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31.
Gultekin Goller D. P. Koty S. N. Tewari M. Singh A Tekin 《Metallurgical and Materials Transactions A》1996,27(11):3727-3738
Metal-matrix composites have been prepared by pressure-infiltration casting of copper-base alloy melts into microporous carbon
preforms. The carbon preforms contained varying proportions of amorphous carbon and graphite. Load dependence of the wear
and friction behavior of the composite pins has been examined under ambient conditions against cast-iron plates, using a pin-on-plate
reciprocating wear tester. The wear resistance of the composite is significantly improved, as compared with the base alloy.
Contrary to the normally expected behavior, the addition of graphite to the amorphous carbon does not reduce the friction
coefficient, especially at high loads. The wear and friction behavior of the composites is very sensitive to the size and
distribution of the microstructural constituents. 相似文献
32.
J. John Rajesh J. Bijwe B. Venkataraman U. S. Tewari 《Journal of Materials Science》2002,37(23):5107-5113
Two polyamides (PAs) viz. PA 6 and PA 12 were selected for investigating the influence of water absorption on some physical, mechanical and tribological properties. Erosive wear studies on water treated and untreated samples were carried out at two impact angles viz. 30° and 90°. Tests on tensile strength of untreated and treated polyamides revealed that the water treatment resulted in enhancement for PA12 and reduction for PA 6. Exactly similar trends were reflected in their erosive wear performance also. The water absorption deteriorated the wear performance of PA 6 at both the angles, whereas the erosion wear behaviour of PA 12 improved at both the angles due to water absorption. The improvement was more significant at the angle of 30° rather than 90°. Scanning electron microscopic (SEM) investigations were done to study the topography of worn surfaces and to understand the wear mechanisms. 相似文献
33.
S. N. Tewari Y-Husuan Weng G. L. Ding R. Trivedi 《Metallurgical and Materials Transactions A》2002,33(4):1229-1243
Cellular array morphology has been examined in the shallow cell, deep cell, and cell-to-dendrite transition regime in Pb-2.2
wt pct Sb and Al-4.1 wt pct Cu alloy single-crystal samples that were directionally solidified along [100]. Statistical analysis
of the cellular spacing distribution on transverse sections has been carried out using minimum spanning tree (MST), Voronoi
polygons, radial distribution factor, and fast Fourier transform (FFT) techniques. The frequency distribution of the number
of nearest neighbors and the MST parameters suggest that the arrangement of cells may be visualized as a hexagonal tessellation
with superimposed 50 pct random noise. However, the power spectrum of the Fourier transform of the cell centers shows a diffused
single-ring pattern that does not agree with the power spectrum from the hexagonal tessellation having a 50 pct superimposed
random (uniformly distributed or Gaussian) noise. The radial distribution factor obtained from the cells is similar to that
of liquids. An overall steady-state distribution in terms of the mean primary spacing is achieved after directional solidification
of about three mushy-zone lengths. However, the process of nearest-neighbor interaction continues throughout directional solidification,
as indicated by about 14 pct of the cells undergoing submerging in the shallow cell regime or by an increasing first and second
nearest-neighbor ordering along the growth direction for the cells at the cell-to-dendrite transition. The nature of cell
distribution in the Al-Cu alloy appears to be the same as that in the Pb-Sb. The ratio between the upper and lower limits
of the primary spacing, as defined by the largest and the smallest 10 pct of the population, respectively, is constant: 1.43±0.11.
It does not depend upon the solidification processing conditions. 相似文献
34.
S. N. Tewari 《Metallurgical and Materials Transactions A》1987,18(13):1164-1164
35.
36.
R. Tewari G. K. Dey S. Banerjee N. Prabhu 《Metallurgical and Materials Transactions A》2006,37(1):49-58
The Zr3Al-based intermetallics have been considered as candidate materials for structural components in pressurized heavy water nuclear
reactors due to their attractive properties. However, these alloys could not be used for structural application in the reactors
because of their poor room-temperature ductility and irradiation-induced amorphization. Our recent studies of ternary addition
of niobium in Zr3Al have shown some promising results. The present article reports the microstructural evolution in these alloys upon long-time
annealing treatments. The formation of various phases, temperature regime of their stability, chemical composition, and volume
fraction of these phases during prolonged annealing have also been studied. A pseudobinary phase diagram with varying niobium
concentration has also been developed. The morphology and distribution of the Zr3Al phase have been explained on the basis long-range diffusion as the rate-controlling step.
