Effect of cooling rate on microstructure,microsegregation and mechanical properties of cast Ni-based superalloy K417G

The effects of different solidification rates after pouring on the microstructures,microsegregation and mechanical properties of cast superalloy K417 G were investigated.Scheil-model was applied to calculate the temperature range of solidification.The casting mould with different casting runners was designed to obtain three different cooling rates.The microstructures were observed and the microsegregation was investigated.Also,high temperature tensile test was performed at 900?C and stress rupture test was performed at 950?C with the stress of 235 MPa.The results showed that the secondary dendrite arm spacing,microsegregation,the size and volume fraction of γ'phase and the size of γ/γ'eutectic increased with decreasing cooling rate,but the volume fraction of γ/γ' eutectic decreased.In the cooling rate range of 1.42?C s~(-1)–0.84?C s~(-1),the cast micro-porosities and carbides varied little,while the volume fraction and size of phase and γ/γ' eutectic played a decisive role on mechanical properties.The specimen with the slowest cooling rate of 0.84?C s~(-1) showed the best comprehensive mechanical properties.     

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy

Abstract

Microstructural evolution and flow behaviour during hot compression of twin roll cast ZK60 magnesium alloy were characterised by employing deformation temperatures of 300, 350 and 400°C and strain rate ranging from 10^?3 to 100 s^?1. When compressed at 10^?3 s^?1, all stress–strain curves at different temperatures (300, 350 and 400°C) showed a flow softening behaviour due to active dynamic recrystallisation. When compressed at 10^?2 s^?1 and elevated temperatures (300, 350 and 400°C), all stress–strain curves showed a flow stress drop after peak stress due to twinning for 300 and 350°C deformation and recrystallisation for 400°C deformation. The balance between shear deformation and recrystallisation resulted in a steady flow behaviour after the true strain reached 0·22. When strain rate increased to 10^?1 s^?1, a small fraction of dynamic recrystallisation in shear deformation region was responsible for slight flow softening behaviour during compression. A flow hardening appeared due to basal and non-basal slips when deformed at 100 s^?1. It is suggested that the flow behaviour during hot compression of twin roll cast ZK60 alloy depends on the separating effect or combined effects of shear deformation, twinning and recrystallisation.     

Yield strength anomaly and dynamic strain ageing behaviour of recently developed advanced ultra-supercritical boiler grade wrought Ni-based superalloy IN 740H

The yield strength anomaly (YSA) and dynamic strain ageing (DSA) behaviour of advanced ultra-supercritical boiler grade wrought nickel-based superalloy IN 740H is studied by conducting tensile tests in temperature range 28–930°C and by employing strain rates 1 × 10^?2, 1 × 10^?3, 1 × 10^?4 and 1 × 10^?5 s^?1 followed by extensive electron microscopic examination. Increase in yield strength accompanied by impairment of ductility indicates that YSA exists in alloy IN 740H in temperature range of 650–760°C. The electron microscopic observation confirms that YSA is due to pinning of dislocations by γ′ precipitates and shearing of γ′ precipitates in IN 740H. DSA is observed in the temperature range of 200–500°C and is predominant at 300°C. The nature of serrated plastic flow due to DSA is dependent on the temperature and strain rate.     

Reduced graphene oxide film with record-high conductivity and mobility

Graphene has attracted significant attention in both scientific and industrial fields. The scalable and high-yield chemical functionalization methods have been widely used to produce graphene, such as reduced graphene oxide (RGO). However, previously reported conductivity (<1500 S cm^?1) and mobility (<5 cm² V^?1 s^?1) values for RGO film are relatively low, which limits its application in many fields. In this work, we report a RGO film with a record-high conductivity of 6300 S cm^?1 and a record-high mobility of 320?cm²?V^?1?s^?1, which was reduced by Joule heating at an extremely high temperature of 3000?K. Thermal reduction process challenges of Joule heating were overcome by employing a two-step reduction and a curved RGO film. An investigation into how charge transport properties of RGO film are influenced by the reduction temperature was pursued. As the reduction temperature increases, the oxygen-containing functional groups, acting as dopant sources and scattering centers, are gradually removed, such that the carrier density gradually decreases, and the mobility and conductivity gradually increases. The localization length, corresponding to the size of graphitic sp² domains, is 8.7?nm for the 3000-K-reduced RGO film, which exceeds previously reported values. The unique features of the reported 3000-K-reduced RGO film, such as less defects/impurities and large graphitic sp² domains within a dense structure, enable both record-high conductivity and mobility.     

