Composite ceramic shell mold for directional solidification of niobium-silicon based alloy test bars and method of making

Abstract

The application provides a composite ceramic shell for directional solidification of a niobium-silicon-based alloy test bar and a preparation method thereof, which comprises a pouring cup, a plurality of straight pipes, an outer wall of each of the straight pipes being wrapped with a ceramic shell coating, the ceramic shell coating comprising a plurality of layers of zirconium oxide powder layers and sand layers arranged at intervals, the innermost layer and the outermost layer of the ceramic shell coating being zirconium oxide powder layers, a bottom plate, and the other end of each of the straight pipes being fixed on the bottom plate, the pouring cup, the straight pipes and the bottom plate being formed by machining hexagonal boron nitride, and the application can obtain a niobium-silicon-based alloy columnar crystal test bar with extremely low thickness of a surface interface reaction layer, high surface smoothness and high dimensional accuracy, thereby meeting the evaluation and application of mechanical properties such as high-temperature tensile strength and high-temperature endurance life.

Description

Technical Field

[0001] This invention relates to the field of directional solidification technology, specifically to a composite ceramic shell for directional solidification test bars of niobium-silicon based alloys and its preparation method. Background Technology

[0002] The ultimate temperature resistance limit of nickel-based superalloys used in aero-engines is no more than 1500K, while the turbine inlet temperature of the adaptive cycle aero-engines used in sixth-generation fighter jets, which urgently need development, will reach 2400K. While technologies such as thermal barrier coatings and film cooling can improve the temperature resistance of hot-end components in aero-engines, these methods offer limited advantages. Therefore, there is an urgent need to develop new materials with a higher intrinsic temperature resistance than nickel-based superalloys. Among various refractory alloys, niobium-silicon (NbSi)-based alloys possess high melting points (≥1750℃) and low densities (≤7.2g / cm³). 3 With its significant advantages, such as a temperature resistance approximately 250°C higher than nickel-based alloys, niobium-silicon-based alloy turbine blades have attracted increasing attention and are among the most promising candidate materials for hot-end components such as blades in high thrust-to-weight ratio aero-engines. Therefore, the materials and forming technologies for next-generation high thrust-to-weight ratio aero-engine high-temperature niobium-silicon-based alloy turbine blades have become a current research hotspot.

[0003] Directional solidification is the primary technology for preparing columnar crystalline components of niobium-silicon alloys, and evaluating the mechanical properties of directionally solidified niobium-silicon alloy specimens is the main method for assessing components prepared by directional solidification. Since the directional solidification temperature of niobium-silicon alloys generally exceeds 1800℃, far exceeding the refractoriness of most oxide ceramic materials, and given the high high-temperature reactivity of the niobium-silicon alloy melt, which readily undergoes high-temperature chemical reactions with oxide ceramic materials, the preparation of the required shell shape for directionally solidified niobium-silicon alloy specimens with high surface finish and high dimensional accuracy has become one of the challenges for the practical application of directionally solidified niobium-silicon alloys.

[0004] A search revealed Chinese invention patent application CN109108224A, which discloses a ceramic shell for directional solidification investment casting of niobium-silicon alloy blades and its preparation method. The method includes: Step 1, under stirring, adding a mixture of calcium oxide-stabilized zirconium dioxide and hexagonal boron nitride powder, polyvinyl acetate emulsion, wetting agent, defoamer, dispersant, and preservative to zirconium diacetate sol, and continuing stirring to obtain a surface coating for the ceramic shell; Step 2, under stirring, adding calcium oxide-stabilized zirconium dioxide powder, powdered zirconium oxide fiber, polyvinyl acetate emulsion, defoamer, dispersant, and preservative to zirconium diacetate sol, and continuing stirring to obtain a back coating for the ceramic shell; Step 3, preparing the ceramic shell. This patent also selects hexagonal boron nitride as one of the refractory fillers in the surface slurry to adjust the interfacial wettability between the niobium-silicon alloy melt and the surface layer. However, the ceramic shell surface layer obtained by the slurry dipping and sand sprinkling process is not dense enough. After the niobium-silicon alloy melt solidifies, sand adhesion defects will still occur on the surface of the casting, resulting in high surface roughness. Furthermore, since the shell back layer described in this patent is obtained by bonding with an adhesive and has not reached the degree of sintering, the high temperature strength is generally only 5MPa, and the resistance to deformation at high temperatures is weak, which directly affects the forming and dimensional accuracy of the niobium-silicon alloy-based casting. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a composite ceramic shell for niobium-silicon based alloy directional solidification test bars and its preparation method.

