Previous success in achieving exceptional tensile properties of \u3e1100 MPa ultimate tensile strength and \u3e30% elongation to failure in alloys that exhibit a two-stage transformation induced plasticity mechanism (γ-→ε→α) has prompted the continued development of this alloy system. First principles investigations of stacking fault energy revealed Si has the same effect as Al in decreasing the barrier to nucleate e-martensite from parent austenite while decreasing the relative stability of ε-martensite compared to austenite. Insight from ab-initio calculations has been combined with thermodynamic driving force and Ms temperature calculations to develop two alloys of composition Fe-15.1Mn-1.95Si-1.4Al-0.08C- 0.017N (Fe-15-2-1.4-0.08) and Fe-14.3Mn-3.0Si-0.9Al-0.16C-0.022N (Fe-14-3-1-0.16). The Fe-15-2-1.4-0.08 alloy achieved a triplex hot band microstructure of austenite, e-martensite, and a-martensite, which exhibited two-stage TRIP character and a UTS of 1058 MPa at 29.1% elongation to failure. The work hardening rate in this alloy has been related to a grain refinement mechanism characterized by the γ→ε→α\u27 transformation. The Fe-14-3-1-0.16 alloy achieved a hot band microstructure consisting predominately of ε- and α- martensite. The limited fraction of austenite resulted in the absence of Stage I (γ→ε) TRIP and the material work hardened directly after yielding via Stage II (ε→α) TRIP. Over-stabilization of e-martensite led to incomplete transformation to a-martensite that resulted in premature failure at 726 MPa UTS and 11.0% elongation. It is concluded that the ideal hot band microstructure to achieve exceptional tensile properties via two-stage TRIP behavior is composed primarily of austenite and e-martensite which can be controlled via C, Si, and Al alloying
