Effects of alloying elements on microstructure and properties of titanium alloys

1.Al aluminum has a significant solid solution strengthening effect, and the solid solution in alpha-ti is higher than that in beta Ti, which increases the temperature of the alpha/interconversion and enlarges the alpha phase region, belonging to the alpha stabilizing element.When the mass fraction of Al in the alloy is less than 7%, the strength of the alloy increases with the increase of the content of Al, but the plasticity does not decrease obviously.When the mass fraction of Al in the alloy was over 7%, brittleness Ti3Al appeared in the alloy structure and the plasticity was significantly reduced.

V(Mo, Nb, Ta) vanadium belongs to the eutectic element of beta Ti, which has the function of beta stabilization. It is infinitely solid soluble in beta Ti and has a certain solid solubility in alpha Ti.Vanadium has a remarkable solid solution strengthening effect, and can maintain good plasticity while improving the alloy strengthening.Vanadium can also improve the thermal stability of titanium alloy.

3.Cu copper is a stable element. Part of copper in titanium alloy exists in solid solution state, while the other part forms Ti2Cu or TiCu2 compound.Since the solid solubility of copper in alpha phase decreases significantly with the decrease of temperature, the strength of the alloy can be enhanced by aging precipitation.

4. The eutectoid transition temperature of Si silicon is higher (860℃), which can improve the heat resistance of the alloy.The amount of silicon added in the heat-resisting alloy should not exceed the maximum solid solubility of alpha phase, generally about 0.25%.Due to the large difference in atomic size between silicon and titanium, it is easy for them to converge at the dislocation in solid solution to prevent dislocation movement and thus improve heat resistance.In addition to the solid solution of silicon as a solid soluble element in the matrix, a part of silicon forms the second phase precipitation, which expands the stable temperature range of martensite and improves the hardness of the alloy.For the directional solidification growth of titanium aluminum alloy, a small amount of silicon can improve the creep resistance and oxidation resistance of solidified microstructure, but reduce the fracture toughness.

5.Zr and Sn neutral elements have higher solubility in alpha Ti and beta Ti, which play a complementary and strengthening role.In the heat-resisting alloy, zirconium and tin should be added in addition to aluminum to further improve the heat-resisting property in order to ensure the alloy structure based on alpha phase.Aluminum, zirconium, and tin all inhibit the formation of ohmic phases, and tin reduces susceptibility to hydrogen embrittlement.In ti-sn alloy system, when tin BBB 0 is 18.5%, Ti3Sn will form an ordered phase, which reduces the plasticity and thermal stability.

6.Mn, Fe and Cr have great enhancement effect, strong stability of the beta phase, and lower density than molybdenum and tungsten. Therefore, they are widely used and are the main additives of high-strength substable beta titanium alloy.However, they formed a slow eutectoid reaction with titanium, and the microstructure was unstable and the creep resistance was low under high temperature and long-term working conditions.The addition of beta isocrystalline elements, especially molybdenum, inhibits eutectoid reactions.

7. Role of alloying elements in titanium alloy:

(1) solid solution strengthening effect.Iron, manganese, chromium and silicon are the most significant ones to improve the tensile strength at room temperature.Aluminum, molybdenum, vanadium;But the effect of zirconium, tin, tantalum and niobium is poor.

(2) raise or lower phase transition point, play a stabilizing alpha phase or beta phase.

(3) the addition of stable elements to increase the hardenability of the alloy, so as to enhance the effect of heat treatment.

(4) aluminum, zirconium, tin to prevent the formation of phase effect;Rare earth can inhibit alpha 2 phase precipitation.Beta isocrystalline elements have the effect of preventing the eutectoid decomposition of beta.

Adding aluminum, silicon, zirconium and rare earth elements can improve the heat resistance of the alloy.

Add palladium, ruthenium, platinum and so on to improve the corrosion resistance of the alloy and expand the passivation range.

In addition to aluminum, stable elements such as vanadium, molybdenum and neutral elements such as tin and zirconium are also added. These elements not only enhance the alpha and beta phase strength in the complex phase structure, but also change the decomposition kinetics of the beta phase, improve the dispersion of the effective structure, and significantly improve the strengthening effect of heat treatment.

Editor : Carrie Qi (rowlynti@rowlyn.com)