[0006] According to one aspect of the present invention, a composite ceramic shell for niobium-silicon based alloy directional solidification test bars is provided, comprising:

[0007] Pouring cup;

[0008] Several straight tubes, one end of which is connected to the bottom of the pouring cup, the outer wall of each straight tube is covered with a ceramic shell coating, the ceramic shell coating includes several layers of zirconium oxide powder and sand material arranged at intervals, the innermost and outermost layers of the ceramic shell coating are both zirconium oxide powder layers;

[0009] The chassis, with the other end of the straight tube fixed to the chassis;

[0010] The pouring cup, the straight tube, and the base are all formed by machining sintered hexagonal boron nitride.

[0011] Optionally, the inner diameter of the straight tube is 8-11 mm and the outer diameter is 11-15 mm.

[0012] Optionally, one layer of the ceramic shell coating includes a zirconium oxide powder layer and a sand layer, and the 0.5 layer of the ceramic shell coating includes a zirconium oxide powder layer. The number of layers of the ceramic shell coating is 3.5-5.5.

[0013] Optionally, the bottom of the pouring cup is provided with a plurality of threaded holes with internal threads, one end of the straight tube is provided with a thread that matches the threaded holes, and the chassis is provided with a plurality of mounting holes for the straight tube to be inserted.

[0014] According to another aspect of the present invention, a method for preparing the above-mentioned composite ceramic shell for niobium-silicon based alloy directional solidification test bar is provided, the method comprising:

[0015] Ceramic powder is added to the binder and stirred evenly to obtain a slurry;

[0016] After the surface of the straight tube is roughened by grinding, it is assembled onto the pouring cup and the base plate to form a ceramic shell-shaped frame. Then, the slurry is evenly coated onto the surface of the straight tube through a slurry coating process to form a slurry coating.

[0017] By using a sand-spreading process, sand is evenly adhered to the surface of the slurry coating to form a sand layer;

[0018] Repeat the slurry coating and sand spreading process until a predetermined number of ceramic shell coating layers are formed. Each layer of slurry coating combined with each layer of sand is counted as one layer of ceramic shell coating. If there is only a slurry coating, it is counted as 0.5 layers of ceramic shell coating. The last layer of ceramic shell coating on the outside of the straight pipe is a slurry coating.

[0019] Optionally, in the repeated slurry coating and sand spreading process, the previous coating layer is fully dried before preparing the next coating layer.

[0020] Optionally, after the repeated slurry coating and sand-sprinkling process has been completed until a predetermined number of ceramic shell coating layers are formed, the process further includes: placing the composite ceramic shell in a shell firing furnace at 700-900℃ for firing.

[0021] Optionally, the method has one or more of the following options:

[0022] - The binder is any one of yttrium oxide sol, zirconium diacetate, and ammonium zirconium carbonate;

[0023] - The ceramic powder is either zirconia partially stabilized by fused calcium oxide or zirconia stabilized by fused yttrium oxide;

[0024] - The sand material is made of zirconia partially stabilized by fused calcium oxide or zirconia stabilized by fused yttrium oxide.

[0025] Preferably, the particle size of the ceramic powder is 200-325 mesh.

[0026] Preferably, the particle size of the sand is 46-100 mesh.

[0027] Compared with the prior art, the present invention has at least one of the following beneficial effects:

[0028] This invention uses hexagonal boron nitride to form the pouring cup and straight tube. The interfacial reaction layer between the niobium-silicon alloy melt and the hexagonal boron nitride is extremely thin, ensuring a high surface finish on the niobium-silicon alloy test rod. Due to the excellent thermal conductivity of hexagonal boron nitride, the rapid cooling of the molten metal at the contact point between the niobium-silicon alloy melt and the inner wall of the hexagonal boron nitride straight tube affects the formation of oriented columnar crystals. Compared with most other commonly used ceramic shell materials, such as fused silica and fused alumina, the ceramic shell coating with zirconium oxide as the main material has a lower thermal conductivity, thus effectively reducing the heat dissipation efficiency of the niobium-silicon alloy melt at the tube wall and ensuring the formation of oriented columnar crystal structure in the niobium-silicon alloy test rod. At the same time, the high-temperature bending strength of hexagonal boron nitride is generally not less than 40 MPa, which is much higher than the high-temperature strength of ceramic shells. Therefore, the thin-walled straight tube easily maintains its rigidity and will not undergo significant deformation, thereby ensuring the high dimensional accuracy of the niobium-silicon alloy test rod. This invention can produce niobium-silicon alloy columnar crystal test rods with extremely low surface interface reaction layer thickness, high surface smoothness and dimensional accuracy, thereby meeting the evaluation and application requirements of mechanical properties such as high temperature tensile strength and high temperature creep life. Attached Figure Description

