Effect of heat treatment process on the structure and mechanical properties of high-strength titanium alloy rods

1 Introduction

Titanium and titanium alloys have many excellent properties such as high specific strength, corrosion resistance, and high temperature resistance. With the increasing demand for material performance in aerospace, marine engineering, weapons and equipment, biomedical, petrochemical and other fields, high-strength titanium alloys have developed rapidly  Nearly β-type titanium alloy can obtain excellent properties of high strength and toughness through optimized processing and heat treatment processes  Titanium alloys with a tensile strength of 1300MPa, an elongation of 6%, and a fracture toughness of 50MPa/m2 are usually called ultra-high-strength titanium alloys Research shows that the β grain size, the length and thickness of α lamellae and the α phase at the grain boundary are the main factors affecting the strength and toughness of titanium alloys. The α lamellae within the grains can strengthen the β matrix and prolong the crack expansion path, which is beneficial to optimizing the alloy. Strong toughness. Continuous α grain boundaries will affect the plasticity of the alloy, but when the grain size is larger than the crack tip plastic zone, it will not affect the toughness of the alloy  Niinomi et al. found  that the β grain size of Ti-6Al-2Sn-4Zr-6Mo alloy increases, the crack nucleation resistance increases, and the fracture toughness of the alloy improves. In order to obtain high-strength and high-toughness titanium alloys, titanium alloys are usually heat treated in the β phase zone to obtain larger-sized equiaxed β grains, and the full-lamellar structure is obtained through further aging strengthening to improve the alloy strength 

 

β phase zone treatment forms equiaxed β grains. β grain size and grain boundary strength are the main factors affecting the strength and toughness of ultra-high strength titanium alloys. It is an economical way to control the microstructure of titanium alloys by adjusting the heat treatment process parameters. And effective strengthening means . This paper obtains structures with different β grain sizes and different grain boundary morphologies by regulating the heat treatment system of high-strength titanium alloys. The effect of the solid solution time in the β phase region on the β grain size and the mechanical properties of high-strength titanium alloys is studied. The two-stage The influence of grain boundary coarsening behavior on the mechanical properties of alloys during the solid solution process provides a theoretical reference for the industrial production of high-strength titanium alloys.

2. Experiment

The titanium alloy used in the experiment is a new type of Ti-Al-Mo-V-Cr-X alloy independently developed by the author's research group. The alloy phase transformation point is about 800°C. After three times of vacuum consumable melting, the ingot is obtained. After the β phase zone is opened, the (α+β) phase zone and β phase zone are forged to obtain a large-size titanium alloy rod with a diameter of 400mm.

In order to avoid insufficient uniformity of the large-size bar structure and affect the experimental results, only the core material of the forged bar was sampled. Various heat treatment process designs are shown in Figure 1. The alloy sample was subjected to solid solution treatment in the β phase zone, maintained for 5 to 240 minutes, air-cooled, and then unified at 530°C for 4 hours of aging, as shown in Figure 1a. The alloy is subjected to two-stage solid solution, two-stage solid solution method A: first solid solution at a temperature higher than the phase transformation point (820°C) for 1 hour, then air-cooled to room temperature, and then passed through the (α+β) phase zone at different temperatures (740, 760 and 780℃) respectively, and then air-cooled to room temperature (calculated as 820 + 740, 820 + 760 and 820 +780 respectively) for 1 hour, and finally subjected to 530℃/4h aging treatment, as shown in Figure 1b. Two-stage solid solution method B: Solid solution at 820℃ for 1 hour, then slowly furnace cooled to different temperatures in the (α+β) phase zone (740, 760 and 780℃), kept for 1 hour and then air cooled to room temperature (respectively calculated as 820 ~ 740 , 820 ~ 760 and 820 ~ 780), and finally subjected to 530℃/4h aging treatment, as shown in Figure 1c.

After the above solid solution and aging treated samples were mechanically ground, polished and chemically etched, the alloy structure was observed using a metallographic microscope and TESCAN MIRA3 field emission scanning electron microscope. The INSTRON5982 tensile machine was used to conduct uniform tensile experiments on each group of samples, and the tensile rate was 0. 5mm/min. The SANS-ZBC2452-C metal pendulum impact testing machine was used to conduct the metal Charpy U-shaped opening impact test on each group of samples.

3. Results and discussion

By controlling the solid solution and aging heat treatment regime, a titanium alloy with equiaxed β grains and α lamellae was obtained, and the effects of β grain size and grain boundary morphology on the mechanical properties of ultra-high strength titanium alloys were explored.

