2SL Model for FCC, HCP, and BCC

The reason for using a 2SL model instead of a 4SL model is that the calculations are faster, but on the other hand it can only model one kind of ordered phase(s). For instance, it is not possible to model FCC with both L1₀ and L2₁, or BCC with D0₃ and B2, etc.

In order to get the fully disordered phase to form, constraints are needed as for the 4SL model. For the symmetric phases, e.g. B2 and L1₀, the relation between parameters are:

G_A:B=G_B:A

L_A,B:*=L_*:A,B

For a 2SL asymmetric model including, but not limited to, L1₂, more constraints are needed. This model has many complicated relations between the parameters. The relation between the parameters for the 2SL L1₂ model can be derived from a 4SL model. The conversion from 4SL model parameters to 2SL parameters for L1₂ can be found in Dupin et al. (2001) and for L1₀ in Yuan et al. (2012). Higher-order systems with an L1₂ phase modeled with two sublattices require a lot of ternary and some quaternary interaction parameters in order to make the disordered state stable. These parameters have been derived by e.g. Dupin (1995), also found in Kusoffsky et al. (2001).

Many publications exist on how to model order/disorder transformations using a two-sublattice model, see e.g. Dupin, Ansara (1999), De Keyzer et al. (2009).