Showing papers by "Oak Ridge National Laboratory published in 1983"

The induction of micronuclei as a measure of genotoxicity: A report of the U.S. environmental protection agency Gene-Tox program☆

Abstract: There are many possible micronucleus assays involving different test organisms and tissues. Because micronuclei arise from chromosomal fragments or chromosomes that are not incorporated into daughter nuclei at the time of cell division, the assay detects both clastogens and agents that affect the spindle apparatus. We know of no case in which micronuclei and chromosomal breakage (or loss) have been shown to occur independently of one another in any dividing cell population. This relationship is so close that false-positives and false-negatives (insofar as the detection of tissue-specific chromosome damage is concerned) should be determined primarily by the statistics of sampling. The production of micronuclei in various experimental organisms has been reviewed. Although there are several promising experimental approaches such as the use of meiotic plant cells or human cells in culture, only one form of the assay, the in vivo mammalian bone-marrow polychromatic erythrocyte (PCE) assay, has been sufficiently developed to be considered a standard assay. More than 150 chemicals have been tested in this assay, with varying degrees of rigor. The data from the literature have been summarized and evaluated in light of the work Group's recommendations for an adequate test. The standards for an adequate test are an important part of the recommendations. These standards, although based on the most recent information available to us, are subject to change because this assay is still evolving. The most important recommendations in this report are: (1) at least 500 PCE should be examined from each of 8 animals to detect an increase of about 4‰ (per thousand) PCE when the background is less than 4 per 1000, (2) sampling should be extended to at least 72 h after the initial treatment, with sampling intervals no greater than 24 h, and (3) the highest possible doses should be used. The success rate of the assay to detect chemicals designated by the Environmental Protection Agency (EPA) as carcinogens is difficult to estimate for several reasons. First, few chemicals designated as noncarcinogens were studied, although in routine testing noncarcinogens are expected to be much more common than carcinogens. Hence, the rate of false-positives (insofar as the detection of cancer is concerned), which ought to be one of the strongest features of the assays, could not be estimated. Second, few chemicals have been tested as rigorously as this report recommends. Hence, the rate of false-negatives is almost certainly overestimated. (It is, nevertheless, obvious that false-negatives are to be expected for any tissue-specific in vivo assay like the micronucleus assay. For example diethylnitrosamine, which produces chromosomal aberrations, micronuclei, and cancer in the liver, is not detected in the bone-marrow micronucleus assay.) Third, many carcinogens are species-specific and this fact has not been taken into account. Considering these caveats, the uncorrected detection rate of the chemicals designated as carcinogens by EPA is about 50%. We believe that this would have been significantly higher had all tests been performed according to the test criteria. Further improvements in the assay are to be expected and these may lead to improvements in its success rate. Recent developments are discussed.

ORIGEN2: A Versatile Computer Code for Calculating the Nuclide Compositions and Characteristics of Nuclear Materials

Abstract: ORIGEN2 is a versatile point-depletion and radioactive-decay computer code for use in simulating nuclear fuel cycles and calculating the nuclide compositions and characteristics of materials contai...

Plant-Herbivore Interactions in a North American Mixed-Grass Prairie. I. Effects of Black-Tailed Prairie Dogs on Intraseasonal Aboveground Plant Biomass and Nutrient Dynamics and Plant Species Diversity

