Two-stage transformation-induced plasticity (TRIP) behavior characterized by the martensitic transformations → α′, has produced exceptional tensile strengths and work hardening rates in Fe-14 wt pct Mn alloys containing Al and Si. A regular solution model has been developed to accurately calculate ΔGγ → ε for a given TRIP alloy and the calculated driving force is used to determine the MεS temperature. The regular solution model developed here predicted driving forces that corresponded well with reported microstructures and behavior of seven FeMnAlSiC steels from literature when considered in conjunction with nucleating defect critical size and material process history. The role of available nucleating defects of critical size, n*, has been linked to the stacking fault energy necessary to observe the γ → ε transformation and the MεS temperature. The regular solution model provided excellent correlation between calculated MεS temperatures and those measured experimentally in 89 alloys from literature and suggested n* = 4 is the critical size of a nucleating defect in annealed microstructures. Factors affecting the γ → ε transformation and the MεS temperature have been identified as prior austenite grain size, dislocation substructure due to prior deformation, and solute segregation.