**1. Introduction**

The TA15 titanium alloy (Ti-6.5Al-2Zr-1Mo-1V) is classified as a medium strength titanium alloy with a high aluminum equivalent α type, making it highly desirable for its exceptional specific strength, creep resistance, ductility, fracture toughness, and fatigue properties. Its exceptional qualities make it a preferred material in the aviation field [1–3]. However, the alloy's mechanical properties are highly sensitive to its microstructure characteristics, such as phase composition, morphology, distribution, and grain size [4–6]. Given the high temperature sensitivity of titanium alloys, their microstructure and mechanical properties rely heavily on thermal mechanical processing and heat treatment [7].

In recent years, numerous researchers have explored the microstructure and mechanical properties of forged TA15 titanium alloys [8,9]. Sun et al. [10] investigated the tri-modal microstructure evolution in near-β and two-phase field heat treatments of forged TA15 alloys and found that a microstructure consisting of approximately 15% equiaxed α<sup>p</sup> and a significant amount of thick and long lamellar α<sup>s</sup> exhibited excellent comprehensive mechanical properties. Sun et al. [11] further studied the evolution of the lamellar α phase during two-phase field heat treatment in TA15 alloys and found that the volume fraction of the primary lamellar α decreased, the average thickness increased, and the average length variation trend was complex. Li et al. [12] studied mesoscale deformation mechanisms in relation to slip and grain boundary sliding in TA15 alloys during tensile deformation and found that grain boundary slip occurred at the equiaxed α (αp)/β and lamellar α (αl)/β

**Citation:** Li, L.; Pan, X.; Liu, B.; Liu, B.; Li, P.; Liu, Z. Strength and Toughness of Hot-Rolled TA15 Aviation Titanium Alloy after Heat Treatment. *Aerospace* **2023**, *10*, 436. https://doi.org/10.3390/ aerospace10050436

Academic Editor: Andrea Di Schino

Received: 21 March 2023 Revised: 30 April 2023 Accepted: 6 May 2023 Published: 8 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

boundaries during deformation. Sun et al. [13] conducted a study on the microstructure and mechanical properties of TA15 alloys after thermomechanical processing and successfully fabricated equiaxed and fine-grained TA15 alloys with a mean grain size of 2 μm. Lei et al. [14] examined the microscopic damage development and crack propagation characteristics of TA15 titanium alloys with a trimodal structure consisting of equiaxed α (αp), lamellar α (αl), and β transformed matrix α (βt) for the first time. Xu et al. [15] discovered that heat treatment could improve the mechanical and fatigue properties of TA15 alloy joints produced through friction stir welding. In their study, Wu et al. [16] discovered that using a dual heat treatment approach, which involves near-β heat treatment followed by (α + β) heat treatment, is an effective way to achieve a tri-modal microstructure in TA15 titanium alloys. This particular microstructure was found to have exceptional overall performance. Additionally, Zhao et al. [17] investigated the recrystallization behavior of TA15 titanium alloy sheets with numerous predeformed substructures during annealing and hot drawing at 800 ◦C. Meanwhile, Wu et al. [18] focused on the microstructure and mechanical properties of heat-treated and thermomechanically processed TA15 titanium alloy composites. They discovered that composite microstructures consisting of two or three phases exhibited excellent mechanical properties, while fully lamellar microstructures had the highest fracture toughness.

Despite the numerous studies conducted on the microstructure and mechanical properties of wrought TA15 titanium alloys, there is still a lack of research focusing on the effects of multiple heat treatments at high temperatures [19]. In a study conducted by Zhao et al. [20], the impact of vacuum annealing on the microstructure and mechanical properties of TA15 titanium alloy sheets was investigated. It was discovered that the recovery, recrystallization, and phase transformation of TA15 sheets annealed in different modes significantly affected their microstructure and mechanical properties. Furthermore, the strength of TA15 titanium alloy sheets was observed to improve, and the plastic toughness increased as the annealing temperature increased, although the strength decreased.

In addition, many scholars have studied dynamic recrystallization at high temperatures [21–23]. Wang et al. [24] investigated the thermal deformation mechanism of laserwelded TA15 joints through high-temperature tensile tests ranging from 800 ◦C to 900 ◦C, and found that at 900 ◦C and a strain rate of 0.001 s−1, the maximum elongation was 292%. Vo et al. [25] simulated the flow stress of TA15 titanium alloys in compression tests at temperatures of 975, 1000, 1025, 1060, and 1100 ◦C and strain rates of 0.1 and 1 s−1. Yang et al. [26] studied the hot tensile properties of TA15 by conducting tensile tests at temperatures ranging from 750 ◦C to 850 ◦C at strain rates of 0.001, 0.01, and 0.1 s<sup>−</sup>1, and the metallographic results showed that the primary α phase transformed into equiaxed crystals, while the secondary α phase and the lamellar β phase bent and fractured. Hao et al. [27] conducted tensile tests on TA15 titanium alloys over a wide temperature range from −<sup>60</sup> ◦<sup>C</sup> to 900 ◦C and at a strain rate of 0.001 s<sup>−</sup>1. They used electron backscatter diffraction (EBSD) to analyze the evolution of microstructure and deformation mechanisms at different temperatures. At relatively low temperatures (60 ◦C, 23 ◦C, and 400 ◦C), the dislocation slip mechanism dominated the deformation, even when twinning was detected. However, the spheroidization and dynamic recrystallization (DRX) of α lamellae dominated the deformation mechanism at high temperatures (600 ◦C and 800 ◦C), leading to flow softening. Zhao et al. [17,28] found significant recrystallization behavior in the TA15 alloy during high temperature tensile tests at 800 ◦C. Li et al. [29] studied the effect of strain and temperature on hot deformation using an exponential Zener-Hollomon equation, and tested the effectiveness of the proposed constitutive equation in isothermal compression tests at temperatures ranging from 800 ◦C to 1000 ◦C, with strain rates of 10, 1, 0.1, 0.01, and 0.001 s−<sup>1</sup> and a strain of 0.9. Feng et al. [30,31] studied the tensile properties and fracture mechanisms of TA15 titanium alloys at temperatures ranging from 400 ◦C to 700 ◦C, and attempted to improve its high-temperature plastic deformation ability by adding TiBw. Zhao et al. [32] analyzed the deformation non-uniformity and slip mode of TA15 titanium alloy sheets during hot tensile tests at 750 ◦C along the rolling direction. Li et al. [33] revealed the flow softening and plastic damage mechanisms in the uniaxial thermal tensile tests of titanium alloys at temperatures ranging from 910 ◦C to 970 ◦C and strain rates of 0.01 to 0.1 s<sup>−</sup>1. Zhao et al. [34] studied the evolution of texture in the rolling direction of TA15 sheets through experiments and crystal plasticity simulations in a tensile test at 750 ◦C. Liu et al. [35] investigated the correlation between the α value and strain in TA15 titanium alloys deformed at 750 ◦C.

The research presented above highlights the need to further investigate the impact of different heat treatments on the strength and toughness of forged TA15 titanium alloy sheets. Consequently, several heat treatment experiments were conducted to systematically explore the effect of heat treatment parameters on the microstructure of the forged TA15 titanium alloy sheets. Furthermore, microhardness, room temperature tensile, impact, and bending tests were conducted on TA15 titanium alloy sheets subjected to different heat treatments. The resulting data was analyzed to assess the impact of different heat treatments on the strength and toughness of the forged TA15 titanium alloy sheets.