Abstract: Seasonal dynamics of soil nematodes and root biomass were examined from under western wheatgrass (Agropyron smithii) and little bluestem (Andropogon scoparius) from a heavily grazed prairie dog (Cynomys ludovicianus) colony occupied for 5 to 10 years and an adjacent lightly grazed, uncolonized area in Wind Cave National Park, South Dakota, USA. Nematodes were differentiated into classes of plant-parasitic Tylenchida and Dorylaimida and nonparasitic Dorylamida and Rhabditida. Root-feeding nematodes were generally more numerous from A. smithii than from A. scoparius, while nonparasitic populations were not different in soil from beneath the two plant species. Rhabditida, parasitic Dorylaimida and Tylenchida (from A. scoparius only) were more numerous on the prairie dog colony than from the uncolonized site, but nonparasitic Dorylaimida populations did not differ between the two areas. Mean total (live plus dead) root biomass beneath A. scoparius and A. smithii on the prairie dog colony averaged 71% and 81%, respectively, of values from the uncolonized area. Estimated consumption by root-feeding nematodes averaged 12.6% and 5.8% of annual net root production in the upper 10 cm from the prairie dog colony and uncolonized site, respectively. We conclude that, because of microhabitat modification or reductions in plant resistance to nematodes, heavy grazing by aboveground herbivores apparently facilitates grazing by belowground herbivores. Because heavily grazed plants have less roots than lightly grazed or ungrazed plants, the impact of root-feeding nematodes on primary producers is likely to be greatest in heavily grazed grasslands.

Concentration waves and Fermi surfaces in random metallic alloys

Abstract: On the basis of a new first-principles, electronic model for the forces driving clustering and short-range order in metallic alloys, it is argued that observed concentration-dependent peaks in the x-ray and electron diffuse scattering intensities are due to parallel sheets of flat Fermi surface. The positions of the peaks are directly related to the spanning vector k/sub 0/.

Community assembly and food web stability

Abstract: The ecological assembly of food web is considered as a process of predator colonizations and extinctions. The results of computer simulations using predator-prey equations allow us to identify three types of food web stability and examine how they may change through development of food webs. Species turnover stability increases, stability to extensive species extinction remains constant, and local stability to population fluctuations decreases as food web assembly proceeds.

Overview of tumor promotion in animals.

Abstract: Our present understanding of two-stage carcinogenesis encompasses almost four decades of research. Evidence for chemical promotion or cocarcinogenesis was first provided by Berenblum, who reported that a regimen of croton oil (weak or noncarcinogenic) applied alternately with small doses of benzo(a)pyrene (BP) to mouse skin induced a larger number of tumors than BP alone. Subsequently, Moltram found that a single subcarcinogenic dose of BP followed by multiple applications of croton oil could induce a large number of skin tumors. These investigations as well as a number of others, such as Boutwell, Van Duuren and Hecker, were responsible in defining many important aspects of the initiation and promotion of two-stage carcinogenesis. The initiation stage in mouse skin requires only a single application of either a direct-acting carcinogen or a procarcinogen and is essentially an irreversible step which as data suggests probably involves a somatic cell mutation. The promotion stage in mouse skin can be accomplished by a wide variety of weak or noncarcinogenic agents and is initially reversible later becoming irreversible. Current information suggests that skin tumor promoters are not mutagenic but bring about a number of important epigenetic changes, such as epidermal hyperplasia, and an increase in polyamines, prostaglandins and dark basal keratinocytes as well as other embryonic conditions. Recently, tumor promotion in mouse skin was shown to consist of at least two stages, in which each stage can be accomplished by either a known promoter or a weak or nonpromoting agent. Some of the important characteristics of the first stage of promotion are: (1) only one application of a first-stage promoter, such as phorbol ester tumor promoters, calcium ionophore A23187, hydrogen peroxide and wounding is needed; (2) the action is partially irreversible; (3) an increase in dark basal keratinocytes and prostaglandins is important; and (4) such an increase can be inhibited by antiinflammatory steroids and protease inhibitors. The second stage of promotion is initially reversible but later becomes irreversible. Polyamines and epidermal cell proliferation are important events in the second stage of promotion. A number of weak or nonpromoting agents, such as mezerein, are effective second-stage promoters which can be counteracted by retinoic acid, antiinflammatory steroids and polyamine synthesis inhibitors. Although skin tumor promotion has been extensively studied in mice, not all strains and stocks of mice are susceptible to phorbol ester tumor promoters. In this regard, the C57BL/6 mice appear to be fairly resistant to phorbol ester tumor promoters. In addition, not all species are equally susceptible to phorbol ester tumor promotion. Recently the generality of the two-stage system of inducing tumors has been shown to exist in a number of experimental carcinogenesis systems, such as the liver, bladder, lung, colon, esophagus, stomach, mammary gland, pancreas and cells in culture. In these systems, a wide variety of promoting agents such as diet, bile acids, hormones, saccharin, tryptophan, phenobarbital, polychlorinated biphenyls, polybrominated biphenyls and butylated hydroxytoluene have been used to accomplish the tumor promotion stage. It is not presently known if other experimental carcinogenesis systems and the induction of human cancer involves a series of stages similar to that in the mouse skin.

