Fig. 1 Dimension schematic of superplastic tension specimen (unit: mm)

Fig. 2 Secondary electron (SE) micrographs showing microstructures a, b and 3D topography maps c, d of Ti plate surfaces prior to diffusion bonding: a, c TC4 alloy, b, d SP700 alloy, e schematic sketch showing the initial void height

Fig. 3 SEM images at the bonded interfaces a, b, c and substrates d, e of TC4/SP700 alloys under different pressures: a 3 MPa, b 5 MPa, c 10 MPa, d TC4 layer, e SP700 layer (voids are enveloped by yellow circles)

Fig. 4 EBSD analysis on TC4/SP700 laminate after diffusion bonding at parameters of 800 ℃/1 h/10 MPa: a, b TC4 layer, c, d SP700 layer, e, f bonded interface, a, c grain size maps based on the circle equivalent diameter, b, d inverse pole figure (IPF) maps, e image quality map, f distribution of deformed (red), substructured (yellow) and recrystallized (blue) α grains (β grains are marked by gray, RD: rolling direction, ND: normal direction)

Fig. 5 Engineering stress-strain curves of multilayered TC4/SP700 alloys at various temperatures: a 700 ℃ (specimens before and after superplastic tension in the inset), b 750 ℃, c 800 ℃, d peak stresses with various temperatures and strains

Fig. 6 Activation energy and stress exponent of superplastic deformation based on the relationship between a liner relationship between \(\ln \dot{\varepsilon }\) and \(\ln \sigma\), b liner relationship between \(\ln \sigma\) and 1/T

Fig. 7 EBSD and TEM observations on TC4 a, c, e, f and SP700 b, d alloys after superplastic deformation at different temperatures and strain rates: a, b 700 ℃ and 0.005 s−1, c-f 800 ℃ and 0.0005 s−1. a-d Phase images on top of the image quality maps, e dark-field image showing α precipitates from β grain in TC4 alloy, f pole figure (PF) map showing the symmetrical relationship of crystal planes and crystal directions

Fig. 8 TEM observation showing microstructure and dislocations in TC4 layer a, b and SP700 layer c, d after superplastic deformation at 750 ℃ and 0.001 s−1: a-d bright-field images, (a1-d1) dark-field images, and selected area electron diffraction (SAED)

Fig. 9 EBSD analysis after superplastic deformation at 700 ℃ and 0.005 s−1: a-c SP700 alloy, d-f TC4 alloy, a, d phase images, b, e KAM maps, c, f image quality maps showing lattice distortion

Fig. 10 Tensile strengths of all samples after superplastic deformation

Fig. 11 Fracture morphology of dissimilar TC4/SP700 laminate subjected to superplastic deformation and quasi-static tension: a overview image showing the fracture surface of layers and interface, b TC4 layer, c SP700 layer