Computer Applications in the Development of Steels
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steels are a careful blend of alloying elements designed to give optimal proper ties for a given application. Computer calculations can greatly simplify the task of finding the optimal composition and processing parameters.
Computational Thermodynamics The stable State of a System is demonstrated by its thermodynamic equilibrium. At equilibrium, the State variables such as temperature, chemical potentials of the components, amounts and composition of phases, etc., have well-defined values that are independent of the way in which the equilibrium was reached.
Thermodynamic Models The implementation of Computer modeling has made the relations between quantities such as chemical potential, temperature, enthalpy, and heat capacity much easier to calculate and present graphically in different ways. Together with thermodynamic databases that can describe real, multicomponent materials, this innovation represents a completely new field for thermodynamics. It has been named computational thermody namics (CT). CT does not deal with the atomistic origin of thermodynamic rela tions, but is a connection between the atomistic modeling and macroscopic behavior of a thermodynamic System. CT makes use of Statistical mechanics to formulate the configurational entropy and bond-energy modeis to calculate the en thalpy of a System. Special physical phe nomena, such as magnetic transitions, can be implemented by separate modeis, and each phase in a thermodynamic Sys
tem is described separately, taking into account the way the components mix inside the phase. The basic expression used in CT for the Gibbs energy per mole is Gm =
srf
Gm + cf6Cm +
ph
G m + EGm
(1)
where srfGm is the surface of reference for the Gibbs energy of the phase. This gives the relation between this phase and any other phases in the System and also the Gibbs energy for internal formation of molecules or long-range order. c ' 8 G m , the configurational Gibbs energy, can be ideal if the constituents of the phase are mixed randomly, or it can take into account long- and short-range ordering. ph G m describes any physical phenomena modeled separately, such as the magnetic ordering. Such ordering often has a different com position and temperature dependence from the remaining Gibbs energy. Finally, in the excess Gibbs energy E G m , the re maining Gibbs energy is described. With a good model for the other quantities, this should be very simple; most often, a regulär solution-type model is sufficient. A modern introduction to thermody namics and modeling can be found in the recent book by Hillert. 4
Kinetic Modeling Thermodynamic and kinetic modeling may be combined to predict the time evolution of a nonequilibrium System; for example, Simulation of a heat treatment. The most straightforward extension is to treat diffusional transformations. By considering gradients of the chemical po tentials of a nonequilibrium system as driving forces for diffusion, one may derive the so-called "thermodynamic factor" of the diffusion-coefficient matrix of a multicom
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