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.

The effects of Ca and Ce on the precipitation behaviour and microstructural characteristics of Mg-Zn based alloys are investigated by comprehensive multi-scale characterization and analysis. The elements Ca and Ce are chosen for their potential to enhance (a) precipitation hardening and (b) alloy texture and ductility, and are examined at both alloying and microalloying (< 0.5 wt%) levels. When added individually to Mg-Zn, Ca is found to enhance precipitation, but Ce produces a generally adverse effect on the hardening response. A pre-ageing strategy is proposed to alleviate this negative effect of Ce. The highlight of this work is the double microalloying addition of Ce-Ca to Mg-Zn, as this combination and quantity proves to be the most effective at increasing the age-hardening response, and enhancing microstructural characteristics for improved ductility. Transmission electron microscopy analysis reveals the hardening increase to originate from a refined precipitate microstructure, and the formation of fine-scale basal plate precipitates. These fine precipitates form during early ageing as monolayer GP zones consisting of Ca and Zn. The formation of these GP zones is facilitated by the atomic size difference between those two solutes, and their observed tendency to co-cluster. The monolayer GP zones evolve to multi-layered forms in the peak-aged condition. These precipitates are observed to be uniformly distributed, even where apparent precipitate-free zones are observed for the Mg-Zn type phases in the grain boundary regions. Notably, the size of these precipitate-free zones for the Mg-Zn phases is also reduced in the Ce-Ca microalloyed samples, compared to the binary alloy. The Ce-Ca microalloying additions also promote grain refinement and a weakening of the basal textures, typical of conventional Mg-based alloys, compared to both Mg-Zn and Mg-Zn-Ce. As a result, the tensile behaviour of the alloys with Ce-Ca is similarly enhanced. Considering both the precipitation hardening capability and microstructural characteristics, it is concluded that the double microalloying additions of Ce-Ca can be considered as a new alloy design strategy to successfully achieve improvement in both the strength and ductility of Mg-Zn alloys.

The central goal of this research is to quantitatively characterize the relationships between processing, microstructure, and mechanical properties of important high-pressure die-cast (HPDC) Mg-alloys. For this purpose, a new digital image processing technique for automatic detection and segmentation of gas and shrinkage pores in the cast microstructure is developed and it is applied to quantitatively characterize the effects of HPDC process parameters on the size distribution and spatial arrangement of porosity. To get better insights into detailed geometry and distribution of porosity and other microstructural features, an efficient and unbiased montage based serial sectioning technique is applied for reconstruction of three-dimensional microstructures. The quantitative microstructural data have been correlated to the HPDC process parameters and the mechanical properties. The analysis has led to hypothesis of formation of new type of shrinkage porosity called, gas induced shrinkage porosity that has been substantiated via simple heat transfer simulations. The presence of inverse surface macrosegregation has been also shown for the first time in the HPDC Mg-alloys. An image analysis based technique has been proposed for simulations of realistic virtual microstructures that have realistic complex pore morphologies. These virtual microstructures can be implemented in the object oriented finite elements framework to model the variability in the fracture sensitive mechanical properties of the HPDC alloys.

The effects of Ca and Ce on the precipitation behaviour and microstructural characteristics of Mg-Zn based alloys are investigated by comprehensive multi-scale characterization and analysis. The elements Ca and Ce are chosen for their potential to enhance (a) precipitation hardening and (b) alloy texture and ductility, and are examined at both alloying and microalloying (< 0.5 wt%) levels. When added individually to Mg-Zn, Ca is found to enhance precipitation, but Ce produces a generally adverse effect on the hardening response. A pre-ageing strategy is proposed to alleviate this negative effect of Ce. The highlight of this work is the double microalloying addition of Ce-Ca to Mg-Zn, as this combination and quantity proves to be the most effective at increasing the age-hardening response, and enhancing microstructural characteristics for improved ductility. Transmission electron microscopy analysis reveals the hardening increase to originate from a refined precipitate microstructure, and the formation of fine-scale basal plate precipitates. These fine precipitates form during early ageing as monolayer GP zones consisting of Ca and Zn. The formation of these GP zones is facilitated by the atomic size difference between those two solutes, and their observed tendency to co-cluster. The monolayer GP zones evolve to multi-layered forms in the peak-aged condition. These precipitates are observed to be uniformly distributed, even where apparent precipitate-free zones are observed for the Mg-Zn type phases in the grain boundary regions. Notably, the size of these precipitate-free zones for the Mg-Zn phases is also reduced in the Ce-Ca microalloyed samples, compared to the binary alloy. The Ce-Ca microalloying additions also promote grain refinement and a weakening of the basal textures, typical of conventional Mg-based alloys, compared to both Mg-Zn and Mg-Zn-Ce. As a result, the tensile behaviour of the alloys with Ce-Ca is similarly enhanced. Considering both the precipitation hardening capability and microstructural characteristics, it is concluded that the double microalloying additions of Ce-Ca can be considered as a new alloy design strategy to successfully achieve improvement in both the strength and ductility of Mg-Zn alloys.

