High-pressure heat treatment process for preparing high-temperature titanium alloy with nano hierarchical structure

Abstract

The invention relates to the field of heat treatment, in particular to a high-pressure heat treatment process for preparing a high-temperature titanium alloy with a nano hierarchical structure, which comprises the following steps of: performing cogging forging at 10-80 DEG C above a beta phase transformation point of the titanium alloy to obtain a forging stock; carrying out solution treatment and rapid quenching at the temperature 10-150 DEG C higher than the silicide complete dissolution temperature; carrying out primary tempering at 10-100 DEG C in an alpha + beta two-phase region of which the temperature is lower than the recrystallization temperature; applying static pressure in the same temperature interval to carry out pressure maintaining treatment for more than 1h; and secondary tempering, namely low-temperature destressing tempering, is carried out at the temperature 50-150 DEG C below the primary tempering temperature. According to the method, superhigh-temperature solid solution quenching is combined with high-pressure pressure maintaining at the sub-recrystallization temperature, martensite decomposition and alpha-phase oriented precipitation are induced through a stress field, and nanoscale refinement of an alpha lamella and uniform dispersion distribution of silicides are synchronously achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of heat treatment, in particular to a high-pressure heat treatment process for preparing a nano-structured high-temperature titanium alloy. BACKGROUND

[0002] High-end fasteners (titanium alloy) in the field of aerospace are core structural components for connecting key components, and need to serve in extreme conditions such as high temperature, high pressure and vibration for a long time, which puts high requirements on the comprehensive performance of the materials: not only excellent strength, toughness and fatigue resistance are needed, but also dimensional stability and structural uniformity are needed. Near-alpha titanium alloy has become one of the preferred materials for high-end fasteners in aerospace because of its high strength, good high-temperature creep and fatigue performance, but the size and distribution state of the alpha lamella in the alloy, as well as the size and dispersion degree of the silicide, directly affect the mechanical properties and service reliability of the material, and the low concentration of beta stabilizing elements in the alloy leads to poor hardenability and abnormally narrow forging window, making it difficult to control the size and distribution of lamella and precipitates by conventional processing methods.

[0003] Conventional beta forging and supporting heat treatment technology is the mainstream organization control means, but when applied to near-alpha titanium alloy, it has fatal defects: limited by the hardenability and narrow forging window of the material, only the alpha bundle interlaced basket organization can be obtained, and the intracrystalline contains a large amount of alpha parallel organization and is easy to precipitate large alpha phase, which will seriously cut off the continuity of the matrix, significantly reduce the fatigue performance and crack propagation resistance of the material, and directly shorten the service life of the fastener. More importantly, although some studies have tried to improve the organization by adjusting the forging temperature or optimizing the heat treatment parameters, the fundamental contradiction between poor hardenability and narrow forging window cannot be solved, and the alpha bundle aggregation cannot be avoided, the problem of coordinated control of alpha lamella refinement and silicide dispersion cannot be solved, and the performance requirements of high-end fasteners in aerospace cannot be met. Conventional technology cannot break through the bottleneck of organization control, and cannot realize the ideal basket organization of nano-level long lamellar alpha phase interlaced weaving. SUMMARY

[0004] The technical problem to be solved by the present application is how to solve the problems of coarse basket organization lamella and precipitate phase, lamella bundle, high order degree of basket, poor fatigue and fracture toughness, etc. in the existing near-alpha high-temperature titanium alloy, specifically: the basket organization prepared by conventional beta forging and supporting heat treatment technology not only contains a large amount of alpha parallel organization, but also is easy to precipitate large alpha phase, which significantly reduces the fatigue performance and crack propagation resistance of the material, and directly shortens the service life of the fastener. Conventional beta forging and supporting heat treatment technology cannot further reduce the thickness of alpha lamella to further improve the service performance of the alloy at high temperature. Conventional beta forging and supporting heat treatment technology cannot simultaneously control the size and distribution of lamella and precipitate phase, and cannot avoid the segregation and coarsening of precipitate phase.

