Short crack initiation and growth at 600 °C in notched specimens of Inconel718

The natural initiation and growth of short cracks in Inconel®718 U-notch specimens has been studied at 600 °C in air. U notches were introduced through broaching, and hardness traces and optical microscopy on cross-sections through the U notch broaching showed that the broaching process had introduced a deformed, work hardened layer. Fatigue tests were conducted under load control using a 1-1-1-1 trapezoidal waveform, on specimens with as-broached and polished U-notches. Multi-site crack initiation occurred in the notch root. Many of the cracks initiated at bulge-like features formed by volume expansion of oxidising (Nb,Ti)C particles. In unstressed samples, oxidation of (Nb,Ti)C particles occurred readily, producing characteristic surface eruptions. Scanning electron microscopy on metallographic sections revealed some sub-surface (Nb,Ti)C oxidation and localised matrix deformation around oxidised particles. A mechanism for crack initiation by carbide expansion during oxidation is discussed. Surface short crack growth rates in the notch root of polished specimens were measured using an acetate replica technique. Observed short-crack growth rates were approximately constant across a wide range of crack lengths. However, there was a transition to rapid, accelerating crack growth once cracks reached several hundred micrometers in length. This rapid propagation in the latter stages of the fatigue life was assisted by crack coalescence. Polishing the U-notch to remove broaching marks resulted in a pronounced increase in fatigue life

Understanding the deformation mechanism of individual phases of a ZrTi-based bulk metallic glass matrix composite using in situ diffraction and imaging methods

The plasticity of a ZrTi-based bulk metallic glass composite consisting of glassy matrix and crystalline dendritic phase was studied in-situ under identical tensile loading conditions using scanning electron microscopy and synchrotron X-ray diffraction. A generic procedure was developed to separate the diffraction information of the crystalline phases away from that of the matrix and to precisely calculate the microscopic strains of the two phases at different macroscopic load steps. In this way, the time-evolved quantitative links between shear bands nucleation/propagation and the corresponding microscopic stress fields around them are established, providing more quantitative understanding on (1) how the shear bands are driven by the local stress field, and (2) the critical stresses required for the shear bands to nucleate in the crystalline phase, propagate through the crystalline/matrix interface, and finally into the matrix

Development of EM-CCD-based X-ray detector for synchrotron applications

A high speed, low noise camera system for crystallography and X-ray imaging applications is developed and successfully demonstrated. By coupling an electron-multiplying (EM)-CCD to a 3:1 fibre-optic taper and a CsI(Tl) scintillator, it was possible to detect hard X-rays. This novel approach to hard X-ray imaging takes advantage of sub-electron equivalent readout noise performance at high pixel readout frequencies of EM-CCD detectors with the increase in the imaging area that is offered through the use of a fibre-optic taper. Compared with the industry state of the art, based on CCD camera systems, a high frame rate for a full-frame readout (50 ms) and a lower readout noise (<1 electron root mean square) across a range of X-ray energies (6–18 keV) were achieved

New modeling of the Vostok ice flow line and implication for the glaciological chronology of the Vostok ice core

International audienceWe have used new spaceborne (elevation) and airborne (ice thickness) data to constrain a 2D1/2 model of snow accumulation and ice flow along the Ridge B‐Vostok station ice flow line (East Antarctica). We show that new evaluations of the ice flow line geometry (from the surface elevation), ice thickness (from low‐frequency radar data), and basal melting and sliding change significantly the chronology of the Vostok ice core. This new Vostok dating model reconciles orbital and glaciological timescales and is in good agreement with the Dome Fuji glaciological timescale. At the same time, the new model shows significantly older ages than the previous GT4 timescale for the last glacial part, being thus in better agreement with the GRIP and GISP2 chronologies

