摘要:

Iron and nickel aluminide intermetallics are not only oxidation and corrosion resistant, but also thermodynamically compatible with a wide range of ceramics. This makes them suitable as the matrix for a wide range of composite systems. Among the composites evaluated to date are combinations of FeAl or Ni3Al with WC, TiC, SiC, TiB2, and Al2O3, with ceramic volume fractions ranging from (5 to 90%. A variety of processing techniques has been employed, but conventional liquid phase sintering and pressureless melt infiltration appear to be the most successful ones. Recently, a novel one‐step melt infiltration procedure has been developed to fabricate composites with ceramic volume fractions approaching 90%. Room temperature flexure strengths as high as 1.8 GPa have been obtained. Both FeAl and Ni3Al composites exhibit fracture toughnesses similar to those of WC/Co. It is found that sufficiently thin (< 2μm) ligaments of FeAl tend to fracture in a ductile manner. The absence of cleavage fracture in these thin ligaments is due to the unavailability of sufficiently long dislocation pile‐ups for nucleating cleavage cracks. In addition to the mechanical properties of FeAl and Ni3Al composites, other properties of interest such as wear and corrosion resistance are briefly discussed. The properties of FeAl and Ni3Al composites are seen to complement each other and may be of interest in those applications where WC/Co composites Have limitations.

ISSN:0253-3839    

DOI:10.1080/02533839.1999.9670437

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Phase transformations and microstructural evolution observed in variousγ‐TiAl based alloys are discussed. The mechanical properties are summarized and analyzed in duplex as well as fully‐lamellar materials over a wide range of grain sizes. From the structure‐property relationships, a generalization is made on the desired microstructural features which will result in significant improvements in balanced or specific properties. Further property enhancement, especially in creep resistance achieved by microalloying with carbon and Si, is also discussed. On the basis of the accumulated knowledge and data, the next‐generation TiAl alloy system for higher temperature use is formulated.

ISSN:0253-3839    

DOI:10.1080/02533839.1999.9670438

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

This paper summarizes research efforts at WVU, conducted to comprehensively understand the fundamental mechanism of solidification cracking of high‐strength aluminum alloys for aerospace plate applications. Three important technical approaches were adapted: 1. in‐situ thermal couples drop measurement of DC casting; 2. characterization of thermo‐mechanical properties of cast ingots, correlated with the cast structure; 3. numerical modeling of ingot thermal/stress history. The alloy of interest was 7050. The research efforts focused on both the transient and steady stages of DC casting.

ISSN:0253-3839    

DOI:10.1080/02533839.1999.9670439

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The effects of microstructure on the hydrogen trapping ability and effective diffusivity in steels with different carbon content have been studied. The sites for trapping hydrogen in pure iron, ferrite/pearlite steel, and the martensitic structure etc. have been identified. The hydrogen pick‐up was generally increased with the amount of grain boundary area in the steel. The hydrogen content of a steel after hydrogen charging increases and its hydrogen apparent diffusivity decreases, with a lowering of the transformation temperature. Thus a fully martensitic structure occludes the largest amount of hydrogen and possesses the lowest hydrogen apparent diffusivity. This microstructure retains a large portion of hydrogen even after high temperature outgassing. However, the hydrogen content reduces when the martensitic structure is tempered before charging. The effect of carbon content of the steel on the hydrogen occlusivity will also be discussed.

ISSN:0253-3839    

DOI:10.1080/02533839.1999.9670440

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Brittle‐to‐ductile transition (BDT) temperature (TBD) has been evaluated according to temperature dependence of tensile properties under different strain rates from 10‐5to 10‐1s‐1in a two‐phase Ti‐47Al‐2Mn‐2Nb alloy with near lamellar microstructure. Tensile fractography was observed using a scanning electron microscope while deformation substructures were investigated using a transmission electron microscope. It was found that TBD, when defined as the temperature corresponding to 7.5% elongation, increases from 1023K to more than 1373K, the strain rate increases from 10‐5to 10‐1s‐1. Based on the strain rate dependence of TBD(and using the Zener‐Hollomon factor) an apparent activation energy of 324kJ/mol was obtained, which is approximate to the self‐ and inter‐diffusion activation energies in the y‐TiAl phase. Transgranular fracture and dimple fracture were found to dominate in fracture surfaces below and above TBd, respectively. Furthermore, the most popular 1/2<110] ordinary dislocations were found to begin to climb around TBD. All this evidence, as well as a theoretical calculation using the Nabarro Model, add up to a conclusion that the BDT of the alloy is controlled by dislocation climbing.

