Fig. 1. The drawings of specimen geometry for (a) tensile test and (b) fracture toughness test.

Fig. 2. The mechanical properties of TC4 and TC4-xF alloys in the homogenization treated state, illustrating the variations of (a) yield strength σ0.2, (b) ultimate tensile strength σs, (c) elongation ε, (d) mode I fracture toughness KIC with the amount of Fe alloying. The fitted yield strength and fracture toughness (in hollow circles) are compared with the experimental data (in solid square).

Fig. 3. Optical microstructure of the homogenized TC4 and TC4-xF alloys. (a,a1) TC4, (b-b1) TC4-0.1F, (c-c1) TC4-0.3F, (d-d1) TC4-0.5F, (e-e1) TC4-0.4F, and (f-f1) TC4-0.9F. The prior-β grains are highlighted in black.

Fig. 4. SEM images of lamellar microstructure of the homogenized TC4 and TC4-xF alloys. (a) TC4, (b) TC4-0.1F, (c) TC4-0.3F, (d) TC4-0.5F, (e) TC4-0.4F, and (f) TC4-0.9F.

Fig. 5. Variations of microstructure features with the amount of Fe alloying in the homogenization treated TC4 and TC4-xF alloys (in black solid square). (a) prior-β grain size, (b) colony size, (c) relative volume fraction of colony, (d) relative volume fraction of α phase, (e) relative volume fraction of β phase, (f) average width of β lamellae, (g) average width of α lamellae, (h) average length of α lamellae. The data of as-rolled alloys [34] are compared in red solid dots, the aging-treated alloys are presented in blue solid dots [16], and the data from diffusion couples are indicated by light green dots [35](For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

Fig. 6. TEM images of lamellar α/β microstructure and local distribution of chemical compositions in the homogenization treated TC4 and TC4-xF alloys (a-a1) TC4, (b-b1) TC4-0.3F, (c-c1) TC4-0.9F.

Fig. 7. Variations of the average chemical compositions of α and β phases in the homogenization treated TC4 and TC4-xF alloys with the minor alloying addition of Fe.

Fig. 8. SEM images of crack propagation and phase morphology near the cracks in the homogenization treated TC4 and TC4-xF alloys. The types of crack propagation pathways, namely, across α/β lamellae, along colony boundary, and along α/β phase boundary are shown. (a) TC4, (b) TC4-0.1F, (c) TC4-0.3F, (d) TC4-0.5F, (e) TC4-0.7F, and (f) TC4-0.9F.

Fig. 9. The contributions of the individual strengthening factors to the absolute values of yield strengths σ0.2 of the homogenization treated TC4 and TC4-xF alloys.

Fig. 10. The contributions of the crack propagation resistances from different cracking paths to the overall fracture toughness KIC of the homogenization treated TC4 and TC4-xF alloys.

Fig. 11. The summary of mechanical properties of TC4 and TC4-xF alloys in different thermal-mechanical processing conditions. The variations of mechanical properties with the amount of Fe alloying are presented. The mechanical properties in the homogenization treated state are compared with those of hot-worked and aging treated states (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.).

Fig. 12. The linear regression between the measured fracture toughness of the homogenized TC4-xF alloys and the parameter. $\lambda =\frac{{{\sigma }_{s}}-{{\sigma }_{0.2}}}{\varepsilon }$