Databases for computational thermodynamics and diffusion modeling

Databases for computational thermodynamics and diffusion modeling can be applied to the prediction of phase diagrams and microstructural evolution. These predictions can be used for alloy and process design, as well as to improve modeling capabilities that would decrease the time for new alloy and process development. Databases that are currently available to scientists, engineers, and students need to be expanded and improved. Accordingly, a workshop was held March 21–22, 2002, to identify the database and information delivery tool needs of industry and education. As a result of the workshop a roadmap was developed to show how these needs can be met during the next decade in a cost-effective way through expanded collaborative efforts in education, basic research, and database development. The workshop consisted of a series of invited talks given to the group as a whole followed by general discussions of needs and benefits, and the construction of a roadmap of future activities. A brief report of a follow-up meeting held on May 27, 2003, is given in the Appendix.     

Efficient phase diagram information and computational thermodynamics

Industries that process and use metals, from microelectronics to aerospace, can use phase equilibria data for designing and processing complex alloys (three or more components) and diffusion data for controlling the formation and dissolution of precipitate phases within a matrix or at an interface. The number of images or data tables that would be needed for the graphic representation of such data is prohibitively large for systems with a large number of components. Databases that store thermodynamic and diffusion data as analytical functions provide a compact, retrievable storage method. They also permit the extrapolation of properties from binary and ternary systems to higher-order systems based on a physical model. Special task software that allows the nonexpert to extract the required information has been developed for the casting of superalloys and the solidification of Pb-free solders. Examples of user-friendly data representation and data utilization in advanced applications are presented. This paper was presented at the International Symposium on User Aspects of Phase Diagrams, Materials Solutions Conference and Exposition, Columbus, Ohio, October 18–20, 2004.     

The computational modeling of alloys at the atomic scale: From ab initio and thermodynamics to radiation-induced heterogeneous precipitation

This paper describes a strategy to simulate radiation damage in FeCr alloys wherein magnetism introduces an anomaly in the heat of formation of the solid solution. This has implications for the precipitation of excess chromium in the α′ phase in the presence of heterogeneities. These complexities pose many challenges for atomistic (empirical) methods. To address such issues the authors have developed a modified many-body potential by rigorously fitting thermodynamic properties including free energy. Multi-million atom displacement Monte Carlo simulations in the transmutation ensemble, using the new potential, predict that thermodynamically grain boundaries, dislocations, and free surfaces are not preferential sites for α′ precipitation.     

Understanding management of computational process modeling and simulation

The use of computational process modeling and simulation has become widespread as the real cost of computers has decreased and their capabilities have increased. However, many science and engineering university faculties have failed to treat this area as a formal undergraduate discipline. The result has been generally a self-taught approach by professionals to both the practice and the management of modeling and simulation. This article describes process modeling and simulation techniques and applications. For more information, contact Michael Wadsley, Austherm Pty Ltd., P.O. Box 2049, North Brighton, Victoria 3186, Australia; e-mail wadsley@austherm.com.au.     

Hot rolling mill roll microstructure interpretation: A computational thermodynamics study

The microstructure of the interface of a high chromium cast iron/ductile cast iron composite hot rolling mill roll has been interpreted with the help of Fe-Cr-Si-C laboratory alloys and thermodynamic calculations of their solidification paths. It has been shown that the mill roll presents two limiting microstructures: the ductile cast iron core and the white cast iron shell. While the core is heavily contaminated with chromium, originating from the partial dissolution of the shell during core pouring, the shell shows a typical high chromium white cast iron micro-structure, since it remains solid during processing. The high chromium contents in the core originate a typical “mottled” cast iron microstructure, combining nodular graphite and ledeburite. The region between the two limiting microstructures is called interface. As the distance from the core toward the shell increases in the interface, the content of chromium increases but that of silicon decreases. The resulting microstructural gradient shows ledeburitic M₃C, hexagonal-section M₃C, duplex M₃C/M₇C₃ carbides, and M₇C₃ carbide, which could be well correlated with the calculated solidification paths of the laboratory samples.     

Hot rolling mill roll microstructure interpretation: A computational thermodynamics study

The microstructure of the interface of a high chromium cast iron/ductile cast iron composite hot rolling mill roll has been interpreted with the help of Fe-Cr-Si-C laboratory alloys and thermodynamic calculations of their solidification paths. It has been shown that the mill roll presents two limiting microstructures: the ductile cast iron core and the white cast iron shell. While the core is heavily contaminated with chromium, originating from the partial dissolution of the shell during core pouring, the shell shows a typical high chromium white cast iron micro-structure, since it remains solid during processing. The high chromium contents in the core originate a typical “mottled” cast iron microstructure, combining nodular graphite and ledeburite. The region between the two limiting microstructures is called interface. As the distance from the core toward the shell increases in the interface, the content of chromium increases but that of silicon decreases. The resulting microstructural gradient shows ledeburitic M₃C, hexagonal-section M₃C, duplex M₃C/M₇C₃ carbides, and M₇C₃ carbide, which could be well correlated with the calculated solidification paths of the laboratory samples.     

