Blog

Titanium and titanium alloys have a series of excellent properties such as low density, high strength, good heat resistance, high toughness, good thermal conductivity and fatigue resistance, wide operating temperature range, excellent seawater corrosion resistance and ultra-low temperature performance. Performance engineering structural materials. Therefore, it is widely used in the aerospace field. Titanium and titanium alloys have become one of the pillars of the aerospace industry. Relevant data shows that the application of high-performance titanium and titanium alloys in the aerospace industry accounts for about 70% of the total titanium production. Although the one-time investment of titanium equipment is high, the life-cycle cost is low and the economic benefits are obvious. Currently, high-performance aircraft and tanks are using titanium alloy components. The compressor disks, compressor blades, fan blades and engine parts of advanced engines are also used. The boxes, etc. are all made of titanium alloy.

1. Characteristics and applications of titanium alloys for aerospace use

As a rising material in the aerospace field, titanium alloy has the following advantages:
(1) High specific strength. Titanium alloy has high strength, its tensile strength is 686-1176MPa, and its density is only about 60% of steel, so its specific strength is very high.

(2) Excellent high temperature performance. Titanium alloys can still maintain good mechanical properties at high temperatures, their heat resistance is much higher than that of aluminum alloys, and their operating temperature range is wide.

(3) Strong corrosion resistance. In air below 550°C, a thin and dense titanium oxide film will quickly form on the titanium surface, and its corrosion resistance is better than that of most stainless steels.

2. Factors affecting the performance of high-temperature titanium alloys
   (1) Influence of alloying elements
   Most of the high-temperature titanium alloys currently in use and researched and developed are near α-type titanium alloys, and the main alloying elements added are Al, Sn, Zr, Mo, V, Nb and Si. Compared with β-type titanium alloys, near-α-type titanium alloys have higher high-temperature strength, better thermal stability, and excellent welding properties. In addition, impurity elements C, N, H and O as well as rare earth elements such as Nd also have an important impact on the properties of high-temperature titanium alloys.

(1)Al element
Al is a frequently added element in high-temperature titanium alloys and is also one of the most important solid solution strengthening elements. The addition of Al can reduce the alloy density, increase the recrystallization temperature, strength and (α+β)/β transformation point, and improve the alloy’s oxidation resistance. Al can also improve the atomic bonding force of solid solution and increase the high-temperature strength of the alloy. Since Al atoms exist in the α phase in a substitutional manner, the Al content exceeds its solubility limit in the α phase, which easily leads to the precipitation of the ordered α2 (Ti3Al) phase, causing the alloy to embrittle. When the content is between 6.0 and 7.0%, the alloy has High thermal stability and good weldability.

(2) C element
When the C element with a mass fraction of less than 0.09% is added to a titanium alloy, the C element will be completely dissolved in solid solution and has a solid solution strengthening effect, which can hinder dislocation movement. At the same time, the grains in the alloy structure remain relatively small, thereby significantly improving the strength and high-temperature creep properties of titanium alloys.

However, when the C element content is too much in the titanium alloy, coarse carbides will precipitate in the structure, and the formed carbides will increase the thickness of the α sheet. If carbides are precipitated, a large stress concentration will occur near the alloy during deformation. Carbide is a brittle phase, which mainly precipitates at grain boundaries, which has a negative impact on the plasticity and high-temperature creep properties of the alloy. In addition, the addition of element C is beneficial to improving the service life of titanium alloys.

(3) Rare earth elements

In high-temperature titanium alloys, rare earth elements can form second phase dispersed particles, refine grains, increase dislocation density, etc., thereby producing strengthening effects. However, rare earth elements will seize interstitial oxygen in the solid solution and reduce interstitial solid solution strengthening, thus Produces a softening effect.

Rare earth elements can optimally match the thermal strength and thermal stability of titanium alloys, and can effectively improve the high-temperature creep properties of titanium alloys. Rare earth elements have a relatively high affinity with oxygen, and they can also transfer part of Sn in the alloy to purify the matrix and inhibit the precipitation of α2 phase. However, internal oxidation reduces the interstitial solid solution strengthening effect, resulting in softening effect.

