Showing papers by "University of Science and Technology Beijing published in 2002"

Rapid synthesis of dense Ti3SiC2 by spark plasma sintering

Abstract: Ti3SiC2 was rapidly synthesized and simultaneously consolidated from the starting mixture of Ti/Si/2TiC by spark plasma sintering (SPS) An intensive reaction leading to the formation of Ti3SiC2 occurred at the measured temperature of around 1200 °C, which is several hundreds degrees lower than that of conventional reactive hot pressing The phase composition of the product could be tailored by adjusting the process parameters An axisymmetric preferred orientation of the Ti3SiC2 grains with well-developed (008) planes was formed, resulting in an anisotropic hardness in respect to the textured product

Permanence of an SIR epidemic model with distributed time delays

Abstract: We consider permanence of an SIR epidemic model with distributed time delays. Based on some known techniques on limit sets of differential dynamical systems, we show that, for any time delay, the SIR epidemic model is permanent if and only if an endemic equilibrium exits.

Effect of boron addition on structure and properties of low carbon bainitic steels

Abstract: The effect of trace boron in steels, especially the influence of boron on microstructure and properties of low carbon bainitic steels were summarized. It was pointed out that the hardenability of boron steels is determined by non-equilibrium boron segregation on grain boundaries. The synergistic effect of boron with other elements can enhance the boron effect. The abnormal boron segregation on the moving new grain boundaries is the basic reason of the boron retarding the recrystallization of deformed austenite, the addition of copper and niobium will increase this effect. The addition of boron accelerates the strain induced niobium carbonitride precipitation, and trace boron can refine the microstructure and improve the mechanical properties of HAZ. The mechanisms of boron effect were also expounded preliminarily in this paper.

Effect of Al content and Nb addition on the strength and fault energy of TiAl alloys

Abstract: The yield strengths of Ti–45Al–xNb and Ti–49Al–xNb (x=0, 10) with a nearly-equiaxed gamma microstructure were measured at 900 °C. Nb addition was found to largely increase the yield strengths but the variation of Al content has little influence on the yield strength of binary TiAl alloys. To understand the strengthening effect of Nb, the stacking fault energies in these alloys were measured using weak-beam transmission electron microscope (TEM) techniques. The SISF energy decreases significantly with the decreasing of Al content of γ phase in binary alloys but is practically independent of the Al content in ternary alloys. The implications of the dissociation behavior on the mechanical properties of the material will be discussed.

Deformation enhanced transformation and dynamic recrystallization of ferrite in a low carbon steel during multipass hot deformation

Abstract: Multipass hot deformation with an industry-acceptable pass reduction has been investigated by hot compression. A fine ferrite grain size can be achieved by carefully controlling the interaction of the deformation-enhanced transformation and dynamic recrystallization of ferrite. By the development of a ferrite grain size of 3–5 μm, the yield strength of Q235 grade plain carbon steel can be doubled, without significant loss of ductility, compared with material produced by conventional processing.

Ab initio interionic potentials for NaCl by multiple lattice inversion

Abstract: Based on the Chen-Mobius lattice inversion and a series of pseudopotential total-energy curves, a different method is presented to derive the ab initio interionic pair potentials for B1-type ionic crystals. Comparing with the experimental data, the static properties of B1- and B2-type NaCl are well reproduced by the interionic potentials. Moreover, the phase stability of B1-NaCl has been described by the energy minimizations from the global deformed and disturbed states. The molecular-dynamics simulations for the molten NaCl indicate that the calculated mean-square displacements, radial distribution function, and diffusion coefficients gain good agreements with the experimental results. It can be concluded that the inversion pair potentials are valid over a wide range of interionic separations for describing the structural properties of B1-type ionic crystals. Etot1 ~ a!5ENa-Cl1 ~ a!1ENa-Na ~ a!1ECl-Cl ~ a!1Eiso , ~1! where the E Na-Na (a) and E Cl-Cl (a) are the like-ion contribu- tions on fcc ~face-centered-cubic! sublattice, Eiso is the en- ergy of isolated ions, which is independent on the interionic separation. Only with the Etot1 (a), we cannot separately cal- culate each item of the right side in Eq. ~1!, so three other virtual structures are introduced with regard to B1, which may not exist in nature. Their ab initio total energies could be calculated for a series of lattice constant a. 16 Based on the differences between these total energies, the partial lattice energy as a function of only one kind of interionic pair po- tential can be obtained. It is worth noting that the virtual structures should be constructed properly. For example, in order to derive the Na-Cl pair potential, if a Na fcc and a Cl fcc lattices constitute a B3-NaCl, its total energy is Etot3 ~ a!5ENa-Cl3 ~ a!1ENa-Na ~ a!1ECl-Cl ~ a!1Eiso , ~2! with the identical lattice constant a, the difference between B1 and B3 structures is only the relative displacement of cation and anion sublattices. From B 1t oB3, the Na-Na and Cl-Cl distances are unaffected by this displacement, and their total-energy difference is only about the Na-Cl interactions and can be expressed as DE Na-Cl~ a!5E tot1 ~ a!2E tot3 ~ a!5E Na-Cl1 ~ a!2E Na-Cl3 ~ a!. ~3!

A Coupled Mathematical Model of Microsegregation and Inclusion Precipitation during Solidification of Silicon Steel

Abstract: A coupled mathematical model of microsegregation and inclusion precipitation during solidification is established. The model can be used to calculate the kind of precipitated inclusion, the amount of inclusion precipitation and the growth of inclusion during solidification. The calculation by this model shows that the segregation degree of solute elements is suppressed greatly due to the precipitation of inclusions during solidification. A non-coupled model will overestimate the segregation degree and the amount of inclusion precipitation and even give a wrong prediction for the kind of the inclusions. In this paper, the effects of cooling rate on the precipitation and growth of oxides are also discussed.

Interfacial segregation and cosegregation behaviour in a nickel-base alloy 718

Abstract: The Auger analyses provided clear evidence of sulfur and phosphorus segregation to grain boundaries and carbide/matrix interfaces. The degree of segregation was different for the two elements. Sulfur was preferentially segregated at the carbide/matrix interface while phosphorus was mainly distributed on grain boundaries. It was also found that metal elements, such as Mo and Nb also segregated to grain boundaries, these grain boundary segregation results show the existence of the attractive interactions of phosphorus with metal elements of Mo and Nb at grain boundaries, especially the co-segregation of Mo–P. It did not find any interaction of sulphur with metal elements at grain boundaries.

Thermodynamics on the formation of spinel (MgO·Al2O3) inclusion in liquid iron containing chromium

Abstract: The stability diagram of MgO, spinel solid solution (MgO·(Al X Cr1−X )2O3), and sesquioxide solid solution ((Al Y Cr1−Y )2O3) as a function of Mg, Al, and O contents at a constant chromium content (18 mass pct) in liquid iron is drawn at 1873 K. The interaction parameters between Mg and other solutes (Al, Cr, Ni, Ti, Si, and C) are determined by the experimental method, which assures equilibrium between Mg vapor and liquid iron, were applied to calculate the diagram. Titanium deoxidation is not recommended for the prevention of spinel formation, because Ti accelerates Mg dissolution from refractory or slag due to its high affinity for Mg (e Mg Ti = − 0.64). The standard Gibbs free energies of formation for the three inclusions (periclase, spinel, and sesquioxide solid solutions) and the tielines between two solid solutions were calculated with the aid of the regular solution model and the thermochemical F*A*C*T database computing system, respectively. The phase stability regions and oxygen content in steel for the current Fe-Mg-Al-Cr (18 mass pct)-O system are compared with those of the previous non-Cr system. Detailed information on the spinel composition according to Mg and Al contents is also available from the present stability diagram.

