Showing papers by "University of Science and Technology Beijing published in 2000"

Fluid flow and inclusion removal in continuous casting tundish

Abstract: Three-dimensional fluid flow in continuous casting tundishes with and without flow control devices is first studied. The results indicate that flow control devices are effective to control the strong stirring energy within the inlet zone, and other zones are with much uniform streamline. By dividing tundish into two zones with different inclusion removal mechanisms the inclusion removal is calculated. Three modes of inclusion removal from molten steel in the tundish, i.e., flotation to the free surface, collision and coalescence of inclusions to form larger ones, and adhesion to the lining solid surfaces, are taken into account. The Brownian collision, Stokes collision, and turbulent collision are examined and discussed. The suitable coagulation coefficient is discussed, and a value of 0.18 is derived. Calculation results indicate that, besides flotation, collision of inclusion and adhesion to the lining solid surfaces are also important ways for inclusion removal from molten steel in tundish especially for the smaller inclusions. The flotation removal holds 49.5 pct, and the adhesion removal holds 29.5 pct for the tundish with flow control devices; the collision effect is reflected in improving flotation and adhesion. Finally, industrial experiment data are used to verify the inclusion removal model.

Nanomechanics of individual carbon nanotubes from pyrolytically grown arrays.

Abstract: The bending modulus of individual carbon nanotubes from aligned arrays grown by pyrolysis was measured by in situ electromechanical resonance in transmission electron microscopy (TEM). The bending modulus of nanotubes with point defects was approximately 30 GPa and that of nanotubes with volume defect was 2-3 GPa. The time-decay constant of nanotube resonance in a vacuum of 10(-4) Torr was approximately 85 micros. A femtogram nanobalance was demonstrated based on nanotube resonance; it has the potential for measuring the mass of chain-structured large molecules. The in situ TEM provides a powerful approach towards nanomechanics of fiberlike nanomaterials with well-characterized defect structures.

Morphologies of GaN one-dimensional materials

Abstract: GaN one-dimensional materials with different morphologies were formed on LaAlO3 crystal, silicon crystal and quartz glass substrates through a simple sublimation method. They were characterized by powder X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray (EDX) spectroscopy. FE-SEM images showed that the morphologies of the one-dimensional materials included straight nanorods, curved nanowires, nanoribbons, zigzag nanorods and beaded or capture-tree nanorods. XRD and EDX studies indicated that all the one-dimensional materials were wurtzite GaN.

Modeling of Oxidation Kinetics of Y-Doped Fe–Cr–Al Alloys

Abstract: Studies using advanced analytical techniques indicated that the reactiveelements (RE) segregate along the oxide grain boundaries and at theoxide–alloy interface during oxidation of α-Al2O3forming alloys. The segregation results in inward oxygen diffusion along theoxide grain boundaries as the predominant transport process in the oxidegrowth. The present work establishes a mathematical model based on themechanisms of inward oxygen diffusion along the grain boundaries and oxidegrain coarsening. This model has been used to describe the oxidationkinetics of Y-doped Fe–Cr–Al alloys. The results showed a muchbetter agreement with the experimental data than the parabolic rate law. Byusing this model, the exponential number for the grain coarsening of aluminascales during oxidation was calculated to be ∼3. The activation energyfor oxygen diffusing along the grain boundaries was 450 kJ/mol. They arealso in good agreement with values reported in the literatures.

In-situ transmission electron microscopic observation of corrosion-enhanced dislocation emission and crack initiation of stress corrosion

Abstract: A constant deflection device designed for use within a transmission electron microscope (TEM) was used to study the change in dislocation configuration ahead of a crack tip during stress corrosion cracking (SCC) of brass in water, Ti-24% Al-11% Nb alloy in methanol (CH3OH), and the initiation of SCC. In-situ tensile tests in the TEM also were carried out to assess deformation without the influence of environment. Results showed that corrosion during SCC enhanced dislocation emission, multiplication, and motion as well as produced a dislocation-free zone (DFZ). Nanocracks of SCC initiated in the DFZ or from the crack tip when the corrosion-enhanced dislocation emission and motion reached a certain condition. The action of the corrosion process prompted nanocrack propagation into a cleavage or intergranular microcrack rather than blunting into a void as seen during experiments in the TEM.

