A titanium alloy with high strain hardening capability and a preparation method thereof

Abstract

The application discloses a kind of high strain hardening ability titanium alloy and preparation method.The titanium alloy is by the following components according to mass percentage: Al 5.5~6.5%, V 3.5~4.5%, Mo 1.5~2.5%, Fe 0.4~0.6%, Cr 0.4~0.6%, remainder is Ti and inevitable impurities;Wherein the sum of mass of Fe and Cr is 0.8~1.2%, and the mass ratio of Mo / (Fe+Cr) is 1.7~3.2.By introducing slow diffusion element Mo, synergistic effect with Fe, Cr, combined with near β phase transition region hot rolling and rapid cooling, metastable β phase is obtained.The metastable β phase can simultaneously start stress-induced α' martensitic phase transition, realize multiple plastic deformation mechanism synergy.Tensile strength ≥1125MPa, yield strength ≥712MPa, elongation ≥15.1%, realize the excellent match of high strength, high plasticity and high strain hardening ability.

Description

Technical Field

[0001] This invention relates to titanium alloys and their preparation methods, and particularly to a titanium alloy with high strain hardening capability and its hot rolling process, a titanium alloy material that improves strain hardening performance by obtaining a metastable β phase through the synergistic regulation of Mo-Fe-Cr tri-element combined with near-β phase transformation zone rolling. Background Technology

[0002] Titanium alloys are widely used in aerospace, marine engineering, and biomedical fields due to their high specific strength, excellent corrosion resistance, and high-temperature performance. Among them, Ti-6Al-4V (TC4) is the most widely used α+β type titanium alloy, but its strain hardening ability is relatively low, which makes it prone to early necking during cold forming, limiting the forming performance of complex shaped components.

[0003] To improve the strain hardening capability of titanium alloys, researchers have attempted to introduce β-stabilizing elements through alloying to obtain metastable β phases, utilizing the stress-induced phase transformation or twinning effect of these metastable β phases during deformation to generate sustained work hardening. Fe and Cr, as inexpensive β-stabilizing elements, have attracted considerable attention. However, both Fe and Cr are fast-diffusing elements, prone to compositional segregation during solidification and hot working, leading to inhomogeneous microstructure and localized brittleness. Furthermore, as eutectoid β-stabilizers, excessive addition of Fe and Cr can easily form brittle intermetallic compounds such as TiFe and TiCr2, impairing the alloy's plasticity.

[0004] Mo, as a isomorphous β-stabilizing element, exhibits slow diffusion characteristics, which can effectively suppress compositional segregation and obtain a uniform β-structure. Existing research mainly focuses on high-Mo content systems, but the high Mo content in these alloys significantly increases costs, and excessively high β-stability can suppress the transformation-induced plasticity (TRIP) effect during deformation.

[0005] Therefore, how to obtain a metastable β phase with a uniform microstructure and multiple deformation mechanisms through reasonable composition design and process control, while controlling costs and avoiding segregation, thereby significantly improving the strain hardening ability of titanium alloys, remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] Purpose of the invention: To overcome the shortcomings of existing technologies, such as relatively low strain hardening capacity, compositional segregation and brittle phase precipitation, high cost of high Mo content systems, and the over-stability of the β phase which inhibits the TRIP effect, as well as low strength, plasticity, and strain hardening capacity, this invention provides a high strain hardening capacity titanium alloy and its preparation method.

[0007] This invention introduces the slow-diffusing element Mo as the main β-stabilizer, which works synergistically with trace amounts of fast-diffusing elements Fe and Cr. Combined with hot rolling and rapid cooling processes in the near-β transformation region (Tβ-20℃~Tβ-5℃), a metastable β-phase with a uniform microstructure and a volume fraction of 30%~50% was obtained. This metastable β-phase can simultaneously initiate stress-induced α' martensitic transformation under room temperature deformation conditions, achieving a synergistic effect of multiple reshaping deformation mechanisms. The tensile strength is ≥1125MPa, the yield strength is ≥712MPa, and the elongation is ≥15.1%, achieving an excellent balance of high strength, high plasticity, and high strain hardening capability.

[0008] Technical solution: The high strain hardening titanium alloy of the present invention is composed of the following components: Al: 5.5%–6.5% V: 3.5%~4.5% Mo: 1.5–2.5% Fe: 0.4–0.6% Cr: 0.4–0.6% The balance consists of Ti and unavoidable impurities; wherein the sum of the masses of Fe and Cr is 0.8 to 1.2%, and the mass ratio of Mo to (Fe+Cr) is 1.7 to 3.2.