[0005] The technical solution adopted in this invention is: a high-pressure heat treatment process for preparing nano-hierarchical high-temperature titanium alloys, which is carried out according to the following steps. Step 1: Perform forging at 10-80℃ above the β phase transformation point of the titanium alloy to obtain a forged billet; Step 2: Perform solution treatment and rapid quenching at 10-150°C above the complete dissolution temperature of the silicide; Step 3: Perform the first tempering in the α+β two-phase region at 10-100℃, which is below the recrystallization temperature; Step 4: Apply static pressure within the same temperature range and maintain the pressure for at least 1 hour; Step 5: Perform a second tempering at 50-150℃ below the initial tempering temperature, i.e., low-temperature stress-relief tempering.

[0006] In step one, forging is performed in three directions, with deformation occurring once in each direction, and the deformation amount reaching 50% or more, to obtain a forged billet. This fully breaks down the as-cast structure and initially disperses the silicides.

[0007] In step two, the forging billet is heated to 10-150°C above the temperature at which the silicides completely dissolve. Then, it is kept for a sufficient time according to the size of the titanium alloy to allow the coarse silicides to completely dissolve and the alloying elements to be fully homogenized. Subsequently, it is subjected to rapid water quenching or oil quenching to obtain a supersaturated metastable β phase and high-density nano-acicular martensite, which provides a high-concentration driving force and a uniform nucleation basis for subsequent phase transformation.

[0008] In step three, the first tempering is carried out in the α+β two-phase region at 10-100℃ below the recrystallization temperature to eliminate residual stress from quenching. Martensite begins to decompose and form the initial lamellar α phase, while the β grains do not coarsen significantly.

[0009] In step four, at the same temperature as the initial tempering temperature, a constant axial pressure of 2 GPa or higher is applied by a forging press and held for more than 1 hour. The stress field further promotes the decomposition of martensite and the precipitation of α-lamellae, and strongly promotes the dispersion and precipitation of nano-sized silicides at the α / β phase boundary and within the lamellars.

[0010] In step five, the secondary tempering temperature is 50-150°C below the primary tempering temperature, and the holding time is 4-8 hours. After that, it is taken out and air-cooled to room temperature.

[0011] The beneficial effects of this invention are as follows: By combining "ultra-high temperature solution quenching" with "high pressure holding at sub-recrystallization temperature," this invention utilizes stress field-induced martensite decomposition and directional precipitation of the α phase, simultaneously achieving nanoscale refinement of α lamellae and uniform dispersion of silicides. This results in the preparation of a high-performance near-α titanium alloy with a basketweave structure of nanoscale α lamellae (approximately 0.8 μm thick) and dispersed silicide distribution (size less than 50 nm). This process solves the problems of coarse α lamellae and silicide aggregation and growth in traditional titanium alloy processing, enabling the material to maintain high strength while possessing excellent fracture toughness and high-temperature performance, making it particularly suitable for the manufacture of high-end fasteners in the aerospace field. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the nanosheet basket structure of Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the nanosheet thickness distribution in Embodiment 1 of the present invention; Figure 3 This is a schematic diagram of nanoscale dispersed distribution of nanoscale silicides in Embodiment 1 of the present invention; Figure 4 This is a schematic diagram of the stress-strain curve at 650℃ in Embodiment 1 of the present invention; Detailed Implementation

[0013] Example 1: The present invention will be described in detail below with reference to a specific example. The titanium alloy used is Ti-6Al-3Sn-9Zr-1Mo-1Nb-1W-0.45Si alloy, with a β phase transformation point of approximately 1060℃.

[0014] Step 1: Heat the cylindrical alloy ingot to 1100 degrees Celsius and hold for one hour for billet forging. Forge once in the radial direction and once in the axial direction, with a deformation of 50%. Finally, forge it into a disc-shaped billet along the direction of the largest size. The coarse β grains are broken and the silicides are initially dispersed, but they are still concentrated near the β grain boundaries.

[0015] Step 2: Cut the forging billet into a 35cm×35cm×70cm cuboid and heat it to 1200℃ (solution temperature). Hold it at that temperature for two hours, and then quench it with 15 wt.% brine to obtain a supersaturated metastable β phase and high-density nano-needle martensite. The silicides are completely dissolved and all elements are evenly distributed.

[0016] Step 3: Heat the quenched billet to 770℃ for the first tempering treatment to eliminate residual quenching stress. The martensite begins to decompose and form the initial lamellar α phase, while the β grains do not coarsen significantly.

