**2. Experimental Procedures**

#### *2.1. Materials and Heat Treatment*

Table 1 presents the chemical composition of TA15 titanium alloys, while the rectangular samples used in this study were 100 mm × 77 mm × 5 mm in size. The heat treatment furnace type is an SA2-9-12TP 1200 ◦C box atmosphere furnace. The furnace adopts a double-shell structure and an intelligent programmable temperature control system. The heating element is a resistance wire thermocouple with a temperature range of 0–1200 ◦C, and the temperature control accuracy is ±1 ◦C. Table 2 outlines the various heat treatment methods utilized, including two multiple heat treatment methods (TriAC and TriWQ) to examine the effect of high temperatures on the microstructure and mechanical properties of the TA15 titanium alloy. Additionally, other samples (810AC, 810WQ, and 940WQ) were utilized to evaluate the impact of cooling rates on microstructure and mechanical properties. The samples were polished using silicon carbide paper (300, 600, 1000, 1500, and 2000) and etched using Kohler reagent (H2O:HNO3:HCl:HF is 190:5:3:1). The microstructure was analyzed using an S-7800N Scanning Electron Microscope (SEM), and phase identification was conducted using X-ray diffraction (XRD).

**Table 1.** Chemical composition (wt%) of TA15 titanium alloy used in the study.


**Table 2.** Heat treatment conditions for different samples.


#### *2.2. Mechanical Properties Test*

To determine the microhardness of the TA15 titanium alloy, an indentation test was conducted, measuring the microhardness of each sample at 100 g load and 15 s residence time. Nine indentation tests were performed on each sample at 0.5 mm intervals, and the average value was taken as the microhardness of the material. Due to limitations in the testing equipment, the extensometer was not used in the experiment. Figure 1 displays the dimensions of the sample used for the tensile, bending, and impact toughness tests. The room temperature tensile and bending tests were carried out using a DDL100 electronic universal testing machine, and the calculation formula for bending properties was applied. Impact properties were evaluated using an NI150 metal pendulum impact testing machine (ASTM, E23-2018).

**Figure 1.** Size of samples with different mechanical properties.