The cryopumping of water vapour in the continuum pressure region

Pumping speed measurements in the continuum pressure region, P>10^?3 torr, have been made for water vapour impinging on copper spheres and coils cooled to liquid nitrogen temperatures. Water vapour flow rates between 0.06 mg s^?1 and 420 mg s^?1 were used. The volumetric pumping speed was constant over the pressure range 2 × 10^?3 torr to 2 × 10^?2 torr and was, as expected, higher than that obtained in the free molecular flow region. Above 2 × 10^?2 torr the pumping speed decreased and possible reasons for this were investigated and are discussed. These included the effects of inadequate heat transfer from the liquid nitrogen refrigerant to the cryopump, a poor thermal conductivity of the cryodeposit, and an impurity, nitrogen gas, in the water vapour.     

The Decay of Forced Turbulent Coflow of He II Past a Grid

We present an experimental study of the decay of He II turbulence created mechanically, by a bellows-induced flow past a stationary grid in a 7×7 mm² superfluid wind tunnel. The temporal decay L(t) originating from various steady-states of vortex line length per unit volume, L ₀, has been observed based on measurements of the attenuation of second-sound, in the temperature range 1.17 K<T<1.95 K. Each presented decay curve is the average of up to 150 single decay events. We find that, independently of T and L ₀, within seconds past the sudden stop of the drive, all the decay curves show a universal behavior lasting up to 200 s, of the form L(t)∝(t?t ₀)^?3/2, where t ₀ is the virtual origin time. From this decay process we deduce the effective kinematic viscosity of turbulent He II. We compare our results with the bench-mark Oregon towed grid experiments and, despite our turbulence being non-homogeneous, find strong similarities.     

Decay of Turbulence Generated by Spin-Down to Rest in Superfluid 4He

We report on the extension of the experiments (P.M. Walmsley et al., Phys. Rev. Lett. 99:265302, 2007) on the decay of quasiclassical turbulence generated by an impulsive spin-down from angular velocity Ω to rest of superfluid ⁴He in a cubic container at temperatures 0.15 K–1.6 K. The density of quantized vortex lines L is measured by scattering negative ions. Following the spin-down, the maximal density of vortices is observed after time t～10Ω^?1. By observing the propagation of ions along the axis of the initial rotation, the transient dynamics of the turbulence spreading from the perimeter of the container into its central region is investigated. Nearly homogeneous turbulence develops after time t～100Ω^?1 and decays as L ∝ t ^?3/2. The effective kinematic viscosity in T=0 limit is ν=0.003κ, where κ=10^?3 cm²?s^?1 is the circulation quantum.     

Melt flow properties of polypropylene/EPDM/glass bead ternary composites

The flow properties of polypropylene(PP)/ethylene-propylene-diene monomer copolymer(EPDM)/glass bead (GB) ternary composites were measured in a wide scope of shear rates by using a Rosand capillary rheometer. The apparent shear rates varied from 10 to 10⁵ s^–1, and the test temperature was from 210 to 240°C. The results showed that the flow behavior of the composite melts may be considered to approximate that of a power law fluid although the slope of the melt flow curves somewhat varied around shear rate of 700 s^{– 1}. The dependence of the melt shear viscosity on the test temperature was roughtly consistent with the Arrhenius expression. The melt shear viscosity increased dramatically with increasing the volume fraction (_g) of GB under lower shear rate level, white it increased gently with an addition of _g under higher shear rate level. Furthermore, there were certain effects of the filler surface treatment on the melt shear viscosity and its sentivity to the filler content at higher concentration of the beads at lower shear rates.     

