A method for preparing heterostructures in titanium alloys

Abstract

A method for preparing heterogeneous structures in titanium alloys belongs to the field of titanium alloy preparation technology. First, titanium alloys forged in the α+β phase region are held at 10-15°C below the β / (α+β) phase transformation point for 55-65 minutes and then air-cooled to obtain a bimodal microstructure. Then, hot rolling with a total deformation of 60%-70% is performed in the low-temperature, medium-temperature, and high-temperature regions within the α+β phase. Finally, stabilization and aging treatments are sequentially performed to obtain titanium alloy sheets with a layered heterogeneous structure. This invention achieves a synergistic improvement in the strength-elongation ratio of titanium alloys under room-temperature tensile stress and prepares heterogeneous structures in titanium alloys.

Description

Technical Field

[0001] This invention belongs to the field of titanium alloy preparation technology (a field of thermomechanical deformation controlled by deformation and phase transformation), specifically relating to a titanium alloy sheet with a heterostructure (HS) material and its rolling and heat treatment processes. The titanium alloy sheet with a layered heterostructure has excellent comprehensive properties and will have important applications in the aerospace field. Background Technology

[0002] Titanium alloys have been widely used as structural materials in the aerospace industry due to their excellent mechanical properties. To obtain ideal mechanical properties, different microstructures can be obtained through deformation and heat treatment, but these traditional microstructures suffer from the dilemma of strength-ductility trade-off.

[0003] To achieve synergistic enhancement of strength and ductility, it is necessary to develop a novel microstructure. In recent years, heterostructure (HS) materials have attracted widespread attention in the materials science community, and various heterostructure materials have been developed. Heterostructures generally consist of coarse and fine grains, with the coarse grains surrounded by the fine grains. This results in a strengthening effect at the interface between these two grains (back stress strengthening), and this interface has a very significant hindering effect. In contrast, the biphase microstructure of titanium alloys consists of a soft phase (αp phase) and a hard phase (αs phase). The interface between these two phases hinders dislocations, but their grain sizes are essentially uniform, thus lacking the significant strengthening effect of heterostructure materials. Researchers have prepared heterogeneous sheet structures in commercially available pure titanium. These structures consist of coarse recrystallized α grains surrounded by nanostructured α grains, exhibiting better ductility. However, heterostructures have not yet been observed to be prepared in titanium alloys. This invention patent provides a highly promising heterogeneous structure in titanium alloys, namely a layered heterogeneous structure. Its characteristic is that by combining rolling and heat treatment processes in a traditional bimodal structure, a nanoscale αs phase structure and a micrometer-scale lamellar αp phase structure are prepared, with αs and αp phases arranged in alternating layers. This improves the strength and elongation of the titanium alloy. Moreover, this structure has a very wide range of applications, meaning that it can be prepared in any titanium alloy with a traditional bimodal structure. Summary of the Invention

[0004] The purpose of this invention is to provide a rolling and heat treatment process for preparing layered heterogeneous structures in titanium alloys. The layered heterogeneous titanium alloy sheet prepared by this invention breaks through the traditional microstructure pattern of titanium alloys, representing a novel method for preparing heterogeneous structures in the field of titanium alloys. It possesses excellent comprehensive mechanical properties, thereby improving the service performance of titanium alloys.

[0005] The present invention provides a rolling and heat treatment process for preparing a layered heterostructure in a titanium alloy, which specifically includes the following steps:

[0006] Step 1

[0007] Titanium alloy ingots were obtained using conventional casting methods. The titanium alloy composition, by mass percentage, was: Al: 6.1%, Sn: 3.0%, Zr: 5.1%, Mo: 0.5%, Nb: 1.1%, Ta: 0.9%, Si: 0.4%, Er: 0.2%, with the remainder being Ti. The ingots were then forged in the β single-phase region at 140–155°C (preferably 1150°C) above the β / (α+β) phase transformation point (holding for 2 hours, three upsetting and three drawing cycles). Subsequently, they were precision forged in the (α+β) two-phase region at 40–50°C (preferably 970°C) below the β / (α+β) phase transformation point (holding for 1 hour, repeated upsetting and drawing cycles) to obtain bars of the required dimensions.

[0008] Step Two

[0009] The forged structure obtained in step one is subjected to solution treatment, and the heat treatment temperature is within 10 to 15°C (preferably 990°C) below the β / (α+β) phase transformation point of the titanium alloy, the holding time is 55 to 65 minutes, and then air-cooled.

