High-strength and high-toughness titanium alloy and preparation method thereof

Abstract

The application discloses a high-strength and high-toughness titanium alloy and a preparation method thereof, and aims to solve the technical problems of strength-toughness inversion and deterioration of toughness of continuous grain boundary alpha phase of existing high-strength titanium alloy. The alloy comprises the following components in percentage by mass: Al: 4.5-5.5%, Mo: 4.5-5.5%, V: 4.5-5.5%, Cr: 0.5-1.5%, Fe: 0.5-1.5%, B: 0.05-0.15%, and the balance of Ti and impurities. TiB2 is used as raw material in the preparation, single TiB whisker is generated in situ to refine grains and avoid the introduction of brittle TiC phase. The preparation method comprises smelting, forging, solid solution, segmented cooling, and aging treatment. After the solid solution, the segmented cooling process is adopted, the formation of continuous grain boundary alpha film is effectively inhibited, and the alpha film is distributed in the form of discontinuous point chain. The high-strength and high-toughness titanium alloy and the preparation method thereof realize the synergistic matching of high strength, high plasticity and high fracture toughness.

Description

Technical Field

[0001] This application belongs to the field of alloy preparation technology, and specifically relates to a high-strength and high-toughness titanium alloy and its preparation method. Background Technology

[0002] Near-β titanium alloys are widely used in the aerospace field due to their high specific strength, excellent hardenability, and good deep-machining properties. As aerospace structural components move towards higher damage tolerance designs, higher demands are placed on the strength and toughness of titanium alloys. TC18 is one of the most mature high-alloy near-β titanium alloys currently in use and has been widely applied in key structural components such as aircraft landing gear and load-bearing beams. However, existing TC18-based titanium alloys exhibit a significant inversion in their strength-toughness ratio: when high strength is pursued, the fracture toughness and plasticity of the alloy often decrease significantly.

[0003] To improve the overall performance of near-β type titanium alloys, existing technologies have attempted to modify them through microalloying. For example, Chinese invention patent CN101921930A discloses a multi-component microalloyed titanium alloy, which adds boron carbide and graphite to the composition of TC18, attempting to refine the grain and strengthen the alloy by utilizing in-situ generated TiB short fibers and TiC particles. However, in actual service or under stress, it has been found that this alloy is prone to interfacial debonding or cleavage fracture accompanied by microcracks, which increases the risk of brittle fracture and fails to meet the high fracture toughness requirements of aerospace high-damage-tolerance materials.

[0004] Besides microalloying as a strengthening method, another important reason for the inverted strength and toughness of TC18 titanium alloy lies in the grain boundary α phase (α...) during the solid solution cooling stage. GB The precipitation behavior of the α phase. During the conventional solution cooling stage, the α phase readily nucleates and grows along the original β grain boundaries, forming continuous or semi-continuous grain boundary α films. These continuous grain boundary α films have a different modulus than the matrix and are prone to dislocation accumulation. Under stress, they provide a low-resistance path for rapid crack propagation, leading to low-energy intergranular fracture of the alloy, thus severely degrading the fracture toughness of the alloy.

[0005] Existing technologies have made various attempts to control the α phase at grain boundaries, but all have limitations to varying degrees. For example, Chinese invention patent CN115976441A discloses a heat treatment method for TC18 titanium alloy, employing a single-phase solution cooling followed by two-stage aging. This method obtains a single β-supersaturated solid solution by direct water cooling after solution treatment in the β single-phase region, followed by two-stage aging to precipitate multiple α phase morphologies to improve strength. However, the solution temperature of this method is much higher than T... βThis leads to significant coarsening of β grains at high temperatures. Although direct water cooling after solution treatment can basically suppress the precipitation of α phase at grain boundaries, the excessive cooling rate results in excessively high supersaturation in the β phase. This easily promotes the nucleation of ω phase during subsequent aging, leading to severe insufficiency of the alloy's plasticity. Summary of the Invention

[0006] The technical effect to be achieved by this application is to provide a high-strength and high-toughness titanium alloy and its preparation method, thereby solving the problems existing in the prior art.