Visualization of dynamic fiber-matrix interfacial shear debonding

To visualize the debonding event in real time for the study of dynamic crack initiation and propagation at the fiber–matrix interface, a modified tension Kolsky bar was integrated with a high-speed synchrotron X-ray phase-contrast imaging setup. In the gage section, the pull-out configuration was utilized to understand the behavior of interfacial debonding between SC-15 epoxy matrix and S-2 glass fiber, tungsten wire, steel wire, and carbon fiber composite Z-pin at pull-out velocities of 2.5 and 5.0 m s^?1. The load history and images of the debonding progression were simultaneously recorded. Both S-2 glass fiber and Z-pin experienced catastrophic interfacial debonding whereas tungsten and steel wire experienced both catastrophic debonding and stick–slip behavior. Even though S-2 glass fiber and Z-pin samples exhibited a slight increase and tungsten and steel wire samples exhibited a slight decrease in average peak force and average interfacial shear stress as the pull-out velocities were increased, no statistical difference was found for most properties when the velocity was increased. Furthermore, the debonding behavior for each fiber material is similar with increasing pull-out velocity. Thus, the debonding mechanism, peak force, and interfacial shear stress were rate insensitive as the pull-out velocity doubled from 2.5 to 5.0 m s^?1. Scanning electron microscope imaging of recovered epoxy beads revealed a snap-back behavior around the meniscus region of the bead for S-2 glass, tungsten, and steel fiber materials at 5.0 m s^?1 whereas those at 2.5 m s^?1 exhibited no snap-back behavior.     

Dielectric losses and ac conductivity of Si–LiNbO3 heterostructures grown by the RF magnetron sputtering method

Single-axis <0001> textured polycrystalline LiNbO₃ films were grown on (001) Si substrates by the RF magnetron sputtering method. Dielectric losses that occur in the Si–LiNbO3 heterostructures are caused by the conductivity of the LiNbO3 films. Analysis of temperature and frequency dependence of ac conductivity in the frequency range f = 25/10⁵ Hz has demonstrated that it is expressed by the power law σ(ω) = Aω^s and is described in the framework of the correlated barrier-hopping model. Thermal annealing (TA) of the Si–LiNbO3 heterostructures causes an increase in the density of the localized states in the band gap of LiNbO3 from D = 7 × 10²⁴ m^?3 to D = 2 × 10²⁵ m^?3. The conduction mechanism is changed radically after TA and phonon-assisted tunneling influences ac conductivity at the frequency of up to 800 Hz. At high frequency (f > 800 Hz), dielectric relaxation predominates affecting frequency dependence σ(ω) on relaxation time τ = 6.6 × 10^?5 s.     

Simulating sedimentation of liquid drops

This work was carried out to investigate the effect of fluid properties on the flow pattern and on the sedimentation velocity of an axisymmetric steady flow of a Newtonian fluid past a liquid drop in an unbounded region. The governing equations of motion were solved by the finite element method. The results show that the flow pattern of a liquid drop depends strongly both on the Reynolds number and on the ratio of the viscosity between the drop and the surrounding flowing fluids. The viscosity ratio in the range 0.02<μ^*<50 has appreciable effect on the drag coefficient. Finally, a correlation for the sedimentation velocity is presented. Copyright © 2004 John Wiley & Sons, Ltd.     

Measurement of the Diffusion Coefficient of Water in RP-3 and RP-5 Jet Fuels Using Digital Holography Interferometry

The diffusion coefficient of water in jet fuel was measured employing double-exposure digital holographic interferometry to clarify the diffusion process and make the aircraft fuel system safe. The experimental method and apparatus are introduced in detail, and the digital image processing program is coded in MATLAB according to the theory of the Fourier transform. At temperatures ranging from 278.15 K to 333.15 K in intervals of 5 K, the diffusion coefficient of water in RP-3 and RP-5 jet fuels ranges from 2.6967?×?10 ^?10 m²·s^?1 to 8.7332?×?10 ^?10 m²·s^?1 and from 2.3517?×?10 ^?10 m²·s^?1 to 8.0099?×?10^?10 m²·s^?1, respectively. The relationship between the measured diffusion coefficient and temperature can be well fitted by the Arrhenius law. The diffusion coefficient of water in RP-3 jet fuel is higher than that of water in RP-5 jet fuel at the same temperature. Furthermore, the viscosities of the two jet fuels were measured and found to be expressible in the form of the Arrhenius equation. The relationship among the diffusion coefficient, viscosity and temperature is analyzed according to the classic prediction model, namely the Stokes–Einstein correlation, and this correlation is further revised via experimental data to obtain a more accurate predication result.     