[0009] Furthermore, the sum of the masses of Fe and Cr is 0.8%, and the mass ratio of Mo to (Fe+Cr) is 3.1.

[0010] Furthermore, the sum of the masses of Fe and Cr is 1.0%, and the mass ratio of Mo to (Fe+Cr) is 2.5.

[0011] Furthermore, the sum of the masses of Fe and Cr is 1.2%, and the mass ratio of Mo to (Fe+Cr) is 1.7.

[0012] Furthermore, Mo: 2.0–2.5%.

[0013] Furthermore, the microstructure of the titanium alloy consists of an α phase and a metastable β phase, with the metastable β phase having a volume fraction of 30% to 50%. The metastable β phase is capable of undergoing stress-induced martensitic transformation under room temperature deformation conditions.

[0014] Furthermore, the titanium alloy exhibits high strain hardening capacity, with a tensile strength ≥1125MPa, a yield strength ≥712MPa, and an elongation ≥15.1%.

[0015] On the other hand, the present invention provides a hot-rolled method for preparing the above-mentioned titanium alloy, comprising the following steps: (1) Determination of phase transformation point: The phase transformation point Tβ of the titanium alloy was determined by metallographic method; (2) Heating: Heat the titanium alloy billet to the rolling temperature T, where T is controlled within the range of 20°C to 5°C below Tβ (i.e., near the β phase transformation zone), and hold for 30 to 60 minutes to make the billet temperature uniform; (3) Rolling: The heated billet is hot rolled in multiple passes, and the total deformation is controlled to be 70% to 80%. The final rolling temperature is not lower than 30°C below Tβ. The number of rolling passes is 4 to 5, and the temperature is held for 10 minutes after each pass. The deformation is 10% to 25% per pass. During the rolling process, the deformation heat effect and dynamic phase transformation are used to obtain a metastable β phase with a volume fraction of 30% to 50% in the alloy. (4) Cooling: After rolling, rapid cooling is carried out immediately with a cooling rate of ≥50℃ / s, preferably water cooling, so as to retain the metastable β phase after high temperature deformation to room temperature.

[0016] Furthermore, the heat preservation time in step (2) is 30 to 60 minutes.

[0017] Furthermore, in step (3), the number of rolling passes is 4 to 5.

[0018] Technical principles and synergistic effects: This invention achieves the following synergistic effects through precise component design and process control: (1) Synergistic effect of Mo-Fe-Cr elements Mo, as a isomorphous β-stabilizing element, exhibits slow diffusion characteristics, allowing it to distribute uniformly during alloy solidification and hot working, avoiding localized compositional fluctuations and laying the foundation for a homogeneous β-structure. Fe and Cr, as fast-diffusion eutectoid β-stabilizing elements, although added at low levels (Fe+Cr≤1.2%), can promote dynamic recrystallization and refine grains through micro-enrichment near grain boundaries and phase boundaries. This invention, by controlling the Mo / (Fe+Cr) mass ratio to 1.7–3.2, leverages both the homogenizing effect of Mo and the interfacial activation effect of trace amounts of Fe and Cr, achieving precise control over the stability of the β-phase. This keeps the β-phase in a "metastable critical" state—sufficient to be retained to room temperature during rapid cooling and capable of undergoing stress-induced phase transformation during subsequent deformation.

[0019] (2) Microstructure control during near-β phase transformation rolling This invention selects the near-β phase transformation region (Tβ-20℃ to Tβ-5℃) for hot rolling. Its advantages are: the temperature is higher than Tβ-20℃, ensuring the presence of 10%–20% of the primary α phase, utilizing the α / β phase boundary as a nucleation point for dynamic recrystallization, refining the grains, and simultaneously pinning grain boundaries to prevent β grain coarsening; the temperature is lower than Tβ, avoiding rapid β grain growth after entering the single-phase region. Within this specific temperature range, deformation mainly occurs in the β phase, but due to the presence of the α phase, the deformation energy storage is higher, promoting the retention of the metastable β phase during subsequent cooling.

[0020] (3) Synergy of multiple remodeling deformation mechanisms The metastable β phase obtained in this invention can simultaneously initiate two plastic deformation mechanisms under room temperature deformation conditions: stress-induced α' martensitic transformation (TRIP effect). The TRIP effect consumes local stress concentration through phase transformation, providing continuous work hardening; the TRIP effect divides grains through twin boundaries, generating a dynamic Hall-Petch effect. The synergistic effect of the two mechanisms enhances the strain hardening capability of the alloy of this invention.