Implementing an Office Training

For my capstone project, I chose to develop a training course with corresponding materials regarding the Return to Title IV Funds (R2T4) process in the Financial Aid Office. I have been working in this office for over a year, and I have been the manager and main processor of R2T4 for over six months. When I took a step back, I realized that our office has very little in terms of proper training materials, instead relying on new employees to learn through written out procedures. While this is okay for some of the easier processes, it does not foster an easy learning environment for heavily audited procedures such as R2T4. In order to address this, I’ve created a job aid and training slideshow presentation. Additionally, my supervisor and I recognized that our office would benefit from more understanding on this topic specifically as it would allow for the potential to have additional processors for it rather than just myself. Stakeholders include me, the Financial Aid Office as a whole, the institution, and its students. I have been able to provide something that our office not only needed, but something that can be utilized and iterated upon for other processes as well to foster a better training environment within our department

High speed synchrotron X-ray imaging studies of the ultrasound shockwave and enhanced flow during metal solidification processes

The highly dynamic behaviour of ultrasonic bubble implosion in liquid metal, the multiphase liquid metal flow containing bubbles and particles, and the interaction between ultrasonic waves and semisolid phases during solidification of metal were studied in situ using the complementary ultrafast and high speed synchrotron X-ray imaging facilities housed respectively at the Advanced Photon Source, Argonne National Laboratory, US, and Diamond Light Source, UK. Real-time ultrafast X-ray imaging of 135,780 frames per second (fps) revealed that ultrasonic bubble implosion in a liquid Bi-8 wt. %Zn alloy can occur in a single wave period (30 kHz), and the effective region affected by the shockwave at implosion was 3.5 times the original bubble diameter. Furthermore, ultrasound bubbles in liquid metal move faster than the primary particles, and the velocity of bubbles is 70 ~ 100% higher than that of the primary particles present in the same locations close to the sonotrode. Ultrasound waves can very effectively create a strong swirling flow in a semisolid melt in less than one second. The energetic flow can detach solid particles from the liquid-solid interface and redistribute them back into the bulk liquid very effectively

Mapping the inhomogeneous electrochemical reaction through porous LiFePO&lt;inf&gt;4&lt;/inf&gt;-electrodes in a standard coin cell battery

[Image - see article] Nanosized, carbon-coated LiFePO4 (LFP) is a promising cathode for Li-ion batteries. However, nano-particles are problematic for electrode design, optimized electrodes requiring high tap densities, good electronic wiring, and a low tortuosity for efficient Li diffusion in the electrolyte in between the solid particles, conditions that are difficult to achieve simultaneously. Using in situ energy-dispersive X-ray diffraction, we map the evolution of the inhomogeneous electrochemical reaction in LFP-electrodes. On the first cycle, the dynamics are limited by Li diffusion in the electrolyte at a cycle rate of C/7. On the second cycle, there appear to be two rate-limiting processes: Li diffusion in the electrolyte and electronic conductivity through the electrode. Three-dimensional modeling based on porous electrode theory shows that this change in dynamics can be reproduced by reducing the electronic conductivity of the composite electrode by a factor of 8 compared to the first cycle. The poorer electronic wiring could result from the expansion and contraction of the particles upon cycling and/or the formation of a solid-electrolyte interphase layer. A lag was also observed perpendicular to the direction of the current: the LFP particles at the edges of the cathode reacted preferentially to those in the middle, owing to the closer proximity to the electrolyte source. Simulations show that, at low charge rates, the reaction becomes more uniformly distributed across the electrode as the porosity or the width of the particle-size distribution is increased. However, at higher rates, the reaction becomes less uniform and independent of the particle-size distribution.We acknowledge the Engineering Physical Science Research Council (EPSRC) for a Doctoral Training Account Award (for FCS) and the US Department of Energy (DOE) for support via the NECCES, an Energy Frontier Research Center (DE-SC0001294 and DE-SC0012583). FCS acknowledges the Science and Technology Facilities Council for travel funding through the Global Challenge Network in Batteries and Electrochemical Energy Devices. Synchrotron X-ray beamtime was provided by Diamond Light Source, under experiment number EE8385. We also thank Zlatko Saracevic at the Department of Chemical Engineering at the University of Cambridge for help with the BET experiments and Jon Rickard at the Department of Physics at the University of Cambridge for help with the SEM. Lastly; we thank Charles Monroe and Paul Shearing for discussions on this project.This is the final version of the article. It first appeared from ACS Publications via http://dx.doi.org/10.1021/cm504317