Boundary selectivity of crack paths in corrosion fatigue of stainless steel

Stress corrosion and corrosion fatigue cracks are frequently very branched and there have been extensive attempts to define the characteristics of crack-stopping features. EBSD has been used to examine the full length (~8mm) of a corrosion fatigue crack in stainless steel. The grain boundary character distribution of the cracked boundaries is compared to that of the rest of the material and observations presented on the effect of grain boundary character on the choice of crack path at grain boundary junctions of different configurations and orientations with restect to the principle stres

Effects of radiation damage on the critical resolved shear stresses in zirconium alloys for nuclear applications

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Crystal Structure of the ZrO Phase at Zirconium/Zirconium Oxide Interfaces

Zirconium-based alloys are used in water-cooled nuclear reactors for both nuclear fuel cladding and structural components. Under this harsh environment, the main factor limiting the service life of zirconium cladding, and hence fuel burn-up efficiency, is water corrosion. This oxidation process has recently been linked to the presence of a sub-oxide phase with well-defined composition but unknown structure at the metal–oxide interface. In this paper, the combination of first-principles materials modeling and high-resolution electron microscopy is used to identify the structure of this sub-oxide phase, bringing us a step closer to developing strategies to mitigate aqueous oxidation in Zr alloys and prolong the operational lifetime of commercial fuel cladding alloys

Correlation between microstructure and superconducting properties of MgB2 bulk samples with Mg addition and Mg/hBN co-additions

The microstructure of polycrystalline MgB2 has a strong influence on the current carrying ability, with grain boundaries and non-superconducting nanoparticles acting as good flux pinning centres which improve the local (intrinsic) critical current density (Jc) of the material, whereas porosity and poor connectivity between grains or particles adversely affect macroscopic current transport. Previous studies have found that hexagonal boron nitride (hBN) doping improves intrinsic Jc by introducing nanoscale flux pinning centres, and Mg doping improves extrinsic Jc by liquid-assisted sintering. Here we investigate the effect of co-doping with 5 wt.% Mg and 1 wt.% hBN with the aim of combining the improved intrinsic and extrinsic properties in bulk MgB2 samples fabricated using field assisted sintering. Additionally, the influence of ball milling and processing temperatures on MgB2 samples with only Mg additions is reported. By correlating microstructure with superconducting properties, we show that the presence of Mg liquid during processing of Mg-doped samples accelerates the reaction between BN and MgB2, forming an impurity phase, MgNB9, the presence of which is detrimental to superconducting performance. Nevertheless, we have achieved a considerable improvement in performance of samples doped only with Mg by increasing the sintering temperature

Preparation, microstructure and microwave dielectric properties of sprayed PFA/barium titanate composite films

Frequency dependence of the dielectric properties of polymer-ferroelectric composites at different bands of microwave frequencies was investigated in this work. Perfluoroalkoxy (PFA)/barium titanate (BaTiO3) nanocomposite films were prepared by spray deposition. The spraying process was scaled up to fabricate large area (max. 160 mm × 160 mm) uniform composite sheets out of which a controlled bonding process was introduced to form composite blocks. The microstructure of the composite films was examined by SEM with a microtome sample preparation method to evaluate the effectiveness of the spraying process at producing uniform particle distributions. The dielectric properties of the composite films with various BaTiO3 loadings were characterised by an Impedance Analyzer at frequencies between 10 Hz and 1 MHz and Vector Network Analyzer at 12–18 GHz respectively. The Lichtenecker mixing rule was incorporated to fit the measured dielectric constant data, which gives estimates of dielectric constant of the BaTiO3 nanometer sized particles to be 895 and 571 at 10 kHz and 15 GHz respectively. In comparison, the composite effective dielectric constant was approximately reduced by 25% at 10 kHz than that at 15 GHz

High resolution characterisation of microstructural evolution in Rbx_{x}Fe2−y_{2-y}Se2_{2} crystals on annealing