Aluminum (Al) powder metallurgy (PM) provides a cost effective and environmentally friendly means of creating lightweight, high performance, near net shape components, relative to conventional casting/die casting technology. Unfortunately, the current lack of commercially available Al alloy powder blends has hindered development in this field as a result of the limited scope of mechanical properties available; especially under elevated temperature conditions common to many automotive applications. As such, the objective of this research was to attempt to improve the versatility of current Al PM technology through the incorporation of Fe and Ni transition metal additions into an emerging Al- 4.4Cu-1.5Mg-0.2Sn alloy, as this technique is known to enhance the elevated temperature stability of wrought/cast Al alloys through the formation of stable, Fe/Ni aluminide dispersoids. Initial experimentation consisted of evaluating the feasibility of incorporating Fe and Ni both elementally and pre-alloyed, through a series of tests related to their PM processing behaviour (compressibility, sintering response) and sintered product performance (ambient tensile properties). Results confirmed that pre-alloying of the base Al powder was the most effective means of incorporating Fe and Ni as all such specimens achieved properties similar or slightly superior to the unmodified alloy. Of the pre-alloyed systems considered, that containing 1%Fe+1%Ni displayed the most desirable results in terms of mechanical performance and microstructural homogeneity of the Fe/Ni dispersoid phases present in the sintered product. Bars of the baseline system and that modified with pre-alloyed additions of 1Fe/1Ni were then sintered industrially to gain a preliminary sense of commercial viability and obtain additional specimens for elevated temperature exposure tests. Results confirmed that the sintering response, tensile properties and microstructures were essentially identical in both alloys whether they were sintered in a controlled laboratory setting or an industrial production environment. Furthermore, DSC data indicated that S (Al2CuMg)-type phases were the dominant precipitates formed during heat treatment. The effects of elevated temperature exposure were assessed in the final stage of research. Both alloys were found to exhibit comparable behaviour when exposed to the lowest (120°C) and highest (280°C) temperatures considered. Here, the alloys showed no obvious degradation at 120°C. Conversely, exposure at 280°C prompted a steady decline in yield strength for both alloys with significant precipitate coarsening noted as well. Despite these similarities, differences emerged during isochronal tests at intermediate temperatures. Here, DSC data indicated that the precipitates present in the pre-alloyed material were stable at temperatures up to 160°C while those in the unmodified alloy had begun to overage under the same exposure conditions. These differences were accompanied by increased stability in tensile yield strength for the pre-alloyed material. In all, this study has indicated that the use of Al powder pre-alloyed with Fe/Ni additions is feasible for press-and-sinter PM technology and that the sintered product exhibits improved elevated temperature stability under certain conditions.

No description available.

A continuum plasticity yield function is developed that captures tension/compression asymmetry and its evolution as exhibited by HCP materials such as magnesium alloy sheet. The model, referred to herein as “CPB06ex3ev”, is based upon the CPB06 [1] yield surface which is extended in this research to consider evolution of asymmetry and anisotropy under monotonic loading. The model is further modified to incorporate thermal softening and strain rate effects. Mechanical characterization experiments are performed to acquire uniaxial tensile and compressive stress-strain data along a range of in-plane and through-thickness loading orientations. Experiments are performed for a range of strain rates (0.001-1s-1) and temperatures (23-250°C). A strong, evolving asymmetry is observed at room temperature when comparing tensile and compressive flow stresses and r-values, while asymmetry and anisotropy are reduced dramatically as temperature is increased. AZ31B exhibits moderate strain rate sensitivity at room temperature, however, the rate sensitivity increases with temperature. The CPB06ex3ev model is applied to simulate AZ31B magnesium alloy sheet. An error minimization scheme is used to fit the yield function and evolution coefficients over the entire data set. The calibrated model is shown to capture the evolving asymmetric/anisotropic response of both flow stresses and r-values in tension and compression, while also fitting the flow stress at the biaxial tension and pure shear locations on the yield locus. The model, which uses three stress transformations, is implemented within a user defined material model (UMAT) and linked to the commercial finite element software LS-DYNA. In order to assess the finite element implementation of the CPB06ex3ev model, a series of validation experiments were performed and corresponding finite element models were developed: (i) room temperature three-point bending; (ii) elevated temperature (250°C) limiting dome height experiments; and, (iii) warm cup drawing experiments. The three point bend simulations demonstrated the importance of capturing material asymmetry and the associated shift in neutral axis. Comparison between the warm forming experiments and models revealed qualitative agreement between the predicted punch load-displacement and strain distributions. The CPB06ex3ev formulation was able to capture the anisotropy trends in terms of the differences in strains measured along the sheet rolling versus transverse directions. Beyond the constitutive characterization and modeling effort, the cup draw formability experiments have provided interesting insight into the effect of temperature and temperature distribution within the AZ31B sheet. The current work has served to show the existence of a process window in which the blank center temperature must lie below the die temperature but above the temperature for activation of non-basal slip systems (to avoid low temperature fracture). Two modes of failure have been identified at the process window boundaries in which the cup either fractures due to low temperature (brittle) failure or a high temperature (necking) failure.

