A practical method for identifying intermetallic phase particles in aluminium alloys by electron probe microanalysis

Aluminium alloys that contain Si, Mg, Fe, Mn and/or Cu usually contain one or more types of intermetallic phases that are not readily distinguishable in the microstructure by conventional microscopy methods. It has thus been a challenge to develop a method that will unambiguously identify them. A practical approach has been developed that is based on an inherent linear relationship revealed for the overall distribution of any two elements in a precipitate/matrix geometry and the first-order approximation of electron probe microanalysis (EPMA) results. Application of this approach to a direct chill cast 6082 alloy is demonstrated, and its major limitations are discussed.

Synthesis and structural properties of patellamide a derivatives and their copper(II) compounds

The synthesis, characterization and copper(II) coordination chemistry of three new cyclic peptide ligands, PatJ(1) (cyclo-(Ile -Thr- (Gly)Thz-lle-Thr(Gly)Thz)), PatJ(2) (cyclo-(Ile-Thr(Gly)Thz-(D)-Ile-Thr-(Gly)Thz)), and PatL (cyclo-(Ile-Ser-(Gly)Thz-Ile-Ser(Gly)Thz)) are reported. All of these cyclic peptides and PatN (cyclo-(Ile-Ser(Gly)Thz-Ile-Thr-(Gly)Thz)) are derivatives of patellamide A and have a [24]azacrown-8 macrocyclic structure. All four synthetic cyclic peptides have two thiazole rings but, in contrast to patellamide A, no oxazoline rings. The molecular structure of PatJ1, determined by X-ray crystallography, has a saddle conformation with two close-to-co-parallel thiazole rings, very similar to the geometry of patellamide D. The two coordination sites of PatJ1 with thiazole-N and amide-N donors are each well preorganized for transition metal ion binding. The coordination of copper(II) was monitored by UV/Vis spectroscopy, and this reveals various (meta)stable mono- and dinuclear copper(II) complexes whose stoichiometry was confirmed by mass spectra. Two types of dinuclear copper(II) complexes, [Cu-2(H4L)(OH2)(n)](2+) (n = 6, 8) and [Cu-2(H4L)(OH2)(n)] (n=4, 6; L=PatN, PatL, PatJ1, PatJ2) have been identified and analyzed structurally by EPR spectroscopy and a combination of spectra simulations and molecular mechanics calculations (MM-EPR). The four structures are similar to each other and have a saddle conformation, that is, derived from the crystal structure of PatJ(1) by a twist of the two thiozole rings. The small but significant structural differences are characterized by the EPR simulations.

PrrC from Rhodobacter sphaeroides, a homologue of eukaryotic Sco proteins, is a copper-binding protein and may have a thiol-disulfide oxidoreductase activity

PrrC from Rhodobacter sphaeroides provides the signal input to a two-component signal transduction system that senses changes in oxygen tension and regulates expression of genes involved in photosynthesis (Eraso, J.M. and Kaplan, S. (2000) Biochemistry, 39, 2052-2062; Oh, J.-I. and Kaplan, S. (2000) EMBO J. 19, 42374247). It is also a homologue of eukaryotic Sco proteins and each has a C-x-x-x-C-P sequence. In mitochondrial Sco proteins these cysteines appear to be essential for the biogenesis Of the Cu-A centre of respiratory cytochrome oxidase. Overexpression and purification of a water-soluble and monomeric form of PrrC has provided sufficient material for a chemical and spectroscopic study of the properties of the four cysteine residues of PrrC, and its ability to bind divalent cations, including copper. PrrC expressed in the cytoplasm of Escherichia coli binds Ni2+ tightly and the data are consistent with a mononuclear metal site. Following removal of Ni2+ and formation of renatured metal-free rPrrC (apo-PrrC), Cu2+ could be loaded into the reduced form of PrrC to generate a protein with a distinctive UV-visible spectrum, having absorbance with a lambda(max) of 360 nm. The copper:PrrC ratio is consistent with the presence of a mononuclear metal centre. The cysteines of metal-free PrrC oxidise in the presence of air to form two intramolecular disulfide bonds, with one pair being extremely reactive. The cysteine thiols with extreme O-2 sensitivity are involved in copper binding in reduced PrrC since the same copper-loaded protein could not be generated using oxidised PrrC. Thus, it appears that PrrC, and probably Sco proteins in general, could have both a thiol-disulfide oxidoreductase function and a copper-binding role. (C) 2002 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.

Inherited copper toxicosis in a Bedlington terrier

Inherited copper toxicosis was diagnosed in a Bedlington Terrier using conventional diagnostic methods such as plasma biochemistry, hepatic copper assay, histology and electron microscopy and a recently developed DNA microsatellite marker test. The dog was successfully managed using D-penicillamine and a low copper diet.

