Modification-related porosity formation in hypoeutectic aluminum-silicon alloys

A series of aluminum-10 wt pet silicon castings were produced in sand molds to investigate the effect of modification on porosity formation. Modification with individual additions of either strontium or sodium resulted in a statistically significant increase in the level of porosity compared to unmodified castings. The increase in porosity with modification is due to the presence of numerous dispersed pores, which were absent in the unmodified casting. It is proposed that these pores form as a result of differences in size of the aluminum-silicon eutectic grains between unmodified and modified alloys. A geometric model is developed to show how the size of eutectic grains can influence the amount and distribution of porosity. Unlike traditional feeding-based models, which incorporate the effect: of microstructure on permeability, this model considers what happens when liquid is isolated from the riser and can no longer flow. This simple isolation model complements rather than contradicts existing theories on modification-related porosity formation and should be considered in the development of future comprehensive models.

Ferromagnetism, paramagnetism, and a Curie-Weiss metal in an electron-doped Hubbard model on a triangular lattice

Motivated by the unconventional properties and rich phase diagram of NaxCoO2 we consider the electronic and magnetic properties of a two-dimensional Hubbard model on an isotropic triangular lattice doped with electrons away from half-filling. Dynamical mean-field theory (DMFT) calculations predict that for negative intersite hopping amplitudes (t < 0) and an on-site Coulomb repulsion, U, comparable to the bandwidth, the system displays properties typical of a weakly correlated metal. In contrast, for t > 0 a large enhancement of the effective mass, itinerant ferromagnetism, and a metallic phase with a Curie-Weiss magnetic susceptibility are found in a broad electron doping range. The different behavior encountered is a consequence of the larger noninteracting density of states (DOS) at the Fermi level for t > 0 than for t < 0, which effectively enhances the mass and the scattering amplitude of the quasiparticles. The shape of the DOS is crucial for the occurrence of ferromagnetism as for t > 0 the energy cost of polarizing the system is much smaller than for t < 0. Our observation of Nagaoka ferromagnetism is consistent with the A-type antiferromagnetism (i.e., ferromagnetic layers stacked antiferromagnetically) observed in neutron scattering experiments on NaxCoO2. The transport and magnetic properties measured in NaxCoO2 are consistent with DMFT predictions of a metal close to the Mott insulator and we discuss the role of Na ordering in driving the system towards the Mott transition. We propose that the Curie-Weiss metal phase observed in NaxCoO2 is a consequence of the crossover from a bad metal with incoherent quasiparticles at temperatures T > T-* and Fermi liquid behavior with enhanced parameters below T-*, where T-* is a low energy coherence scale induced by strong local Coulomb electron correlations. Our analysis also shows that the one band Hubbard model on a triangular lattice is not enough to describe the unusual properties of NaxCoO2 and is used to identify the simplest relevant model that captures the essential physics in NaxCoO2. We propose a model which allows for the Na ordering phenomena observed in the system which, we propose, drives the system close to the Mott insulating phase even at large dopings.

Modeling the optical constants of solids using acceptance-probability-controlled simulated annealing with an adaptive move generation procedure

The acceptance-probability-controlled simulated annealing with an adaptive move generation procedure, an optimization technique derived from the simulated annealing algorithm, is presented. The adaptive move generation procedure was compared against the random move generation procedure on seven multiminima test functions, as well as on the synthetic data, resembling the optical constants of a metal. In all cases the algorithm proved to have faster convergence and superior escaping from local minima. This algorithm was then applied to fit the model dielectric function to data for platinum and aluminum.

Laboratory wear testing of polyurethane elastomers

A dry sand-rubber wheel abrasion test was used to investigate the wear behaviour of polyurethanes. The dry sand-rubber wheel abrasion test (DSRW test) is an approved ASTM test designed primarily for testing metals, therefore, in this study the set of test conditions was optimized for use with polyurethane elastomers. The wear performance of polyurethanes was assessed for the range of Shore hardness 85A to 65D, and a correlation was identified between the wear rate and the sample hardness. Polyurethane elastomers can be separated into three classes according to their hardness and wear performance, and each class shows a different dependence on the specimen temperature. This work has implications for use of the DSRW test for the prediction of field performance of polyurethanes. (C) Elsevier Science S.A.

