A method for refining carbides in cast TiAl alloys

Abstract

This invention belongs to the field of heat treatment of TiAl alloys, specifically relating to a method for refining carbides in cast TiAl alloys. The method for refining carbides in cast TiAl alloys includes the following steps: (1) subjecting the as-cast TiAl alloy to high-temperature remelting treatment; the high-temperature remelting treatment temperature is 1350–1500℃, followed by furnace cooling to obtain a remelted TiAl alloy; (2) subjecting the remelted TiAl alloy to medium-temperature cyclic annealing, wherein the medium-temperature cyclic annealing treatment is performed 1–10 times; the medium-temperature annealing treatment temperature is 1000–1300℃, followed by furnace cooling or air cooling. After treatment by this invention, the micron-sized primary coarse carbides in the cast alloy are refined into high-density nano-sized secondary carbides, reducing the carbide size by 94%, and the secondary carbides are thermodynamically more stable Ti2AlC.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment of TiAl alloys, and specifically relates to a method for refining carbides in cast TiAl alloys. Background Technology

[0002] TiAl intermetallic compound alloys are a new type of lightweight high-temperature structural material. Due to their low density, high melting point, high specific strength and specific modulus, as well as their excellent physical and mechanical properties such as high resistance to oxidation and creep at high temperatures, they are considered to be the most promising material to replace nickel-based superalloys in the temperature range of 650℃ to 800℃ for the production of low-pressure turbine blades.

[0003] High-Nb-TiAl alloys are TiAl alloys with higher operating temperatures, exhibiting high strength and low creep rate, with potential operating temperatures reaching 850-900℃. Solid solution and precipitation strengthening resulting from the addition of carbon (C) significantly improve the high-temperature strength and high-temperature creep resistance of high-Nb-TiAl alloys. Industrially, TiAl alloys are primarily produced through casting, which is the most economical method. Using this method, the amount of C is generally controlled within the alloy's solid solubility limit. This only adds trace amounts of C to provide limited solid solution strengthening. Second-phase precipitation strengthening is another effective method for metal strengthening. In as-cast TiAl alloys, when the C content slightly exceeds the alloy's solid solution limit, carbides precipitate during solidification. However, under the high-temperature conditions of liquid-solid coexistence, C atoms accumulate at the solidification front of the alloy, and the resulting carbides grow rapidly, leading to relatively coarse primary carbides. Coarse carbides formed during casting are detrimental to the improvement of the alloy's mechanical properties; extremely coarse carbides can even destroy the alloy's mechanical properties. Numerous studies have shown that high-density, fine-grained precipitates in alloys often have a positive strengthening effect on their properties. Therefore, it is necessary to obtain high-density, fine-grained carbide precipitates in the as-cast microstructure of high-Nb-TiAl alloys to achieve strengthening through a combination of solid solution and precipitate precipitation.

[0004] Chinese invention patent application CN114921735A discloses a thermal control method for improving the mechanical properties of high Nb-TiAl alloys for casting. The method involves determining the α-phase transformation temperature, performing hot isostatic pressing at 10℃~30℃ below the α-phase transformation temperature and at a pressure of 140MPa~180MPa, followed by aging heat treatment at 750℃~950℃, and furnace cooling to obtain carbide precipitates, thereby strengthening the high Nb-TiAl alloy.

[0005] However, the above methods for controlling the precipitation of strengthening carbides rely on expensive hot isostatic pressing equipment. The low C content in the alloy and the absence of carbides in the initial casting microstructure result in a small amount of carbides precipitated during subsequent aging treatment, limiting the strengthening effect on the alloy. High-density, uniformly fine carbides are key factors for further improving the alloy's high-temperature strength and creep resistance, but currently, heat treatment technologies for eliminating primary coarse carbides and precipitating high-density secondary fine carbides in cast TiAl alloys have not yet been developed. Summary of the Invention

[0006] The purpose of this invention is to provide a method for refining carbides in cast TiAl alloys, providing an achievable heat treatment technology for eliminating primary coarse carbides and precipitating high-density secondary fine carbides in cast TiAl alloys, and effectively controlling the size, distribution and content of carbides in cast alloys.

[0007] To achieve the above objectives, the technical solution adopted by this invention is as follows:

[0008] A method for refining carbides in a cast TiAl alloy, wherein the TiAl alloy has the following atomic percentage composition: Al 42-49%, Nb 2-9%, C 0.2-1.5%, with the remainder being Ti; the refining method includes the following steps:

[0009] (1) The as-cast TiAl alloy is subjected to high-temperature remelting treatment; the temperature of the high-temperature remelting treatment is 1350~1500℃, the time is 5~120min, and the furnace is cooled to obtain the remelted TiAl alloy.