Pb(II)-catalysed cleavage of the sugar-phosphate backbone of yeast tRNAPhe--implications for lead toxicity and self-splicing RNA.

Abstract: Pb(II) is extremely efficient at depolymerizing RNA1–5 and studies on tRNAs have shown that site-specific cleavages in these molecules can be brought about by the action of Pb(II)6. We have observed, by difference Fourier analysis7,8, sugar–phosphate strand scission between residues 17 and 18 in crystals of yeast tRNAPhe soaked in dilute Pb(II) solution at pH 7.4. We have also deduced the structure of the Pb(II)–tRNAPhe derivative at pH 5.0 where this cleavage reaction is considerably slower and report that, in this structure, the sugar–phosphate backbone remains intact. We have, therefore, a picture of the reactants (at pH 5.0) and products (at pH 7.4) of this cleavage reaction. From this crystallographic study, and associated biochemical work, we have formulated a possible mechanism for the cleavage reaction and also present here some general ideas on the action of metal ions on nucleic acids.

Optical functions of silicon between 1.7 and 4.7 eV at elevated temperatures

Abstract: Polarization modulation ellipsometry has been used to determine the optical functions of silicon at elevated temperatures up to 1000 K. The ${E}_{1}$, ${E}_{0}^{\ensuremath{'}}$, and ${E}_{2}$ features move monotonically to lower energies as the temperature is increased. A fit of the ${E}_{0}^{\ensuremath{'}}$ and ${E}_{2}$ peak positions to the empirical formulation of Varshni is obtained; it is found that the critical points of the joint density of states for the ${E}_{0}^{\ensuremath{'}}$ and ${E}_{2}$ gaps move somewhat differently from the indirect gap, although the difference is not large.

Frog Embryo Teratogenesis Assay: Xenopus (FETAX) — A Short-Term Assay Applicable to Complex Environmental Mixtures

Abstract: There is no question that a rapid and inexpensive screening tool is needed to assess potential teratogenicity. The now classical Ames test (Ames, 1975) and other tests screen for potential mutagenicity and carcinogenicity. Although a number of relatively rapid bioassays, ranging from the invertebrates through lower vertebrates to mammals, as well as cell, organ, and embryo culture systems, have been used to examine teratogenicity, none has been adequately validated or widely accepted for routine use. The tier-testing approach, which uses rapid screens to detect potential hazard and indicate need for further testing, has been useful and expedient. Comparable tests, however, are not yet applicable for teratogenesis testing despite the fact that there are a number of methods available. Among these is one which we submit may be a valid model for preliminary assessment of potential teratogens. This model, referred to as FETAX (Frog Embryo Teratogenesis Assay: Xenopus), has been applied to examine the relative teratogenic risk of a variety of chemicals and complex mixtures. The mixtures we have tested come from coal-conversion and shale-oil technologies and their effects have been compared to those of similar materials derived from natural petroleums.

Plant-herbivore interactions in a North American mixed-grass prairie : II. Responses of bison to modification of vegetation by prairie dogs.