[0021] Compared with the prior art, the present invention has the following beneficial effects: 1. Reasonable composition design and controllable cost: By introducing the slow-diffusion element Mo as the main β stabilizer (1.5~2.5%), the amount of Fe and Cr is greatly reduced (Fe+Cr≤1.2%), which avoids the risk of segregation and brittle phase caused by high Fe and Cr, and controls the cost of alloy.

[0022] Uniform microstructure and high metastable β phase content: The homogenization effect of Mo combined with the interfacial activation effect of trace Fe and Cr, combined with the near-β phase transformation zone rolling process, yielded a uniform microstructure with a metastable β phase of 30% to 50% by volume, which is much higher than the metastable β phase content (about 10 to 15%) of conventional two-phase zone rolled TC4.

[0023] 2. Significantly improved strain hardening capacity: The room temperature tensile strain hardening capacity of the alloy of this invention is improved; at the same time, the tensile strength is ≥1125MPa, the yield strength is ≥712MPa, and the elongation is ≥15.1%, achieving an excellent match of high strength, high plasticity and high strain hardening capacity.

[0024] 3. Synergistic effect of multiple deformation mechanisms, excellent cold forming performance: The metastable β phase initiates the TRIP effect during deformation, giving the alloy good cold bending performance and resistance to necking, making it suitable for applications in industries requiring high strength and toughness titanium alloys.

[0025] 4. Wide process window, suitable for industrial production: The near-β phase transformation zone rolling process adopted in this invention is stable and controllable, requires no special equipment, and is easy to promote and apply on existing titanium alloy processing production lines. Attached Figure Description

[0026] Figure 1 The EBSD phase diagram of the titanium alloy prepared in Example 1 of this invention is shown. The red area represents the α phase, the green area represents the stable β phase, and the blank area represents the metastable β phase with high dislocation density. The volume fraction of the metastable β phase is approximately 46%. Figure 2 The EBSD phase diagram of the titanium alloy prepared in the comparative example of this invention is shown. Red represents the α phase, and green represents the stable β phase with a volume fraction of about 30%, while the metastable β content is very low. Figure 3 The figures show a comparison of the actual room temperature tensile stress-strain curves of Examples 1-3 and Comparative Example 1 of the present invention, demonstrating that the alloy of the present invention has a longer uniform plastic deformation stage. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be further described below.

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Alloy preparation and testing methods Alloys were prepared according to the mass percentages shown in Table 1, and melted three times in a vacuum arc furnace. The ingots were then forged to obtain 20 mm thick plate blanks. The phase transformation point Tβ of each alloy was determined by metallography.

[0030] Room temperature tensile testing was performed according to GB / T 228.1 standard, with a strain rate of 1×10⁻⁶. -3 s -1 Tissue observation was performed using an optical microscope (OM). The volume fraction of the β phase was statistically analyzed using Image-Pro Plus image analysis software.

[0031] Example 1: Alloy composition: Al 6.0%, V 4.0%, Mo 2.5%, Fe 0.4%, Cr 0.4%, Fe+Cr=0.8%, Mo / (Fe+Cr)=3.1%, balance Ti. The phase transformation point Tβ of the titanium alloy was determined by metallography; Tβ: 850℃.

[0032] The billet is heated to 835℃ (Tβ-15℃) and held for 60 minutes; then it is hot rolled in 5 passes on a hot rolling mill, and after each pass it is held at the same temperature for 10 minutes, with a total deformation of 70% and a final rolling temperature of 830℃ (≥Tβ-25℃); and then immediately water-cooled after rolling.

[0033] The obtained boards were subjected to microstructure observation and mechanical property testing, and the results are shown in Table 2.

[0034] Example 2: Alloy composition: Al 6.0%, V 4.0%, Mo 2.5%, Fe 0.5%, Cr 0.5%, Fe+Cr=1.0%, Mo / (Fe+Cr)=2.5%, balance Ti. The phase transformation point Tβ of the titanium alloy was determined by metallography; Tβ: 845℃.

[0035] The billet was heated to 830℃ (Tβ-15℃) and held for 60 minutes; after each rolling pass, it was held at the same temperature for 10 minutes, with a total rolling deformation of 70% and a final rolling temperature of 820℃; then water-cooled. Performance is shown in Table 2.

[0036] Example 3: Alloy composition: Al 6.0%, V 4.0%, Mo 2.5%, Fe 0.6%, Cr 0.6%, Fe+Cr=1.2%, Mo / (Fe+Cr)=1.7, balance Ti. The phase transformation point Tβ of the titanium alloy was determined by metallography; Tβ: 860℃.

