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In the present investigation, MMCs have been fabricated using the creep-resistant AE42 magnesium alloy as matrix and reinforcing it with saffil short fibres (essentially δ-Al2O2) and SiC particles in various combinations. These MMCs have been investigated for their creep, wear and corrosion behaviour. The above properties of the matrix AE42 alloy have also been investigated for comparison. Further, laser surface melting has been carried out on a creep-resistant MRI 230D Mg alloy and the corrosion and wear behaviour of this alloy before and after laser surface melting has been investigated. The creep tests on the AE42 alloy were carried out in the temperature range of 1500 to 2400C at the stress levels ranging from 40 to 120 MPa and the composites were tested in the temperature range of 1750C to 3000 at the stress levels ranging from 60 to 140 MPa both in the longitudinal direction (LD) and in the transverse direction (TD). Wear tests were conducted on a pin-on-disc set-up under dry sliding condition at a constant sliding velocity of 0.837 m/s for a constant sliding distance of 2.5 km in the load range of 10 to 40 N for the AE42 alloy and the composites, which were tested both in LD and TD, and for a constant sliding distance of 1km in the load range of 5 to 20 N for the MRI 230D alloy before and after laser melting. All the materials were subjected to electrochemical corrosion tests in a 5 wt.% NaCl solution having ph value 11 for 22 hours. All the composites in both LD and TD exhibit lower creep rate as compared to the AE42 alloy and it is higher in TD than in LD. The creep resistance of the hybrid composites, in which saffil short fibres are partially replaced by SiC particles, is observed to be comparable , i.e., of the same order of magnitude , to that of the composite reinforced with Saffil short fibres alone at all the temperatures and stresses employed in both LD and TD. Wear rate of all the composites in both LD and Td is found to be lower than the alloy at all the loads employed and it is higher in TD than LD, Wear rate progressively decreases with the partial replacement of Saffil short fibres by Sic Particles, and is lowest for the composites reinforced with 10 vol.% Saffil short fibres and 15 vol.% Sic particles in both LD and TD. It is 34% and 35% lower than the 20% Saffil composite at 40 N load in LD and TD, respectively. The Ae42 alloy exhibits the best corrosion resistance and the addition of the Saffil short fibres and/or Sic particles in the AE42 alloy deteriorates its corrosion resitance. The composite reinforced with Saffil short fibres alone exhibits slightly better corrosion resitance than the hybrid composites. However, there is no systematic trend of corrosion resistance with SiC particles content. The laser surface melting is found to improve the corrosion, hardness and wear resistance of the MRI 230D alloy. High temperature climb of dislocation is found to be the dominant creep mechanism in the AE42 alloy in the stress and temperature range employed. Various glide and climb of dislocation are found to be the dominant creep mechanisms for all the composites in both LD and TD in the stress and temperature range employed. The presence of SiC particles in the hybrid composites improves the wear resistance in both LD and TD since these particles remain intact and retain their load bearing capacity even at the highest load employed in the present investigation. They promote the formation of iron-rich transfer layer and they also delay the fracture of Saffil short fibres to higher loads in case of the composites in LD. Under the experimental conditions used in the present investigation, the dominant wear mechanism is found to be abrasion for the AE42 alloy and its composites in both LD and TD. It is accompanied by severe plastic deformation of surface layers in case of the alloy, the fracture of Saffil short fibres as well as the formation of iron-rich transfer layer in case of the composites in Ld, and the fracture and pull-out of the Saffil short fibres in case of the composites in TD. The lower corrosion resistance of all the composites is not caused by the galvanic coupling between reinforcements and matrix, and is related to the microstructural changes, such as, distribution of precipitates and the nature of the film formed at the surface. The improved corrosion resistance following laser surface melting is due to the absence of the Al2Ca phase at the grain boundary, microstructural refinement and increased solid solubility, particularly of Al, owing to rapid solidification; the improved hardness and wear resistance is due to grain refinement and solid solution strengthening. To conclude, the creep resistance of the hybrid composites is comparable, wear resistance is better and corrosion resistance is slightly inferior to the composite reinforced with Saffil short fibres alone. Therefore, from the commercial point of view, the use of the hybrid composites, replacing a part of the expensive Saffil short fibres by cheap SiC particles, is beneficial. The laser surface melting is beneficial for the corrosion and wear resistance of the MRI 230D alloy.

