Deformation twinning in magnesium using nano-indentation

Su, Tina Yu-Ting (2014). Deformation twinning in magnesium using nano-indentation. University of Birmingham. M.Phil.

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Abstract

This study describes an investigation into nano-indentation induced deformation twinning in pure magnesium.
The working objectives of this study were to clarify: 1. Whether deformation twinning is a statistical certainty 2. The positive effects of high stress on deformation twinning 3. Whether deformation twins in magnesium are strain-rate sensitive 4. The role of grain and pre-existing twin boundaries on deformation twinning 5. The efficacy of the Schmid criterion (CRSS) in predicting deformation twinning.
Deformation twins induced by nano-indentation using a Berkovich tip on magnesium grains were studied using a combination of optical microscopy, SEM and electron back scattering diffraction (EBSD).
All nano-indents made on the magnesium grains in this study induced twinning, mainly of the {101 2} tension twin type. The twins induced were highly reproducible in terms of twin type, size, shape, amount and orientation with respect to the indent.
Consistent with findings from other studies, deformation twins were found to react positively to increasing contact pressure. The evidence shows that twin nucleation and growth are both higher for indents made with higher stress. Moreover, deformation twins appear to be insensitive to strain-rate. Whilst pre-existing twin boundaries play no apparent role in promoting further twinning, grain boundaries twins were found to promote twinning in neighbouring grains. Finally, the twin variants formed by indenting in different crystal orientation suggested a break-down of the Schmid criterion, hence invalidates of the CRSS evaluation for deformation twinning in magnesium. These results elucidate existing knowledge gaps in this area.

Type of Work:

Thesis (Masters by Research > M.Phil.)

Award Type:

Masters by Research > M.Phil.

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