Understanding crystallographic defects in hard materials : a study in ECCI and EBSD

Student thesis: Doctoral Thesis

Abstract

This thesis investigates the crystallographic defects inherent in two hard materials, tungsten carbide embedded in a cobalt matrix (WC-Co) and diamonds. The scanning electron microscopy (SEM) techniques of electron channelling contrast imaging (ECCI), cathodoluminescence (CL) imaging and electron backscatter diffraction (EBSD) allowed for the investigation of the crystallographic defects. Initial work on WC identified the dislocations and subgrain structures in grains of known crystallographic orientation. By combining ECCI and EBSD, new insights into the microstructure of WC grains were revealed. The advantage of using these techniques over the more common TEM based techniques is that ECCI and EBSD allow for analysis over a large field of view, in this case 75 μm × 75 μm. These large scale analyses reveal the microstructure and misorientation and dislocations throughout an entire WC grain. ECCI exposed dislocations while EBSD data revealed small changes in crystallographic orientation resulting from dislocations and subgrains, thereby corroborating the ECCI data. Further investigation of dislocations in WC categorized the type of dislocations found in grains of known crystallographic orientation using ECCI and EBSD. Dislocations and their Burgers vectors are described. The type of dislocations found in a particular grain are discussed in relation to their orientation with respect to neighbouring grains. Using ECCI and EBSD to categorize dislocations in the basal and prismatic planes helps explain how WC grains deform plastically. Plastic deformation is studied by indenting WC grains and studying the damage with ECCI and EBSD analysis. Misorientation surrounding the indent is investigated by EBSD for two different indenter tips. Using ECCI to reveal slip bands around indents in grains exposes different defects and deformation depending on the crystallographic orientation of grains.This novel ECCI work agrees with previous TEM data on slip systems in indented WC grains. Using ECCI presents an advantage over TEM in this case in that damage in grains can be imaged over a wider field of view with simpler sample preparation. The characterization of twinned gem-quality diamonds, also known as macles, using ECCI, EBSD and CL reveals that twinned diamonds are all composed of two sibling twins with around 180° between the siblings. CL is helpful in understanding the initial twin nucleation and distinct periods of crystallization in the diamond’s history while EBSD is used to corroborate CL. This study shows that the twinning plane is not linear, but jagged, corroborating previous research indicating that these are not true contact twins, rather they contain some intergrowths. EBSD confirms that these are undeniable twins as the plane separating the two siblings is a twin Coincident Site Lattices (CSL) grain boundary. CL and EBSD together are powerful tools for characterizing the twinning plane and growth mechanism in diamonds and this work describes the novel use of these techniques to understand diamond crystallization.
Date of Award23 Jun 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SupervisorCarol Trager-Cowan (Supervisor) & Robert Martin (Supervisor)

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