Transformation induced plasticity (TRIP) behavior was studied in steel with
composition Fe-0.07C-2.85Si-15.3Mn-2.4Al-0.017N that exhibited two TRIP
mechanisms. The initial microstructure consisted of both {\epsilon}- and
{\alpha}-martensites with 27% retained austenite. TRIP behavior in the first 5%
strain was predominately austenite transforming to {\epsilon}-martensite (Stage
I), but upon saturation of Stage I, the {\epsilon}-martensite transformed to
{\alpha}-martensite (Stage II). Alloy segregation also affected the TRIP
behavior with alloy rich regions producing TRIP just prior to necking. This
behavior was explained by first principle calculations that revealed aluminum
significantly affected the stacking fault energy in Fe-Mn-Al-C steels by
decreasing the unstable stacking fault energy and promoting easy nucleation of
{\epsilon}-martensite. The addition of aluminum also raised the intrinsic
stacking fault energy and caused the {\epsilon}-martensite to be unstable and
transform to {\alpha}-martensite under further deformation. The two stage TRIP
behavior produced a high strain hardening exponent of 1.4 and led to ultimate
tensile strength of 1165 MPa and elongation to failure of 35%.Comment: submitted to Met. Mater. Trans. A manuscript E-TP-12-953-