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41.
Véronique J. Michaud Andreas Mortensen 《Metallurgical and Materials Transactions A》1992,23(8):2263-2280
In a previous article, [1] a theoretical analysis was developed to describe the infiltration of fiber preforms by a binary
alloy, and its solution was given for unidirectional adiabatic infiltration under constant applied pressure. This article
further develops the analysis by proposing a model to predict the permeability of fibrous preforms containing solidified primary
metal, by deducing the final composite microstructure from processing parameters, and by addressing the influence of external
cooling on macrosegregation within the composite. Experimental procedures estab- lished for the infiltration of fiber preforms
by pure aluminum are modified to produce samples infiltrated under nearly adiabatic conditions. Samples of SAFFIL alumina
fiber preforms infil- trated adiabatically under constant applied pressure with Al-4.5 wt pct Cu show longitudinal variations
in copper concentration, which are well predicted by theory presented in Part I. [1] The microstructures in the infiltrated
composite samples also agree with analysis: the grain size is small where solid and liquid matrix coexisted during infiltration,
whereas it is large where remelting occurred, indicating that SAFFIL fibers do not promote nucleation of Al-Cu. The model
proposed here for permeability of the preform in the presence of solidified metal yields infiltration rates in agreement with
experimental data. Finally, samples produced under non- adiabatic conditions exhibit transverse macrosegregation; this is
explained for simple limiting cases of heat transfer at the die wall. 相似文献
42.
43.
A study of d.c. and microwave conductivity and thermoelectric power of the organic conductor (TMTSF)2BrO4 is presented. The transport properties are in qualitative agreement with charge transport via variable-range hopping among localized states. The localization is attributed to the anions, which are ordered only over short ranges, probably due to impurities. 相似文献
44.
45.
FV Mortensen JS Rasmussen O Viborg S Laurberg EM Pedersen 《Canadian Metallurgical Quarterly》1998,164(8):599-604
OBJECTIVE: To validate a transit time ultrasound flowmeter (CardioMed CM 4000) for measuring blood flow in isolated colonic mesenteric arteries. DESIGN: Experimental and clinical study. SETTING: Teaching hospital, Denmark. ANIMALS AND SUBJECTS: One female pig, and 6 patients being operated on for carcinoma of the sigmoid colon and rectum. INTERVENTIONS: Volume blood flow measured by Cardiomed CM 4000 and pump withdrawal flow recordings. MAIN OUTCOME MEASURES: Correlation between the two methods. RESULTS: There was good agreement between transit time flow recordings and pump withdrawal flow recordings (correlation coefficient of 1.0). Of the differences between the two methods, 95% were between -0.16 ml min(-1) and 1.29 ml min(-1), mean 0.57 ml min(-1), or (in percentages) 105, 95% lying between 97-115. There was also good reproducibility in transit time flow recordings, the mean difference between repeated measurements being 0.06 ml min(-1), 95% lying between -0.66 ml min(-1) and 0.78 ml min(-1). CONCLUSION: Ultrasound transit time flow recordings gave precise measurements of blood flow in isolated colonic mesenteric arteries. 相似文献
46.
JC Nielsen HR Andersen PE Thomsen L Thuesen PT Mortensen T Vesterlund AK Pedersen 《Canadian Metallurgical Quarterly》1998,97(10):987-995
OBJECTIVE: In the present study, physical and sexual abuse among incarcerated youth were examined with respect to sexual activity, contraceptive use, and pregnancy. METHOD: The self-report survey data were collected from 62 females and 334 males who were incarcerated in Nevada youth correctional facilities in the summer of 1994. Of the males, 46.8% (n = 156) reported a history of physical abuse and 9.9% (n = 33) reported sexual abuse. A surprisingly high 73% (n = 46) of the females reported a history of physical abuse and 68.3% (n = 43) reported sexual abuse. RESULTS: The analyses indicated that females who reported a history of sexual abuse had an earlier mean age of first intercourse than those who reported no sexual abuse. In addition, male and female respondents with a history of physical and/or sexual abuse reported using no method of contraception as compared to youth with no history of abuse. CONCLUSIONS: Implications of these results for detention-based counseling and prevention programming are discussed. 相似文献
47.