Mechanical milling (alloying) is one of the non-equilibrium techniques used to prepare alloys with exceptional properties. This technique was employed in this research to develop a new class of Al- and Mg-based nanocomposite alloys using SPEX high energy milling. These nanocomposites are characterized by the dispersion of nanocrystals in an amorphous matrix. Zirconium was added to the Al-Mg alloys for the purpose of promoting glass formability. As-milled samples were annealed at 400°C for 1 hour to investigate the thermal stability of the nanostructure. The phase evolution of the resulting alloys was studied using XRD and TEM/EDS, which showed a strong dependence of the resulting metastable phases on the starting alloys compositions. / The nanocomposite structure was developed at Zr concentrations of 20 and 35 at.% regardless of the Al/Mg ratio and with some traces of oxidation. However, the amount of amorphous phase was varied in each case depending on the Al concentration into the alloy, since in low Al-containing alloys the amount of amorphous phase was less pronounced. It was found that higher Zr concentrations will lead to greater refinement of the nanostructure. These nanocomposites showed improved mechanical properties, in terms of higher hardness values, in addition to improved thermal stability. The improvement in thermal stability was attributed to the presence of Al3Zr which proved to contribute significantly to retarding grain growth via grain boundary pinning. / Additionally, the employment of mechanical alloying was beneficial in producing Al3Zr in the cubic L12 ordered structure which improves the ductility of the alloy. Moreover, the homogeneity ranges of gamma-Al 12Mg17 and Al3Zr were extended significantly due to the nature of the non-equilibrium processing. In this research, the alloy with the maximum hardness was Al40Mg25Zr35, which has an average hardness value close to 780 HV and average crystallite size of about 10 nm. A common observation in the alloys that showed a higher hardness values combined with improved thermal stability, is that they contain higher Al and Zr concentrations. / Le broyage mécanique est une technique hors équilibre qui permet la fabricationde nouveaux alliages avec des propriétés exceptionnelles. Lors de cette recherche, unbroyeur SPEX 8000 a été utilisé pour développer une nouvelle classe denanocomposites à base d'aluminium et de magnésium. Ces nanocomposites tirent leurspécificité de leur dispersion de nanocrystaux dans une matrice amorphe. Duzirconium a été ajouté aux alliages d'aluminium et de magnésium pour promouvoirl'amorphisation. Les échantillons de poudres broyées ont été recuits à 400°C pour 1heure pour évaluer la stabilité thermique des différentes phases. Leur évolution a étécaractérisée par diffraction par rayon-X et par MEBIEDS. TI fut démontré que lesphases métastables obtenues dépendent fortement de la composition des alliages dedépart.

The kinetics of incongruent reduction between sapphire and oxygen-controlled Mg-Al melts was studied by measuring spinel-layer thickness, sample-weight change and sample-thickness change as a function of time at various temperatures. To eliminate the crucible contamination caused by impurities in commercial MgO crucibles, self-made high-purity MgO crucibles were achieved by gelcasting method, which is an attractive ceramic-forming technique for making high-purity ceramic parts. The oxygen-controlled alloys were obtained by the three-phase-equilibrium experiments at various temperatures. To avoid MgO formation, the oxygen-controlled alloys prepared at relatively lower temperatures were used for incongruent reaction at relatively higher temperatures. That is to say, the oxygen-controlled alloys prepared at 900°C, 1000°C, and 1100°C were used for spinel formation at 1000°C, 1100°C, and 1200°C, respectively. The experiments were conducted in a vertical furnace, and sapphire wafers were hung vertically in high-purity MgO crucibles so that the natural convection induced by the density change in the melt could be investigated. Experimental results obtained at 1000°C, 1100°C, and 1200°C showed that the spinel layer thickness on two kinds of sapphire wafers, namely {0001} and , followed orientation-independent parabolic kinetics, indicating the diffusion in spinel was one of the rate-limiting steps. In addition, the spinel layer thickness was not a function of position. The results of sample-thickness- change measurements also indicated that the effect of natural convection could be neglected. XPS, XRD, and TEM were also employed to characterize some samples in this study. Based on a simple model where the diffusion in spinel was the only rate-limiting step, the governing partial differential equations for diffusion and fluid dynamics were solved by the finite element method. The calculated theoretical parabolic constants at various temperatures were compared with these experimental results, and a good agreement was obtained. Some preliminary studies were also made on the morphologies of spinel particles at the nucleation stage. It was found that the triangular {111} faces of spinel particles were parallel to the surface of {0001} sapphire substrate. The product shape was consistent with the tetrahedron composed of {111} faces. The morphology of spinel particles on a sapphire substrate was more complicated in that the triangular {111} faces of spinel had to be inclined at a certain angle to the substrate in order to maintain the orientation relationship.

Thesis (Ph. D.)--Carleton University, 2004. / Includes bibliographical references (p. 123-134). Also available in electronic format on the Internet.