Papers by Author: Peter Skeldon

Abstract: Ultramicrotomy is used extensively as a specimen preparation method for transmission electron microscopy (TEM) in the biological and polymer fields, where relatively soft materials are sectioned to generate electron transparent specimens. Additionally, in corrosion control studies of light metals, e.g. aluminium and magnesium and their alloys, ultramictrotomy has been progressed at Manchester for characterisation of the metallic materials and their filming behaviour as well as the propagation of corrosion into the material interior at selected sites. The benefits of ultramicrotomy include the ability to generate uniform thicknesses of multiphase specimens with relatively large observation areas that include, for example, randomly distributed intermetallic particles in the alloy. However, mechanical sectioning with a diamond knife generates artefacts that include chattering and local damage; localised corrosion of the thin slices may also result from their residence on a water bath at the rear of the knife prior to collection for TEM study. Recently, ultramicrotomy has also been utilised to assist high resolution imaging in the scanning electron microscope (SEM); the generation of relatively flat specimens removes roughness effects from the secondary electron signal and improves the backscattered electron yield due to removal of an oxidised or carbon contaminated surface. The combination of ultramicrotomy and low voltage scanning electron microscopy has also enabled generation of high resolution, three dimensional images using sectioning and subsequent imaging of the fresh surface by SEM. However, importantly, recent instrumental developments, i.e. the GATAN 3View System, now enable ultramicrotomy to be performed in-SEM. Consequently, rapid in-SEM sectioning and imaging can now be undertaken with ready reconstruction of electron tomographs for light metallic materials. Here, the application of the Gatan 3View system in a Quanta 250 FEG-ESEM is presented, with consideration of artefacts introduced by the electron beam for serial block face sectioning imaging of light alloys.

Abstract: High strength aluminium alloys are widely used in the civil and military aerospace industry due to their low weight and high mechanical properties, achieved by selected alloying elements and heat treatments. The resulting multiphase alloy system, a solid solution of alloying elements in the aluminium matrix and a variety of second phase material, requires specific anticorrosion measures in order to prevent localized corrosion, which is promoted by microgalvanic coupling between the different metallographic phases. Traditionally, the anticorrosion performances are achieved by chromic acid anodizing (CAA), followed by painting. However, environmental issues and associated costs for the disposal of chromate wastes, require the development of new approaches for anodizing of aluminium alloys. In this work, the potential for tailoring the porous anodic film morphology through the film thickness by controlled variations of the anodizing potential is inspected. The procedure developed is, in principle, applicable to any aluminium alloy in any anodizing electrolyte and results in the generation of innovative graded porous anodic film morphologies which promise improvement of anticorrosion properties and replacement of CAA .