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Thesis (M.Sc. (Physics)) -- University of Limpopo, 2021 / First-principles density functional theory has been used to study the stabilities of binary TiPt, TiTa, TiNi and TiCo potential shape memory alloys. Furthermore, ternary alloys Ti50Pt50-xMx with V, Ta, Hf and quaternary Ti50(PtCo)50-xTax systems were also investigated. The structural, electronic and mechanical properties were deduced to mimic the stabilities of these alloys. Furthermore, their vibrational stability, x-ray diffraction and temperature dependence have been examined. The structures were subjected to full geometry optimization to obtain equilibrium lattice constants. It was found that the equilibrium lattice parameters for all the binary systems are in good agreement with experimental results to within 5%. The heats of formation (ΔHf) were calculated to determine the thermodynamic stability of the B2 TiM systems. It was revealed that TiPt is the most energetically favourable (most stable) whereas TiTa is the least favourable due to high ΔHf value (less stable). In addition, electronic properties suggest that TiPt, TiNi and TiCo systems are stable with TiTa being the least favourable consistent with the ΔHf. The elastic properties were also calculated to mimic the mechanical stability of these alloys. TiNi, TiCo and TiTa were found to be mechanical stable whereas TiPt is unstable. This behaviour is consistent with the phonon dispersion curves for TiPt and TiCo. TiCo structure, in particular is the most stable in line with the predicted phonon dispersion. The effect of alloying on Ti50Pt50-xMx (M = V, Ta, Hf) ternary system was carried out using the supercell approach. It was observed that the lattice parameters decrease minimally with an increase in V and increases with an increase in Ta and Hf content. The structures ii become thermodynamically less stable with an increase in V, Ta and Hf content, as depicted by heats of formation. The shear modulus (C′) of Ti50Pt50-xMx increases with an increase in M (V, Ta and Hf) concentration suggesting mechanical stability of these alloys. This has been confirmed from the phonon curves where the phonon soft modes are reduced and tend to disappear with increasing content of the alloying elements. Thus the results suggest that the V, Ta and Hf addition reduces the transformation temperatures of the TiPt alloy as indicated by its higher shear modulus C′. Furthermore, it was observed that the lattice parameters of the quaternary system decrease with an increase in Ta content in the system. Thus ΔHf of the B2 and B19 Ti50Pt43.75-xCo6.25Tax and B19 Ti50Pt31.25-xCo18.75Tax alloy system showed that the 6.25 at.% Ta addition is energetically most favourable (ΔHf<0). The DOS behaviour confirms that the 6.25 at.% Ta as least favourable whereas for B19, the 6.25 at.% Ta is most favourable. The elastic constants for B19 and B2 show the positive shear modulus (mechanical stability). Moreover, the phonon dispersions and phonon density of states for the B2 and B19 Ti50Pt43.75-xCo6.25Tax and Ti50Pt31.25-xCo18.75Tax were calculated and are consistent with the elastic constant. The LAMMPS code was employed to investigate the temperature dependence of the B19 Ti50Pt43.75-xCo6.25Tax and Ti50Pt31.25-xCo18.75Tax structures. The martensitic to austenite transformation temperature decreases with an increase in Ta concentration. Temperature variations of the XRD patterns for the B19 are in reasonable agreement with predicted lattice parameters. / National Research Foundation (NRF) and Titanium centre of competence (TiCoC)

