Low Temperature Plasticity

At low temperatures (<0.3-0.4 Tm), deformation becomes limited by dislocation motion 21, 22. The barrier to their motion can either be via obstacles (e.g. precipitates, solute atoms, etc.) or by the lattice resistance itself. In tungsten carbide this resistance should be relatively high due to carbon solute atoms expanding the lattice, leading to solid solution strengthening. The covalent-metallic-ionic type bonding present will also lead to stronger resistance to dislocation motion than would be seen in a metal. Diffusional Creep At very low stresses, creep can be the result of vacancy diffusion. When a stress is applied to a material, there will be regions of compressive and tensile stress. This leads to vacancies diffusing towards the tensile faces and away from the compressive faces, resulting in a flux of matter and permanent deformation 20. Nabarro-Herring creep is the result of diffusion through the bulk. This is usually observed at temperatures around 0.8-0.9 Tm and at very low stresses 23, 24. Coble creep results from diffusion along grain boundaries. This is the dominant mechanism in polycrystalline materials with a fine grain size. Diffusion is faster along the grain boundary and the activation energy required is lower, leading to Coble creep occurring at lower temperatures 25. In transition metal carbide ceramics, both the anion (carbon) and cation (metal) will have a diffusion rate, which depends on the vacancy concentration on the associated sublattice. In such carbides, the number of carbon vacancies usually exceeds the number of metal vacancies, due to its lower formation energy. This leads to the rate controlling mechanism for diffusion being the slower diffusion rate of the anions and cations (cations in the case of transition metal carbides) along the fastest path (bulk or grain boundary) 20, 26. When the grain size of a material is very small, diffusional creep may be accommodated by Lifshitz sliding. The grain elongation that results from diffusional creep would, without any other mechanisms occurring, produce large cavities, voids and cracks between the grains. This is not the case in reality because the grains are able to slide over each other and maintain their nearest neighbours through Lifshitz sliding 20, 27.