Fig. 1 a Standard tensile sample, b in-situ tensile sample for 2D digital image correlation (2D-DIC) analysis

Fig. 2 a, b Microstructure of the base metal; c vertical section of the Ti-Al-Nb phase diagram at 21 at.% Al, calculated by Pandat; d XRD pattern of the base metal

Fig. 3 BSE micrographs of the joint under typical welding conditions for a WZ, b TMAZ, c HAZ

Fig. 4 a EBSD IPF map of the weld zone, b misorientation distribution and the low-/high-angle grain boundaries of the WZ, c sub-grain boundary and dislocation arrays in the WZ

Fig. 5 EBSD maps of the thermo-mechanically affected zone (TAMZ): a1 IPF map, a2 complete recrystallized and substructured grain distribution, a3 grain boundary distribution. Bright-field TEM images of b O phase, c B2 and O phases. d The corresponding SAED patterns in c

Fig. 6 Bright-field TEM morphology of the HAZ: a lamellar O phases and SAED patterns showing the orientation relationship of O-B2, b HRTEM morphologies of specific regions of a, c, d the corresponding FFT patterns of specific regions of b revealing the O phase precipitated from the B2 phase. EBSD phase maps of e BM, f HAZ

Fig. 7 Interfacial Vickers hardness distribution across the joint

Fig. 8 In-situ SEM images of the microstructure evolution in the HAZ, TMAZ and WZ a. a1-a3 HAZ, TMAZ and WZ under strain ~ 3%, b1-b3 HAZ, TMAZ and WZ under strain ~ 6%, c1-c3 HAZ, TMAZ and WZ under strain ~ 8%

Fig. 9 a Micro-strain distribution map of the base metal, b the axial micro-strain distribution of a along the tensile direction, c the micro-strain distribution of the joint, d the axial micro-strain distribution of c along the tensile direction

Fig. 10 Results of in-situ SEM analysis. a-c Deformed zones from the HAZ to the WZ with emerging slip bands: a strain ~ 3%, b strain ~ 6%, c strain ~ 8%. d Crack initiation in the grain boundary, e crack propagation and intergranular crack in the HAZ. f Multi-slip bands in the HAZ close to the TMAZ

Fig. 11 Microstructure of fracture regions at a low magnification, b high magnification. SEM micrographs of the fracture surface of the joints at c low magnification, d high magnification