Heat treatment method for improving structure uniformity and performance of double-state Ti2AlNb-based alloy

Abstract

The invention discloses a heat treatment method for improving structure uniformity and performance of a double-state Ti2AlNb-based alloy, the problems of poor plasticity, uneven structure, serious segregation and the like of the Ti2AlNb-based alloy are solved through heat treatment of a primary solid solution process, a secondary solid solution process and an aging process, and the structure uniformity and performance of the double-state Ti2AlNb-based alloy are improved through a heat treatment method of double-solid-solution coordinated regulation and graded aging strengthening. Matching of different mechanical properties can be achieved by regulating and controlling the size of B2 / beta matrix crystal grains and the size, morphology and volume fraction of alpha 2 and O phases, so that the alloy structure is homogenized, and the mechanical properties are remarkably improved.

Description

Technical Field

[0001] The present invention relates to the field of heat treatment technology, and specifically relates to a heat treatment method for improving the uniformity of the structure and performance of a dual-state Ti2AlNb-based alloy. Background Art

[0002] The demand for the development of high thrust-to-weight ratio aero-engines is driving the service temperature of advanced titanium alloys to expand into the high temperature range. When traditional near-α-type titanium alloys are in service for a long time at temperatures above 600°C, their oxidation resistance and creep resistance deteriorate significantly. Although Ti3Al-based alloys can achieve long-term service at 650-700°C and withstand instantaneous temperatures of 1000°C, the lack of room temperature plasticity caused by intrinsic brittleness restricts their engineering applications. By introducing an ordered O phase strengthening mechanism, Ti2AlNb-based alloys combine excellent room temperature ductility with outstanding high temperature strength. In the service window of 650-750°C, they exhibit mechanical properties comparable to those of nickel-based high-temperature alloys, while reducing their density by approximately 40%, making them highly competitive lightweight high-temperature structural materials.

[0003] The microstructural evolution and performance response of this alloy show significant process sensitivity, involving the macrosegregation of the Nb element during the casting process, the narrow window of hot working parameters, and the complexity of the heat treatment system. In particular, the large melting point difference between the Al and Nb components during the smelting process leads to compositional segregation and microstructural heterogeneity. In the microstructural control after thermomechanical processing, the heat treatment system, as a key control method, requires precise coordination of the competing mechanisms of solid solution strengthening, phase transformation kinetics, and precipitation strengthening. Although the traditional single-stage solid solution + aging treatment can achieve basic performance control, existing research lacks a systematic understanding of its microstructural evolution mechanism and performance control laws.

[0004] This invention establishes a novel triple heat treatment process system combining dual solid solution synergistic regulation and graded aging strengthening. Based on thermodynamic equilibrium phase diagram analysis and multiphase transformation kinetics, this process achieves a synergistic effect of O-phase nano-precipitation strengthening and lath microstructure optimization by staged manipulation of the B2 / O ratio, the a2 phase volume fraction, the precipitate morphology distribution, and grain boundary characteristics. This process provides a scientific basis for overcoming the inverse relationship between strength and ductility in Ti2AlNb-based alloys, significantly improving their reliability in engineering applications. Summary of the Invention

[0005] In order to solve the above problems, a heat treatment method for improving the uniformity and performance of the dual-state Ti2AlNb-based alloy is provided. This method better adapts to the characteristics of component segregation and microstructural inhomogeneity of Ti2AlNb-based alloy. Combined with a large number of heat treatment experiments, the material thermodynamics and equilibrium theory and the complex phase transformation mechanism of Ti2AlNb-based alloy are used to homogenize the alloy structure and significantly improve the mechanical properties.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a heat treatment method for improving the microstructure uniformity and performance of a dual-state Ti2AlNb-based alloy, comprising the following steps:

[0007] S1. Single solid solution: First, the annealed Ti2AlNb-based alloy is subjected to solid solution treatment. A box-type resistance furnace is used for solid solution treatment. The furnace is heated to 40-50°C below the β phase transformation point at a heating rate of 10°C / min. Then the heating rate is reduced and the temperature is slowly raised to 25-30°C below the β phase transformation point of the alloy, so that the phase region is in the a2+B2 high two-phase region, and the temperature is kept for 1-2 hours. Then the sample is cooled to room temperature by oil cooling. The cooling rate of oil cooling is faster than that of air cooling to ensure phase stability.

