Phase diagrams and mechanical properties of TiC-SiC solid solutions from first-principles

First-principles calculations were carried out in order to investigate the stability and properties of the random Ti_1-xSi_xC solid solutions (alloys) with both the B1 and B3 structures. Lattice parameter, total energy, formation energy, phonon spectra, and elastic properties were studied as functions of composition. The phase diagram, in particular, binodal and spinodal curves were calculated. It was established that at 0 K the B1 alloys are energetically favorable at 0 ≤ x < 0.5, while the B3 alloys are favorable at 0.5 ≤ x ≤ 1.0. It was found that the contribution to the Gibbs free energy coming from the lattice vibrations strongly reduces the critical temperature of stabilization of the solid solutions. Calculated elastic moduli, Debye temperature, and Vickers hardness do not point to any strength enhancement of the alloys compared to TiC and SiC. Analysis of the spatial distribution of the Young and shear moduli shows that the B3 alloys exhibit much more spatial anisotropy of the elastic moduli than the B1 alloys.     

Ensemble mating selection in evolutionary many-objective search

Traditional multi-objective evolutionary algorithms have encountered difficulties when handling many-objective problems. This is due to the loss of selection pressure incurred by the growing size of objective space. A variety of environmental selection operators have been proposed to address the issue, each has its distinct benefits and drawbacks. We develop a novel ensemble framework to enhance the effectiveness and robustness of many-objective optimization. The framework incorporates multiple environmental selection operators to guide the search, which are then viewed as voters to construct a mating pool. We design an ensemble mating selection strategy that makes decisions based on the preference information provided by the voters: individuals elected by more voters will be assigned larger possibilities to enter the mating pool. By doing so, high quality offspring can be reproduced from the elected promising candidates. To accommodate the multiple selection operators for voting, the framework maintains multiple parallel populations, where each population is updated by one of the selection operators. An instantiation of the framework with three popular operators is presented as a prime example. Extensive experiments have been conducted on a number of many-objective problems to examine the effectiveness of the proposed approach. Experimental results show that the mating selection strategy is capable of improving the quality of the obtained solution set.     

Experimental investigation and thermodynamic modeling of the Fe−Si−Zr system

The Fe−Si−Zr alloys have attracted considerable interests in recent decades. It is important to study this ternary phase diagram for experimental design. In this paper, the Fe−Si−Zr ternary system is investigated by combining the experiments and thermodynamic calculations. Liquidus surface projection of the Fe−Si−Zr system is characterised using X-ray powder diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS) and differential thermal analysis (DTA). The liquidus projection of this ternary system is constructed by identifying primary crystallization phases and invariant reaction temperatures in the as-cast alloys. Eleven different primary solidification regions are observed. Based on the experimental results of this work and the data from the previous work in the literature, the thermodynamic calculation of the Fe−Si−Zr system is performed using CALPHAD (CALculation of PHAse Diagrams) technique. A set of self-consistent thermodynamic parameters of the Fe−Si−Zr system is obtained. The calculated results are in good agreement with the experimental data. This study provides a set of reliable thermodynamic parameters to the Fe-based thermodynamic database, and a cost-effective tool to design experiments and manufacturing processes.     

Characterization of ternary phase diagrams by means of thermal and rheological analyses

Context: Mixtures made of oil, water and surfactants give rise to a wide range of structure with different characteristics and phase manifestations.

Objective: Aim of this paper is to build up and understand the phase diagram of a model ternary system (Water, Polysorbate 80 and isopropyl myristate) by the use of common techniques such as thermal analysis and rheology, in comparison with visual assessment and polarized light microscopy.

Methods: Different ternary systems were prepared and analyzed by means of DSC and rheology in order to highlight the state of water (free, interphasal, bound water) and the samples structural characteristics.

Results: The resultant phase diagram is divided into four different zones. Bound water zone is predominant at elevated surfactant/oil ratios, while as the surfactant/oil ratio decreases, DSC reveals the presence of free water. Interphasal water prevails at intermediate water and surfactant content which corresponds with gels systems. Mechanical spectra allow to discern between cubic (true gel) and lamellar mesophases (weak gel), while flow curves allow to distinguish among microemulsions, emulsions or lamellar mesophases.

Discussion: A deeper characterization of a model ternary phase diagram is possible, with respect to the simple visual inspection, by the use of thermal analysis and rheology. The state of water molecules and the viscoelastic characteristics of the system allow to obtain important structural considerations.

Conclusions: In conclusion, the knowledge of the state of water and of the viscoelastic characteristics of the systems allow a deeper understanding of the structural features of the ternary phase diagram.     

Thermodynamic calculations of the Au–Sc and Fe–Sc systems

Thermodynamic optimization of the Au–Sc and Fe–Sc systems was carried out by means of the CALPHAD (CALculation of PHAse Diagram) method on the basis of the available experimental data in literature. Redlich–Kister polynomials were used to describe the excess Gibbs energy of solution phases, and all the compounds are treated as stoichiometric ones. The Au–Sc system was described thermodynamically for the first time, and the Fe–Sc system was re-optimized by considering the new experimental data about enthalpies of mixing of the liquid phase. A set of self-consistent parameters was obtained for each of these two binary systems, respectively.     

Effect of temperature on fast forging process of Mg-Ni samples for fast formation of Mg2Ni for hydrogen storage

Fast-Forging was used as a Severe Plastic Deformation technique to process Mg/Ni fine powder mixtures at a ratio corresponding to the eutectic composition. The samples were processed at different temperature, increased successively from room temperature up to above 500 °C. The one shock forging operation led to a reduction rate comprised between 80 to more than 90% depending of the applied temperature. Interestingly, a threshold temperature was pointed out for which amounts of the binary Mg₂Ni alloy were directly synthesized in increasing proportions when increasing temperature. A maximum amount of Mg₂Ni was synthesized according the nominal proportions at the highest applied temperature. Besides, numerical simulations were developed to consider and integrate to the forging process, the heat arising from the mechanical energy at deformation. Interestingly the total temperature at shock – heat applied to and heat developed in – indicates that the threshold temperature correspond well with the eutectic temperature as reported in the phase diagram. Early hydrogenation cycles suggest that both mechanical defects in brittle Mg and presence of amounts of Mg₂Ni as catalyst should be combined to optimize the hydrogenation characteristics.     

Freeze-dried ammonia borane-polyethylene oxide composites: Phase behaviour and hydrogen release

A solid-state hydrogen storage material comprising ammonia borane (AB) and polyethylene oxide (PEO) has been produced by freeze-drying from aqueous solutions from 0% to 100% AB by mass. The phase mixing behaviour of AB and PEO has been investigated using X-ray diffraction which shows that a new ‘intermediate’ crystalline phase exists, different from both AB and PEO, as observed in our previous work (Nathanson et al., 2015). It is suggested that hydrogen bonding interactions between the ethereal oxygen atom (–O–) in the PEO backbone and the protic hydrogen atoms attached to the nitrogen atom (N–H) of AB molecules promote the formation of a reaction intermediate, leading to lowered hydrogen release temperatures in the composites, compared to neat AB. PEO also acts to significantly reduce the foaming of AB during hydrogen release. A temperature-composition phase diagram has been produced for the AB-PEO system to show the relationship between phase mixing and hydrogen release.