Effect of alloying elements on microstructure and properties of titanium alloys

1.Al aluminum has a significant solid solution strengthening effect, and the solid solubility in α-Ti is greater than that in β-Ti, which increases the α / mutual transition temperature and expands the βα phase region, and belongs to the α stabilized element. When the mass fraction of Al in the alloy is less than 7%, the strength of the alloy increases with the increase of Al content, but the plasticity does not decrease significantly. When the mass fraction of Al in the alloy exceeds 7%, brittle Ti3Al appears in the alloy structure, and the ductility decreases significantly.

2.V(Mo, Nb, Ta) vanadium belongs to β-Ti isomorphous element, has β stabilization, infinite solid solution in β-Ti, and also has a certain solid solubility in α-Ti. Vanadium has significant solid solution strengthening effect and can maintain good plasticity while improving alloy strengthening. Vanadium also improves the thermal stability of titanium alloys.

3.Cu copper is a β-stabilized element. Part of the copper in titanium alloy exists in a solid solution state, and the other part forms Ti2Cu or TiCu2 compounds. TiCu2 has thermal stability and plays a role in improving the thermal strengthening of the alloy. Since the solid solubility of copper in α phase decreases significantly with the decrease of temperature, the strength of the alloy can be improved by aging precipitation strengthening.

4.Si silicon has a higher eutectoid transition temperature (860℃), which can improve the heat resistance of the alloy. The amount of silicon added to the heat-resistant alloy should not exceed the maximum solid solubility of the α phase, generally about 0.25%. Due to the large difference in atomic size between silicon and titanium, it is easy to segregation at the dislocation in the solid solution, preventing the dislocation movement, thereby improving the heat resistance. In addition to being solidly dissolved in the matrix as a solidly soluble element, some of the silicon formed a second phase precipitation, which expanded the stable existence temperature range of martensite and increased the hardness of the alloy. For the directional solidification growth of Ti-Al alloy, a small amount of silicon can improve the creep resistance and oxidation properties of the solidification structure, but reduce the fracture toughness. 5.Zr and Sn neutral elements have large solubility in α-Ti and β-Ti, which play a complementary and strengthening role. In heat resistant alloys, in order to ensure that the alloy structure is based on α phase, zirconium and tin need to be added in addition to aluminum to further improve the heat resistance, while the adverse effect on plasticity is smaller than aluminum, so that the alloy has good pressure workability and weldability. Aluminum, zirconium, tin can inhibit the formation of ω phase, and tin can reduce the sensitivity to hydrogen embrittlement. In the Ti-Tin alloy system, when tin >18.5%, the ordered phase Ti3Sn will be formed, which reduces the plasticity and thermal stability.

6.Mn, Fe, Cr strengthening effect is large, stable β phase ability is strong, density is smaller than molybdenum, tungsten, so more applications, is the main additive of high strength metastable β titanium alloy. However, they form a slow eutectoid reaction with titanium, and the structure is unstable and the creep resistance is low under long-term working conditions at high temperature. The addition of β isomorphic elements, especially molybdenum, can inhibit the eutectoid reaction.