Improving the performance of structural materials definitely leads to their better use for several applications, and consequently to a decrease in the use of natural resources or in the harmful environmental consequences. For example, reducing the weight of cars or planes while improving their structural performances will also bring about a decrease of their fuel consumption and of their level of emitted CO2. The key issue is thus to find and to fabricate better materials for these applications. Our research project deals with the influence of thermal, mechanical and thermomechanical parameters on the thermodynamics, kinetics, crystallography and mechanics of phase transformations occurring in different metallic alloys presenting finely grained multiphase microstructures in order to design new alloys or microstructures exhibiting mechanical properties never reached before. It encompasses the characterisation and the understanding of the relationships between the mechanisms dictating the elastic-plastic properties and the phase transformations and recrystallisation of different engineering metallic materials including steels, iron alloys and titanium alloys in order to improve their structural performances. We focus, on the one hand, on how the microstructure of engineering alloys can be designed and controlled through the combined actions of heat and deformation, and on the other hand, on the discovery and the understanding of how phenomena operating at different scales dictate the macroscopic mechanical behaviour. By controlling the defects structures generated at several levels ranging from 1nm to 100µm in the engineered microstructures, it is possible to attain large improvements of the mechanical properties instead of premature embrittlement.

The C54 TiSi2 thin films are widely applied in semiconductor devices due to the low electric resistance and high thermal stability. Through the annealing processing in this study, the metastable C49 TiSi2 with an electric resistivity of 219.3 £g£[-cm transforms to the stable C54 TiSi2 phase at a higher annealing temperature, with a resistivity of 30.5 £g£[-cm. Hence the transformation temperature of C49 ¡÷ C54 is of great concern in metallization of gates and local interconnections. In this thesis, it is found that the oxygen content and Ag addition impose significant influence on the transformation temperature of C49 ¡÷ C54. The as-sputtered TiSi2 thin films are confirmed to be amorphous. After annealing at 600oC or 900oC, the silicides would transform to the metastable C49 TiSi2 or C54 TiSi2 phase, respectively. The current transformation temperatures are much higher than 200oC and 600oC for the normal TiSi2 system, due to high oxygen content in the current films (up to 15-20 at% as a result of our old sputtering system). Nevertheless, the co-sputtered TiSi2 thin films with 5 and 20 at% Ag can decrease the formation temperature of C54 TiSi2 phase to 800oC. Compare with the as-sputtered TiSi2 thin films, the desirable electric resistivity of the C54 phase in the 20 at% Ag thin films is also further reduced to 22.9 £g£[-cm. The time-dependent mechanical responses of the amorphous, crystalline C49, and C54 TiSi2 thin films are investigated by room-temperature nanoindentation at the different loading rates ranging from 0.0125 to 5 mN/s. The anelasticity response plays an important role in the current TiSi2 thin films and is found to be sensitive to the loading rate. The displacement of time-dependent anelasticity recorded during the period of hold time increases with increasing loading rate. The anelasticity behavior can be analyzed by the Kelvin model. The as-deposited amorphous phase, with a lower atomic packing density and higher degree of defects and free volumes, exhibits the higher anelasticity deformation and longer relaxation time.

Ti3Al based intermetallic alloys are attractive for aerospace and aircraft applications due to their superior high temperature properties. Excellent high temperature superplasticity in the Ti3Al-Nb based alloy has been widely published. However, the alloys become brittle and hard to deform at temperatures below 600oC so that low temperature superplasticity is difficult to develop. In the current super

Pressureless-sintered lanthanum gallate (LaGaO3) ceramics have been analyzed using X-ray diffractometry (XRD), scanning electron and transmission electron microscopy (SEM and TEM), and electron-backscatter diffraction (EBSD). Transformation-induced twin domains are generated by solid-state phase transition upon cooling from the rhombohedral (r, space group R c) to orthorhombic (o, space group Pnma) symmetry at 145oC. Four types of transformation twins {101}, {121},

By means of molecular simulation, we propose possible packing models for £\ and £\¡¬ phases in poly(9,9-di-n-hexyl-2,7-fluorene) (PFH). Simulated multi-chain unit cell structures are compared with experimental diffraction patterns of PFH where the unit cell structure (and the space group) of the high-temperature £\ crystals was identified as monoclinic (C2) and that of £\¡¬ phase (kinetically favored upon programmed cooling) triclinic (P1). Results show that £\ phase prefers to adopt bi-radial side-chain conformation whereas the £\¡¬ phase prefers tetra-radial one. Both models exhibit embracing behavior between adjacent chains in spite of differences in inter-chain distance. A group of embracing chains aligned along b-axis in £\ phase and the comparatively greater inter-chain distance in £\¡¬ phase are consistent with the observed faceting along (100) planes and the tensile cracking along the a-axis. A qualitative analysis of co-existing £\ and £\¡¬ phases reproduce the [001] SAED pattern quite well. In addition, we also show that arbitrary alternation of 40o and 140o in dihedral angle between neighboring monomers generates equally stable single-chain conformations in this case of linear alkyl side-chains.

