Measurement and modeling of Alloy-Spinel-Corundum equilibrium in the Ni-Mn-Al-0 system at 1873 K

The oxygen content of liquid Ni-Mn alloy equilibrated with spinel solid solution, (Ni,Mn)O. (1 +x)A12O3, and α-Al2O3 has been measured by suction sampling and inert gas fusion analysis. The corresponding oxygen potential of the three-phase system has been determined with a solid state cell incorporating (Y2O3)ThO2 as the solid electrolyte and Cr + Cr2O3 as the reference electrode. The equilibrium composition of the spinel phase formed at the interface of the alloy and alumina crucible was obtained using EPMA. The experimental data are compared with a thermodynamic model based on the free energies of formation of end-member spinels, free energy of solution of oxygen in liquid nickel, interaction parameters, and the activities in liquid Ni-Mn alloy and spinel solid solution. Mixing properties of the spinel solid solution are derived from a cation distribution model. The computational results agree with the experimental data on oxygen concentration, potential, and composition of the spinel phase.

Alloy oxide equilibria in the system Ca Al O at 1373 K

The tie lines delineating equilibria between different oxides of the Ca-Al-O system and liquid Ca-Al alloy has been determined at 1373 K. Equilibration of the alloy with two adjacent oxide phases in the CaO-Al2O3 pseudo-binary system was established in a closed cell made of iron. Equilibrium oxide phases were confirmed by x-ray analysis and alloy compositions were determined by chemical analysis. The compound 12CaO.7Al2O3 Ca12Al14O33 was found to be a stable phase in equilibrium with calcium alloys. The experimental diagram is consistent with that calculated from the free energies of formation of the oxide phases and activities in liquid Ca-Al alloys at 1373 K reported in the literature.

Solid state electrochemical sensor for monitoring Mg in Al refining process

Novel solid-state electrochemical sensors have been designed using the Mg2+ cation conductors incorporating novel solid-state reference electrodes for in-line monitoring of Mg in molten Al during the refining process and also for in-line monitoring of Mg content in molten Al in the alloying process. In this paper we report the preparation of Mg2+ ion conductors, MgAl2O4 and MgZr4(PO4)6, by the solid state ceramic synthesis route, measurement of their electrical properties using ac-impedance spectroscopy and application of the above cation conductors for designing novel electrochemical sensors for monitoring Mg dissolved in molten Al. The activation energy for Mg2+ ion conduction in MgAl2O4 is 2.08 eV and in MgZr4(PO4)6 is 1.7 eV, respectively. The sensors have been found to respond rapidly to change in Mg content in molten aluminium around 1000 K.

Thermodynamics of Al(1-x)Ga(x)N solid solution: Inclination for phase separation and ordering

Although Al(1-x)Ga(x)N semiconductors are used in lighting, displays and high-power amplifiers, there is no experimental thermodynamic information on nitride solid solutions. Thermodynamic data are useful for assessing the intrinsic stability of the solid solution with respect to phase separation and extrinsic stability in relation to other phases such as metallic contacts. The activity of GaN in Al(1-x)Ga(x)N solid solution is determined at 1100 K using a solid-state electrochemical cell: Ga + Al(1-x)Ga(x)N/Fe, Ca(3)N(2)//CaF(2)//Ca(3)N(2), N(2) (0.1 MPa), Fe. The solid-state cell is based on single crystal CaF(2) as the electrolyte and Ca(3)N(2) as the auxiliary electrode to convert the nitrogen chemical potential established by the equilibrium between Ga and Al(1-x)Ga(x)N solid solution into an equivalent fluorine potential. Excess Gibbs free energy of mixing of the solid solution is computed from the results. Results suggest an unusual mixing behavior: a mild tendency for ordering at three discrete compositions (x = 0.25, 0.5 and 0.75) superimposed on predominantly positive deviation from ideality. The lattice parameters exhibit slight deviation from Vegard's law, with the a-parameter showing positive and the c-parameter negative deviation. Although the solid solution is stable in the full range of compositions at growth temperatures, thermodynamic instability is indicated at temperatures below 410 K in the composition range 0.26 <= x <= 0.5. At 355 K, two biphasic regions appear, with terminal solid solutions stable only for 0 <= x <= 0.26 and 0.66 <= x <= 1. The range of terminal solid solubility reduces with decreasing temperature. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Chemical and shifted mechanical thresholds in Al-Ge-Te glass system

We report the results of the electrical switching studies performed on the bulk Al20GexTe80-x (2.5 less than or equal to x less than or equal to 15) chalcogenide glasses. The well known topological features, mechanical and chemical thresholds are observed. Mechanical threshold is seen at a mean coordination number of atoms, < r > = 2.50 (x = 5) a clear shift rom the mean field value of < r > = 2.4 whereas the chemical threshold is observed at < r > = 2.65 (x = 12.5) as predicted by the chemically ordered covalent network model These experiments are a sequel to similar experiments on Al20AsxTe80-x glasses in which mechanical threshold was seen at < r > = 2.60 and no chemical threshold was observed These results am well understood by a chemical bond picture developed in this article.

