The present thesis consists of a number of studies on the elastic properties of covalently bonded crystals, the ideal strengths of a selection of face-centred cubic (FCC) metals and the anomalously large shear modulus of iridium.
Semi-empirical formulae for both bulk modulus B and shear modulus G of covalent crystals were elaborated in terms of bond length and ionicity fraction of the bonding. The resulting equations can be applied to a broad selection of covalent materials and their predictions are in good agreement with the experimental data and the results from first-principles calculations. Furthermore, the correlation between the ratio G/B and the aforementioned bonding parameters was investigated. The analysis of this relationship demonstrates that the bond length is the predominant parameter responsible for the brittle features of covalent materials.
The ideal shear and tensile strengths of FCC transition metals were examined through density functional theory (DFT)-based computations. These results allowed establishing a new indicator of ductility, namely the ratio of ideal shear strength to ideal tensile strength taum/sigma <111>. Moreover, it was found that palladium can sustain a surprisingly long range of deformation. Based on the analysis of density of states (DOS) curves, it is suggested that creation of angular features is responsible for the notable amount of distortion suffered by palladium.
The electronic origin of the anomalously large elastic modulus and intrinsic brittleness of FCC iridium were studied using ab initio DFT-based calculations. The electron localisation function (ELF) and bond orders (BO) of iridium and a selection of FCC metals were calculated and then used to evaluate the directionality and the strength of the bonds. The analysis of the trend of bond strength versus elastic moduli, Cauchy pressure and the ratio of shear modulus over bulk modulus suggests that the bond strength is the primary factor that causes the abnormally high modulus of iridium and its intrinsic brittleness.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/27595 |
| Date | January 2008 |
| Creators | Kamran, Sami |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 110 p. |
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