Titanium alloys exhibit different properties based on their composition and structure. Titanium has two crystal structures: α-titanium, with a hexagonal lattice below 882°C, and β-titanium, with a body-centered cubic structure above 882°C. By adding appropriate alloying elements, the phase content and transition temperatures can be manipulated to obtain various titanium alloy types. At room temperature, titanium alloys can be classified into three categories.
1. α Titanium Alloy: This single-phase alloy consists of α-phase solid solution. It maintains its α-phase structure at both normal and elevated temperatures. The α titanium alloy exhibits stable organization, lower wear resistance compared to pure titanium, and excellent oxidation resistance. Although it retains its strength and creep resistance between 500-600°C, it cannot be strengthened through heat treatment. The room temperature strength of α titanium alloy is not particularly high.
2. Beta Titanium Alloy: This single-phase alloy is composed of β-phase solid solution. It possesses high strength even without heat treatment. Furthermore, the alloy can be further strengthened through processes like quenching and aging. The tensile strength of beta titanium alloy at room temperature can reach 1372-1666 MPa.
3. Alpha-Beta Titanium Alloy: This duplex alloy exhibits excellent overall performance, including good organizational stability, toughness, plasticity, and high-temperature deformation properties. It is well-suited for hot pressure processing, quenching, and aging to enhance its strength. The heat-treated alpha-beta titanium alloy demonstrates a 50-100% increase in strength compared to the annealed state. It can withstand long-term operation at temperatures of 400-500°C and exhibits remarkable thermal stability, second only to alpha titanium alloy.
Among these three types of titanium alloys, the most commonly used are α titanium alloy and alpha-beta titanium alloy. In terms of machinability, α titanium alloy offers better performance, followed by alpha-beta titanium alloy, while beta titanium alloy lags behind. The corresponding codes for these alloys are TA for α titanium alloy, TB for beta titanium alloy, and TC for alpha-beta titanium alloy.
Performance Characteristics of Titanium Alloys:
1. High Strength: Titanium alloys have a density of approximately 4.51 g/cm³, which is only 60% of steel. Some high-strength titanium alloys surpass the strength of many alloy structural steels. Consequently, the specific strength (strength/density) of titanium alloys exceeds that of other metal structural materials. These alloys are ideal for manufacturing lightweight components with high strength and rigidity, such as aircraft engine parts, skeletons, skins, fasteners, and landing gear.
2. High Thermal Strength: Titanium alloys can withstand higher temperatures compared to aluminum alloys. They can maintain their required strength even at medium temperatures and exhibit exceptional strength between 150-500°C. In contrast, aluminum alloys experience significant strength reduction at 150°C. The working temperature range of titanium alloys extends up to 500°C, while aluminum alloys are limited to temperatures below 200°C.
3. Excellent Corrosion Resistance: Titanium alloys possess superior corrosion resistance in humid atmospheres and seawater, outperforming stainless steel. They exhibit robust resistance to pitting corrosion, acid corrosion, and stress corrosion. Titanium alloys also demonstrate excellent resistance to alkalis, chlorides, chlorine organic substances, nitric acid, sulfuric acid, etc. However, they exhibit poor corrosion resistance to reducing environments containing oxygen and chromium salts.
4. Good Low-Temperature Performance: Titanium alloys maintain their mechanical properties even in low and ultra-low temperatures. Due to their low thermal expansion coefficient, certain titanium alloys, such as TA7, retain a degree of plasticity even at -253°C. Thus, titanium alloys are crucial structural materials for low-temperature applications.
5. Significant Chemical Activity: Titanium exhibits high chemical activity, reacting strongly with atmospheric elements like oxygen, nitrogen, hydrogen, carbon monoxide, carbon dioxide, water vapor, and ammonia. For instance, when the carbon content exceeds 0.2%, hard titanium carbides (TiC) form within the alloy. Similarly, at higher temperatures, the reaction with nitrogen leads to the formation of hard titanium nitride (TiN) surface layers. Titanium readily absorbs oxygen above 600°C, resulting in the formation of a hardened layer. Additionally, increased hydrogen content can lead to the development of a brittle layer. These reactions can cause adhesion phenomena with friction surfaces.
6. Low Thermal Conductivity and Elasticity: Titanium possesses low thermal conductivity (approximately 15.24 W/(m·K)). Its thermal conductivity is about 1/4 of nickel, 1/5 of iron, and 1/14 of aluminum. Titanium alloys exhibit even lower thermal conductivity compared to pure titanium.
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