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Understanding Texture Weakening in Magnesium Rare Earth Alloys

Griffiths, David Glyndwr John

[Thesis]. Manchester, UK: The University of Manchester; 2015.

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Abstract

Magnesium has the lowest density of any structural metal making it a strong candidate for weight savings in the aerospace and automotive industries. However, strong crystallographic textures combine with anisotropic deformation modes to severely limit formability in wrought magnesium alloys. Recently improved formability has been achieved by the addition of small concentrations of solute rare earth elements which reduce the intensity of recrystallisation textures. Developing a mechanistic understanding of this effect is critical in leading alloy design towards a new class of highly formable wrought magnesium alloys.In this study the static recrystallisation mechanism of rolled magnesium rare earth alloys, which causes the texture weakening, is examined with a particular emphasis on the contrasting texture weakening effects in binary and tertiary magnesium rare earth alloys. In binary magnesium-rare earth alloys the `rare-earth' texture is simply a weakened deformation texture, while recrystallisation of magnesium-zinc-rare earth alloys produces unique `rare-earth' texture components. In the binary alloys weakened recrystallisation textures are attributed to the generation of `off-basal' orientations within regions of high strain localisation during deformation. These orientations recrystallise and subsequently dominate the recrystallised texture. Texture weakening by this mechanism is also thought to be observed in non-rare earth magnesium alloys where dynamic recrystallisation is suppressed by cold rolling. The unique rare-earth texture components in magnesium-zinc-rare earth alloys are found to be determined by the orientation of shear bands in the material. Similarly to texture weakening in the binary alloys, nuclei for these orientations are thought to develop during deformation as a result of strain incompatibilities within shear bands. The mechanism forming these orientations remains unclear, however it is postulated that a complex change in recovery behaviour within shear bands, as a result of rare earth and zinc additions, may be the cause.Retarded dynamic recrystallisation is suggested to be of critical importance in the texture weakening mechanisms of all magnesium alloys, both rare earth and non-rare earth. In rare earth alloys dynamic recrystallisation is suppressed by the segregation of rare earth atoms to grain boundaries. A combination of high resolution TEM and EDX shows rare earth atoms form clusters approximately 2nm in diameter on grain boundaries which are expected to retard dynamic recrystallisation through a solute drag mechanism.