GumDrop at the DISRPT2019 Shared Task: A Model Stacking Approach to Discourse Unit Segmentation and Connective Detection

In this paper we present GumDrop, Georgetown University's entry at the DISRPT 2019 Shared Task on automatic discourse unit segmentation and connective detection. Our approach relies on model stacking, creating a heterogeneous ensemble of classifiers, which feed into a metalearner for each final task. The system encompasses three trainable component stacks: one for sentence splitting, one for discourse unit segmentation and one for connective detection. The flexibility of each ensemble allows the system to generalize well to datasets of different sizes and with varying levels of homogeneity.Comment: Proceedings of Discourse Relation Parsing and Treebanking (DISRPT2019

On the Influence of Alloy Composition on the Additive Manufacturability of Ni-Based Superalloys

The susceptibility of nickel-based superalloys to processing-induced crack formation during laser powder-bed additive manufacturing is studied. Twelve different alloys—some of existing (heritage) type but also other newly-designed ones—are considered. A strong inter-dependence of alloy composition and processability is demonstrated. Stereological procedures are developed to enable the two dominant defect types found—solidification cracks and solid-state ductility dip cracks—to be distinguished and quantified. Differential scanning calorimetry, creep stress relaxation tests at 1000 °C and measurements of tensile ductility at 800 °C are used to interpret the effects of alloy composition. A model for solid-state cracking is proposed, based on an incapacity to relax the thermal stress arising from constrained differential thermal contraction; its development is supported by experimental measurements using a constrained bar cooling test. A modified solidification cracking criterion is proposed based upon solidification range but including also a contribution from the stress relaxation effect. This work provides fundamental insights into the role of composition on the additive manufacturability of these materials

Assessment of corrosive attack of Fe9Cr1Mo alloys in pressurised CO2 for prediction of breakaway oxidation

To provide clarity on the poorly-understood mechanism of breakaway oxidation, corrosion of Fe9Cr1Mo steel in pressurised CO is quantified and modelled. The temperature range 400–640 , relevant to nuclear power plants, is emphasised. Attack is in the form of combined oxide scale growth and internal carburisation of the metal. Carbon activity in the metal at its surface exhibits a strong time dependence consistent with the kinetically-limited transport of carbon due to the slow Boudouard reaction. Breakaway is associated with the approach to saturation of the steel with respect to carbon. Diffusion modelling agrees well with steel carbide precipitation observations

Modelling of diffusional phenomena in structural alloys for high temperature applications

This thesis is concerned with the physical metallurgy/corrosion of high temperature structural alloys, of the type used for some of the most demanding engineering applications yet devised by mankind. Both iron-based steels and nickel-based superalloys are used for such applications; the common thread in this thesis from the science perspective is the role played by defects such as vacancies which lead to degradation due to diffusional phenomena such as oxidation and creep. Modelling for typical diffusional behaviour is carried out at various length scales in this work. First, the vacancy formation free energy in fcc-nickel is calculated by using highly-accurate ab initio thermodynamics including relevant temperature effects such as phonon vibrations and magnetisms. Predictions show good agreement with experiments, with non-Arrhenius behaviour due to the anharmonicity suggested by calculations. Second, the oxidation kinetics is predicted for the Ni-Cr-O system by coupling CALPHAD thermodynamics and diffusional kinetics. A general (oxidation) diffusion model coupled with the homogenisation model is applied, allowing the treatment of metal, oxide and metal/oxide interface to be unified. Predicted oxidation kinetics considering oxide phases (halite, corundum and spinel) and metallic phases (fcc and bcc) show reasonable agreement with experiments. Proposed quantification of entropy-production potentially allows the long-debated question concerning the rate-controlling steps (phases) in high temperature corrosion process to be answered. Last, the failure (breakaway oxidation) mechanisms of a critical Fe9Cr1Mo nuclear component exposed to CO2 are studied by high-resolution characterisations. Based upon theories of carbon diffusion in ferrite and Boudouard surface reaction, the temperature-dependent lifetime â linked with carbon saturation in metal â is rationalised at accelerated conditions and is extrapolated to service conditions. </p

Modelling of diffusional phenomena in structural alloys for high temperature applications

This thesis is concerned with the physical metallurgy/corrosion of high temperature structural alloys, of the type used for some of the most demanding engineering applications yet devised by mankind. Both iron-based steels and nickel-based superalloys are used for such applications; the common thread in this thesis from the science perspective is the role played by defects such as vacancies which lead to degradation due to diffusional phenomena such as oxidation and creep. Modelling for typical diffusional behaviour is carried out at various length scales in this work. First, the vacancy formation free energy in fcc-nickel is calculated by using highly-accurate ab initio thermodynamics including relevant temperature effects such as phonon vibrations and magnetisms. Predictions show good agreement with experiments, with non-Arrhenius behaviour due to the anharmonicity suggested by calculations. Second, the oxidation kinetics is predicted for the Ni-Cr-O system by coupling CALPHAD thermodynamics and diffusional kinetics. A general (oxidation) diffusion model coupled with the homogenisation model is applied, allowing the treatment of metal, oxide and metal/oxide interface to be unified. Predicted oxidation kinetics considering oxide phases (halite, corundum and spinel) and metallic phases (fcc and bcc) show reasonable agreement with experiments. Proposed quantification of entropy-production potentially allows the long-debated question concerning the rate-controlling steps (phases) in high temperature corrosion process to be answered. Last, the failure (breakaway oxidation) mechanisms of a critical Fe9Cr1Mo nuclear component exposed to CO2 are studied by high-resolution characterisations. Based upon theories of carbon diffusion in ferrite and Boudouard surface reaction, the temperature-dependent lifetime — linked with carbon saturation in metal — is rationalised at accelerated conditions and is extrapolated to service conditions. </p

