Modelling of interstitials in the bcc phase

There are several widespread thermodynamic datasets which produce a spurious bcc interstitial solution at high temperature and high X content (X is an interstitally dissolved element). The reason for this is the standard model for bcc interstitial solutions (M(V a,X)₃), which requires careful selection of optimising parameters to minimise spurious appearances of the bcc phase. In this work the model M(V a,X)₁ is suggested as an alternative. This model is much easier to handle and its parameters can be directly compared with those of the fcc phase. The two models are compared for the Fe–C and Nb–N systems. In the Fe–C system almost identical results are achieved. In Nb–N there are some differences for high N content, but there is no experimental data to clearly support any model.     

Solvus Composition Paths in Multicomponent Alloys—Experimental Approach and Correlation with Calphad Calculations for the Example Al–Mg–Si

An experimental approach employing temperature and concentration gradients is presented that is suitable for determining a sequence of solvus concentrations in a single experimental cycle. The Al–Mg–Si system is used as an example. Al solid solutions (“(Al)”) with different compositions and Mg₂Si precipitates were equilibrated in the temperature range between 350 and 550 °C. The equilibrium compositions of (Al) were measured by energy dispersive X‐ray spectroscopy in the transmission electron microscope. The coupled temperature‐concentration values represent a path on the ternary solvus surface. Calphad calculations using different thermodynamic data sets were carried out to relate the experimental results to phase diagram evaluations. The measured solvus path agrees well with solvus data calculated with most recent thermodynamic parameter sets. The experimental approach can be applied to multicomponent alloys irrespective of their number of components with the same experimental effort. The efficiently generated solubility data are suitable to support phase diagram evaluations of multicomponent systems by the Calphad method.     

CeO2−CoO Phase Diagram

Phase equilibria in the CeO₂−CoO system at temperatures above 1500°C were investigated. The microstructures and the phase compositions of the DTA (differential thermal analysis) samples and the quenched solid pellets were analyzed using SEM (scanning electron microscope), EDX (energy dispersive X-ray), and WDX (wavelength dispersive X-ray). A eutectic reaction was found at 1645 ± 5°C. The eutectic point was calculated to be at 82 ± 1.5 mol% CoO. The eutectic phases were the CeO₂-rich phase (containing <5 mol% CoO) and the CoO-rich phase (containing ∼0.5 mol% CeO₂). At 1580°C, the solubility of CoO in CeO₂ was ∼3 mol%.     

Molar volumes of Al,Li, Mg and Si

The molar volumes of the fcc, bcc, hcp and liquid phases of Al, Li, Mg and Si as well as diamond Si have been evaluated as functions of temperature based on experimental data from the literature. The molar volume of each element in each structure is described by a single polynomial expression as a function of temperature. These polynomials can be used above around 150 K. The molar volumes of the liquids were described by a linear temperature dependence. The molar volumes of nonstable structures were evaluated with the help of lattice parameter measurements of the corresponding solid solutions. A large majority of the solid solutions studied showed negative excess volumes. The molar volumes of the relatively closely packed fcc, bcc and hcp structures were always found to be very close to each other, and a reasonably good approximation would be to set them as equal.     

Thermodynamic assessment of the lanthanum-oxygen system

The data on the thermodynamic properties of La₂O₃ have been reviewed and optimized using the CALPHAD method. A consistent set of parameters is presented. Data on this system are scarce and, with the exception of a few datapoints on substoichiometric La₂O_3−x and one measurement of oxygen solubility in La metal, limited to the properties of pure La and pure La₂O₃. Using the optimized parameters, a tentative phase diagram and stability diagram have been calculated.     

Experimental Phase Diagram Study and Thermodynamic Optimization of the Ag-Bi-O System

The Ag-Bi-O system has been experimentally studied using differential thermal analysis (DTA) and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), and thermodynamically optimized. The temperatures of the eutectic, monotectic, and Bi₂O₃ allotropic transformations have been measured in N₂, in air, and in O₂ by DTA. There are no ternary phases stable at ambient pressure. Presently measured transformation temperatures have been combined with existing oxygen activity measurements in the metal liquid to optimize thermodynamic parameters describing the liquid phase. The resulting fit is excellent. EDX measurements of the composition in the oxide liquid have a rather low precision but confirm the thermodynamic optimization. However, some uncertainties remain concerning the liquid composition at the eutectic transformation and the shape of the miscibility gap at higher temperatures.