Plastic flow and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys during hot deformation

Abstract

The hot deformation behaviour and microstructural evolution in Ti–6Al–2Zr–1Mo–1V alloys have been studied using isothermal hot compression tests. The processing map was developed at a true strain of 0·7 in the temperature range 750–950°C and strain rate range 0·001–10 s^?1. The corresponding microstructures were characterised by means of a metallurgical microscope. Globularisation of lamellae occurring to a greater extent in the range 780–880°C and 0·001–0·01 s^?1 had a peak power dissipation efficiency of 58% at about 850°C and 0·001 s^?1. The specimens deformed in 750–880°C and 0·01–10 s^?1 showed an instability region of processing map, whereas the specimens deformed in 880–950°C and 1–10 s^?1 indicated three kinds of flow instabilities, i.e. macro shear cracks, prior beta boundary cracks and flow localisation bands.     

3He-3He and 4He-4He Cross Sections in Matter at Low Temperature

The Galitskii-Migdal-Feynman (GMF) formalism is applied to liquid ³He and (for the first time) to liquid ⁴He. The effective total, diffusion and viscosity cross sections, as well as the effective scattering length and the effective range, are calculated. For liquid ³He, it is found that S-wave scattering dominates for wave number k<0.5 Å^?1. At the Fermi momentum k _F, the effective partial cross section σ _? (and thus the total cross section σ _T) has a singularity (virtual state). This singularity may be interpreted as a signature of superfluidity or a quasi-bound state. For k>2 Å^?1, the effective total cross section is nearly constant. On the other hand, it is found in liquid ⁴He that S-wave scattering dominates for k<0.3 Å^?1, and a peak exists in σ _T arising from a peak in the effective D-wave cross section. This resonance corresponds to a quasi-bound state trapped by the ?=2 centrifugal barrier. The most prominent features of our calculations are a resonance and a Ramsauer-Townsend minimum in the matter cross section at low temperatures. This effect is absent in the ³He gas. It is, therefore, a purely many-body effect in liquid ³He. With increasing energies, the matter results approach the vacuum results. This indicates that the high-energy behavior is dominated by the self-energy contribution; the many-body effects can be neglected.     

Comparative analysis of the factors associated with citation and media coverage of clinical research

Our objective was to study the relationship between the design and content of randomized clinical trials (RCTs) and the subsequent number of citations in the medical literature and attention in online news and social media. We studied RCTs published during 2014 in five highly cited medical journals. This was a retrospective review focusing on characteristics of the individual trials and measures of citation and lay media attention. Primary outcome measures included citation count and Altmetric^® scores (a composite score measuring attention in news, blogs, Twitter^®, and Facebook^®). Two hundred and forty two RCTs were included in the final analysis. Trial characteristics that were positive predictors of citation count included investigation of Hepatitis C treatment (r = 0.35, p < 0.001), private funding (r = 0.24, p < 0.001), mortality-related endpoint (r = 0.22, p < 0.001), and research setting within the United States (r = 0.13, p < 0.001). The trial characteristic that positively predicted Altmetric score was the population size potentially affected (r = 0.39, p < 0.001). The only negative predictor of citation count was the size of the population potentially affected (r = ?0.21, p < 0.001). Negative predictors of the Altmetric score included investigation of Hepatitis C treatment (r = ?0.21, p < 0.001) and private funding (r = ?0.13, p < 0.001). While correlation magnitudes were weak, the predictors of biomedical literature citation and non-academic media coverage were different. These predictors may affect editorial decisions and, given the rising influence of health journalism, further study is warranted.     

Crystallographic, thermoelectric, and mechanical properties of polycrystalline type-I Ba8Al16Si30-based clathrates

Crystallographic, thermoelectric, and mechanical properties of polycrystalline Ba₈Al₁₆Si₃₀-based samples with type-I clathrate structure prepared by combining arc melting and spark plasma sintering methods were investigated. The major phase of the samples was a type-I clathrate with an actual Al/Si ratio of ~15/31, strongly suggesting that framework deficiency was absent or was present in very low concentration in the samples. The Hall carrier concentration n of the samples was approximately 1 × 10²¹ cm^?3, which is lower than the values reported so far for the Ba₈Al₁₆Si₃₀ system. Other important material parameters of the samples were as follows: the density-of-states effective mass m* = 2.3m ₀, Hall mobility μ = 7.4 cm² V^?1 s^?1, and the lattice thermal conductivity κ _L = 1.2 W m^?1 K^?1. The thermoelectric figure of merit ZT reached approximately 0.4 (900 K) for a sample with n = 9.7 × 10²⁰ cm^?3. Simulation using the experimentally determined values of material parameters showed that ZT reached values >0.5 if the carrier concentration is optimized at about 3 × 10²⁰ cm^?3. Young’s, shear, and bulk moduli were estimated to be approximately 98, 39, and 117 GPa, respectively, and Poisson’s ratio was found to be 0.25 from the longitudinal and transverse velocities of sound, v _L = 6038 m/s and v _T = 3503 m/s, respectively, for a sample with ZT = 0.4. The coefficient of thermal expansion (CTE) ranged from approximately 8 × 10^?6 K^?1 to 10 × 10^?6 K^?1 (330–690 K), which is smaller than the values reported for Ba₈Ga₁₆Ge₃₀ and Sr₈Ga₁₆Ge₃₀ clathrates.     

