Fig. 1 Tensile properties (including tensile strength (Rm), yield strength (RP0.2), elongation (A), and section shrinkage (Z)) of the Ti-Mo-Cr-V-Nb-Al series titanium alloys at room temperature

Fig. 2 a Fatigue crack growth rate test specimen; b compact tensile specimen size diagram

Fig. 3 Microstructures of the Ti-Mo-Cr-V-Nb-Al titanium alloy: a-c BW morphologies; d-f LM morphologies; g-i BM morphologies

Fig. 4 a Fatigue crack propagation rate curves for the three microstructures; b Paris formula fitting; c BW threshold test results; d BM threshold test results

Fig. 5 Fitted curves of fatigue crack propagation rates for the three microstructures: a fitted curve of the BW material; b fitted curve of the LM material; c fitted curve of the BM material; d comparison of three curves

Fig. 6 BW fracture morphologies: a-d fracture morphologies near the fatigue source region, cutting through the αP particles for propagation, and secondary cracks along the αP/β-phase interface; e-h fracture morphologies in the steady state propagation region, with a large number of secondary cracks and more obvious fatigue striations; i-l fracture morphologies at the late stage of the steady state propagation and the rapid propagation region, with a large number of macroscopic shear bands and dimples

Fig. 7 LM fracture morphologies: a-d fracture morphologies near the fatigue source area, cut through the β-grain propagation, crack deflection at the grain boundary, small secondary cracks and fatigue striations can be observed in the magnified image; e-g fracture morphologies in the steady state propagation area, a large number of secondary cracks and apparent fatigue striations; h-l fracture morphologies in the late stage of steady state propagation and rapid propagation area, deep secondary cracks and prominent quasi-cleavage surfaces are observed, and a large number of macroscopic shear zones and terrace features are also found

Fig. 8 BM fracture morphologies: a, b fracture morphologies near the fatigue source region, cutting through the αP particles and producing secondary cracks at the same time; c-h fracture morphologies in the steady state propagation region, with a large number of secondary cracks and fatigue striations, and a spherical crater produced by winding around the equiaxed αP particles is observed; i-l fracture morphologies in the late stage of the steady state propagation and the rapid propagation region, with a large number of horizontally extended macroscopic shear bands, and the enlarged image can see that it is shear elongation dimples

Fig. 9 Microscopic features of BW fracture profile: a-c macroscopic crack propagation paths of the profile with increasing crack propagation rate; d-g secondary cracks produced along αGB or αP particles during steady state propagation; h, i deeper secondary cracks produced along αGB during rapid propagation

Fig. 10 Microscopic features of LM fracture profiles: a-c macroscopic crack propagation paths of the profile with increasing crack propagation rate; d-f crack deflection at grain boundaries during steady state propagation; g-i deeper secondary cracks produced during the rapid propagation phase

Fig. 11 Microscopic features of the BM fracture profile: a-c macroscopic crack propagation paths of the profile with increasing crack propagation rate; d secondary cracks generated during steady state propagation; e-g cracks expand around equiaxed αP particles during steady state propagation; h, i microcracks with micropores generated below the main cracks during the rapid propagation phase

Fig. 12 Microstructural characteristics of three microstructures near the fracture: a-c IPF orientation, phase, and KAM diagrams of BW structure; d-f IPF orientation, phase, and KAM diagrams of LM structure; g-i IPF orientation, phase, and KAM diagrams of BM structure

Fig. 13 a-c α-phase and β-phase polar plots of BM, LM, and BM microstructures

Fig. 14 a-c Crystal arrangements of the α-phase of BW, LM, and BM structures, respectively; d-f frequency statistics of the angle between the c-axis of the α-phase and the loading direction of BW, LM, and BM microstructures, respectively

Fig. 15 a-c Variations of Schmidt factor for BW, LM, and BM, respectively, when the α-phase c-axis is at different angles to the loading direction

Fig. 16 Distributions of slip systems corresponding to the maximum value of the α-phase Schmidt factor in BW a, LM b, and BM c, respectively

Fig. 17 Secondary crack in BW that develops below the primary crack during the rapid propagation stage and expands parallel to the primary crack direction

Fig. 18 EBSD microscopic characterization of secondary cracks in BW, including IPF orientation and analysis of α particles orientation and slip traces around cracks

Fig. 19 EBSD microscopic characterization of BW secondary crack: a GND diagram; b phase diagram; c crystal arrangement; d α-phase orientation distribution

Fig. 20 Schematic diagram illustrates the closure of cracks in BW a, LM b, and BM c within the near-threshold region