Method for improving processability of gadolinium-rich nickel-based alloy and modified gadolinium-rich nickel-based alloy

Abstract

The application discloses a method for improving the processing performance of a gadolinium-rich nickel-based alloy. The gadolinium-rich nickel-based alloy comprises an austenitic matrix and a second-phase compound Ni5Gd distributed along the interdendritic of the austenitic matrix. The processing performance of the gadolinium-rich nickel-based alloy is improved by using the following method: the gadolinium-rich nickel-based alloy is subjected to an oxidation treatment, so that part of the second-phase compound Ni5Gd in the gadolinium-rich nickel-based alloy is converted into gadolinium oxide. The application also discloses a modified gadolinium-rich nickel-based alloy. Compared with the prior art, the gadolinium-rich nickel-based alloy is modified by a simple and easy oxidation process, so that part of the second-phase compound Ni5Gd in the gadolinium-rich nickel-based alloy is converted into finer gadolinium oxide, thereby effectively inhibiting the generation of processing cracks and greatly improving the fracture elongation of the material, and providing an effective and low-cost way for the industrial application of the gadolinium-rich nickel-based alloy.

Description

Technical Field

[0001] This invention relates to an alloy modification method, specifically to a method for improving the processing performance of gadolinium-rich nickel-based alloys. Background Technology

[0002] With the rapid development of the world economy, efficiently addressing energy demand, reducing greenhouse gas emissions, and mitigating air pollution are of paramount importance. Nuclear energy, as a clean, efficient, and stable energy source, has become a strategic priority in the long-term energy planning of various countries. However, the continuous accumulation of spent fuel may become one of the key factors restricting the sustainable development of nuclear power. The closed-loop cycle of spent fuel requires reprocessing to recover usable elements. This inevitably increases the processes involved in spent fuel disposal, transfer, processing, and storage. Given the special properties of spent fuel, it is crucial to prevent it from reaching a subcritical state during transportation or storage. Neutron-absorbing materials are key materials for constructing spent fuel storage and transportation baskets and partitions. Currently, spent fuel containers typically have a shell 12-38 cm thick, composed of materials such as steel and concrete. With this material and thickness, the full load weight is around 150 tons, but it can only carry 20 tons of spent fuel. Therefore, accelerating the research and development of structurally functional integrated materials is particularly important for the lightweighting of spent fuel storage and transportation containers.

[0003] Currently, the main neutron absorbing materials include concrete, polymer composites, B4C / Al-based composites, and boron-containing stainless steel. Concrete materials were among the earliest used spent fuel storage containers and have seen good development, with relatively low prices and good shielding effects. However, their large size and weight make transportation very inconvenient, greatly limiting their future prospects. Lead-boron polyethylene (B4C / Al) is composed of B4C, lead, and polyethylene, but this material has poor mechanical properties and aging resistance, and is not fire-resistant, which limits its application to some extent. B4C / Al-based composites are a research hotspot. Researchers have used stir casting technology to prepare Al-B4C metal, but its tensile strength and elongation are poor, and it easily produces harmful second phases such as Al3BC, Al4C3, AlB2, and AlB. 12 C2; Boron-containing austenitic stainless steel has excellent thermal neutron attenuation capabilities, but the reaction of neutrons with boron produces helium, which leads to irradiation expansion and deterioration of mechanical properties. Due to the significant differences in physical and mechanical properties, the effects of non-metallic fillers on non-metallic substrates and non-metallic fillers on metallic substrates are not ideal.

