Aspects of discontinuous precipitation reactions in Ag-7.5Cu

Since the crystallographic orientation of a cellular colony derives from the grain it grows away from, experimental interest in discontinuous precipitation (DP) has centred on how nucleation and growth rates are affected by the misorientation of the initiating grain boundary. There has been little attention paid to the nature of the interface at the reaction front. Ageing experiments on Ag-7.5Cu at 2500C showed there to be two DP colony populations with distinctly different growth behaviours. EBSD studies have shown that as the colonies grow there is a build-up of misorientation behind the reaction front and eventually the fast growing colonies are surrounded by interfaces close to particular low Σ coincident site lattice (CSL) misorientations with respect to the grain into which they are growing. On the other hand, slow growing colonies are characterised by misorientations close to a different set of low ΣCSLs. The growth mechanisms behind this behaviour will be discussed

Microstructures of ancient and historic silver

The microstructures of silver-copper alloys from archaeological and historical contexts have been of particular interest since age-related changes at grain boundaries were first mooted as an indicator of antiquity and authenticity. Subsequent discussion has focused on how such structures might be reproduced by appropriate heat treatments but there was only limited experimental investigation of these precipitation phenomena. A second strand of interest has been the embrittlement of archaeological silver by segregation of impurities to grain boundaries. More recently industrial interest has developed in silver-copper alloys because of their use as solders and in electrical contact, and a growing number of papers on sterling and other silver alloy microstructures is being published. To interpret the microstructures of ancient and historic silver the key question is how to distinguish between the respective contributions of manufacture, age and the environment. This paper will describe and discuss a series of heat treatment experiments on wrought Britannia and Sterling silver and also on cast Sterling, the microstructure of cast silver being hitherto a rather neglected topic. The simple eutectic silver-copper system can exhibit a variety of precipitate morphologies and these have been characterised using optical microscopy, scanning and transmission electron microscopy, electron backscatter diffraction, and microhardness and nanoindenter testing. As a test case for discriminating between the effects of manufacture and age a series of medieval Islamic silver coins with a range of mint technologies has been examined in detail and the results presented here. The data will also be used to highlight the limits within which age-related modifications of the microstructure can be expected to be observed

Measurement of residual stresses in a dissimilar metal welded pipe

Dissimilar metal welds (DMW) are used in light water reactor power plants to join ferritic and austenitic steel piping components. High residual stresses remaining in this type of welded joint can significantly increase its susceptibility to stress corrosion cracking (SCC) under water reactor chemistry conditions. This degradation mechanism has compromised the integrity of many nuclear power plants throughout the world over the past 10 years. The Open University (OU) is undertaking a programme of research aimed at improving the reliability of residual stress measurements in DMWs using neutron diffraction. AREVA, the French nuclear power plant constructor, has developed an improved narrow gap DMW weld using a nickel-based corrosion resistant filler (alloy-52) to eliminate the risk of SCC in next generation nuclear plant. Through-wall neutron measurements have been carried out on a full-size DMW mock-up (352 mm OD, 40 mm thick) using the ENGIN-X instrument at the ISIS Facility in the UK. The results have shown that the largest tensile stress components in the welded component lie in the hoop direction, and have values of around 250 MPa and 225 MPa in the austenitic and alloy-52 materials respectively. These measured stresses were in reasonable agreement with those obtained from deep-hole drilling and numerical simulations. A notable finding in the experimental work was a wide scatter in the measured unstressed lattice parameters within a range equivalent to micro-strains of ≈ 430, 400 and 600 for austenitic stainless steel, ferritic steel and alloy-52 respectively. This scatter was reduced by two orders of magnitude through making additional measurements whilst slowly rotating the stress-free reference cubes. The OU is undertaking systematic studies to identify the origins of the observed measurement scatter in order to improve the reliability of measurements