[0010] (2) The TiAl alloy in the remelted state is subjected to medium-temperature cyclic annealing, wherein the medium-temperature cyclic annealing is performed 1 to 10 times; the temperature of the medium-temperature annealing is 1000 to 1300℃, the time is 10 min to 48 h, and the furnace is cooled or the air is cooled.

[0011] The method for refining carbides in cast TiAl alloys of the present invention involves dissolving primary carbides at high temperature followed by precipitation of secondary carbides through medium-temperature cyclic annealing. The high-density secondary carbides formed after several medium-temperature cyclic annealing cycles are small in size and regularly distributed, and the content of secondary precipitated carbides is controllable, which can effectively eliminate the adverse effects of primary carbides precipitated during solidification on material properties.

[0012] After treatment with this invention, the micron-sized primary coarse carbides in the cast alloy are refined into high-density nano-sized secondary carbides, reducing the carbide size by 94%. The secondary carbides are distributed at the lamellar interface, the γ-γ lamellar interface, and the α2-γ lamellar interface, and are thermodynamically more stable Ti2AlC. Furthermore, this method is characterized by simple process steps and low equipment requirements, making it highly valuable for application.

[0013] To more effectively control the size, distribution, and content of carbides, preferably, the size and distribution of carbides are controlled by adjusting the temperature, time, cooling method, and / or number of cycles of the intermediate-temperature annealing treatment. More preferably, the size of the carbides is reduced and the distribution of the carbides is optimized by increasing the number of cycles.

[0014] The following describes typical methods for controlling carbide size and distribution:

[0015] Preferably, the temperature of the medium-temperature annealing treatment is 1250-1300℃, the time is 10 min-24 h, the furnace cooling or air cooling is used, and the number of cycles is 1-4.

[0016] Preferably, the medium-temperature annealing treatment is performed at a temperature of 1250–1300°C for 10 min–6 h, followed by furnace cooling, and the cycle is repeated 4 times. Alternatively, the medium-temperature annealing treatment is performed at a temperature of 1250–1300°C for 30 min–1 h, followed by air cooling, and the cycle is repeated 4 times.

[0017] Preferably, the high-temperature remelting treatment is performed at a temperature of 1380–1410°C for 1–2 hours, followed by furnace cooling.

[0018] More preferably, the atomic percentage composition of the TiAl alloy is: Al 45-46%, Nb 7-8%, C 0.8-1.2%, with the remainder being Ti. The TiAl alloy using the above formulation has a simple composition and low production cost.

[0019] Because of the above-mentioned technical means, the invention has the following advantages:

[0020] (1) Low equipment requirements; conventional muffle furnaces can meet the heat treatment needs.

[0021] (2) This process can eliminate the primary coarse and disorderly distributed carbides precipitated during the casting solidification process and precipitate high-density fine and uniform secondary carbides by adjusting the temperature, time, number of cycles and cooling method of heat treatment. The size of the carbides is reduced by 94%, and the secondary carbides are Ti2AlC, which is thermodynamically more stable.

[0022] (3) The process is simple, safe and efficient. Attached Figure Description

[0023] Figure 1 The microstructure diagrams show the morphology, size, and distribution of carbides in the cast TiAl alloys of Examples 1-4 of this invention; wherein: Figure 1 (a) is a diagram of the as-cast microstructure of the TiAl alloy containing coarse carbides in Example 1 of the present invention; Figure 1(b) is a solid solution microstructure of the TiAl alloy in Example 1 of the present invention (after furnace cooling at 1380°C for 2 hours); Figure 1 (c) is a microstructure diagram of the TiAl alloy of Example 1 of the present invention (after furnace cooling at 1250°C for 24 hours without cyclic annealing); Figure 1 (d) is a microstructure diagram of the TiAl alloy of Example 2 (after furnace cooling at 1250°C for 6 hours and 4 cycles of annealing); Figure 1 (e) is a microstructure diagram of the TiAl alloy of Example 3 of the present invention (after furnace cooling at 1250°C for 1 hour and 4 cycles of annealing); Figure 1 (f) is a microstructure diagram of the TiAl alloy of Example 4 of the present invention (after annealing at 1250°C for 30 min, air cooling, and 4 cycles).

