Fig. 1. (a) Schematic of the thermomechanical history of the as-received material with illustrated microstructures and the subsequent experimental plan, (b) the specimen dimension of SR and tensile experiments. (Unit: mm).

Fig. 2. (a) SR curves with different initial strain levels at four temperatures, (b) variations of relaxed stress percentage, defined by the ratio of the total relaxed stress to the initial stress, with initial strain levels at 700 °C and 800 °C, which are derived from the SR experimental results of 2400 s.

Fig. 3. (a) Evolution of the YS of Ti-6Al-4V during the whole SR, (The hollow symbols indicate elastic strain and the solid symbols indicate plastic strain.) and (b) YS loss in different stages of the whole SR. (The percentage is defined as the strength loss at the current stage divided by the YS of as-received material.).

Fig. 4. Microstructural evolution in Ti-6Al-4V alloy from SEM observations, (a) as-received state, (b) after SR at 700 °C/10%, (c) after loading at 800 °C/0.5%, (d) after SR at 800 °C/0.5%, (e) after loading at 800 °C/10%, (f) after SR at 800 °C/10%.

Fig. 5. The percentage evolution of α phase during SR under different test conditions.

Fig. 6. (a) KAM map at 700 °C/10%/0s, (b) GND map at 700 °C/10%/0s and (c) Evolution of GND density during SR for different temperatures and initial strain levels.

Fig. 7. Orientation map for samples with different SR conditions, (a) as-received, (b) 800 °C/0.5%/2400 s, (c) 800 °C /10%/1200 s, (d)-(f) the development of misorientation curves along the line direction. (PTP, point to point, represents adjacent misorientations, and PTO, point to origin, represents cumulative misorientations.).

Fig. 8. Map scanning results of some lamellae with the same orientation for different SR conditions.

Fig. 9. Line scanning results of β phase and adjacent α phases for different SR conditions.

Fig. 10. HRTEM characterization of Ti-6Al-4V alloy, (a) HRTEM image of lamellar structure, (b) and (c) are the atomic images of α and β phases, respectively, (d) and (e) are the inverse fast Fourier transform (IFFT) images of α and β phases, respectively, (f) magnified image of two-phase interface in frame Ⅲ, (g) the IFFT images in frame Ⅲ, (h) magnified image of two-phase interface in frameⅣand (i) IFFT image of yellow frame in (g) filtered, showing the dislocations at the interface.

Fig. 11. Characterization of dislocation morphology and phase structure, (a) TEM image of dislocation at 700 °C/0.5%/0 s, (b) TEM image of fractured β phase and dislocation at 700 °C/10%/200 s, (c)-(e) two-beam images using three imaging conditions of g=[0002], g=[11¯01] and g=[11¯00], respectively, (f) dislocation slip induced micro-shear banding and fragmentation of β phase, (g)-(i) the elemental distribution of Al, V and Ti after 700 °C/10%/200 s, respectively.

Fig. 12. Evolution of the GND gradient as indicated by the yellow arrow in (d)-(f) with different relaxation times, (a)-(c) secondary electron image, (d)-(f) GND distribution, (g)-(i) the GND curves along arrow scan in the same colony.

Fig. 13. The development of the stress exponent with different initial strain levels during SR. (The hollow symbols indicate elastic strain and the solid symbols indicate plastic strain.).

Fig. 14. Schematics of microstructure evolution during SR with different temperatures and initial strains. (Elastic and Plastic, respectively represent elastic and plastic loadings.) The σρ is the dislocations strength, and the σH-P is the Hall-Petch strength of interfaces. (The horizontal line represents that the strength is hardly affected by the relaxation stage; the up and down arrows represent the increase and decrease of strength, respectively.).

Fig. 15. The contribution of the individual calculated terms to the total YS of Ti-6Al-4V alloy.