Influence of homogenization on the processing map for hot working of as-cast Mg-2Zn-1Mn alloy

Processing maps have been developed for hot deformation of Mg-2Zn-1Mn alloy in as-cast condition and after homogenization with a view to evaluate the influence of homogenization. Hot compression data in the temperature range 300-500degreesC and strain rate range 0.001-100 s(-1) were used for generating the processing map. In the map for the as-cast alloy the domain of dynamic recrystallization occurring, at 450degreesC and 0.1 s(-1) has merged with another domain occurring at 500degreesC and 0.001 s(-1) representing grain boundary cracking. The latter domain is eliminated by homogenization and the dynamic recrystallization domain expanded with a higher peak efficiency occurring at 500 degreesC and 0.05 s(-1). The flow localization occurring at strain rates higher than 5 s(-1) is unaffected by homogenization.

Modeling of wheeled mobile robots using dextrous manipulation kinematics

Abstract—This document introduces a new kinematic simulation of a wheeled mobile robot operating on uneven terrain. Our modeling method borrows concepts from dextrous manipulation. This allows for an accurate simulation of the way 3-dimensional wheels roll over a smooth ground surface. The purpose of the simulation is to validate a new concept for design of off-road wheel suspensions, called Passive Variable Camber (PVC). We show that PVC eliminates kinematic slip for an outdoor robot. Both forward and inverse kinematics are discussed and simulation results are presented.

The determination of stem cell fate by 3D scaffold structures through the control of cell shape

Stem cell response to a library of scaffolds with varied 3D structures was investigated. Microarray screening revealed that each type of scaffold structure induced a unique gene expression signature in primary human bone marrow stromal cells (hBMSCs). Hierarchical cluster analysis showed that treatments sorted by scaffold structure and not by polymer chemistry suggesting that scaffold structure was more influential than scaffold composition. Further, the effects of scaffold structure on hBMSC function were mediated by cell shape. Of all the scaffolds tested, only scaffolds with a nanofibrous morphology were able to drive the hBMSCs down an osteogenic lineage in the absence of osteogenic supplements. Nanofiber scaffolds forced the hBMSCs to assume an elongated, highly branched morphology. This same morphology was seen in osteogenic controls where hBMSCs were cultured on flat polymer films in the presence of osteogenic supplements (OS). In contrast, hBMSCs cultured on flat polymer films in the absence of OS assumed a more rounded and less-branched morphology. These results indicate that cells are more sensitive to scaffold structure than previously appreciated and suggest that scaffold efficacy can be optimized by tailoring the scaffold structure to force cells into morphologies that direct them to differentiate down the desired lineage. Published by Elsevier Ltd.

Solubility and activity of oxygen in liquid antimony

The solubility limit of oxygen in liquid antimony has been measured by a novel isopiestic technique in the temperature range 995--1175 deg K. The results can be expressed by the equation log c = --5500/T + 3.754 ( plus/minus 0.04) with c in at.% O and T in deg K. The oxygen potential over Sb + O alloys equilibrium with Sb2O3 has been measured by a solid state cell using a fully stabilized CaO.ZrO2 electrolyte. The cell was designed to contain the Sb + Sb2O3 mixture in a closed volume, that the vaporization of the oxide can be minimized and true equilibrium attained. The Gibbs free energy of the reaction 2 Sb(s) + 3/2 O2 = Sb2O3(s) is Delta G deg = --719560 + 274.51 T( plus/minus 500) and Sb(l) + 3/2 O2 = Sb2O3(l), Delta G deg = --704711 + ( plus/minus 500) ( Delta G deg in J/mole, T in deg K). The combination of these results with Sieverts' law yields the standard free energy of solution of oxygen in liquid antimony according to the reaction 1/2 O2 = \O\Sb,at.% as Delta G deg = --129620 + 14.23 T ( plus/minus 950). The standard enthalopy and entropy of the solution of oxygen in Sb are compared with values for other metal- oyxgen systems, and with the standard enthalpies of formation of corresponding oxides. The resulting correlations permit the estimation of the standard free energy of solution of oxygen in pure metals for which experimental information is lacking. 24 ref.--AA

Three phase equilibria for activity determinations: system Co Mn

A solid state e.m.f. cell with yttria-doped thoria as the electrolyte and a mixture of Cr + Cr sub 2 O sub 3 as the reference electrode, was employed for the measurement of the activity of manganese in the Co--Mn system at 1760 deg K, for 0.3 > X sub Mn > 0.05. The liquid alloy was contained in an alumina crucible and saturated with MnAl sub 2+2x O sub 4+3x . The cell can be represented by Pt, W, (Co--Mn) + MnAl sub 2+2x O sub 4+3x + Al sub 2 O sub 3 /ThO sub 2 --Y sub 2 O sub 3 /Cr + Cr sub 2 O sub 3 , Pt. The activity of manganese shows negative deviations from Raoult's law. The activities in the Co--Mn system are intermediate between those in the Fe--Mn and Ni--Mn systems. The Gibbs' energies of mixing in these systems follow the trends given by Miedema's model. 14 ref.--AA.

