The TEM analysis, focused on the grain boundary precipitates and Widmanstätten plates, is shown in Fig. 3. The isothermal holding temperature of the high Mn steel in Fig. 3 is 1023 K and the holding period is 100 h. A bright-field (BF) image in Fig. 3(a) illustrates the appearance of precipitates in the austenite (γ). Three second-phase grains (C) connect together along the austenite grain boundary. Another BF image in Fig. 3(b) shows a dark Widmanstätten plate (C) existing in the austenite matrix. Fig. 3(c) shows the corresponding selected area diffraction (SAD) pattern taken from the grain boundary precipitate in the middle of the three C grains in Fig. 3(a)