The temperature dependence of plasticity in molybdenum

This thesis aims to add to the knowledge of the mechanical properties of molybdenum, with particular emphasis on the temperature and size dependence of plasticity. The experimental findings of this thesis will help to build a picture of the small scale properties of Mo for future studies on irradiated thin layers. Bulk scale microhardness testing of [001] orientated single crystal Mo from -196 to 872◦C formed a basis for a size-dependent nanoindentation study from -31 to 800◦C and maximum loads from 2 to 300mN. The size dependent behaviour was consistent with the approach of BCC metals to FCC behaviour upon heating due to the thermal activation of the kink pair mechanism, as the size dependence increased up to the knee temperature to the level expected of an FCC metal. A strain gradient plasticity model was used with success above the knee temperature, but was not suitable at lower temperatures. It is proposed that two different mechanisms act to control the characteristic length scale of indentation above and below the knee temperature. Bulk scale four point bend tests were carried out on single crystal Mo from -165 to 328◦C with sample edges parallel to {001} to provide a comparison with microcantilever bend tests with the same crystallographic orientation from -30 to 571◦C. The microscale tests showed much higher flow stresses at all temperatures. This is most likely due to the high strain gradients imposed on the smaller scale bend specimens, although a further experiment covering a range of beam widths would be necessary to explore this area further. Additionally, polycrystalline samples were annealed until cold work was observed to decrease significantly and tested from -40 to 522◦C. These samples had very similar behaviour to the single crystal samples, but reduced flow stresses compared to a previous study on unannealed samples, suggesting the grain growth and reduced cold work measured by EBSD were responsible.