Knudsen effusion measurements on Cr Mn alloys

The equilibrium partial pressures of Mn over bcc Cr--Mn alloys have been measured using Knudsen cell technique in the temp. range 1200-1500K. The alloys in particulate form were contained in thoria crucibles inside Knudsen cells made of tungsten. The rates of mass loss of each cell under vacuum was monitered as a function of time at constant temp. using a microbalance. Activities exhibit mild negative deviations from Raoult's law, contrary to indications from an earlier study using a fused salt emf technique. The Cr--Mn system is characterized by negative enthalpy and excess entropy of mixing. There is close similarity between the composition dependence of enthalpy and excess entropy. These findings suggest strong vibrational and negligible magnetic contributions to excess entropy of mixing in bcc phase at high temp. 10 ref.--AA

Thermodynamic properties and phase boundaries of Co O solutions

The thermodynamic properties of liquid unsaturated Co--O solutions have been determined by electrochemical measurements using (Y sub 2 O sub 3 )ThO sub 2 as solid electrolyte. The cell can be represented as, Pt. MoO sub 2 + Mo | (Y sub 2 O sub 3 )ThO sub 2 | O sub Co , tungsten, Pt, Emf of the cell was measured as a function of oxygen concentration in liquid Co at 1798, 1873 and 1973K. Least-mean squares regression analysis of the experimental data gives for the free energy of solution of diatomic oxygen in liquid Co Delta G exp 0 sub O(Co) = --84935--7.61 T ( plus/minus 400) J/g-atom and self interaction parameter for oxygen epsilon exp O sub O = --97240/T + 40.52 ( plus/minus 1) where the standard state for O is an infinitely dilute solution in which the activity is equal to atomic percent. The present data are discussed in comparison with those reported in the literature and the phase diagram for the Co--O system. 18 ref.--AA.

Thermodynamic properties of Cr Mo solid alloys

The activity of Cr in solid Cr-Mo alloys has been measured at 1873 K using a metal-oxide-gas equilibrium technique. Thin foils of Mo were equilibrated with solid Cr203 under flowing gas mixtures of argon, hydrogen and watervapourof known composition. The equilibrium concentration of Cr in Mo was determined by chemical analysis. These measurements indicate positive deviations from Raoult's law. The activity data obtained in the study at 1873 K are combined with free energy of mixing at 1471 K, calorimetric enthalpy of mixing at 1673 K, and experimental evidence of phase separation at lower temperatures, reported in the literature, to obtain an optimised set of thermodynamic parameters for the Cr-Mosystem in the solid state.

Oxygen content of liquid cobalt in equilibrium with CoO.(1+x)Al2O3 and -Al2O3

The oxygen concentration of liquid cobalt in equilibrium with cobalt aluminate and a-alumina has been measured by suction sampling and crucible quenching techniques at temperatures between 1770 and 1975 K. Experiments were made with cobalt of high and low initial oxygen contents, and with and without the addition of cobalt aluminate. The effect of temperature on the equilibrium oxygen content is represented by the equation, log (at.% 0) = -10,4001T(K) + 4.64 (±0.008). The composition of the spinel phase, CoO.(1+x)AI20 3, saturated with alumina, has been determined by electron probe microanalysis. The values of x are 0.22 at 1770 Kand 0.28 at 1975 K. The oxygen potential corresponding to the three-phase equilibrium between cobalt, aluminate and alumina, and the standard Gibbs' energy of formation of nonstoichiometric cobalt aluminate are evaluated by combining the results of this study with recently published data on the activity of oxygen in liquid cobalt. Implications of the present results to aluminium deoxidation of liquid cobalt are discussed.

Measurement of the Gibbs energy of formation of URh3 using an oxide solid electrolyte

The Gibbs' energy offormation of the intermetallic compound URh3has been measured in the temperature range 980 to 1320 K using an oxide solid state cell incorporating yttria-doped thoria as the solid electrolyte and a mixture of manganese and manganese oxide as the reference electrode. The cell can be represented as Pt, Mn + MnO I (Y203)Th02 I Rh + URh3 + U02 + x' Rh, Pt The reversible emf of the cell was a linear function of temperature E = 15.60 +0.0237 T (±0.8) mY. Using auxiliary thermodynamic data for MnO and U02+ x the Gibbs' energy of formation of URh3 from component metals has been computed. The results can be expressed by the equation L'.G?< URh3 > = -316240 + 13.22 T (± 3000) J mol-1. The "third-law" enthalpy of formation of URh3at 298 K is -293.2 (± 4) kJ mol-1, significantly more negative than the value of -181.5 kJ mol-1 calculated using Miedema's model.

