Fig. 1. (a) Schematic of isothermal compression bonding tests. (b) Shape and dimensions of the round bar-shaped specimen for isothermal compression bonding tests. (c) Area selected for microstructural characterization of interfacial grain boundaries (IGBs). (d) Location and dimensions of the tensile test specimen.

Fig. 2. Optical microscopy images of IN718 joints at 1000 °C with a strain rate of (a) 0.001 s-1, (b) 0.01 s-1, (c) 0.1 s-1, and (d) 1 s-1. (The bonding zone is highlighted with yellow lines.).

Fig. 3. Representative inverse pole figure maps of the bonding area of the joints obtained at 1000 ℃ with a strain rate of (a) 0.001 s-1, (b) 0.01 s-1, (c) 0.1 s-1, and (d) 1 s-1. Effects of deformation strain rates on (e) the volume fraction of different types of grains (recrystallized, substructured, and deformed) and (f) the average grain size at various strain rate levels.

Fig. 4. (a) Demonstration for calculating interfacial bonding ratio. (b) Effect of strain rate on interfacial bonding ratio (ΨBonding) in the bonding area.

Fig. 5. Representative inverse pole figure maps of the bonding area in the joints obtained at strain of (a) 0.20, (b) 0.30, (c) 0.40, and (d) 0.50, under constant strain rate and PDB temperature of 0.01 s-1 and 1000 °C, respectively. Evolution of (e) volume fraction of different types of grains (i.e. recrystallized, substructured, and deformed) and (f) average grain size at various strain levels.

Fig. 6. (a, b) Inverse pole figure maps, (c, d) Grain boundary maps and (e, f) Kernel average misorientation maps of the bonding interface bonded at 1000 °C under deformation strains of: (a, c, e) 0.20 and (b, d, f) 0.40. (g) Geometrically necessary dislocation density (ρGND) profile along the lines ‘AB’ and ‘CD’ marked in panels (e) and (f), respectively. (h) Geometrically necessary dislocation density (ρGND) profile from point G1 to G2, (marked in inset of panel (h)).

Fig. 7. (a) Inverse pole figure map, (b) kernel average misorientation map, and (c) TEM image of the subgrains evolved in the vicinity of IGBs at 1000 °C and a deformation strain of 0.10. (d) Misorientation angle (Δθ) and corresponding (e) geometrically necessary dislocation density (ρGND) profiles along the line ‘AB’ marked in panel (a).

Fig. 8. (a) Room temperature tensile stress-strain curves of the wrought and PDBed specimens deformed at different deformation strain rates. (b) Comparison between yield strengths (YSs), ultimate tensile strengths (UTSs), and elongations (ELs) extracted from the tensile curves in panel (a).

Fig. 9. Schematic diagram explaining bonding mechanism through nucleation and growth of DRXed grains in the bonding area: (a) Subgrains, acting as nuclei for DRX, are initially generated within the parent grain on both sides of the interface under continuous deformation. (b) Boundaries of DRXed grains move into the opposite side of the IGBs in order to bond the interface. (c) Nucleation and growth of DRXed grains in the vicinity of the IGBs leading to a fully bonded joint.