In this study, firstly, in order to achieve fine grains in solid solution strengthened AZ31 magnesium alloy by friction stir processing (FSP), various efforts have been made. It has found that with a newly designed cooling system, the microstructure of commercial AZ31 alloy can be refined dramatically by carefully controlling the FSP parameters. It is of scientific interest that nanometer grains have been observed in the resultant microstructure for the AZ alloy experienced by two-pass FSP. Besides, in order to modify the microstructure and mechanical properties, FSP is also applied to incorporate AZ31 Mg alloy with nano-ZrO2 particles, nano-SiO2 particles and different fractions of Al and Zn elements. The microstructure and mechanical properties of the modified alloy and composite samples are investigated and compared. By one-pass FSP coupled with rapid heat sink from liquid nitrogen cooling approach, the ultrafine grain size in AZ31 Mg alloy is successfully achieved. The grain boundaries are well defined and the mean grain size can be refined to 100~300 nm from the initial 75 £gm of commercial AZ31 Mg alloys sheets. The ultrafine grained structure can drastically increases the microhardness from the initial 50 up to 120 Hv, or an increment factor of 2.4 times. Furthermore, the nanometer grains can be even achieved by two passes FSP coupled with rapid heat sink. The resulting microstructure exhibits equiaxed grains ranging from 40 nm to 200 nm with an average grain size of less than 100 nm. The nanocrystalline grains can be characterized by the TEM observations and the diffraction rings in SAD patterns. The highest hardness point can reach ~150 Hv which is equal to triple of the AZ31 matrix, and the mean hardness also increases up to around 134 Hv. Bulk Mg-AZ31 based composites with 10~20 vol% of nano-ZrO2 particles and 5~10 vol% of nano-SiO2 particles are also successfully fabricated by FSP. The average grain size of the resultant composites could be effectively refined to 2~4 £gm, and it demonstrates much higher hardness values compared to commercial AZ31 billet. Moreover, for the Mg/ZrO2 composite fabricated by one pass and subsequent cooling pass FSP, the recrystallized grain size could be further refined to 0.4 £gm with the hardness value of 135 Hv. As for multi-element Mg base alloys fabricated by FSP, high fractions of Al and Zn elements can result in apparent grain refinement, this can be proved by the broadening of diffraction peaks. Multi-passes FSP can induce the appearance of intermetallic compounds, however, some of them are quasi-crystals with icosahedral point group symmetry. The average hardness of the resultant alloys reachs nearly 350 in Hv scale due to the generation of intermetallic compounds and grain refinement.

<p>The purpose of this thesis has been to investigate the elastic properties of an fcc FeMg alloy with 10 at.% magnesium under high pressure. Recent research has shown that magnesium can be a possible candidate for light element impurities in the Earth’s inner core, something that was previously not considered possible because of the low miscibility of magnesium in iron at ambient pressure. Gaining knowledge about the composition of the Earth’s core can help us better understand such phenomena as seismic activity and the fluctuations of the Earth’s magnetic field.</p><p>The elastic constants of the FeMg alloy was calculated using ab-initio methods based on Density Functional Theory. The Exact Muffin-Tin Orbitals method was used in conjunction with the Coherent Potential Approximation.</p><p>The FeMg alloy was found to be overall considerably softer than pure iron, and the softening effect on the elastic constants was also found to increase with pressure. The results also showed that 10% Mg alloying increased the anisotropy with about 40% compared to pure iron.</p>