[0017] Step 4: Place the billet into the preheated forging die, and apply constant axial pressure at 770℃ using a forging press (hydraulic pressure 170 bar; piston diameter 250 mm; pressure head contact area 210 mm²). 2 The sample chamber pressure was 3.9 GPa, and the pressure and temperature were maintained for 2 hours. The stress field further promoted martensite decomposition and α-lamellae precipitation, and strongly promoted the dispersion and precipitation of nano-sized silicides at the α / β phase boundary and within the lamellae. Device Description: The pressure device mentioned in this step can be, but is not limited to, an isothermal forging press, a hot forging press, a six-sided high-pressure device, a hot-pressing sintering furnace, or a special heat treatment furnace with pressurization function. Its core function is to apply a controllable and stable pressure to the sample or workpiece at the specified precise temperature.

[0018] Pressure parameter description: At the sub-recrystallization temperature, the main function of applied pressure is to provide a stress field sufficient to drive directional precipitation of the α-phase, lamellar interleaving, and suppress silicide segregation, with a critical value of approximately 2 GPa. Below this value, the stress-induced effect is insignificant, and the microstructure refinement and homogenization effects are insufficient. Theoretically, given sufficient equipment conditions, applying higher pressures (e.g., 6 GPa, 8 GPa, or higher) is expected to further promote the above processes, resulting in a more uniform and fine microstructure. The specific upper limit of the pressure is primarily limited by the capabilities of the industrial plant, rather than the process principle itself. Therefore, the core of this invention lies in identifying and utilizing the physical metallurgical mechanism of "applying static pressure not lower than the induction precipitation critical value at the sub-recrystallization temperature," and its protection scope should not be limited to the specific pressure value used in the current embodiment.

[0019] Step 5: Place the pressurized billet into a heat treatment furnace at 650℃ and hold for 6 hours, then remove and air-cool to room temperature.

[0020] Tissue characteristics: The final tissue consists of three-dimensionally interwoven ultrafine α-lamellae with an average thickness of approximately 800 nm, such as... Figure 2 As shown, a typical non-bundled nanobasket structure is formed, such as... Figure 1 As shown, the silicides are uniformly dispersed in the α-layer and at the phase boundaries as particles with a diameter of ~50 nm, without segregation, as... Figure 3 As shown.

[0021] Room temperature mechanical properties: The room temperature yield strength can reach over 1100 MPa, and the elongation remains at around 8%. This nanobasket structure exhibits excellent fracture toughness KIC = 80 MPa·m. 1 / 2 .

[0022] High-temperature performance: The fine microstructure and dispersed reinforcing phase enable the material to maintain a yield strength of 850 MPa at 650℃, such as... Figure 4 As shown.

[0023] Example 2 differs from Example 1 in that the solution temperature in step two is 1180 ℃ and the holding time is 1 h.

[0024] In this embodiment, the alloy has an α-lamellae thickness of 1.1 μm, and the silicides tend to segregate at the β grain boundaries, with a maximum size of 0.5 μm. The room temperature yield strength is reduced to 1020 MPa, the elongation is 6%, and the fracture toughness (KIC) is 60 MPa·m. 1 / 2 The yield strength at 650 ℃ is 760 MPa. Uneven elemental distribution leads to silicide segregation, coarsening of alpha layers, and reduced three-dimensional interlacing, resulting in a decrease in overall mechanical properties.

[0025] Example 3 differs from Example 1 in that the tempering temperature in step three and the pressure holding temperature in step four are set to 700°C.

[0026] In this embodiment, the α-lamellae thickness of the alloy was 0.7 μm. Due to the low tempering and holding pressure temperatures, atomic diffusion was weakened. The nucleation rate of the α-phase increased, but growth was strongly suppressed. The final microstructure showed a decrease in α-lamellae thickness, but the lamellae were shorter, and the degree of three-dimensional interlacing was slightly lower than in Example 1 due to insufficient driving force. The silicide precipitates were also reduced in size, but due to the low temperature, their distribution uniformity was slightly poor, showing a tendency to accumulate along specific interfaces. The room temperature yield strength increased to 1150 MPa, the elongation decreased to 9%, and the fracture toughness KIC was 60 MPa·m. 1 / 2 The yield strength decreases to 700 MPa at 650 ℃.