[0004] Gadolinium has attracted considerable attention from scholars both domestically and internationally in recent years due to its large equivalent thermal neutron absorption cross section, which is dozens of times larger than that of boron. It also exhibits thermal neutron radiation stability and good thermal stability. Robino et al. studied 316 stainless steel with different gadolinium contents, while Ha and Kim et al. investigated the corrosion resistance and mechanical properties of gadolinium-rich 304L alloys. However, because gadolinium is also insoluble in iron-based austenitic materials such as 304 and 316 stainless steels, it forms as a low-melting-point compound (Fe, Cr, Ni) 3Gd, with a melting point around 1060℃. This widens the solidification temperature range and reduces the hot working window, making the material difficult to process and prone to solidification cracking during welding. Therefore, it is difficult to fabricate large components that meet thermal neutron absorption requirements. To address the issue of low-melting-point second phases, researchers have proposed adding gadolinium to nickel-based superalloys to obtain gadolinium-rich nickel-based alloys. For example, in 2005, the Idaho National Laboratory, in collaboration with Lehigh University, developed various gadolinium-rich nickel-based alloys with Ni-Mo-Cr as the matrix; in 2022, Shanghai University developed gadolinium-rich nickel-based alloys with different gadolinium contents based on Ni-Cr-Fe and Ni-Cr-W, as well as gadolinium-rich iron-nickel-based alloys based on Ni-Cr-Mo-Fe. In these gadolinium-rich nickel-based alloys, Gd is also insoluble in the matrix, but exists as the high-melting-point compound Ni5Gd and is distributed along the dendrites of the austenite matrix. Its eutectic temperature is around 1260℃, which greatly reduces the solidification temperature range and expands the hot working window, making it very advantageous for hot working.

[0005] However, existing gadolinium-rich nickel-based alloys still suffer from severe cracking problems during hot working and welding, primarily due to the presence of coarse and brittle second phase (Ni5Gd). This makes it difficult to process large plates and causes solidification cracking during hot working and welding, seriously hindering the industrial application of gadolinium-rich nickel-based alloys as neutron absorbers. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a method for improving the processing performance of gadolinium-rich nickel-based alloys. The method modifies the gadolinium-rich nickel-based alloys by oxidation to suppress the generation of processing cracks and improve the fracture elongation of the material.

[0007] The present invention specifically adopts the following technical solutions to solve the above-mentioned technical problems:

[0008] A method for improving the processing properties of a gadolinium-rich nickel-based alloy, wherein the gadolinium-rich nickel-based alloy comprises an austenitic matrix and a second-phase compound Ni5Gd distributed along the dendrites of the austenitic matrix; the processing properties of the gadolinium-rich nickel-based alloy are improved by oxidizing the gadolinium-rich nickel-based alloy, thereby converting a portion of the second-phase compound Ni5Gd into gadolinium oxide.

[0009] Preferably, the oxidation treatment is performed by placing the gadolinium-rich nickel-based alloy to be treated in an atmosphere with an oxygen content not exceeding an oxygen content threshold, and holding it at a temperature below 1260°C for a period of time; the oxygen content threshold is the oxygen content required to convert all the second-phase compounds Ni5Gd in the gadolinium-rich nickel-based alloy to be treated into gadolinium oxide.

[0010] More preferably, the temperature is maintained at 1000℃~1250℃.

[0011] Preferably, after the oxidation treatment, the oxidation depth of the gadolinium-rich nickel-based alloy along the Ni5Gd distribution direction is 100μm to 500μm.

[0012] Based on the same inventive concept, the following technical solutions can also be obtained:

[0013] A modified gadolinium-rich nickel-based alloy is obtained by treating the gadolinium-rich nickel-based alloy using the processing performance improvement method described in any of the above technical solutions, wherein a portion of the second-phase compound Ni5Gd in the gadolinium-rich nickel-based alloy is converted into gadolinium oxide.

[0014] Compared with the prior art, the present invention has the following technical effects:

[0015] This invention modifies gadolinium-rich nickel-based alloys through a simple and easy oxidation process, which transforms some of the second-phase compound Ni5Gd into finer gadolinium oxides. This effectively suppresses the generation of processing cracks and significantly improves the fracture elongation of the material, providing a low-cost and effective way for the industrial application of gadolinium-rich nickel-based alloys.

[0016] By using the scheme of this invention to modify gadolinium-rich nickel-based alloys, the material's processing performance is greatly improved while its thermal neutron absorption capacity is not affected. On the contrary, due to the increased elongation of the material, it is possible to consider adding more Gd elements to further enhance the thermal neutron absorption capacity. Attached Figure Description

[0017] Figure 1 Microscopic images of the cross-sections of gadolinium-rich nickel-based alloy (Hastelloy N+Gd) tensile samples after being subjected to different oxidation treatments and then fractured at room temperature; where (a), (b), and (c) are samples subjected to oxidation treatment at 1250℃ / 24h, 1250℃ / 48h, and 1250℃ / 72h, respectively.