Abstract: Studies were conducted during the 1979 growing season to examine how North American bison (Bison bison) use prairie dog (Cynomys ludovicianus) colonies in Wind Cave National Park, South Dakota. Objectives included (1) determining whether bison selected for prairie dog towns parkwide; (2) characterizing in greater detail bison use patterns of a 36-ha colony in Pringle Valley as a function of time since prairie dog colonization; and (3) relating these bison use patterns to measured changes in structure and nutritional value of vegetation on and off the dog town.During midsummer, prairie dog towns were one of the most frequently used habitats by bison parkwide. Day-long observations at Pringle Valley revealed that bison exerted strong selection (nearly 90% of all habitat use and feeding time) for the dog town, which occupied only 39% of the valley. While there, they partitioned their use of the colony by grazing in moderately affected areas (occupied 26 years occupation).Prairie dogs facilitate bison habitat selection for a shortgrass successional stage in this mixed-grass community by causing a broad array of compositional, structural, and nutritional changes in the vegetation.

Genotoxicity of tobacco smoke and tobacco smoke condensate

Abstract: This report reviews the literature on the genotoxicity of mainstream tobacco smoke and cigarette smoke condensate (CSC) published since 1985 CSC is genotoxic in nearly all systems in which it has been tested, with the base/neutral fractions being the most mutagenic In rodents, cigarette smoke induces sister chromatid exchanges (SCEs) and micronuclei in bone marrow and lung cells In humans, newborns of smoking mothers have elevated frequencies of HPRT mutants, translocations, and DNA strand breaks Sperm of smokers have elevated frequencies of aneuploidy, DNA adducts, strand breaks, and oxidative damage Smoking also produces mutagenic cervical mucus, micronuclei in cervical epithelial cells, and genotoxic amniotic fluid These data suggest that tobacco smoke may be a human germ-cell mutagen Tobacco smoke produces mutagenic urine, and it is a human somatic-cell mutagen, producing HPRT mutations, SCEs, microsatellite instability, and DNA damage in a variety of tissues Of the 11 organ sites at which smoking causes cancer in humans, smoking-associated genotoxic effects have been found in all eight that have been examined thus far: oral/nasal, esophagus, pharynx/larynx, lung, pancreas, myeoloid organs, bladder/ ureter, uterine cervix Lung tumors of smokers contain a high frequency and unique spectrum of TP53 and KRAS mutations, reflective of the PAH (and possibly other) compounds in the smoke Further studies are needed to clarify the modulation of the genotoxicity of tobacco smoke by various genetic polymorphisms These data support a model of tobacco smoke carcinogenesis in which the components of tobacco smoke induce mutations that accumulate in a field of tissue that, through selection, drive the carcinogenic process Most of the data reviewed here are from studies of human smokers Thus, their relevance to humans cannot be denied, and their explanatory powers not easily dismissed Tobacco smoke is now the most extreme example of a systemic human mutagen

Transport Effects Induced by Resistive Ballooning Modes and Comparison with High- β p ISX- B Tokamak Confinement

Abstract: The transport effects induced by resistive ballooning modes are estimated from a theory, and are found to be mainly thermal electron conduction losses. An expression for electron thermal diffusivity ${\ensuremath{\chi}}_{e}$ is derived. The theoretical predictions agree well with experimental values of ${\ensuremath{\chi}}_{e}$ obtained from power balance for the ISX-$B$ plasmas at high poloidal beta.

Repair of O6-methylguanine in DNA by demethylation is lacking in Mer- human tumor cell strains.

Abstract: The ability of extracts of human tumor cells to demethylate O6-methylguanine (O6-MeG) in DNA was assayed using the synthetic DNA polymer poly(dC,dG,m6dG). Cell strains proficient in repair of O6-MeG in vivo (Mer+ phenotype) contained a methyltransferase activity while repair deficient cells (Mer- phenotype) had little or no activity. Mixing extracts of different Mer- strains did not result in the appearance of the activity. Extracts of Mer- cells did not inhibit the activity in extracts of Mer+ cells. Both Mer+ and Mer- strains contained methylnitrosourea-damage-specific endonuclease activity. The data suggest that the Mer- strains are deficient in methyltransferase and that this is the fundamental reason for their hypersensitivity to the cytotoxic effects of DNA alkylation. The activity was partially purified from a Mer+ colon carcinoma cell strain. Its kinetics parallel the repair of O6-MeG in DNA in vivo and suggest that the activity is inactivated during repair of DNA.