Measurement of thermal diffusivity for alumina borosilicate glass bearing TRISO fuel particles: experiment and modelling correlation

A long-term disposal of a nuclear waste package requires a matrix material that possesses a high thermal conductivity in order to release the decay heat from the fission products. In this study, the thermal diffusivity (α) of alumina borosilicate glass (ABG), encapsulating surrogated tristructural-isotropic (TRISO) particles, has been measured experimentally using the laser flash analysis (LFA) methodology. Image-based models were developed using X-ray computed tomography for glass samples bearing different proportions of TRISO particles. Simpleware software generated finite element (FE) models which then were solved using Abaqus software. The matrices were examined at different waste loadings (10, 20 and 30 wt%) at a temperature of 50 °C. The modelling results were in close agreement with the experimental results and the deviations were within the bounds of standard numerical error (<5 %). The thermal diffusivity of the samples was increased with increasing proportion of TRISO particles. For the maximum mechanically stable matrix, the thermal diffusivity was found to be 0.92 ± 0.021 × 10^?6 and 0.94 × 10^?6 m² s^?1 measured by LFA and modelling, respectively.     

Microstructure and thermoelectric properties of tungsten trioxide ceramics doped with a low amount of terbium dioxide

The effect of the nonstoichiometric compound terbium dioxide (Tb₄O₇) on the thermoelectric properties of tungsten trioxide (WO₃) ceramics was investigated. Among the sintered ceramics, the sample doped with 0.1 mol% Tb₄O₇ showed the maximum grain size and density. Doping with Tb₄O₇ also increased the electrical conductivity (σ) of the ceramics by about two orders of magnitude, and the sample doped with 0.1 mol% Tb₄O₇ showed the highest electrical conductivity. The absolute value of Seebeck coefficient (|S|) of the doped samples increased as well. Consequently, the power factor (σs ²) markedly increased. The sample doped with 2.0 mol% Tb₄O₇ demonstrated the maximum σs ² of 2.88 μW m^?1 K^?2 at 973 K, which was larger than the highest recorded σs ² for WO₃ ceramics (2.71 μW m^?1 K^?2 at 1,023 K). In addition, the low-doped sample (0.1 mol%) exhibited excellent thermoelectric properties.     

The Hot Workability of SiCp/2024 Al Composite by Stir Casting

The hot workability of SiC_p/2024 Al composite was explored by conducting hot compression simulation experiments on Gleeble-3500 under temperatures of 300–500 °C and strain rates of 10^?3–1 s^?1. Constitutive equation was developed through hyperbolic sine function, and the activation energy was calculated to be 151 kJ mol^?1. The hot processing maps referring to dynamic material model were drawn in a true strain range from ?0.2 to ?0.8. At the strain of ?0.8, the recommended regions in processing map contained two domains: superplastic domain (500 °C, 10^?3 s^?1) with an efficiency of about 0.72 and DRX domain (500°C, 1 s^?1) with an efficiency of about 0.45. Together with macrostructure and microstructure observations, it was suggested to remove the DRX region.     

Thermometry in Normal Liquid 3He Using a Quartz Tuning Fork Viscometer

We have developed the use of quartz tuning forks for thermometry in normal liquid ³He. We have used a standard 32 kHz tuning fork to measure the viscosity of liquid ³He over a wide temperature range, 6 mK<T<1.8 K, at SVP. For thermometry above 40 mK we used a calibrated ruthenium oxide resistor. At lower temperatures we used vibrating wire thermometry. Our data compare well with previous viscosity measurements, and we give a simple empirical formula which fits the viscosity data over the full temperature range. We discuss how tuning forks can be used as convenient thermometers in this range of temperatures with just a single parameter needed for calibration.