Titanium is an important structural metal that developed in the 1950s. Titanium alloys are widely used in various fields due to their high specific strength, good corrosion resistance and high heat resistance. Many countries around the world have recognized the significance of shovel alloy materials and have successively conducted research and development on them, which have been put into practical application.

The first practical titanium alloy was the Ti-6Al-4V alloy developed in the United States in 1954. Due to its excellent heat resistance, strength, plasticity, toughness, formability, weldability, corrosion resistance and biocompatibility, it has become the top alloy in the titanium alloy industry. The usage of this alloy now accounts for 75% to 85% of all titanium alloys. Many other titanium alloys can be regarded as modified versions of Ti-6Al-4V alloy.

In the 1950s and 1960s, the main focus was on developing high-temperature titanium alloys for aero engines and structural titanium alloys for aircraft bodies. In the 1970s, a batch of corrosion-resistant titanium alloys were developed. Since the 1980s, corrosion-resistant titanium alloys and high-strength titanium alloys have been further developed. The service temperature of heat-resistant titanium alloys has increased from 400℃ in the 1950s to 600-650 ℃ in the 1990s. The emergence of A2(Ti3Al) and r (TiAl) -based alloys has led to titanium being pushed from the cold end of the engine (fans and compressors) to the hot end (turbines) in the engine's application areas. Structural titanium alloys are developing towards high strength, high plasticity, high strength and high toughness, high modulus and high damage tolerance.

In addition, since the 1970s, shape memory alloys such as Ti-Ni, Ti-Ni-Fe, and Ti-Ni-Nb have emerged and have been increasingly widely applied in engineering.

At present, hundreds of titanium alloys have been developed in the world, among which 20 to 30 are the most famous. Such as Ti-6Al-4V, Ti-5Al-2.5Sn, Ti-2Al-2.5Zr, Ti-32Mo, Ti-Mo-Ni, Ti-Pd, SP-700, Ti-6242, Ti-1023, Ti-10-5-3, Ti-1023, BT9, BT20, IMI829, IMI 834 et al. [2,4].

Alloying of titanium

Titanium alloy is an alloy composed of titanium as the base and other elements added. Titanium has two allocrystals: α titanium with a close-packed hexagonal structure below 882℃ and β titanium with a body-centered cubic structure above 882℃.

Alloying elements can be classified into three categories based on their influence on the phase transformation temperature:

Elements that stabilize the α phase and increase the phase transition temperature are α -stabilizing elements, including aluminum, carbon, oxygen and nitrogen, etc. Among them, aluminum is the main alloying element of titanium alloys. It has a significant effect on enhancing the alloy's strength at both room and high temperatures, reducing its specific gravity, and increasing its elastic modulus.

Elements that stabilize the β phase and lower the phase transition temperature are β -stabilizing elements, which can be further classified into two types: isomorphic and eutectoid. The former includes molybdenum, niobium, vanadium, etc. The latter includes chromium, manganese, copper, iron, silicon, etc.

③ Elements that have little effect on the phase transition temperature are neutral elements, such as zirconium and tin.

Oxygen, nitrogen, carbon and hydrogen are the main impurities in titanium alloys. Oxygen and nitrogen have a relatively high solubility in the α phase, which has a significant strengthening effect on titanium alloys, but it reduces plasticity. It is generally stipulated that the content of oxygen and nitrogen in titanium should be below 0.15 to 0.2% and 0.04 to 0.05% respectively. Hydrogen has a very low solubility in the α phase. Excessive hydrogen dissolution in titanium alloys can form hydrides, making the alloy brittle. The hydrogen content in titanium alloys is usually controlled below 0.015%. The dissolution of hydrogen in titanium is reversible and can be removed by vacuum annealing.

Classification of titanium alloys

Titanium is an allotrope with a melting point of 1720℃. Below 882℃, it presents a close-packed hexagonal lattice structure and is known as α -titanium. At temperatures above 882℃, it presents a body-centered cubic lattice structure and is called β -titanium. By taking advantage of the different characteristics of the above two structures of titanium and adding appropriate alloying elements, the phase transformation temperature and phase content are gradually changed to obtain titanium alloys with different structures (titanium alloys). At room temperature, titanium alloys have three types of matrix structures, and titanium alloys are thus classified into the following three categories: α alloys,(α+β) alloys, and β alloys. China is represented by TA, TC and TB respectively.

α titanium alloy is a single-phase alloy composed of α phase solid solutions. It remains in the α phase both at normal temperatures and at higher practical application temperatures. It has a stable structure, higher wear resistance than pure titanium, and strong oxidation resistance. At temperatures ranging from 500℃ to 600℃, it still maintains its strength and creep resistance, but it cannot be strengthened by heat treatment, and its strength at room temperature is not high.

β titanium alloy is a single-phase alloy composed of β phase solid solutions. It has high strength without heat treatment. After quenching and aging, the alloy is further strengthened, and its room-temperature strength can reach 1372 to 1666 MPa. However, it has poor thermal stability and is not suitable for use at high temperatures.