Thermodynamics of Aluminium -Strontium Alloys

The activity of strontium in liquid Al-Sr alloys (X(Sr) less-than-or-equal-to 0.17) at 1323 K has been determined using the Knudsen effusion-mass loss technique. At higher concentrations (X(Sr) greater-than-or-equal-to 0.28), the activity of strontium has been determined by the pseudoisopiestic technique. Activity of aluminium has been derived by Gibbs-Duhem integration. The concentration - concentration structure factor of Bhatia and Thornton at zero wave vector has been computed from the thermodynamic data. The behaviour of the mean square thermal fluctuation in composition and the thermodynamic mixing functions suggest association tendencies in the liquid state. The associated solution model with Al2Sr as the predominant complex can account for the properties of the liquid alloy. Thermodynamic data for the intermetallic compunds in the Al-Sr system have been derived using the phase diagram and the Gibbs' energy and enthalpy of mixing of liquid alloys. The data indicate the need for redetermination of the phase diagram near the strontium-rich corner.

A thermodynamic study of liquid Cu-Cr alloys and metastable liquid immiscibility

he thermodynamic acitivity of chromium in liquid Cu-Cr alloys is measured in the temperature range from 1473 to 1873 K using the solid state cell: Pt, W, Cr + Cr2O3 |(Y2O3) ThO2|Cu - Cr + Cr2O3, Pt The activity of copper and the Gibbs energy of mixing of the liquid alloy are derived. Activities exhibit large positive deviations from Raoult's law. The mixing properties can be represented by a pseudo-subregular solution model in which the excess entropy has the same type of functional dependence on composition as the enthalpy of mixing: ΔGE = XCr(1 - XCr)[60880 - 18750 XCr)-- T(16.25 - 7.55 XCr)]J mol-1 Pure liquid Cu and Cr are taken as the reference states. The results predict a liquid-liquid metastable miscibility gap, with TC = 1787 (±3) K and XCr = 0.436 (±0.02), lying below the liquidus. The results obtained in this study are in general agreement with experimental information reported in the literature, but provide further refinement of the thermodynamic parameters.

Structural and relaxor characteristics of Ca0.18Sr0.226Ba0.594Nb2O6

Polycrystalline Ca0.18Sr0.226Ba0.594Nb2O6 (CSBN18) was synthesized via the solid-state reaction route. X-ray structural studies confirmed it belonged to the tetragonal tungsten bronze family. Rietveld refinement of the X-ray data has been carried out for CSBN18 where the atomic positions and site occupancy factors for A-sites have been determined. The dielectric properties of CSBN18 ceramic were studied as a function of temperature in the 100 Hz - 1 MHz frequency range. The dielectric relaxation followed the Vogel-Fulcher relation wherein E-a = 37.4 meV; T-f = 131.5 degrees C and omega(0) = 4.31 x 10(9) rad s(-1). A high pyroelectric coefficient of similar to 250 mu C m(-2).K was obtained around the transition temperature (similar to 150 degrees C). This is significantly higher than that reported for polycrystalline SrxBa1-xNb2O6 (SBN). However, the piezoelectric coefficient (d(33)) of the title composition was as low as 6 pC N-1.

Tutorial: implementing a pedestrian tracker using inertial sensors

Shoe-mounted inertial sensors offer a convenient way to track pedestrians in situations where other localization systems fail. This tutorial outlines a simple yet effective approach for implementing a reasonably accurate tracker. This Web extra presents the Matlab implementation and a few sample recordings for implementing the pedestrian inertial tracking system using an error-state Kalman filter for zero-velocity updates (ZUPTs) and orientation estimation.

Isothermal transformation of beta-phase in Cu-rich Cu-Al-Sn alloys

Microstructural changes resulting from isothermal decomposition of the beta-phase have been studied in Cu-rich binary Cu-Al and ternary Cu-Al-Sn alloys containing up to 3 at.% Sn at temperatures from 873 to 673 K. Results are presented as TTT diagrams. The decomposition occurs in several stages, each of which involves the establishment of metastable equilibrium between beta and one or more of the product phases alpha, beta(1) and gamma(2). Addition of Sn has been shown to increase the stability of the ordered beta(1)-phase in relation to beta. In alloys containing more than 2 at.% Sn, the beta(1) emerges as a stable phase. At low Sn concentrations beta(1) is metastable. An important new finding is the existence of three-phase equilibrium microstructure containing alpha, beta(1) and gamma(2). Increasing addition of Sn alters the morphology of beta(1) from rosettes to dendrites and finally to Widmanstatten needles.