The superconducting and magnetic properties of phase-separated A$_x$Fe$_{2-y}$Se$_2$ compounds are known to depend on post-growth heat treatments and cooling profiles. This paper focusses on the evolution of microstructure on annealing, and how this influences the superconducting properties of Rb$_x$Fe$_2-y$Se$_2$ crystals. We find that the minority phase in the as-grown crystal has increased unit cell anisotropy (c/a ratio), reduced Rb content and increased Fe content compared to the matrix. The microstructure is rather complex, with two-phase mesoscopic plate-shaped features aligned along {113} habit planes. The minority phase are strongly facetted on the {113} planes, which we have shown to be driven by minimising the volume strain energy introduced as a result of the phase transformation. Annealing at 488K results in coarsening of the mesoscopic plate-shaped features and the formation of a third distinct phase. The subtle differences in structure and chemistry of the minority phase(s) in the crystals are thought to be responsible for changes in the superconducting transition temperature. In addition, scanning photoemission microscopy has clearly shown that the electronic structure of the minority phase has a higher occupied density of states of the low binding energy Fe3d orbitals, characteristic of crystals that exhibit superconductivity. This demonstrates a clear correlation between the Fe-vacancy-free phase with high c/a ratio and the electronic structure characteristics of the superconducting phase.Comment: 6 figures v2 is exactly the same as v1. The typesetting errors in the abstract have been correcte

Microstructural evolution in multiseeded YBCO bulk samples grown by the TSMG process

Superconducting single-grain YBCO bulk samples with the ability to trap high magnetic fields can be grown using the top-seeded melt-growth process. Multiseeding techniques have the potential to enable larger diameter bulks to be grown, but the performance of these materials is not yet comparable to the single-seeded bulks. Here we carry out detailed three-dimensional microstructural characterisation on a multiseeded sample grown with the seeds aligned in the 0°-0° geometry using high resolution microanalysis techniques. Chemical and structural variations have been correlated with the trapped field distribution in three separate slices of the sample. The top slice of the sample shows four peaks in trapped field, indicating that the current flows in four separate loops rather than in one large loop within the sample. This has been explained by the build-up in insulating Y-211 particles where the growth fronts from the two seeds meet, forming a barrier to current flow, as well as the low Y-211 content (and hence low $J_c$) of the large $c$-axis growth sector.Engineering and Physical Sciences Research Council (Grant ID: EP/K02910X/1

The effect of Sn concentration on oxide texture and microstructure formation in zirconium alloys

AbstractThe development of oxide texture and microstructure formed on two zirconium alloys with differing Sn contents (Zr–1Nb–1Sn–0.1Fe, i.e. ZIRLO™ and Zr–1.0Nb–0.1Fe) has been investigated using transmission Kikuchi diffraction (TKD) in the scanning electron microscope (SEM) and automated crystal orientation mapping in the transmission electron microscope (TEM). Bulk texture measurements were also performed using electron backscatter diffraction (EBSD) in order to quantify and compare the oxide macrotexture development. The Sn-free alloy showed significantly improved corrosion performance by delay of the transition region and reduced levels of hydrogen pickup. The macroscopic texture and grain misorientation analysis of the oxide films showed that the improved corrosion performance and reduced hydrogen pick up can be correlated with increased oxide texture strength, the improved oxide grain alignment resulting in longer, more protective columnar grain growth. A lower tetragonal phase fraction is also observed in the Sn-free alloy. This results in less transformation to the stable monoclinic phase during oxide growth, which leads to reduced cracking and interconnected porosity and also to the formation of larger, well-aligned monoclinic grains. It is concluded that the Zr–1.0Nb–0.1Fe alloy is more resistant to hydrogen pickup due the formation of a denser oxide with a larger columnar grain structure

Non-steady state operation of polymer/TiO2 photovoltaic devices

We present data on the initial period of operation of Gilch-route NMH-PPV/TiO2 composite solar cells (CSCs) which show that during this period the CSCs operate in a non-steady state regime. The behavior is complex and may include a gradual rise of the open circuit voltage (V-oc) and of the short-circuit current density (J(sc)) with time, a passage through a maximum of either or both parameters, and even a sign reversal. The mechanisms most probably contributing to the transient processes are: i) diffusion driven redistribution of charges resulting in the build up of a quasi steady state charge density profile across the device; ii) photo-doping resulting in a relatively slow increase of the average charge carrier concentration and consequently of the conductivity of the device. The latter is responsible for a strong decrease in V-oc, and is evidenced by the significant increase in dark current after device illumination