Fully austenitic Fe-28Mn-10Al-1.0C steel with high stacking fault energy exhibited exceptionally high uniform elongations (85 to 100 pct) and total elongations (100 to 110 pct) at room temperature. The origin of such exceptional room-temperature ductility was rationalized in terms of strain accommodation mechanisms of reduction of glide plane spacing in Taylor lattice (TL) formation at low strains and TL rotation forming domain boundaries (DBs) and microbands (MBs) at high strains.Transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) associated with austenite are attractive for enhancing the mechanical properties of steels. [1] Phase transformation of austenite to martensite in TRIP and mechanical twin formation in TWIP depend on the austenite stability and stacking fault energy (SFE). TRIP and TWIP dominantly occur when SFE of austenite is <20 mJ/m 2 and 20 to 50 mJ/m 2 , respectively. [2,3] Meanwhile, by investigating deformation of a fully austenitic steel having high SFE (~90 mJ/m 2 ), the present authors [4] recently suggested a new plasticity called ''microband induced plasticity (MBIP)'' in which microbands (MBs) formed by planar glide accommodate a large amount of strain. The MBIP steel exhibited exceptionally high uniform elongation of~80 pct and total elongation of~100 pct at room temperature, which cannot be attained in any class of existing steels. The present study is aimed at rationalizing the origin of such extraordinary high elongation of the fully austenitic steel with the high SFE by correlating the microstructural evolution during deformation to its strain hardening behavior in detail in light of MBIP.The steel having composition of 0.98 C, 28.2Mn, 9.95Al, 0.003P, 0.0038S, and the balance Fe in wt pct was supplied in the form of 12-mm-thick hot-rolled plates (Kwangyang Mill, POSCO, Kwangyang, Korea). The plates were fully recrystallized (1000°C for 1 hour), cold rolled (65 pct reduction), solution treated (1000°C to 1200°C for 1 hour), and water quenched. Roomtemperature tensile tests were performed on the samples (the gage dimension of 25.4 mm 9 6 mm 9 2 mm) taken from quenched plates using a universal testing machine (Instron* model 4484) at the initial strain rate of 10 À3 s À1 . Some tests were interrupted at predetermined strains to observe the microstructures developed at the different strain levels. Microstructures were optically examined by etching the mechanically polished samples with 2 pct Nital. Phase identification was performed by using X-ray diffraction (XRD, Rigaku**, D/max 2500H). Submicrostructures of deformed samples were observed using a transmission electron microscope (TEM, JEOL 3011) operated at 300 kV. Figure 1 shows the optical micrographs of the steel solution treated at 1000°C to 1200°C and the corresponding XRD profiles. The XRD results revealed that the steel was fully austenitic due to the high Mn content. The austenite grain size was~63 ± 7 lm,~114 ± 14 lm, and~350 ± 52 lm for 1000°C, 1100°C, and 1200°C, respectively, and most grains cont...