Advances in the computational modeling of thermal-plasma processing

Significant advances have been made in the theoretical foundations and the associated computational modeling of thermal-plasma processes, particularly in the areas of nonequilibrium effects, multicomponent transport, chemical kinetics, and radiative transport. The comprehensive computational tools developed in the course of this work have been used to perform numerical simulations of a variety of complex thermal-plasma processes.     

Point defect thermodynamics and diffusion in Fe3C: A first-principles study

The point defect structure of cementite (Fe₃C) is investigated using a combination of the statistical mechanical Wagner–Schottky model and first-principles calculations within the generalized gradient approximation. Large 128-atom supercells are employed to obtain fully converged point defect formation energies. The present study unambiguously shows that carbon vacancies and octahedral carbon interstitials are the structural defects in C-depleted and C-rich cementite, respectively. The dominant thermal defects in C-depleted and stoichiometric cementite are found to be carbon Frenkel pairs. In C-rich cementite, however, the primary thermal excitations are strongly temperature-dependent: interbranch, Schottky and Frenkel defects dominate successively with increasing temperature. Using the nudged elastic band technique, the migration barriers of major point defects in cementite are also determined and compared with available experiments in the literature.     

Quantitative phase-field modeling for two-phase solidification process involving diffusion in the solid

A quantitative phase-field model for two-phase solidification processes is developed based on the anti-trapping current approach with the free energy functional formulated to suppress the formation of an extra phase at the interface. This model appropriately recovers the free boundary problem for the motion of interface in the thin-interface limit and, importantly, it is applicable to the solidification process in binary alloy systems with arbitrary values of the solid diffusivities and interfacial energies. The performance of the present model is investigated for the peritectic reaction process in carbon steel. The present model exhibits excellent convergence behavior with respect to the interface thickness.     

考虑热力学和动力学相关性的二元合金凝固界面动力学建模

在同时考虑碰撞限制生长模式和短程扩散限制生长模式的情况下,提出一个更加完善且具有可变动力学前因子的二元合金固?液界面动力学模型.与上述两种生长模式相耦合,提出4种潜在的热力学和动力学相关性,并将其应用于平界面迁移和枝晶凝固.其中,有效热力学驱动力与有效动力学能垒间的线性相关性更符合物理实际.基于此线性热力学和动力学相关...     

Integrated design of Nb-based superalloys: Ab initio calculations,computational thermodynamics and kinetics,and experimental results

An optimal integration of modern computational tools and efficient experimentation is presented for the accelerated design of Nb-based superalloys. Integrated within a systems engineering framework, we have used ab initio methods along with alloy theory tools to predict phase stability of solid solutions and intermetallics to accelerate assessment of thermodynamic and kinetic databases enabling comprehensive predictive design of multicomponent multiphase microstructures as dynamic systems. Such an approach is also applicable for the accelerated design and development of other high performance materials. Based on established principles underlying Ni-based superalloys, the central microstructural concept is a precipitation strengthened system in which coherent cubic aluminide phase(s) provide both creep strengthening and a source of Al for Al₂O₃ passivation enabled by a Nb-based alloy matrix with required ductile-to-brittle transition temperature, atomic transport kinetics and oxygen solubility behaviors. Ultrasoft and PAW pseudopotentials, as implemented in VASP, are used to calculate total energy, density of states and bonding charge densities of aluminides with B2 and L2₁ structures relevant to this research. Characterization of prototype alloys by transmission and analytical electron microscopy demonstrates the precipitation of B2 or L2₁ aluminide in a (Nb) matrix. Employing Thermo-Calc and DICTRA software systems, thermodynamic and kinetic databases are developed for substitutional alloying elements and interstitial oxygen to enhance the diffusivity ratio of Al to O for promotion of Al₂O₃ passivation. However, the oxidation study of a Nb–Hf–Al alloy, with enhanced solubility of Al in (Nb) than in binary Nb–Al alloys, at 1300 °C shows the presence of a mixed oxide layer of NbAlO₄ and HfO₂ exhibiting parabolic growth.     