[0029] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0030] Figure 1 This is a schematic diagram of the composite ceramic shell structure in one embodiment of the present invention;

[0031] Figure 2 This is a schematic diagram of the pouring cup in one embodiment of the present invention;

[0032] Figure 3 This is a schematic diagram of the chassis structure in one embodiment of the present invention;

[0033] Figure 4 This is a schematic diagram of the structure of a straight tube in one embodiment of the present invention.

[0034] In the diagram: 1 is the pouring cup, 11 is the threaded hole, 2 is the base plate, 21 is the mounting hole, and 3 is the straight pipe. Detailed Implementation

[0035] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0036] This invention provides a composite ceramic shell for niobium-silicon based alloy directional solidification test bars, referring to... Figure 1-4 The composite ceramic shell includes a pouring cup 1, several straight tubes 3, and a base plate 2. One end of the straight tube 3 is connected to the bottom of the pouring cup 1, and the other end of the straight tube 3 is fixed to the base plate 2. The outer wall of each straight tube 3 is wrapped with a ceramic shell coating. The ceramic shell coating includes several layers of zirconium oxide powder and sand material arranged at intervals. The innermost and outermost layers of the ceramic shell coating are both zirconium oxide powder layers. The zirconium oxide powder layer and sand material layer have excellent heat insulation performance. The pouring cup 1, straight tubes 3, and base plate 2 are all formed by machining sintered hexagonal boron nitride. Specifically, they are all made by machining bulk hexagonal boron nitride.

[0037] In some embodiments, the bottom of the pouring cup 1 is provided with a plurality of threaded holes 11 with internal threads. The threaded holes 11 are located between the center point and the edge of the bottom of the pouring cup 1 and are evenly distributed at the bottom of the pouring cup 1. One end of the straight tube 3 is provided with a thread that matches the threaded holes 11 of the pouring cup 1. The base plate 2 is provided with a plurality of mounting holes 21 for the straight tube 3 to be inserted, so that the pouring cup 1, the straight tube 3 and the base plate 2 can be connected to form a composite ceramic shell frame.

[0038] In some embodiments, the composite ceramic shell is assembled as follows: straight tubes 3 are connected to the pouring cup 1 by screw rotation. After all the straight tubes 3 are connected to the pouring cup 1, the lower end of the straight tube 3 is inserted into the mounting hole 21 of the base plate 2. A ceramic shell coating is prepared on the outer surface of the straight tube 3 to obtain a composite ceramic shell that can be used for calcination.

[0039] In some embodiments, the inner diameter of the straight pipe 3 is 8-11 mm, and the outer diameter is 11-15 mm. The selection of the wall thickness of the straight pipe 3 is generally based on the ability to machine an external thread with a depth of not less than 1.5 mm, so the preferred wall thickness is not less than 3 mm as a basic indicator.

[0040] In some embodiments, a layer of zirconia powder and a layer of sand are used as a ceramic shell coating layer, that is, a layer of zirconia powder is a half layer (0.5 layer) of ceramic shell coating, and the number of ceramic shell coating layers is 3.5-5.5 layers.

[0041] In some embodiments, the number of ceramic shell coating layers covering the outside of the straight tube 3 of the composite ceramic shell type with zirconium oxide as the main material is 3.5-5.5, and its thickness can generally reach 3.5-5.5 mm. This can achieve the purpose of reducing the heat dissipation rate of directional solidification of niobium-silicon alloy test rod and ensuring the formation of directional columnar crystal structure of test rod.

[0042] In some embodiments, the ceramic shell coating on the outside of the straight tube 3 of the composite ceramic shell type, which is mainly made of zirconium oxide, has 3.5-5.5 layers. The last 0.5 layer is a slurry layer. The purpose of the slurry layer is to cover the ceramic particles of the sand layer to prevent some of the large ceramic particles from peeling off.

[0043] In some embodiments, the composite ceramic shell has a strength of not less than 40 MPa to meet the requirements for directional solidification of niobium-silicon alloy test rods.