Optical, electrical and structural characterization of ZnO:Al thin films prepared by a low cost sol-gel method

ZnO:Al thin films were prepared on glass and silicon substrates by the sol-gel spin coating method. The x-ray diffraction (XRD) results showed that a polycrystalline phase with a hexagonal structure appeared after annealing at 400 degrees C for 1 h. The transmittance increased from 91 to about 93% from pure ZnO films to ZnO film doped with 1 wt% Al and then decreased for 2 wt% Al. The optical band gap energy increased as the doping concentration was increased from 0.5 wt% to 1 wt% Al. The metal oxide semiconductor (MOS) capacitors were fabricated using ZnO films deposited on silicon (100) substrates and electrical properties such as current versus voltage (I-V) and capacitance versus voltage (C-V) characteristics were studied. The electrical resistivity decreased and the leakage current increased with an increase of annealing temperature. The dielectric constant was found to be 3.12 measured at 1 MHz. The dissipation value for the film annealed at 300 degrees C was found to be 3.1 at 5 V. (C) 2011 Elsevier Ltd. All rights reserved.

Effect of Pt on interdiffusion and mechanical properties of the gamma and gamma ` phases in the Ni-Pt-Al system

The effect of Pt on the growth kinetics of the gamma'-Ni(Pt)](3)Al ordered intermetallic phase and the gamma- Ni(Pt, Al) solid solution diffusion rates of the species, hardness and elastic modulus was examined by employing the diffusion couple experimental technique. Experiments were conducted by using the beta-Ni(Pt)Al phase and Ni(Pt) alloy couples, each of which had a fixed amount of Pt (5, 10 and 15 at. %) in both the end members so that the Pt content is more or less constant throughout the interdiffusion zone. The results suggest that the growth kinetics of both phases and the average effective interdiffusion coefficients of Ni and Al increase with the increase in Pt content. Nanoindentation studies across the compositional gradients show that the mechanical properties of the intermetallic phase in the superalloy are relatively insensitive to the presence of Pt but are more sensitive to the Ni/Al ratio. In contrast, the marked variation in the hardness of the gamma phase were noted, increasing markedly with Al concentration in a given couple and also increasing with increasing Pt content. Possible causes for the observed variations are discussed.

Influence of Bottom Bubbling Rate on Formation of Metal Emulsion in Al-Cu Alloy and Molten Salt System

In steel refining process, an increase of interfacial area between the metal and slag through the metal droplets emulsified into the slag, so-called ``metal emulsion'', is one prevailing view for improving the reaction rate. The formation of metal emulsion was experimentally evaluated using Al-Cu alloy as metal phase and chloride salt as slag phase under the bottom bubbling condition. Samples were collected from the center of the salt phase in the container. Large number of metal droplets were separated from the salt by dissolving it into water. The number, surface area, and weight of the droplets increased with the gas flow rate and have local maximum values. The formation and sedimentation rates of metal droplets were estimated using a mathematical model. The formation rate increased with the gas flow rate and has a local maximum value as a function of gas flow rate, while the sedimentation rate is independent of the gas flow rate under the bottom bubbling condition. Three types of formation mode of metal emulsion, which occurred by the rupture of metal film around the bubble, were observed using high speed camera. During the process, an elongated column covered with metal film was observed with the increasing gas flow rate. This elongated column sometimes reached to the top surface of the salt phase. In this case, it is considered that fine droplets were not formed and in consequence, the weight of metal emulsion decreased at higher gas flow rate.

Texture and formability studies on AA7020 Al alloy sheets

This paper deals with the influence of crystallographic texture on room temperature mechanical behavior of the sheets of the aluminum alloy AA7020 processed to different thicknesses. Three different thicknesses of the alloy sheet, namely 1, 1.85, and 3.6 mm, corresponding to different textures were investigated. Tensile tests were carried out at 0°, 45° and 90° with respect to sheet rolling direction and the resulting in-plane anisotropy in 0.2 proof stress, work hardening and plastic strain ratio (r-value) were determined. Texture derived r-values are also calculated and discussed vis-à -vis the experimentally obtained r-values. Finally the formability of the optimal alloy was studied using forming limit diagrams. Effect of natural aging, with a simulated heat treatment of 70 °C for 2 h on FLD was studied and compared with the as solutionized samples. It was observed that, the strain levels in the bi-axial region of the FLD were not much affected by the heat treatment. © 2012 Elsevier B.V. All rights reserved.