Effect of milling conditions on the synthesis of chromium carbides by mechanical alloying

The synthesis of chromium carbides, Cr7C3 and Cr3C2, by mechanically allowing chromium and carbon powders has been investigated. Milling conditions were found to have a strong influence on the evolution of microstructure, with high collision energies being required to form carbide phases. Milling at intermediate energy levels resulted in the formation of an amorphous phase, and with low energy conditions only grain size refinement of Cr occurred with no evidence of any reaction between Cr and C. The amorphous phase was found to be the precursor to carbide formation. (C) 1997 Elsevier Science S.A.

The secondary passive film for type 304 stainless steel in 0.5 M H2SO4

An examination has been carried out of the secondary passive film on Type 304 stainless steel in 0.5 M H2SO4. The characterization techniques used were electrochemical (potentiodynamic; potentiostatic, and film reduction experiments) and surface analytical. A bilayer model for the secondary passive film is proposed. It appears that next to the metal, there is a modified passive film which controls the electrochemical response; i.e., governs the current for any applied potential. On top of this modified passive film, the experimental data are consistent with a ''porous'' corrosion-product film which adds to the total film thickness but has little influence on the electrochemical response. The composition of the secondary passive film corresponds most probably to a mixed Fe/Cr oxide/hydroxide enriched in Cr3+, With a composition similar to a primary passive film.

The effects of copper on the microbial community of a coral reef sponge

Marine sponges often harbour communities of symbiotic microorganisms that fulfil necessary functions for the well-being of their hosts. Microbial communities associated with the sponge Rhopaloeides odorabile were used as bioindicators far sublethal cupric ion (Cu2+) stress. A combined strategy incorporating molecular, cultivation and electron microscopy techniques was adopted to monitor changes in microbial diversity. The total density of sponge-associated bacteria and counts of the predominant cultivated symbiont (alpha -proteobacterium strain NW001) were significantly reduced in response to Cu2+ concentrations of 1.7 mug l(-1) and above after 14 days of exposure. The number of operational taxonomic units (OTUs) detected by restriction fragment length polymorphism (RFLP) decreased by 64% in sponges exposed to 223 mug l(-1) Cu2+ for 48 h and by 46% in sponges exposed to 19.4 mug l(-1) Cu2+ for 14 days. Electron microscopy was used to identify 17 predominant bacterial morphotypes, composing 47% of the total observed cells in control sponges. A reduction In the proportion of these morphotypes to 25% of observed cells was evident in sponges exposed to a Cu2+ concentration of 19.4 mug l(-1). Although the abundance of most morphotypes decreased under Cu2+ stress, three morphotypes were not reduced in numbers and a single morphotype actually increased in abundance. Bacterial numbers, as detected using fluorescence in situ hybridization (FISH), decreased significantly after 48 h exposure to 19.4 mug l(-1) Cu2+. Archaea, which are normally prolific in R. odorabile, were not detected after exposure to a Cu2+ concentration of 19.4 mug l(-1) for 14 days, indicating that many of the microorganisms associated with R. odorabile are sensitive to free copper. Sponges exposed to a Cu2+ concentration of 223 mug l(-1) became highly necrosed after 48 h and accumulated 142 +/- 18 mg kg(-1) copper, whereas sponges exposed to 19.4 mug l(-1) Cu2+ accumulated 306 +/- 15 mg kg(-1) copper after 14 days without apoptosis or mortality. Not only do sponges have potential for monitoring elevated concentrations of heavy metals but also examining changes in their microbial symbionts is a novel and sensitive bioindicator for the assessment of pollution on important microbial communities.

Critical review of acid in situ leach uranium mining: 1. USA and Australia

The technique of in situ leach (ISL) uranium mining is well established in the USA, as well as being used extensively in Eastern Europe and the former Soviet Union. The method is being proposed and tested on uranium deposits in Australia, with sulfuric acid chemistry and no restoration of groundwater following mining. Test sites in the USA were required to restore groundwater to ascertain the extent of impacts and compare costs to alkaline ISL mines. The problems encountered include expensive and difficult restoration, gypsum precipitation, higher salinity and some heavy metals and radionuclides after restoration. One of the most critical issues is whether natural attenuation is capable of restoring groundwater quality and geochemical conditions in an acid leached aquifer zone. The history of acid ISL sites in the USA and Australia are presented in this study, with a particular focus on the demonstration of restoration of groundwater impacts.

