The microstructure and mechanical properties of Titanium Alloy are determined by the processing technology. In the production practice, people often carry on the back inference, that is, according to the required mechanical properties, judge the best structure state, and then according to the required structure state, carry on the alloy hot working and heat treatment process optimization. In this process, the relationship between organization and performance becomes the most critical step. The General Relationship Between Organization and performance, through a lot of testing and practice, has formed a certain consensus. The general relationship of mechanical properties of four typical microstructures of titanium alloys is shown in Table 1. Each microstructure has its corresponding mechanical properties advantages and disadvantages. However, the comprehensive service conditions of modern aviation flight materials increasingly require that the materials have better comprehensive mechanical properties. Therefore, the main contradiction of titanium alloy material processing and engineering application is that the high demand of comprehensive mechanical properties and material properties can only meet part of the superior properties. In order to alleviate this contradiction, people constantly through the process of innovation, for the optimization of the organization. For example, the introduction of near-β, quasi-β, and multi-heat treatment is to some extent to sacrifice the convenience of the process to obtain the optimal microstructure and thus obtain the best comprehensive mechanical properties.

The effect of microstructure on the Mechanical Properties 01 the effect of microstructure on the strength and plasticity at room temperature it is generally believed that the strength decreases and the plasticity increases with the decrease of primary α phase, therefore, from equiaxed structure to bi-state structure to lamellar structure, the ductility decreases and the strength increases gradually. The mechanical properties of TC4 alloy at room temperature under different microstructures are shown in Table 2.

02 The effect of microstructure on fracture toughness and crack growth rate the effect of microstructure on fracture toughness and crack growth rate of titanium alloy has been studied and some laws have been obtained, the lamellar structure obtained by deformation in β region or heat treatment in β region can obtain higher fracture toughness and crack propagation rate. It is explained that the secondary cracks are easily bifurcated due to the influence of the original β grain boundary and α cluster, so the propagation path of the cracks in the lamellar structure is more tortuous, which leads to the increase of the total length of the cracks and more energy consumption. The properties of TC4 alloys with two typical microstructures are shown in table 3. The properties of TC 11 alloys with different microstructures are shown in table 4.

03 microstructure affects the thermal strength of titanium alloy the thermal strength of reactive materials can resist deformation at high temperature. The most widely studied properties are transient strength, rupture strength and creep strength at high temperature. A large number of studies have shown that the thermal strength of the lamellar structure is higher than that of the spherical structure. When the grain size increases and the grain structure changes from spherical to flaky, the rupture strength increases and then decreases, and the creep resistance increases with the increase of β grain size. Among the four typical microstructures of titanium alloy, the net-basket microstructure has the best thermal strength, that is, the net-basket microstructure has the best comprehensive properties of tensile strength, rupture strength and creep strength at high temperature. Widmanstatten structure is next, equiaxed structure has the lowest thermal strength. The relationship between thermal strength and microstructure of TC11 alloy is shown in Table 5.

The effect of microstructure on fatigue properties of smooth specimen under the action of high frequency stress of symmetrical cycle, the fatigue strength of equiaxed microstructure is better than that of flaky microstructure. Under four typical microstructure conditions, equiaxed microstructure has the best fatigue properties, then double-state microstructure, then net-basket microstructure and widmanstatten microstructure have the worst fatigue properties. Table 6 shows the effect of different microstructures on the fatigue properties of TC6 alloy.

With the rapid development of modern aircraft, the design of Titanium Alloy microstructure has put forward new requirements for the application performance of materials, that is, five basic factors of structural design and material selection in modern aviation industry: Static strength and stiffness of “Undamaged”material; fatigue property of “Undamaged”material; creep, durability and thermal stability in high temperature service; The static strength of the damaged material; the fatigue property of the damaged material. The relationship between material selection criteria and microstructure and properties is shown in Table 7. It can be seen that these properties have an irreconcilable contradiction to the requirements of the microstructure, so in practical engineering applications, it is necessary to design the microstructure according to the required mechanical properties of the alloy, the special structure or “Intermediate”structure is designed according to the requirements to meet the requirements of the Alloy mechanical properties. The microstructure design of two typical types of titanium alloys —— -- High temperature titanium alloys and high strength and toughness titanium alloys is described. Table 7 relationship between material selection criteria and microstructure and properties