Laser surface modification of AZ31B Magnesium alloy changes surface composition and roughness to provide improved surface bio-wettability. Laser processing resulted in phase transformation and grain refinement due to rapid quenching effect. Furthermore, instantaneous heating and vaporization resulted in removal of material, leading the textured surface generation. A study was conducted on a continuum-wave diode-pumped ytterbium laser to create multiple tracks for determining the resulting bio-wettability. Five different laser input powers were processed on Mg alloy, and then examined by XRD, SEM, optical profilometer, and contact angle measurement. A finite element based heat transfer model was developed using COMSOL multi-physics package to predict the temperature evolution during laser processing. The thermal histories predicted by the model are used to evaluate the cooling rates and solidification rate and the associated changes in the microstructure. The surface energy of laser surface modification samples can be calculated by measuring the contact angle with 3 different standard liquid (D.I water, Formamide, and 1-Bromonaphthalen). The bio-wettability of the laser surface modification samples can be conducted by simulated body fluid contact angle measurement. The results of SEM, 3D morphology, XRD, and contact angle measurement show that the grain size and roughness play role for wetting behavior of laser processing Mg samples. Surface with low roughness and large grain size performs as hydrophilicity. On the contrast, surface with high roughness and small grain size performs as hydrophobicity.

Al-Mg solid solution alloys of various grain sizes were prepared by friction stir processing (FSP). The mechanical properties and micro-structure evolution were studied. The results show that the mechanical properties including tensile strength and ductility are improved by increasing Mg weight fraction. The homogeneous deformation is enhanced by fined slip bands within the grains. On the other hand, Dynamic strain aging or serrated flow stress has been wildly investigated in Al-Mg alloys. Effects of strain rate and magnesium content on dynamic strain aging are also discussed.

The microstructures and mechanical properties of the AZ31 Mg tubes fabricated by one-pass forward piercing tube extrusion operated at 250-400oC and 10-2-100 s-1 are examined. The grain size is refined from the initial ~75

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<div>Magnesium alloys are regarded as the most promising structure materials in transportation and aerospace fields because of their low densities and high specific strengths. However, the unsatisfactory mechanical performance and corrosion resistance restrict their applications. Grain refinement is an effective way to improve the mechanical properties and widen the applications. Among which, ultrasonic shot peening shows a great potential in producing refined grains or even nanocrystalline. A nanocrystalline forms at the surface after ultrasonic shot peening treatment. The formed nanocrystalline has been proved to dramatically affect the mechanical properties, such as hardness, mechanical stress, wear resistance and fatigue life. </div><div><br></div><div> </div><div>In this dissertation, the microstructure evolution of AZ31 Mg alloy after the ultrasonic shot peening as well as its effect on the mechanical properties are investigated. The grain size, the twin structure, the surface roughness and the residual stress distribution after ultrasonic shot peening are characterized. A gradient nanostructure is achieved through ultrasonic shot peening and the thickness of this gradient nanostructure increases with prolonging the treated time. The grains at the top surface after 5 min treatment is refined to 45 nm and further refined to 42 nm for 10 min and 37nm for 15 min treatment from the XRD result. A lamellar nanocrystaline is below the top surface and a lot of tensile twins are found at the heavily deformed grains below the nanocrystalline layer. Below the twinned layer, a residual stress is distributed as deep as 400 $\mu$m in the matrix. A rough surface is obtained and the surface roughness of the 5 min treated sample was 5.934 $\mu$m, increased to 6.161 $\mu$m for10 min and 6.236 $\mu$m for 15 min. The nanocrystalline leads to the improvement of the microhardness, from 65 HV of the as-received to 123 HV, 127 HV and 145 HV for 5 min, 10 min and 15 min treatment, respectively. The tensile stress and compression stress are also improved remarkably. The yield stress is increased from 127.7 MPa of as-received to 198 MPa of 10 min treated sample and the compression stress is improved from 73 MPa to 100 MPa. The improved yield stress is attributed to the grain refinement and the work hardening of the nanocrystalline. </div><div><br></div><div> </div><div> </div><div>The wear resistance of AZ31 Mg alloy is improved greatly after ultrasonic shot peening process. The coefficient of friction and the wear rate of the ultrasonic shot peening treated sample are both lower than that of as-received. The width of the wear track of ultrasonic shot peening treated is also narrower than that of as-received, and the worn surface has a lower surface roughness. In as-received samples, abrasion and oxidation dominate the wear mechanism at low sliding speed and low applied load. The increase of sliding speed or applied load resulted in the delamination. Severe wear such as thermal softening happens with the further increase of load value or sliding speed. In ultrasonic shot peening treated samples, oxidation, the abrasion and delamination are also existing while no severe wear is found. The improved wear resistance of the ultrasonic shot peened sample is due to the improved hardness and a higher activity of oxidation during wear process. The nanocrystalline on the top surface leads to the the transition boundary between the mild wear and severe wear to a higher sliding speed and higher applied load. </div><div><br></div><div><br></div><div>The corrosion resistance of AZ31 Mg alloy before and after ultrasonic shot peening is tested in 3.5 $\%$ NaCl solution. The corrosion resistance after ultrasonic shot peening is reduced greatly because of the Fe particles at the top surface, which was exfoliated from the shot during the treating process. After a 40 $\mu$m thick polishing, Fe particles are removed totally and the corrosion resistance is improved, compared with that of as-received. The anodic current density of the nanocrystallized surface after polishing is reduced because of the compression residual stress and a rapid formation of protective layer. Meanwhile, the grain boundary acts as a physical barrier for corrosion and reduces the corrosion rate. </div><div><br></div><div><br></div><div>The bending behavior of AZ31 Mg alloy before and after ultrasonic shot peening is studied by a V-bending test. The ultrasonic shot peening treated sample has a similar bending performance even though the ductility has been reduced after ultrasonic shot peening. A single side ultrasonic shot peening (either at the inner side or the outer side) changed the bending behavior because of its asymmetric structure. The ultrasonic shot peening at the inner side for 5 min improves the bendability and longer treated, such as 10 min and 15 min degenerates the bendability to as-received. The improved bendability of 5 min treated sample is due to the drawing back of the neutral layer. The ultrasonic shot peening at the outer side for 5 min also improves the bending performance and a longer treatment of 15 min further enhanced the bendability. The improved bendability after outer side treatment is due to the high yield stress of nanocrystalline at the convex, resulting in the smaller strain here. </div>