3.1  Effect of β grain size on mechanical properties of alloys

The titanium alloy was kept at 820°C for 5 to 240 minutes, then air-cooled to room temperature, and then aged at 530°C for 4 hours. The microstructure is shown in Figure 2. When the solid solution time is 5 and 10 min, due to the short holding time, recrystallized β grains have not been observed, and the curved original β grain boundaries can still be observed, and the grain diameters are 154 and 143 μm respectively. When the solid solution time is 20 minutes, some recrystallized β grains with straight grain boundaries can be observed. The size of the recrystallized grains is small, with a diameter of 55 μm. When the solid solution time is 30 minutes, all β grain boundaries of the alloy are straight and clear, indicating that the alloy has completely recrystallized, and at the same time, the recrystallized β grains have grown, and their diameter is about 88 μm. Discontinuous grain boundaries can be observed when the solid solution is dissolved for 5 and 10 minutes, as shown in Figures 3a and 3b; when the solid solution time is 20 minutes, straight grain boundaries that have recrystallized can be observed (Figure 3c), which are not completely re-crystallized. A small amount of primary α phase can still be observed during crystallization; when the solid solution time is 30 minutes, only straight grain boundaries exist (Figure 3d), and the alloy is completely recrystallized at this time.

As shown in Figure 2e ~ 2f, as the solid solution time increases, the recrystallized β grains grow uniformly and the size gradually increases. When the solid solution time reaches 240 min, the β grain size is the largest, with a diameter of approximately 186 μm. The growth of β grains is the result of interface migration. The driving force of interface migration at constant temperature can be expressed as formula (1):

In the formula, γ is the interface energy (J/m2); R is the radius of curvature of the interface (m); p is the interface migration driving force (J/m3). The smaller the grain radius, the greater the interface curvature. The smaller the interface curvature radius, the greater the phase change driving force and the greater the interface migration rate. The migration process of the interface is the process of reducing the curvature of the interface and reducing the free energy difference on both sides of the interface [15]. Therefore, during the isothermal solid solution process, as the holding time increases, some of the larger grains continue to grow, while the smaller grains gradually shrink and eventually disappear as the grain boundaries migrate.

Therefore, a large difference in grain size is observed in the microstructure with a solid solution time of 60~240min. The relationship between β grain size and solid solution time is shown in Figure 4. When the alloy is completely recrystallized, the grain size becomes significantly larger as the solid solution time increases. The change of mechanical properties of ultra-high-strength titanium alloy with solution time is shown in Figure 5. When the solution time is 5~20 minutes, the strength of the alloy is higher, but the plasticity and toughness are lower. This is because the solution time is short and the alloy is not completely formed. After recrystallization, some of the original tissue is still retained. When the solid solution time is in the range of 30 to 240 minutes, the alloy undergoes complete recrystallization. As the solid solution time increases, the β grain diameter increases from 88 to 186 μm. The mechanical properties of the alloy and the solid solution time do not conform to the linear law. Analysis shows that when the solid solution time is 60 minutes, a large number of small-sized grains formed due to interface migration cause local stress concentration. At this time, the alloy yield strength and tensile strength reach the highest. , respectively 1346 and 1391MPa. The solid solution time has no significant effect on the elongation, area reduction and impact toughness of the alloy. This is because the grain size of the alloy is smaller after a short period of solid solution, which increases the possibility of intergranular cracking. At the same time, at a certain The crack expansion path is extended to a certain extent; as the solid solution time increases, the grain diameter increases and the number of grain boundaries decreases, reducing the possibility of intergranular cracking and improving the plasticity and toughness of the alloy. Therefore, based on the above two factors, the solid solution time has no significant impact on the plasticity and toughness of the alloy.

3.2  Effect of grain boundary morphology on mechanical properties of alloys

In order to study the influence of grain boundary morphology on the mechanical properties of the alloy, the two-stage solid solution method A and B was used to obtain the grain boundary coarsened structure. After aging treatment, the lamellar secondary α phase will precipitate in the β matrix and conform to the Burgers orientation relationship with the matrix. The size and thickness of the secondary α lamellae depend on the aging temperature and time. In order to avoid the impact of the morphology of the secondary α lamellae on the mechanical properties of the alloy, the solid solution structures obtained by the two methods were unified at 530°C and aged for 4 hours to obtain the final The microstructure is shown in Figure 6. Micron-level secondary α lamellae of similar size and thickness are evenly arranged in the matrix. The coarsening phenomenon of the grain boundaries can be observed under different heat treatment conditions. The lower the solid solution temperature of the (α+β) phase region, the coarsening phenomenon of the grain boundaries. The more obvious it is.