[0044] In this embodiment of the invention, hexagonal boron nitride is used to form the pouring cup and straight tube. The interfacial reaction layer between the niobium-silicon based alloy melt and the hexagonal boron nitride is extremely thin, ensuring a high surface finish on the niobium-silicon alloy test rod. The contact area between the niobium-silicon based alloy melt and the inner wall of the hexagonal boron nitride straight tube is prone to solidification, affecting the formation of oriented columnar crystals. The coating can effectively reduce the heat dissipation efficiency of the niobium-silicon alloy melt at the tube wall, ensuring the formation of oriented columnar crystal structure in the niobium-silicon alloy test rod. At the same time, the thin-walled straight tube made of hexagonal boron nitride easily maintains rigidity and will not undergo significant deformation, thereby ensuring the high dimensional accuracy of the niobium-silicon alloy test rod.

[0045] Another embodiment of the present invention provides a method for preparing the composite ceramic shell for the above-mentioned niobium-silicon based alloy directional solidification test bar, the method comprising:

[0046] S1. Add ceramic powder to the binder and stir evenly to obtain a slurry;

[0047] S2. After the surface of the straight tube is roughened by grinding, it is assembled onto the pouring cup and the base plate to form a ceramic shell-shaped frame. Then, the slurry is evenly coated onto the surface of the straight tube through the slurry coating process to form a slurry coating layer.

[0048] S3. By using a sand-spreading process, sand is evenly adhered to the surface of the slurry coating to form a sand layer;

[0049] S4. Repeat the slurry coating and sand spreading process until a predetermined number of ceramic shell coating layers are formed. Each layer of slurry coating and each layer of sand coating are combined to form one layer of ceramic shell coating. If there is only slurry coating, it is counted as 0.5 layers of ceramic shell coating. The last layer of ceramic shell coating on the outside of the straight pipe is slurry coating.

[0050] In some implementations, in step S2, the surface of the straight pipe can be roughened by sanding with 16# sandpaper to facilitate the application of slurry to the surface of the straight pipe.

[0051] In some embodiments, in step S4, the previous coating layer is thoroughly dried before preparing the next coating layer. To achieve a final slurry coating, after reaching the specified number of layers, the slurry is applied without sanding, which is counted as half a layer (slurry application and sanding constitute one layer), and then it is thoroughly dried.

[0052] In some embodiments, after repeating the slurry coating and sand-sprinkling process until a predetermined number of ceramic shell coating layers are formed, the process further includes: placing the composite ceramic shell in a shell firing furnace at 700-900°C for firing, so that the coating on the outer wall of the straight tube develops sufficient strength, thereby obtaining a composite ceramic shell that can be used for the directional solidification preparation of niobium-silicon alloy test rods.

[0053] In some embodiments, the binder is any one of yttrium oxide sol, zirconium diacetate, and ammonium zirconium carbonate. The ceramic powder is either fused calcium oxide partially stabilized zirconium oxide or fused yttrium oxide stabilized zirconium oxide, with a particle size of 200-325 mesh. The slurry coating in step S2 is the zirconium oxide powder layer. The abrasive is either fused calcium oxide partially stabilized zirconium oxide or fused yttrium oxide stabilized zirconium oxide, with a particle size of 46-100 mesh. This improves the coating quality and facilitates the acquisition of niobium-silicon alloy columnar crystal specimens with extremely low surface interface reaction layer thickness, high surface smoothness, and high dimensional accuracy.

[0054] The composite ceramic shell and preparation method for directional solidification test bars of niobium-silicon based alloys provided in this invention have the following advantages: Due to the very low wettability of hexagonal boron nitride with the alloy melt, and the absence of significant high-temperature interfacial reactions with various alloying elements under vacuum, the niobium-silicon based alloy melt, when poured into the hexagonal boron nitride pouring cup and flowing into the hexagonal boron nitride straight tube, results in a very low interfacial reaction layer thickness, ensuring a high surface finish on the test bar, even though the directional solidification preparation time is relatively long (generally exceeding 30 minutes). Furthermore, the thermal conductivity of hexagonal boron nitride... The niobium-silicon-based alloy melt exhibits excellent heat insulation properties, making it prone to solidification at the contact point with the inner wall of the hexagonal boron nitride tube. This hinders the formation of oriented columnar crystals. Therefore, the several layers of zirconia coating with excellent thermal insulation properties applied to the outer wall effectively reduce the heat dissipation efficiency of the niobium-silicon-based alloy melt at the tube wall, ensuring the formation of the oriented columnar crystal structure in the specimen. Although the hexagonal boron nitride tube has a thin wall, hexagonal boron nitride, as a non-oxide ceramic, has a temperature resistance under vacuum far exceeding 2000℃, thus maintaining its rigidity and preventing significant deformation, thereby ensuring high dimensional accuracy of the specimen. This invention is used for the directional solidification forming of niobium-silicon-based alloy specimens of various compositions, yielding niobium-silicon alloy columnar crystal specimens with extremely low surface interface reaction layer thickness, high surface smoothness, and high dimensional accuracy, meeting the evaluation requirements for mechanical properties such as high-temperature tensile strength and high-temperature creep life.