[0037] The billet was heated to 843℃ (Tβ-17℃) and held for 60 minutes; after each rolling pass, it was held at the same temperature for 10 minutes, with a total rolling deformation of 70% and a final rolling temperature of 840℃; then water-cooled. Performance is shown in Table 2.

[0038] Comparative example (conventional two-phase rolling): The same alloy composition as in Example 1 was used, but the rolling temperature was changed to 800℃ (Tβ-50℃, conventional two-phase region), and other processes were the same. Properties are shown in Table 2.

[0039] Test results: Table 1 Alloy composition (mass fraction %) of the Examples and Comparative Examples

[0040] Table 2. Microstructure and mechanical properties of the examples and comparative examples

[0041] Results analysis: The following conclusions can be drawn from the results in Table 2: (1) The key role of near-β rolling process The comparative example used the same composition as Example 1, but the rolling temperature was reduced to the conventional two-phase region (Tβ-50℃). The β phase volume fraction was 30%, but the metastable β phase was significantly reduced. This indicates that even with proper composition optimization, without a near-β phase transformation rolling process, it is impossible to obtain a sufficient volume fraction of metastable β phase, let alone activate the multiple deformation mechanism. The synergistic effect of composition and process is fully verified here.

[0042] In summary, this invention achieves an optimal balance between microstructure uniformity and metastable β phase by controlling the Mo content within a specific range of 2–2.5%, the total Fe+Cr content within 0.8–1.2%, and the Mo / (Fe+Cr) ratio within a range of 2–3.2, combined with a near-β phase transformation zone rolling process. This results in excellent strain hardening properties. The effects are unattainable by adjusting the composition or process alone, demonstrating significant synergistic effects and inventiveness.

[0043] The above are merely preferred embodiments of the present invention and do not constitute any limitation on the present invention. Any equivalent substitutions or modifications made by those skilled in the art to the technical solutions and content disclosed in the present invention without departing from the scope of the present invention shall be deemed to have remained within the protection scope of the present invention.

Claims

1. A titanium alloy with high strain hardening capability, characterized in that, By weight percentage, it consists of the following components composition: Al：5.5～6.5% V：3.5～4.5% Mo: 1.5–2.5% Fe: 0.4–0.6% Cr：0.4～0.6% The balance consists of Ti and unavoidable impurities; wherein the sum of the masses of Fe and Cr is 0.8 to 1.2%, and the mass ratio of Mo to (Fe+Cr) is 1.7 to 3.

2.

2. The high strain hardening titanium alloy according to claim 1, characterized in that, The sum of the masses of Fe and Cr is 0.8%, and the mass ratio of Mo to (Fe+Cr) is 3.

1.

3. The high strain hardening titanium alloy according to claim 1, characterized in that, The sum of the masses of Fe and Cr is 1.0%, and the mass ratio of Mo to (Fe+Cr) is 2.

5.

4. The high strain hardening titanium alloy according to claim 1, characterized in that, The sum of the masses of Fe and Cr is 1.2%, and the mass ratio of Mo to (Fe+Cr) is 1.

7.

5. The high strain hardening titanium alloy according to claim 1, characterized in that, Mo: 2.0–2.5%.

6. The high strain hardening titanium alloy according to claim 1, characterized in that, The microstructure of the titanium alloy consists of an α phase and a metastable β phase, with the metastable β phase having a volume fraction of 30% to 50%. The metastable β phase is capable of undergoing stress-induced martensitic transformation under room temperature deformation conditions.

7. The high strain hardening titanium alloy according to claim 1, characterized in that, The titanium alloy has high strain hardening capacity, tensile strength ≥1125MPa, yield strength ≥712MPa, and elongation ≥15.1%.

8. A hot-rolling method for preparing the high strain hardening titanium alloy according to any one of claims 1-7, characterized in that, Includes the following steps: (1) Determine the phase transformation point: The phase transformation point Tβ of the titanium alloy is determined by metallographic method; (2) Heating: The titanium alloy billet is heated to the rolling temperature T, which is controlled within the range of 20°C to 5°C below Tβ, and the billet temperature is kept uniform; (3) Rolling: The heated billet is hot rolled in multiple passes, and the temperature is kept for 10 minutes after each pass, the total deformation is controlled to be 70% to 80%, and the final rolling temperature is not lower than 30°C below Tβ; (4) Cooling: The rolling is immediately followed by rapid cooling at a rate of ≥50°C / s, and the metastable β phase after high temperature deformation is kept to room temperature.

9. The hot rolling method according to claim 8, characterized in that, The heat preservation time in step (2) is 30 to 60 minutes.

10. The hot rolling method according to claim 8, characterized in that, In step (3), the rolling passes are 4 to 5.

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