Fig. 1. (a) Influence of Ta and Nb on the phase stability of Ti alloys calculated by ab initio method; (b) Phase stability diagram based on $\overline{Bo}$-$\overline{Md}$ parameters (adapted and redrawn from Ref. [16]).

Fig. 2. Schematic representation of the processing route. (RT: Room temperature, HT: Homogenization treatment, and AC: Air cooling).

Fig. 3. Microstructures of as-cast samples: OM images of (a) T3102, (b) T3132, and (c) T5132 alloys; (d) XRD spectra; TEM micrographs of (e) T3102, (f) T3132, and (g) T5132 alloys (the inset is the corresponding dark-filed image); (h, i) Corresponding EDS results of regions A and B in (f), respectively.

Fig. 4. (a) Stress-strain curves of three alloys measured by compressive procedure; the stress-strain curves of cycle compression for (b) T3102, (c) T3132 and (d) T5132 alloys in the same states. The cyclic compressive strain during loading-unloading testing was 3%, 6%, 9%, 12%, 15%, 18%, 21%, 24%, and 27%, respectively (εE, εSE and εR denote elastic recovery, superelastic recovery, and residual strain, respectively); (e) Comparison of strain after unloading. Ti-xNb-yTa-2ZrE, Ti-xNb-yTa-2ZrSE, and Ti-xNb-yTa-2ZrR are the values of Ti-xNb-yTa-2Zr specimens respectively; (f) Relationship between compressive strength and strain of various β/(α+β) Ti alloys and compressive properties of the present alloys.

Fig. 5. TEM and HRTEM images of as-cast T3132 alloy after compression: (a) TEM image of a longitudinal section of the sample; (b) Magnified view of α" martensite in (a); (c) Corresponding diffraction patterns of α, β, and α" phases in (a); (d) Aberration-corrected HRTEM image of the interface between α and β phases in (a), with the inset showing the corresponding FFT pattern; (e, f) IFFT images in (d) along [0001]α zone axis; (g) Aberration-corrected HRTEM image of the interface between β and α" phases in (b); (h) Corresponding IFFT image of (b); (i, j) Magnified view of marked region in (h).

Fig. 6. TEM and HRTEM images of as-cast T5132 alloy after compression: (a) Low-magnification overview of the lamellae microstructure; (b) Magnified view of ω phase and dislocations in (a); (c) HRTEM image of the interface between the α and β phases in (a); (d) Corresponding IFFT image of (c).

Fig. 7. (a) XRD profiles of the cold-rolled T3102, T3132, and T522 alloys; (b-d) IPF maps viewed in the normal direction of the alloys, respectively; (e) Legend of the IPF maps; (f-h) Phase maps, respectively.

Fig. 8. TEM images of cold-rolled T3102 alloy: (a) An overview of the deformation region; (b) Morphology of martensite and dislocations; (c) Diffraction patterns of α, β, and α" phases; (d) Shear bands; (e) Interfaces of α, β, and α" phases; (f) Ultrafine grains formed by rolling. (RD: rolling direction).

Fig. 9. TEM analysis of cold-rolled T3132 alloy: (a) Low-magnification overview of the deformed microstructure; (b) Bright-filed image of the stripe structure; (c, d) HRTEM images of stripes of areas A and B, respectively; (e, f) Bright-filed and dark-filed images of the martensite phases, respectively; (g) Corresponding SAED of (e); (h) Corresponding energy dispersive spectroscopy (EDS) of circled regions A, B, C, and D in (b) and (e), respectively. (RD: rolling direction).

Fig. 10. TEM images of cold-rolled T5132 alloy: (a) Low-magnification overview of the deformed microstructure; (b) Magnified view of ω phase and dislocations in (a); (c, d) Bright -filed and drak-filed images of the nanograins, respectively.

Fig. 11. (a) Stress-strain curves of the as-rolled alloys; (b) Comparison of modulus of elasticity of β/(α+β) Ti alloys; (c) Comparison of the strength-to-modulus ratio of representative biomedical materials.

Fig. 12. APT analysis of as-cast T3132 alloy before compressive deformation. (a) LEAP used in the APT test; (b) 3D reconstruction of Ti (black), Nb (yellow), Ta (blue) and Zr (green) atom distribution in the T3132 sample; (c) Proxigram of L1 showing the average concentration of various elements as a function of the distance from the α/β phase interface (L1: Lamellar 1); (d) 1D concentration profile of L1 along with the cyan cylinder (20 nm diameter) indicated in (b); (e, f) 1D concentration profile of L1 indicated in (d), showing clearer element distribution.