions. Stiffness and X-ray measurements supported results obtained from SEM photomicrographs indicating that the CT specimens loaded to a stress intensity of 22 ksi-in^.5 were not susceptible to SCC in this environment. These alloys were also evaluated for their SCC resistance in boiling MgCl₂ and NaCl solutions. Results of this study indicated that alloys containing higher nickel contents were more resistant to chloride SCC. / Master of Science

A computer model for diffusion-controlled internal precipitation was used to simulate the corrosion behavior of Ni-based superalloys in carburizing atmospheres. The model is based on Crank-Nicholson finite difference solution of the diffusion equation. The code also includes the ternary cross diffusion effect due to substitutional alloying elements that are preferentially oxidized. The model can treat two sets of boundary conditions corresponding to the presence or absence of a protective oxide scale. It accounts for internal precipitation of corrosion products whenever thermodynamic solubility limits are exceeded. Up to four different carbides can be treated simultaneously. The solubility product is computed for each reaction and the amount of carbon that reacts is removed from the diffusion process. Under non-protective conditions, the predictions of model were compared to carburization profiles obtained experimentally in H₂-CH₄ environment tests. Results are presented for the formation of Cr, Mo, Ti, W, and Nb carbides in Ni-based superalloys. The predicted corrosion profiles are in qualitative agreement with experimental data. / Master of Science

Mode I and Mode III fatigue tests were performed on copper nickel alloys in helium, salt water environments. The hydrogen, oxygen, two alloys used air and in this investigation were 90-10 and 70-30 copper nickel. Both alloys contained iron which was added to improve the erosion corrosion resistance. The extent of cracking varied with the test environment. Tests showed that oxygen and humid air promoted cracking while salt water helium was used as the baseline retarded cracking when environment. Hydrogen promoted cracking when compared to helium but retarded cracking if comparisons were made with oxygen or humid air. The environmental effects (helium as the base case} in the Mode I tests in gaseous environments were manifested in the form of shorter fatigue lives, easier crack initiation, marginally higher crack growth rates and the development of intergranular fracture at the surface. These effects were accompanied by a change in the near surface deformation characteristics. The increases in fatigue life induced by testing in aqueous environments were greatly extended if the copper nickel was galvanically coupled to steel. Mode III tests showed the same ranking of environmental effects as Mode I tests and also showed multiple initiation, brittle fracture and secondary cracking. Two models were proposed to explain the observed results. One was based on the dilation-aided diffusion of oxygen ahead of the crack tip and subsequent oxidation of internal iron particles. The oxidation caused a volume expansion which produced internal tensile strains and facilitated fracture. The other mechanism was based on dilation-aided transport of hydrogen with subsequent accumulation of hydrogen at interfaces, resulting in a lowering of the interfacial strength and promoting intergranular fracture. The observed increases in life in the aqueous environments were rationalized by the reduced oxygen content available in the stagnant solutions. These observations suggest that the presence of iron accelarates fatigue in copper nickel alloys exposed to aggressive environments. Thus, any application involving fatigue loading with simultaneous exposure to aggressive environments should attempt to ensure that the iron content of the copper nickel alloys is minimized. / Doctor of Philosophy

Shape Memory Alloys (SMA’s) are intermetallic materials (chemical compounds of two or more elements) that are able to sustain a residual deformation after the application of a large stress, but they “remember” the original shape to which they creep back, without the application of any external force, after they are heated above a certain critical temperature. A general one-dimensional dynamic mathematical model is presented which accounts for thermal coupling, time-dependent distributed and boundary inputs and internal variables. Well-posedness is obtained using an abstract formulation in an appropriate Hilbert space and explicit decay rates for the associated linear semigroup are derived. Numerical experiments using finite-dimensional approximations are performed for the case in which the thermodynamic potential is given in the Landau-Devonshire form. The sensitivity of the solutions with respect to the model parameters is studied. Finally, an alternative approach to the stress-strain laws is presented which is able to capture the dependence on the strain history. / Ph. D.