Microstructural modification of austenitic stainless steels by rapid solidification

Abstract: The microstructural modifications in three austenitic stainless steels (types 308, 310, and 312) were evaluated after rapid solidification. These three steels are commonly used weld filler metals. Two methods of rapid solidification were investigated, autogenous laser welding and arc-hammer splat quenching. The structure of 310 stainless steel was found to be 100 pct austenite, and did not vary over the range of conditions studied. On the contrary, the structures of types 308 and 312 steels were very sensitive to the cooling rates and solidification conditions. With the highest cooling rates, the type 308 structure was fully austenitic while the type 312 structure was fully ferritic. At lower cooling rates, the structures were duplex ferrite plus austenite. The results were interpreted in terms of faster kinetics of solidification of austenite compared to ferrite under the conditions examined. A comparison of the structures produced by the two rapid solidification techniques indicated the cooling rates are comparable.

Chromosome assignment of genes encoding the alpha and beta subunits of glycoprotein hormones in man and mouse.

Abstract: The chromosomal locations of the genes for the common alpha subunit of the glycoprotein hormones and the beta subunit of chorionic gonadotropin in humans and mice have been determined by restriction enzyme analysis of DNA isolated from somatic cell hybrids. The CG alpha gene (CGA), detected as a 15-kb BamHI fragment in human DNA by hybridization to CG alpha cDNA, segregated with the chromosome 6 enzyme markers ME1 (malic enzyme, soluble) and SOD2 (superoxide dismutase, mitchondrial) and an intact chromosome 6 in human-rodent hybrids. Cell hybrids containing portions of chromosome 6 allowed the localization of CGA to the q12 leads to q21 region. The greater than 30- and 6.5-kb BamHI CGB fragments hybridizing to human CG beta cDNA segregated concordantly with the human chromosome 19 marker enzymes PEPD (peptidase D) and GPI (glucose phosphate isomerase) and a normal chromosome 19 in karyotyped hybrids. A KpnI-HindIII digest of cell hybrid DNAs indicated that the multiple copies of the CG beta gene are all located on human chromosome 19. In the mouse, the alpha subunit gene, detected by a mouse thyrotropin (TSH) alpha subunit probe, and the CG beta-like sequences (CG beta-LH beta), detected by the human CG beta cDNA probe, are on chromosomes 4 and 7, respectively.

Effect of Microstructure on the Properties of TiB2 Ceramics

Abstract: Samples of a TiB2 ceramic containing 0 to 10 wt% Ni were fabricated by hot-pressing. Several properties, including fracture strength, indentation fracture toughness, and thermal expansion (between 25° and 1000°C) were measured. Resulting data were correlated with sample microstructure and composition.

Role of participate organic matter in decreasing accumulation of polynuclear aromatic hydrocarbons byDaphnia magna

Abstract: Accumulation of benzo[a]pyrene (BaP) and anthracene (A) byDaphnia magna in the presence of suspended yeast cells was analyzed using multicompartment models. The rate coefficient for uptake of polynuclear aromatic hydrocarbon (PAH) due to ingestion of yeast cells laden with sorbed chemical was only 3 to 15% of the rate coefficient for uptake of dissolved PAH. Uptake and accumulation of BaP was reduced 97% due to sorption of PAH to naturally occurring organic matter (humic acids). Accumulation of hydrophobic chemicals in aqueous systems appears to depend on the amount of chemical in solution and on the amount of chemical sorbed to particles entering the food chain. Chemicals sorbed to suspended organic matter, including dissolved or colloidal organic matter, have greatly reduced availability.