In the two-stage solid solution method A, the β-type titanium alloy obtains equiaxed β grains with straight grain boundaries after solid solution in the β phase zone, and the α phase is formed in the second phase (α+β) phase zone solid solution stage. , due to the high content of β-stabilizing elements in ultra-high-strength titanium alloys and the relatively stable β matrix, the energy required for nucleation with the help of grain boundaries is much lower than the energy required for uniform nucleation within the crystal itself [16,17], so the α phase takes the lead in the crystal. It forms at the boundary and gradually grows into the grain, forming a coarsened α grain boundary, as shown in Figure 6a~6c. When the solid solution temperature of the (α+β) phase region is 740°C, a small amount of primary α phase is observed in the crystal in Figure 6a. According to the thermodynamic equilibrium phase diagram of titanium alloy, the proportion of α phase increases as the solid solution temperature decreases. The temperature at 740°C solid solution is lower, and the larger degree of supercooling increases the nucleation driving force and promotes the nucleation of α phase. When the solid solution temperature increases, the critical nucleation work provided by supercooling cannot overcome the nucleation energy barrier. Therefore, the existence of primary α phase in the crystal cannot be observed during solid solution at 760 and 780°C, and as (α+β ) The solid solution temperature of the phase region decreases, and the degree of grain boundary coarsening is obvious. In the two-stage solid solution method B, after solid solution in the β phase zone, the furnace is slowly cooled to the (α+β) phase zone and kept at different temperatures. The cooling rate of the furnace cooling is about 1°C/min.

Due to the low cooling rate, there is neither composition nor temperature fluctuation in the β matrix in the initial stage of the slow cooling process, making it difficult for the primary α phase to nucleate in the β grains. As the cooling time increases, the solid solution temperature gradually decreases, and the primary α phase precipitates along the β grain boundaries with part of the β grain boundaries as heterogeneous nucleation points, and gradually grows into the interior of the grains. In titanium alloys, the proportion of primary α phase increases as the solid solution temperature of the (α+β) phase region decreases. In solution mode B, the lower the solid solution temperature of the (α+β) phase region, the grain boundaries coarsen. The greater the number.

However, in Figures 6d~6f, it is not observed that the solid solution temperature of the (α+β) phase region has a significant impact on the coarsening degree of the grain boundary. This is because the lower the solid solution temperature, the lower the kinetic driving force that promotes the growth of α grain boundaries, which is not conducive to the growth of α grain boundaries.

The alloy was solid-solubilized in two ways and then uniformly aged. Its tensile properties and impact properties are shown in Figure 7. There are similar rules under the two solid solution modes. As the solid solution temperature in the (α+β) phase zone increases, the strength and plasticity of the alloy increase, but the change in impact toughness is not obvious. Combined with the microstructure analysis, it is believed that in the two-stage solid solution mode A, the lower the solid solution temperature of the (α+β) phase region, the more significant the grain boundary coarsening effect; while in the two-stage solid solution mode B, ( The lower the solid solution temperature in the α + β) phase region, the more coarsened grain boundaries there are, and at the same time the strength and plasticity of the alloy decrease, indicating that grain boundary coarsening makes the grain boundaries weaker and affects the strength and plasticity of the alloy. This is because the coarsened grain boundaries are composed of primary α phase, and the α lamellae formed after aging strengthen the β matrix, making its strength much higher than that of the grain boundary α phase. Therefore, the coarsened α grain boundaries deform preferentially during the tensile experiment, while the strengthened β matrix is difficult to deform. As the degree of deformation increases, the stress is concentrated on the α grain boundary interface to form cracks, and the cracks are prone to propagate along the grain boundaries and cause intergranular fracture, resulting in a simultaneous decrease in the strength and plasticity of the alloy [17]. The alloy only undergoes solid solution and aging in the β phase region without coarsening the β grain boundaries. Its yield strength is 1346MPa, the tensile strength is 1391 MPa, and the elongation is 5%, as shown in Figure 5 after 60 minutes of solid solution. In contrast, after the two-stage solid solution coarsens the grain boundaries, the strength and plasticity are reduced to varying degrees. Among them, the grain boundary coarsening effect of solid solution method A is more significant than that of solid solution method B, so the overall strength of the alloy is lower. .

Figure 8 shows the tensile fracture morphology of two types of two-stage solid solution and unified aging. It is observed that there are a large number of flat facets on the macroscopic fracture of the sample. Further observation of high-magnification photos of the fracture shows that these facets are smooth and flat, which are typical along the edges. There are also some shallow-depth dimples on the crystalline fracture surface, indicating that the fracture mechanism is a mixed fracture mechanism with intergranular fracture. Through comparison, it was found that more smooth and flat intergranular fracture surfaces were observed in the two-stage solid solution method A, which also verified that samples with significant grain boundary coarsening effects are more likely to undergo intergranular fractures, making the alloy strength and plasticity worse at the same time. . Therefore, the coarsening of α grain boundaries with lower strength should be avoided in the actual production process, and the problem of weak grain boundaries needs to be studied in depth.