The 3-D computational modeling of shear-dominated ductile failure in steel

This paper presents recent advances in the computational analysis of the failure mechanisms in high-strength steel. Computational issues are described regarding modeling of the geometry, distribution, and material behavior of the dispersed phases present in the microstructure of steel. The investigation of the failure mechanisms using computational cell model methodology in two and three dimensions is then presented with an emphasis on microvoid-induced shear failure occurring at the scale of submicrometer grain-refining carbide precipitates. The failure of a three-dimensional particle cluster extracted from tomographic analysis of an engineering alloy is simulated. Finally the cell model results are used to simulate the failure of the material at the macro-scale.     

Mathematical modeling of the formation of diffusion coatings with periodic structure

To obtain coatings of thickness to 0.7 mm or more with controllable porosity, having in contrast with the traditional coatings a more uniform chemical composition and properties that are uniform through the thickness, we propose to create on the surface of the part that is being coated a periodic relief of a definite geometry, consisting of alternating protuberances (ribs) and depressions, after which diffusional saturation is performed. The inter-rib channels provide for accelerated diffusion of the saturating element through the entire thickness of the coating, equal to the rib height. Thus, the structure of the diffusion coating will be determined not only by the technological factors of diffusional saturation (temperature, duration, chemical composition of the medium and of the material being coated) but also by the geometry of the relief, which makes it possible to broaden the capabilities for varying the structural state of the diffusion coatings. The multifactor nature of the proposed method leads to certain difficulties in developing the regimes for obtaining coatings with the desired periodic structure. In this connection it is advisable to formulate a mathematical model that will make it possible to predict the structure of the obtained coatings. The mathematical model consists of a system of differential equations that can be solved numerically and describe the changes in time of the concentration of the coating elements and the location of the interphase boundaries. The adequacy of the proposed mathematical model is confirmed herein.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 6–9, June, 1994.     

Adapting thermodynamics for materials science problems

Some recent developments in the applications of thermodynamics to problems in materials science have required adaptations that are in the inventive spirit of classical thermodynamics. Diffuse interfaces and the problems that crystal lattices pose are taken as examples.     

Nonlinear comminution process modeling based on GA–FNN in the computational comminution system

A new concept named computational comminution is firstly proposed in this paper. Based on information technology, the structure of a computational comminution system (CCS) is built. The study on CCS is very different from the traditional ones for comminution, such as the study based on theoretic models, or the study based on experimental models. As one of the key technologies in CCS, a modeling framework for comminution processes is implemented particularly by employing GA–FNN that can model complex nonlinear processes such as the comminution process of cement by integrating artificial neural networks, fuzzy sets and genetic algorithms. Application results of this modeling method to the Horomill cement comminution process show that the modeling framework discussed in this paper is efficient.     

Mathematical modeling of the diffusion chromium plating of steels in stationary and moving alloying mixtures

Metal Science and Heat Treatment -     

An integrated computational tool for precipitation simulation

Computer aided materials design is of increasing interest because the conventional approach solely relying on experimentation is no longer viable within the constraint of available resources. Modeling of microstructure and mechanical properties during precipitation plays a critical role in understanding the behavior of materials and thus accelerating the development of materials. Nevertheless, an integrated computational tool coupling reliable thermodynamic calculation, kinetic simulation, and property prediction of multi-component systems for industrial applications is rarely available. In this regard, we are developing a software package, PanPrecipitation, under the framework of integrated computational materials engineering to simulate precipitation kinetics. It is seamlessly integrated with the thermodynamic calculation engine, PanEngine, to obtain accurate thermodynamic properties and atomic mobility data necessary for precipitation simulation.     

On the Potential for Improving Equilibrium Thermodynamic Databases with Kinetic Simulations

In this work, the possibilities for improving the accuracy of thermodynamic databases based on kinetic simulations are explored. With a new model for the simulation of precipitation kinetics in multi-component alloys, calculations are performed with all unknown parameters of the simulation obtained from independent thermodynamic and kinetic databases. Since no fitting parameters are used, the simulations are considered as having ‘predictive character’. The corresponding methodology is outlined. Based on the comparison of the predicted precipitation kinetics and experimental information, the potential for improving the accuracy of thermodynamic databases is explored. This article was presented at the Multi-Component Alloy Thermodynamics Symposium sponsored by The Alloy Phase Committee of the joint EMPMD/SMD of the Minerals, Metals, and Materials Society (TMS), held in San Antonio, Texas, March 12-16, 2006, to honor the 2006 William Hume-Rothery Award recipient, Professor W. Alan Oates of the University of Salford, UK. The symposium was organized by Y. Austin Chang of the University of Wisconsin, Madison, WI, Patrice Turchi of the Lawrence Livermore National Laboratory, Livermore, CA, and Rainer Schmid-Fetzer of the Technische Universitat Clausthal, Clauthal-Zellerfeld, Germany.