<p> The recrystallization behavior of three Al-Mg-Si-Cu alloys with varying iron and manganese additions was studied by differential power scanning calorimetry under nonisothermal annealing conditions. The influence of cold deformation on the precipitation sequence and its interaction with recrystallization was also investigated. The DSC experiments were complemented by hardness measurements and microstructural studies by optical and electron microscopy. The DSC signals, after optimization of the baseline, were used for the calculation of the kinetic parameters of the recrystallization process. Two different modeling approaches based on global JMAK kinetics were implemented. The first approach utilizes the classical isothermal JMAK expression directly, while the second approach introduces a path variable related to the thermal history of the material in the JMAK description. Model-independent estimates of the activation energy were also evaluated using the Flynn-Wall-Ozawa integral isoconversion method. </p> <p> The results show that the initial stages of recrystallization are not affected by the preceding precipitation processes and recrystallization always follows the precipitation of the Q' phase. However, during recrystallization enhanced coarsening of the Q' phase takes place leading to its transformation to the more stable Q phase. The Q phase exerts a Zener pinning pressures on the migrating boundaries preventing the formation of an equilibrium grain structure. Moreover, for high Fe and Mn additions, discontinuous precipitation of Mg2Si overlaps with the end of recrystallization and exerts an additional pinning pressure on the boundaries. Varying the Fe and Mn content significantly affects the recrystallization kinetics. PSN is promoted in alloys with the higher Fe and Mn content and the recrystallization temperature shifts to lower values. The modeling results show that the recrystallization process conforms to the classical JMAK type behavior. The course of the reaction was reproduced successfully by the path variable approach and the evaluated activation energies were in good agreement with the isoconversional model-independent estimates. However, when the classical JMAK expression was applied directly to non-isothermal measurements, a dependency of the recrystallization process on thermal history was observed. </p> / Thesis / Doctor of Philosophy (PhD)

<p> A microscopic study of two samples of the Gowganda Formation, both having undergone pure shear deformation at greenschist f acies metamorphism has shown: Sample A -Reduction in quartz grain size, and pressure solution shadows were evidence for strong pressure solution activity. Pre-lithification fractures provided channelways for the removal of quartz and water out of the system. Matrix quartz was not recrystallized. Sample B -Extensive local recrystallization of quartz due to pressure solution activity caused metamorphic segregation and the formation of a cleavage. Cleavage behaviour differs in the matrix from that observed in an area of contact strain, as produced by a buckled quartz vein. There is no evidence for removal of quartz from the system. </p> <p> A comparison between the two samples suggests that (a) water, in this case at least, is necessary to produce metamorphic segregation, (b) the system has to be closed to reach metamorphic segregation and (c) original features may be preserved through limited metamorphism if the rock is dried out early in its history. </p> / Thesis / Bachelor of Science (BSc)

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This dissertation presents a numerical model constructed to investigate the dynamics and structures of the Earth’s mantle. Deformation of the Earth’s mantle, which is composed of solid silicate minerals, is strongly governed by the constitutive relation-ship among multiple length-scale structures and properties. To explain the realistic consti-tutive behavior of the silicate mantle, an Internal State Variable (ISV) theory that is an advanced and novel constitutive approach for history-dependent elastoviscoplasticity was applied. The ISV constitutive model was, in turn, implemented into a three-dimensional geodynamic code, TERRA3D, which uses the Finite Element method developed for the mantle convection problem. The sequential studies performed in this dissertation are presented in the follow-ing order: i) a comprehensive summary of the mantle material structures (compositions and microstructural features) and its mechanical properties (elasticity and rheology), ii) a development of a recrystallization and grain size dependent ISV constitutive model for the polycrystalline materials such as minerals and metals, which explains comprehensive mineral physics occurring under the conditions of pressure, temperature, and strain rate within the mantle and their history dependence, and iii) an application of the recrystalli-zation and grain size dependent ISV model to the Earth’s mantle convection problem us-ing the TERRA3D for an investigation of the grain size and dynamic recrystallization efect on the mantle dynamics. The applied ISV constitutive model within the TERRA3D Finite Element frame-work captures the subscale dynamics (dislocation density evolution, dynamic and static recrystallization, grain growth, and grain refinement) and their effect on the large-scale rheology and dynamics of the Earth’s mantle. The numerical investigations reveal that the potential for the mechanical instability and weakening within the mantle arises from the kinetics of grain size and recrystallization and their rheological effect. This mechanical instability leads to the mantle convection entering the episodic overturn regime. The TERRA3D-ISV mantle convection model herein also provides some insightful discover-ies regarding the dynamics and structures within the mantle, explaining its complex rhe-ology caused by the kinetics of recrystallization, grain size, hardening, dislocation recov-ery, and diffusion in the geological settings.

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No description available.