[0007] To achieve the above-mentioned technical effects, this application provides a high-strength and high-toughness titanium alloy, which comprises the following components by weight percentage: Al: 4.5%~5.5%; Mo: 4.5%–5.5%; V: 4.5%~5.5%; Cr: 0.5%~1.5%; Fe: 0.5%~1.5%; B: 0.05%~0.15%; The balance consists of Ti and impurities.

[0008] As a preferred option, the high-strength and high-toughness titanium alloy has a B content of 0.08% to 0.12% by mass percentage.

[0009] To achieve the above-mentioned technical effects, this application also provides a method for preparing a high-strength and high-toughness titanium alloy, comprising the following steps: The raw materials are weighed according to the mass percentage of the high-strength, high-toughness titanium alloy composition described in any of the above schemes, mixed, smelted, and cast to obtain a titanium alloy ingot; wherein element B is used in the following manner. Added in powder form; The titanium alloy ingot is sequentially subjected to Phase zone forging and α+ Two-phase forging is used to obtain forged billets; The forged billet is heated to the solution temperature and held at that temperature, followed by segmented cooling. The solution-treated billet is heated and held at a constant temperature, and then cooled to room temperature to obtain the high-strength and high-toughness titanium alloy.

[0010] As a preferred option, the segmented cooling includes a first cooling segment, a second cooling segment, and a third cooling segment; The first cooling section cools at a cooling rate of 15–30 °C / min to Below 250℃~ Below 220℃; The second cooling section is Below 250℃~ The temperature is maintained at 220℃ for 15–25 minutes. The third cooling section cools to below 550°C at a cooling rate of 3–8°C / min, and then cools naturally to room temperature; in, The β-transformation temperature is the value of the high-strength, high-toughness titanium alloy.

[0011] Preferably, the heating temperature for forging the β-phase region is: Above 30℃~ At temperatures above 50℃, the heat preservation time is 1–2 hours, and the total deformation is not less than 60%. The heating temperature for forging of the α+β two-phase region is: Below 50℃~ At temperatures below 30℃, the total deformation shall not be less than 50%.

[0012] Preferably, the solution temperature is Below 60℃~ At 30℃ or below, the heat preservation time is 1 to 2 hours.

[0013] As a better option, the aforementioned The purity of the powder is not less than 99.5%, and the particle size is 10-50 μm.

[0014] As a preferred option, the melting process employs vacuum consumable arc melting, and the melting process is performed at least three times.

[0015] As a preferred option, the heating and heat preservation of the solution-treated billet includes: heating the solution-treated billet to 460-510°C and holding it for 8-12 hours.

[0016] The beneficial effects of this application are as follows: 1. The solution provided in this application introduces trace amounts of B element using TiB2 powder as raw material, thereby generating uniformly dispersed TiB whisker reinforcing phase in situ in the alloy matrix. This effectively refines the β grain size and improves the uniformity of nucleation distribution of the secondary α phase, resulting in a high-strength and high-toughness titanium alloy with fine grains, uniform structure, and dispersed intragranular TiB whiskers.

[0017] 2. The solution provided in this application uses a segmented cooling process to cool the grain boundary α phase (α...) during the solution treatment stage. GB The precipitation behavior of α phase at grain boundaries was actively regulated, which effectively suppressed the formation of continuous grain boundary α film, solved the adverse effect of continuous distribution of grain boundary α phase on fracture toughness in conventional continuous cooling process, and achieved a synergistic match between high strength and high fracture toughness.

[0018] 3. The solution provided in this application combines vacuum melting, multi-fire forging and solution aging treatment. The preparation process is mature and can effectively control the process parameters of each step. The resulting material has a uniform microstructure and significantly improves the performance stability of the alloy when used as an aerospace structural material. Detailed Implementation

[0019] The embodiments of the technical solution of this application will be described in detail below. The following embodiments are only used to illustrate the technical solution of this application more clearly, and are therefore only examples, and should not be used to limit the scope of protection of this application.

[0020] This application provides a high-strength, high-toughness titanium alloy, comprising the following components by weight percentage: Al: 4.5%~5.5%; Mo: 4.5%–5.5%; V: 4.5%~5.5%; Cr: 0.5%~1.5%; Fe: 0.5%~1.5%; B: 0.05%~0.15%; The balance consists of Ti and impurities.

