Introduction to Titanium Alloy Properties
Titanium alloys are a class of metals, characterized by their performance, which is influenced by the presence of impurities such as carbon, nitrogen, hydrogen, and oxygen. The purest form of titanium has an impurity content of less than 0.1%, resulting in high plasticity but low strength. Industrial pure titanium, with a purity of 99.5%, exhibits the following properties: density (ρ) of 4.5 g/cm3, melting point of 1725°C, thermal conductivity (λ) of 15.24 W/(m·K), tensile strength (σb) of 539 MPa, elongation (δ) of 25%, section shrinkage (ψ) of 25%, elastic modulus (E) of 1.078×105 MPa, and a hardness (HB) of 195.
1. Low Density and High Strength: High-strength titanium alloys typically have a density of approximately 4.5 g/cm3, which is only 60% that of steel. Pure titanium exhibits strength comparable to ordinary steel, while certain high-strength titanium alloys surpass the strength of many alloy structural steels. Consequently, titanium alloys possess a significantly higher specific strength (strength/density ratio) than other metal structural materials. This characteristic allows the production of lightweight parts and components with high unit strength, rigidity, and durability. Titanium alloys find applications in engine components, skeletons, skins, fasteners, and landing gear.
2. High Thermal Strength: Titanium alloys can maintain their required strength at elevated temperatures, surpassing the capabilities of aluminum alloys by several hundred degrees Celsius. Between 150°C and 500°C, they maintain their high specific strength, whereas aluminum alloys see a notable drop in specific strength at 150°C. Titanium alloys can operate at temperatures up to 500°C, whereas aluminum alloys are limited to temperatures below 200°C.
3. Excellent Corrosion Resistance: Titanium alloys exhibit superior corrosion resistance compared to stainless steel in moist atmospheres and seawater environments. They particularly excel in resisting pitting corrosion, acid corrosion, and stress corrosion. Additionally, titanium alloys have remarkable resistance to sulfuric acid, nitric acid, chlorides, and chlorinated organic compounds. However, in decreasing oxygen and chromium salt conditions, titanium has a low corrosion resistance.
4. Good Low-Temperature Performance: Titanium alloys retain their mechanical properties at low and ultra-low temperatures. Certain titanium alloys, such as TA7, perform exceptionally well at low temperatures and keep some of their plasticity even at -253°C. Hence, titanium alloys are crucial structural materials for low-temperature applications.
5. Chemical Reactivity: Titanium exhibits significant chemical activity, readily reacting with oxygen, nitrogen, hydrogen, carbon monoxide, carbon dioxide, water vapor, and ammonia gas present in the atmosphere. Hard TiC forms in titanium alloys at higher carbon contents (over 0.2%). When TiN interacts with nitrogen at high temperatures, a hard surface layer is formed. Titanium absorbs oxygen above 600°C, resulting in the formation of a hardened layer with high hardness. Increased hydrogen content leads to the formation of an embrittlement layer. The depth of the hardened brittle surface caused by gas absorption can reach 0.1-0.15 mm, with a hardening degree of 20%-30%. Titanium also exhibits a significant chemical affinity, easily forming adhesion with friction surfaces.
6. Thermal Conductivity and Elastic Modulus: Titanium possesses low thermal conductivity, approximately one-fourth that of nickel, one-fifth that of iron, and one-fourth that of aluminum. The thermal conductivity of different titanium alloys is approximately 50% lower compared to that of pure titanium. Because titanium alloys have an elastic modulus that is around half that of steel, they are less stiff and more prone to deformation. As a result, thin rods and components with thin walls have to be avoided since cutting and processing surfaces have a large amount of rebound volume—roughly two to three times that of stainless steel. This rebound can cause intense friction, adhesion, and bonding wear on the tool surface.
Titanium alloys are composed of titanium as the base metal, supplemented with other elements. Two types of titanium crystal structures exist α-titanium, which exhibits a close-packed hexagonal structure below 882°C, and β-titanium, which possesses a body-centered cubic structure above 882°C.
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