Alexander D. Warren

Metal electrodeposition in batteries is fundamentally unstable and affected by different instabilities depending on operating conditions and chemical composition. Particularly at high charging rates, a hydrodynamic instability called electroconvection sets in that aggravates the situation by creating non-uniform ion flux and preferential deposition at the electrode. Here, we experimentally investigate how oligomer additives interact with the hydrodynamic instability at a cation selective interface. From electrochemical measurements and direct visualization experiments, we find that electroconvection is delayed and suppressed at all voltage in the presence of oligomers. Our results also reveal that it is important to consider the role of polymers at the interface, in addition to their bulk effects, to understand the stabilization effect and its mechanism.

The current generation of advanced gas-cooled nuclear reactors (AGRs) in the United Kingdom are approaching the end of their intended operating periods. Whilst awaiting the commissioning of new plant, there is a desire to extend the operational lifetime of the AGR fleet to maintain electricity supply to the UK grid. One of the factors determining lifetime extension of the AGRs is

Contact-mode high-speed atomic force microscopy (HS-AFM) has been utilised to measure in situ stress corrosion cracking (SCC) with nanometre resolution on AISI Type 304 stainless steel in an aggressive salt solution. SCC is an important failure mode in many metal systems but has a complicated mechanism that makes failure difficult to predict. Prior to the in situ experiments, the contributions of microstructure, environment and stress to SCC were independently studied using HS-AFM. During SCC measurements, uplift of grain boundaries before cracking was observed, indicating a subsurface contribution to the cracking mechanism. Focussed ion beam milling revealed a network of intergranular cracks below the surface lined with a thin oxide, indicating that the SCC process is dominated by local stress at oxide-weakened boundaries. Subsequent analysis by atom probe tomography of a crack tip showed a layered oxide composition at the surface of the crack walls. Oxide formation is posited to be mechanistically linked to grain boundary uplift. This study shows how in situ HS-AFM observations in combination with complementary techniques can give important insights into the mechanisms of SCC.

There is still a need to develop reliable and robust matrix deposition methods for matrix-assisted laser desorption/ionisation mass spectrometry that are applicable to a range of matrices, solvents and analyte types. This paper presents a robust methodology for the airbrush application of matrices along with the implications of varying the set-up and airbrush parameters. A small number of organic analytes and metal salts are analysed in both positive and negative ion modes to exemplify this methodology. In the analyses with the airbrush deposited matrices, performance was enhanced when compared to standard pipette deposition with the need for a search for sweat spots greatly diminished due to the increase homogeneity of the matrix surface and resultant analyte spots. As expected, the graphite matrices were shown to specifically outperform the organic matrices in negative ion mode.

A Type 316H austenitic stainless steel component containing Cr and impurity element-rich localised regions arising from component fabrication was aged for a prolonged period during service at a temperature of approximately 550 °C. These regions make up approximately 5% of the total volume of the microstructure. Previous work has shown that these regions contain ferrite and carbide precipitates and a finer austenite grain size than the adjacent matrix. The present study has used high-resolution transmission electron microscopy combined with compositional microanalysis to show that these regions have a highly complex microstructure containing G phase, chi phase and intragranular γ′ precipitates within the austenite grains. There is phosphorus migration to the chi austenite phase boundary, and the basis for this equilibrium impurity segregation is discussed. A Cr-depleted region was observed surrounding the chi phase precipitates, and the impact of this on the other precipitates is considered. The diversity of precipitates in these Cr-rich regions means that they behave significantly differently to the bulk material under long-term creep conditions leading to preferred nucleation and growth of creep cavities and the formation of localised creep cracks during service.

The UK’s Advanced Gas Cooled Reactor (AGR) nuclear power generating plant operates at temperatures up to 550 °C, where creep life is important. We consider Type 316H austenitic stainless steel headers and tubes with thermo-mechanical fabrication histories that result in significantly different initial microstructures. The heat affected zone of weldments, in these thick section headers and thin walled boiler tubes, have been found to be susceptible to creep damage leading to cracking during service. In this work we explore these differences in the long term service aged microstructure and the link to overall creep life of these components. To achieve this, samples containing weldments have been removed from plant after extended periods of service. Specifically parent and HAZ regions have been examined to determine the types of precipitates arising from the long term ageing. In particular, thin foils have been examined in a JEOL ARM transmission electron microscope operating at 200KeV fitted with an Oxford Instruments energy dispersive spectrometer to allow comparison between high resolution images and chemical composition. The thin foils were removed from predetermined locations using gallium ion milling and finally thinned using a low ion current to minimise ion damage. Differences between the distributions and types of precipitates are considered in the context of the initial microstructure arising from the thermo-mechanical history on the loss of creep strength and initiation of creep cavities at grain boundaries. The impact on overall service life is addressed.