Assessment of different techniques for quantification of α-Al(FeMn)Si and β-AlFeSi intermetallics in AA 6xxx alloys

During homogenisation of AA 6xxx aluminium alloys, the platelike β-AlFeSi intermetallic phase will transform to a less Si-rich and more spheroidised α-Al(FeMn)Si phase which is more favourable for extrusion. In this study, several quantitative methods, which determine the relative volume fraction of α-Al(FeMn)Si and β-AlFeSi, are compared and an assessment of each method is made. The methods used are optical microscopy, scanning electron microscopy (SEM) in combination with electron dispersive X-ray (EDX) using polished samples, and X-ray diffraction (XRD) on intermetallics, extracted through selective dissolution of the Al matrix. The highest accuracy is obtained by using SEM/EDX analysis and applying two criteria.     

Production of rapidly solidified Al/SiC composites

Rapidly solidified metal matrix composites have been produced on a laboratory scale either by (1) melt spinning a composite after introduction of the ceramic phase and extrusion of the flakes obtained, or (2) blending melt-spun powder (basic alloy) with the ceramic phase and subsequent extrusion. AlMg(Si) alloys were used as matrix material while SiC particles with diameters of 3 or 20 m were used as the ceramic phase. For the composites prepared by route 1 it was found that the basic alloy was reinforced by the addition of 3 m particles whereas for the 20m particles reinforcement was observed only for very ductile matrices. The bond between SiC particles and matrix was good. A diffusion and wetting bond was formed. For the composites prepared by route 2 it was found that reinforcement did not occur and that the bond between particles and matrix was weak. Debonding of the particles took place in the case of tensile fracture. The advantage of a rapidly solidified matrix for these composites is that relatively high ductilities are combined with good reinforcement effects. Prior contact of the ceramic phase and the aluminium melt is needed for a good bond between SiC and the matrix material. It is concluded that route 1 should be preferred for the production of rapidly solidified aluminium matrix composites.     

Current and future materials in advanced gas turbine engines

Future gas turbine engines will have better fuel efficiencies and lower operating costs. This will require new and advanced materials with higher temperature capabilities. This paper discusses some of the presently applied materials in the turbine section of gas turbines, and reviews the material developments that are occurring and will be necessary for the near and long term futures. Reprinted from “Sermatech Review, Number 51, Spring 1995≓, Sermatech International Incorporated, 155 S. Limerick Road, Limerick, PA 19464-1699. Tel: (610) 948-5100, Fax: (610) 948-0811. The acknowledgment section indicates where this paper originally appeared.     

The Influence of an Ozone,Chlorine and Chlorine Dioxide Treatment on Mutagentic Activity in (Drinking) Water

Three disinfectants, viz. chlorine, chlorine dioxide, and ozone, were compared with respect to the formation of organic mutagens and halogenated organic compounds in drinking water. Chlorine increased the mutagenic activity (Ames test) drastically, as well as the level of halogenated organics. High chlorine dioxide doses (5 – 15 mg/L C10₂) also showed a drastic increase of mutagenic activity, while relatively low doses of chlorine dioxide (< 5 mg/L CIO₂) in most cases showed no effect or a slight increase in mutagenic activity. Hardly any increase in the level of halogenated organics was observed after this treatment.     

Structure and Replication of yDNA: A Novel Genetic Set Widened by Benzo‐Homologation