[0010] Step 3

[0011] The alloy material with a dual-phase structure obtained in step two is hot-rolled. It is held at the α+β phase region of the titanium alloy (920-990℃, such as 920℃, 960℃, 990℃, preferably 960℃) for 20 minutes, and the first hot rolling with a deformation of 10% is performed. Then, it is held at this temperature for 5 minutes, and the next hot rolling with a deformation of 15% is performed. The rolling is repeated, and the deformation of each pass during the repeated rolling is 10%-20% (preferably 15%). Finally, the total deformation of the hot rolling needs to be controlled at 60%-70%.

[0012] This step is mainly to transform the equiaxed αp phase into a lamellar αp phase, and to gradually reduce the αp phase content as the rolling temperature increases, thereby breaking down and refining the αs grains, thus obtaining layered heterostructure titanium alloy plates with different αp phase contents.

[0013] Step Four

[0014] The sheet material obtained in step three is subjected to stabilization treatment and aging heat treatment in sequence. The stabilization process is 800℃ / 1h / AC; the aging process is 700℃ / 5h / AC.

[0015] This invention yields high-performance layered heterostructure titanium alloy plates with a room temperature strength of over 1119 MPa and an elongation of over 15%.

[0016] The titanium alloy prepared by this invention has a nanoscale αs phase structure and a micrometer-scale lamellar αp phase structure, and the αs and αp phases are arranged in alternating layers to form a heterojunction. The micrometer-scale lamellar αp phase structure is transformed from the equiaxed αp phase, which improves the strength and elongation of the titanium alloy. Moreover, this structure has a very wide range of applications, that is, it is possible to prepare titanium alloys with traditional dual-state structures.

[0017] The present invention has the following beneficial effects:

[0018] This invention controls the rolling process to maintain the soft phase (αp phase) grain size while transforming it into a lamellar shape (the lamellar αp phase also has a certain strengthening effect); while the hard phase (αs phase) becomes fine grains. Through this rolling process (i.e., the rolling process mentioned in the invention description of this patent), we retain the phase interface and make it the interface between large and small grains, creatively adding back stress strengthening on the basis of phase boundary strengthening, to jointly achieve a strengthening effect. In other words, this invention prepares a layered heterostructure of titanium alloy through hot rolling and subsequent heat treatment. The unique structure of this layered heterostructure causes a large change in flow stress from one region (αp phase) to the next region (αs phase), thereby achieving a synergistic enhancement of strength and ductility. The layered heterostructure can alleviate the attraction concentration caused by the precipitation of silicides and α2 phase. Because the layered αp phase contains a large number of unit volume interfaces, appropriate heat treatment processes can control the precipitation of precipitates along the interfaces as uniformly as possible, thereby controlling the precipitation location, reducing stress concentration, and improving overall mechanical properties. Its tensile strength reaches 1119 MPa, yield strength reaches 1069 MPa, and elongation reaches 15%, making it a novel titanium alloy microstructure with excellent comprehensive mechanical properties. Attached Figure Description

[0019] Figure 1 The microstructure of the forged structure obtained after the forging process in step one.

[0020] Figure 2 The microstructure of the bimorphic structure obtained after step two solution treatment (left) and the EBSD size angle grain boundary distribution map (right).

[0021] Figure 3 Example 1: The morphology of the layered heterostructure after hot rolling and heat treatment (left) and the distribution of EBSD grain boundaries (right).

[0022] Figure 4 Example 2: The morphology of the layered heterostructure after hot rolling and heat treatment (left) and the distribution of EBSD size angle grain boundaries (right).

[0023] Figure 5Example 3: The morphology of the layered heterostructure after hot rolling and heat treatment (left) and the distribution of EBSD size angle grain boundaries (right).

[0024] Figure 6 Stress-strain curves of dual-state microstructures and layered heterogeneous structural plates. Detailed Implementation

[0025] The present invention will be further described below with reference to the embodiments, but the present invention is not limited to the following embodiments.

[0026] Example 1

[0027] The titanium alloy in this embodiment has the following composition by mass percentage: Al: 6.1%, Sn: 3.0%, Zr: 5.1%, Mo: 0.5%, Nb: 1.1%, Ta: 0.9%, Si: 0.4%, Er: 0.2%, with the remainder being Ti. A titanium alloy ingot was obtained using conventional casting methods, and the β / (α+β) phase transformation point was determined to be 1001℃ using DSC (Differential Scanning Calorimetry). The first step involved forging the alloy through a 1150℃ β single-phase region, followed by precision forging at 970℃ to obtain an asphalt-structured bar. The second step involved heat treatment at 990℃ for 1 hour to obtain a bimodal microstructure with a primary α phase content of 13%. Figure 2 As shown. The third step involves holding the bimorphic structure obtained in the previous step at 920℃ for 20 minutes, followed by a first hot rolling with a deformation of 10%. This is then held at this temperature for 5 minutes, followed by another hot rolling with a deformation of 15%. This 15% rolling process is repeated until the total hot rolling deformation reaches 67%, yielding a titanium alloy rolled sheet. The fourth step involves sequentially performing stabilization and aging heat treatments. The stabilization process is 800℃ / 1h / AC; the aging process is 700℃ / 5h / AC, resulting in a layered heterogeneous titanium alloy sheet, as shown. Figure 3 .