[0021] Specifically, the titanium alloy described in this application is based on TC18 titanium alloy (nominal composition Ti-5Al-5Mo-5V-1Cr-1Fe), with trace amounts of boron added. During the smelting process, boron reacts in situ with Ti to form TiB whiskers, as shown in the following reaction formula: During solidification, TiB whiskers are distributed at the β grain boundaries, playing a role in heterogeneous nucleation and refining β grains. Simultaneously, during subsequent hot deformation, they inhibit abnormal β grain growth through a pinning effect. The selection of the boron (B) content is based on extensive experimental verification. The applicant found that when the B content is below 0.05%, the number of TiB whiskers generated is insufficient to produce a significant grain refinement effect; when the B content is above 0.15%, the size and aspect ratio of the TiB whiskers increase significantly. Excessively coarse TiB whiskers may fracture under stress, becoming crack nucleation sources and negatively impacting the alloy's plasticity and fracture toughness. The β transformation temperature (Tβ) of this alloy is approximately 870–880℃, which can be determined by metallographic methods according to the national standard GB / T 23605.

[0022] Preferably, the high-strength, high-toughness titanium alloy has a boron content of 0.08% to 0.12% by mass. Within this preferred range, the morphology and size of the TiB whiskers are relatively ideal, which is beneficial for fully utilizing the grain refinement and crack deflection toughening effects while maintaining good plasticity.

[0023] This application also provides a method for preparing a high-strength, high-toughness titanium alloy, comprising the following steps: S1. Weigh the raw materials according to the mass percentage of the high-strength and high-toughness titanium alloy composition described in any of the above embodiments, mix them, and then melt and cast them to obtain a titanium alloy ingot. Specifically, in the raw materials of the high-strength, high-toughness titanium alloy S1, Al is added in the form of sponge titanium-aluminum master alloy or pure aluminum strips, Mo is added in the form of AlMo master alloy, V is added in the form of AlV master alloy, Cr is added in the form of metallic chromium blocks, and Fe is added in the form of iron nails or electrolytic iron. B is added in the form of TiB2 powder because TiB2 can fully dissolve in the titanium melt and react in situ with Ti to generate only TiB, avoiding the inevitable simultaneous formation of the brittle TiC phase when using B4C. After weighing the above raw materials according to the designed distribution ratio, a consumable electrode is pressed. Optionally, the purity of the TiB2 powder is not less than 99.5%, and the particle size is 10–50 μm. Particles that are too coarse (>50 μm) will result in uneven distribution of TiB2 particles in the electrode, affecting the melting uniformity; particles that are too fine (<10 μm) are prone to oxidation and agglomeration.

[0024] Preferably, the melting process employs vacuum arc remelting (VAR), with at least three melting operations. Multiple melting operations ensure a sufficiently uniform distribution of element B and other alloying elements within the ingot. After each melting operation, the electrodes are flipped, and the ingot, after cooling, can be inspected for macroscopic metallurgical quality using ultrasonic testing.

[0025] S2. The titanium alloy ingot is forged sequentially in the β phase region and the α+β two-phase region to obtain the forged billet; Specifically, the purpose of this step is to break up the coarse columnar β grains in the as-cast state, so that the TiB whiskers can be redistributed uniformly, and to refine the β grains and establish the initial equiaxed α phase through forging in the two-phase region.

[0026] Preferably, the heating temperature for forging the β-phase region is: Above 30℃~ At temperatures above 50℃, the holding time is 1–2 hours, and the total deformation is not less than 60%. Within this temperature range, the alloy is in a single β-phase region, which is conducive to large deformation and fractured as-cast structure.

[0027] The heating temperature for forging of the α+β two-phase region is: Below 50℃~ At temperatures below 30℃, the total deformation should not be less than 50%. Forging in the two-phase region can form a certain volume fraction of equiaxed primary α phase (αp), and further refine the β grains through the pinning effect of TiB whiskers on the β grain boundaries. The target microstructure of the forged billet is: equiaxed αp distributed within the refined β grains and around the TiB whiskers, with the β grain size significantly smaller than that in the as-cast state.