Chung, Kam Chuen = 鐵基/鐵三鋁基/鋁基金屬基複合材料的合成和表徵 / 鍾錦銓. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2007. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / Chung, Kam Chuen = Tie ji/tie san lü ji/lü ji jin shu ji fu he cai liao de he cheng he biao zheng / Zhong Jinquan. / Abstract --- p.i / 摘要 --- p.iii / Acknowledgement --- p.v / Table of contents --- p.vi / List of tables --- p.x / List of figures --- p.xi / Chapter Chapter 1 --- Introduction --- p.1-1 / Chapter 1.1. --- Metal matrix composites (MMCs) --- p.1-1 / Chapter 1.1.1. --- Introduction --- p.1-1 / Chapter 1.1.2. --- Matrix materials --- p.1-1 / Chapter 1.1.3. --- Reinforcements --- p.1-2 / Chapter 1.1.4. --- Fabrication techniques --- p.1-3 / Chapter 1.1.5. --- Applications --- p.1-7 / Chapter 1.2. --- MMCs in this work --- p.1-9 / Chapter 1.2.1. --- Metal matrices --- p.1-9 / Chapter 1.2.2. --- Reinforcements --- p.1-11 / Chapter 1.3. --- Previous works --- p.1-13 / Chapter 1.4. --- Objectives and current works --- p.1-15 / Chapter 1.5. --- Thesis layout --- p.1-16 / References --- p.1-18 / Chapter Chapter 2 --- Methodology and Instrumentation --- p.2-1 / Chapter 2.1. --- Powder metallurgy (PM) --- p.2-1 / Chapter 2.1.1. --- Mixing --- p.2-1 / Chapter 2.1.2. --- Compacting --- p.2-1 / Chapter 2.1.3. --- Sintering --- p.2-2 / Chapter 2.2. --- Sample preparation --- p.2-3 / Chapter 2.2.1. --- Mixing and compacting --- p.2-3 / Chapter 2.2.2. --- Tube furnace sintering --- p.2-3 / Chapter 2.2.3. --- Arc melting --- p.2-4 / Chapter 2.3. --- Sample characterization --- p.2-4 / Chapter 2.3.1. --- DTA and DSC --- p.2-5 / Chapter 2.3.2. --- XRD --- p.2-6 / Chapter 2.3.3. --- SEM --- p.2-6 / Chapter 2.3.4. --- TEM --- p.2-6 / Chapter 2.3.5. --- Microhardness test --- p.2-7 / Chapter 2.3.6. --- VSM --- p.2-7 / References --- p.2-9 / Chapter Chapter 3 --- Synthesis of magnetic hercynite in Fe-based MMC --- p.3-1 / Chapter 3.1. --- Introduction --- p.3-1 / Chapter 3.2. --- Experiments --- p.3-2 / Chapter 3.3. --- Results and discussion --- p.3-2 / Chapter 3.3.1. --- DTA and XRD results --- p.3-2 / Chapter 3.2.2. --- SEM and EDS results --- p.3-3 / Chapter 3.3.3. --- Reaction mechanisms --- p.3-5 / Chapter 3.3.4. --- Thermodynamic model for the reactions --- p.3-8 / Chapter 3.3.5. --- Saturation magnetization --- p.3-9 / Chapter 3.3.6. --- Microhardness --- p.3-11 / Chapter 3.4. --- Conclusions --- p.3-11 / References --- p.3-13 / Chapter Chapter 4 --- Synthesis of reinforced Fe3Al-based MMC --- p.4-1 / Chapter 4.1. --- Introduction --- p.4-1 / Chapter 4.2. --- Experiments --- p.4-2 / Chapter 4.3. --- Results and discussion --- p.4-4 / Chapter 4.3.1. --- AI2O3-reinforced samples --- p.4-4 / Chapter 4.3.2. --- MgO-reinforced samples --- p.4-8 / Chapter 4.3.3. --- MgAl204-reinforced samples --- p.4-11 / Chapter 4.3.4. --- Microhardness and densities --- p.4-14 / Chapter 4.4. --- Conclusions --- p.4-16 / References --- p.4-18 / Chapter Chapter 5 --- Formation of Al-Fe intermetallics in Al-based MMC…… --- p.5-1 / Chapter 5.1. --- Introduction --- p.5-1 / Chapter 5.2. --- Experiments --- p.5-2 / Chapter 5.3. --- Results and discussion --- p.5-3 / Chapter 5.3.1. --- DTA and XRD results --- p.5-3 / Chapter 5.2.2. --- "SEM, TEM and EDS results" --- p.5-4 / Chapter 5.3.3. --- Reaction mechanisms --- p.5-9 / Chapter 5.3.4. --- Phase transformation in solidification --- p.5-11 / Chapter 5.3.5. --- Microhardness --- p.5-13 / Chapter 5.4. --- Conclusions --- p.5-14 / References --- p.5-15 / Chapter Chapter 6 --- Conclusions and future work --- p.6-1 / Chapter 6.1. --- Conclusions --- p.6-1 / Chapter 6.2. --- Future work --- p.6-3

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.

Thesis (Ph. D.)--Lehigh University, 2003. / Includes vita. Includes bibliographical references (leaves 198-205).

