Structure and stability of Al-Cu-Ru face-centered icosahedral alloys

The phases and microstructures in rapidly solidified Al-Cu-Ru alloys were investigated in this study. A chemically and topologically disordered icosahedral (i) phase grows dendritically from the liquid as the primary solidification product over the entire compositional region studied. The as-solidified i-phase is metastable and transforms to crystalline products at ≈500°C. The i-phase was not found as a product of the exothermic transformation for any composition, indicating that it is not the low temperature stable phase in the Al-Cu-Ru system;A chemically and topologically ordered i-phase was found to be an equilibrium phase at temperatures above ≈670°C and exists over a compositional region of several atomic percent. Once formed, this phase was easily retained at lower temperatures because of kinetic limitations of the transformation to the low temperature crystalline phase;Crystalline phases which from diffraction results appear structurally similar to the i-phase were also found in the Al-Cu-Ru system. These approximant phases aid in the determination of the atomic structure of i-phases by having common structural units. A simple cubic structure (a = 12.38 A, Pm3) containing a bcc network of icosahedral clusters was discovered. Comparisons of this phase with the i-phase indicated that strong similarities exist between the two structures;A rhombohedral approximant phase was also found. It exists as a transition state between the low-temperature crystalline phase and the high-temperature i-phase. This approximant phase also contains local icosahedral symmetry. The strong presence of icosahedral clusters in approximant phases in the Al-Cu-Ru system points to the distinct possibility that the i-phase is a quasiperiodic packing of icosahedral clusters of atoms

Effect of combined metal-carbon additions on the microstructure and structure of Sm2Fe17

The effect of combined alloying additions on the structure and scale of rapidly solidified Sm–Fe alloys was investigated. Transition metal additions tend to promote the formation of the disordered TbCu7-type structure in Sm2Fe17 alloys, as determined by monitoring the long-range order parameter. Essentially no order was observed for M = Ti, Zr, V, or Nb. Thus, the structure was close to the prototypical TbCu7-type structure. With M = Si, a large amount of order was observed (S = 0.62), resulting in a structure closer to the well-ordered Th2Zn17-type. The microstructural scale was also affected by alloying. In this case, refinement depended on the substituent and also on carbon for microstructural refinement. The scale of the as-solidified grain structures ranged from 100 nm for SiC-modified alloys to 13 nm for NbC-modified alloys. The degree of refinement was directly related to the atomic size of the M addition. The refinement was the result of solute partitioning to grain boundaries, resulting in a solute drag effect that lowered the growth rates

Effect of Nb and C Additives on the Microstructures and Magnetic Properties of Rapidly Solidified Sm-Co Alloys

Highly coercive Sm-Co-based permanent magnets have been achieved through simple modification of binary Sm12Co88 alloys with Nb, C or combined Nb and C at concentrations ranging from 1 to 10 atomic percent processed via rapid solidification. Melt spinning at 40 m/s resulted in the formation of the metastable TbCu7-type structure in all alloys. While the unalloyed, as-solidified Sm12Co88 alloy displayed a coercivity of 0.5 kOe, alloying additions resulted in a systematic and profound increase in coercivity. Nb additions resulted in as-solidified coercivities up to 9 kOe, C additions up to 37 kOe, and combined NbC additons 8 kOe. The Nb and NbC additions led to a reduction in grain size, while C additions altered the morphology, producing a grain-boundary phase that effectively isolated the magnetic grains from one another. The magnetization processes for Nb- and NbC-modified Sm–Co were determined to be nucleation-controlled, while a transition to pinning-controlled magnetization was observed for the C-modified alloy

Sources of unburned carbon in the fly ash produced from low-NOx pulverized coal combustion

Journal ArticleThe unburned carbon in the fly ash produced from low-NOx pulverized coal combustion is shown to consist of a mixture of soot and coal char. The soot was identified by the presence of chains or aggregates of 10-50-nm-diameter primary particles in electron microscope images of both laboratory samples and a sample of fly ash from a power plant operating low-NOx burners. Laboratory samples showed increasing carbon content with decreasing nitrogen oxide (NOx) concentration. The experiments included a high-NOx base case and four low-NOx cases consisting of (1) staged combustion with short (0.5 s) residence time, (2) staged combustion with long (1.5 s) residence time, (3) a low-NOx burner with slow mixing, and (4) reburning using coal as the reburning fuel. Comparison of the base case that used premixed coal and air with the long-residence-time staged combustion case shows a decrease in the NOx from over 900 ppm to below 200 ppm and an increase in the carbon in the ash from 4% to over 30%. The fly ash from staged combustion was a mixture of large soot aggregates, porous char, and spherical particles of mineral ash, whereas the ash from reburning lacked the large aggregates. For all laboratory conditions, the carbon content in the particle fraction with an aerodynamic diameter over 10 lm was higher than in the 1-2.5- lm-diameter fraction. Both soot aggregates and char contributed to the high carbon in the large particle fraction. The difference in carbon burnout between the two staging conditions was consistent with published soot oxidation rates. Both char burnout and soot formation need to be considered in studies of the carbon content of pulverized coal fly ash

Rocks and Hard Places: Exploring Educational Psychologists’ Perspectives on “Off-Rolling” or Illegal Exclusionary Practices in Mainstream Secondary Schools in England

Research being undertaken by the Universities of Exeter and Plymouth is exploring educational psychologists’ knowledge of, and perspectives on, exclusionary practices in schools in England, particularly illegal practices referred to as “off-rolling”. Preliminary findings from the survey element of a mixed methods research project are reported here. The role of business models in the provision of educational psychology services to schools is considered through the conceptual lens of Giroux, Agamben and Ball to highlight ambiguities around the client relationship and to recast individualised ethical dilemmas as systemic features that inhibit direct challenges to school practices relating to inclusion. It is suggested that traded and privatised services risk implicating educational psychologists in schools’ management of the (in)visibility of “off-rolling” and the manufactured legitimacy of varied exclusionary practices

Anomalous Eutectic Microstructures in Mg-Al Structural Alloy Prepared by Rapid Solidification

Magnesium is the lightest engineering metal 1 However, conventional Mg alloys typically suffer from low strength and poor deformability due to very few slip systems and easy twinning 3 Alloying Mg with other materials and microstructural engineering are promising approaches to increase ductility and strength of Mg In the current work, non equilibrium solidification conditions were applied to induce a transition from regular to anomalous eutectic in Mg Al eutectic alloy such that four distinguished microstructures were acquired and the corresponding formation mechanisms were investigate

Texture formation in FePt thin films via thermal stress management

The transformation variant of the fcc to fct transformation in FePt thin films was tailored by controlling the stresses in the thin films, thereby allowing selection of in- or out-of-plane c-axis orientation. FePt thin films were deposited at ambient temperature on several substrates with differing coefficients of thermal expansion relative to the FePt, which generated thermal stresses during the ordering heat treatment. X-ray diffraction analysis revealed preferential out-of-plane c-axis orientation for FePt films deposited on substrates with a similar coefficients of thermal expansion, and random orientation for FePt films deposited on substrates with a very low coefficient of thermal expansion, which is consistent with theoretical analysis when considering residual stresses