[0018] Figure 2This is a comparison of the cross-sectional cracking of gadolinium-rich nickel-based alloy (Hastelloy N+Gd) tensile samples after room temperature fracture, with (a) showing the cracking of the tensile sample after oxidation treatment at 1250℃ / 24h, and (b) showing the cracking of the tensile sample after unoxidized treatment.

[0019] Figure 3 Comparison of room temperature fracture elongation after different oxidation treatments for tensile samples of gadolinium-rich nickel-based alloy (Hastelloy N+Gd);

[0020] Figure 4 Microscopic images of the cross sections of gadolinium-nickel-based alloy (ASTM B932-04 alloy) tensile samples after being subjected to different oxidation treatments and then fractured at room temperature; where (a), (b), and (c) are samples subjected to oxidation treatment at 1250℃ / 24h, 1250℃ / 48h, and 1250℃ / 72h, respectively.

[0021] Figure 5 This is a comparison of the room temperature elongation at break of tensile samples of gadolinium-rich nickel-based alloy (ASTM B932-04 alloy) after different oxidation treatment processes. Detailed Implementation

[0022] Existing gadolinium-rich nickel-based superalloys are typically obtained by adding less than 10% Gd to conventional nickel-based superalloys. These superalloys primarily consist of an austenitic matrix and a second-phase compound, Ni5Gd, distributed along the dendrites of the austenitic matrix. Taking the Hastelloy C-4+Gd superalloy based on Hastelloy C-4 as an example, 2% Gd is added. The main function of Gd is to enhance the alloy's thermal neutron absorption capacity. However, Gd is insoluble in the austenitic matrix and exists as a coarse and brittle second-phase Ni5Gd. Due to the presence of Ni5Gd, cracks preferentially initiate in Ni5Gd, leading to severe cracking during forging, rolling, and welding of gadolinium-rich nickel-based superalloys.

[0023] During their research on the cracking mechanism of gadolinium-rich nickel-based superalloys during forging, rolling, and welding, the inventors accidentally discovered that Ni5Gd is preferentially oxidized before the matrix under oxidizing conditions. The coarse surface Ni5Gd is oxidized into a finer phase. Since gadolinium oxide has a much higher melting point than the alloy, this helps to solve the solidification cracking phenomenon caused by the lower-melting-point Ni5Gd. Furthermore, because Ni5Gd is often the origin of cracks, its finer size after transformation into Gd oxide optimizes the material's mechanical properties. Further research showed that the elongation of the tensile sample after oxidation treatment was more than doubled compared to the unoxidized tensile sample. Based on this discovery, the inventors proposed the following technical solution:

[0024] A method for improving the processing performance of a gadolinium-rich nickel-based alloy, wherein the gadolinium-rich nickel-based alloy comprises an austenitic matrix and a second-phase compound Ni5Gd distributed along the dendrites of the austenitic matrix; the processing performance of the gadolinium-rich nickel-based alloy is improved by oxidizing the gadolinium-rich nickel-based alloy, thereby converting a portion of the second-phase compound Ni5Gd into gadolinium oxide.

[0025] Since the second-phase compound Ni5Gd preferentially oxidizes in an oxidizing environment compared to the matrix, the oxygen content in the environment can be controlled to convert some of the second-phase compound Ni5Gd in the gadolinium-rich nickel-based alloy into gadolinium oxide, while the austenitic matrix remains unoxidized. Furthermore, the eutectic temperature of the austenitic matrix and Ni5Gd is 1260℃. Based on these considerations, the following preferred technical solution can be obtained:

[0026] The oxidation treatment is performed by placing the gadolinium-rich nickel-based alloy to be treated in an atmosphere with an oxygen content not exceeding an oxygen content threshold, and holding it at a temperature below 1260°C for a period of time; the oxygen content threshold is the oxygen content required to convert all the second-phase compounds Ni5Gd in the gadolinium-rich nickel-based alloy to be treated into gadolinium oxide.

[0027] The oxidation rate is related to the oxygen content in the environment, as well as the ambient temperature and oxidation time. To accelerate the oxidation process as much as possible and improve the practicality of the process, oxidation at a higher ambient temperature can be considered. This leads to the following further preferred solutions:

[0028] Keep warm at a temperature of 1000℃~1250℃.

