Structural evolution of alumina-γ-aluminium oxynitride composites during high-temperature compression creep

TEM studies of Al₂O₃--AION composites show the presence of precipitates in the -AION phase, due to a partial decomposition of this metastable phase formed during heat treatment. Thermal treatment (1650°C) has no effect on the microstructure, while compressive creep deformation (1650°C, 10–30 MPa) leads to a decrease of the -AION content. This phase decomposes, probably into alumina or alumina-poor AION. The precipitation process can be activated by the presence of dislocations, and is associated with a slight increase in strain rate during creep.     

In-plane rigid-body vibration mode characterization with a nanometer resolution by stroboscopic imaging of a microstructured pattern

This article introduces an improved approach for the characterization of in-plane rigid-body vibration, based on digital processing of stroboscopic images of the moving part. The method involves a sample preparation step, in order to pattern a periodic microstructure on the vibrating device, for instance, by focused ion beam milling. An image processing method has then been developed to perform the optimum reconstruction of this a priori known object feature. In-plane displacement and rotation are deduced simultaneously with a high resolution (10-2 pixel and 0.5 x 10(-3) rad, respectively). The measurement principle combines phase measurements-that provide the high resolution-with correlation-that unwraps the phase with the proper phase constants. The vibration modes of a tuning fork are used for demonstrating the capabilities of the method. For applications allowing the sample preparation, the proposed methodology is more convenient than common interference methods or image processing techniques for the characterization of the vibration modes, even for amplitudes in the nanometer range.     

Influence of Residual Impurities on Phase Partitiioning and Grain Growth Processes of Y-TZP Materials

Microstructural evolution at 1450°C of two 3 mol% Y₂O₃─ZrO₂ tetragonal polycrystals (Y-TZP) containing different residual impurity amounts has been investigated by analytical and scanning electron microscopy in order to clarify the role of the amorphous intergranular phases. Phase partitioning is interpreted as a process based on dissolution/reprecipitation reactions involving mass transport via the liquid impurity phase. Grain growth is inhibited by yttria migration before the phase equilibrium is reached, then increases drastically.     

A comparison of the flow boiling performance characteristics of partially-heated cross-ribbed channels with different rib geometries

Measured local heat transfer data and the results of flow visualization experiments are reported for convective boiling of R-113 and methanol in vertical cross-ribbed channels similar to geometries used in formed-plate compact heat exchangers. Experiments are conducted using a test section in which one wall of the channel is heated while the opposite wall is adiabatic and transparent to allow visual observation of the flow. Heat transfer data are obtained for three different rib crossing patterns at coolant mass flux values between 40 and 230 kg m⁻² s⁻¹ and mass qualities between 0.10 and 0.80. Comparison of these data, together with those for a fourth geometry previously reported, indicate that variations of the rib spacing and angle have a significant effect on boiling heat transfer performance. Variation of the rib crossing pattern in these channels is found to have only a small effect on heat transfer. The swirl and agitation induced by the ribs are found to increase the convective boiling heat transfer coefficient by as much as a factor of 4 above that for a round tube under comparable flow conditions. Methods of correlating the heat transfer data for annular film-flow boiling in these geometries are also discussed.     

EDI技术在企业进出口申报业务中的应用

随着经济的飞速发展,信息的交换速度变得非常重要。作为电子化数据的交换工具———EDI技术是现今应用的重点。本文对当前的EDI技术做了一个综述,介绍了EDI技术的概念、构成要素和目前研究的核心技术,并从现实出发,介绍了EDI技术在海关、外经贸局等部门的应用,以期进一步推动EDI技术在应用领域的研究工作。     

Evolution of grain shape and size during high-temperature creep of a yttria-doped fine-grained alumina

A semi-automatic method of image analysis was used to characterize the evolution of the microstructure of a polycrystalline alumina in terms of grain size and grain shape during different thermomechanical treatments. This study showed that for this material normal grain growth occurs during compressive creep and annealing; in addition, during creep under certain conditions grains flatten in the direction of the applied stress. Three-dimensional estimation of the grain shape was performed by analysing differently oriented plane sections of the specimens.     

Temperature coefficient of the high-frequency guided acoustic mode in a photonic crystal fiber

High-frequency guided acoustic Brillouin modes have recently been observed in small-core silica photonic crystal fibers. In this paper, we investigate the temperature dependence of the optical sideband frequency generated by one of these guided acoustic waves. The experimental results show a temperature coefficient of 100 kHz/°C at an acoustic resonance frequency of 1.15 GHz and are in very good agreement with the theoretical predictions. This coefficient demonstrates a temperature sensitivity 10 times larger than that previously reported in conventional single-mode fibers, which is promising in view of potential applications to optical fiber sensors.     

Effect of Grain Boundary Impurities on the Mechanical and Tribological Properties of Zirconia Surfaces

It is desirable to establish a relationship between the processing of ceramics, their mechanical properties that can be rapidly measured, for instance by indentation (hardness and toughness), their abrasion and scratch resistance, and their tribological performance. This paper examines such relationships for transformation-toughened zirconia. Experiments were performed on two zirconium oxides of similar doping with yttria (3%), one of high purity and one containing grain boundary impurities. The results are compared with those of previous work, in which the composition of the zirconium oxide was changed, but the processing and grain boundary purity were constant. It is found that the relationship between mechanical properties and triboiogical performance is complex but understandable in terms of the scale of the material responses (plastic deformation and fracture) with respect to its microstructure. The yttria content of the zirconia has a large influence on the wear resistance of the material (which increases with the fourth power of toughness). At constant yttria content, impurities produce relatively small changes in mechanical properties and wear behavior. The impurities weaken the grain boundaries with the following consequences: zirconia with pure grain boundaries behaves much like a brittle continuum, the material is hard, and macroscopic cracks develop at the corners of indentations and underneath wear tracks. Grain boundary impurities lower the hardness and increase the apparent (macroscopic) toughness by crack diffusion. At low bearing load, wear occurs by microchipping, and grain boundary phases have no effect. In water, wear is increased by intergranular fracture and the presence of grain boundary phases increases the wear rate. At high loads (44.5N), macroscopic cracks underneath the wear track develop early in the pure material; these cracks are retarded by intergranular fissuration in the material with grin boundary impurities.     

Dedensification and Anomalous Grain Growth during Sintering of Undoped Barium Titanate

Dedensification (up to 10% of the theoretical density in some instances) occurs at temperatures of .1320°C during the sintering of commercial undoped BaTiO₃ powders. The expansion of the rigid, interconnected network of large, rounded grains that grow by the Ostwald ripening effect can explain the dedensification that is observed. The isotropic anomalous grain growth results from the appearance of a liquid phase at } 1320°C and induces tensile stresses on the residual porous fine-grained matrix. No dedensification was observed at temperatures of, 1320°C, despite anisotropic anomalous grain growth.     

Internal stresses due to dislocation walls around second phase particles

Nickel base superalloys with large γ′ volume fractions exhibit a high threshold stress during a tensile test, and a large internal stress when tested in high temperature creep. Arrays of regularly spaced edge dislocations which develop during the first stages of creep have been observed. They lie in the γ-γ′ interfaces and form dipolar arrangements on opposite sides of the γ′ cuboids. The various components of the stress tensor are calculated for such a dipolar wall configuration and mapped by drawing equistress lines. The stress field resulting from such dislocation configurations has a very strong component along the tensile axis and opposes the applied stress. The shear component in the glide plane of similar dislocations is also very large between the precipitates and would tend to repel any new dislocation, in the absence of an external stress. The structural instability of these alloys under creep strain is also interpreted by this model.