01 microstructure design of high temperature titanium alloy in practical application, high temperature titanium alloy requires good matching of room temperature property, high temperature strength, creep property, thermal stability, fatigue property and fracture toughness, etc. , the most important is the matching of thermal stability, high temperature creep property and fatigue property, which are contradictory to the composition and structure of materials. Therefore, IMI834 alloy is treated by solution treatment at the upper limit temperature of the two-phase region with 15% of primary α phase in order to give consideration to both creep and fatigue properties. Oil Cooling is recommended for billet with section size > 15mm, and Air Cooling is recommended for billet with section size < 15mm, and then aging at 700 °C. The addition of 0.06% °C in the alloy is also to enlarge the temperature range of the two-phase heat treatment and to control the content of the primary α phase better during solution heat treatment. The optimum heat treatment zone and its corresponding microstructure of IMI834 alloy are shown in Fig. 1.

As a high temperature titanium alloy, TI1100 alloy forgings are recommended to be produced by β forging and direct aging to obtain the lamellar structure. That is, forging is carried out in the range of 25 ~ 55 °C above the phase transition point, and aging treatment is carried out directly at 600 °c/8h after r forging. Such a process reduces a high-temperature heat treatment, simplifies the process, and reduces production costs. Fig. 2 shows the microstructure of TI1100 alloy in service condition. 02 high strength and toughness titanium alloy in the microstructure design table 7, except the creep, durability and thermal stability at high temperature, all the properties of high strength and toughness titanium alloy are emphasized. Therefore, in order to coordinate the relationship between different properties, the high strength and toughness titanium alloy not only refines the grains, but also improves the strength and toughness of the alloy, as far as possible, choose an organization state between equiaxed organization and lamellar organization. A more complex combination of forging and heat treatment is usually employed. For example, TI62222 alloy, in order to obtain a good combination of tensile properties, fracture toughness and fatigue crack growth rate, a triple treatment system is adopted: First Weight, T β + 28 °c/0.5 or HFC, cooling rate from 20 ~ 67 °c/min to 482 °c; The second weight, t-β-39 °c/1 HAC or FC, and the third weight, 538 °c/8 H AC. After triple treatment, the properties are σb = 1034mpa, Σ0.2 = 931MPA, δ = 6% , ψ = 42% ー48% , KIC = 77MPA M1/2. For TI17 alloy, the heat treatment process of double solid solution + aging was also used after forging in double state zone to obtain a comprehensive microstructure. For a new titanium alloy TI5553 with high strength and toughness, the optimum comprehensive mechanical properties were obtained by adjusting the precipitation position and morphology of aging α phase through double aging treatment. The most typical examples of microstructure design for high strength and toughness titanium alloys are the advanced and comprehensive application of near Β, quasi β and water cooling technology after forging, the application idea of near β forging in high strength and toughness titanium alloy is to further reduce and refine the content of primary equiaxed α phase on the basis of dual state structure, and to obtain a mixed structure by increasing secondary strip α and aging α, the toughness and anti-fatigue crack growth ability of the alloy were improved by adding secondary strip α, aging α and aging β on the basis of no obvious decrease in plasticity. The quasi-β forging process is also based on the basket-net structure with the best comprehensive mechanical properties. Although the basket organization is not the best among the various performance indicators, but for all the performance, the basket-net structure is superior or medium (see table 1) , so the basket-net structure is the best one when the comprehensive mechanical properties are required. In order to construct this structure, the quasi-β forging process is proposed. At the same time, the post-forging water-cooling technology is mainly aimed at high strength and toughness titanium alloy, which can improve the fracture toughness and anti-crack propagation ability of the alloy by water-cooling refining lamellar and strip α. 

Source: New Alloy material-titanium Alloy by Zhao Yongqing Xinshe Wei Chen Yongnan Mao Xiaonan

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