α+β titanium alloy is a duplex alloy with excellent comprehensive performance, good microstructure stability, good toughness, plasticity and high-temperature deformation performance. It can be well processed by hot pressure and strengthened by quenching and aging. The strength after heat treatment is approximately 50% to 100% higher than that in the annealed state. It has high high-temperature strength and can work for a long time at temperatures ranging from 400℃ to 500℃. Its thermal stability is second only to that of α titanium alloy.

Among the three titanium alloys, the most commonly used ones are α titanium alloy and α+β titanium alloy. The machinability of α titanium alloy is the best, followed by α+β titanium alloy, and β titanium alloy is the worst. The code for α titanium alloy is TA, that for β titanium alloy is TB, and that for α+β titanium alloy is TC.

Titanium alloys can be classified by their applications into heat-resistant alloys, high-strength alloys, corrosion-resistant alloys (such as titanium-molybdenum and titanium-palladium alloys), low-temperature alloys, and special functional alloys (such as titanium-iron hydrogen storage materials and titanium-nickel memory alloys). The composition and properties of typical alloys are shown in the table.

Different phase compositions and structures can be obtained by adjusting the heat treatment process of heat-treated titanium alloys. It is generally believed that fine equiaxed structures have better plasticity, thermal stability and fatigue strength. The needle-like structure has high creep strength, creep strength and fracture toughness. The mixed structure of equiaxed and needle-like structures has better comprehensive performance.

The properties of titanium alloys

Titanium is a new type of metal. The properties of titanium are related to the content of impurities such as carbon, nitrogen, hydrogen and oxygen it contains. The purest titanium iodide has an impurity content of no more than 0.1%, but it has low strength and high plasticity. The properties of 99.5% industrial pure titanium are as follows: density ρ=4.5g/cm ³, melting point 1725℃, thermal conductivity λ=15.24W/(m · K), tensile strength σb=539MPa, elongation δ= 25%, reduction of area ψ= 25%, elastic modulus E=1.078×105MPa, hardness HB195.

The density of high-strength titanium alloys is generally around 4.5g/cm ³, which is only 60% of that of steel. The strength of pure titanium is close to that of ordinary steel. Some high-strength titanium alloys exceed the strength of many alloy structural steels. Therefore, the specific strength (strength/density) of titanium alloys is much greater than that of other metal structural materials, as shown in Table 7-1. It can be used to produce components and parts with high unit strength, good rigidity and light weight. At present, titanium alloys are used in the engine components, frames, skins, fasteners and landing gears of aircraft.

(2) High thermal strength: The service temperature is several hundred degrees higher than that of aluminum alloys. It can still maintain the required strength at medium temperatures and can work for a long time at temperatures ranging from 450 to 500℃. These two types of titanium alloys still have very high specific strength within the range of 150℃ to 500℃, while the specific strength of aluminum alloys drops significantly at 150℃. The operating temperature of titanium alloy can reach 500℃, and that of aluminum alloy is below 200℃.

(3) Good corrosion resistance: Titanium alloys have much better corrosion resistance than stainless steel when working in humid air and seawater media. It has particularly strong resistance to pitting corrosion, acid corrosion and stress corrosion. It has excellent corrosion resistance to alkali, chlorides, organic substances containing chlorine, nitric acid, sulfuric acid, etc. However, titanium has poor corrosion resistance to reducing oxygen and chromium salt media. (4) Good low-temperature performance: Titanium alloys can still maintain their mechanical properties at low and ultra-low temperatures. Titanium alloys with excellent low-temperature performance and extremely low interstitial elements, such as TA7, can still maintain a certain degree of plasticity at -253℃. Therefore, titanium alloy is also an important low-temperature structural material.

(5) High chemical reactivity: Titanium has high chemical reactivity and undergoes intense chemical reactions with gases such as O, N, H, CO, CO2, water vapor, and ammonia in the atmosphere. When the carbon content is greater than 0.2%, hard TiC will form in titanium alloys. When the temperature is relatively high, it will also form a hard TiN surface layer when reacting with N. When the temperature is above 600℃, titanium absorbs oxygen to form a very hard hardened layer. An increase in hydrogen content can also form a brittle layer. The depth of the hard and brittle surface layer formed by absorbing gas can reach 0.1 to 0.15 mm, and the degree of hardening is 20% to 30%. Titanium also has a high chemical affinity and is prone to adhering to friction surfaces.

(6) Low thermal conductivity and low elastic modulus: The thermal conductivity of titanium is λ=15.24W/ (m · k).

It is approximately one fourth of nickel, one fifth of iron, and one fourteenth of aluminium. Moreover, the thermal conductivity of various titanium alloys is about 50% lower than that of titanium. The elastic modulus of titanium alloy is approximately half that of steel, so it has poor rigidity and is prone to deformation. It is not suitable for making slender rods and thin-walled parts. During cutting, the rebound of the machined surface is very large, about 2 to 3 times that of stainless steel, causing severe friction, adhesion and bonding wear on the rear face of the tool.