Zr-2.5Nb is used as the pressure tube material for 2nd and 3rd generation CANDU reactors. The physical properties of pressure tubes in service, including strength, dimensional stability, and delayed hydride cracking resistance, are largely dependent upon the crystallographic texture of the hcp α-phase, whose texture is predominantly developed during the extrusion stage of manufacturing. During extrusion and subsequent cooling, the formation of α may occur by transformation of the bcc β-phase to α according to the Burgers relationship and influenced by variant selection – that is, a preference for one or more of the twelve possible orientations of the hcp lattice relative to the bcc lattice. Variant selection has been observed in other Zr alloys, including the heat-treated zone in pressure tube welds and the bulk texture of heat-treated pressure tubes. Further, it has been proposed as a possible explanation for texture characteristics in pressure tubes that are not explained by the deformation mechanics of extrusion. However, the criteria for variant selection are unclear. In this work, an understanding of the criteria for variant selection is developed through observations of the differing mechanisms at play during both directions of transformation, from α-β and β-α. Transformation via the Burgers relationship was confirmed; the existence of variant selection is also established. In thermal cycles to the β-regime, this selection manifests as the selection of a new (0002) variant, as driven by anisotropic thermal stresses generated during heating. Upon cooling, the high-temperature β texture is inherited by the α grains via the Burgers relationship; the magnitude of the texture maxima is driven by elastic transformation strains. Further thermal cycles to the β regime demonstrate texture memory, with some development of cubic symmetry due to grain growth during the hold in the β-phase. No texture changes are observed if samples are not heated fully into the beta regime. Finally, a study of the biasing effects of both residual and external stresses is discussed. While the external stress did not appear to be capable of biasing variant selection during either heating or cooling, some texture changes were observed, likely due to deformation at high temperature. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2010-09-23 20:44:24.784

The current understanding of the atomic scale phenomenon associated with the influence of beta phase instabilities on the evolution of microstructure in titanium alloys is limited due to their complex nature. Such beta phase instabilities include phase separation and precipitation of nano-scale omega and alpha phases in the beta matrix. The initial part of the present study focuses on omega precipitation within the beta matrix of model binary titanium molybdenum (Ti-Mo) alloys. Direct atomic scale observation of pre-transition omega-like embryos in quenched alloys, using aberration-corrected high resolution scanning transmission electron microscopy and atom probe tomography (APT) was compared and contrasted with the results of first principles computations performed using the Vienna ab initio simulation package (VASP) to present a novel mechanism of these special class of phase transformation. Thereafter the beta phase separation and subsequent alpha phase nucleation in a Ti-Mo-Al ternary alloy was investigated by coupling in-situ high energy synchrotron x-ray diffraction with ex-situ characterization studies performed using aberration corrected transmission electron microscopy and APT to develop a deeper understanding of the mechanism of transformation. Subsequently the formation of the omega phase in the presence of simultaneous development of compositional phase separation within the beta matrix phase of a Ti-10V-6Cu (wt%) alloy during continuous cooling has been investigated using a combination of transmission electron microscopy and atom probe tomography. The results of these investigations provided novel insights into the mechanisms of solid-state transformations in metallic systems by capturing the earliest stages of nucleation at atomic to near atomic spatial and compositional resolution.