<p>Sheet specimens of alloy AZ31 were cross-rolled to equivalent strains of 0.05, 0.10, 0.30, 0.40, 0.56, and 0.77. The microstructure evolution was examined using a combination of optical metallography (OM), Electron Backscattered Diffraction (EBSD), Transmission Electron Microscopy (TEM) and X-ray diffraction (XRD). The results revealed significant activity of basal and non-basal slip as well as twinning. The twins were mainly of the contraction and double-twin (contraction-extension) types. In addition to the micron scale (1-5μm) twins observed on EBSD patterns, nano-scale twins were observed. The nano twins had a width of less than 0.20μm and existed either as individual/isolated twins or as twin-bundles that are several microns thick. The number of nano twin-bundles increased with increasing strain. Shear bands were also observed to form at high strains and eventually led to the failure of the sheet. As for the texture evolution, analysis of the pole figures shows an evident strengthening of the basal texture during the cross-rolling.</p> <p>Specimens of Mg alloy AZ31 cold-rolled to equivalent strains of 0.10 and 0.30 were selected and annealed at 250^oC. The progress of recrystallization was followed using OM, EBSD and TEM with special emphasis on the nucleation of recrystallization. The distribution of recrystallization nuclei was very heterogeneous due to the heterogeneity of the as-deformed microstructure. Twin/grain-boundary and twin/twin intersections as well as twin interiors were the dominant recrystallization nucleation sites. Significant recovery was observed in the non-recrystallized regions and this limited the growth of the recrystallized grains.</p> / Master of Applied Science (MASc)

The Thin Slab Cast Direct Rolling (TSCDR) process offers several economic and environmental advantages. The elimination of slab reheating and roughing deformation, however, leave fewer opportunities for grain refinement and some large grains persist in the microstructure. To solve this problem, a new chemistry which leads to a two-phase mixture of ferrite and austenite over a wide temperature range was introduced by Zhou et al. The two phase mixture is highly resistant to grain coarsening leading to a small initial grain size compared with the grain size of conventional TSCDR slab. In addition, ferrite and austenite co-exist over wide range of temperature in many third generation steels, making it extremely important to understand the hot deformation behavior of these materials, which have traditionally received less attention in the literature. In order to investigate the microstructure evolution of ferrite-austenite mixtures during thermomechanical processing, an Al containing model alloy, for which the two phases co-exist over a wide temperature range, was designed. Two types of experiments were carried out: the first involved single hit hot compression tests; and the second involved stress relaxation tests. According to the microstructure observation the main change of austenite microstructure under deformation conditions was a decrease in the spacing of the austenite particles within the ferrite matrix. In other words the austenite phase behaved as hard particles inside a soft ferrite matrix. Hot deformation led to the static recrystallization of the ferrite matrix. The most favourable nucleation sites were in the vicinity of the old grain boundaries and the around austenite particles. The recovery and recrystallization kinetics of ferrite were analyzed using the stress relaxation test. Based on analysis of the stress relaxation tests, more than 95% of stored energy was consumed by recovery, while static recrystallization consumed less than 5% of the stored energy. The retardation of recrystallization in the model alloy is attributed to both the high rate of recovery in BCC materials and texture effects. / Thesis / Master of Applied Science (MASc) / The Thin Slab Cast Direct Rolling (TSCDR) process offers several economic and environmental advantages. The elimination of slab reheating and roughing deformation, however, leave fewer opportunities for grain refinement and some large grains persist in the microstructure. To solve this problem, a new chemistry which leads to a two-phase mixture of ferrite and austenite over a wide temperature range was introduced by Zhou et al. The two phase mixture is highly resistant to grain coarsening leading to a small initial grain size compared with the grain size of conventional TSCDR slab. In addition, ferrite and austenite co-exist over wide range of temperature in many third generation steels, making it extremely important to understand the hot deformation behavior of these materials, which have traditionally received less attention in the literature. In order to investigate the microstructure evolution of ferrite-austenite mixtures during thermomechanical processing, an Al containing model alloy, for which the two phases co-exist over a wide temperature range, was designed. Two types of experiments were carried out: the first involved single hit hot compression tests; and the second involved stress relaxation tests. According to the microstructure observation the main change of austenite microstructure under deformation conditions was a decrease in the spacing of the austenite particles within the ferrite matrix. In other words the austenite phase behaved as hard particles inside a soft ferrite matrix. Hot deformation led to the static recrystallization of the ferrite matrix. The most favourable nucleation sites were in the vicinity of the old grain boundaries and the around austenite particles. The recovery and recrystallization kinetics of ferrite were analyzed using the stress relaxation test. Based on analysis of the stress relaxation tests, more than 95% of stored energy was consumed by recovery, while static recrystallization consumed less than 5% of the stored energy. The retardation of recrystallization in the model alloy is attributed to both the high rate of recovery in BCC materials and texture effects.

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No description available.