[0029] During the oxidation process, the oxidation of Ni5Gd begins from the surface of the material and gradually extends deeper along the Ni5Gd distribution direction. Therefore, the oxidation depth along the Ni5Gd distribution direction can be used as an indicator to measure the degree of oxidation of Ni5Gd in the material. Extensive experiments have shown that when the oxidation depth is in the range of 100μm to 500μm, the elongation at break of the material is significantly improved. Therefore, preferably, after the oxidation treatment, the oxidation depth of the gadolinium-rich nickel-based alloy along the Ni5Gd distribution direction is 100μm to 500μm.

[0030] To facilitate public understanding of the technical solution and effects of this invention, the technical solution of this invention will be described in detail below through specific embodiments and in conjunction with the accompanying drawings:

[0031] Example 1:

[0032] The gadolinium-rich nickel-based alloy in this embodiment is obtained by adding gadolinium to the nickel-based superalloy Hastelloy N. It is referred to as Hastelloy N+Gd in this paper. Its composition, by mass percentage, is as follows: C≤0.05, Mn≤0.5, Si≤0.5, Fe≤5, Cr: 6~8, Mo: 16~18, Gd: 2, with the balance being Ni and unavoidable impurities. The preparation method of this gadolinium-rich nickel-based alloy is as follows: the alloy is cast in a vacuum induction furnace, and the ingot is homogenized for 6 hours in the temperature range of 1050℃-1250℃. Then, it is forged and rolled in the temperature range of 1050℃-1230℃ to obtain a sheet metal. Further heat treatment at 1170℃~1200℃ / 30min±10min yields the final finished alloy sheet metal. Finally, the alloy sheet metal is prepared into tensile test specimens with dimensions of 60mm×12mm×1mm.

[0033] A set of tensile specimens was subjected to different degrees of oxidation treatment, and then compared with unoxidized tensile specimens for room temperature elongation at break. The cross-sections of the oxidized samples were analyzed using optical microscopy, and the surface cracks after fracture of the oxidized and unoxidized specimens were observed and statistically analyzed in the cross-sectional direction using scanning electron microscopy. The oxidation treatment method used in this embodiment is as follows: three sets of tensile specimens were sealed in vacuum-sealed quartz tubes with a vacuum level of 10... -2Below Pa; then, the quartz tube is sealed using a vacuum sealing device - a quartz glass tube rotary sealer; first, the muffle furnace is heated to 1250℃, and then the vacuum-sealed quartz tube is placed in it for high-temperature heat treatment experiment. The holding times for the three groups of samples are 24h, 48h and 72h respectively. During the holding process, the residual oxygen and the oxygen decomposed by the quartz tube at high temperature will provide oxidation conditions. Since Ni5Gd is more prone to oxidation reaction, the Ni5Gd on the sample surface begins to oxidize and continues to oxidize along the "distribution channel" of Ni5Gd; after the oxidation test is completed, it is quickly taken out and air-cooled.

[0034] Microscopic images of the cross-sections of three groups of tensile specimens after room temperature tensile fracture following oxidation treatment at 1250℃ / 24h, 1250℃ / 48h, and 1250℃ / 72h, respectively. Figure 1 As shown; (a), (b), and (c) are samples oxidized at 1250℃ / 24h, 1250℃ / 48h, and 1250℃ / 72h, respectively, with oxidation depths along the Ni5Gd distribution direction of 222μm, 266μm, and 320μm, respectively. A comparison of the cracking patterns of these three tensile specimens and the unoxidized tensile specimens after room temperature fracture revealed that the oxidized tensile specimens showed almost no cracking and exhibited necking, while the unoxidized tensile specimens showed numerous cracks along the Ni5Gd distribution without necking. Figure 2 As shown. Figure 3 As shown, the elongation at break of the unoxidized tensile sample was 3.77%, while the elongation at break of the three groups of oxidized tensile samples were 9.98%, 9.61%, and 6.12%, respectively, all of which were much higher than that of the unoxidized tensile sample.

[0035] Example 2:

[0036] The gadolinium-rich nickel-based alloy in this embodiment is the existing American ASTM B932-04 material, and its composition by mass percentage is as follows: C≤0.01, Mn≤0.5, Si≤0.08, Fe≤1, Co≤2, Cr: 14.5~17.1, Mo: 13.1~16.0, Gd: 1.9~2.1, with the balance being nickel and unavoidable impurities.

