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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Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / TM in title is superscripted on title page. Typescript. Vita. Includes bibliographical references (leaves 100-103). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1997. / TM in title is superscripted on title page. Typescript. Vita. Includes bibliographical references (leaves 100-103). Also available on the Internet.

Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, June 2004. / Thesis Advisor(s): Terry McNelley. Includes bibliographical references (p. 41-43). Also available online.

This project explores the fundamental mechanisms of grain boundary sliding (GBS) with an emphasis on its role in superplasticity, using both micro- and macro-mechanical testing methods. GBS plays an important role in the deformation of polycrystalline materials, especially at high homologous temperatures (above half of the melting point). Classical models for GBS (Rachinger sliding and Lifshitz sliding) assume that all grains and grain boundaries undergo the same process, but recent research has shown this is not true. Individual grain boundaries differ in their ability to participate in sliding and diffusion. Therefore, it is important to investigate the response of individual grain boundaries to stress. This project uses microcantilevers, loaded using a nanoindenter, to investigate the response to stress of individual grain boundaries in Sn-1%Bi, which is expected to exhibit GBS at room temperature. The response of individual grain boundaries are correlated with grain boundary characters determined using electron backscattered diffraction (EBSD). On the macroscopic scale, both in-situ and ex-situ shear tests are conducted to investigate the superplastic behaviour of this material. The strain rate sensitivity index of the material with a grain size of 8.5 μm is found to be around 0.45. Surface marker lines have quantitatively revealed grain boundary sliding. The investigation from surface studies is expanded to the interior of bulk material in 3D by conducting an in-situ tensile test coupled with diffraction contrast tomography (DCT) at a synchrotron facility. The microcantilever tests enable grain boundary sliding and diffusion creep to be investigated separately by varying the normal and shear stresses on the grain boundary plane. GBS is dependent on grain boundary structure (misorientation angle, rotation axis and grain boundary plane orientation). The microcantilever size is similar to the grain size used in the macro-mechanical tests. It is demonstrated that the shear stress for steady-state GBS is comparable in micro- and macro-tests. Grain neighbour switching events have been identified in the interior of bulk material in 3D for the first time.

Magnesium is the lightest structural metal and magnesium alloys are therefore obvious candidates in weight critical applications. The environmental imperative to reduce vehicle emissions has recently led to intensified research interest in magnesium, since weight reduction is one of the most effective ways of improving fuel efficiency. The hexagonal close-packed structure of magnesium results in poor room temperature formability. However, on heating, several magnesium alloys show superplastic properties, with the ability to deform to very high strains (up to 3000%). This opens up the possibility of forming complex components directly by superplastic forming (SPF). As a result, SPF of magnesium is a highly active research topic. The most widely used class of magnesium alloys contain aluminium as the major alloying addition, which has a relatively high solubility in magnesium, and manganese, which has a less solubility. The effect of these elements on the deformation behaviour and failure mechanisms operating in the superplastic regime is not yet well understood. The objective of this work was to gain fundamental insights into the role of these elements. To do this, alloys with different aluminium content (AZ31 and AZ61) and manganese levels have been studied in-depth.After casting, all alloys were subject to a hot rolling procedure that produced a similar fine grain size and texture in each material. Hot uniaxial testing was performed at temperatures between 300 to 450 degC and at two strain rates to investigate the material flow behaviour, elongation to failure and failure mechanism. All of the alloys exhibited flow curves characterised by an initial hardening and extensive flow softening region. Dynamic recrystallization did not occur, and the flow softening was attributed to grain growth and cavity formation. Increasing the level of aluminium in solution was observed to increase the grain growth rate, and also reduce the strain rate sensitivity. The elongation to failure, however, depended strongly on the manganese level but not on the aluminium content. This attributed to the role of manganese in forming coarse particles that act as sites for cavitation.To study cavity formation and growth, and its effect on failure, a series of tests were conducted to different strain levels followed by investigation of cavitation in 3-dimensions using X-ray tomography. New methods were developed to quantify the correlation between cavities and coarse particles using X-ray tomography data and it was shown that over 90% of cavities are associated with particles. Cavity nucleation occurred continuously during straining, with progressively smaller particles forming cavities as strain increased. The mechanism of cavity formation and growth was identified, and it has been demonstrated that particle agglomerates are effective sites for cavity formation even when the individual particles in the agglomerates are below the critical size predicted by theory for cavity nucleation sites. These results suggest that to improve the ductility of magnesium alloys in the superplasticity regime, it is most critical to minimise the occurrence of particle agglomerates in the microstructure.

Superplastic behavior of two ultrafine-grained (UFG) magnesium alloys was investigated in this thesis. Commercial Mg-4Y-3RE (wt.%) alloy was prepared by equal channel angular pressing (ECAP) and new experimental alloy Mg-23Zn-2Y (wt.%) was prepared by extrusion and ECAP. Eight passes of Mg-4Y-3RE through ECAP resulted in grain refinement down to ~340 nm and formation of a high volume fraction of fine secondary phase particles. UFG microstructure with an average grain size of 3.2 µm after extrusion and 1.6 µm after Ex-ECAP was achieved in Mg-23Zn-2Y alloy. The microstructure of Mg-23Zn- 2Y was observed by scanning and transmission electron microscopy. The thermal stability of both alloys was measured by microhardness tests. Superplastic behavior was investigated in the temperature range of 250-450 řC and strain rate range of 5x10-4 s-1 - 10-1 s-1 . The results revealed a high strain rate superplasticity in Mg-4Y-3RE alloy. Deformation to fracture exceeded 1000% for several deformation conditions, even at the strain rate of 10-1 s-1 . The highest elongation of 656 % in Mg-23Zn-2Y alloy was achieved in extruded state at the strain rate of 10-3 s-1 .