[0028] S3. Heat the forged billet to the solution temperature and hold it thereafter, then cool it in stages. Preferably, the solution temperature is Below 60℃~ The temperature is below 30℃, and the holding time is 1 to 2 hours. Within this temperature range, a certain volume fraction of equiaxed αp is retained in the alloy to provide plasticity reserves, while a large amount of β phase is in a metastable state, providing a basis for the subsequent precipitation of αs and αt during aging.

[0029] Preferably, the segmented cooling includes a first cooling segment, a second cooling segment, and a third cooling segment; The first cooling section cools at a cooling rate of 15–30 °C / min to Below 250℃~ Below 220℃; the function of this section is to quickly pass through the sensitive temperature range for the precipitation of the α phase (αGB) at the grain boundary (approximately 50℃ to 200℃ below Tβ), shorten the residence time of the alloy in this temperature range, and inhibit the nucleation and growth of continuous αGB films along the β grain boundary.

[0030] The second cooling section is Below 250℃~ The temperature was maintained at 220℃ for 15–25 min. Within this temperature range, the small number of short αGBs formed in the first cooling section underwent morphological instability evolution driven by surface energy, tending to break and become discontinuously distributed.

[0031] The third cooling section cools to below 550°C at a cooling rate of 3–8°C / min, and then cools naturally to room temperature. The slow cooling in this section promotes the uniform nucleation and moderate growth of secondary αs in the β matrix (including near the TiB whisker / β interface), forming αs bundles with a certain lamellar thickness.

[0032] in, The β-transformation temperature is the value of the high-strength, high-toughness titanium alloy.

[0033] S4. The billet after solution treatment is heated and kept at a constant temperature, and then cooled to room temperature to obtain the high-strength and high-toughness titanium alloy.

[0034] Preferably, the heating and heat preservation of the solution-treated billet in step S4 includes: The solution-treated billet is heated to 460–510℃ and held for 8–12 h. This step drives the precipitation of nanoscale needle-like αt phase in the residual metastable β phase, further improving the alloy strength. The aging temperature of 460–510℃ is chosen based on the following considerations: the precipitation temperature range of the ω phase in the TC18 alloy is approximately 200–350℃. Aging above 460℃ allows the ω phase to transform into αt, avoiding embrittlement caused by the residual ω phase; simultaneously, this temperature range ensures a high αt nucleation density, and the existing αs phase does not undergo significant coarsening (the Ostwald ripening rate of αs is low below 510℃). After aging, the billet is air-cooled to room temperature.

[0035] The technical solutions and beneficial effects of this application are further illustrated below through specific embodiments and comparative examples. Experimental methods not specifying specific conditions in the embodiments are generally performed according to conventional conditions in the art or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all conventional products available on the market.

[0036] The β transformation temperature (Tβ) of the TC18 titanium alloy used in the following examples was determined to be 875±5℃ by metallographic method.

[0037] Example 1 Composition: The ingredients are prepared according to the following mass percentage: Ti-5.0Al-5.0Mo-5.0V-1.0Cr-1.0Fe-0.10B. B is added in the form of TiB2 powder (purity 99.5%, particle size 20-40 μm).

[0038] S1: Press the prepared raw materials into a consumable electrode, under a vacuum degree ≤5×10 -2 Three vacuum consumable arc melting processes were carried out under Pa conditions, with the electrodes flipped after each melting process to obtain a uniform alloy ingot.

[0039] S2: β-phase region forging: The ingot is heated to 910℃ (35℃ above Tβ), held for 1.5 h, and then upset forging is performed, with a total deformation of 65%. α+β two-phase region forging: The billet is heated to 835℃ (40℃ below Tβ), and then upset forging is performed, with a total deformation of 55%. The resulting billet has dimensions of approximately Φ100 mm × 120 mm.

[0040] S3: Heat the forged billet to 835℃ (40℃ below Tβ) and hold for 1.5 h. Then perform segmented cooling: First cooling section: Cooling to 635℃ at 20℃ / min (240℃ below Tβ); Second cooling section: Hold isothermally at 635℃ for 20 minutes; Third cooling section: After furnace cooling to 540°C at 5°C / min, remove and air cool to room temperature.

