Fig. 1. Mechanical properties of the Cu-free and Cu-containing steels: (a) engineering tensile stress-strain curves of the steels in the as-quenched and 60-h aged conditions, (b) work hardening rate curves and true stress-strain curves of the steels in the 60-h aged condition, and (c) elongations and Charpy impact energies in the 60-h aged condition.

Fig. 2. Microstructure of Mo-enriched and Cu-rich precipitates: (a) bright-field TEM image of the Cu-containing steel in the 60-h aged condition, HR-TEM images of (b) 9R Cu and (d) Laves phase, (c) and (e) are the corresponding FFT patterns of (b) and (d), respectively, (f) APT microstructure of Cu-rich and Mo-enriched precipitates, and (g) and (h) are the proximity histograms of Cu-rich and Mo-enriched precipitates, respectively.

Fig. 3. Microstructure of reverted austenite before deformation: XRD patterns of the (a) Cu-free and (b) Cu-containing steels in the as-quenched and aged conditions, (c) volume fractions of reverted austenite as a function of aging time, (d) EBSD phase maps for the Cu-containing steel in the 60-h aged condition.

Fig. 4. Microstructure of reverted austenite before deformation: (a) the TEM micrograph of the Cu-containing steel in the 60-h condition, (b) the SAED pattern corresponding to reverted austenite and matrix in (a), (c) and (d) are the bright-field TEM image and corresponding TEM/EDS elemental mapping of Ni, Cu and Mo, respectively, and (e) and (f) APT microstructure and compositions of Cu-rich precipitates and reverted austenite.

Fig. 5. Microstructure of reverted austenite after deformation: (a) XRD pattern, (b) EBSD phase maps, and (c) bright-field TEM image of the Cu-containing steel after tensile deformation.