[0024] Figure 2 This is a comparison diagram of the size and morphology of secondary carbides and primary carbides in the cast TiAl alloy in Example 4; wherein, Figure 2 (a) is a comparison chart of carbide sizes; Figure 2 (b) is a diagram of the coarse carbide morphology in the as-cast structure of the TiAl alloy in Example 1 of the present invention; Figure 2 (c) is a diagram of the morphology of fine secondary carbides in Example 4 of the present invention;

[0025] Figure 3 The diffraction pattern showing the distribution and phase determination of the secondary carbides in Example 4; Figure 3 (a) is a diagram of secondary carbides distributed at the lamellar cluster interface; Figure 3 (b) is a diagram of secondary carbides distributed at the interface between γ and γ lamellae; Figure 3 (c) is a diagram of secondary carbides distributed at the interface between α2 and γ sheets; Figure 3 (d) is the diffraction pattern of the secondary carbide Ti2AlC. Detailed Implementation

[0026] To address the problem of coarse and disordered primary carbides in the as-cast microstructure of TiAl alloys, which severely degrades material properties, this invention provides the following method for refining carbides in cast TiAl alloys. The atomic percentage composition of the TiAl alloy is: Al 42-49%, Nb 2-9%, C 0.2-1.2%, with the remainder being Ti. This method further ensures the amount of carbide precipitation, thereby optimizing the precipitation strengthening effect. Preferably, the C content in the TiAl alloy is 0.8-1.2%, more preferably 0.9-1.2%.

[0027] The method for refining carbides in cast TiAl alloys includes the following steps:

[0028] Step 1: Preparation of conventionally cast TiAl alloy ingots.

[0029] The conventional cast TiAl alloy ingot (with coarse and randomly distributed carbides) was wire-cut into 10*10*10mm samples, cleaned and dried, and then placed in an alumina ceramic crucible for later use.

[0030] Step 2: High-temperature remelting of conventionally cast TiAl alloy ingots.

[0031] The sample placed in an alumina ceramic crucible was subjected to high-temperature remelting treatment in a muffle furnace; the high-temperature remelting treatment was performed at a temperature of 1350-1500℃ for 5-120 min, followed by furnace cooling, to obtain the remelted TiAl alloy.

[0032] Step 3: The dissolved TiAl alloy is subjected to medium-temperature cyclic annealing, resulting in the secondary precipitation of fine and regularly distributed carbides.

[0033] The sample, after being remelted at high temperature in an alumina ceramic crucible, is subjected to medium-temperature cyclic annealing in a muffle furnace. The medium-temperature cyclic annealing involves performing the medium-temperature annealing treatment 1 to 10 times; the temperature of the medium-temperature annealing treatment is 1000 to 1300°C, and the time is 10 min to 48 h, followed by furnace cooling or air cooling.

[0034] Step 4: After heat treatment, the sample is taken out, the wire-cut section is ground and electrolytically polished, and then the morphology, size, distribution and content of carbides are observed by SEM and the phase is determined.

[0035] The above-mentioned refining method involves resolving the primary carbide at high temperature, followed by precipitating high-density, fine secondary carbides through medium-temperature cyclic annealing. After several cycles of medium-temperature cyclic annealing, the secondary carbides are small in size and regularly distributed, and the content of the secondary precipitated carbides is controllable. This effectively eliminates the deterioration of the properties caused by the primary carbides precipitated during solidification and provides precipitation strengthening effect to the alloy.

[0036] The implementation process of the present invention will be described in detail below with reference to specific embodiments.

[0037] I. Specific Embodiments of the Method for Refining Carbides in Cast TiAl Alloys of the Present Invention

[0038] Example 1

[0039] The method for refining carbides in the cast TiAl alloy of this embodiment includes the following steps:

[0040] (1) The atomic percentage composition of the carbon-containing TiAl alloy ingot is: Al 45.6%, Nb 7.8%, C 0.9%, with the remainder being Ti, i.e., Ti45.6Al7.8Nb0.9C. Conventionally cast carbon-containing TiAl alloy ingots were wire-cut into 10*10*10mm samples, cleaned and dried, and then placed in an alumina ceramic crucible for later use. At this time, the material is in the as-cast state, and the microstructure is shown in the diagram below. Figure 1 As shown in a.

[0041] (2) The sample placed in the alumina ceramic crucible underwent high-temperature resolution treatment in a muffle furnace. The high-temperature resolution parameters were: temperature 1380℃, time 2h, and furnace cooling. At this time, the material was in a solid solution state, and the microstructure was as follows: Figure 1 As shown in b.

[0042] (3) The sample, after high-temperature remelting in an alumina ceramic crucible, was subjected to medium-temperature cyclic annealing in a muffle furnace. The parameters for medium-temperature cyclic annealing were: temperature 1250℃, time 24h, furnace cooling, and no circulation. At this time, the microstructure of the material is shown in the figure. Figure 1 As shown in c.

[0043] (4) After the control is completed, the sample is taken out, the wire-cut section is ground and electrolytically polished, and the carbides are observed.