Magnesium is the lightest metal that can be used for structural applications. For the reasons of weight saving, there has been an increasing demand for magnesium from the automotive industry. However, poor formability at room temperature, due to a limited number of slip systems available owing to its hexagonal close packed crystal structure, imposes severe limitations on the application of Mg and its alloys in the wrought form. One possibility for improving formability is to form the components superplastically. For this, it is necessary to refine the grain structure. A fine-grained material is also stronger than its coarse grain counterpart because of grain size strengthening. Moreover, fine-grained magnesium alloys have better ductility as well as a low ductile to brittle transition temperature, thus their formability at room temperature could be improved. In addition to grain refinement, the issues pertaining to poor formability or limited ductility of Mg alloys can be addressed by controlling the crystallographic texture. Recently, it has been shown that warm equal channel angular extrusion (ECAE) of magnesium led to reduction in average grain size and shear texture formation, by virtue of which subsequent room temperature rolling was possible. Based on the literature, it was also certain that, in order to make magnesium alloys amenable for processing, grain refinement needs to be carried out and the role of shear texture needs to be explored. Since processing at higher temperature would lead to relatively coarser grain size, large strain deformation at lower temperatures is desirable. The present thesis is an attempt to address these issues. The thesis has been divided in to eight chapters. The chapters 1 and 2 are dedicated to introduction and literature review on the subject that provides the foundation and motivation to the present work. Subsequent chapters deal with the research methodology, experimental and simulation results, discussion, summary and conclusion. In the present investigation, two single phase alloys were chosen, the commercially pure magnesium and the magnesium alloy AM30. These materials were subjected to suitable processing techniques, detailed posteriori. A systematic analysis of microstructure and texture for each of the as-processed materials was performed by electron backscattered diffraction (EBSD) using a field emission gun scanning electron microscope (FEG-SEM). Bulk texture measurement by X-ray diffraction, neutron diffraction and local texture measurement by synchrotron X-rays were also carried out. In addition, dislocation density was measured using X-Ray diffraction line profile analysis (XRDLPA). The experimental textures were validated by using Visco-Plastic Self Consistent (VPSC) simulation. The details of experimental as well simulation techniques used in the present investigation is described in chapter 3. To understand the philosophy of large strain deformation by shear in magnesium and its alloy, free end torsion tests could provide a guide line. Based on the understanding developed from these tests, further processing strategy could be planned. Therefore, a rigorous study of deformation behaviour under torsion was carried out. In chapter 4, the results of free end torsion tests carried out at different temperatures, 250⁰C, 200⁰C and 150⁰C and strain rates, 0.01 rad.s-1, 0.1 rad.s-1, 1 rad.s-1 are presented for both the alloys. In addition to the analysis of stress-strain behaviour, a thorough microstructural characterization including texture analyses pertaining to deformation and dynamic recrystallization was performed. Both pure Mg and the AM30 alloy exhibit similar ductility under the same deformation condition, while the strength of AM30 was more. The strain hardening rate decreased with temperature and increased with strain rate for both the materials. However, the strain hardening rate was always higher in case of the alloy AM30. Large amount of dynamic recrystallization (DRX) was observed for both the alloys. The initial texture had an influence on the deformation behaviour under torsion and the resulting final texture. The initial non-axisymmetric texture of pure Mg samples led to nonaxisymmetric deformation producing ear and faces along the axial direction, and the final texture was also non-axisymmetric. An examination of the texture heterogeneity was carried out in one of the pure Mg torsion tested samples by subjecting it to EBSD examination at different locations of the surface along the axial direction. The strain induced on the ear portion was maximum, and in the face was lower. This has been attributed to the orientation of basal planes in the two regions. The axisymmetric initial texture in case of the alloy AM30 led to the formation of axisymmetric texture with no change in the shape of the material. Owing to this simplicity, the occurrence of dynamic recrystallization (DRX) was studied in more detail for this alloy. The mechanism of texture development due to deformation as well as dynamic recrystallization could be tracked at every stage of deformation. A typical shear texture was observed with respect to the strain in each case. Very low fraction of twins was observed for all the cases indicating slip dominated deformation, which was validated by VPSC simulation. It was found that with the increase in strain during torsion, the fraction of dynamically recrystallized grains increased. The recrystallization mechanism was classified as “continuous dynamic recovery and recrystallization” (CDRR) and is characterized by a rotation of the deformed grains by ~30⁰ along c-axis. After developing an understanding of large strain deformation behaviour of pure Mg and the alloy AM30 through torsion tests, the possibility of low temperature severe plastic deformation for both the materials by equal channel angular extrusion (ECAE) was explored. The outcome of this investigation has been presented in chapter 5. At first, ECAE of pure magnesium was conducted at 250⁰C up to 4 passes and then the temperature was reduced by 50⁰C in each subsequent pass. In this way, ECAE could be carried out successfully up to 8th pass with the last pass at room temperature. A grain size ~250 nm and characteristic ECAE texture with the fibres B and C2 were achieved. The AM30 alloy subjected to similar processing schedule as pure Mg, however, could be deformed only up to 6th pass (TECAE=150⁰C) without fracture. An average grain size ~ 420 nm and a texture similar to ECAE processed pure Mg was observed for this alloy. The difference in the deformation behaviour of the two alloys has been explained on the basis of the anisotropy in the stacking fault energy (SFE) in the case of pure Mg. Neutron diffraction was carried out to confirm and validate the microtexture results obtained from the EBSD data, while the local texture measurement by synchrotron radiation was carried out at different locations of the ECAE samples to give a proper account of the heterogeneity in texture. The effect of grain refinement was examined, deconvoluting the effect of shear in improving the strength and ductility using another severe plastic deformation technique, namely multi axial forging (MAF). In this process, the material was plastically deformed by a combination of uniaxial compression and plane strain compression subsequently along all the three axes. The details of this investigation has been presented in chapter 6. By this method, the alloy AM30 could be deformed without fracture up to a minimum temperature of 150⁰C leading to ultra-fine grain size (~400 nm) with very weak texture. A room temperature ductility ~55% was observed for this material. Finally, a comparison of room temperature mechanical properties of the alloy AM30 was carried out for the ECAE and MAF processed conditions having similar grain size in order to observe the effect of texture formed during both the processes. A similar strength and ductility for both the cases was attributed to the orientation obtained from both the ECAE and MAF, which is away from the ideal end orientation for tensile tests. The final outcomes of the thesis has been summarized in chapter 7.

