Three problems in precipitation at interphase boundaries are examined. 1) The classu argument as to the particular phase in
which such a precipitate nucleates is shown to be irrelevant; except in a special situation, the critical nucleus must normally
penetrate both phases forming the interphase boundary. 2) The relative penetrations into the two phases achieved during growth
can be very different than those expected during nucleation; hence, deductions about the nucleation process based upon observations
on growth morphologies can be quite misleading. 3) The observations of Honeycombe and others that the nucleation of carbides
at austenite: ferrite boundaries occurs predominantly at the low energy, immobile broad faces of ledges rather than at the
higher energy, mobile risers of ledges are accounted for theoretically on the basis of the high velocities of the risers preventing
nucleation. Example calculations on a Ti-Ni alloy indicate that precipitation at the risers of ledges may become possible
in substitutional systems, but only at lower homologous temperatures, and if the migration of these boundaries is still controlled
by volume diffusion while nucleation is controlled by interfacial diffusion. 相似文献
The immunity of Holstein-Friesian dairy cows vaccinated against Coxiella burnetii was challenged with 4 X 10(8) infective guinea pig doses of viable rickettsiae. Cows that were vaccinated had normal full-term calves, whereas 2 nonvaccinated cows aborted late in pregnancy. Intrauterine infection of the fetus was indicated by recovery of the organism from tissues of the fetus. Coxiella burnetii was recovered from milk, colostrum, and placenta of vaccinated and nonvaccinated cows after challenge inoculation, but the rickettsiae recovered were as many as 1,000 times more numerous in nonvacinated cows. 相似文献
In the past 25 years, the numbers of registered motorcycles in the United States and California have increased about 1000 and 1100%, respectively. In the same period, the motorcycle collision death rate per million population more than doubled. The purposes of the study were to examine time trends in deaths due to motorcycle collisions, examine a methodologic problem in the study of motor vehicle collision death rates, and to determine driver and vehicle factors which may discriminate in the production of motorcycle crash related injurites.
With the exception of 1974, the increase in the ratio of registered motorcycles per 100,000 population in the United States corresponded to an increase in the crude death rate per million population. A similar pattern was also found in California.
Stepwise discriminant analysis was used to determine an optimum set of factors associated with motorcycle collision injuries. The analysis indicated that for male drivers age was the single factor most significantly related to motorcycle collision injuries. Other factors, in addition to age, which added to the power of the discrimination included number of prior motorcycle driving violations, frequency of motorcycle use, number of prior motorcycle crashes, motorcycle drivers' training, and height of the drivers. 相似文献
The composition WC-(W,Cr)2C-Ni is one of the standard compositions used for the preparation of thermally sprayed coatings by high velocity oxy-fuel
(HVOF) spraying. Surprisingly, this composition has been poorly investigated in the past. Frequent use of commercial designations
WC-‘CrC’-Ni, WC-Cr3C2-Ni, and WC-NiCr indicates the insufficient knowledge about the phase compositions of these powders and coatings. The properties
of these coatings differ significantly from those of WC-Co and WC-CoCr coatings. In this paper, the results of different series
of experiments conducted on HVOF-sprayed WC-(W,Cr)2C-Ni coatings are compiled and their specific benefits pointed out. The focus of this study is on the analysis of the microstructures
and phase compositions of the feedstock powders and coatings. Unlike WC-Co and Cr3C2-NiCr, WC-(W,Cr)2C-Ni is not a simple binary hard phase—binder metal composite. The phase (W,Cr)2C with unknown physical and mechanical properties appears as a second hard phase, which is inhomogeneously distributed in
the feedstock powders and coatings. As examples of coating properties, the oxidation resistance and dry sliding wear properties
are compared with those of WC-10%Co-4%Cr coatings. 相似文献
The comparison of the structure, composition, hardness and friction behavior of DCMS and HiPIMS W-C:H coatings made by hybrid reactive deposition in the presence of C2H2 and H2 in Ar atmosphere revealed a transition from textured to columnar-like structures with the increased addition of acetylene in DCMS coatings whereas only amorphous-like structures were produced by HiPIMS. The hybrid sputtering involves acetylene and hydrogen dissociation and recombination reactions in the plasma resulting in the generation of carbon as well as –CH3 and =CH2 radicals which are chemisorbed at the growing coating surface. The concentration of hydrogen in carbon matrix seems to be controlled by –CH3 and =CH2 radicals and not by the amount of molecular hydrogen. The main advantage of HiPIMS compared to DCMS technology includes coefficients of dry friction below 0.1 attributed to the formation of lubricious transfer layer at moderate level of hardness (around 20?GPa) over the studied acetylene additions range. Hybrid HiPIMS technology seems to offer possibilities for substantial COF decrease without hardness degradation which results in widening of the potential of W-C:H coatings for practical engineering applications. 相似文献
Polypropylene (PP) was modified with maleic anhydride (MAH) via heterogeneous solid-state radical grafting performed in a fluidized polymerization reactor. The effect of PP morphology and different concentration of initiator (dibenzoyl peroxide, DBP) on the course of the reaction and grafting efficiency was evaluated with using infrared spectroscopy, scanning electron microscopy (SEM) and particle surface analysis (BET method). With respect to obtained results, the reaction course can be strongly affected by the reaction conditions and PP morphology. Unlike the reactive modification in melt, heterogeneous reaction exhibits lower extent of β-scission albeit the reaction conversion is lower. Solid-state grafting can be used as an alternative method for the surface modification of nonpolar surfaces with polar monomers under mild conditions. 相似文献
The work deals with the preparation of dense SiC based ceramics with high electrical conductivity. SiC samples with different content of conductive TiNbSiCO based phase were hot pressed at 1820 °C for 1 h in Ar atmosphere under mechanical pressure of 30 MPa. The conductive phase is a mixture of 50 wt% TiNbC (molar ratio of Ti/NbC is 1:1.8) and 50 wt% eutectic composition of Y2O3SiO2. Composite with 30% of conductive TiNbSiCO phase showed the highest electrical conductivity 28.4 S mm?1, while the good mechanical properties of SiC matrix were preserved (fracture toughness KIC = 5.4 MPa m1/2 and Vickers hardness 17.8 GPa).The obtained results show that the developed additive system is suitable for the preparation of SiC-based composite with sufficient electrical conductivity for electric discharge machining. 相似文献
The aim of the study was to explore texture changes of Dutch-type cheese at its different locations during 126-day ripening at the temperature common for maturation of this type of cheese (10°C). Each block of cheese was divided into 3 slices and texture properties were studied in 13 segments evenly distributed in each slice. Due to cheese brining, a significant increase of hardness and elastic component was reported especially in the edge segments. During the subsequent ripening, both hardness and elastic component of natural cheese decreased gradually in all segments and slices. Studied parameters of individual parts of cheese did not demonstrate uniform rate of changes during the ripening period. In the whole experiment (even after 126-day ripening), hardness and elastic component of the cheese in core showed lower values compared to those obtained in the edge of all slices. 相似文献