[0008] S2, double solid solution: The alloy obtained in S1 is subjected to solid solution treatment again. The box-type resistance furnace is used for solid solution treatment. The furnace is heated to the solid solution temperature of 950-970℃ and heated for 2-3 hours. The phase region is a2+B2+O high three-phase region. Then the sample is cooled to room temperature by air cooling.

[0009] S3. Aging process: The alloy obtained from S2 is subjected to aging treatment. A box-type resistance furnace is used for aging treatment. The furnace is heated at a heating rate of 10℃ / min. Gradual aging is adopted. First, the temperature is raised to 780℃ and kept warm for 6~8h, then the temperature is raised to 800℃ and kept warm for 6~8h, then the temperature is raised to 820℃ and kept warm for 6~8h, and then the sample is cooled to room temperature by air cooling.

[0010] Preferably, the Ti2AlNb-based alloy comprises, by mass, 9.4% to 13.2% Al, 38.2% to 46% Nb, and 0% to 1.4% Mo, with the remainder being Ti and other unavoidable impurity elements. The alloy exhibits a dual-structure structure consisting of an equiaxed a2 phase and a lath O phase. High-temperature titanium alloys with this dual-structure typically exhibit excellent strength-ductility matching.

[0011] Preferably, the temperature of the furnaces in S1, S2 and S3 is lower than 200°C, and the heating method is to heat up along with the furnace, so as to ensure that the alloy is heated sufficiently, prevent the sample from cracking, and achieve the purpose of homogenization treatment.

[0012] Preferably, the temperature in S1 is raised to 40-50°C below the β phase transformation point, and the heating rate is reduced to 5°C / min, so as to achieve sufficient stay in the high two-phase region and avoid the precipitation of continuous a2 phase at the grain boundary due to excessively fast heating rate. Solid solution is carried out at 20-30°C below the β phase transformation point. On the one hand, the a2 phase content is reduced, paving the way for the subsequent increase in the O phase ratio, but at the same time a certain proportion of the a2 phase, especially the a2 phase precipitated at the grain boundary, is retained, so that the room temperature and high temperature strength of the alloy are excellent.

[0013] Preferably, the quenching transfer time in S1 is ≤30s to ensure that the temperature deviation is small.

[0014] Preferably, the solution temperature adopted by S2 is set to 960° C., the primary O phase laths inside the B2 grains are coarsened, thereby improving the plastic deformation ability of the alloy and the a2 phase size does not change with the solution temperature.

[0015] Preferably, the temperature in S3 is raised to 780° C. and held for 6 to 8 hours to precipitate fine and dispersed secondary needle-shaped O phase to achieve the effect of dispersion strengthening. The gradual increase in aging temperature and sufficient holding time can effectively control the size of the O phase.

[0016] Preferably, the aging process adopted in S3 is selected as graded aging, first keeping the temperature at 780°C for 6 to 8 hours, then heating to 800°C for 6 to 8 hours, and then heating to 820°C for 6 to 8 hours. The purpose of the above aging treatment is to precipitate fine and dispersed secondary needle-shaped O phase from the B2 matrix, further improve the stability and strength of the alloy, especially the high-temperature strength, and achieve a good match between the strength and plasticity of the alloy.

[0017] Preferably, the temperature in S3 is raised to 780°C and kept warm for 6 to 8 hours in order to precipitate fine and dispersed secondary needle-shaped O phase from the B2 matrix to achieve the effect of dispersion strengthening. The gradual increase in aging temperature and sufficient holding time can effectively control the size of the O phase.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention proposes a heat treatment method for improving the uniformity and performance of the dual-state Ti2AlNb-based alloy structure, overcoming the problems of poor plasticity, uneven structure, severe segregation, etc. of the Ti2AlNb-based alloy. Through the heat treatment method of dual solid solution synergistic regulation + graded aging strengthening, by regulating the size of the B2 / β matrix grains and the size, morphology and volume fraction of the precipitated phase (a2 and O phase), the matching between different mechanical properties can be achieved, so that the alloy structure is homogenized, and the mechanical properties are significantly improved.