[0041] S4: Heat the solution-treated billet to 490℃, hold for 10 hours, and air cool to room temperature.

[0042] Examples 2-7 The difference between Examples 2-7 and Example 1 lies in the amount of substances added in each step and the process control. Everything else is the same as in Example 1. See Table 1 for details.

[0043] Comparative Example 1 The only difference between this comparative example and Example 1 is that it does not contain element B, and the alloy composition is Ti-5.0Al-5.0Mo-5.0V-1.0Cr-1.0Fe (i.e., standard TC18 composition, without B). The remaining steps and parameters are exactly the same as in Example 1.

[0044] Comparative Example 2 The only difference between this comparative example and Example 1 is the cooling method in step S3. This comparative example omits segmented cooling; after solution treatment and holding at 835°C for 1.5 h, it is directly furnace cooled to room temperature at a single rate of 5°C / min. All other parameters are identical to those in Example 1.

[0045] Comparative Example 3 The only difference between this comparative example and Example 1 is the cooling method in step S3. After solution treatment and holding at 835°C for 1.5 h, this comparative example was directly water-cooled (at a rate of approximately 600°C / min or higher) to room temperature. The parameters of the remaining steps were exactly the same as those in Example 1.

[0046] Comparative Example 4 The only difference between this comparative example and Example 1 is that B is added in the form of B4C powder (equivalent to a B content of 0.10%, with the introduction of a corresponding proportion of C), instead of TiB2 powder. The remaining steps and parameters are exactly the same as in Example 1.

[0047] Table 1. Control parameters for each step in Examples 1-7 and Comparative Examples 1-4 Effect testing and data analysis The high-strength and high-toughness titanium alloys prepared in Examples 1-7 and Comparative Examples 1-4 were subjected to performance tests. The test items and methods included: (1) Room temperature tensile properties test: In accordance with GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature", room temperature tensile tests were conducted on a universal testing machine. The specimens were processed into proportional specimens (d0=5 mm, L0=25 mm) according to GB / T 6397 standard. The tensile strength Rm (MPa), yield strength Rp0.2 (MPa) and elongation after fracture δ (%) were determined. Three specimens were taken from each group, and the average value was taken.

[0048] (2) Fracture toughness (KIC) test: Following GB / T 4161-2007 "Test Method for Plane Strain Fracture Toughness (KIC) of Metallic Materials", compact tensile specimens (CT specimens) with a thickness B = 20 mm and a width W = 40 mm were used. Fracture toughness was tested on a universal testing machine to determine the plane strain fracture toughness (KIC) in MPa·m^(1 / 2). Two specimens were taken from each group, and the average value was recorded. The validity conditions were verified according to GB / T 4161 standard.

[0049] (3) Microstructure observation: The microstructure morphology of the alloy was observed using an optical microscope (OM) and a scanning electron microscope (SEM), including: β grain size, morphology and volume fraction of equiaxed αp, continuity and distribution of grain boundary α phase (αGB), lamellar thickness and distribution characteristics of secondary αs, and morphology and distribution of TiB whiskers. The continuity of αGB was quantitatively assessed by the proportion of the αGB coverage length along the β grain boundary to the perimeter of a single grain in the SEM-BSE image.

[0050] (4) Identification of the reinforcing phase: X-ray diffraction (XRD) was used to identify the type of reinforcing phase in Example B and Comparative Example 4.

[0051] The test results are shown in Table 2 below: Table 2 Test results of Examples 1-7 and Comparative Examples 1-4 Results analysis: According to the test results in Table 2, a comparison between Examples 1-3 and Comparative Example 1 shows that after adding B, the β grain size was refined from ≤90μm to ≤40-65μm, and the alloy strength increased by approximately 50-120MPa. With increasing B content, the alloy strength showed an increasing trend, but the plasticity and KIC decreased slightly. Example 1, with a B content of 0.10%, exhibited the best overall performance (Rm=1380MPa, δ=12.5%, KIC=68MPa·m^1 / 2).

