L1₀-disorder phase equilibria in the Fe-Pd system investigated by phenomenological calculation

The creep strength of advanced ferritic heat-resistant steels for ultra super critical (USC) power plants is significantly improved by the addition of Pd. It has been found by a detailed transmission electron microscopy (TEM) observation that fine precipitation of an Fe-Pd based L1₀-ordered phase within a lath-martensite grain possessing a certain crystallographic orientation relationship with the matrix is responsible for the strengthening mechanism. Phenomenological calculation by combining the cluster variation method (CVM) with a Lennard-Jones (L-J) type pair interaction energy is herein attempted for the Fe-Pd binary system to evaluate L1₀-disorder phase equilibria as an initial step to investigate ternary systems. Some of the unknown parameters in the Lennard-Jones pair interaction energies are determined with the help of computations based on a combination of a thermodynamic database and experimental measurement of the latent heat for the L1₀-disorder transition, the latter from differential scanning calorimetry. The experimental L1₀-disorder transition temperature is well reproduced by incorporating a tetragonal distortion of the L1₀-ordered phase into the calculation. In addition, detailed atomistic information, which is indispensable in designing and controlling the morphology of the L1₀-ordered phase, is obtained by the present calculation.