[0044] Example 2

[0045] The method for refining carbides in the cast TiAl alloy in this embodiment is basically the same as the method in Example 1, with the differences explained below:

[0046] In step (3), the parameters for medium-temperature cyclic annealing are: temperature 1250℃, time 6h, furnace cooling, and 4 cycles, meaning a total of 4 cycles of medium-temperature annealing at 1250℃ for 6h and furnace cooling are performed. At this point, the microstructure of the material is as follows: Figure 1 As shown in d.

[0047] Example 3

[0048] The method for refining carbides in the cast TiAl alloy in this embodiment is basically the same as the method in Example 1, with the differences explained below:

[0049] In step (3), the parameters for the medium-temperature cyclic annealing are: temperature 1250℃, time 1h, furnace cooling, and 4 cycles, meaning a total of 4 cycles of medium-temperature annealing at 1250℃ for 1h and furnace cooling are performed. At this point, the microstructure of the material is as follows: Figure 1 As shown in e.

[0050] Example 4

[0051] The method for refining carbides in the cast TiAl alloy in this embodiment is basically the same as the method in Example 1, with the differences explained below:

[0052] In step (2), the parameters for high-temperature remelting treatment are: temperature 1410℃, time 1h, and furnace cooling.

[0053] In step (3), the parameters for the medium-temperature cyclic annealing are: temperature 1250℃, time 30min, air cooling, and 4 cycles, meaning a total of 4 cycles of medium-temperature annealing at 1250℃ for 30min and air cooling are performed. At this point, the microstructure of the material is as follows: Figure 1 As shown in figure f, a comparison diagram of the size of fine secondary carbides and coarse carbides in the casting structure is shown. Figure 2 The distribution and phase determination of secondary carbides, such as Figure 3 .

[0054] Figure 1 In (a) to (f), the controllable content of carbides is manifested as an increase in the number of carbides and a finer size. Figures 2-3 It can be seen that by using the method of Example 4, the micron-sized primary coarse carbides in the cast alloy can be refined into high-density nano-sized secondary carbides, and the size of the carbides is reduced by 94%. The secondary carbides are distributed at the lamellar interface, the γ-γ lamellar interface, and the α2-γ lamellar interface, and the secondary carbides are the thermodynamically more stable Ti2AlC.

Claims

1. A method for refining carbides in cast TiAl alloys, characterized in that, The TiAl alloy has the following atomic percentage composition: Al 42-49%, Nb 2-9%, C 0.2-1.5%, with the remainder being Ti; the refining method includes the following steps: (1) The as-cast TiAl alloy is subjected to high-temperature remelting treatment; the temperature of the high-temperature remelting treatment is 1350~1500℃, the time is 5~120min, and the furnace is cooled to obtain the remelted TiAl alloy. (2) The TiAl alloy in the remelted state is subjected to medium-temperature cyclic annealing, wherein the medium-temperature cyclic annealing is performed 4 to 10 times in a cycle; the temperature of the medium-temperature annealing is 1250 to 1300°C, the time is 10 min to 48 h, and the furnace is cooled or the air is cooled. The primary carbide is dissolved at high temperature, and then the secondary carbide Ti2AlC is precipitated by medium-temperature cyclic annealing.

2. The method for refining carbides in cast TiAl alloys as described in claim 1, characterized in that, The size and distribution of carbides can be controlled by adjusting the temperature, time, cooling method, and / or number of cycles of the intermediate-temperature annealing treatment.

3. The method for refining carbides in cast TiAl alloys as described in claim 2, characterized in that, By increasing the number of cycles, the size of the carbides is reduced, and the distribution of the carbides is optimized.

4. The method for refining carbides in cast TiAl alloys as described in claim 1, characterized in that, The medium-temperature annealing process takes 10 minutes to 24 hours and is carried out by furnace cooling or air cooling.

5. The method for refining carbides in cast TiAl alloys as described in claim 4, characterized in that, The medium-temperature annealing process takes 10 minutes to 6 hours, followed by furnace cooling, and is repeated 4 times.

6. The method for refining carbides in cast TiAl alloys as described in claim 4, characterized in that, The medium-temperature annealing process takes 30 minutes to 1 hour, followed by air cooling, and is repeated 4 times.

7. The method for refining carbides in cast TiAl alloys as described in any one of claims 1 to 6, characterized in that, The high-temperature remelting treatment is performed at a temperature of 1380–1410°C for 1–2 hours, followed by furnace cooling.

8. The method for refining carbides in cast TiAl alloys as described in claim 7, characterized in that, The atomic percentage composition of the TiAl alloy is: Al 45-46%, Nb 7-8%, C 0.8-1.2%, with the remainder being Ti.

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