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. 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. 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.

Ultrafine-grained(UFG) AZ31 Mg alloy was obtained by equal-channel angular extrusion(ECAE) and subsequent annealing at elevated temperatures. The basal texture component for ECAEed material is located on the Z plane of the ECAEed billets. Tensile tests were performed at temperatures between room temperature and 125¢J, and strain rates used ranging from 3*10-5 to 6*10-2 s-1. The experimental results showed that a high tensile yield stress of 394 MPa was obtained at room temperature under a strain rate of 3*10-3 s-1. Strengths of UFG AZ31 specimens were greatly improved due to grain refinement. It was found that strain rate sensitivity of UFG AZ31 alloy increased significantly from 0.024 to 0.321 with increasing temperature. The constant k of Hall-Petch equation, £m=£m0 +kd-1/2, decreased with increasing temperature, and decreasing strain rate. Negative k values were ontained at 75¢J and 100¢J under a strain rate 3*10-5 s-1. When compressed along X, Y and X45Z billet orientations, strain localization within shear bands was found in UFG AZ31 specimens. Shear bands are formed inclined near 45 to the compression axis. The smaller the grain size, the thinner the shear band. Different Hall-Petch constant k were found in specimens deformed along different orientations, which is caused by different deformation mechanisms. The formation of tension twins is the primary deformation mechanism for compressed X and Y samples, and basal slip is responsible for the deformation of X45Z sample. tension twins were found in 0.46 £gm grain size specimens.

Magnesium alloys have attracted considerable attention owing to its low density of ~1.7 g/cm3. On the other hand, the carbon fiber (CF) reinforced polyether ether ketone (PEEK) polymer composites possess extraordinary specific strength and stiffness along the longitudinal (or fiber) direction. It follows that the combination of Mg/CF/PEEK would offer an alternative in forming a high specific strength and stiffness composite. In the first part of this study, the low density and high performance Mg-based laminated composites were fabricated by means of sandwiching the AZ31 Mg foils with the carbon-fiber/PEEK prepreg through hot pressing. Proper surface treatments of AZ31 sheet using CrO3 base etchants are necessary in order to achieve good interface bonding characteristics. The resulting Mg base laminated composite, with a low density of 1.7 g/cm3, exhibits high modulus of 75 GPa and tensile strength of 932 MPa along the longitudinal direction. The experimentally measured tensile modulus and strength data along both the longitudinal and transverse direction are within 90-100% of the theoretical predictions by rule of mixtures, suggesting that the bonding between layers and the load transfer efficiency are satisfactory. The flexural stress and modulus along the longitudinal direction are 960 MPa and 54.6 GPa, respectively, suggesting a sufficiently high resistance against bending deflection. The peel strengths are about 2.75 and 4.85 N/mm along the longitudinal and transverse directions, respectively, superior to that of the epoxy-resin-adhered and carbon-fiber-reinforced aluminum laminated composites. Polymer nanocomposites have attracted considerable attention during the past decade due to their versatile and extra-ordinary performances. The polymer nanocomposites can be prepared by the well-known sol-gel method. It is well known that PEEK is a good solvent resistant polymer. Hence, it is impossible to fabricate the PEEK nanocomposite by means of sol-gel method. In the second part of this study, the PEEK nanocomposites filled with nano-sized silica or alumina measuring 15-30 nm to 2.5-10 weight percent were fabricated by vacuum hot press molding at 400oC. The resulting nanocomposites with 5-7.5 wt% SiO2 or Al2O3 nanoparticles exhibit the optimum improvement of hardness, elastic modulus, and tensile strength by 20-50%, with the sacrifice of tensile ductility. With no surface modification for the inorganic nanoparticles, the spatial distribution of the nanopartilces appears to be reasonably uniform. There seems no apparent chemical reaction or new phase formation between the nanoparticle and matrix interface. The crystallinity degree and thermal stability of the PEEK resin with the addition of nanopartilces were examined by X-ray diffraction, differential scanning calorimetry, and thermogravity analyzer, and it is found that a slight decrease in crystallinity fraction and a higher degradation temperature would result in as compared with the prestine PEEK.