Micromechanical Creep - Improving Experimental and modelling capabilities

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This project aims to develop a method for measuring creep deformation of in-core nuclear components in the presence of irradiation damage. Creep deformation is time-dependent permanent deformation of materials under load nominally at temperatures higher than half the material melting point. Creep deformation plays a crucial role in the structural integrity of engineering components that work at high temperature such as those in aerospace propulsion and energy generation. While it is known to be one of the main life limiting factors of nuclear fission power plants that work at high temperature little work has been applied to fusion relevant materials. This is despite the fact that any future nuclear fusion power systems rely on the development of materials which can withstand some of the most extreme engineering environments. These include temperatures up to 1500oC, high fluxes of high energy neutrons and effects of gaseous elements produced by transmutation and implantation from the plasmas. Due to efforts to minimise the production of nuclear waste by such reactors the elements which may be used in structural components is limited and in many cases there is a lack of understanding of the basic deformation processes occur in ether pure materials or alloys and importantly how these are effected by temperature, radiation damage and gas content. Due to the long time periods required for neutron irradiation campaigns, plus the associated cost and difficulty of working with active materials there is a need to develop robust methods for the characterisation of irradiated materials on the microscale. There are two advantages to this. Firstly it allows the maximum data return from small volumes of neutron irradiated materials. Secondly it allows the use of heavy ion irradiation to mimic neutron damage. In this case while the damage is similar to that of neutrons and can be built up in much shorter time frames the damage is only over a few 10's of microns. This precludes the use of traditional mechanical testing methods. While the methods required for understanding micro-scale plastic deformation are well developed micro-fracture testing has lagged behind. While there have been extensive efforts to develop micromechanical methods for measuring all major mechanical properties including, fracture stress and toughness, yield behaviour and elastic properties. Much less effort has been focused on creep. This is due to the fact that traditional micromechanical methods have only been able to operate at room temperature. This project will aim to develop an experimentally validated model of creep deformation on the microscale. This will be fed into a bigger project aiming to develop multiscale model for the complete nuclear industry.We now have the ability to carry out these tests routinely up to 1273K in Oxford on three different systems. One of these systems allows simultaneous imaging using the secondary electron microscope, for imaged based analysis. This project will use these nanoindenters in conjunction with novel and newly designed micro-scale test specimens cut using focused ion beam machining to provide well describe geometries with simple stress states. Creep tests will be performed, both in and ex-situ, on fusion materials (initially reduced activation steels and 304 stainless steel, which are both also of wider interest in the nuclear field) and then the results validated by development of discrete dislocation dynamics modelling and finite element analysis. Once calibrated on unirradiated materials the method will be applied to irradiated steels. This will be the first time microscale creep tests on irradiated materials has been undertaken. This data will be used to inform high scale models being developed as part of a wider EPSC grant. This project is funded by the EPSRC CDT in Science and Technology of Fusion Energy. This project falls within the EPSRC Energy research area.