Papers by Keyword: Computational Thermodynamics

Abstract: Steel alloys with high Mn and low C, low Cr wt.%, were designed based on the composition system for traditional high toughness, creep resistance, and longevity for high-temperature applications. In terms of energy resource utilization during production and refining, CALPHAD strategical optimization is preferable for all steel alloys. Thermo-Calc software calculates the phase diagrams α-BCC (Ferrite), and M₂₃C₆ (carbide) phases. The vital temperatures which are highlighted in this work are Ac₃ (threshold temperature at which ferrite is fully transformed into austenite (α→γ)), and A₄ (the threshold temperature at which austenite is fully transformed into Delta ferrite (γ→δ)) are essential for phase transformations. JMatPro software is used to predict the mechanical properties of steel alloys. The interfacial energies with regards to alloying elements for M₂₃C₆ are calculated to be between ~0.272 J/m^-2 to ~0.328 J/m^-2 for α-BCC) matrix, while γ-FCC has interfacial energy ranges to be between ~0.132 J/m^-2 to ~0.168 J/m^-2. This paper focuses on investigating the effect of alloying elements on phase transformations, interfacial energy, coarsening rate of carbides, and many other mechanical properties such as toughness at high-temperature applications using CALPHAD strategies.

Abstract: Heusler alloys have been considered as one of the most promising thermoelectric materials for electrical power generation in a temperature range of 500–800 °C. Establishment of phase diagrams allows one to predict formation, equilibria, and stability of phases in of these ternary alloys. In this work we report on the simulation and investigation of phase diagram and phase equilibria in ternary Ti-Fe-Sb system which is of considerable interest for thermoelectric applications. The simulation was carried out using the CALPHAD method in Pandat software. The existence of the thermoelectric Heusler TiFe_1.5Sb phase was revealed in a temperature range from 970 to 1070 K. The equilibria between TiFe_1.5Sb and other phases were determined. The entropy of formation was calculated for the phases existing at 970, 1020 and 1070 K using a fitting approach. A narrow equilibrium region containing pure body centered cubic Fe and TiFe_1.5Sb was found.

Abstract: This chapter presents a novel method for analysis and optimization of the in-situ formation of TiC-reinforced composite surface layers (TRL) on a ductile cast iron substrate during the laser surface alloying process, combining the experimental approach with the computational thermodynamics. The microstructure of the TRLs has been assessed by light optical microscopy, scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction. The results of thermodynamic calculations with the Scheil-Gulliver model showed a good agreement with the experimental results, indicating that the actual solidification path for the analyzed Fe-C-Si-Ti alloy systems under the investigated range of laser processing conditions is close to the Scheil-Gulliver assumption.

Abstract: Numerical simulation of multicomponent alloy solidification demands accuracy of thermophysical properties in order to obtain a numerical representation as close as possible to the physical reality. Some alloy properties are only seldom found in the literature. In this paper, a solution of Butler’s formulation for surface tension is presented for Al-Cu-Si ternary alloys, allowing the Gibbs-Thomson coefficient to be calculated as a function of Cu and Si contents. The importance of the Gibbs-Thomson coefficient is related to the reliability of predictions furnished by predictive microstructure growth models and of numerical computations of solidification thermal variables that will be strongly dependent on the values of the thermophysical properties adopted in the calculations. A numerical model based on Powell hybrid algorithm and a finite difference Jacobian approximation was coupled with a ThermoCalc TCAPI interface to assess the excess Gibbs energy of the liquid phase, permitting the surface tension and Gibbs-Thomson coefficient for Al-Cu-Si hypoeutectic alloys to be calculated. The computed results are presented as a function of the alloy composition.

Abstract: Casting simulation requires high quality information about the thermophysical properties of the alloy, but the number of alloys for which such information is available is limited. In this paper, a solution of Butler’s formulation for surface tension is presented for Al-Cu-Si ternary alloys and consequently, permitting the Gibbs-Thomson coefficient to be determined. The importance of the Gibbs-Thomson coefficient is related to the reliability of predictions furnished by predictive microstructure growth models and of numerical computations of solidification thermal variables, which will be strongly dependent on the values of the thermophysical properties adopted in the calculations. The Gibbs-Thomson coefficient for ternary alloys is seldom reported in the literature. A numerical model based on Powell hybrid algorithm and on a finite difference Jacobian approximation was coupled with a ThermoCalc TCAPI interface to assess the excess Gibbs energy of the liquid phase, permitting the surface tension and Gibbs-Thomson coefficient for Al-Cu-Si hypoeutectic alloys to be calculated. The computed results are presented as a function of the alloy composition.

Abstract: This paper summarizes the development of new cast and wrought magnesium alloys using computational thermodynamics tools and experimental approach. The Mg-Al-Ca alloys show excellent creep resistance due to the formation of high-temperature (Mg,Al)₂Ca phase. The Mg-Al-Sn alloys are designed for mechanical properties and corrosion resistance through the optimization of Mg₁₇Al₁₂ and Mg₂Sn phases in the microstructure. In the Mg-Zn-Ce system, Zn provides strength through solid solution strengthening while Ce increases the ductility via improved texture. Mg-Nd-Zn is a heat-treatable alloy system based on the precipitation hardening of Mg₁₂Nd phase.

Abstract: The tetragonal lattice relaxation has been included in the thermodynamics of the fcc→L1₀ ordering to produce a first-order character of the transition within the mean field description of the binary solution energetics. In view of growing interest in such systems e.g. Fe-Pd and Co-Pt alloys, which display a wide range of applications relevant to current and futuristic technologies, the fcc→L1₀ two-phase field is re-examined utilizing a generalized Bragg-Williams approach including first and second nearest neighbor interactions. The thermodynamic behavior is examined in the limit of T→0K and discussed in terms of the implications of the Third Law of Thermodynamics.

Abstract: The Thermo-Calc and DICTRA software/database/programming-interface packages, through many successful applications in the fields of Computational Thermodynamics and Kinetics, have tremendously contributed to quantitative conceptual design and processing of various advanced materials. Materials scientists and engineers can efficiently apply such unique and comprehensive tools in calculating material properties, predicting material structures and simulating material processes, which are of wide-ranging industrial and academic importance.

Abstract: In order to be used for applications, the thermodynamic stability of a candidate hydrogen storage material should be suitable for hydrogen sorption at room conditions. By mixing different hydrides, it is possible to promote the hydrogenation/dehydrogenation processes. On the other hand, small changes in composition allow a tailoring of thermodynamic stability of hydrides. Knowledge of thermodynamic stability of hydrides is thus fundamental to study the hydrogenation/dehydrogenation processes and useful to rationalize synthesis reactions and to suggest possible alternative reaction routes. The purpose of this work is to develop a consistent thermodynamic database for hydrogen storage systems by the CALPHAD approach. Experimental data have been collected from the literature. When experimental measurements were scarce or completely lacking, estimations of the energy of formation of hydrides have been obtained by ab initio calculations performed with the CRYSTAL code. Several systems of interest for hydrogen storage have been investigated, including metallic hydrides (M-H) and complex hydrides. The effect on thermodynamic properties of fluorine-to-hydrogen substitution in some simple hydrides is also considered. Calculated and experimental thermodynamic properties of various hydrides have been compared and a satisfactory agreement has been achieved.