Changes in zinc and copper homeostasis in human livers and kidneys associated with exposure to environmental cadmium

This study was undertaken to assess changes in zinc and copper homeostasis in human tissues that could be attributed to human exposure to environmental cadmium, using samples of lung, liver and kidney cortex of 61 Queensland residents, aged 2 to 89 years, who had died of accidental causes. None of the subjects were exposed to cadmium in the workplace. Levels of zinc in liver and kidney cortex samples showed inverse associations with donor age whereas zinc in lung only showed inverse association with gender. Lung zinc levels in females were 14% lower than in males. Zinc in liver and kidney cortex samples were found to exist in at least two pools; one was associated with cadmium that bound to metallothionein (MT) and the other was associated with non - MTbound copper. In liver, the amounts of zinc in the MT pool were smaller compared to those in non-MT pool given that only 7% of zinc variations were explained by cadmium whereas 22% of the liver zinc Variations were accounted for by non - MT bound copper. In sharp contrast, larger amounts of zinc in kidney cortex samples were in the MT pool, compared to those in the non-MT pool given that cadmium was found to explain 69% of total zinc variation whereas copper explained only 17% of kidney zinc variations. The levels of copper in liver were found to be increased by 45-50% in subjects with high cadmium exposure level, compared to subjects of similar ages with medium exposure level. The levels of zinc and copper in kidney cortex samples in the subjects with high cadmium exposure were both found to be significantly elevated compared to those found in the medium-exposure group whereas copper contents were about 19-23% greater than in medium- as well as low-exposure groups. Taken together these results indicate increased sequestration of zinc and copper in liver and kidney cortex samples. The increases in metal sequestrations were observed in liver samples having cadmium contents of greater than 1 mug/g wet weight and in kidney cortex having cadmium contents of greater than 26 mug/g wet weight. Zinc and copper contents in lung of this sample group, however, were not associated with cadmium due probably to lower exposure levels compared to those of liver and kidney.

Establishment of native ecosystems after mining - Australian experience across diverse biogeographic zones

The Australian minerals industry, which is dominated by coal, gold, bauxite, iron ore, base metals and mineral sand operations, is widely scattered across a continent which has a wide range of climatic zones ranging from moist temperate in the south through hot deserts in the centre to moist tropical in the north. There is an emphasis at most mines on establishing native ecosystems after mining, and technologies have had to be developed to ensure successful establishment and stability of these ecosystems under often adverse climatic conditions. This paper describes some of the innovative practices used to establish native ecosystenms in bauxite, mineral sand and coal operations across diverse biogeographic zones. Additionally, brief reference is made to an ecosystem function analysis, which has been developed to assess the success of establishment of these ecosystems. (C) 2001 Elsevier Science B.V. All rights reserved.

Coupled experimental and thermodynamic study of the Zn-Fe-Si-O system

Experimental and thermodynamic modeling studies have been carried out on the Zn-Fe-Si-O system. This research is part of a wider program to characterize zinc/lead industrial slags and sinters in the PbO-ZnO-SiO2-CaO-FeO-Fe2O3 system. Experimental investigations involve high-temperature equilibration and quenching techniques followed by electron probe X-ray microanalysis (EPMA). Liquidus temperatures and solid solubilities of the crystalline phases were measured in the temperature range from 1200 °C to 1450 °C (1473 to 1723 K) in the zinc ferrite, zincite, willemite, and tridymite primary-phase fields in the Zn-Fe-Si-O system in air. These equilibrium data for the Zn-Fe-Si-O system in air, combined with previously reported data for this system, were used to obtain an optimized self-consistent set of parameters of thermodynamic models for all phases.

Determination of eutectic solidification mode in Sr-modified hypoeutectic Al-Si alloys by EBSD

The effect of eutectic modification by strontium on nucleation and growth of the eutectic in hypoeutectic Al-Si foundry alloys has been investigated by electron back-scattering diffraction (EBSD) mapping. Specimens were prepared from three hypoeutectic AlSi base alloys with 5, 7 and 10 mass%Si and with different strontium contents up to 740 ppm for modification of eutectic silicon. By comparing the orientation of the aluminium in the eutectic to that of the surrounding primary aluminium dendrites? the growth mode of the eutectic could be determined. The mapping results indicate that the eutectic grew from the primary phase in unmodified alloys. When the eutectic was modified by strontium, eutectic grains nucleated separately from the primary dendrites. However, in alloys with high strontium levels, the eutectic again grew from the primary phase. These observed effects of strontium additions on the eutectic solidification mode are independent of silicon content in the range between 5 and 10 mass%Si.