EXPERIMENTAL CHARACTERIZATION OF DEFORMATION MECHANISMS IN MAGNESIUM AND THE EFFECT OF ALLOYING ON DUCTILITY

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Date
2020-03-25
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Johns Hopkins University
Abstract
Magnesium (Mg) is a promising structural material with low density and high specific strength. The use of wrought Mg products has been stunted by poor formability, which is related to underlying anisotropic deformation mechanisms. The number of independent easy glide basal slip systems does not satisfy the Von Mises criterion, and activation of <c + a> slip and twining holds the key to creating formable Mg. The current study provides experimental observations of the deformation mechanisms activated in commercial and developmental Mg alloys. <c + a> dislocations in pure Mg were observed to non-conservatively dissociate onto the basal plane, resulting in a sessile configuration. By contrast, the dislocations in AZ31 do not dissociate, and the dissociation in Mg-3Er is also much smaller than for pure Mg. HREM images revealed that the suppressed dissociation may be related to the spreading of partial dislocation cores on pyramidal planes. The more compact <c + a> dislocations maintain their mobility and lead to improved ductility. Extension twinning is another significant deformation mechanism that affects the ductility of Mg. Observations made in this study indicate that the strain concentration at the tip of a twin can be released by the formation of dislocations. The high density of dislocations observed in and around twin tips indicate that twin-dislocation and twin-GB interactions play an important role in twin propagation. For Mg-0.1Ca, texture weakening was observed to be a critical mechanism that improves ductility. Ca was observed to segregate at high-angle grain boundaries, which retards GB mobility and activates the nucleation of static recrystallization in deformed grains. The GB energy of the decorated boundaries are less dependent on misorientation, which enables the growth of randomly orientated grains and weakens the texture. In total, investigations of the deformation mechanisms in hot-rolled polycrystalline pure Mg, AZ31, Mg-3Er and Mg-0.1Ca presented in this thesis have elucidated the mechanisms that govern ductility. Control of <c + a> dislocation cores, deformation twinning, and texture reduction highlight promising paths for the design of ductile Mg alloys.
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Magnesium, <c+a> dislocations, extension twinning, texture weakening
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