Investigation of microstructural evolution by real-time SEM of high-temperature specimens.

This thesis presents the results of a project to investigate the growth of grains and movement of grain boundaries in face centred cubic metals, using Environmental Scanning Electron Microscopy (ESEM).The original aim proved impractical without considerable modification to the microscope technique. The result of this was an imaging technique suitable for "real-time" . characterisation of dynamic microstructures, evolving as materials are heated, cooled or held at high temperatures in the SEM. The technique is adaptable to both conventional "high-vacuum" SEM and environmental SEM. The development of the technique is described, and its application to hot metal specimens.The technique has been applied to various metals, but most notably to steel. The project has yielded probably the first "real-time" images of grain growth with time in steel, the first images of Austenite decomposition and phase change occurring in steel, the first images of grain growth in a bulk gold alloy and images of grain growth in an aluminium alloy.It is shown that the motion of grain boundaries in polycrystalline metal bulks is discontinuous ("jerky") and that this jerky motion occurs independent of grain boundary grooving.It is also shown that the first manifestation of austenite decomposition is an as-yet unexplained micron-scale "cellular" sub-structure within the austenite grain.It is further shown that in cooling of steel at slow-to-moderate speeds, the first appearance of permanent non-austenite structure is the precipitation of relatively large carbides at surfaces. Unexpectedly, this observation is in a slightly hypo-eutectoid steel, in which a slight excess of ferrite would be expected, leading to the logical but erroneous expectation that pro-eutectoid ferrite should be the first phase to precipitate.In slow-to-moderate cooling of near-eutectoid steel, it is shown that the number of nuclei initiating the austenite-to-pearlite transformation is small by comparison to the number of austenite grains present and that the austenite-to-pearlite transformation front sweeps from grain to grain with relative ease

Determination of the Cause of the Differing Ballistic Performance of 9mm DM11 Bullets from Two Manufacturers

In London, firearm threats faced by police during criminal activity include 9mm handguns and submachine guns. The UK Home Office body armour standards have included 9mm DM11 A1B2, manufactured originally by Dynamit Nobel under RWS branding, for over a decade. The recently published 2017 UK Home Office body armour standard continues to specify the 9mm DM11 A1B2, however, the specified manufacturer has changed to Metallwerk Elisenhütte GmbH (MEN). The DM11 A1B2 bullet comprises a copper coated steel full metal jacket with a lead core and bullets from both are specified to the same drawings and dimensional tolerances. However, during empirical testing against soft armour systems differences have been observed in the Vmean measured by CPA for the 2 bullets. As a result, body armour systems designed to pass the standard tests using the RWS 9mm DM11 A1B2 bullet manufactured may have a lesser safety margin when subject to impact with the equivalent MEN bullet. This paper reports on the results of an investigation in to the causes of the differing performance of the two sources of 9mm DM11 A1B2 bullets. It includes a study of the metallurgy of the steel jacket, dimensional and mass comparisons and a range of high strain rate testing to compare the properties and deformation behaviour of the two bullet types. Ballistic tests have been performed to demonstrate how the difference in performance may be related to the observed differences in the steel jacket metallurgy and the resulting differing deformation behaviour. The study has shown that the root cause of the differing performance is due primarily to differences in the steel used for the jackets by the different manufacturers. This work has important consequences for the UK body armour industry and others testing with the 9mm DM11 round

Results from in-situ, real-time SEM observations of grain growth in polycrystalline metal

The development of the converter plate detector has allowed the generation of real-time, in-situ image sequences of evolving microstructures in model and industrial polycrystalline metals. The principal metals investigated were steel, aluminium and gold. The raw video results allow a number of qualitative statements to be made about grain growth behaviour in these systems and some simplistic quantitative statements. However, the sheer volume and variety of data available in the time-stream of micrographs presents its own problems in identifying and extracting the information most useful for rigorous characterisation of the behaviour of a dynamically evolving microstructure. We present and discuss our approach to the analysis of this data and results of that analysis

Investigation of microstructural evolution by real-time SEM of high-temperature specimens

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Backscattered SEM imaging of high-temperature samples for grain growth studies in metals

A novel technique is presented for SEM backscattered electron imaging of hot specimens, in-situ, in real time. The technique has been applied to recrystallisation, grain growth and phase transformation studies in metals, principally steel. Temperatures attained were in the order of 850degreesC and were limited by the capability of the specimen heater, not the detector. Representative results are presented as stills and video