Electron backscatter diffraction (EBSD) measurement of accumulated strain

Reliable life prediction depends on a sound knowledge of the accumulated strain in components subject to creep. Electron backscatter diffraction (EBSD) is now well-established for estimating/measuring plastic strain and there have been a number of different EBSD metrics proposed for this. Microstructure has a strong effect on the calibration of most of these, limiting their use in critical areas such as around welds where microstructure is inhomogeneous. During the service life of materials such as 316 steel there is extensive precipitation but most published applications of EBSD are on precipitate-free materials. A systematic study has been made on the applicability of different EBSD metrics to both solution-annealed and service-aged 316H stainless steel subject to a range of plastic and creep strains between 0 and 35% to determine the conditions for maximum strain sensitivity of each. A simple new method of assessing cumulative strain, ‘deformed grain fraction’ (DGF) is presented and DGF has been compared with more conventional EBSD strain metrics. In 316H steel with a range of microstructures the effects of plastic and creep strains are additive for all metrics. DGF is relatively insensitive to changes in microstructure and its use is demonstrated in measuring total plastic strain in ex-service welded components

Spatially resolved texture analysis of Napoleonic War era copper bolts

The spatial resolution achievable by a time-of-flight neutron strain scanner has been harnessed using a new data analysis methodology (NyRTex) to determine, nondestructively, the spatial variation of crystallographic texture in objects of cultural heritage. Previous studies on the crystallographic texture at the centre of three Napoleonic War era copper bolts, which demonstrated the value of this technique in differentiating between the different production processes of the different types of bolts, were extended to four copper bolts from the wrecks of HMS Impregnable (completed 1786), HMS Amethyst (1799), HMS Pomone (1805) and HMS Maeander (1840) along with a cylindrical `segment' of a further incomplete bolt from HMS Pomone. These included bolts with works stamps, allowing comparison with documentary accounts of the manufacturing processes used, and the results demonstrated unequivocally that bolts with a `Westwood and Collins' patent stamp were made using the Collins rather than the Westwood process. In some bolts there was a pronounced variation in texture across the cross section. In some cases this is consistent with what is known of the types of hot and cold working used, but the results from the latest study might also suggest that, even in the mature phase of this technology, some hand finishing was sometimes necessary. This examination of bolts from a wider range of dates is an important step in increasing our understanding of the introduction and evolution of copper fastenings in Royal Navy warships

An EBSD study of the deformation of service-aged 316 austenitic steel

Electron backscatter diffraction (EBSD) has been used to examine the plastic deformation of an ex-service 316 austenitic stainless steel at 297K and 823K (24 °C and 550 °C)at strain rates 3.5x10-3 to 4 x 10-7 s-1. The distribution of local misorientations was found to depend on the imposed plastic strain following a lognormal distribution at true strains 0.1. At 823 K (550 °C), the distribution of misorientations depended on the applied strain rate. The evolution of lattice misorientations with increasing plastic strain up to 0.23 was quantified using the metrics kernel average misorientation, average intragrain misorientation, and low angle misorientation fraction. For strain rate down to 10-5 s-1 all metrics were insensitive to deformation temperature, mode (tension vs. compression) and orientation of the measurement plane. The strain sensitivity of the different metrics was found to depend on the misorientation ranges considered in their calculation. A simple new metric, proportion of undeformed grains, is proposed for assessing strain in both aged and unaged material. Lattice misorientations build up with strain faster in aged steel than in un-aged material and most of the metrics were sensitive to the effects of thermal aging. Ignoring aging effects leads to significant overestimation of the strains around welds. The EBSD results were compared with nanohardness measurements and good agreement established between the two techniques of assessing plastic strain in aged 316 steel

The use of size distributions in determining growth mechanisms: the growth of grain boundary precipitates in cobalt-20 iron

Accurate prediction of microstructural stability in an alloy depends not only on a sound knowledge of the thermodynamics of the system but also of the kinetics of the phase changes involved. Conventionally, precipitate growth mechanisms have been inferred from the variation with aging time of various single parameters such as the mean, mode or maximum of the precipitate size distribution, which has then been compared to theoretical models of growth of an individual precipitate. In the present study, the development, with aging time at 1003 K (730 °C), of the size and shape distributions of grain boundary precipitates in Co-20Fe has been examined to determine the rate-controlling processes, and the conclusions compared to those from conventional analysis. The growth of the precipitates was well described by the grain boundary-dependent collector plate mechanism of Brailsford and Aaron. As the precipitates grew, low-energy facets were formed, which could move only by the propagation of ledges, and thickening was inhibited. The precipitates’ diffusion fields in the grain boundary overlapped and the size distributions of the longest aged specimens showed that local coarsening occurred under partial interface control

