Effect of Rotating Magnetic Field on Microstructure in AlCuSi Alloys

The solidification of AlCuSi alloys with Mn and Fe was studied by rotating a magnetic field to understand the effect of melt flow. The specimens solidified with a forced convection, low cooling rate and low temperature gradient. Electromagnetic stirring generated by an electric coil around the specimens caused a transformation from equiaxed dendritic to rosette morphology, occasionally with spheroids and minor dendrites. The transformation was quantitatively observed with a specific surface Sv, that decreased for almost all alloys and marked the flow effect on α-Al. The computer coupling of phase diagrams and thermochemistry (CALPHAD) technique was applied for the calculation of phase diagrams and property diagrams. Forced convection decreased secondary dendrite arm spacing λ2 in almost all alloys, while it increased slightly in one studied alloy. The length of detrimental β-Al5FeSi phases decreased in the alloy, where β starts to precipitate in the presence of α-Al, while increasing in alloys where β starts as first and grows in the fully liquid melt. The average overall dimension of the Mn-rich phases increased in almost all alloys, and the number density decreased under flow. The modification of spacing for AlSi-eutectics and Al2Cu was analyzed. It was found that the occurrence of Al2Cu does not influence the fluid flow and vice versa.

Calphad Modeling of LRO and SRO Using ab initio Data

Results from DFT calculations are in many cases equivalent to experimental data. They describe a set of properties of a phase at a well-defined composition and temperature, T, most often at 0 K. In order to be practically useful in materials design, such data must be fitted to a thermodynamic model for the phase to allow interpolations and extrapolations. The intention of this paper is to give a summary of the state of the art by using the Calphad technique to model thermodynamic properties and calculate phase diagrams, including some models that should be avoided. Calphad models can decribe long range ordering (LRO) using sublattices and there are model parameters that can approximate short range ordering (SRO) within the experimental uncertainty. In addition to the DFT data, there is a need for experimental data, in particular, for the phase diagram, to determine the model parameters. Very small differences in Gibbs energy of the phases, far smaller than the uncertainties in the DFT calculations, determine the set of stable phases at varying composition and T. Thus, adjustment of the DFT results is often needed in order to obtain the correct set of stable phases.

Thermodynamic Approach to the Development and Selection of Hardfacing Materials in Energy Industry

The overall study objection is selection and optimization all available thermodynamic data required for using calculation of phase diagram (CALPHAD) technique within the Fe-C-Cr-Mn-Si-Ti system. Such data collected in the thermodynamic database can be used for predicting the phase constitution states of a given composition for Fe-based hardfacing materials, which often use in energy industry in order to increase the abrasion and impact wear resistance of equipment parts. In order to compare theroretical calculation results with experimental data, four different types of hardfacing were deposited using flux-cored arc welding. Microstructure and chemical composition of deposited layers was investigated using optical and scanning electron microscopy together with energy dispersive X-ray spectroscopy. Comparison of experimental and computed results shows that they are in good agreement in meaning of presence of all-important phase equilibrium regions. The developed database can be used for rational selection of hardfacing materials for energy industry equipment and reasonable choice of new alloying systems.

Thermodynamic and Experimental Studies on Al Addition of 253MA Steel

To solve the nozzle clogging issue in the continuous casting process of 253MA steel, a method of modifying solid inclusions to liquid phases is proposed. The CALPHAD technique was employed to predict the liquid region of the Al2O3-SiO2-Ce2O3 system. Then a thermodynamic package based on the extracted data during the phase diagram optimization process was developed. This package was then used to compute the appropriate aluminum addition, which was 0.01% in 253MA steel. The Si-Al alloy was chosen as the deoxidant according to the thermodynamic analysis. The solid inclusions were ultimately modified to liquid phases at 1500 °C when cerium was added through the equilibrium experiments in a MoSi2 tube furnace.

Ginzburg-Landau Modeling for Martensitic Transformation Coupled with Composition Redistribution

The Ginzburg-Landau (G-L) model possesses the thermodynamic foundation of energy minimization and is available for many dynamic formalisms, thus holds great potential for investigating the complex materials behaviors. The common ingredient in energy spawns the real-time control of diffusion potential and chemical mobility by integrating G-L model with CALPHAD technique. The coupling between martensitic transformation and dislocation evolution is achieved by mean of continuous mechanism. The updated G-L model is then validated against the martensitic transformation coupled with composition redistribution in Fe-C binary system. The modeling allows some deeper insights into the mechanisms of coupling effects behind the observed phenomena. It has been proven that the partitioning of carbon in steels is an ordinary diffusion governed by instantaneous diffusion potential and chemical mobility. The rough twin boundaries and retained austenite within the martensite should be attributed to the effect of dislocations. Although the developed model in this chapter has deficiencies, it sheds some lights on the integration of multi-physics models for a complex phase transformation.

A thermodynamic description of metastable c-TiAlZrN coatings with triple spinodally decomposed domains

A critical thermodynamic assessment of the metastable c-TiAlZrN coatings, which are reported to spinodally decompose into triple domains, i.e., c-TiN, c-AlN, and c-ZrN, was performed via the CALculation of PHAse Diagram (CALPHAD) technique based on the limited experimental data as well as the first-principles computed free energies. The metastable c-TiAlZrN coatings were modeled as a pseudo-ternary phase consisting of c-TiN, c-AlN and c-ZrN species, and described using the substitutional solution model. The thermodynamic descriptions for the three boundary binaries were directly taken from either the CALPHAD assessment or the first-principles results available in the literature except for a re-adjustment of the pseudo-binary c-AlN/c-ZrN system based on the experimental phase equilibria in the pseudo-ternary system. The good agreement between the calculated phase equilibria and the experimental data over the wide temperature range was obtained, validating the reliability of the presently obtained thermodynamic descriptions for the c-TiAlZrN system. Based on the present thermodynamic description, different phase diagrams and thermodynamic properties can be easily predicted. It is anticipated that the present thermodynamic description of the metastable c-TiAlZrN coatings can serve as the important input for the later quantitative description of the microstructure evolution during service life.

Thermodynamic Assessment of Mushy Zone in Directional Solidification

Solidification of AlSiFe alloys was studied using a directional solidification facility and the CALPHAD technique was applied to calculate phase diagrams and to predict occurring phases. The specimens solidified by electromagnetic stirring showed segregation across, and the measured chemical compositions were transferred into phase diagrams. The ternary phase diagrams presented different solidification paths caused by segregation in each selected specimen. The property diagrams showed modification in the sequence and precipitation temperature of the phases. It is proposed in the study to use thermodynamic calculations with Thermo-Calc which enables us to visualize the mushy zone in directional solidification. 2D maps based on property diagrams show a mushy zone with a liquid channel in the AlSi7Fe1.0 specimen center, where significant mass fraction (33%) of β-Al5FeSi phases may precipitate before α-Al dendrites form. Otherwise liquid channel occurred almost empty of β in AlSi7Fe0.5 specimen and completely without β in AlSi9Fe0.2. The property diagrams revealed also possible formation of α-Al8Fe2Si phases.