[0037] Similar to Example 1, three sets of tensile specimens were sealed in vacuum-sealed quartz tubes with a vacuum level of 10. -2Below Pa; then, the quartz tube is sealed using a vacuum sealing device - a quartz glass tube rotary sealer; first, the muffle furnace is heated to 1250℃, and then the vacuum-sealed quartz tube is placed in it for high-temperature heat treatment experiment. The holding times for the three groups of samples are 24h, 48h and 72h respectively. During the holding process, the residual oxygen and the oxygen decomposed by the quartz tube at high temperature will provide oxidation conditions. Since Ni5Gd is more prone to oxidation reaction, the Ni5Gd on the sample surface begins to oxidize and continues to oxidize along the "distribution channel" of Ni5Gd; after the oxidation test is completed, it is quickly taken out and air-cooled.

[0038] Microscopic images of the cross-sections of three groups of tensile specimens after room temperature tensile fracture following oxidation treatment at 1250℃ / 24h, 1250℃ / 48h, and 1250℃ / 72h, respectively. Figure 4 As shown in the figure; (a), (b), and (c) are samples oxidized at 1250℃ / 24h, 1250℃ / 48h, and 1250℃ / 72h, respectively, with oxidation depths along the Ni5Gd distribution direction of 185μm, 255μm, and 310μm, respectively. A comparison of the cracking patterns of these three groups of tensile specimens and the unoxidized tensile specimens after room temperature fracture revealed that the oxidized tensile specimens showed almost no cracking and exhibited necking, while the unoxidized tensile specimens showed numerous cracks along the Ni5Gd without necking. Figure 5 As shown, the elongation at break of the unoxidized tensile sample was 5.27%, while the elongation at break of the three groups of oxidized tensile samples were 6.69%, 8.27%, and 8.05%, respectively, all of which were much higher than that of the unoxidized tensile sample.

[0039] As can be seen from the above embodiments, for gadolinium-rich nickel-based alloys mainly composed of an austenitic matrix and a second-phase compound Ni5Gd distributed along the dendrites of the austenitic matrix, modification by oxidation to convert some of the second-phase compound Ni5Gd into gadolinium oxide can effectively suppress the generation of processing cracks and significantly improve the elongation of the material. While maintaining the neutron absorption capacity, the processing performance of gadolinium-rich nickel-based alloys is greatly improved, providing a low-cost and effective approach for the industrial application of gadolinium-rich nickel-based alloys. Furthermore, since the modified material exhibits increased elongation due to the present invention, the addition of a higher content of Gd can be considered to further enhance the material's thermal neutron absorption capacity.

Claims

1. A method for improving the processing properties of gadolinium-rich nickel-based alloys, wherein the gadolinium-rich nickel-based alloy comprises an austenitic matrix and a second-phase compound Ni5Gd distributed along the dendrites of the austenitic matrix; characterized in that, The processing properties of the gadolinium-rich nickel-based alloy are improved by oxidizing the alloy to convert some of the second-phase compound Ni5Gd into gadolinium oxide.

2. The method for improving the processing properties of gadolinium-rich nickel-based alloys as described in claim 1, characterized in that, The oxidation treatment is performed by placing the gadolinium-rich nickel-based alloy to be treated in an atmosphere with an oxygen content not exceeding an oxygen content threshold, and holding it at a temperature below 1260°C for a period of time; the oxygen content threshold is the oxygen content required to convert all the second-phase compounds Ni5Gd in the gadolinium-rich nickel-based alloy to be treated into gadolinium oxide.

3. The method for improving the processing performance of gadolinium-rich nickel-based alloys as described in claim 2, characterized in that, Keep warm at a temperature of 1000°C to 1250°C.

4. The method for improving the processing performance of gadolinium-rich nickel-based alloys as described in claim 1, characterized in that, After the oxidation treatment, the oxidation depth of the gadolinium-rich nickel-based alloy along the Ni5Gd distribution direction is 100μm to 500μm.

5. A modified gadolinium-rich nickel-based alloy, characterized in that, The gadolinium-rich nickel-based alloy is obtained by treating it with the method for improving the processing performance of gadolinium-rich nickel-based alloy as described in any one of claims 1 to 4, wherein a portion of the second-phase compound Ni5Gd in the gadolinium-rich nickel-based alloy is converted into gadolinium oxide.

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