4 Conclusion

(1) For ultra-high-strength titanium alloys that are solid-soluted above the phase transformation point (800°C), and then subjected to aging treatment at 530°C, as the solid-solution time increases, the average β-grain size of the alloy gradually increases, and the β-grain boundaries of the isothermal solid-solution process Migration, some grains grow up, and some grains shrink until they disappear. This process will cause a large difference in grain size. Insulating the β phase zone for 30 to 240 minutes has no significant effect on the tensile properties and impact properties of the alloy.

(2) The ultra-high-strength titanium alloy is subjected to two-stage solid solution and 530°C aging treatment. In the solid solution method A (the β phase zone is cooled to room temperature and then the (α+β) phase zone is solid solutioned), as In the second stage (α+β), the solid solution temperature in the phase region decreases, and the grain boundary coarsening effect gradually becomes apparent. Due to the large supercooling during solid solution at 740°C, primary α phase is formed inside some β grains; in the solid solution mode In B (the β phase area is solidly dissolved and then the furnace is cooled to the (α+β) phase area), the slow cooling rate only coarsens part of the grain boundaries. As the solid solution temperature decreases, the α phase in the grain boundary grows. The chemical driving force is reduced, so the grain boundary coarsening effect is equivalent under different solid solution temperatures, but the lower the solid solution temperature, the greater the number of coarsened grain boundaries.

(3) The α strength of the coarsened grain boundary is low, and it deforms preferentially during the deformation process, resulting in stress concentration and cracking. The crack propagates along the grain, causing the strength and plasticity of the alloy to decrease at the same time. The more obvious the effect of grain boundary coarsening, the greater the impact on the strength of the alloy. And the greater the influence on plasticity, since the grain size does not change, there is no significant impact on the impact toughness of the alloy.

ReferencesReferences

[1] WANG H, ZHAO Q, XIN S, et al. Materials Science and Engineer-ing:A[J], 2021, 3: 141626.

[2] KANG LM, YANG C. Advanced Engineering Materials[J], 2019,21(8): 1801359.

[3] CHENG J, LI JS, YU S, et al. Metals[J], 2021, 11(1): 11.

[4] CHENG J, LI JS, YU S, et al. Frontiers in Materials[J], 2020,7: 114.

[5] CHENG J, WANG HC, LI JS, et al. Frontiers in Materials[J], 2020, 7: 228.

[6] Chen Wei, Liu Yunxi, Li Zhiqiang. Journal of Aeronautical Materials[J], 2020, 40(3): 63-76.

CHEN W, LIU YX, LI Z Q. Journal of Aeronautical Materials[J], 2020, 40(3):63-76.

[7] Xin Shewei, Zhou Wei, Li Qian, et al. Progress in Materials in China [J], 2021, 40(6):441-445.

XIN SW, ZHOU W, LI Q, et al. Materials China[J], 2021, 40 (6):441-445.

[8] WANG Y, CHEN R, CHENG X, et al. Journal of Materials Science & Technology[J], 2019, 35(2): 403-408.

[9] ZHOU W, GE P, ZHAO YQ, et al. Rare Metal Materials and Engineering[J], 2017, 46(8): 2076-2079.

[10] Niinomi M, Inagaki I, Kobayashi T. Materials Science and Technology[J], 1988, 4(9): 803-8

[11] GAO X, ZHANG L, CHEN X, et al. Materials Characterization [J], 2020, 167:110492.

[12] FENG X, QIU JK, MA YJ, et al. Journal of Materials Science & Technology[J], 2016, 32(4):362.

[13] Chemistry, Chemistry, Chemistry.New Journal of Chemistry[ J], 2018, 39(4):31-36.

GUO P, ZHAO YQ, HONG Q. Transactions of Materials and Heat Treatment[J], 2018, 39(4):31-3

[14] PEDERSON R, NIKLASSON F, SKYSTEDT F, et al. Materials Sci- ence and Engineering:A[J], 2012, 552: 555-565.

[15] LIU X, ZHUANG K, LIN S, et al. Crystals[J], 2017, 7(5):128-140.

[16] HUANG S S, ZHANG J H, MAY J, et al. Journal of Alloys and Compounds[J], 2019, 791: 575-585.

[17] GAO X, ZENG W, ZHANG S, et al. Acta Materialia[J], 2017,122: 298-309.

[18] SHEKHAR S, SARKAR R, KAR S, et al. Materials & Design[J],2015, 66: 596-610.