Fig 1. (a) The preparation route for the desired pure titanium (Ti) laminates with a novel gradient structured (GS)/heterogeneous lamella structured (HLS), including the schematic of final GS/HLS Ti laminates. (b) and (c) are the EBSD orientation maps of the hot-pressed GS Ti and the resulting GS/HLS Ti laminates, respectively.

Fig 2. (a) schematic of the location of the microhardness test; (b) dimensions of the (quasi) in-situ tensile specimen (mm); (c) schematic of the OM-DIC target area and SEM-DIC target area of a (quasi) in-situ tensile specimen.

Fig 3. (a) and (b) are the EBSD orientation maps of the fine grains (less than 1 μm) and coarse grains (greater than 3 μm) in the different regions of GS/HLS Ti laminates, respectively. (c) TEM images of the fine grains in the surface layer. (d) the finer EBSD map in the blue rectangle in Fig. 3(b) clearly showed the FG around the CG. (e) the variation in the area fraction of fine grains and coarse grains in the surface layer, middle layer, and center layer. (f) the area-weighted fraction of the equivalent circle diameter of grains in the surface layer, middle layer, and center layer.

Fig 4. (a) Microhardness (HV) gradient in the Ti laminates with the gradient structure /heterogeneous lamella structure, the point plot in (a) showed the average HV changing from the surface into the center; (b)-(g) exhibited the EBSD map colored with equivalent circle diameter around the point 1-6 in (a), respectively. (h) the variation in the area fraction of fine grain (less than 1 μm) and coarse grain (greater than 3 μm) in (b)-(g).

Fig 5. (a) and (b) Tensile engineering stress-strain curves and tensile true stress-strain curves in GS/HLS Ti laminates at an initial quasi-static strain rate of 5 × 10-4 s-1 in comparison with FG Ti laminates and CG Ti laminates. (c) Fracture morphologies of GS/HLS Ti laminates: Zone 1 and Zone 2 showed the magnified regions of the surface layer and center layer, respectively.

Fig 6. (a) LUR stress-strain curves of GS/HLS Ti laminates. (b) The first three hysteresis loops of (a). The segment B1C1(or B2C2, B3C3) were the linear (elastic) part of the unloading stress. (c) Schematic of calculating HDI stress [16]. (d) HDI stress and effective stress versus applied strain.

Fig 7. (a) Optical microscope combined with digital image correlation (OM-DIC) speckle image obtained from the lateral surface (RD-ND) of the GS/HLS Ti laminates: typical speckle patterns (a-1) and the detailed distribution of grayscale in a single correlation subset (91 × 91 pixel2) (a-2); (b) OM image of the unique GS/HLS structure and the responding schematic. The evolution of local stain (εxx) map of the resulting GS/HLS Ti laminates acquired by OM-DIC at the macrostrains ε: 0.02% (c), 0.1% (d), 0.2% (e), 0.4% (f). In the coordinate, X is the tensile direction, and Y is the sample thickness direction.

Fig 8. (a) and (c) were TEM images of typical regions in the surface layer and center layer of the GS/HLS Ti laminates without tensile deformation, respectively. (b) and (d) showed TEM images of typical regions in the surface layer and center layer of the GS/HLS Ti laminates undergoing a tensile strain of 1%, respectively.

Fig 9. (a) Scanning electron microscope combined with digital image correlation (SEM-DIC) speckle image obtained from the lateral surface (RD-ND) of the final GS/HLS Ti laminates: typical speckle patterns (a-1), enlarged images of the speckle patterns (a-2) and the detailed distribution of grayscale in a single correlation subset (51 × 51 pixel2) (a-3); (b) EBSD map colored with equivalent circle diameter of the final GS/HLS Ti laminates. (c)-(e) are EBSD-DIC maps produced by superimposing the local strain (εxx) maps at the macrostrain ε of 0.1%-1% obtained by SEM-DIC on the corresponding EBSD band contrast maps. SEM-DIC analysis was anchored within the red rectangle (120 μm far from the surface) shown in Fig. 2(b).

Fig 10. Quantitative analysis of average local strain (εxx) in CG regions with increasing macrostrains (ε). The orange, green and red dot-dash lines indicated the average local strain (εaxx) of the entire acquired region when the macrostrain is 0.1%, 0.5%, and 1%, respectively.

Fig 11. Distribution of schmid factor about a-type slip systems of GS/HLS Ti laminates: (a) {0001} <11-20>; (b) {1-100} <11-20>; (c) {1-101} <11-20>; (d)-(f) are the schmid factor statistics corresponding to (a)-(c), respectively; (g) Schmid factor distribution of {1-100} <11-20> prismatic slip system of grains with the equal diameter circle diameter greater than 5 μm, and the slip plane trace and slip direction of the active slip systems are marked as blue lines and red arrows, respectively; (h) Local strain distribution map at the macrostrain of 0.1%, and slip traces in (g) marked with blue lines.

Fig 12. KAM maps of GS/HLS Ti laminates at the macrostrain of 0.1% (a) and 1% (b); (c) TEM image at a tensile strain of 0.5%, showing the pile-up of dislocations in coarse grain surround by undeformed fine grains. (d) TEM image at a tensile strain of 2%, exhibiting the pile-up of dislocations in both coarse grain and fine grains.