<p> The a-β phase transformation was studied in hydrothermally grown crystals of A1PO4 by measuring the spin-Hamiltonian parameters of Fe^3+ as a function of temperature. </p> <p> The theory of the Blume-Orbach mechanism for the zero-field splitting of s-state ions was generalized and used to calculate the D-tensor of the spin-Hamiltonian. The experimentally observed temperature variation of the spin-Hamiltonian was interpreted in terms of a temperature-dependent point-multipole model of the charge distribution in the crystal lattice. </p> <p> A similar study of the a-S phase transformation in quartz was attempted but E.P.R. measurements could only be taken up to 450°C (123°C below the transformation temperature) because of the instability of the Fe^3+ center at higher temperatures. </p> / Thesis / Doctor of Philosophy (PhD)

It is a well-known fact that drugs can exist in different solid-state forms. These solid-state forms can be either crystalline or amorphous. Furthermore, significant differences are identified between the different solid-state forms of the same drug. Physico-chemical properties that are affected by the solid-state include: melting point, solubility, dissolution rate, stability, compressibility, processability, to name but a few. During the last two decades a significant amount of attention was directed towards the amorphous solid-state forms of drugs. The amorphous form is the direct opposite of the crystalline solid-state. While crystalline forms are constituted by unit cells arranged in a repetitive and structured nature, amorphous forms do not have a long-range order. This lack of order leads to an increase in the Gibbs free energy of such compounds which in turn leads to increased dissolution and solubility. The advantage of improved aqueous solubility and dissolution is a sought after characteristic within the pharmaceutical industry. Improved solubility ultimately could lead to improved bioavailability of a drug. In this study the amorphous nature and stability of amorphous azithromycin was studied. Although previous studies reported that amorphous azithromycin can be easily prepared, there is not a significant amount of data available on the stability of the amorphous form. Furthermore, the effect of milling, mixing, compression, handling and storage on the amorphous form was also investigated. This study showed that amorphous azithromycin remains stable during milling, mixing and compression. A compatibility study on azithromycin when mixed with tableting excipients showed some incompatibilities and this was helpful information to assist with the choice of excipients to be included in the tablet formulation. During the formulation study it became evident that good formulation strategies can greatly improve the flow properties of a drug. The stability of amorphous azithromycin was also studied. During this phase of the study an atypical stability indicating method was used in order to determine and demonstrate the stability of amorphous azithromycin. Dissolution studies were used to illustrate the stability of amorphous azithromycin due to the fact that dissolution is the only method that indicates the phenomena of solution-mediated phase transformation of an amorphous form to a stable crystalline form. During the stability study of six months at 40°C ± 75% RH no recrystallisation of the amorphous form to the crystalline form occurred. It was concluded that amorphous azithromycin will remain stable during processing steps, product formulation and manufacturing as well as during storage for a period of six months at elevated temperature and humidity. / MSc (Pharmaceutics), North-West University, Potchefstroom Campus, 2015

It is a well-known fact that drugs can exist in different solid-state forms. These solid-state forms can be either crystalline or amorphous. Furthermore, significant differences are identified between the different solid-state forms of the same drug. Physico-chemical properties that are affected by the solid-state include: melting point, solubility, dissolution rate, stability, compressibility, processability, to name but a few. During the last two decades a significant amount of attention was directed towards the amorphous solid-state forms of drugs. The amorphous form is the direct opposite of the crystalline solid-state. While crystalline forms are constituted by unit cells arranged in a repetitive and structured nature, amorphous forms do not have a long-range order. This lack of order leads to an increase in the Gibbs free energy of such compounds which in turn leads to increased dissolution and solubility. The advantage of improved aqueous solubility and dissolution is a sought after characteristic within the pharmaceutical industry. Improved solubility ultimately could lead to improved bioavailability of a drug. In this study the amorphous nature and stability of amorphous azithromycin was studied. Although previous studies reported that amorphous azithromycin can be easily prepared, there is not a significant amount of data available on the stability of the amorphous form. Furthermore, the effect of milling, mixing, compression, handling and storage on the amorphous form was also investigated. This study showed that amorphous azithromycin remains stable during milling, mixing and compression. A compatibility study on azithromycin when mixed with tableting excipients showed some incompatibilities and this was helpful information to assist with the choice of excipients to be included in the tablet formulation. During the formulation study it became evident that good formulation strategies can greatly improve the flow properties of a drug. The stability of amorphous azithromycin was also studied. During this phase of the study an atypical stability indicating method was used in order to determine and demonstrate the stability of amorphous azithromycin. Dissolution studies were used to illustrate the stability of amorphous azithromycin due to the fact that dissolution is the only method that indicates the phenomena of solution-mediated phase transformation of an amorphous form to a stable crystalline form. During the stability study of six months at 40°C ± 75% RH no recrystallisation of the amorphous form to the crystalline form occurred. It was concluded that amorphous azithromycin will remain stable during processing steps, product formulation and manufacturing as well as during storage for a period of six months at elevated temperature and humidity. / MSc (Pharmaceutics), North-West University, Potchefstroom Campus, 2015