Ultrahigh-temperature metamorphism from an unusual corundum plus orthopyroxene intergrowth bearing Al-Mg granulite from the Southern Marginal Zone, Limpopo Complex, South Africa

The Southern Marginal Zone of the Limpopo Complex is composed of granite-greenstone cratonic rocks reworked by a Neoarchean high-grade tectono-metamorphic event. Petrographic and mineral chemical characterization of an Al-Mg granulite from this zone is presented here. The granulite has a gneissic fabric with distinct Al-rich and Si-rich layers, with the former preserving the unusual lamellar (random and regular subparallel) intergrowths of corundum and symplectic intergrowth of spinel with orthopyroxene. The Al-rich layer preserves mineral assemblages such as rutile with orthopyroxene + sillimanite +/- A quartz, Al-rich orthopyroxene (similar to 11 wt%), spinel + quartz, and corundum in possible equilibrium with quartz, while the Si-rich layer preserves antiperthites and orthopyroxene + sillimanite +/- A quartz, all considered diagnostic of ultrahigh-temperature metamorphism. Application of Al-in-opx thermometry, ternary feldspar thermometry and construction of suitable pressure-temperature phase diagrams, compositional and model proportion isopleth results indicate P-T conditions as high as similar to 1,050-1,100 A degrees C, and similar to 10-12 kbars for the Al-Mg granulite. Our report of ultrahigh-temperature conditions is significant considering that the very high temperature was reached during decompression of an otherwise high-pressure granulite complex (clockwise P-T path), whereas most other ultrahigh-temperature granulites are linked to magma underplating at the base of the crust (counterclockwise P-T path).

Colossal nonsaturating linear magnetoresistance in two-dimensional electron systems at a GaAs/(Al,Ga)As heterointerface

Engineering devices with a large electrical response to magnetic field is of fundamental importance for a range of applications such as magnetic field sensing and magnetic read heads. We show that a colossal nonsaturating linear magnetoresistance (NLMR) arises in two-dimensional electron systems hosted in a GaAs/AlGaAs heterostructure in the strongly insulating regime. When operated at high source-drain bias, the magnetoresistance of our devices increases almost linearly with magnetic field, reaching nearly 10 000% at 8 T, thus surpassing many known nonmagnetic materials that exhibit giant NLMR. The temperature dependence and mobility analysis indicate that the NLMR has a purely classical origin, driven by nanoscale inhomogeneities. A large NLMR combined with small device dimensions makes these systems an attractive candidate for on-chip magnetic field sensing.

A new method for fracture toughness determination of graded (Pt,Ni)Al bond coats by microbeam bend tests

A novel method is proposed for fracture toughness determination of graded microstructurally complex (Pt,Ni)Al bond coats using edge-notched doubly clamped beams subjected to bending. Micron-scale beams are machined using the focused ion beam and loaded in bending under a nanoindenter. Failure loads gathered from the pop-ins in the load-displacement curves combined with XFEM analysis are used to calculate K-c at individual zones, free from substrate effects. The testing technique and sources of errors in measurement are described and possible micromechanisms of fracture in such heterogeneous coatings discussed.

Microstructural Evolution of A356 Al Alloy During Flow Along a Cooling Slope

Cooling slope (CS) has been used in this study to prepare semi-solid slurry of A356 Al alloy, keeping in view of slurry generation on demand for Rheo-pressure die casting process. Understanding the physics of microstructure evolution during cooling slope slurry formation is important to satisfy the need of semi-sold slurry with desired shape, size and morphology of primary Al phase. Mixture of spherical and rosette shaped primary Al phase has been observed in the samples collected during melt flow through the slope as well as in the cast (mould) samples compared to that of dendritic shape, observed in case of conventionally cast A356 alloy. The liquid melt has been poured into the slope at 650 A degrees C temperature and during flow it falls below the liquidus temperature of the said alloy, which facilitates crystallization of alpha-Al crystals on the cooling slope wall. Crystal separation due to melt flow is found responsible for nearly spherical morphology of the primary Al phase.

Effect of length scale on mechanical properties of Al-Cu eutectic alloy

This paper attempts a quantitative understanding of the effect of length scale on two phase eutectic structure. We first develop a model that considers both the elastic and plastic properties of the interface. Using Al-Al2Cu lamellar eutectic as model system, the parameters of the model were experimentally determined using indentation technique. The model is further validated using the results of bulk compression testing of the eutectics having different length scales. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.4761944]