Copper transfer from the Cu(I) chaperone, CopZ, to the repressor, Zn(II)CopY: Metal coordination environments and protein interactions

Extracellular copper regulates the DNA binding activity of the CopY repressor of Enterococcus hirae and thereby controls expression of the copper homeostatic genes encoded by the cop operon. CopY has a CxCxxxxCxC metal binding motif. CopZ, a copper chaperone belonging to a family of metallochaperones characterized by a MxCxxC metal binding motif, transfers copper to CopY. The copper binding stoichiometries of CopZ and CopY were determined by in vitro metal reconstitutions. The stoichiometries were found to be one copper(l) per CopZ and two copper(l) per CopY monomer. X-ray absorption studies suggested a mixture of two- and three-coordinate copper in Cu(1)CopZ, but a purely three-coordinate copper coordination with a Cu-Cu interaction for Cu(1)(2)CopY. The latter coordination is consistent with the formation of a compact binuclear Cu(l)-thiolate core in the CxCxxxxCxC binding motif of CopY. Displacement of zinc, by copper. from CopY was monitored with 2,4-pyridylazoresorcinol. Two copper(l) ions were required to release the single zinc(II) ion bound per CopY monomer. The specificity of copper transfer between CopZ and CopY was dependent on electrostatic interactions. Relative copper binding affinities of the proteins were investigated using the chelator, diethyldithiocarbamic acid (DDC). These data suggest that CopY has a higher affinity for copper than CopZ. However, this affinity difference is not the sole factor in the copper exchange: a charge-based interaction between the two proteins is required for the transfer reaction to proceed. Gain-of-function mutation of a CopZ homologue demonstrated the necessity of four lysine residues on the chaperone for the interaction with CopY. Taken together, these results suggest a mechanism for copper exchange between CopZ and CopY.

Growth and compaction behaviour of copper concentrate granules in a rotating drum

In order to understand the growth and compaction behaviour of chalcopyrite (copper concentrate), batch granulation tests were carried out using a rotating drum. The granule growth exhibited induction-type behaviour, as defined by Iveson and Litster [AIChE J. 44 (1998) 15 10]. There were two consecutive stages during granulation: the induction stage, during which the granules are gradually being compacted and little or no growth occurs, and the rapid growth stage, which starts when the granules have become surface wet and are rapidly growing. In agreement with earlier findings. an increased amount of binder liquid shortened the induction time. The compaction behaviour was also investigated. A displaced volume method was adopted to determine the porosity of the granules. It was shown that this technique had a limitation as it was unable to detect the reduction of the volumes of the granule pores after the granules had become surface wet. Due to this, some of the measurements were not suited for fitting a three-parameter empirical model. Attempts were made to determine whether the rapid growth stage started with the pore saturation exceeding a certain critical value, but due to the scatter in the porosity measurements and the fact that some of the measurements could not be used, it was not possible to determine a critical pore saturation, However, the porosity measurements clearly demonstrated that the porosity of the granules decreased during the induction stage of an experiment and that when rapid growth occurred, the granules had a pore saturation was around 0.85. This value was slightly lower than unity, which is most likely due to trapped air bubbles. (C) 2002 Published by Elsevier Science B.V.

Columnar to equiaxed transition of eutectic in hypoeutectic aluminium-silicon alloys

Directional solidification of unmodified and strontium modified binary, high-purity aluminium-7 wt% silicon and commercial A356 alloys has been carried out to investigate the mechanism of eutectic solidification. The microstructure of the eutectic growth inter-face was investigated with optical microscopy and Electron Backscattering Diffraction (EBSD). In the commercial alloys, the eutectic solidification inter-face extends in the growth direction and creates a eutectic mushy zone. A planar eutectic growth front is observed in the high-purity alloys. The eutectic aluminium has mainly the same crystallographic orientation as the dendrites in the unmodified alloys and the strontium modified high-purity alloy. A more complex eutectic grain structure is found in the strontium modified commercial alloy. A mechanism involving constitutional undercooling and a columnar to equiaxed transition explains the differences between pure and commercial alloys. It is probably caused by the segregation of iron and magnesium and the activation of nucleants in the commercial alloy. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

An age hardening model for Al-7Si-Mg casting alloys

Yield strength (YS) ageing curves have been modelled for A356 and A357 aluminium casting alloys below the solvus temperature of the main hardening precipitate. Predictions are based on the Shercliff and Ashby methodology (Acta MetaH. Mater. 38 (1990) 1789) for wrought alloys. Differences between strengthening in wrought and cast Al-Si-Mg alloys are considered. A Brinell hardness to YS conversion incorporating strain hardening has been established to enable YS ageing curves to be predicted with reduced experimental effort. (C) 2002 Elsevier Science B.V. All rights reserved.

Characteristic zirconium-rich coring structures in Mg-Zr alloys

The most characteristic feature of the microstructure of a magnesium alloy that contains more than a few tenths per cent soluble zirconium is the zirconium-rich cores that exist in most grains. The morphology, distribution and composition of cores observed in a Mg-0.56%Zr alloy and the small particles present in them were investigated. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.