Thermodynamics and microstructure of Ti–ZrO2 metal–ceramic functionally graded materials

Abstract: The standard Gibbs free energy functions of Ti2O and Ti5O9 have been predicted according to the quasi-parabola rule. To determine the synthesis conditions of Ti–ZrO2 composite, the thermodynamic stability of various oxides in Ti–Zr–O system and the co-existing region of Ti and ZrO2 have been obtained by thermodynamic calculation. According to the thermodynamic analysis, it can be deduced that Ti could probably react with ZrO2 in the synthesis processes of Ti–ZrO2 functionally graded materials (FGMs). But the experimental results show that there is no reaction product in the Ti/ZrO2 interfaces and the bonding state between Ti and ZrO2 is physical in the composite. ZrO2 (3 mol.% Y2O3-stabilized zirconia) mainly exists as tetragonal phase in the prepared Ti–ZrO2 FGMs. The presence of Ti exerts an important influence on the transformation of ZrO2 from tetragonal to monoclinic phase. Ti–ZrO2 system functionally graded materials (FGMs) have been prepared by powder metallurgical method. The microstructural features of Ti–ZrO2 system FGMs have been appropriately interpreted by the heat percolation model.

Grain boundary segregation in ultra-low carbon steel

Abstract: Segregation of phosphorus and sulfur to grain boundaries in ultra low carbon steel (IF-steel) was investigated by Auger electron spectroscopy (AES) and scanning electron microscopy (SEM). The segregation kinetics of phosphorus and sulfur while holding at 600 degrees C have the characteristics of non-equilibrium segregation and a maximum peak was presented for each element in the initial hour, which could be reasonably explained by diffusion of phosphorus-vacancy complexes and sulfur-vacancy complexes, respectively. With longer holding at 600 degrees C, the concentration of phosphorus would increase and then approached an equilibrium value. whereas the concentration of sulfur almost remained at a low level. Temperature dependence of phosphorus and sulfur concentration on grain boundaries showed that the concentration of phosphorus decreased continuously with increasing temperature: however, the concentration of sulfur increased slightly, which could be explained by site competition and chemical interaction between phosphorus and sulfur. Interaction between phosphorus and sulfur was evaluated using the Guttmann model of a regular solution with size competition for the ternary system. It clearly reveals that the repulsive interaction between phosphorus and sulfur in grain boundaries is very strong and that the interaction coefficient alpha p(s) decreases with increasing temperature. From the interactive coefficient, the free energies of segregation of sulfur at 600 and 700 degrees C were approximately estimated. (C) 2000 Elsevier Science S.A. All rights reserved.

In-situ TEM study of fracture mechanisms of polysynthetically twinned (PST) crystals of TiAl alloys

Abstract: The fracture mechanisms of polysynthetically twinned (PST) crystals of Ti–49at.% Al in different lamellar orientations have been investigated by using in-situ transmission electron microscopy (TEM). The results indicated that the fracture mechanisms and crack propagation behavior depended strongly on the lamellar orientation of PST crystals. When the tensile axis was nearly parallel to the lamellae, the main crack propagated discontinuously by nucleation, growth and linkage of microcracks formed ahead of crack tip. Slip band decohesion, fracture along prism plane of α2 plates and twinning-induced microcracks are three modes of nucleation of microcracks. When the loading axis was perpendicular to the lamellae, the crack parallel to the lamellae propagated by tearing and shear of shear ligament. Interfacial microcrack usually occurred ahead of the main crack tip. Sometimes, the main crack propagated along (111) cleavage plane within γ lamellae.