The rare earth element Nd is added to the Ti-60 alloy. Nd will form a rare earth phase rich in Nd, Sn and O through internal oxidation, thereby reducing the oxygen in the matrix, purifying the matrix and improving the thermal stability of the alloy. ; Research shows that only when certain types and appropriate amounts of rare earth elements are added to titanium alloys can their hot workability be improved. For example, adding less than 0.05% of the rare earth elements Y and Er can significantly reduce edge cracks formed during rolling of Ti-6Al-4V alloy.

Rare earth elements have certain direct or indirect effects on the structure and properties of high-temperature titanium alloys. Some of these effects are beneficial and some are harmful. It is related to the type, addition amount, alloy type and alloying elements of rare earth elements. Among them, the addition amount of rare earth elements is the most critical, because it determines the existence state and distribution of rare earth elements. Only by adding an appropriate amount of rare earth elements to titanium alloy can beneficial effects be obtained.

(2) Effect of microstructure on the properties of high-temperature titanium alloys

The type of microstructure and phase size parameters have a great influence on the properties of high-temperature titanium alloys. Adjusting the type and parameters of the microstructure is an important way to obtain a good match of strength, plasticity and other performance properties. The mesh basket structure is obtained when the deformation of the two-phase titanium alloy starts around the β phase transition temperature, or starts deformation in the β single-phase region, and is completed in the two-phase region, and the deformation amount reaches 50% to 80%. The fatigue of the mesh basket structure The intensity is relatively high.

In titanium alloys, the structures and performance characteristics of the equiaxial structure and the dual-state structure are similar. The only difference is the number of primary α phases contained. The equiaxed structure and the two-state structure have high plasticity and fatigue properties. Its disadvantages are lower high temperature properties and fracture toughness. Research has found that the optimal size of equiaxed α grains in the equiaxed structure is 2 μm to 4 μm. At this time, an alloy with good matching of strength, plasticity and fatigue properties can be obtained. The lamellar α phase interface can effectively hinder dislocation movement and improve the alloy’s fracture toughness, high-temperature creep strength and resistance to crack expansion, but its plasticity and low-cycle fatigue strength are low.

(3) Effect of high temperature heat treatment

High-temperature titanium alloys containing different alloying elements can obtain different structural states and properties after different heat treatments. In order to make the structure and properties of the alloy meet the requirements, it can be improved by selecting an appropriate heat treatment system. Commonly used heat treatments in high-temperature titanium alloys include annealing, solution and aging. After long-term in-depth research on high-temperature titanium alloys, its production technology is mature and it is widely used in aviation, aerospace, medical and other fields. However, the poor impact toughness of titanium alloys severely limits its wider application. Titanium alloys are composed of α and β phases at room temperature. They can be heat treated by annealing, solid solution and aging. Different heat treatment processes can obtain different phase compositions and microstructures. In the ordinary annealed state, the flake structure The mechanical properties of the two-state structure are better than those of the two-state structure, but in the high stress area, the mechanical properties of the two-state structure are better than those of the flake structure.

3. Conclusion

High-temperature titanium alloys have been used in the aviation field for a long time. With the rapid development of social economy and science and technology, the temperatures used by high-temperature titanium alloys are getting higher and higher, and their performance requirements are getting higher and higher, especially high-temperature strength and Thermal stability has become an obstacle restricting the development of high-temperature titanium alloys. In order to improve the performance of high-temperature titanium alloys when used at high temperatures, it is necessary to understand the factors that affect their performance. Therefore, this article briefly analyzes and summarizes some factors that affect high-temperature titanium alloys. More factors and research will require the joint efforts of scientific researchers in the future. I believe that with the joint efforts of many scientific researchers, the service temperature of high-temperature titanium alloys will be improved. It will get higher and higher.