*3.5. Tensile Properties*

Figure 8 illustrates the impact of different heat treatments on the tensile properties of TA15 titanium alloys. The data in Table 3 suggests that heat treatment results in a reduction in elongation (ε), with the highest elongation of approximately 27.92% observed in the initial sample. The yield strength (YS) of 810AC and 810WQ samples decreases slightly and insignificantly, while the ultimate tensile strength (UTS) increases significantly, with values of 987 MPa and 984 MPa, respectively. Notably, the tensile strengths are similar to the bending strengths observed in Figure 7. However, for the samples subjected to higher heat treatment temperatures (940WQ, TriAC, and TriWQ), YS, UTS, and elongation all decrease significantly. Specifically, the YS of TriAC and TriWQ samples are 529 MPa and 364 MPa, respectively, while their UTS are 272 MPa and 325 MPa, respectively. Furthermore, the very low elongation of TriAC and TriWQ samples suggests that the fracture mechanism is close to a brittle fracture.

**Figure 8.** Effects of different heat treatments on tensile properties of the TA15 titanium alloy.

#### *3.6. Fracture Analysis*

To gain a thorough understanding of the fracture mechanism during a tensile test, an analysis was performed on the fracture characteristics of TA15 tensile samples, as presented in Figure 9. The fracture surface in Figure 9a exhibits numerous evenly distributed dimples, indicative of the sample's strong toughness and typical ductile fracture. In Figure 9b, a ductile fracture with numerous large depressions is shown. The size and depth of the dimples have increased significantly compared to the initial sample, and many shallow and small depressions are present, indicating that the ductile fracture is the primary fracture mechanism of the 810WQ sample. Lastly, Figure 9c depicts the tensile fracture morphology of the 810WQ sample. Compared to the None sample, the fracture surface undulation of the 810WQ TA15 titanium alloy is larger and contains many deep dimples, with a large number of small dimples distributed around them, which are indicative of ductile fractures [37]. As shown in Figure 9d, the fracture morphology of the 940WQ sample exhibits an obvious river-like tearing ridge, and the fracture is similar to a quasi-cleavage fracture. These results indicate that the plasticity of the TA15 titanium alloy after 940WQ treatment is significantly reduced, as the αSr strongly hinders slip and makes it more difficult to deform the sample, with the dislocation almost unable to cross the α/β boundary. Furthermore, Figure 9e shows that the cleavage steps, cleavage plane, and tear edges of the TriAC sample cover most of the fracture plane, with no dimple observed, and an unstable fracture tear edge existing on the cleavage plane. A portion of the fracture direction is nearly perpendicular to the tensile direction, resulting in a uniform fracture surface. Brittle fracture is the predominant mechanism in the fracture process, which aligns with the elongation value of 5.59%. As illustrated in Figure 9f, the TriWQ sample's fracture mainly exhibits fluvial tearing ridges, with micropores present at the grain boundary, representing a prominent characteristic of brittle intergranular fractures. These results indicate that the TriWQ sample exhibits low toughness and is prone to breakage, consistent with its ultimate tensile strength of 364 MPa and an elongation of 2.9% (refer to Table 4). Therefore, it can be inferred that the ductility of the TA15 titanium alloy is significantly reduced after triple heat treatment, as a result of a large number of αS and layered α+β structures that hinder slip.

**Figure 9.** Tensile fracture morphology of the TA15 titanium alloy under different heat treatment methods: (**a**) None, (**b**) 810AC, (**c**) 810WQ, (**d**) 940WQ, (**e**) TriAC, (**f**) TriWQ.


**Table 4.** Measured mechanical properties of TA15 under different conditions.

The impact fracture surfaces displayed in Figure 10a–c demonstrate distinct ductile fracture characteristics. The surface of the fracture exhibits multiple depressions of varying sizes with minimal surface undulations. The region situated between the deeper dimples is characterized by small dimples, which is a classic feature of ductile fractures. This type of fracture is a result of extensive plastic deformation before the sample's eventual rupture. In contrast, Figure 10d–f indicate that the impact fracture morphology displays a prominent cleavage plane, indicating a brittle fracture. However, some regions of the cleavage plane are overlaid with shallow dimples, representing a brittle ductile mixed fracture. After annealing at 810 ◦C, the dimple size of the sample increases significantly compared to the original sample, indicating excellent ductility and impact toughness. Conversely, at temperatures exceeding 940 ◦C, the area of the fracture cleavage plane of the sample increases significantly, and the size and number of dimples decrease, leading to a considerable reduction in impact toughness, as illustrated in Figure 6.

**Figure 10.** Impact fracture morphology of TA15 titanium alloys under different heat treatment methods: (**a**) None, (**b**) 810AC, (**c**) 810WQ, (**d**) 940WQ, (**e**) TriAC, (**f**) TriWQ.

Figure 11 illustrates the mechanism and schematic diagram of the impact of post-heat treatment on the microstructures and mechanical properties of TA15 titanium alloys. A lower heat treatment temperature has minimal effect on the mechanical properties. The transformed α phase and acicular α phase lead to a slight increase in microhardness (Figure 5). However, the impact toughness (Figure 6), tensile elongation (Figure 8), and bending strength (Figure 7) experience a significant decrease. Despite the reduction in toughness and plasticity, both the tensile fracture surface and impact test fracture surface's dimples indicate that it is a typical ductile fracture. However, a higher heat treatment temperature induces a complete transformation in the sample's microstructure morphology. The transformed α phase leads to a significant increase in microhardness (Figure 5) but a substantial reduction in plasticity (Figure 6). Consequently, the mechanical properties, including impact toughness (Figure 6), tensile elongation, tensile strength (Figure 8), and bending strength (Figure 7), undergo a considerable decrease due to the reduction in plasticity. Moreover, the fracture mode changes from a ductile fracture with lower heat treatment temperatures to a brittle fracture with higher heat treatment temperatures.

**Figure 11.** Mechanism and schematic diagram of the effect of post heat treatment on the microstructures and mechanical properties of TA15 titanium alloys.
