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991.
M. K. Miller P. A. Beaven G. D. W. Smith 《Metallurgical and Materials Transactions A》1981,12(7):1197-1204
The redistribution of carbon atoms during the early stages of ageing and tempering of iron-carbon martensites has previously
been studied only by indirect methods. The computer-controlled atom probe field ion microscope permits the direct, quantitative
determination of carbon concentrations at the atomic level, and thus all the stages of the martensite decomposition process
become amenable to direct study. Analyses of a low-carbon martensite, Fe-1.0 at. pct C, (Fe-0.21 wt pct C), water quenched
and tempered for 10 min at 150 °C, showed a matrix carbon content of only 0.14 at. pct. Analysis of a 2 nm diam area centered
on a lath boundary showed a local concentration of 2.01 at. pct C. There is some evidence that this carbon level is associated
with the presence of a thin film of retained austenite at the boundary. In the case of a higher carbon martensite, Fe-0.64
at. pct Mn, 3.47 at. pct C, (Fe-0.65 wt pct Mn-0.78 wt pct C) water quenched and aged for approximately 24 h at room temperature,
analysis of twinned regions showed a matrix carbon level of 2.7 at. pct and a concentration enrichment to 6.9 at. pct in a
region 2 nm diam, centered on the coherent twin interface. Assuming the segregated carbon to be located in a single atomic
layer at the twin interface, this result indicates that a carbon concentration of 24 at. pct exists locally at the boundary.
These results appear to be the first direct demonstration of the segregation of carbon atoms to lattice defects in carbon
martensites. Tempering of the higher carbon martensite for 1 h at 160 °C produced further segregation of carbon to the region
of twin interfaces. The matrix carbon content fell to 1.5 at. pct and the average carbon content over a 2 nm diam region at
the interface rose to 8.7 at. pct. The width of the carbon segregated regions also increased, which seems to imply that incipient
carbide precipitation in the plane of the twin boundaries is occurring at this stage of the tempering process.
Formerly with the Department of Metallurgy and Science of Materials, University of Oxford 相似文献
992.
Stereochemical requirements for intercalation of platinum complexes into double-stranded DNA's 总被引:1,自引:0,他引:1
The complexes 1,10-phenanthrolineethylenediamineplatinum(II) and 2,2'-bipyridineethylenediamineplatinum(II) have a planar, aromatic ligand system that facilitates intercalation, as shown by their ability to unwind closed circular duplex DNA. Nonbonded steric interactions can rotate the pryidine ligands out of the coordination plane in bis(pyridine)ethylenediamineplatinum(II), thus preventing intercalation. Fiber x-ray diffraction patterns of the two metallointeracalators indicate that the binding is governed by the neighbor exclusion principle. 相似文献
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Measurements are reported of the frequency dependence of the maximum negative conductance which can be realised by a Gunn device operating as an oscillator in the delayed-domain mode. A theoretical value is found for the frequency at which the maximum negative conductance reaches a peak value, and this is found to be in good agreement with the measurements. 相似文献
996.
In 1970-72, 456 (53.4%) of 854 drivers responsible for a motor vehicle accident in which either they or other persons were killed had been drinking beforehand. Of surviving culpable drivers tested for blood alcohol concentration 81.7% were found to be legally impaired; however, only 24.8% of these drivers were tested. Because surviving culpable drivers pose a possible future hazard to highway safety it is important that data on this group be collected and analysed. 相似文献
997.
A laboratory procedure for synthesizing an alkylaryl polyamide curing agent for liquid epoxy resins from commercially available,
low cost materials is given. The various chemical reactions required to obtain the desired end-product are described, and
several properties of the novel polyamide are listed. This agent was found suitable for producing foamed or frothed epoxy
products. 相似文献
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