Columnar dendritic solidification in a metal- matrix composite 总被引:14,自引:0,他引:14
Andreas Mortensen J. A. Cornie M. C. Flemings 《Metallurgical and Materials Transactions A》1988,19(3):709-721
Results are presented of a study on columnar dendritic solidification of the matrix of a fibrous metal-matrix composite, the
fibers of which are aligned SiC fibers 140 m in diameter and the matrix of which is Al-4.5 wt pct Cu. Samples were produced
by pressure infiltration of the metal-matrix into a preform of the fibers. The matrix was subsequently remelted and resolidified
under controlled thermal gradient and growth rate. Dendrite growth begins in the center of the interstices left between the
fibers. The dendrite tip temperature is not significantly influenced by the fibers, but the usual linear dependency of dendrite
arm spacing ont
1/3
(wheret is time during solidification) is altered significantly in the narrower interstices at long solidification times. The underlying
mechanism is dendrite arm coalescence which takes place at a sufficiently rapid rate in the composite that the microstructure
gradually becomes nondendritic. The solid/liquid interface then is parallel to the matrix/fiber interface. A model is presented
for the kinetics of dendrite arm coalescence and compared with experimental results. The amount of microsegregation that was
found in the matrix within interstices is significantly less than that found in the usual cast alloy, especially at long solidification
times (low cooling rates). The mechanism responsible for the observed reduction in microsegregation is solid-state diffusion
which is enhanced in the composite by the fact that the fibers place an upper limit on the dendrite arm spacing, and hence
on the required diffusion distance. 相似文献
48.
D. C. Dunand J. L. Sommer A. Mortensen 《Metallurgical and Materials Transactions A》1993,24(10):2161-2170
Preforms of nickel powder or nickel/alumina powder blends were infiltrated with molten aluminum to produce nickel aluminides.
Application of a pressure of 3.6 and 6.9 MPa on the melt allowed infiltration of preforms with nickel powder particles between
5 and 15 μm in diameter, which could not be infiltrated under the sole action of capillary forces. By varying the initial
preform temperature from 705 °C to 280 °C, the diameter of nickel powder particles from 15 to 150 μm, and the volume fraction
of alumina from 0 to 34 vol pct, pressure-infiltrated samples with large variations in the extent of reaction between nickel
and aluminum were pro- duced. The range of microstructure extended from fully reacted nickel aluminide to essentially unreacted
nickel/aluminum samples containing low levels of final porosity. In particular, it was found that reducing the initial preform
temperature below the melting point of aluminum results in a reduction of the rate and extent of aluminide formation, allowing,
in turn, reduction of macrosegregation within the resulting infiltrated material.
Formerly Research Assistant, Department of Materials Science and
Formerly Research Assistant, Department of Materials Science and 相似文献
49.
A Dalsgaard HF Mortensen K M?lbak F Dias O Serichantalergs P Echeverria 《Canadian Metallurgical Quarterly》1996,34(5):1189-1192
In the present study, 19 strains of Vibrio cholerae O1 biotype El Tor isolated during outbreaks of cholera in Guinea-Bissau in 1987, 1994, and 1995 were characterized to investigate a possible epidemiological relationship among the isolates. On the basis of ribotyping with the restriction enzyme BglI, 5 strains isolated in 1987 showed two closely related ribotypes, while 14 strains isolated in 1994 and 1995 showed the same ribotype that was distinct from the ribotypes of strains isolated in 1987. Southern blot hybridization of BglI-digested genomic DNA with a cholera toxin probe demonstrated that the strains isolated in 1987 showed an identical cholera toxin genotype, whereas O1 strains isolated in 1994 and 1995 showed the same genotype that was distinct from the genotype of strains isolated in 1987. These results were supported by the results of antibiotic susceptibility testing, in which strains isolated in 1987 showed resistance to polymyxin B only, while each of the strains from 1994 and 1995 showed resistance to polymyxin B, trimethoprim-sulfamethoxazole, and the vibriostatic agent O/129. Although our results are based on a limited number of V. cholerae O1 strains, they suggest that the epidemic in Guinea-Bissau in 1994 and 1995 was due to the introduction of a new strain to the country. 相似文献
50.
Andreas Mortensen 《Metallurgical and Materials Transactions A》1989,20(2):247-253
An analytical treatment of the influence of coarsening by ripening on microsegregation is presented. Comparison of results
from the model with data on Al-4.5 wt pct Cu and Sn-Bi alloys indicates that an overestimation of the reduction in microsegregation
results if it is assumed that ripening proceeds throughout the solidification range. This is attributed to the changing nature
of coarsening processes, in which coalescence processes predominate over ripening at higher volume fractions solid. 相似文献