Due to its excellent biocompatibility and mechanical properties, Ti-6Al-4V alloy has been extensively used in the medical field, especially as a material for hard tissue replacement. Owing to the unique shape memory and superelastic properties, NiTi shape memory alloy (SMA, with 50.8 at.% of Ni) has been investigated for load-bearing applications in orthopedics and dentistry. Since the longevity of current metal implants is approximately 10 to 15 years, many patients need to have revision surgeries in their lifetime. Therefore, there is great interest in the long-term stability, biocompatibility, bioactivity and other properties of Ti-6Al-4V and NiTi SMA implants. Implant-associated infections also pose serious threat to the success of metal implants. The goal of this project was to investigate several low-temperature surface modification techniques, including anodization and electrochemical deposition, and formulate coatings for potential clinical applications. Accordingly, several types of coatings were synthesized on Ti-6Al-4V and NiTi SMA substrates. Various aspects of the coatings, such as morphology, chemical composition, crystallinity, phase and bioactivity, were analyzed. Firstly, a systematic study on the formation of titania nanotubes on Ti-6Al-4V by anodization was performed. Anodizing voltage and time were varied for comparisons. A dense and compact titania nanotube layer was synthesized on Ti-6Al-4V by anodizing at 25 V for 20 min. The titania nanotubes formed were rutile. After annealing at 500oC for 1 h, the titania nanotubes became anatase. The anatase phase exhibited better wettability than the rutile phase. Secondly, dense and compact apatite coatings were formed on NiTi SMA samples through electrochemical deposition using mainly double-strength simulated body fluid (2SBF) as the electrolyte. The deposition conditions were varied and apatite coating characteristics studied. With the inclusion of collagen molecules (0.1 mg/ml) in the electrolyte (2SBFC), apatite/collagen composite coatings were fabricated. Collagen fibrils were not only observed on the surface of composite coatings but also were embedded inside in the coatings and at the coating-substrate interface. Results obtained from transmission electron microscopic and X-ray diffraction analyses showed that the apatite crystals in apatite coatings and apatite/collagen composite coatings were calcium-deficient carbonated hydroxyapatite. Apatite/collagen composite coatings exhibited excellent hydrophilicity, whereas apatite coatings displayed hydrophobic surfaces. Finally, gentamicin-loaded, tobramycin-loaded, and vancomycin-loaded apatite coatings and apatite/collagen composite coatings were synthesized on NiTi SMA samples through electrochemical deposition using different drug concentrations in the electrolytes. A comparative study of apatite coatings and apatite/collagen composite coatings as drug delivery vehicles were conducted. Different aspects of antibiotic-loaded coatings (surface characteristics, chemical composition, wettability, etc.) and in vitro release behaviour were investigated. The antibiotics were physically embedded in coatings during coating formation. Upon sample soaking in phosphate-buffered saline (PBS), the release profiles established for antibiotic-loaded coatings demonstrated different levels of initial burst release and subsequent steady release characteristics. Apatite coatings and apatite/collagen coatings displayed preferential incorporation of specific antibiotics. For instance, apatite/collagen coatings showed better vancomycin incorporation than apatite coatings and the incorporation of vancomycin was better than tobramycin for apatite/collagen coatings. Apatite coatings demonstrated better tobramycin incorporation than apatite/collagen composite coatings. / published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

In the present study, dilute alloys in the Mg-rich corner of the Mg-Ce-In ternary system in the composition range 0 to 3% In and 0 to 1.5% Ce were synthesized. Cooling curve analysis was used to determine the liquidus points in order to construct the liquidus surface of the ternary phase diagram in the Mg corner. Energy dispersive spectroscopy (EDS), wavelength dispersive spectroscopy (WDS), and x-ray diffraction (XRD) techniques were used to examine phases present at the compositions studied. A thermal arrest presumed to represent a eutectic transformation was discovered at 580°C. Two new intermetallic compounds, designated tau and theta, were found. Trace silicon present in the alloys was found to concentrate in one of the intermetallic compounds. / To further investigate these compounds, an induction furnace was used to synthesize alloys containing the concentrations of Ce and In seen in electron probe micro-analysis (EPMA) examinations of these compounds. The alloys were examined using the cooling curve technique and XRD, and proved to contain the compounds already observed with some variation in dissolved indium content. In addition, differential scanning calorimetry (DSC) was used to confirm the liquidus and solidus values determined using cooling curve analysis. / A diffusion couple with terminal compositions of pure Ce and a Mg-In alloy was prepared in order to determine the equilibrium phases present in the system between these two compositions at 390°C. EPMA was used to identify the zones obtained, and confirmed the presence of several Mg-Ce compounds with 1 at% dissolved indium, as well as a ternary compound corresponding to the theta compound found in the dilute alloys. / Finally, literature values and experimental data were used to calculate a preliminary ternary phase diagram using FACTSage, in collaboration with the CTRC at Ecole Polytechnique, in order to affirm the validity of the experimentally determined values as well as to project the diagram beyond the studied composition range.

Thesis (Ph.D.)--University of Western Australia, 2003.