Fig. 1. (a) EBSD inverse pole figure (IPF) map of the martensite initial microstructure. (b) Schematic illustration of the thermomechanical processing routes (I, II and III) used in this study to obtained three series of bimodal microstructures with αp grain sizes of 0.8, 2.4 and 5.0 μm, respectively.

Fig. 2. Typical BSE images of the bimodal microstructures with primary α grain sizes of 0.8 μm (a)-(c), 2.4 μm (d)-(f) and 5.0 μm (g)-(i). The intercritical annealing temperatures were 830 °C for (a), (d), (g), 910 °C for (b), (e), (h) and 970 °C for (c), (f), (i). Some examples of αp grains and βtrans areas were highlighted by red and yellow dotted lines, respectively.

Fig. 3. Representative engineering stress-strain curves of each serial of bimodal microstructures with primary α grain sizes 0.8 μm (a), 2.4 μm (b) and 5.0 μm (c). The location of peak stress was indicated by red arrow for each curve. The corresponding work-hardening rate (broken lines) and true stress-strain curves (solid lines) are shown in (d), (e) and (f), respectively. A distinctive difference in the shapes of the engineering stress-strain curves as well as the work-hardening rate curves was found for bimodal microstructures with intercritical annealing temperatures below and above 910 °C.

Fig. 4. The evolution of yield strength (σp0.2 %) (a) and uniform elongation (εu) (b) with intercritical annealing temperature in bimodal microstructures with different primary α grain sizes. (c) The evolution of microstructural characteristics, including the volume fractions of primary α grains (vol%(αp)), transformed β areas (vol%(βtrans)) and αp/βtrans interface length densities (ρ) with intercritical annealing temperature in bimodal microstructures with different primary α grain sizes. The intercritical annealing temperatures were divided into two regions (region I: 830-910 °C and region II: 910-970 °C) as indicated by red dashed lines in each figure.

Fig. 5. Strain distribution analysis of bimodal microstructures with a primary α grain size of 5.0 μm and intercritical annealing temperatures of (a) 870 °C, (b) 910 °C and (c) 950 °C. The first row of this figure showed the BSE images of undeformed microstructures with micro-grids patterns. In (a-i) and (b-i), the βtrans areas were delineated by yellow dotted lines and in (c-i), the αp grains were delineated by green dotted lines. The second row of this figure showed the local strain mappings obtained by the micro-DIC technique after tensile deformation by 1.0 %, superimposed on the SEM images in which the transformed β areas in (a-ii), (b-ii), and the primary α grains in (c-ii) were delineated by white dotted lines, following the first row. The third and fourth rows showed the local strain mappings after tensile deformation by 2.5 % and 4.0 %, respectively. The scale bar of the Von-Mises mapping was shown in the right-hand side of this figure. Note that the BM950 specimen was not deformed to 4.0 % as necking already occurred after plastically deformed by 3.0 %.

Fig. 6. Strain histograms of bimodal microstructures with a αp grain size of 5.0 μm, and intercritical annealing temperatures of (a) 870 °C, (b) 910 °C and (c) 950 °C. The first, second and third rows of this figure showed the strain histograms of αp grains (in blue) and βtrans areas (in red) after tensile deformed by (i) 1.0 %, (ii) 2.5 % and (iii) 4.0 % of plastic strains. The BM950 specimen was not deformed to 4.0 % as necking already occurred after plastically deformed by 3.0 %.

Fig. 7. (a) Typical BSE image showing the deformed BM910 microstructure (D(αp) = 5.0 μm) with pre-polished surface, in which intensive slip lines inside αp grains and micro-shear bands across βtrans areas were clearly identified. (b) A magnified BSE image showing the details of micro-shear bands which shuffled across several secondary α lamellae. (c) Fractions of each slip system in the BM910 and BM950 microstructures with primary α grain sizes of 2.4 μm and 5.0 μm after tensile deformation. For the BM910 and BM950 microstructures with the primary α grain size of 5.0 μm, the plastic strains were 4.0 %. For theBM910 and BM950 microstructures with the primary α grain size of 2.4 μm, the plastic strains were 2.5 %. At least 80 primary α grains were checked for each specimen for statistical confidence.

Fig. 8. (a) Work-hardening rate curves of BM910 and BM950 microstructures with a αp grain size of 5.0 μm, together with the indications of plastic strains (0.2 %, 1.0 %, 2.5 %, 4.0 % and 6.0 %.) where the tensile deformation was interrupted. (b) Evolution of the fractions of basal, prismatic and pyramidal I slip systems with deformation strain for BM910 and BM950 microstructures. (c) Evolution of the fractions of pyramidal II slip system with deformation strain for BM910 and BM950 microstructures.

Fig. 9. Characterization of dislocation morphologies in the BM910 microstructure with a primary α grain size of 2.4 μm after tensile deformed to different plastic strains. (a) A typical BSE image of the BM910 microstructure, in which examples of αp/αp grain boundary and αp/βtrans interface boundary were highlighted by red and yellow dotted lines, respectively. (b) Work-hardening rate curves of the bimodal microstructures with different intercritical annealing temperatures, in which the curve of BM910 microstructure was indicated by the arrow. (c-i)-(c-iv) Typical dislocation morphologies near the αp/βtrans interface boundary after tensile deformed to the plastic strains of 0.2 % (c-i), 1.0 % (c-ii), 2.5 % (c-iii) and 4.0 % (c-iv). (d-i)-(d-iv) Typical dislocation morphologies near the αp/αp grain boundaries after tensile deformed to the plastic strains of 0.2 % (d-i), 1.0 % (d-ii), 2.5 % (d-iii) and 4.0 % (d-iv).

Fig. 10. Characterization of dislocation morphologies in the BM950 microstructure with a primary α grain size of 2.4 μm after tensile deformed to different plastic strains. (a) A typical BSE image of the BM950 microstructure, in which the αp/βtrans interface boundary was found to be predominant. (b) Work-hardening rate curves of the bimodal microstructures with different intercritical annealing temperatures, in which the curve of BM950 microstructure was indicated by the arrow. (c-i)-(c-iv) Typical dislocation morphologies near the αp/βtrans interface boundaries after tensile deformed to the plastic strains of 0.2 % (c-i), 1.0 % (c-ii), 2.5 % (c-iii).

Fig. 11. Dislocation type analysis in the BM910 specimen with a primary α grain size of 2.4 μm after tensile deformed to a plastic strain of 2.5 %. (a) TEM bright-field (BF) image of the primary α grain of interest. (b) [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20]] zone axis of α phase was used for the two-beam diffraction condition. TEM bright-field (c) and dark field (d) images using the g vector of 0002 are shown. TEM bright-field (e) and dark field (f) images using the g vector of 10-10 are shown. Most of the dislocations inside primary α grain were visible at both imaging conditions, confirming the predominance of <c+a> dislocations.

Fig. 12. Dislocation type analysis in the BM950 specimen with a primary α grain size of 2.4 μm after tensile deformed to a plastic strain of 2.5 %. (a) TEM bright-field (BF) image of the grain of interest. (b) [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20]] zone axis of α phase was used for the two-beam diffraction condition. TEM bright-field (c) and dark field (d) images using the g vector of 0002 are shown. TEM bright-field (e) and dark field (f) images using the g vector of 10-10 are shown. Most of the dislocations within the primary α grain became invisible when the g vector was 0002, indicating most of them were only <a> type dislocations.

Fig. 13. Schematic illustrations of the deformation mechanisms of bimodal microstructures obtained by intercritical annealing in region I and region II.