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盛文斌 《中国有色金属学会会刊》2006,16(B02):719-722
A model was established based on the combination of the equation of continuity, the equation of conservation of momentum and the equation of general energy to describe the filling and solidification of TiAl melt by permanent mold centrifugal casting. The model was solved numerically and the filling and solidification processes in the centrifugal field were discussed. The results indicate that the centrifugal field essentially influences the filling and solidification processes of TiAl melt. The melt will first fill the cavity along the back boundary until it reaches the end. After the end is fully filled, the whole cavity will be filled gradually by the way that free surface of the melt moves towards the entrance, hence the entrance is the last part to be filled. Furthermore, the mechanism by which internal defects can be formed in centrifugally cast TiAl components were interpreted. 相似文献
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A statistical-thermodynamic model for binary nonstoichiometric A5B3 with D81-structure has been established on the basis of the grand canonical ensemble. In the model the average chemical potentials according to the composition and a new extra equation as a boundary condition are proposed for more accuracy and free experimental data calculations. The model can be used in any A5B3 type compounds with D81 structure. The establishment method of the statistical-thermodynamic model can be extended to other complicated structures. We apply the statistical-thermodynamic model to α-Nb5Si3 and the parameters used in the model are obtained from the first-principles' calculations. From these parameters the expression for the point defect concentrations as a function of compositions and temperatures is obtained by numeral calculations. The constitutional defects and thermal defects in α-Nb5Si3 are discussed. 相似文献
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《Expert systems with applications》2014,41(17):7878-7888
For monitoring online manufacturing processes, the proportion of weights imposed on each type of product’s defects (nonconformities or demerits) has a profoundly effective impact on control charts’ performance. Apparently, the demerit-chart approach is superior than the widely-used c-chart scheme, because it allows us to place relative precise weights (real numbers) on defects according to their distinctly inferior degrees affecting the product quality so that the abnormal variations of processes can be literally exposed. However, in many applications, the seriousness of defects is evaluated partially or entirely by the inspectors’ perceptive judgement or knowledge, so with the precise-weight assignment, the demerit rating mechanism is considered to be somewhat constrained and subjective which inevitably leads to the targeted manufacturing process with limited and possibly biased information for online surveillance. To cope with the drawback, a demerit-fuzzy rating system and monitoring scheme is proposed in this paper. We first incorporate fuzzy weights (fuzzy numbers) to properly reflect the severity measures of defects which are categorized linguistically. Then, based on properties of fuzzy set theory and proposed approaches for fuzzy-number ranking, we develop the demerit-fuzzy charting scheme which is capable of discriminating process conditions into multi-intermittent statuses between in-control and out-of-control. This approach improves the traditional process control techniques with the binary-classification restraint for the process conditions. Finally, the proposed demerit-fuzzy rating system, monitoring scheme, and classification is elucidated by an application in garment industry to monitor textile-stitching nonconformities conditions. 相似文献
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It is suggested that some relaxation processes observed in crystalline polyethylene are consequences of the diffusive motion of a particular defect called a point dislocation or twist dispiration loop along the polyethylene stems in lamellar crystals. The motion of the defect, characterized by a diffusion coefficient and a mobility, is described by solutions of the Smoluchowski diffusion equation with boundary conditions that constrain the defect to move along routes that produce experimentally observable results. The fact that passage of the defect causes both a 180° rotation of the chain and moves an extra CH2 group in the direction of the chain axis is important to the interpretation of the data according to this model. The diffusion coefficient for a defect is estimated to be around 2 × 10?9 cm2 s?1 at 70°C. This value is shown to be reasonable both from the viewpoint of detailed computer modelling of defect motion and contemporary ideas about scaling. 相似文献
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Structural characterization by transmission electron microscopy of metal nanoparticle coatings on oxide nanospheres has been
employed to point out the potential of such materials for use as porous support model catalysts. On silica and titania nanospheres
of various origins surface-mediated metal deposition at mild temperature conditions has been utilized to fabricate Pt, Pd,
Ag, and Au nanoparticle coatings on the oxide supports. Promising coating characteristics have been achieved for Ag and Au
nanoparticles by direct reduction on terminating hydroxyl-rich St?ber silica. A high-resolution electron microscopy analysis
directed to surface stress, lattice contraction and planar lattice defects of the latter particles revealed no strong metal-support
interaction. 相似文献
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This paper reviews the brittle fracture behavior of dielectric ceramics such as barium titanate, and describes some of the relationships between defects such as cracks and electrical degradation and failure of multilayer capacitors. Stresses arising from the ferroelectric phase transformation in these dielectric materials are shown to play a part as a driving force for crack growth. In addition, possible contributions to failure from stresses arising from thermal excursions in the capacitor are discussed. Low-voltage failures arising from a short between the electrodes in multilayer capacitors are shown to be related to the growth of cracks in the dielectric. A technique for predicting the onset of these types of failures based upon fracture mechanics techniques is described. Possible effects of the electric field itself in promoting or retarding the growth of cracks are discussed. 相似文献