[0028] Example 2

[0029] The titanium alloy in this embodiment has the same alloy composition as in Example 1. A titanium alloy ingot was obtained using conventional casting methods, and then the β / (α+β) phase transformation point of the alloy was determined to be 1001℃ using DSC (Differential Scanning Calorimetry). First, the ingot was forged in the β single-phase region at 1150℃, followed by precision forging at 970℃ to obtain a forged bar. Second, a heat treatment at 990℃ for 1 hour was performed to obtain a bimodal microstructure with a primary α phase content of 13%, as shown below. Figure 2As shown. The third step involves holding the bimorphic structure obtained in the previous step at 960℃ for 20 minutes, followed by a first hot rolling with a deformation of 10%. This is then held at this temperature for 5 minutes, followed by another hot rolling with a deformation of 15%. This rolling process is repeated until the total hot rolling deformation is controlled to be within 67%, yielding a titanium alloy rolled sheet. The fourth step involves sequentially performing stabilization and aging heat treatments. The stabilization process is 800℃ / 1h / AC; the aging process is 700℃ / 5h / AC, resulting in a layered heterogeneous titanium alloy sheet, as shown. Figure 4 .

[0030] Example 3

[0031] The titanium alloy in this embodiment has the same alloy composition as in Example 1. A titanium alloy ingot was obtained using conventional casting methods, and then the β / (α+β) phase transformation point of the alloy was determined to be 1001℃ using DSC (Differential Scanning Calorimetry). First, the alloy was forged at 1150℃ to open the β single-phase region, followed by precision forging at 970℃ to obtain a forged bar. Second, a heat treatment at 990℃ for 1 hour was performed to obtain a bimodal microstructure with a primary α phase content of 13%, as shown below. Figure 2 As shown. The third step involves holding the bimorphic structure obtained in the previous step at 990℃ for 20 minutes, followed by a first hot rolling with a deformation of 10%. This is then held at this temperature for 5 minutes, followed by another hot rolling with a deformation of 15%. This rolling process is repeated until the total hot rolling deformation is controlled to be within 67%, yielding a titanium alloy rolled sheet. The fourth step involves sequentially performing stabilization and aging heat treatments. The stabilization process is 800℃ / 1h / AC; the aging process is 700℃ / 5h / AC, resulting in a layered heterogeneous titanium alloy sheet, as shown. Figure 5 .

[0032] The microstructure of the plates obtained in the above three embodiments is as follows: Figure 3 , Figure 4 and Figure 5 As shown, the main difference between Examples 1, 2, and 3 is the rolling temperature. However, the difference in rolling temperature has a significant impact on the microstructure. On the one hand, because the rolling is carried out in the (α+β) two-phase region, and the temperature is controlled in the low-temperature region (920℃), the medium-temperature region (960℃), and the high-temperature region (990℃) of the (α+β) two-phase region, this will lead to significant differences in the content and morphology of the αp phase. On the other hand, as the rolling temperature increases, the dynamic recrystallization of the material will be enhanced, which will lead to a stronger equiaxed tendency of the slender αp phase. Therefore, the αp phase is not as slender as it is at the other two temperatures when rolled at 990℃.

[0033] In the technical background section, we introduced a fundamental characteristic of heterostructure materials: the presence of small grains surrounding large grains. Our fabricated layered heterostructure perfectly matches this characteristic, where large lamellar αp grains are surrounded by fine αs grains, such as... Figure 3 ,4,5 (right), and there is a clear interface between the two. Furthermore, this interface perfectly coincides with the phase interface between the αp and αs phases. Therefore, the interface of our titanium alloy layered heterostructure is not merely the grain size interface of other heterostructure materials, but a perfect composite of the grain size interface and the phase interface. In titanium alloys, the αp phase is a soft phase, so dislocations preferentially occur within it. When dislocations reach the phase interface, this composite phase interface greatly hinders dislocation movement. The stress concentration caused by this hindering effect does not promote crack initiation because there is a limit to the strengthening at this interface. Exceeding this limit will break the interface, thereby releasing dislocation pile-up and stress concentration. These dislocations will enter the αs phase. The grains in the αs phase are fine, and the presence of numerous grain boundaries further hinders dislocation movement, resulting in fine-grain strengthening. The heat-treated plates from the three examples were processed into tensile specimens according to GB / T228.1-2010, and their tensile mechanical properties were tested on a tensile testing machine, as shown in Table 1.