[0055] The composite ceramic shell for preparing the above-mentioned niobium-silicon based alloy directional solidification test bar and its preparation method are further explained with more specific embodiments.

[0056] Example 1

[0057] In this embodiment, the composite ceramic shell mold used for preparing the niobium-silicon-based alloy directional solidification test rod consists of three hexagonal boron nitride straight tubes with an inner diameter of 11 mm and an outer diameter of 15 mm. During the formation of the ceramic shell coating, yttrium oxide sol is used as the binder, fused yttrium oxide-stabilized zirconia powder is used as the powder, and fused yttrium oxide-stabilized zirconia sand is used as the sand. The ceramic shell coating consists of 5.5 layers, and the firing temperature is 700℃.

[0058] Three niobium-silicon alloy directionally solidified specimens with a diameter of 10.92 mm were obtained using this composite ceramic shell mold. The surface reaction layer thickness was ≤6 μm. After sandblasting, the surface roughness was 2.0 μm, and no deformation was detected. The longitudinally cut specimens did not show fine equiaxed crystals near the outer surface. In contrast, the niobium-silicon alloy specimens prepared with a hexagonal boron nitride shell mold without a shell coating showed equiaxed crystal structures with a thickness of approximately 200-500 μm near the outer surface; the specimens prepared with a high-temperature resistant zirconia shell mold showed severe sand adhesion and interface reaction defects on the surface, as well as visible deformation.

[0059] Example 2

[0060] In this embodiment, the composite ceramic shell mold used for preparing niobium-silicon-based alloy directional solidification test rods consists of four hexagonal boron nitride straight tubes with an inner diameter of 9 mm and an outer diameter of 12.5 mm. During the formation of the ceramic shell coating, zirconium diacetate is used as the binder, fused calcium oxide stabilized zirconium oxide powder is used as the powder, and fused calcium oxide stabilized zirconium oxide sand is used as the sand. The ceramic shell coating consists of 4.5 layers, and the firing temperature is 800℃.

[0061] Four niobium-silicon alloy directionally solidified specimens with a diameter of 8.94 mm were obtained using this composite ceramic shell mold. The surface reaction layer thickness was ≤4 μm. After sandblasting, the surface roughness was 2.2 μm, and no deformation was detected. The specimens were longitudinally cut, and no fine equiaxed crystals were found near the outer surface. In contrast, the niobium-silicon alloy specimens prepared with a hexagonal boron nitride shell mold without a shell coating showed equiaxed crystal structures with a thickness of approximately 200-500 μm near the outer surface; the specimens prepared with a high-temperature resistant zirconia shell mold showed severe sand adhesion and interface reaction defects on the surface, as well as visible deformation.

[0062] Example 3

[0063] In this embodiment, the composite ceramic shell used for preparing the niobium-silicon-based alloy directional solidification test rod consists of 5 hexagonal boron nitride straight tubes with an inner diameter of 8 mm and an outer diameter of 11 mm. During the formation of the ceramic shell coating, the binder is ammonium zirconium carbonate, the powder is fused calcium oxide stabilized zirconium oxide powder, and the sand is fused calcium oxide stabilized zirconium oxide sand. The number of layers is 3.5, and the calcination temperature is 900℃.

[0064] Five niobium-silicon alloy directionally solidified specimens with a diameter of 8.91 mm were obtained using this composite ceramic shell mold. The surface reaction layer thickness was ≤10 μm. After sandblasting, the surface roughness was 2.8 μm, and no deformation was detected. The longitudinally cut specimens did not show fine equiaxed crystals near the outer surface. In contrast, the niobium-silicon alloy specimens prepared with a hexagonal boron nitride shell mold without a shell coating showed equiaxed crystal structures with a thickness of approximately 200-500 μm near the outer surface; the specimens prepared with a high-temperature resistant zirconia shell mold showed severe sand adhesion and interface reaction defects on the surface, as well as visible deformation.