[0019] (1) The present invention adopts a furnace heating method during solution or aging, especially slowing down the heating rate during the first solution. This method ensures that the alloy component is fully heated, achieves the purpose of composition homogenization and well controls the proportion of the a2 phase.

[0020] (2) Compared with thermal deformation, subsequent heat treatment process is still one of the effective means to regulate the microstructure and mechanical properties of the alloy. The heat treatment process of dual solid solution synergistic regulation + graded aging strengthening adopted in the present invention achieves the purpose of uniform organization and improved mechanical properties. Secondly, this method is simple to operate, has low technical requirements, low cost, and low equipment requirements. The overall idea of this method also has certain guiding significance for the engineering practice of Ti2AlNb-based alloys.

[0021] (3) The heat treatment method proposed in the present invention transforms the original dual-state structure consisting of equiaxed a2 phase and lamellar O phase into a ternary structure consisting of equiaxed a2 phase, coarse lamellar O phase and fine lamellar O phase. The dispersion strengthening effect of the fine O phase and the grain boundary strengthening mechanism of the a2 phase make the room temperature and high temperature strength of the alloy excellent. The B2 phase and the coarse lamellar O phase indirectly ensure the plasticity of the alloy. The optimized alloy has strong thermal stability and creep performance, with room temperature tensile strength greater than 1100MPa, high temperature tensile strength greater than 800MPa at 750℃, high temperature endurance of 750℃ / 250MPa≥29h and other excellent mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the heat treatment system.

[0023] Figure 2 This is a 200-fold metallographic image of the original microstructure of the Ti2AlNb alloy.

[0024] Figure 3 This is a 10,000x SEM image of the original microstructure of the alloy.

[0025] Figure 4 This is a 500x SEM microstructure image of the Ti2AlNb alloy obtained by the heat treatment method in Example 1.

[0026] Figure 5 This is a 10,000x SEM microstructure image of the Ti2AlNb alloy obtained by the heat treatment method in Example 1.

[0027] Figure 6 This is a 500x SEM microstructure image of the Ti2AlNb alloy obtained by the heat treatment method in Example 2.

[0028] Figure 7 This is a 10,000x SEM microstructure image of the Ti2AlNb alloy obtained by the heat treatment method in Example 2. DETAILED DESCRIPTION

[0029] The Ti2AlNb-based alloy used in the present invention comprises 9.4% to 13.2% Al, 38.2% to 46% Nb, and 0% to 1.4% Mo by weight, with the remainder being Ti and other unavoidable impurities. The microstructure is a dual-modal structure consisting of an equiaxed a2 phase and a lath O phase. High-temperature titanium alloys with a dual-modal structure typically exhibit a good balance of strength and ductility. Metallographic measurements show that the β-phase transition point of the Ti2AlNb-based alloy is 1055°C. After the dual solution and multi-stage aging heat treatment process described in the present invention, the alloy develops a tri-modal structure with uniform structure and excellent mechanical properties.

[0030] Two bars with a diameter of 300 mm and a thickness of 40 mm were selected as samples for the following examples.

[0031] Table 1 shows the room temperature and high temperature tensile properties of the original Ti2AlNb-based alloy, as follows:

[0032]

[0033] Table 2 shows the high temperature stress rupture properties of the original Ti2AlNb-based alloy, as follows:

[0034]

[0035] In order to make the technical problems, technical solutions and advantages of the present invention more clear, they will be described in detail below with reference to specific examples, but the protection scope of the present invention is not limited to these specific examples.

[0036] Unless otherwise defined, all terms used hereinafter shall be interpreted according to the meanings commonly understood by those skilled in the art. The terms used herein are only used to describe specific embodiments and are not intended to limit the scope of protection of the present invention.