This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,”
which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural
Materials Committee. 相似文献
37.
The effect of insulin-like growth factor (IGF) on tumor necrosis factor (TNF)-induced cell killing was determined for mouse BALB/c3T3 fibroblasts in vitro. Cells maintained in 0.5% fetal bovine serum (FBS) were killed by TNF within 6 h in a concentration-dependent manner, an effect that was prevented by IGF-I. TNF-induced cytotoxicity of 3T3 cells that overexpress the human IGF-I receptor (p6 cells) was prevented by IGF-I alone in the absence of serum. TNF-induced cell death was associated with the morphologic features of apoptosis and the release of low-molecular-weight DNA, both of which were prevented by IGF-I. Neither epidermal growth factor (EGF) nor platelet-derived growth factor (PDGF) protected p6 cells from TNF-induced apoptosis. The specific protective action of the IGF-I receptor was demonstrated further by the marked sensitivity to TNF of embryo fibroblasts derived from mice with targeted disruption of the IGF-I receptor (R-cells) but not of fibroblasts derived from wild-type littermates or R-cells transfected with the cDNA for the human IGF-I receptor. Cycloheximide or actinomycin D markedly reduced the protection offered by IGF-I. IGF-I protection of BALB/c3T3 cells persisted for up to 5 days in the presence of PDGF and EGF, whereas IGF-I lost its effectiveness after 2 days in the absence of growth factors. IGF-I did not prevent TNF-induced release of arachidonic acid. The results demonstrate a specific role for the IGF-I receptor in the protection against TNF cytotoxicity. This action of the IGF-I receptor is mediated by protective cytosolic proteins that exhibit a high rate of turnover and whose levels are regulated principally by factors within serum other than IGF-I. 相似文献
38.
Sporolactobacillus cellulosolvens (NCIMB 12173) isolated from an anaerobic digester and characterised biochemically is being reported for homofermentative lactic acid production from molasses in a batch culture. The effect of various process parameters on lactic acid production were optimized. A maximum lactic acid (24.2 g/l) and yield coefficient (0.79) was achieved using 3% (v/v) inoculum of 36 h old culture in molasses medium containing sugars (5%; w/v) supplemented with peptone (2.5 g/l) and (NH4)2SO4 (7.5 g/l), pH 6.5 at 40 degrees C after 72 h of fermentation. 相似文献
39.
Jun Hui R. Tiwari X. Wu S. N. Tewari R. Trivedi 《Metallurgical and Materials Transactions A》2002,33(11):3499-3510
Pb-2.2 wt pct Sb and Pb-5.8 wt pct Sb alloys have been directionally solidified from a single-crystal seed with its [100]
orientation parallel to the growth direction, to examine the primary dendrite distribution and disorder of the dendrite arrays.
The dendrite distribution and ordering have been investigated using analysis techniques such as the Gauss-amplitude fit to
the frequency distribution of nearest and higher-order spacings, minimum spanning tree (MST), Voronoi polygon, and Fourier
transform (FT) of the dendrite centers. Since the arrangement of dendrites is driven by the requirement to accommodate side-branch
growth along the 〈100〉 directions, the FT images of the fully developed dendrite centers contain spots which indicate this
preferred alignment. A directional solidification distance of about three mushy-zone lengths is sufficient to ensure a steady-state
dendritic array, in terms of reaching a constant mean primary spacing. However, local dendrite ordering continues throughout
the directional solidification process. The interdendritic convection not only decreases the mean primary spacing, it also
makes the dendrite array more disordered and reduces the ratio of the upper and lower spacing limits, as defined by the largest
5 pct and the smallest 5 pct of the population. 相似文献
40.