Showing papers by "University of Science and Technology Beijing published in 1994"

Structure and growth of platelets in graphite spherulites in cast iron

Abstract: The structure and orientation of graphite platelets in graphite spherulites (GS) in cast irons modified by either cerium or magnesium plus cerium were investigated by TEM and micro-diffraction. The platelets have a rhombohedral structure with their [001] directions nearly parallel to the radius of the GS, but each platelet is twisted about 2 ° over a 1 μm length. Randomly orientated graphite is located in the interplatelet areas, in which most of the graphite has a hexagonal structure rather than rhombohedral structure. Based on the crystallographic characterization of the graphite and the cerium and magnesium elemental distribution in the GS, the model for the growth of GS as originally proposed by Double and Hellawell is re-examined.

Modeling of the influence of the abrasive waterjet cutting parameters on the depth of cut based on fuzzy rules

Abstract: Currently, the abrasive waterjet cutting parameters for the milling operation have to be determined by a combination of prior experience and trial and error. It is shown that the selection of the abrasive waterjet cutting parameters for a required depth of cut in the given material can be effectively done by applying the principles of the fuzzy set theory. This approach will eliminate the need for extensive experimental work in order to select the magnitudes of the most influential abrasive waterjet parameters on the depth of cut. Fuzzy logic provides a methodology and imitation of a human's way of making decisions which is very useful in such applications where the mathematical model of the process does not exist, and one of such processes is indeed abrasive waterjet cutting. A number of case studies are performed to verify the validity of the proposed methodology for selecting the abrasive waterjet cutting parameters in order to achieve the predetermined depth of cut.

Stress-field pinning induced by lattice mismatch in Y-123 superconductors

Abstract: A pinning mechanism of a stress field has been proposed in high-Tc superconductors. The studies focus on the interaction between magnetic flux and lattice mismatch induced by substitution of Ho for Y in YBa2Cu3O7−δ. By establishing an effective defect model, the stress distribution, centered on the substitutional dopant Ho, is briefly described. Moreover, a calculation on the pinning strength has been performed for the flux lines in such a stress field. The calculated result further demonstrates that rare-earth dopants for Y can become the effective pinning centers under the appropriate doping level.

Stress Corrosion Cracking of Type 304 Stainless Steel Weldments in the Active State

Abstract: Slow strain rate tests (SSRT) were conducted in solutions of hydrochloric acid (HCl) + sodium chloride (NaCl) at ambient temperature on type 304 (UNS S30400) stainless steel (SS) weldments that exhibited a duplex ferrite-austenite structure in the weld fusion zone. Results indicated the weld fusion zone corroded preferentially. Stress corrosion cracking (SCC) initiated and propagated along the ferrite-austenite interphase. Austenitic dendrite was observed by scanning electron microscopy (SEM) in the fracture surface morphology. This interfacial cracking was attributed to the formation of a complex cell structure consisting of weld fusion zone/parent metal with delta-ferrite/austenite in the weld zone. The delta-ferrite was microanodic phase. The proposed model of SCC for type 304 SS weldments in HCl + NaCl was film formation-slip (film rupture)-dissolution-crack propagation. Because of the presence of the complex cell structure, the surface film was nonuniform, which was favorable for crack initi...

Correlation-effects, generalized brownian-motion and anomalous diffusion

Abstract: We investigate the diffusion motion of a Brownian particle which is acted upon by both a friction force with memory effect and a noise. The noise is expressed as f(X, t) is similar to X-sigma F(t), sigma > 0, where X and t are the displacement and time, respectively, and F(t) has the long-time correlation effect [F(0) F(t)] is similar to t(-beta), 0 < beta < 1, beta = 1, 1 < beta < 2. The generalized Langevin equation, the corresponding Fokker-Planck equation and its solution at large time are established. A variety of anomalous diffusion patterns are proposed. The correlation effects of noise may bring about that the effective diffusion coefficient is dependent on both the displacement and time, and the probability density for finding the Brownian particle is a non-Gaussian distribution. O'Shaughnessy and Procaccia's results on the diffusion on fractals can be derived from our model.

The in-situ observations of microcrack nucleation and bluntness in ductile fracture

Abstract: The in-situ tensile test in a transmission electron microscope (TEM) is a powerful method for studying the mechanism of the initiation and propagation of microcracking and voiding in ductile fracture For fcc materials, it is generally recognized that voids or microcracks with a length of a micrometer initiate and grow through the thinning of a thin zone resulting from multiple slip and link with the main crack resulting in a zig-zag crack propagation The microcracks reported here, however, were in fact voids with sizes from hundreds of nanometers to several micrometers Until now, there are no reports about the initiation of a microcrack with nanometer sizes, and blunting into a void with micrometer sizes Neither is it clear where and how a microcrack nucleates and changes into a void It has been generally accepted and there is a dislocation free zone (DFS) ahead of a crack tip, though the argument about it still persists The principle topics of this paper are to study the possibility of initiating a microcrack with nanometer size in the DFZ of the ductile materials and to clarify how a microcrack transforms into a void through an in-situ tensile test The ductile material used for thismore » investigation was a stable austenitic stainless steel of type 310« less

Relationship between J1c and fractal value of fracture surface of ductile materials

Abstract: When the fracture appearance of materials can be considered as approximately fractal, a relationship between j{sub 1c} and fractal value of the fracture surface for ductile materials can be established The new theoretical formula is J{sub 1c} = ({sigma}{sub 0}/3)ln((4{sup (1{minus}D{sub F}{sup {minus}1})}{minus} 1)/12f{sub p})l{sub 0} where {sigma}{sub 0} is the tensile strength of the material, D{sub F} is the fractal value of the fracture surface, f{sub p} is the volume fraction of void initiating particles, l{sub 0} is the characteristic distance The formula shows that the higher the fractal value of the fracture surface, the larger the J{sub 1c} value There is a correlation between hydrogen-induced J{sub 1c} decrease and hydrogen decreases in the fractal value of the fracture surface, which is consistent with experimental observations

On the prebainitic phenomenon in some alloys

Abstract: The prebainitic phenomenon in several copper alloys and ferrous alloys was investigated by means of internal friction measurement and composition analysis. The internal friction peaks of zinc or carbon atoms diffusion inβ brasses or in austenite of ferrous alloys, respectively, were related to the zinc or carbon-depleted region formation within the bainitic incubation period observed by composition analysis. The possibility of bainite shear nucleation in solute atoms’ depleted regions, formed by diffusing of solute atoms to a certain level, was pointed out.

The in-situ TEM observation of microcrack nucleation in titanium aluminide

Abstract: Rice and Thomson assumed that the emissions of dislocation from a crack tip would lead to plastic blunting of the crack tip, and the materials were categorized as intrinsically cleavage if a condition for decohesion of the crystal plane ahead of the crack was reached before those for dislocation emission from the crack. For the ductile material, the TEM in-situ observation showed that a main crack tip might still be sharp after the dislocations had been emitted from the crack tip and the dislocation free zone (DFZ) had been formed; thus, the local stress near the crack tip in the DFZ was high enough to equal the cohesive strength, and then a microcrack with size of nanometers nucleated in the DFZ. The microcracks initiated in the DFZ would blunt into voids through the movement of the existing dislocation and/or those emanated from the crack tip and the nearby dislocation source. The discontinuous void resulting from the blunting of the microcracks in the DFZ would link with each other and with the main crack, resulting in ductile fracture. For brittle materials, the authors wonder whether the DFZ also exist and whether a microcrack with the size of nanometers nucleated preferably inmore » the DFZ. If such occurred, the microcrack initiated in the DFZ should propagate into a cleavage crack rather than blunt into void. The goal of the present work was to check the possibility of dislocation emissions, DFZ formation, and initiation and propagation of a microcrack in the DFZ for the brittle material. For the titanium aluminides, e.g., Ti-24Al-11Nb or TiAl, a brittle fracture occurred by slow stable crack growth characterized by a well-defined resistance curve, i.e., the resistance of the cleavage crack propagation increased with crack growth.« less

Stress Corrosion Cracking of Austenitic Type 304 Stainless Steel in Solutions of Hydrochloric Acid + Sodium Chloride at Ambient Temperature

Abstract: Austenitic type 304 (UNS S30400) stainless steel is susceptible to stress corrosion cracking (SCC) in acidic chloride (Cl−) solutions at ambient temperature. In this study, a layer of adso...

Non-equilibrium segregation of solute to grain boundary

Abstract: Mechanisms for the non-equilibrium segregation of solutes to static grain boundary during cooling (quenching-induced segregation) and to moving grain boundary during recrystallization (moving-induced segregation) are proposed. For quenching-induced segregation, in consideration of the local equilibrium among vacancies, solute atoms and vacancy-solute atom complexes, as well as the influence of equilibrium grain-boundary segregation, the theoretical dynamic formulae for this non-equilibrium segregation have been derived on the basis of the vacancy-dragging mechanism. Theoretical calculations have been carried out for the non-equilibrium segregation of boron to austenitic grain boundaries during isothermal holding and continuous cooling after heating at high temperature; the results agree well with those obtained from experiments. The model has also successfully explained the different behaviours of boron segregation during cooling in α-Fe and in γ-Fe. For moving-induced segregation, based on the interaction between dislocations and the moving boundaries during recrystallization, a dislocation relaxation and widening grain-boundary mechanism of solute segregation on moving boundaries is proposed. Applying this model, we have calculated the boron segregation on moving boundaries during recrystallization in Fe-3% Si alloy; the results of these calculations agree with experimental results.

Superconductivity, microstructure and pressure effect on Tc of Hg- based compounds prepared under high pressure

Abstract: High-temperature superconductors of the mercury-based system were synthesized under high temperature and high pressure. The measurements of resistance and DC magnetic susceptibility for the samples prepared at 20-35 kbar and 850-degrees-C for 1-3.5 h show that the as-synthesized sample (1) prepared at 20 kbar, 850-degrees-C for 1.5 h was a multiphase of Hg-1212 and Hg-1223 phases and showed superconducting transitions of both 132 and 120 K. The sample (2) prepared at 30 kbar, 850-degrees-C for 3.5 h has Hg-1212 as the major phase with only a small amount of impurity phase. The intergrowth of Hg-1212/Hg-1223 was found in the sample (1). The dependence of pressure on T(c) for the Hg-1212 and Hg-1223 phases, measured under hydrostatic pressure, shows that dT(c)/dp is 0.17 and 0.23 K/kbar, respectively, in the pressure range below 10 kbar.

Two edge-disjoint hamiltonian cycles in graphs

Abstract: E. Schmeichel and D. Hayes showed that ifG is a 2-connected graph withd(u) +d(v)?n ?1 for every pair of nonadjacent vertices andv, then G has a Hamiltonian cycle unlessG is the graph of Fig. 2 (b). In this paper, it is proved that, under almost the same conditions as Schmeichel and Hayes's Theorem, namely,G is a 2-connected graph of ordern (n ? 40) with ?(G) ? 7 for every pair of nonadjacent vertices andv, G has two edge-disjoint Hamiltonian cycles unlessG is one of the graphs in Fig. 1 or Fig. 2, and this conclusion is best possible.