About Composition Variables

This section or topic is specific to Console Mode.

For Graphical Mode, see Diffusion Module Terminology in Graphical Mode.

Two types of composition variables are used within the program: site-fractions and u-fractions, although you can also use weight fraction or mole fraction, for example.

The site-fractions are used to set up the problem and interface with POLY-3 (in Console Mode). The site fraction of a species k, yk, is the fraction of equivalent lattice sites occupied by species k. It is the only concentration variable that uniquely defines the composition and the constitution of a phase and it is necessary to use the site fraction in the storage of the data during a simulation.

The u-fractions are used in the diffusion equations. The u-fraction of a component k is defined as

The summation is only performed over the substitutional elements. The choice of the volume fixed frame of reference in the calculations make it convenient to use a composition variable which is related to the volume. The definition of the u-fraction is based upon the assumption that a species may or may not contribute to the volume of a phase. An interstitial element is assumed to not contribute to the volume. Substitutional elements are assumed to contribute equally to volume.