Crack and cavity nucleation at interfaces during creep

Abstract: Athermal nucleation of microcracks and thermal nucleation of cavities during creep deformation are reviewed with an emphasis on effects of solute segregation to grain boundaries and cavity surfaces. The magnitude and the duration of stress concentration at a triple grain junction or at a grain boundary inclusion are estimated for transient Coble creep and steady state power-law creep conditions. Stable configurations of wedge-type microcracks are predicted by a Griffith-like crack model. The rate for thermal nucleation of cavities is obtained by the Fokker-Planck equation for vacancy clusters. Cracks and cavities are interdependent, and cavity nucleation occurs continuously throughout the three creep stages. The local stress concentration enhances microcrack and cavity nucleation. The cavity nucleation rate is generally increased as a result of solute segregation to the surfaces and interfaces and/or gas precipitation into cavity volume. This enhanced nucleation is more effective in a system with mobile solutes than with immobile solutes. Immobile solute or trace elements may affect the nucleation rate also by changing the grain boundary diffusivity. Experimental techniques for quantitative analyses of cavity nucleation processes are discussed.

Formation of benzo(a)pyrene/DNA adducts and their relationship to tumor initiation in mouse epidermis

Abstract: The tumorigenicity of benzo(a)pyrene [B(a)P] applied topically as a skin tumor initiator in Sencar mice and the formation of epidermal B(a)P/deoxyribonucleoside adducts were compared over a similar range of doses (50 to 1600 nmol). The tumor-initiating activity of B(a)P, its covalent binding to mouse epidermal DNA, and the formation of the major hydrocarbon/deoxyribonucleoside adduct showed approximately parallel dose-response curves. The major hydrocarbon/deoxyribonucleoside adduct formed cochromatographed with marker adducts of (N2-(10S-[7R,8S,9R-trihydroxy-7,8,9,10-tetrahydrobenzo(a)pyrene]y) deoxyguanosine while other minor adducts also were observed. The disappearance of DNA-bound products in the epidermis was followed for 21 days after an initiating dose of B(a)P (100 nmol) was applied topically to the mice. The half-lives of the B(a)P/deoxyribonucleoside adducts and the total radioactivity bound to the DNA were 4.5 and 5.5 days, respectively. However, in spite of the loss of measurable DNA-bound material, the tumor yield was unchanged regardless of whether promotion was begun 7 or 21 days after initiation. The results suggest a possible causal relationship between B(a)P/deoxyribonucleoside adduct formation and papilloma formation in mouse skin.

Empirical scaling formulas for critical current and critical field for commercial NbTi

Abstract: Short sample 4.2 K experimental facilities are plentiful, but equipment for measurements of current as functions of temperature and field is scarce. An analysis has been made of published data comprising at least six manufacturers and spanning a range of critical current density at 4.2 K, 8 T of 50 to 108 kA/cm2, and linear equations have been found to fit the data over a wide range of field B and temperature T. For a constant temperature of 4.2 K, the following expression holds for B in the range of 3 to 10 T: j c (B, T = 4.2 K) = j o [1 - 0.096B], where [B c2 (4.2 K)]-1= 0.096 with a standard deviation of 3% for ten samples. The constant j o can be determined for any sample from a single point measurement at a convenient field. For a constant field of 8 T, the following expression holds for T in the range of 2 to 5.5 K: j c (B = 8 T, T) = j' o [1 - 0.177T], where [T c (8 T)]-1= 0.177 with a standard deviation of less than 1%. Linear equations have also been obtained for higher fields and lower temperatures. The critical field vs temperature is B c2 (T) = B c2 (0) [1 - (T/T c (0))n], where B c2 (0) = 14.5 T, T c (0) = 9.2 K, and n = 1.7 (not 2, which is used in theoretical derivations). For more accurate critical temperature calculations above 10 T, this equation can be used with the modification B c2 (0) = 14.8 T. No one simple power law for the upper critical field holds over the whole temperature range.