The influence of sulfur on stress-rupture fracture in inconel 718 superalloys

Abstract: The effects of sulfur, with content variations of 15 to 175 ppm, on the stress-rupture and tensile properties in nickel-base alloy 718 are reported. The stress-rupture life dramatically decreased with increasing sulfur content. This was especially noticeable in the ductility loss at 650 °C. Auger electron spectroscopy of stress-rupture tested specimens provided direct evidence of sulfur and phosphorus segregation to grain boundaries and carbide/matrix interfaces. The stress-rupture life and fracture morphology were both found to correlate with the segregation of sulfur at grain boundaries in alloy 718. Sulfur was also preferentially segregated at the carbide matrix surfaces, and phosphorus was found to be distributed on grain boundaries. However, the phosphorus segregation did not correlate with stress-rupture behavior. Sulfur contents in the range of 15 to 50 ppm had little effect on the stress-rupture life. However, the stress-rupture life decreased dramatically with increasing sulfur content above 50 ppm.

3‐D RAY Tracing Method Based on Graphic Structure

Abstract: The shortest ray path method based on graphic structure overcomes the drawbacks and limitations of traditional ray-tracing methods, but the computation time and accuracy of the results need to be improved This paper introduces Bresenham line algorithm into the traveltime calculating of nodes to improve the accuracy of traveltime and ray path results To speed up computation, it uses QuickSort algorithm and Insertion Sort algorithm instead of usually used HeapSort algorithm We take advantage of the node configuration presented in this paper to introduce the velocity interface, and accordingly get the algorithm of reflection ray tracing Numerical tests proved that it is an flexible and efficient ray tracing method

Stress Corrosion Cracking Caused by Passive Film-Induced Tensile Stress

Abstract: α-Ti foil with a protective layer formed on one side was deflected as a result of corrosion in a methanol (CH3OH) solution containing 0.6 mol/L potassium chloride (KCl) because of tensile ...

A study of nickel-based corrosion resisting alloy layer obtained by double glow plasma surface alloying technique

Abstract: In this paper, Ni–Cr–Mo–Nb multi-element surface alloying has been investigated on a substrate of three kinds of industrial steels (industrial pure iron, low carbon steel and 304 stainless steel) by means of the double glow plasma surface alloying technique The results showed that highly alloyed nickel-based surface alloying layers were formed on the surface of the three kinds of steels The alloying layer was uniform, continuous and compact The alloying layers consisted of a γ phase or γ matrix with several precipitates (P or Laves phase, and NbC carbide, etc) A method of eliminating these precipitates has also been discussed The corrosion resistance of the nickel-based surface alloying layer was investigated by an electrochemical method in a 35% NaCl and 5% HCl solution, and 200-h exposure tests in a 20% H2SO4 and 20% HCl solution The corrosion resistance of surface alloying layer formed on stainless steel substrate was better than that of the nickel-based alloy Inconel 625 and 304 stainless steel The experimental results indicated that this is an effective way to obtain a high quality corrosion resisting surface alloying layer on the surfaces of steels using the double glow plasma surface alloying technique

Interdiffusion involved in SHS welding of SiC ceramic to itself and to Ni-based superalloy

Abstract: In order to reach a better understanding of the mechanism of joining of SiC ceramic to itself and to Ni-based superalloy by pressure-aided self-propagating high-temperature synthesis (SHS) welding process, microstructural study and energy dispersive X-ray microanalysis (EDX) were carried out with the welded samples. The wettability between SiC ceramic and the liquid reaction products of the filler is very good. A composition transition layer forms at the SiC/filler interface mainly due to the diffusion of Ti from the filler to the ceramic. The affinity between Ti and SiC is better than that between Ni and SiC. The components of the Ni-based superalloy remarkably diffuse into the reaction products of the filler. The microstructure shows an excellent bonding at the interfaces.