Cláudio G. Schön
Dept. Metallurgical and Materials Engineering, Escola Politécnica da Universidade de São Paulo, Av. Prof. Mello Moraes, 2463 – CEP 05508-030 São Paulo, Brazil.
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Thermodynamics and physical properties in iron aluminides
Physical properties of materials are always defined in situations where mass, momentum or energy transport takes place in the system, i.e. necessarily outside thermodynamical equilibrium. Nevertheless, equilibrium thermodynamics has an impact over these properties, by setting the boundary conditions which constrain the solution of the non-equilibrium problem. In the case of aluminides, for example, this is particularly complex. Long-range (LRO) and short-range order (SRO) effects introduce non-linear contributions to the composition dependency of the chemical potentials and internal energy, which impose non-trivial composition dependencies for the properties. This will be illustrated by considering two technologically important non-equilibrium processes: ternary diffusion and antiphase boundary formation. Ab-initio calculations using the Full Potential- Linear Augmented Plane Wave (FP-LAPW) method are performed in systems Fe-Al-M (M=Ti,Mo,Nb) and are employed to obtain the basic parameters for the complete thermodynamic model of the body centered cubic (bcc) phase and of its superlattices, in the framework of the irregular tetrahedron cluster approximation of the cluster variation method (CVM). By employing specific algorithms, isopotential lines and antiphase boundary isoenergy lines are calculated and plotted against the (eventually metastable) aluminide stability field in the phase diagram. It is shown that complex non-linear effects, like slope discontinuities, complex curvatures and isoline crowding take place in special regions of the phase diagram. These results are discussed with regard to the shape of diffusion paths in diffusion couples and to superdislocation geometry and its impact over plastic deformation.