[0037] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0038] Example 1:

[0039] S1. First, the obtained Ti2AlNb-based alloy is subjected to solid solution treatment. A box-type resistance furnace is used for solid solution treatment. The furnace is charged at 25°C. The heating method is to heat with the furnace until the temperature reaches 40°C below the β phase transition point at a heating rate of 10°C / min. Then, the heating rate is reduced to 5°C / min and the temperature is slowly raised to 25°C below the β phase transition point of the alloy (i.e., reaching 1030°C) so that the phase region is in the a2+B2 high two-phase region. The temperature is kept for 2 hours, and then the sample is cooled by oil cooling until the sample is cooled to room temperature.

[0040] S2. The alloy obtained in S1 is subjected to solution treatment again. The solution treatment is carried out in a box-type resistance furnace, which is installed at 25°C and heated to the solution temperature (i.e., reaching 960°C) so that the phase region is in the a2+B2+O high three-phase region. The temperature is kept for 2 hours, and then the sample is cooled by air cooling until the sample cools to room temperature.

[0041] S3. The alloy obtained from S2 was subjected to aging treatment. A box-type resistance furnace was used for aging treatment. The furnace was charged at 100°C and the temperature was increased with the furnace at a heating rate of 10°C / min. The aging process adopted graded aging. The temperature was first increased to 780°C and kept for 6 hours, then increased to 800°C and kept for 6 hours, then increased to 820°C and kept for 6 hours, and then the sample was cooled to room temperature by air cooling.

[0042] The Ti2AlNb-based alloy obtained by the above heat treatment process has a three-state structure consisting of equiaxed a2 phase, coarse lath O phase, and fine lath O phase. The alloy has uniform structure and excellent performance, which greatly improves the problems caused by the characteristics of Ti2AlNb-based alloy.

[0043] The obtained alloy is sampled in the chord direction and subjected to at least two experimental tests according to room temperature tensile (GB / T228.1-2021), high temperature tensile (GB / T228.2-2015 and high temperature 750°C / 250MPa rupture (GB / T2039-2012).

[0044] Table 3 shows the room temperature and high temperature tensile properties of the Ti2AlNb-based alloy in Example 1, as follows:

[0045]

[0046] Table 4 shows the high temperature durability of the Ti2AlNb-based alloy in Example 1, as follows:

[0047]

[0048] Example 2:

[0049] S1 firstly carried out solution treatment on Ti2AlNb-based alloy. The solution treatment used a box-type resistance furnace, which was loaded at 200℃. The heating method was to heat up with the furnace until the temperature reached 50℃ below the β phase transformation point at a heating rate of 10℃ / min. Then the heating rate was reduced to 5℃ / min and the temperature was slowly raised to 30℃ below the β phase transformation point of the alloy (i.e. 1025℃) so that the phase region was in the a2+B2 high two-phase region. The temperature was kept for 1h, and then the sample was cooled by oil cooling until the sample cooled to room temperature.

[0050] S2: The alloy obtained in S1 is subjected to solution treatment again. The solution treatment is carried out in a box-type resistance furnace at 25°C and heated to the solution temperature (i.e., 960°C) so that the phase region is in the a2+B2+O high three-phase region. The temperature is kept for 3 hours, and then the sample is cooled by air cooling until the sample is cooled to room temperature.

[0051] S3: The alloy obtained from S2 is subjected to aging treatment. A box-type resistance furnace is used for aging treatment. The furnace is charged at 25°C and the temperature is increased with the furnace at a heating rate of 10°C / min. The aging process is selected as step aging. The temperature is first kept at 780°C for 8 hours, then raised to 800°C for 8 hours, and then raised to 820°C for 8 hours. The sample is cooled to room temperature.

[0052] The Ti2AlNb-based alloy obtained by the above heat treatment process has a three-state structure consisting of equiaxed a2 phase, coarse lath O phase, and fine lath O phase. The alloy has uniform structure and excellent performance, which greatly improves the problems caused by the characteristics of Ti2AlNb-based alloy.