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The AM60 Mg alloy has been used in the automotive industry to help achieve higher fuel efficiency. However, its products, mostly fabricated via high pressure die casting process, are inherently plagued with porosity issues. The presence of porosity impairs mechanical properties, especially fatigue properties, and thus affects the product reliability. We have therefore studied the effect of porosity on the fatigue behavior of samples drawn from a prototype AM60 shock tower by conducting strain-controlled fatigue test along with X-ray computed tomography (XCT). The 3D analysis of porosity by XCT showed discrepancies from 2D metallographic characterization. Fatigue testing results showed the machined surface is the preferential site for crack initiation to occur, on which pores are revealed after specimen extraction. A large scatter in fatigue life was observed as crack initiating at a large pore situated on the surface will result in a significantly shorter fatigue life. SEM fractography showed fracture surfaces are generally flat and full of randomly orientated serration patterns but without fatigue striations. The observations and measurements of porosity and fatigue cracks made by XCT were confirmed by SEM, supporting it as a reliable characterization tool for 3D objects and has value in assisting the failure analysis by SEM. Fatigue life was found to decrease with the increase of fatigue-crack-initiating pore size. The same trend was also found between the fatigue life and the volume fraction of porosity. The pore shape and pore orientation should be taken into account when determining the pore size as they can result in the difference in pore size between 2D and 3D measurement. / Thesis / Master of Applied Science (MASc) / The AM60 Mg alloy has been used in the automotive industry to help achieve higher fuel efficiency. However, its products, mostly fabricated via high pressure die casting process, are inherently plagued with porosity issues. The presence of porosity impairs mechanical properties, especially fatigue properties, and thus affects the product reliability. We have therefore studied the effect of porosity on the fatigue behavior of samples drawn from a prototype AM60 shock tower by conducting strain-controlled fatigue test along with X-ray computed tomography (XCT). The 3D analysis of porosity by XCT showed discrepancies from 2D metallographic characterization. Fatigue testing results showed the machined surface is the preferential site for crack initiation to occur, on which pores are revealed after specimen extraction. A large scatter in fatigue life was observed as crack initiating at a large pore situated on the surface will result in a significantly shorter fatigue life. SEM fractography showed fracture surfaces are generally flat and full of randomly orientated serration patterns but without fatigue striations. The observations and measurements of porosity and fatigue cracks made by XCT were confirmed by SEM, supporting it as a reliable characterization tool for 3D objects and has value in assisting the failure analysis by SEM. Fatigue life was found to decrease with the increase of fatigue-crack-initiating pore size. The same trend was also found between the fatigue life and the volume fraction of porosity. The pore shape and pore orientation should be taken into account when determining the pore size as they can result in the difference in pore size between 2D and 3D measurement.

The aim of this thesis is to determine influence of porosity and shape-factor over mechanical properties, performed on cast samples from Aalen foundry (Germany), that were high pressure die casted from AZ91 D Alloy. Evaluation of these quantities was done using computer software Olympus Five. Measured values were subjected to statistical analysis in order to eliminate gross errors and determine linear dependence between mechanical properties and porosity eventually voids shape factor.