[0027] Example 4 differs from Example 1 in that the oil pressure is reduced to 10 bar in step four, and the pressure inside the sample chamber is approximately 2.3 GPa.

[0028] In this embodiment, the α-lamellae thickness of the alloy is 1 μm. Due to the reduced pressure, the α-phase nucleation rate decreases, resulting in a certain coarsening motive force. The final microstructure shows an increased α-lamellae thickness, with a slightly lower degree of three-dimensional interlacing and density compared to Example 1. The silicide precipitate size is also increased to approximately 70 nm, while the uniformity is similar to that of Example 1. The room temperature yield strength decreases to 1050 MPa, the elongation increases by 5%, and the fracture toughness (KIC) is 55 MPa·m. 1 / 2 The yield strength at 650℃ is similar to that of Example 1, at 800 MPa. Example 5 differs from Example 1 in that: in step five, the pressurized billet is placed in a heat treatment furnace at 600 °C and held for 4 hours, then removed and air-cooled to room temperature. Compared with Example 1, the morphology, size, and distribution of the α-laminates and silicides in this embodiment did not change significantly, indicating that the main microstructure characteristics were basically determined in the holding pressure step, and the performance differences were small. This embodiment shows that the final heat treatment temperature can be adjusted within a certain range, and its main function is to stabilize the microstructure, eliminate stress, and optimize performance consistency without changing the core nano-basket morphology determined by the previous key steps.

[0029] Example 6: Same as Example 1, except that the alloy composition is changed to Ti-1100 (nominal composition Ti-6Al-2.75Sn-4Zr-0.4Mo-0.45Si). The alloy microstructure obtained in this embodiment has α-lamellae thickness and three-dimensional interlacing degree comparable to that of Example 1. However, due to the lower Zr and Si content and lower silicide content, the room temperature yield strength is reduced to 1000 MPa, the elongation is increased to 10%, and the high-temperature strength at 650°C is reduced to 800 MPa. The fracture toughness is similar to that of Example 1 with KIC = 75 MPa·m. 1 / 2 .

[0030] Comparative Example 1 differs from Example 1 in that the solid solution temperature in step two is set to 1100 °C, which is lower than the silicide dissolution temperature, and the holding time is 2 hours.

[0031] In the alloy prepared in this embodiment, the α-lamellae thickness is 1.2 μm, the silicides are mainly concentrated at the β grain boundaries, and the maximum size can reach 1 μm. The room temperature yield strength is reduced to 1020 MPa, the elongation is 3.2%, and the fracture toughness KIC is 50 MPa·m. 1 / 2 The yield strength at 650 ℃ is 660 MPa. Uneven element distribution leads to silicide segregation and coarsening, which becomes a crack initiation point, resulting in a comprehensive reduction in overall mechanical properties. This embodiment illustrates that achieving uniform element distribution through solid solution at the silicide dissolution temperature is an essential and crucial step.

[0032] Comparative Example 2 differs from Example 1 in that the tempering temperature in step three and the holding temperature in step four are set to 850°C, which is higher than the recrystallization temperature.

[0033] In the alloy prepared in this embodiment, the α-lamellae are significantly coarsened to 2 μm, the degree of interlacing is reduced, and some parallel α-clumps appear. When the temperature exceeds the recrystallization temperature, the atomic mobility increases dramatically, and the microstructure tends to reach a thermodynamic equilibrium state, which disrupts the metastable, fine-grained state constructed by the martensite precursor. The silicides are significantly coarsened (100-200 nm) and tend to segregate at α / β phase boundaries or triplex grain boundaries.

[0034] The room temperature yield strength decreases to 950 MPa, the elongation remains the same, and the fracture toughness KIC is 40 MPa·m. 1 / 2The yield strength at 650 ℃ is 550 MPa. The coarsening of the α-lamellae leads to a decrease in strength. The dispersion strengthening is weakened by the coarsening of the silicide, and the coarsened silicide becomes a brittle crack source, thus the overall performance deteriorates.