[0034] As shown in Table 1, the titanium alloy of this invention with a layered heterostructure obtained through hot rolling and heat treatment processes not only exhibits improved tensile strength and significantly increased yield strength compared to the same titanium alloy with a biphase structure, but also shows a significant increase in elongation. Furthermore, Table 1 shows that the layered heterostructure in the titanium alloy reaches its maximum value at 920℃, and the elongation also reaches its maximum value. This is impossible for traditional biphase structures, where the αp phase content gradually decreases, strength gradually increases, and elongation gradually decreases with increasing temperature in the (α+β) two-phase region. This clearly contradicts the phenomenon observed in our prepared layered heterostructure where both strength and elongation reach their maximum values ​​after rolling and heat treatment at 920℃. This is because, as mentioned earlier, with increasing rolling temperature, the dynamic recrystallization of the material intensifies, increasing the equiaxed degree of the αp phase. At 990℃, it tends to become equiaxed, resulting in a less elongated αp phase compared to that at 920℃. Therefore, 920℃ achieves the best synergistic improvement in strength and elongation, which also demonstrates that a more elongated αp phase possesses better mechanical properties. In summary, the superior comprehensive mechanical properties of this layered heterogeneous structure are due to two main factors. First, the combined effect of the grain-to-grain and phase interfaces in the heterogeneous structure enhances the resistance to dislocations, increases the stress required for dislocation movement, and increases the energy required for crack propagation. Second, the layered αp phase contains a large number of interfaces per unit volume. Therefore, appropriate heat treatment processes can control the precipitation of precipitates along the interfaces as uniformly as possible, thereby controlling the precipitation location, reducing stress concentration, and improving overall mechanical properties.

[0035] Table 1. Tensile properties of dimorphic and heterogeneous structures

[0036]

Claims

1. A method of producing a heterogeneous structure in a titanium alloy, characterized by, Includes the following steps: Step 1 Titanium alloy ingots were obtained using conventional casting methods. The titanium alloy composition, by mass percentage, was: Al: 6.1%, Sn: 3.0%, Zr: 5.1%, Mo: 0.5%, Nb: 1.1%, Ta: 0.9%, Si: 0.4%, Er: 0.2%, with the remainder being Ti. The ingots were then forged in the β single-phase region at 140–155°C above the β / (α+β) phase transformation point, and subsequently precision forged in the (α+β) two-phase region at 40–50°C below the β / (α+β) phase transformation point to obtain bars of the required dimensions. Step Two The forged structure obtained in step one is subjected to solution treatment, and the heat treatment temperature is within 10 to 15°C below the β / (α+β) phase transformation point of the titanium alloy, the holding time is 55 to 65 minutes, and then air-cooled. Step 3 The alloy material with a dual-phase structure obtained in step two is hot-rolled. It is held at 920-960℃ for 20 minutes in the α+β phase region of the titanium alloy, and then hot-rolled with a first deformation of 10%. After holding at this temperature for 5 minutes, the next hot-rolled pass with a deformation of 15% is performed. This rolling process is repeated, with each pass involving a deformation of 10%-20%. Finally, the total hot-rolling deformation must be controlled to be between 60% and 70%. Step Four The sheet material obtained in step three is subjected to stabilization treatment and aging heat treatment in sequence. The stabilization process is 800℃ / 1h / AC; the aging process is 700℃ / 5h / AC.

2. The production method according to claim 1, characterized by, Step 1: Forge the ingot in the β single-phase region at 1150℃.

3. The preparation method according to claim 1, characterized in that, Step 1: Precision forging in the (α+β) two-phase region at 970℃.

4. The preparation method according to claim 1, characterized in that, Step two, the heat treatment temperature is 990℃.

5. The preparation method according to claim 1, characterized in that, Step 3: Hot rolling temperature is 960℃.

6. The preparation method according to claim 1, characterized in that, In step three, the deformation amount per pass during repeated rolling is 15%.

7. The preparation method according to claim 1, characterized in that, The resulting titanium alloy has a nanoscale αs phase structure and a micrometer-scale lamellar αp phase structure, with αs and αp phases arranged alternately in layers to form a heterojunction. The micrometer-scale lamellar αp phase structure is transformed from an equiaxed αp phase.

8. A titanium alloy prepared according to any one of claims 1-7.

9. The titanium alloy prepared according to any one of claims 1-7, wherein the titanium alloy is a high-performance layered heterostructure titanium alloy sheet with a room temperature strength of 1119 MPa or higher and an elongation of 15% or higher.

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