Ordering phase relationships in ternary iron aluminides

One of the key issues in the development of iron aluminides is the thermodynamic modeling of alloying effects on the long-range and short-range order states of the underlying bcc phase, needed for the proper description of their effects upon phase equilibria and physical properties of multicomponent alloys. The present work describes results obtained by the present research group in the development of a thermodynamic database using the cluster variation method (CVM) in the irregular tetrahedron approximation, combined with ab initio results obtained from FP-LAPW electronic structure calculation in the GGA approximation, as embodied in the WIEN2k package. The ordering phase equilibria in isothermal sections of systems Fe–Al–Mo, Fe–Al–Nb and Fe–Al–Ti are compared. These equilibria, particularly in the technologically important iron-rich corner, are characterized by radically different behaviors, ranging from very large solubility of Ti in the L21/D03 and B2 phases, to a very small solubility of Mo. The behavior of Nb is somewhat intermediate between these two extremes, and shows a limited solubility in the B2 phase, which is, however, found in metastable equilibrium with a L21 phase. It can be shown that these different behaviors can be understood as a consequence of the different metastable equilibria in the binary Fe–Mo, Fe–Nb and Fe–Ti systems. The results are discussed in reference with experimental data on the stable and metastable ordering equilibria in these systems and are illustrated by their impact of aluminide physical properties, like diffusion and APB energies, with its implications for plastic deformatio