Discontinuous precipitation in silver-copper alloys

Alongside well-studied intragranular precipitation reactions, age-hardening silver copper alloys undergo a range of slower grain boundary initiated transformations including chemically-induced grain boundary migration and cellular precipitation reactions. These are of great interest both for their effect upon mechanical properties and because it has been suggested that the observation of such grain boundary effects offers a means of distinguishing ancient silver objects from modern ones. Optical and scanning electron microscopy have been used to find the temperature dependence of the overall growth rates of cellular colonies in Ag-7.5Cu and Ag-4.16Cu. Electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) have been used to determine the crystallographic relationships between the different transformation products and the parent grains. The extent of transformation varies greatly from boundary to boundary and EBSD has shown that the extent of transformation depends on the boundary plane and the orientation relationship between adjacent grains: extensive cellular growth only proceeds where a colony can form a highly mobile interface with at least one of the grains

Analysis and metallography of copper and brass components from Alum Bay

[About the book] In 1991, sports divers discovered a previously unknown section of wooden shipwreck, subsequently named Alum Bay 1, lying in the sheltered waters of Alum Bay on the north-west coast of the Isle of Wight. The identity of the vessel was initially unknown but it was strongly linked to the loss of the 38-gun frigate HMS Pomone on the nearby Needles in 1811, an identification formally confirmed by the research detailed in this monograph. Archaeological work on the site since 1993 has comprised a seabed survey of the site, targeted excavation of specific areas and sampling of structural remains for dendrochronological and metallurgical analysis. In 2001, a second shipwreck was discovered a short distance away and the focus of archaeological work shifted to this new set of remains, named Alum Bay 2. This vessel proved to be a much smaller vessel that was upturned on the seabed and covered by a thin layer of sediment. This vessel was also subject to archaeological survey and investigation, including dendrochronological analysis. On the basis of the ship structure surviving on the seabed, Alum Bay 2 has been classified as a relatively small vessel that was likely to have been involved in local transport or coastal trade in the very late 18th century and early decades of the 19th century. The role of public engagement in the management of such archaeological sites was developed further in the mid-2000s when a dive trail was established around the two Alum Bay shipwrecks. The dive trail in Alum Bay provides an interesting case study in this form of archaeological interaction with the diving public. In concert with such outreach work, further archaeological survey was been undertaken across Alum Bay in the light of a number of isolated finds being reported by sports divers including parts of cannon carriages and hull elements. Investigative work in Alum Bay has also encompassed the broken remains of the Victorian Pier that was constructed in 1887 to serve the growing boom in seaside tourism. The two shipwrecks of Alum Bay 1 and 2 provide a snapshot of two different aspects of English shipbuilding, naval and merchant, in the very late 18th and early 19th century. The archaeological work conducted in their investigation forms the core of this monograph, with further chapters that discuss the wider searches of Alum Bay and also the installation and use of the public Alum Bay Dive Trail. Such an account represents the results of twenty years of archaeological investigation within Alum Bay by the Maritime Archaeology Trust (which incorporates the Hampshire and Wight Trust for Maritime Archaeology). This work has been undertaken across a time span in which maritime archaeology in the UK has seen tremendous changes, becoming ever more integrated into the wider heritage discipline and with increasing numbers of professional archaeologists working within the marine zone. Throughout this period, archaeological work in Alum Bay has brought together professional and a-vocational archaeologists, who have worked successfully alongside each other. The various fieldwork seasons have provided extensive opportunities for people to receive archaeological training and develop their experience. This monograph therefore represents the last stage of this work, addressing the processing, analysis, interpretation and finally publication and dissemination