[0052] Comparative Example 2 formed a continuous / semi-continuous grain boundary α film with a KIC of only 48 MPa·m^1 / 2, which is about 29% lower than that of Example 1. Although Comparative Example 3 suppressed the grain boundary α film, αs precipitation was insufficient, δ decreased to 6.5%, and KIC was only 45 MPa·m^1 / 2. Example 1 adopted a segmented cooling process, and αGB was distributed in a discontinuous point chain, achieving a synergistic match between strength, plasticity and fracture toughness, proving that the three-segment segmented cooling is the key process for controlling the αGB morphology and overcoming the strength-toughness inversion.

[0053] Example 4 showed the highest Rm, but a relatively low KIC; Example 5 showed the highest KIC, but Rm dropped to 1310 MPa. Example 1 (aged at 490℃) showed the best strength-toughness match. All three examples met the performance design objectives of this application.

[0054] Example 6 showed a high αp volume fraction, with δ reaching 14.5% and KIC reaching 70 MPa·m^1 / 2, but a slightly lower Rm. Example 7 showed a lower αp volume fraction, with Rm increasing to 1410 MPa, but δ and KIC decreasing accordingly. All three examples achieved the performance targets within the scope of the claims, demonstrating that the strength-to-toughness ratio can be flexibly adjusted according to actual application requirements within the solution temperature range of 30°C to 60°C below Tβ.

[0055] The above embodiments and comparative examples fully demonstrate the outstanding beneficial effects of the technical solution of this application.

[0056] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A high-strength, high-toughness titanium alloy, characterized in that, By weight percentage, it consists of the following components composition: Al：4.5%～5.5%； Mo: 4.5%–5.5%; V：4.5%～5.5%； Cr：0.5%～1.5%； Fe: 0.5%~1.5%; B：0.05%～0.15%； The balance consists of Ti and impurities.

2. The high-strength, high-toughness titanium alloy according to claim 1, characterized in that, The chemical composition of the high-strength and high-toughness titanium alloy, by mass percentage, has a boron content of 0.08% to 0.12%.

3. A method for preparing a high-strength, high-toughness titanium alloy, characterized in that, Includes the following steps: Raw materials are weighed according to the mass percentage of the components of the high-strength, high-toughness titanium alloy described in claim 1 or 2, mixed, smelted, and cast to obtain a titanium alloy ingot; wherein element B is used in the following manner. Added in powder form; The titanium alloy ingot is sequentially subjected to Phase zone forging and α+ Two-phase forging is used to obtain forged billets; The forged billet is heated to the solution temperature and held at that temperature, followed by segmented cooling. The solution-treated billet is heated and held at a constant temperature, and then cooled to room temperature to obtain the high-strength and high-toughness titanium alloy.

4. The method according to claim 3, characterized in that, The segmented cooling system includes a first cooling segment, a second cooling segment, and a third cooling segment; The first cooling section cools at a cooling rate of 15–30 °C / min to Below 250℃~ Below 220℃; The second cooling section is Below 250℃~ The temperature is maintained at 220℃ for 15–25 minutes. The third cooling section cools to below 550°C at a cooling rate of 3–8°C / min, and then cools naturally to room temperature; in, The β-transformation temperature is the value of the high-strength, high-toughness titanium alloy.

5. The preparation method according to claim 4, characterized in that, The heating temperature for forging in the β phase region is: Above 30℃~ At temperatures above 50℃, the heat preservation time is 1–2 hours, and the total deformation is not less than 60%. The heating temperature for forging the α+β two-phase region is: Below 50℃~ At temperatures below 30℃, the total deformation shall not be less than 50%.

6. The preparation method according to claim 3, characterized in that, The solution temperature is Below 60℃~ At 30℃ or below, the heat preservation time is 1 to 2 hours.

7. The method according to claim 3, characterized in that, The The purity of the powder is not less than 99.5%, and the particle size is 10-50 μm.

8. The method according to claim 3, characterized in that, The melting process employs vacuum consumable arc melting, and the melting process is carried out at least three times.

9. The method according to claim 3, characterized in that, The heating and heat preservation of the solution-treated billet includes: The solution-treated billet is heated to 460–510℃ and held for 8–12 hours.

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