[0053] The obtained alloy is sampled in the chord direction and subjected to at least two experimental tests according to room temperature tensile (GB / T228.1-2021), high temperature tensile (GB / T228.2-2015 and high temperature 750°C / 250MPa rupture (GB / T2039-2012).

[0054] Table 5 shows the room temperature and high temperature tensile properties of the Ti2AlNb-based alloy in Example 2, as follows:

[0055]

[0056]

[0057] Table 6 shows the high temperature stress-cracking performance of the Ti2AlNb-based alloy in Example 2, as follows:

[0058]

[0059] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A heat treatment method for improving the uniformity and performance of a dual-state Ti2AlNb-based alloy, characterized in that: The following steps are involved: S1. Single solid solution: First, the annealed Ti2AlNb-based alloy is subjected to solid solution treatment. A box-type resistance furnace is used for solid solution treatment. The furnace is heated to 40-50°C below the β phase transformation point at a heating rate of 10°C / min. Then the heating rate is reduced and the temperature is slowly raised to 25-30°C below the β phase transformation point of the alloy, so that the phase region is in the a2+B2 high two-phase region. The temperature is kept for 1-2 hours, and then the sample is cooled to room temperature by oil cooling. S2, double solid solution: The alloy obtained in S1 is subjected to solid solution treatment again. The box-type resistance furnace is used for solid solution treatment. The furnace is heated to the solid solution temperature of 950-970℃. The phase region is a2+B2+O high three-phase region, and the temperature is kept for 2-3 hours. Then the sample is cooled to room temperature by air cooling. S3. Aging process: The alloy obtained from S2 is subjected to aging treatment. A box-type resistance furnace is used for aging treatment. The furnace is heated at a heating rate of 10℃ / min. Gradual aging is adopted. First, the temperature is raised to 780℃ and kept at this temperature for 6~8h, then the temperature is raised to 800℃ and kept at this temperature for 6~8h, and then the temperature is raised to 820℃ and kept at this temperature for 6~8h. Then the sample is cooled to room temperature by air cooling.

2. The heat treatment method according to claim 1, wherein: The mass percentages of the Ti2AlNb-based alloy are: Al: 9.4% to 13.2%, Nb: 38.2% to 46%, MO: 0% to 1.4%, and the remainder is Ti and other inevitable impurity elements. The structure is a dual-state structure consisting of an equiaxed a2 phase and a thin lath O phase.

3. The heat treatment method according to claim 1, characterized in that The temperature of the furnaces in S1, S2 and S3 is all below 200°C, and the heating method is to heat up along with the furnace to ensure that the alloy is fully heated to achieve the purpose of homogenization treatment.

4. The heat treatment method according to claim 1, characterized in that: In S1, the temperature is raised to 40-50°C below the β phase transformation point, and the heating rate is reduced to 5°C / min, so as to achieve sufficient stay in the high two-phase region and avoid the precipitation of continuous a2 phase at the grain boundary due to excessively fast heating rate. Solid solution is carried out at 20-30°C below the β phase transformation point. On the one hand, the a2 phase content is reduced, paving the way for the subsequent increase in the O phase ratio, but at the same time a certain proportion of a2 phase, especially the a2 phase precipitated at the grain boundary, is retained, which makes the alloy have excellent room temperature and high temperature strength.

5. The heat treatment method according to claim 1, characterized in that The quenching transfer time in S1 is ≤30s to ensure that its temperature deviation is small.

6. The heat treatment method according to claim 1, characterized in that The solution temperature in S2 is set to 960℃, and the primary O phase laths inside the B2 grains are coarsened, thereby improving the plastic deformation ability of the alloy, and the a2 phase size does not change with the solution temperature.

7. The heat treatment method according to claim 1, characterized in that The temperature in S3 is raised to 780℃ and kept for 6 to 8 hours to precipitate fine and dispersed secondary needle-shaped O phase from the B2 matrix to achieve the effect of dispersion strengthening. The gradual increase of aging temperature and sufficient holding time can effectively control the size of the O phase.

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