Fig. 1. Mechanical properties of (TiZrHf)87Ta13 and (TiZrHf)85Ta15. (a) The tensile true and engineering stress-strain curves of Ta13 (red) and Ta15 (blue). Inset shows a comparison of the normalized work hardening rate ((dσT/dεP)/G) as a function of elongation between the present alloys and other high-performance Ti alloys [31, [47], [48], [49], [50]] and TWIP steels [45,46]. (b) work hardening rate curves and (c) work hardening exponent curves of Ta13 (red) and Ta15 (blue) as a function of true strain. Five deformation stages are identified and characterized as stages I-V (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 2. Microstructural evolution of (TiZrHf)87Ta13 and (TiZrHf)85Ta15 with increase of strain. XRD patterns of Ta13 and EBSD phase maps of Ta13 and Ta15 subjected to (a) 0%, (b) 3%, (c) 6% and (d) 9% true strain.

Fig. 3. OR analysis with misorientation profile of α" martensites acquired from EBSD of different strained Ta13. (a) Euler angle map of α" phase superimposed on image quality (IQ) map of 3% strained Ta13. The insets are α" features used for OR analysis. (b) Stereographic projections of white arrow highlighted martensites (i-ii): top boxed area in (a); (iii-iv): bottom boxed area in (a)). (c) Euler angle map of α" phase superimposed on IQ map of 6% strained Ta13. The inset is α" features used for OR analysis. (d) Stereographic projections of white arrow highlighted martensites from the boxed area in (c). All crystallographic orientation data of highlighted martensites are marked with corresponding Euler angle color, and stereographic projection figures are constructed with directions of three-axis and key poles of interest.

Fig. 4. Deformation microstructure of Ta13 at different strains. (a) Martensitic twins at 9% true strain in Ta13 revealed by EBSD. IQ and Euler angle map of α" phase with yellow, blue and red lines representing twinning boundaries of {351}α" type I twinning, {111}α" type I twinning and "{351}"< $\bar{2}11$ >α" type II twinning, respectively. (b) KAM map of the α" phase. (c) Microstructural image of Ti13 at 20% strain (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 5. TEM images of deformation bands in the deformed Ta13 specimen at 6% strain. (a) BFI of the deformation band. (b) SADPs taken from the twinning boundary (marked by red circle in (a)) taken along the [$1\bar{1}2$]α" zone axis. The α" matrix (M) and twinned α" variant (T) diffraction spots are marked with red and green open circles, respectively. (c) DFIs of the twinning, and APB-like stacking faults with g* = 110 in [$\bar{1}10$]α" zone axis.

Fig. 6. TEM images of the re-tilted deformation bands in Fig. 5(a), (a) corresponding SADPs taken along the [100]α"M//[011]α"T zone axis. The α" matrix (M), twinning (T) and secondary twinning (ST) diffraction spots are marked with red, green and orange open circles, respectively. (b, c) DFIs of the primary twinning and secondary twinning. (d) SADPs taken along the [$2\bar{1}1$]α"M//[001]α"T zone axis. The diffraction spots corresponding to α" matrix, α" twinning and parent β grain are marked with red, green and yellow open circles, respectively. (e, f) DFIs of the α" matrix and twinned variant, inset is DFI of the retained parent β grain.

Fig. 7. Atomic-scale characterization of the "{351}"< $\bar{2}11$ >α" type II twinning in deformed Ta13 specimen at 6% strain. (a) HRTEM image of the {351}α" twinning boundary. Inset shows the Fast Fourier transform (FFT) pattern of the twinning interface (red box marked region). (b) Magnified HRTEM image of the twinning boundary (boxed region in a). Insets are Wiener filtered image and schematic atomic configuration of the α" matrix. The atomic columns in the α" matrix and columns shuffled with (001)<010>α" displacement are highlighted by blue and orange dots, respectively (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 8. TEM images of complex deformation bands in 9% strained Ta13. (a) BFI of the complex deformation bands. (b) and (c) are SADPs taken next to the twinning boundary (red circled area in (a)), along the [$\bar{1}01$]α"V1//[$\bar{1}10$]α"V2//[$\bar{1}21$]α"V3&V4 and [$\bar{2}11$]α"V1–3//[001]α"V4 zone axes, respectively. DPs of V1-V4 are highlighted with blue, green, red and orange open circles, respectively. (d–f) DFIs correspond to V1-V4 α" variants, respectively (taken from the spots of V1-V4 in (b)) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 9. Crystallographic orientations of the α" phase represented as stereographic projections used to determine the ORs between α" matrix and twinned variants. (a) FFT and simulated SADPs of the yellow dash boxed area in Fig. 8(a) imaged along the [$\bar{1}01$]α"V1//[$\bar{1}10$]α"V2//[$\bar{1}12$]α"V3 zone axis. (b–d) Simulated DPs of [$\bar{1}01$]α"V1, [$\bar{1}10$]α"V2 and [$\bar{1}12$]α"V3. (e-g) Stereographic projection figures of V1-V3 are constructed respectively from the orientation data acquired from FFT in (a).

Fig. 10. Microstructural evolution of SA "{351}"< $\bar{2}11$ >α" type II twinning to mechanical {351}α" type I twinning in 9% strained Ta13. (a) HRTEM image of the yellow boxed area in Fig. 8(a). Insets are FFTs of red boxed regions. (b) HRTEM image of the same area in (a), acquired from [$\bar{2}11$]α"V1&V3 direction. Insets are FFTs of red boxed regions. (c) Schematic illustration of atomic movements associated with the twinning evolution. Blue and orange dots indicate atoms at the α" matrix and shuffled with (001)< 010>α" (lattice corresponds to { $1\bar{1}0$ }<110>β) displacement, respectively. Images from i-iii (left to right) are projections of "{351}"< $\bar{2}11$ >α" type II twinning along [$1\bar{1}2$]α" and [$\bar{2}11$]α" directions, {351}α" type I twinning along [$\bar{2}11$]α" direction, respectively (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 11. TEM analysis of the microstructure in Ta13 with a strain of 20%. (a) BFI of the intersected deformation bands, (b) corresponding SADPs. DPs of four α" martensite variants are marked by red, blue, green and yellow open circles, respectively. The variants are then divided into M and V groups for the {111}α" and {011}α" twinning related variants, respectively. (c-f) DFIs of the four α" martensite variants. Insets in (c) and (e) are the corresponding SADPs of the twinning for M and V groups, respectively; (g, h) Stereographic projection figures constructed from crystallographic orientation taken from SADPs in (b) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 12. Schematic illustrations of (a) microstructural evolution and deformation mechanisms occurring during deformation and (b) schematic sketches illustrating the nucleation of mechanical {351}α" type I twins within "{351}"< $\bar{2}11$ >α" type II twinned α" variants. (c) The dependence of < $\bar{2}11$ >α" type II twinning mode selection on the c/a and b/a ratios. The critical c/a and b/a ratios for activation of three rational < $\bar{2}11$ >α" type II twinning modes are highlighted. The c/a and b/a ratios of Ta13, Ta15 and Ti-20Nb [87] are calculated and represented by red, blue and green squares, respectively (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).