In a functioning genetic system, the information‐encoding molecule must form a regular self‐complementary complex (for example, the base‐paired double helix of DNA) and it must be able to encode information and pass it on to new generations. Here we study a benzo‐widened DNA‐like molecule (yDNA) as a candidate for an alternative genetic set, and we explicitly test these two structural and functional requirements. The solution structure of a 10 bp yDNA duplex is measured by using 2D‐NMR methods for a simple sequence composed of T–yA/yA–T pairs. The data confirm an antiparallel, right‐handed, hydrogen‐bonded helix resembling B‐DNA but with a wider diameter and enlarged base‐pair size. In addition to this, the abilities of two different polymerase enzymes (Klenow fragment of DNA pol I (Kf) and the repair enzyme Dpo4) to synthesize and extend the yDNA pairs T–yA, A–yT, and G–yC are measured by steady‐state kinetics studies. Not surprisingly, insertion of complementary bases opposite yDNA bases is inefficient due to the larger base‐pair size. We find that correct pairing occurs in several cases by both enzymes, but that common and relatively efficient mispairing involving T–yT and T–yC pairs interferes with fully correct formation and extension of pairs by these polymerases. Interestingly, the data show that extension of the large pairs is considerably more efficient with the flexible repair enzyme (Dpo4) than with the more rigid Kf enzyme. The results shed light on the properties of yDNA as a candidate for an alternative genetic information‐encoding molecule and as a tool for application in basic science and biomedicine.     

7‐Azidomethoxy‐Coumarins as Profluorophores for Templated Nucleic Acid Detection

Templated nucleic acid detection is an emerging bioanalytical method that makes use of the target DNA or RNA strand to initiate a fluorogenic reaction. The Staudinger reduction holds particular promise for templated sensing of nucleic acids because the involved functional groups are highly chemoselective. Here, the azidomethoxy group, which can be removed under Staudinger conditions, is used to cage 7‐hydroxycoumarin fluorophores. Reduction by phosphines and subsequent loss of the azidomethoxy substituent induce a significant bathochromic shift of the major absorbance band in the near UV region. When excited at the appropriate wavelength, this change in the absorbance spectrum translates into a substantial fluorescence turn‐on signal. The described profluorophores are readily conjugated to amino‐modified DNAs and are rapidly uncaged by a triphenylphosphine–DNA probe under the control of a DNA template. In addition, turnover of the probes on the target strand occurs and yields substantial signal amplification.     

A study on the β′ and β″ formation kinetics in AA6063 using differential scanning calorimetry

In this work the formation of β″ and β′ precipitates in a commercial AA6063 alloy was investigated using differential scanning calorimetry (DSC) and in situ heat treatments. The DSC used is equipped with fast heating and quenching facilities. The samples were solutionized, isothermally aged to induce formation of β″ or β′, rapidly quenched, and subjected to a DSC scan. The heat treatment cycle was performed entirely in the DSC, to improve the reproducibility of the dissolution and precipitation peaks. The effect of the aging treatment on the size and location of the β″ and β′ peaks was studied. It was found that after aging at 458 K (185 °C) the size of the β″ dissolution peak decreased with aging time, which is correlated with the amount of β″ already precipitated. Similar results were obtained for β′ precipitation at 523 K (250 °C) and 573 K (300 °C). An isothermal transformation diagram for β″ and β′ formation is presented, which is constructed from these experiments. The in situ method proved to be a viable method for studying the transformation sequence and kinetics in this type of alloy.     

Constitutive behavior of as-cast AA1050, AA3104, and AA5182

Recent thermomechanical modeling to calculate the stress field in industrially direct-chill (DC) cast-aluminum slabs has been successful, but lack of material data limits the accuracy of these calculations. Therefore, the constitutive behavior of three aluminum alloys (AA1050, AA3104, and AA5182) was determined in the as-cast condition using tensile tests at low strain rates and from room temperature to solidus temperature. The parameters of two constitutive equations, the extended Ludwik equation and a combination of the Sellars-Tegart equation with a hardening law, were determined. In order to study the effect of recovery, the constitutive behavior after prestraining at higher temperatures was also investigated. To evaluate the quantified constitutive equations, tensile tests were performed simulating the deformation and cooling history experienced by the material during casting. It is concluded that both constitutive equations perform well, but the combined hardening-Sellars-Tegart (HST) equation has temperature-independent parameters, which makes it easier to implement in a DC casting model. Further, the deformation history of the ingot should be taken into account for accurate stress calculations.