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This paper investigates the focal location effects on the penetration depth of molten region surrounding a paraboloid of revolution-shaped cavity (i.e. keyhole of this model) irradiated by a moving focused energy beam, which profile of intensity is assumed to be Gaussian distribution. Considering the momentum balance at the base of the keyhole, a quasi-steady-state thermal model relative to a constant-speed moving high-energy beam and paraboloid of revolution-shaped cavity is developed in a parabolic coordinate system. The analytical solution is obtained for this model with the adiabatic condition directly set on the workpiece surface for semi-infinite domain instead of the image method for infinite domain using the separation-of-variables method. The analytical solution of this model gives a reasonable prediction for the cavity temperatures. The predicted relation of the penetration depth to the focal location agrees with the available measured data. The effects of focal convergence angle and spot size on the penetration depth are also discussed.
The effects of stress-aging processing on corrosion resistance of an Al–Zn–Mg–Cu alloy were investigated. It is found that the one-stage stress-aged alloy is strongly sensitive to the electrochemical corrosion. The poor corrosion resistance of the one-stage stress-aged alloy can be attributed to fine intragranular aging precipitates and continuous distribution of grain boundary precipitates. Meanwhile, the incomplete precipitation of solute atoms results in high electrochemical activity of aluminum matrix. However, when the alloy is two-stage stress-aged, the corrosion resistance is greatly improved. Furthermore, the corrosion resistance decreases firstly and then increases with increasing the first stage stress-aging temperature. Increasing external stress can enhance the corrosion resistance of the two-stage stress-aged alloy. These phenomena are mainly related to aging precipitates within grains and along grain boundaries. The coarse and relatively low-density intragranular aging precipitates, as well as the discontinuously distributed grain boundary precipitates can enhance the corrosion resistance of the stress-aged alloy.
Generally, the obvious work hardening, dynamic recrystallization (DRX), and dynamic recovery behaviors can be found during hot deformation of Ni-based superalloys. In the present study, the classical dislocation density theory is improved by introducing a new dislocation annihilation item to represent the influences of DRX on dislocation density evolution for a Ni-based superalloy. Based on the improved dislocation density theory, the peak strain corresponding to peak stress and the critical strain for initiating DRX can be determined, and the improved DRX kinetics equations and grain size evolution models are developed. The physical framework and algorithmic idea of the improved dislocation density theory are clarified. Moreover, the deformed microstructures are characterized and quantitatively correlated to validate the improved dislocation density theory. It is found that the improved dislocation density-based models can precisely characterize hot deformation and DRX behaviors for the studied superalloy under the tested conditions.
The effects of pre-treatments on the precipitate microstructures of an Al–Zn–Mg–Cu alloy are investigated. Meanwhile, the creep-rupture behavior of the under-aged and peak-aged alloys are comparatively analyzed. Additionally, the effects of pre-treatment on the fracture mechanisms are discussed. It is found that the precipitate microstructures are sensitive to pre-treatments. The intragranular precipitates of the peak-aged alloy are larger than those of the under-aged. The precipitate free zone of the peak-aged alloy is wider than that of the under-aged. Some large intergranular precipitates appear on the grain boundaries of the under-aged alloy, and induce the nucleation of microvoids. Eventually, the creep fracture of the under-aged alloy is accelerated. Therefore, the differences in microstructures lead to the shorter creep-rupture life of the under-aged alloy, compared to the peak-aged alloy.
Gamma Ray Bursts (GRBs) have puzzled astronomers since their discovery more than 20 years ago. As no counterparts at wavelengths other than X- and γ-rays have yet been found the identification of the sources is still missing. Theoretical explanations range from colliding comets (1993) and merging neutron stars (1982) to more exotic objects, such as superconducting cosmic strings (1988). Data accumulated until now still do not discriminate between these models, although results from the BATSE (Burst and Transient Source Experiment) instrument aboard the Compton Gamma Ray Observatory (CGRO) strongly favor extragalactic models.
The Energetic Gamma Ray Experiment Telescope (EGRET) aboard CGRO has s ofar detected photons from 5 GRBs with its spark chamber. These are the highest energy γ-rays associated with GRBs to date. In this work we review previously published data and summarize the properties of these events. Elsewhere we present possible constraints from the data on the models proposed to explain GRBs.
The strain hardening effect and dynamic recovery behavior of a Ni-based superalloy are studied by isothermal compressive tests. A new unified dislocation-density based constitutive model is developed to characterize the strain hardening effect and dynamic recovery behavior of the studied superalloy. In the developed constitutive model, some material parameters (yield stress, strain hardening coefficient, and dynamic recovery coefficient) are assumed as functions of initial grain size, deformation temperature, and strain rate. An iterative algorithm is designed to predict the high-temperature deformation behaviors under time-variant hot working conditions. The hot deformation parameters and material parameters can be updated in each strain increment. Comparisons between the experimental and calculated flow stresses indicate that the developed constitutive model can accurately describe the high-temperature deformation behavior of the studied superalloy. Furthermore, the developed constitutive model is also successfully used for analyzing time-variant hot working processes.
