Creep is a time-dependent deformation of materials under stress at elevated temperatures. The phenomenon of creep allows materials to plastically deform gradually over time, even at stress levels below its yield point or below its transformation temperature. The issues involving creep are especially significant for magnesium alloys, since they are susceptible to creep deformation from temperatures as low as 100 ºC, which inhibits their potential application in areas such as automotive engines. The University of Canterbury has developed a significant level of experience and infrastructure in the field of Electron Backscatter Diffraction (EBSD). EBSD allows microstructures to be characterized by imaging the crystal structure and its crystallographic orientation at a given point on a specimen surface, whereby the process can be automated to construct a crystallographic “orientation map” of a specimen surface. In light of this, the creep of magnesium and its alloys was studied using a novel technique, in which a conventional tensile creep test was interrupted at periodic intervals, and the EBSD was used to acquire the crystallographic orientation maps repeatedly on a same surface location at each interruption stages. This technique allows simultaneous measurement of the rate of creep deformation and the evolution of the specimen microstructure at various stages of creep, bringing further insight into the deformation mechanisms involved. This thesis summarizes the study of the microstructural and crystallographic texture evolution during creep of pure magnesium and a creep resistant magnesium alloy Mg- 8.5Al-1Ca-0.3Sr. Pure magnesium exhibit a conventional “power-law” type creep, and although its creep properties are well established in the past literatures, there has been little in terms of reconciliation between the observed creep rates and the underlying deformation mechanisms. The alloy Mg-8.5Al-1Ca-0.3Sr, on the other hand, is a modern die casting alloy used in the automotive industry for engine and gearbox applications, and despite its superior creep resistance, little is known about the microstructural contributions to its creep properties. This research was conducted to provide a link between the creep properties, observed microstructures, and theories of creep deformation by the use of advanced microscopy techniques. For the first time, the detailed, sequential microstructural development of magnesium and its alloys during creep has been revealed.

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Numerical modeling of failure was performed for magnesium alloys with circular and square cross-sections under axial compression. The failure criterion was employed using material model 124, where failure was simulated using the element deletion method. LS-DYNA material model 124 (MAT_124) was calibrated using stress-strain curves in compression and tension. This approach, combined with MAT_124, captures the material asymmetry. Comparisons with experiments showed that the failure criterion accurately predicted the stress-strain behavior during axial compression tests of the round tubes of magnesium alloy, AZ31. A parametric study was also performed to investigate the effects of various phenomena on simulated results. Numerical modeling of square magnesium tubes during bending was also simulated for extruded magnesium alloys AZ31, AM30 and AM60. The failure criterion, based on element erosion, was used in these models to simulate fracture for all three alloys. Comparisons with experiments, for all three alloys, showed that the proposed numerical model accurately predicted the force-displacement curves during bending. Engineering strain at failure was found from the tensile test curves for the three magnesium alloys (AZ31, AM30 and AM60). Simulations were done to predict local strain at the necking region at this engineering strain. The necking strain was incorporated in the failure criterion, which considerably improved results for the bending simulations. Numerical modeling of slow and fast axial compression tests were also performed for AM30, AM60 and AZ31 magnesium tubes with square cross-section. Comparisons with experiments, for all three alloys, showed that the proposed numerical model accurately predicted the force-displacement curves during quasi-static and high-speed crush tests. Furthermore, the predicted fracture locations and patterns were in good agreement with experimental observations. Finally, new failure criteria was employed to improve the crashworthiness behavior of magnesium alloys by several tube design variations. Magnesium tubes cladded with aluminum and magnesium tubes with alternating strips of aluminum were simulated. Magnesium tubes with thinned sections and spirals were also simulated. Results showed that most of the design modifications increased the crashworthiness of magnesium alloys tubes.

Thesis (PhDAppliedScience)--University of South Australia, 2003.

Typescript (photocopy) / x, 152 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.) Dept. of Physical and Inorganic Chemistry, University of Adelaide, 1981

Thesis (PhDAppliedScience)--University of South Australia, 2003.

There has been much confusion evident in the literature in terms of the influence of impurity elements on grain refinement of magnesium alloys. This thesis addresses how impurity elements such as iron, manganese, carbon and beryllium affect grain refinement in magnesium alloys. The thesis starts with an investigation into the effect of the uptake of iron on grain refinement of Mg-Zr alloys. The highly detrimental influence of the uptake of iron on grain refinement in Mg-Zr alloys has been confirmed. The gradual loss of grain refinement of Mg-Zr alloys partly arises from the consumption of Zr by the formation of Fe2Zr via the reaction between soluble Zr and Fe picked up from mild steel crucibles. (Settling of undissolved Zr particles also partly attributes to the gradual loss of grain refinement.) The morphological evolution of Zr-rich cores from circular to rosette-like has been reported here for the first time. In contrast to the detrimental effect in Mg-Zr alloys, a positive effect of iron has been observed in grain refinement of Mg-Al based alloys. The addition of iron in the form of anhydrous FeCl3 produces significant grain refinement of high-purity Mg-Al alloys. Obvious grain refinement was also achieved through the uptake of iron from steel crucible surfaces; however, the addition of Fe powder in the form of an ALTABTM Fe75 powder compact (75%Fe, 15%Al and 10% Na-free flux) did not give rise to grain refinement. The results obtained from both the grain refinement tests conducted in aluminium titanite crucibles and an ultra-low carbon 316L stainless crucible indicate that the grain refinement of Mg-Al alloys by iron inoculation has little to do with the Al4C3 hypothesis. The nucleant particles have been clarified to be Fe- and Al-rich intermetallics. The effect of manganese on the grain refinement of high purity Mg-Al based alloys and commercially available AZ31 alloys has been investigated using an Al-60%Mn master alloy splatter at 730 „aC in aluminium titanite crucibles. Grain refinement was readily achievable in these alloys. Electron microprobe analyses revealed that prior to the addition of extra manganese the majority of the intermetallic particles found in AZ31 are of the Al8Mn5 type. However, after the addition of extra manganese in the range of 0.1% to 1.0%, the predominant group of intermetallic particles changed to the metastable AlMn type. This leads to a hypothesis that the metastable AlMn intermetallic particles are more effective than Al8Mn5 as nucleation sites for magnesium grains. The hypothesis was supported by the observation that a long period of holding at 730 „aC led to an increase in grain size, due probably to the transformation of the metastable AlMn to the stable Al8Mn5. Native grain refinement in magnesium alloys has been clarified. Based on the fact that native grain refinement is an exclusive feature of high purity Mg-Al alloys, it is hypothesized that Al4C3 particles act as nucleation centres. This is also the mechanism of carbon grain refinement of Mg-Al alloys. A trace of beryllium leads to dramatic grain coarsening in Mg-Al alloys at normal cooling rates. Apart from Mg-Al alloys, a trace of beryllium also causes considerable grain coarsening in Mg-Zn, Mg-Ca, Mg-Ce, Mg-Nd and also hinders grain refinement of magnesium alloys by Zr. Modelling grain refinement to predict the final grain size has been made on the basis of understanding of existing models. The modified model has resolved a fundamental gap in the relative grain size model using a more universal expression of solute concentration in the liquid.

Diss. (sammanfattning) Linköping : Univ., 2005. Diss. (sammanfattning) Jönköping : Högsk., 2005. / Härtill 8 uppsatser.

Thesis (M. Eng. Sc.)--University of Adelaide, Dept. of Chemical Engineering,1991. / Includes bibliographical references (leaf 95).