Deformation mechanism at large strains in a high-Nb-containing TiAL at room temperature

Abstract: This paper investigates the deformation behavior and nano-scale structural characteristics after large deformation (29%) at room temperature of a Ti–45Al–9Nb–2.5Mn (at.%) alloy with duplex microstructure. Dislocation glide is the main deformation mode at low strains (such as 2%). The interaction of moving dislocations reduces the mobility of dislocations by pinning, and when deformation strain reachs 5%, both dislocation glide and mechanical twining are active. With further increase in strain, deformation twining becomes the dominant deformation mode, dividing the original γ grains and lamellar structure into nano-scale. The characteristic feature of the deformation twin in γ grains after large deformation at room temperature is the bent deformation twin (DT or DT′). There are many sub-structures in the DT, which makes the (111)T not exactly straight. High resolution TEM observations showed that (111)DL plane of the DT twins exhibits a large misorientation angle with the (111)M plane of the γ matrix. The orientation relationship between the matrix and the DT or DT′ is actually not the expected true twin relationship. The existence of numerous 1/3[111] Frank partial dislocations was regarded as accommodating the bending of the boundaries of DT. Local zigzag bending of the barrier twin boundaries can be observed due to twin intersections. Intersection induced lattice distortion in the intersection area for type I twin intersection is more severe than that for type II intersection. As results of the relaxation of the heavy lattice distortion, nano-scale subgrains boundaries may form together with a rectangular region near the intersection area. Nanotwins could be observed in the rectangular region for type I intersection. The nanotwins are formed with a homogeneous 1/6[11 2 ] twinning dislocation glide mechanism. For type II twin intersection, the intersection induces the formation of secondary twinning ST in the barrier twin. No region with nanotwins can be observed near the intersection area. The local stress concentration in type II twin intersection may be relaxed by the emission of dislocations from the intersection area.

A comparison in performance of diamond coated cemented carbide cutting tools with and without a boride interlayer

Abstract: Instead of depleting cobalt from the surface of cemented carbide substrates or introducing a cobalt-free interlayer between the substrates and diamond coatings, a boride interlayer could be employed to modify the catalytic characteristics of cobalt in the substrate surface, preventing cobalt from interfering with the deposition of diamond coatings. This paper compares deposition of diamond coatings and performance of diamond coated cemented carbide cutting tools, with and without such a boride interlayer. We will show that a profound effect of using the interlayer is that it allows the deposition of adherent diamond coatings within a relatively broad temperature range, which improves reproducibility of the coating processes. With the interlayer present, an increase in tool life of 20-fold has been obtained, in comparison with that of uncoated tools. Such an increase in tool life can be attributed to the improved adhesion of the diamond coatings.

The Influence of Ti on the Hot Ductility of Nb-bearing Steels in Simulated Continuous Casting Process

Abstract: The hot ductility of as-cast Nb, Ti and Nb-Ti bearing low-carbon steels has been assessed. Hot tensile testing of in-situ melted specimens, replica examinations and thermodynamic modeling showed that addition of 0.014-0.04% Ti to 0.1 %C-0.03%Nb-0.005% N steel leads to a large volume fraction of fine strain-induced precipitates at temperatures up to 1 000°C which seriously deteriorate the hot ductility, in spite of the type of prior thermal history. Generally, three types of precipitates, i.e. coarse boundary precipitate, coarse frond-like and fine strain-induced precipitates, were found in Ti-Nb microalloyed steels with the different sizes and compositions, and the latter could be described by thermodynamic modeling. The current results are different from the previous ones published and a thermodynamic model was employed to explain this discrepancy. Accordingly, the beneficial effect of Ti can only be achieved in some instances at high nitrogen and low titanium contents, i.e. a low Ti/N ratio, which greatly encourages coarse precipitation at high temperatures and reduces the fine strain-induced precipitation. Therefore, only for electric arc steels typically with a high nitrogen level, a small addition of Ti might be considered to be advantageous to the hot ductility.