[0065] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention. The above preferred features can be used in any combination without conflict.

Claims

1. A composite ceramic shell for niobium-silicon based alloy directional solidification test bars, characterized in that, include: Pouring cup; Several straight tubes are connected at one end to the bottom of the pouring cup. The outer wall of each straight tube is coated with a ceramic shell coating. The ceramic shell coating includes several layers of zirconium oxide powder and sand material arranged at intervals. The innermost and outermost layers of the ceramic shell coating are both zirconium oxide powder layers. One layer of the ceramic shell coating includes one layer of zirconium oxide powder and one layer of sand material. 0.5 layers of the ceramic shell coating include one layer of zirconium oxide powder. The number of ceramic shell coating layers is 3.5-5.5 layers. The purpose of the last 0.5 layer is a slurry layer, which is to cover the ceramic particles of the sand layer to prevent some large ceramic particles from peeling off. The thickness of the ceramic shell coating with 3.5-5.5 layers reaches 3.5-5.5 mm, which can reduce the heat dissipation rate of directional solidification of the niobium-silicon alloy test rod and ensure the formation of directional columnar crystal structure of the test rod. The chassis, with the other end of the straight tube fixed to the chassis; The pouring cup, the straight tube, and the base are all formed by machining sintered hexagonal boron nitride.

2. The composite ceramic shell mold for niobium-silicon based alloy directional solidification test bars according to claim 1, characterized in that, The inner diameter of the straight pipe is 8-11 mm, and the outer diameter is 11-15 mm.

3. The composite ceramic shell mold for niobium-silicon based alloy directional solidification test bars according to claim 1, characterized in that, The bottom of the pouring cup has several threaded holes with internal threads, one end of the straight tube has a thread that matches the threaded holes, and the base has several mounting holes for inserting the straight tube.

4. A method for preparing a composite ceramic shell mold for a niobium-silicon based alloy directional solidification specimen as described in any one of claims 1-3, characterized in that, include: Ceramic powder is added to the binder and stirred evenly to obtain a slurry; After the surface of the straight tube is roughened by grinding, it is assembled onto the pouring cup and the base plate to form a ceramic shell-shaped frame. Then, the slurry is evenly coated onto the surface of the straight tube through a slurry coating process to form a slurry coating. By using a sand-spreading process, sand is evenly adhered to the surface of the slurry coating to form a sand layer; Repeat the slurry coating and sand spreading process until a predetermined number of ceramic shell coating layers are formed. Each layer of slurry coating combined with each layer of sand is counted as one layer of ceramic shell coating. If there is only a slurry coating, it is counted as 0.5 layers of ceramic shell coating. The last layer of ceramic shell coating on the outside of the straight pipe is a slurry coating.

5. The method for preparing the composite ceramic shell for niobium-silicon based alloy directional solidification specimens according to claim 4, characterized in that, The repeated slurry coating and sand spreading process includes: before preparing the next coating layer, the previous coating layer is fully dried.

6. The method for preparing the composite ceramic shell mold for niobium-silicon based alloy directional solidification specimens according to claim 4, characterized in that, The repeated slurry coating and sand-sprinkling process, after forming a predetermined number of ceramic shell coating layers, further includes: placing the composite ceramic shell in a shell firing furnace at 700-900℃ for firing.

7. The method for preparing the composite ceramic shell mold for niobium-silicon based alloy directional solidification specimens according to claim 4, characterized in that, One or more of the following options are available: - The binder is any one of yttrium oxide sol, zirconium diacetate, and ammonium zirconium carbonate; - The ceramic powder is either zirconia partially stabilized by fused calcium oxide or zirconia stabilized by fused yttrium oxide; - The sand material is made of zirconia partially stabilized by fused calcium oxide or zirconia stabilized by fused yttrium oxide.

8. The method for preparing the composite ceramic shell mold for niobium-silicon based alloy directional solidification test bar according to claim 4, characterized in that, The particle size of the ceramic powder is 200-325 mesh.

9. The method for preparing the composite ceramic shell mold for niobium-silicon based alloy directional solidification specimens according to claim 4, characterized in that, The particle size of the sand is 46-100 mesh.

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