Magnesium alloys are a popular research topic for structural applications because they have a lower density than conventional structural materials, including steel, titanium, and aluminum; however, the reliability and safety of their mechanical properties must be further proven. An important mechanical property for this purpose is fracture toughness, which is the measure of the material's resistance to crack propagation. In this study, a model of an experiment to investigate the fracture toughness of a magnesium alloy WE43 before and after friction stir processing (FSP) is developed, and the results are compared to those produced by a digital image correlation (DIC) system during an experiment from another paper. The model results of the material before FSP matched well with the DIC results, but the model of the material after FSP only partially matches the DIC results. In addition, a theoretical approach to calculating the standard fracture toughness value, KIc, from the modeling results is proposed, and is found to be a conservative approach.

Severe plastic deformation methods are of great interest in industrial forming applications, as they give rise to significant refinement in microstructures and improvements in mechanical and physical properties. In the &ldquo / Equal Channel Angular Extrusion (ECAE)&rdquo / , which is the most common method for production of ultrafine grained bulk samples, very high plastic strains are introduced into the bulk material without any change in cross section. This study is composed of two main parts. Part I focuses on the plastic deformation behavior of Al alloys by modeling ECAE with Msc. Marc finite element software. A series of numerical experiments were carried out for the die angles of 90&deg / , 120&deg / , and 150&deg / , different friction conditions, and different round corners. Besides, the effects of strain hardening characteristics of the material, strain hardening coefficient (K) and exponent (n) of Hollomon&rsquo / s law, on corner gap formation and strain homogeneity in equal channel angular pressing process were investigated quantitatively. The results were compared and verified with those of the upper bound analysis. The numerical results showed that the process performance can be improved by modifying the die corner curvature accordingly, without running time consuming simulations. On the other hand, the aim of Part 3 is to investigate the texture evolution, mechanical response and the corresponding mechanisms, in terms of the flow stress anisotropy and tension-compression asymmetry in the ZK60 Mg alloy. The alloy was processed using ECAE, with different processing routes and temperatures, in order to produce samples with a wider variety of microstructures and crystallographic textures. Several mechanical tests and microstructure examinations were carried out / and the flow stress anisotropy and tension-compression asymmetry of the as-received and processed samples were measured. It was found that the initial texture has a strong effect on the resulting textures / and the textures, combined with the microstructure effect, define the mechanical properties of processed samples. Thus, the tension-compression asymmetry and the flow stress anisotropy variations in the processed samples are attributed to the generated textures and it is possible to control these properties by controlling the processing route and temperature.

The corrosion behaviour of cast Al-Mg alloy A535 and its composites containing 10 wt.% and 15 wt.% fly ash, and 10 wt.% hybrid reinforcement (5 wt.% fly ash + 5 wt.% SiC) was investigated using weight-loss and electrochemical corrosion tests, optical microscopy, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). The tests were conducted in fresh water collected from the South Saskatchewan River and 3.5 wt.% NaCl solution at room temperature. The pH of the salt solution varied from 3 to 9. For comparison, two other aluminum alloys, AA2618 and AA5083-H116, were tested in the same electrolytes. The results of the weight-loss corrosion test showed that unreinforced A535 alloy had a lower corrosion rate in fresh water and seawater environments than the composites at all the tested pH values. The corrosion rate of the composites increased with increasing fly ash content. As expected, the corrosion rates of A535 alloy and the composites tested in fresh water were lower than those in salt solution. The results of the potentiodynamic and cyclic polarization electrochemical tests showed that the corrosion potential (Ecorr) and pitting potential (Epit) of the alloy were more positive than those of the composites. The corrosion and pitting potentials of the composites became more negative (active) with increasing fly ash content. The composites showed more positive (noble) repassivation or protection potential (Erp) than the matrix alloy, with the positivity increasing with fly ash content. Analysis of the electrochemical noise data showed that pitting corrosion was the dominant mode of corrosion for the alloy in 3.5 wt.% NaCl solution. Optical microscopy and SEM revealed that Mg2Si phase and Al-Mg intermetallics corroded preferentially to the matrix. The EDS data indicated that the protective oxide film formed on A535 contained Al2O3 and MgO.