Fig. 1. The results of Charpy V-notch impact tests for the high-manganese steels. (a) Appearances of Charpy V-notch impact test specimens after fracture; (b), (c) the impact toughness as a function of grain sizes and testing temperatures, respectively.

Fig. 2. SEM images of the samples with different grain sizes after Charpy impact testing at LNT, -80?°C, RT and 200?°C, respectively. Note that the microstructures of the samples before impact testing are also presented for comparison.

Fig. 3. XRD of the areas close to impact fracture in the samples with different grain sizes at (a) LNT; (b) -80?°C; (c) RT and (d) 200?°C, respectively.

Fig. 4. (a) A typical low magnification TEM micrograph showing the microstructures of the area close to impact site of fracture in the 2.7?μm/LNT sample; (b) a high magnification TEM micrograph showing the SFs.

Fig. 5. (a) A typical low magnification TEM micrograph showing the microstructures of the area close to impact site of fracture in the 47.8?μm/LNT sample. (b) A high magnification TEM micrograph showing the deformation εhcp-twins and SFs. (c) a SAED pattern of the black circle area in (b). Note that the black lines in (b) represent the basal planes of εhcp lamellae and their twins. The SAED pattern in (c) is acquired along the [20]ε zone axis.

Fig. 6. HRTEM images showing two types of twin boundaries in the 47.8?μm/LNT sample viewed along the [20] direction. (a) {102} coherent twin boundaries and BP interfaces; (b) BP and PB interfaces. Green arrows indicate basal SFs within the parent and twin. Note that the inset fast Fourier transformation (FFT) patterns in (a) and (b) show that the angles between the basal planes of the parent and twin are 86.3° and 90°, respectively.

Fig. 7. TEM micrographs showing the microstructures of the area close to impact site of fracture in the 47.8?μm/LNT sample. (a) A typical TEM micrograph showing the intersections of two ε-martensite variants. εhcp-twins and γR-austenite are formed in ε1 laths. (b) and (c) are magnified images of areas marked by “b” and “c” in (a), respectively. (d) Multi-structures produced by the ε-ε intersections, which are rendered by colours. Note that both εhcp-twins and γR-austenite are formed in ε1 and ε2 laths, respectively. (e) Stereographic projection of crystal indices showing ORs of multi-structures.

Fig. 8. Microstructures of the area close to impact fracture in the 47.8?μm/-80?°C sample. (a) A low magnification TEM micrograph and the inset SAED pattern is acquired from the area marked by a circle; (b) A magnified image of an area marked by “b” in (a). (c) and (d) are magnified images of areas marked by “c” and “d” in (b), respectively. Note that the inset FFT pattern in (c) shows ～1.3° deviation from the Pitsch OR.

Fig. 9. Microstructures of the area close to impact fracture in the impact samples deformed at RT. (a) A low magnification TEM micrograph of the 2.7?μm/RT sample; (b) A low magnification TEM micrograph showing the massive lamellae in a 47.8?μm/RT sample; (c) A high magnification bright-field image of lamellae in a 47.8?μm/RT sample; (d) The corresponding SAED patterns of (c); (e) Dark-field image of γ-twins taken with the reflection of (1)γT; (f) Dark-field image of εhcp-martensite taken with the reflection of (011)ε.

Fig. 10. Microstructures of the area close to impact fracture in (a) the 2.7?μm/200?°C sample and (b) the 47.8?μm/200?°C sample, respectively.

Fig. 11. The calculated SFE of austenitic matrix as a function of temperatures.

Fig. 12. Schematic illustration for the dependence of impact toughness and deformation mechanism on grain size and temperature in the steel.