The Y‐Ba‐Cu‐0 (YBCO) films were grown on (100)Mg0 substrate by the high pressure DC diode sputtering process. The targets were YBCO compounds made by solid‐state reaction. The sputtering gas was Ar‐50%02, and total pressure was 1.5 torr. As‐deposited superconducting YBCO films can be prepared at low substrate temperature at high discharge current. For Y1Ba2Cu30x target, the atomic ratio Ba/Y in the films almost remains constant (Ba/Y = 1.7) and Cu content monotonically increases with increasing discharge current. The Cu content in the film approaches to that of the target at low discharge current. Concentrations of 0 and 02+ in plasma markedly increase during increase of discharge current. High Cu content at large discharge current may be caused by action of electrical field on Ba+ and Y+.
An AlGaAs/InGaAs HEMT grown on Si substrate with Ge/GexSi1−x buffer is demonstrated. The Ge/GexSi1−x metamorphic buffer layer used in this structure was only 1.0 μgm thick. The electron mobility in the In0.18Ga0.82 As channel of the HEMT sample was 3,550 cm2/Vs. After fabrication, the HEMT device demonstrated a saturation current of 150 mA/mm and a maximum transconductance of 155 mS/mm. The well behaved characteristics of the HEMT device on the Si substrate are believed to be due to the very thin buffer layer achieved and the lack of the antiphase boundaries (APBs) formation and Ge diffusion into the GaAs layers.
In the present study, pure titanium (Ti) plates were firstly treated to form various types of oxide layers on the surface and then were immersed into simulated body fluid (SBF) to evaluate the apatite-forming ability. The surface morphology and roughness of the different oxide layers were measured by atomic force microscopy (AFM), and the surface energies were determined based on the Owens–Wendt (OW) methods. It was found that Ti samples after alkali heat (AH) treatment achieved the best apatite formation after soaking in SBF for three weeks, compared with those without treatment, thermal or H2O2 oxidation. Furthermore, contact angle measurement revealed that the oxide layer on the alkali heat treated Ti samples possessed the highest surface energy. The results indicate that the apatite-inducing ability of a titanium oxide layer links to its surface energy. Apatite nucleation is easier on a surface with a higher surface energy.
The isothermal oxidation behavior of Zr2Al3C4 in the temperature range of 500 to 1000 °C for 20 h in air has been investigated. The oxidation kinetics follow a parabolic law at 600 to 800 °C and a linear law at higher temperatures. The activation energy is determined to be 167.4 and 201.2 kJ/mol at parabolic and linear stages, respectively. The oxide scales have a monolayer structure, which is a mixture of ZrO2 and Al2O3. As indicated by x-ray diffraction and Raman spectra, the scales formed at 500 to 700 °C are amorphous, and at higher temperatures are α-Al2O3 and t-ZrO2 nanocrystallites. The nonselective oxidation of Zr2Al3C4 can be attributed to the strong coupling between Al3C2 units and ZrC blocks in its structure, and the close oxygen affinity of Zr and Al.
Layered stacking characteristics of ternary Zr–Al–C carbides were investigated using scanning transmission electron microscopy (STEM). Three previously unknown compounds, i.e., Zr4Al3C6, Zr5Al6C9, and Zr7Al6C11 were identified. The present study extends the structural information of ternary Zr–Al–C ceramics. The influence of the thickness of the NaCl-type Zr-C slab on the elastic properties of ternary Zr–Al–C ceramics is discussed based on first-principles calculations. In addition, direct atomic-resolution observations illustrate the process for forming the unique layered crystal structures of ternary Zr–Al–C ceramics. These results also provide insights into the formation mechanism of layered ternary Zr–Al–C carbides.
Bulk Ta4AlC3, a new layered-ternary carbide in the Ta–Al–C system, was synthesized and characterized. Transmission electron microscopy investigations on this new phase are reported here. Selected area electron diffraction and convergent beam electron diffraction analyses indicated that this ternary carbide crystallized with the space group P63/mmc. Atomic-scale microstructures of Ta4AlC3 were achieved by means of high-resolution transmission electron microscopy and Z-contrast scanning transmission electron microscopy. The experimental crystal structural parameters agreed well with the theoretical values obtained using density-functional calculations.
An electromigration failure mechanism in flip chip solder joints is reported in this communication. The solder joints failed by a very rapid, asymmetrical, and localized dissolution of the Cu metallization on the cathode side. The average dissolution rate was about 1 μm/min. The dissolved Cu included not only the Cu under bump metallurgy but also the on-chip Cu conducting trace. From the location and geometry of the dissolved Cu, it can be concluded that current crowding plays a critical role in the rapid dissolution. The dissolved Cu atoms were driven to the anode side by electromigration, and a large amount of Cu6Sn5 was formed there.
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