ISSN:0253-3839    

DOI:10.1080/02533839.1999.9670441

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

An ongoing goal of multilayer capacitor research is to lower the firing temperature of the dielectric. This paper gives a comprehensive study of sintering BaTiO3with LiF flux which lowers the firing temperature through liquid phase sintering. A detailed set of experiments is discussed concerning microstructural evolution and corresponding dielectric properties under two processing variables: amount of LiF and sintering temperature. Different scales of chemical inhomogeneity were observed in this system which reflect two underlying mechanisms: solution reprecipitation with limited grain growth at low temperatures, which resulted in distinct core‐shell structures, and flux volatility, which gave rise to microscopic chemical inhomogeneity at higher sintering temperatures.

ISSN:0253-3839    

DOI:10.1080/02533839.1999.9670442

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

An alternative expression has been derived for the variance factors when estimating weights according to the Iterated Almost Unbiased Estimation (IAUE) technique. The variance factors are expressed in terms of the redundancy numbers of the observed quantities. A variance factor can also be approximately estimated by finding the ratio of the redundancy numbers at any two successive interations. The numerical example of the first‐order leveling network of Taiwan indicates: that stabilization of the redundancy number of an observation occurs as the variance factor associated with it converges to unity; and that for those variance factors which fail to converge, the redundancy numbers of the corresponding observations tend to decrease monotonically as the iteration proceeds.

ISSN:0253-3839    

DOI:10.1080/02533839.1999.9670443

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The measurement of accuracy is one of the most important features of data quality. Many statistical and practical criteria have been set for paper map digitization under various situations and considerations. This article explores the theoretical relationship between statistical and practical accuracy tests, and proposes a framework of cost‐accuracy analysis for choosing appropriate digitization methods. Eight trials using three digitization methods on two Taiwan zoning maps with different original qualities were performed to illustrate the differences between statistical and practical methods. The results showed that (1) the test for the mean of the errors should be included in the practical criteria, (2) the error tolerance standard for maps in poor condition can be raised from 0.5mm to 0.3mm, (3) it is sufficient to digitize maps in good condition using the 100dpi‐screen method, (4) appropriate digitization methods can be varied according to different map qualities, accuracy criteria, and budget.

ISSN:0253-3839    

DOI:10.1080/02533839.1999.9670444

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

This paper presents the table‐modeling technique for circuit simulation. First, an a‐Si TFT analytical model based on its operational physics is presented. The table‐modeling technique requires current and capacitance tables for different bias voltages. These tables were obtained from an analytical model. Using pixel simulation, we found that the table‐modeling technique is as accurate as the a‐Si analytical model for the same given conditions. Second, the 2‐D numerical model can be used as a good approximation to simulate an a‐Si TFT device. The current and capacitance tables were generated from the 2‐D numerical model. The simulation with an the a‐Si circuit application using the table‐modeling technique was very close to that using the 2‐D numerical model. The impact of non‐quasi static effect on the table‐modeling technique is also discussed.

ISSN:0253-3839    

DOI:10.1080/02533839.1999.9670445

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The flow behaviors of granular materials in a shear cell were experimentally studied. Different masses of granular materials were put in an annular shear device with a stationary upper wall and a constantly rotated lower wall. The image processing technique and the particle tracking method were employed to measure average particle velocities and fluctuating velocities in the streamwise and transverse directions. The granular flows consisted of a low shear rate region with higher and more uniform velocities in the lower test section, and a high shear rate region in the upper part. The velocities decreased with the height, while the fluctuations, granular temperature and shear rate increased with the height. The velocity fluctuations were anisotropic and were greater in the streamwise direction. The slip velocity and the low shear layer thickness were greater for the case with greater total granular mass.

ISSN:0253-3839    

DOI:10.1080/02533839.1999.9670446

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor