Non-Equilibrium structural factors in experimental U–Pd phase diagram determinations and thermodynamic evaluations

One of the basic assumptions of a phase diagram is that each point on a phase boundary line in theory is in equilibrium between the adjacent phases. In practice there are deviations of experimental points from the real equilibrium conditions, especially when a process is too slow to reach equilibrium. These processes can be physical or chemical, or a combination of them. Some processes can be very slow, such as the diffusion of atoms in a solidified alloy in U–Pd system. In addition, the formation velocity of some intermetallic compounds is so fast that some free atoms or vacancies can be locked in the compound microstructure. These atoms lose their chance to diffuse and react with their would-be counterparts and, therefore, a so-called variable composition compound with thin or wide 'homogeneity' was defined. Such ambiguous concepts hinder reasonable analyses of phase equilibria and are attempted examined in this paper.