THERMODYNAMIC CONSIDERATION OF THE TETRAGONAL LATTICE DISTORTION OF THE L1(0) ORDERED PHASE

In an attempt to understand the thermodynamic consequences of the tetragonal distortion accompanying the L1(0) ordering, we have conducted a theoretical investigation by treating the problem of a rigid lattice free energy model. The problem is treated as an elasticity problem and a generalized formulation is proposed in a form which can be directly utilized to study incoherent two phase equilibria. By use of the Static Concentration Wave (SCW) mean field model as a rigid lattice free energy model, the formulation is applied to the case of our interest. We show that the thermodynamic stability of the L1(0) phase may be significantly influenced by the tetragonal lattice distortion, depending on the magnitude of the associated strain energy relative to the competing positive entropic contribution in the free energy of the stress free L1(0) phase. In association with this, we suggest that the neglect of tetragonal distortion (i.e. the use of a rigid lattice free energy model) could be a source of serious errors particularly for alloys with lower L1(0) ordering transition temperatures (e.g. CuAu and InMg). A prototype phase diagram of the [001]f.c.c.* special point structures, calculated with the tetragonal distortion taken into account, has indeed displayed topological features which are fundamentally different from those of a rigid lattice phase diagram and, furthermore, has reproduced the main topological features of the AuCu-Au3Cu and the InMg-In3Mg sides of their respective phase diagrams.