Effect of strain rate on tribological properties of pre-deformed alloys via work function analysis

• Author / Creator

  Kumar,Vikas

• In industries such as automobile, aerospace and shipping, components made of magnesium and copper alloy could have the chance to bump against other objects during transportation applications. The damage caused by local deformation at high strain rates when other damage processes are involved, e.g. wear or fretting, may result in local micro-cracking and trigger failure of the entire system. The effect of severe deformation on defect formation, e.g., dislocations and microcracks/voids, is influenced by the strain rate. It is therefore, important to study the influences of strain-rate dependent pre-deformation on tribological properties of such alloys.
  The first part of the thesis reports a study on effects of tensile strain rate (fast and slow) on hardness, EWF, and tribological properties, including wear and corrosive wear, of pre-deformed AZ31 alloy. It was observed that the strain rate affected the effect of pre- deformation on these properties of the worked AZ31 alloy. A higher strain rate resulted in more deterioration of the alloy’s strength and lower resistance due to increased fraction ratio of micro-cracks/voids to dislocations. Corresponding XRD diffraction patterns were obtained in order to determine whether it was possible to obtain relevant information on the fraction ratio of dislocations to micro-voids based on residual strain and understand the observed phenomena. Microstructures of the deformed samples were also examined with electron microscopy. First-principle calculations were conducted to understand relevant mechanisms.
  In the second part of the thesis, effects of strain rate and sample thickness on properties of cold-worked brass samples were investigated. We studied influences of the strain rate and sample thickness on strain-softening of deformed brass thin sheets and corresponding wear behavior. Defect-induced strain-softening was observed for cold-worked brass thin sheets, which was affected by the strain rate and sample thickness. A lower strain rate caused smaller decrease in hardness than a higher strain rate for a thicker sheet. However, the situation was reversed for a thin sheet, leading to different variations in the wear behavior.
  A residual strain analysis with XRD diffraction was conducted which, however, did not give clear information to explain the above observed phenomena. The samples were then characterized using SEM, which showed that micro-voids or clusters of vacancies existed in the samples along with dislocations. In order to explain the observed phenomena, electron work functions (EWF) of the deformed samples were measured for supplementary information which demonstrated that this parameter can generate meaningful information to help understand the formation of defects and their role in influencing material performance. It has been shown that local change in electron density is caused by the lattice distortion associated with the formation of defects. Both dislocations and micro-cracks/voids decrease EWF. It turned out that EWF is a promising indicative parameter, providing very useful clues to explain the observed opposite trends.

• Subjects / Keywords

  • Strain rate
  • Electron work function
  • Hardness
  • Corrosion and Wear.

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3H41K34K

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.