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Twinning induced plasticity (TWIP) steels containing nitrogen as the interstitial alloying element were designed, and the effects of N and Al on stacking fault energy (SFE), microstructure, and mechanical properties were investigated by using X-ray diffraction and transmission electron microscopy to determine the stacking fault formation probability. In addition, the microstructure and mechanical properties have been characterized. The results indicate that both N and Al decrease the stacking fault formation probability due to the increase of the SFE. The probability of localized stacking fault determined by a new method using electron diffraction spot shift reveals that stress-or strain-induced martensitic transformations (c fcc fi e hcp fi a¢ bcc ) during deformation may consume a large number of stacking faults and give rise to a more marked decrease in the stacking fault formation probability of TWIP steels with lower SFE than of higher one(s). Based on their magnitude of SFEs, the temperature-dependent mechanical properties of four TWIP steels were analyzed.