Papers by Author: Peter E.J. Flewitt

Abstract: t is increasingly a requirement to be able to determine the mechanical properties of materials: (i) at the micro-scale, (ii) that are in the form of surface coatings and (iii) that have nanoscale microstructures. As a consequence micro-scale testing is an important tool that has been developed to aid the evaluation of the mechanical properties of such materials. In this work cantilever beam specimens (typically 2μm by 2μm by 10μm in size) have been prepared by gallium ion milling and then deformed in-situ within a FEI Helios Dual Beam workstation. The latter is achieved using a force probe with a geometry suitable for loading the micro-scale test specimens. Thus force and displacement can be measured together with observing the deformation and fracture of the individual specimens. This paper considers the evaluation of the mechanical properties in particular elastic modulus, yield strength and fracture strength of materials that result in relatively large deflections to the micro-scale cantilever beams. Two materials are considered the first is linear elastic single crystal silicon and the other elastic-plastic nanocrystalline (nc) nickel. The results are discussed with respect to the reproducibility of this method of mechanical testing and the evaluated properties are compared with those derived by alternative procedures.

Abstract: Residual macro-stresses have to be evaluated using trial samples that comprise a CMSX4 superalloy coated with either a RT22 or a CN91 bond-coat. The samples were exposed in air to a matrix of temperatures, in the range 850°C to 1050°C, and times extending upto 4000hrs to produce thermally grown oxide. This oxide is essentially Al2O3 which allows stresses to be measured by photoluminescence spectroscopy. In addition, Raman spectroscopy and X-ray diffraction have been used to characterise the oxides formed. The results are discussed with respect to confidence in the measurements, changes in stress with temperature and exposure time and the potential for photoluminescence spectroscopy to be used to measure stresses for service components.

Abstract: As part of the requirement to demonstrate safe operation of the steel reactor pressure vessels for the UK Magnox fleet, surveillance samples were installed at the time of construction. These were representative of the steels and weld metals used for the manufacture of the vessels. The history of sample removal and mechanical property testing is briefly reviewed. The factors leading to changes in the fracture resistance and tensile properties will be discussed. The link is described between the mechanisms leading to changes in mechanical properties and the statistically based approach adopted to predict values of fracture toughness for the neutron dose and temperatures at key vessel locations. Predictions are compared with test results obtained from samples removed from a decommissioned reactor and also surveillance and accelerated radiation samples removed towards the end of reactor life.