Fig. 1. Tensile properties of the investigated MMS after intercritical annealing at various temperatures. (a) Tensile engineering stress-strain curves. The corresponding ultimate strength points are marked by squares. The inset shows the close-up view of the yield point region. (b) Yield strength vs. uniform elongation for the investigated MMS, along with data for other high-performance steels [3,35,[37], [38], [39], [40], [41], [42], [43], [44]].

Fig. 2. Microstructures of the IA650 and IA660 samples prior to tensile testing. (a, b) EBSD phase map with grain boundaries and the GROD map for the IA650 sample. (c, d) EBSD phase map with grain boundaries and the GROD map for the IA660 sample. γ-austenite and α-ferrite are indicated by red region and cyan region, respectively and high-angle boundary (≥ 15°), low-angle boundary (2°?15°), and twin boundary are denoted by black lines, fuchsia lines, and blue lines, respectively.

Fig. 3. Element distributions of the IA650 sample prior to tensile testing. (a, b) Near-atomic level chemical element distributions between γ and α, as revealed by 3D-APT. The phase boundary is marked by a 6 at.% Mn iso-concentration surface. (c, d) STEM image and the corresponding Mn distribution mapping. (e) Mn content distribution of various austenite grains.

Fig. 4. Full-field strain distributions of the IA650 sample measured by DIC. (a) Contour maps of axial strain at various applied strains. Number above each map: εapp. Color scale bar at the right ride for each map. (b) Distributions of εL along normalized axial position at varying applied strains, e.g. white vertical line in the first contour in (a).

Fig. 5. Full-field strain distributions of the IA660 sample measured by DIC. (a) Contour maps of axial strain at various applied strains. (b) Distributions of εL along normalized axial position at varying applied strains.

Fig. 6. (a) True stress-strain curves and volume fractions of austenite phase as a function of applied strain for both the IA650 and IA660 samples after several localized deformation bands pass through the entire gage length. (b) Distributions for volume fraction of austenite along the axial direction in the IA650 sample prior to tensile testing and at an interrupted tensile strain of 8%. The inset shows the schematic diagram of measuring positions. (c) Strain duration for each localized deformation band (εLDB) and normalized change of volume fraction of austenite by each εLDB (VLDB/εLDB) for both the IA650 and IA660 samples.

Fig. 7. (a) GND density evolutions of both the IA650 and IA660 samples after several localized deformation bands pass through the entire gage length. (b) Magnitudes of strain gradient at the front of localized deformation band as a function of applied strain for both the IA650 and IA660 samples.

Fig. 8. TEM observations for the IA650 sample at an interrupted strain of 27%. (a) Martensite transformation process. (b) Dark-field image in (a). (c) Ferrite phase.

Fig. 9. TEM observations for the IA650 sample after ultimate strength point.