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Titanium alloy is an alloy based on titanium with other elements added. Titanium alloy has the characteristics of low density, high specific strength, good corrosion resistance and good process performance. It is an ideal structural material for aerospace engineering. Many countries in the world have recognized the importance of titanium alloy materials, and have successively conducted research and development on them and have obtained practical applications.
History of the development of titanium alloys
① The first practical titanium alloy was the Ti-6Al-4V alloy successfully developed by the United States in 1954. Due to its good heat resistance, strength, plasticity, toughness, formability, weldability, corrosion resistance and biocompatibility, it has become the trump alloy in the titanium alloy industry. The use of this alloy has accounted for 75% to 85% of all titanium alloys. Many other titanium alloys can be regarded as modifications of Ti-6Al-4V alloy.
② In the 1950s and 1960s, high-temperature titanium alloys for aircraft engines and structural titanium alloys for fuselages were mainly developed. 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 operating 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 pushed the use of titanium in the engine from the cold end (fan and compressor) of the engine to the hot end (turbine) of the engine. Structural titanium alloys are developing towards high strength, high plasticity, high strength and high toughness, high modulus and high damage tolerance.
③ In addition, since the 1970s, shape memory alloys such as Ti-Ni, Ti-Ni-Fe, and Ti-Ni-Nb have also appeared and have been increasingly widely used in engineering.
There are hundreds of titanium alloys developed in the world, and there are 20 to 30 most famous alloys, 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-10-5-3, Ti-1023, BT9, BT20, IMI829, IMI834, etc.
Corrosion forms of titanium alloys
It is well known that titanium has the characteristics of corrosion resistance, but in actual production environments, titanium alloys will undergo different types of corrosion. Let's take a look at what the indestructible titanium alloy is most afraid of:
1. Crevice corrosion
In the gaps or defects of metal components, local corrosion occurs due to the stagnation of electrolytes forming electrochemical cells[i]. In neutral and acidic solutions, the probability of contact corrosion in the gaps of titanium alloys is much greater than that in alkaline solutions. Contact corrosion does not occur on the entire gap surface, but eventually leads to local perforation damage.
2. Pitting phenomenon
In most salt solutions, titanium does not have pitting corrosion. It mainly occurs in non-aqueous solutions and boiling high-concentration chloride solutions. Halogen ions in the solution corrode the passivation film on the titanium surface[ii] and diffuse into the titanium to cause pitting corrosion. The pitting aperture is smaller than its depth, and some organic media also pit with titanium alloys in halogen solutions. Pitting corrosion of titanium alloys in halogen solutions usually occurs in high-concentration and high-temperature environments. In addition, pitting corrosion in sulfides and chlorides requires specific conditions and is limited.
3. Hydrogen embrittlement
Hydrogen embrittlement, also known as hydrogen-induced cracking or hydrogen damage, is one of the causes of early damage and failure of titanium alloys. The passivation film on the surface of titanium and its alloys has high strength. The hydrogen embrittlement sensitivity increases with the increase of strength, so the hydrogen embrittlement sensitivity of the passivation film is very high.
4. Contact corrosion
The passive oxide film on the surface promotes the displacement of titanium potential to positive potential, which improves the corrosion resistance of titanium materials to acid and aqueous media. Due to the high potential on the surface of titanium alloy, it is bound to cause other metals in contact with it to form an electrochemical circuit and cause contact corrosion.
Titanium alloys are prone to contact corrosion in the following two types of media: the first type is tap water, salt solution, seawater, atmosphere, HNO3, acetic acid, etc. The stable electrode potential of Cd, Zn, and Al in this solution is more negative than that of Ti, and the rate of anodic corrosion increases by 6 to 60 times; the second type is H2SO4, HCl, etc. Titanium may be in a passivated state or an activated state in these solutions. The first type of solution corrosion is common in the actual contact corrosion process. Anodizing is usually used to form a modified layer on the surface of the substrate to prevent contact corrosion.
The main limitation of titanium and titanium alloys is their poor chemical reactivity with other materials at high temperatures. This property forces titanium alloys to be different from the general traditional refining, melting and casting technologies, and even often causes damage to the mold, making the price of titanium alloys very expensive. Therefore, it was mostly used in high-tech industrial fields such as aircraft structures, aircraft, and petroleum and chemical industries at the beginning.
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