[0035] Comparative Example 2 clearly demonstrates that if the tempering and holding temperature is increased above the recrystallization temperature, even with the same ultra-high temperature solution quenching steps, the final microstructure will completely deviate from the design objective of this invention. The high-temperature thermal activation process dominates, leading to uncontrollable coarsening and aggregation of the α-lamellae and silicides. The resulting coarse basketweave structure not only exhibits reduced strength but also severely deteriorates its most critical fracture toughness. This conversely proves that the core step of this invention—"stress-induced holding" within the sub-recrystallization temperature range—is an irreplaceable key technical feature for achieving nanoscale α-lamellae refinement and silicide dispersion, thereby synergistically improving strength and toughness.

[0036] Comparative Example 3 differs from Example 1 in that step four uses conventional multi-directional forging instead of pressure holding heat treatment, with a forging temperature of 770 ℃, and deformation is performed once in each of the three directions, with a deformation amount of 30% per step.

[0037] In the alloy prepared in this embodiment, under dynamic deformation, the initial lamellar rudiments are mechanically broken, bent, and even partially melted and recrystallized. The final microstructure is a mixture of broken, short rod-shaped or equiaxed α-phase and deformed β-matrix, with extremely uneven lamellar thickness distribution. The high defect density introduced by deformation becomes the driving force for rapid microstructure recovery and coarsening at high temperatures. During secondary tempering, the α-phase and silicides are significantly coarsened, resulting in severe deterioration of mechanical properties. Therefore, "applying static or quasi-static pressure for holding treatment in the sub-recrystallization temperature range" is an irreplaceable technical feature of this invention for achieving a nanoscale uniform basket microstructure, which is fundamentally different from any form of conventional forging.

Claims

1. A high-pressure heat treatment process for preparing nano-hierarchical high-temperature titanium alloys, characterized in that: Follow these steps Step 1: Perform forging at 10-80℃ above the β phase transformation point of the titanium alloy to obtain a forged billet; Step 2: Perform solution treatment and rapid quenching at 10-150°C above the complete dissolution temperature of the silicide; Step 3: Perform the first tempering in the α+β two-phase region at 10-100℃, which is below the recrystallization temperature; Step 4: Apply static pressure within the same temperature range and hold for at least 1 hour. Step 5: Perform a second tempering at 50-150℃ below the initial tempering temperature, i.e., low-temperature stress-relief tempering.

2. The high-pressure heat treatment process for preparing nano-hierarchical high-temperature titanium alloys according to claim 1, characterized in that: In step one, forging is performed in three directions, with deformation occurring once in each direction, and the deformation amount is 50% or more. The forging temperature is 10-80℃ above the β phase transformation point to obtain a forged billet. This fully breaks down the as-cast structure and initially disperses the silicides.

3. The high-pressure heat treatment process for preparing nano-hierarchical high-temperature titanium alloys according to claim 1, characterized in that: In step two, the forging billet is heated to 10-150°C above the temperature at which the silicides completely dissolve. Then, it is kept for a sufficient time according to the size of the titanium alloy to allow the coarse silicides to completely dissolve and the alloying elements to be fully homogenized. Subsequently, it is subjected to rapid water quenching or oil quenching to obtain a supersaturated metastable β phase and high-density nano-acicular martensite, which provides a high-concentration driving force and a uniform nucleation basis for subsequent phase transformation.

4. The high-pressure heat treatment process for preparing nano-hierarchical high-temperature titanium alloys according to claim 1, characterized in that: In step three, the first tempering is carried out in the α+β two-phase region at 10-100℃ below the recrystallization temperature to eliminate residual stress from quenching. Martensite begins to decompose and form the initial lamellar α phase, while the β grains do not coarsen significantly.

5. The high-pressure heat treatment process for preparing nano-hierarchical high-temperature titanium alloys according to claim 1, characterized in that: In step four, at the initial tempering temperature, a constant axial pressure of 2 GPa or higher is applied by a forging press and held for more than 1 hour. The stress field further promotes the decomposition of martensite and the precipitation of α-lamellae, and strongly promotes the dispersion and precipitation of nano-sized silicides at the α / β phase boundary and within the lamellars.

6. The high-pressure heat treatment process for preparing nano-hierarchical high-temperature titanium alloys according to claim 1, characterized in that: In step five, the secondary tempering temperature is 50-150°C below the primary tempering temperature, and the holding time is 4-8 hours. After that, it is taken out and air-cooled to room temperature.

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