Luminescence from oxygen vacancies in MgO crystals thermochemically reduced at high temperatures

Abstract: Experimental measurements and theoretical calculations are reported which may resolve some of the remaining difficulties associated with photoemission from anion vacancies in thermochemically reduced MgO crystals. The intensities of the 2.3- and 3.2-eV luminescence bands are found to be strongly influenced by both the concentrations of ${\mathrm{H}}^{\ensuremath{-}}$ ions and anion vacancies present, and also by the intensity of the \ensuremath{\sim}5.0-eV exciting light. Theoretical calculations predict that for a $^{1}A_{1g}$ relaxation of the surrounding ions the $^{1}T_{1u}$ and $^{3}T_{1u}$ electronic states of the $F$ center are almost degenerate and \ensuremath{\sim}0.05 eV above the $^{3}A_{1g}$ state. The $^{1}T_{1u}\ensuremath{\rightarrow}^{1}A_{1g}$ and the $^{3}T_{1u}\ensuremath{\rightarrow}^{1}A_{1g}$ transition energies are predicted to be 2.2 and 2.9 eV, respectively.

Precipitation reactions during the heat treatment of ferritic steels

Abstract: The precipitation reactions in two ferritic steels, 9Cr-1Mo-V-Nb and 12Cr-1Mo-V-W, were studied. Analytical electron microscopy, optical microscopy, electrolytic extractions, and hardness measurements were used to determine the types, amounts, and effects of precipitates formed as a function of the heat treatment. The effect of variations in the austenitizing treatment was ascertained. In addition to variations in the austenitizing time and temperature, different cooling rates after austenitization were also used. Air cooling after austenitization (normalization) resulted in little precipitation in both alloys. Precipitation in the 12Cr-1Mo-V-W alloy after furnace cooling was found in all cases examined. Under certain conditions precipitation was also found after furnace cooling the 9Cr-1Mo-V-Nb alloy. However, when compared to the amount of precipitate in the fully tempered state, the 9Cr-1Mo-V-Nb showed a much greater variation in the degree of precipitation following furnace cooling. In addition, the matrix microstructure of the 9Cr-1Mo-V-Nb alloy was very sensitive to cooling rate. The precipitation reactions during tempering after a normalizing treatment were followed as a function of tempering treatment. Tempering temperatures were varied from 400 to 780 °C. The carbide precipitation was essentially complete after one hour at 650 °C for both alloys. Analytical microscopy was used to identify the precipitates. In the 9Cr-1Mo-V-Nb alloy, a combination of chromium-rich M23C6 and vanadium-niobium-rich MC carbides was found. The carbides in the 12Cr-1Mo-V-W alloy were identified as chromium-rich M23C6 and vanadium-rich MC. The results give an indication of the sensitivity of these alloys to heat treatment variations.

Calculation of self-consistent potentials for substitutionally disordered systems with application to the Ag x − Pd 1 − x alloy series

Abstract: Previous Korringa-Kohn-Rostoker coherent-potential-approximation electronic-structure calculations for substitutionally random alloys have been based on ad hoc potentials. The lack of procedures suitable to provide self-consistent, parameter-free potentials prevented computations for systems consisting of dissimilar atoms and is also the reason why quantities like, for example, cohesive energies or lattice constants, have not so far been evaluated for systems of similar constituents. We present in full detail a generally applicable scheme devised for calculating the self-consistent electronic structures of substitutionally disordered systems. Its feasibility is demonstrated by presenting the results obtained for the ${\mathrm{Ag}}_{x}{\mathrm{Pd}}_{1\ensuremath{-}x}$ alloy series. They are compared with those of former non-self-consistent calculations which use Mattheiss prescription potentials and the $\ensuremath{\alpha}=1$ Slater exchange, whereas the von Barth---Hedin expression is employed in our work. The differences are perceptible and have to be understood as combined self-consistency and exchange-correlation effects.