Commonly added alloy elements in titanium alloys: aluminum, tin, zirconium, molybdenum, vanadium, chromium, iron, silicon, copper, rare earth, of which aluminum is the most used. aluminum:

Except for industrial pure titanium, aluminum is added to almost all types of titanium alloys. Aluminum mainly plays a role of solid solution strengthening. For every 1% Al added, the tensile strength at room temperature increases by 50MPa.

The limit solubility of aluminum in titanium is 7.5%; after exceeding the limit solubility, the ordered phase Ti3Al (α2) appears in the structure, which is unfavorable to the plasticity, toughness and stress corrosion of the alloy, so the amount of aluminum added generally does not exceed 7%.

Aluminum improves the oxidation resistance, aluminum is lighter than titanium, can reduce the density of the alloy, and significantly increase the recrystallization temperature, such as adding 5% Al can increase the recrystallization temperature from pure titanium 600 ℃ to 800 ℃. Aluminum improves the bonding force between atoms in the titanium solid solution, thereby improving the thermal strength. Adding about 3% aluminum to the heat-treated β alloy can prevent the brittleness caused by the ω phase generated by the decomposition of the metastable β phase. Aluminum also increases the solubility of hydrogen in α-Ti and reduces the sensitivity to hydrogen embrittlement caused by hydrides. Tin and zirconium:

It is a neutral element and has a relatively high solubility in α-Ti and β-Ti. It is often added at the same time as other elements to supplement and strengthen it.

In order to ensure that the heat-resistant alloy obtains a single-phase α structure, in addition to aluminum, zirconium and tin are added to further improve the heat resistance; at the same time, the adverse effect on plasticity is smaller than that of aluminum, so that the alloy has good pressure workability and weldability.

Tin can reduce the sensitivity to hydrogen embrittlement. In the titanium-tin alloy, the ordered phase Ti3Sn is formed after tin exceeds a certain concentration, which reduces the plasticity and thermal stability.

In order to prevent the appearance of the ordered phase Ti3X (α2 phase), considering the influence of aluminum and other elements on the precipitation of α2 phase, Rosenberg proposed the aluminum equivalent formula.

As long as the aluminum equivalent is less than 8-9%, the α2 phase will not appear

Molybdenum, vanadium:

The β-stabilizing element is the most used, solid-solution strengthening the β phase, and significantly reducing the phase transition point and increasing the hardenability, thereby enhancing the heat treatment strengthening effect. Titanium alloys containing vanadium or molybdenum do not undergo eutectoid reactions and have good structural stability at high temperatures; but when vanadium is added alone, the heat resistance of the alloy is not high, and its creep resistance can only be maintained to 400°C; molybdenum improves creep resistance The effect is higher than vanadium, but the density is higher; molybdenum also improves the corrosion resistance of the alloy, especially the crevice corrosion resistance of the alloy in the chloride solution. Manganese, chromium:

The strengthening effect is large, the ability to stabilize the β phase is strong, and the density is smaller than that of molybdenum, tungsten, etc., so it is widely used and is the main added element of the high-strength metastable β-type titanium alloy. However, they form a slow eutectoid reaction with titanium. When working at high temperature for a long time, the structure is unstable and the creep resistance is low; when β isomorphic elements are added at the same time, especially molybdenum, it can inhibit the eutectoid reaction. silicon:

The eutectoid transformation temperature is high (860°C). Adding silicon can improve the heat resistance of the alloy. Therefore, an appropriate amount of silicon is often added to the heat-resistant alloy. The amount of silicon added should not exceed the maximum solid solubility of the α phase, generally 0.25% about. Due to the large difference in atomic size between silicon and titanium, it is easy to segregate at dislocations in solid solutions to prevent dislocation motion, thereby improving heat resistance. Rare earth:

Improve alloy heat resistance and thermal stability. The internal oxidation of rare earth forms fine and stable RExOv particles, which produce dispersion strengthening. The internal oxidation reduces the oxygen concentration in the matrix and promotes the transfer of tin from the alloy to the rare earth oxide, which is beneficial to suppress the precipitation of brittle α2 phase. In addition, rare earths also strongly inhibit the growth of β grains and refine grains, thus improving the overall properties of the alloy.

Summary: The role of alloying elements:

⑴Solid solution strengthening: The most significant elements to increase the strength at room temperature are iron, manganese, chromium, and silicon, followed by aluminum, molybdenum, and vanadium, while the strengthening effect of zirconium, tin, tantalum, and niobium is poor.

⑵ Stable α phase or β phase: alloying elements increase or decrease the phase transition point.

⑶Enhance heat treatment strengthening effect: β stable element increases alloy hardenability.

⑷ Eliminate harmful effects: aluminum and tin prevent ω phase, rare earth inhibits the precipitation of α2 phase, and β isomorphic elements prevent β phase eutectoid decomposition.

⑸Improve the heat resistance of the alloy: Add aluminum, silicon, zirconium, rare and so on.

⑹Improve the corrosion resistance of the alloy and expand the passivation range: add palladium, ruthenium, platinum, molybdenum, etc.