Long-term and short-term duration of thienopyridine therapy after coronary stenting in patients with chronic kidney disease a meta-analysis of literature studies

The study aimed to compare the efficacy and safety outcome associated with a short and a prolonged duration of thienopyridine therapy in patients with chronic kidney disease (CKD) after coronary stenting. We systematically searched PubMed, EMBASE and the Cochrane Library from their inception to 1 January 2019 for studies comparing short and prolonged thienopyridine therapy in patients with CKD. Ischemic and bleeding events were considered as the clinical endpoints in this analysis. Odds Ratios (OR) with 95% confidence intervals (CIs) were used as estimates of effect size in random-effect models. Seven studies comprising a total of 17,628 CKD patients were included in the evaluation. Prolonged duration of thienopyridine use, when compared to short-term thienopyridine, was associated with reduced risk of all-cause mortality (odds ratio 0.75, 95% confidence interval: 0.70–0.81, P< .001) and stent thrombosis (OR: 0.54, 95% CI 0.32 to 0.89; P< .001), but the odds of myocardial infarction (OR: 0.91, 95% CI: 0.77–1.07; P = .23) and stroke (OR: 0.91, 95% CI 0.73 to 1.13; P = .38) did not differ according to different duration of thienopyridine. As for bleeding events, long-term thienopyridine therapy did not significantly increase the bleeding (OR: 0.95, 95% CI 0.79 to 1.14; P = .58). In these patients with CKD following PCI, prolonged thienopyridine therapy compared with short-term therapy, was associated with reduced all-cause mortality and stent thrombosis, without any significant difference in myocardial infarction, stroke, and bleeding. Thienopyridine prolongation decisions for CKD patients should be individualized after careful consideration of the benefit–risk balance

Additive manufacturability of superalloys:Process-induced porosity, cooling rate and metal vapour

Digital technology such as metal additive manufacturing (AM) provides flexible process design freedom to fabricate intricate three-dimensional structures layer-by-layer. However, its manufacturability relies on the fundamental understanding of melt pool physics and fluid (metal) dynamics. The effect of metal vapour and porosity induced during the laser-materials interaction can influence the additive manufacturability. In this work, composition-process relationship of laser-based powder-bed fusion (L-PBF) AM is studied via computational fluid dynamics modelling to rationalise solid-liquid-vapour transformation where empirical-based approach is used to generate thermo-physical property of about 100 nickel-based superalloys at the liquid state. It is found that with larger vapor mass loss, the porosity tends to be higher. However, the higher vapour mass loss means faster cooling rate. This is indicated that the thermal-fluid flow process, which is also governed by the thermo-physical property, strongly affects the additive manufacturability. Additive manufacturability map based upon porosity, cooling rate from liquid to solid, volatile mass loss criteria has been established to link the composition in nickel-based superalloys with their thermo-physical property. This offers a thermal-fluid science based tool in designing compositions of novel superalloys for AM applications

Additive manufacturability of superalloys:Process-induced porosity, cooling rate and metal vapour

Digital technology such as metal additive manufacturing (AM) provides flexible process design freedom to fabricate intricate three-dimensional structures layer-by-layer. However, its manufacturability relies on the fundamental understanding of melt pool physics and fluid (metal) dynamics. The effect of metal vapour and porosity induced during the laser-materials interaction can influence the additive manufacturability. In this work, composition-process relationship of laser-based powder-bed fusion (L-PBF) AM is studied via computational fluid dynamics modelling to rationalise solid-liquid-vapour transformation where empirical-based approach is used to generate thermo-physical property of about 100 nickel-based superalloys at the liquid state. It is found that with larger vapor mass loss, the porosity tends to be higher. However, the higher vapour mass loss means faster cooling rate. This is indicated that the thermal-fluid flow process, which is also governed by the thermo-physical property, strongly affects the additive manufacturability. Additive manufacturability map based upon porosity, cooling rate from liquid to solid, volatile mass loss criteria has been established to link the composition in nickel-based superalloys with their thermo-physical property. This offers a thermal-fluid science based tool in designing compositions of novel superalloys for AM applications

Oxidation of iron at 600 °C – experiments and simulations

Pure iron has been oxidized at 600 °C and 1 bar in dry O2 (oxygen partial pressure 0.05, bal. N2) and the mass gain as well as the thicknesses of the individual oxide phases have been measured. The oxidation process has been simulated using a modified version of the homogenization model as implemented in Dictra; this has helped to rationalize the kinetics of oxide scale formation and in particular the evolution of the hematite (Fe2O3), magnetite (Fe3O4), and wustite (FeO) which form. Independently assessed thermodynamic and kinetic Calphad databases are needed for the calculations; details of these are given. Reasonable agreement between simulation results and experimental data is obtained, though it is concluded that the large influence of grain boundary diffusion on the oxidation rate needs further consideration