Superplastic alloy

Abstract

This superplastic alloy contains transition metal carbides and dissimilar metal elements with respect to tungsten in amounts of 0.1-20 wt% for the dissimilar metal elements and 0.2-5.0 wt% for the transition metal carbides. One or more of the dissimilar metal elements exist as a solid solution or in a segregated manner at grain boundaries in a tungsten parent phase. One or more of the transition metal carbides are finely deposited at grain boundaries, within grains, or both at grain boundaries and within grains.

Description

superplastic alloy

[0001] The present invention relates to a superplastic alloy containing at least tungsten.

[0002] When a metal material is normally stretched, the center of the material test piece is locally squeezed, causing a necking, and ultimately, in most cases, it breaks in a cup-and-cone shape due to ductility. The elongation at this time is, for example, around 30-40% for mild steel, and around 50-60% for soft metals such as aluminum and copper, and does not exceed 100%.

[0003] When solid metallic materials, including alloys, are deformed at high temperatures at a relatively low strain rate, for example by tension, they can elongate by hundreds to thousands of percent. This phenomenon is called superplasticity (also known as superplastic deformation or superplastic phenomenon). Superplasticity can be classified into two types: transformation superplasticity, which is primarily caused by phase transformation of the material, and fine-grained superplasticity, which occurs in polycrystalline materials with grain sizes of a few microns or less. In fine-grained superplasticity, the strain rate sensitivity index (m value) of flow stress, which corresponds to the gradient of the log-log stress-strain rate curve, is high. Generally, an m value of 0.3 or greater and a fracture elongation of 200% or greater are considered to be the criteria for the occurrence of superplastic behavior. When superplasticity occurs, the flow stress also decreases, and methods utilizing this phenomenon have been put into practical use for forming high-strength, difficult-to-process materials such as nickel-based superalloys.

[0004] Here, the main metallic materials having superplasticity are aluminum alloys and titanium alloys used in the integral forming technology of structures in the automotive and aerospace industries, as well as magnesium alloys, cobalt alloys, stainless steel alloys, super heat-resistant alloys, etc. For example, as an example of an aluminum alloy having superplasticity, Japanese Patent Laid-Open Publication No. 06-240395 (Patent Document 1) discloses an aluminum alloy for superplastic forming. The aluminum alloy described in Patent Document 1 is an aluminum alloy for superplastic forming (see Claim 1 in Patent Document 1, etc.), characterized in that it contains 4.0 to 7.0% magnesium, more than 1.5% but not more than 2.5% manganese, 0.00005 to 0.01% beryllium, the impurity iron is restricted to less than 0.2%, and the balance is aluminum and other unavoidable impurities. It has good superplastic properties, and at the same time, its corrosion resistance and weldability are far superior to those of conventional aluminum alloy sheets for superplastic forming, and therefore can fully satisfy the performance required for various building materials, other containers, and further various structural materials, etc.

[0005] Furthermore, in titanium alloys, an example of evaluating cooperative grain boundary sliding, which plays an important role in superplasticity, using a Ti-4.5Al-3V-2Fe-2Mo alloy (SP-700 alloy) is disclosed in Non-Patent Document 1. In Non-Patent Document 1, in particular, the temperature is 750°C and the strain rate is 1×10 －２ s －１ It is stated that under the conditions, an elongation of 1200% or more was obtained.

[0006] However, the aluminum alloy described in Patent Document 1 is derived from a light metal, and therefore has a low melting point and is not suitable for products that require heat resistance. Also, the titanium alloy described in Non-Patent Document 1 has superior strength compared to aluminum alloys, but is difficult to use at high temperatures of 1000°C or higher. When using at high temperatures of 1000°C or higher, alloys based on heavy metals (transition metals) are considered as one solution.

[0007] Among heavy metals (transition metals), tungsten, for example, has the properties of high melting point, low thermal expansion coefficient, high thermal conductivity, discharge resistance, chemical stability at high temperature, etc. Furthermore, when tungsten is alloyed, focusing on the melting point, the melting point of this tungsten alloy is as high as 3000°C or higher, compared with the melting point of the titanium alloy mentioned above, which is 1500-1600°C, and the melting point of the nickel-based alloy (such as Inconel (registered trademark)), which is 1300-1400°C. However, due to its brittleness and difficulty in processing, the application range of tungsten in industry has been limited.

[0008] In tungsten as brittle material, there are known techniques such as adding rhenium and other elements to solid-solution in order to improve mechanical properties, or adding rhenium and other elements to improve electrical resistivity at room temperature; however, as mentioned above, the tungsten material including tungstenalloy has the advantage that it can be used at high temperature, but its electrical resistivity is small, and there are few heterogeneous metals that can be solid-solutioned to greatly improve electrical resistivity; and furthermore, there are concerns about low-temperature embrittlement, recrystallization embrittlement (the embrittlement caused by recrystallization under high temperature), or the irradiation embrittlement caused by radiation (such as neutron beam, etc.).As the technique of improving this kind of embrittlement, for example, International Publication No. 2013 / 018714 (Patent Document 2) discloses the alloy that the carbide of III-VI group transition metal elements is solid-solution segregated and finely precipitated in the grain boundary of tungsten.

[0009] The technology described in Patent Document 2 relates to an alloy having high strength and high toughness and improved embrittlement resistance, and a method for producing the alloy, which is obtained by treating a mixed powder of a transition metal carbide and tungsten by a mechanical alloying (MA) method and a hot isostatic pressing (HIP) method, and further applying a grain boundary sliding based microstructural modification (GSMM) method.

[0010] Here, Patent Document 2 states that "in practice, grain boundary sliding treatment and superplastic deformation are performed for the purpose of grain boundary strengthening to resolve brittleness," and "W-TiC (tungsten-titanium carbide) exhibits large superplastic deformation (160% or more at 1600°C) through grain boundary sliding treatment" (see paragraphs

[0044] to

[0046] of Patent Document 2). However, in reality, as a result of grain boundary sliding treatment, the grain size increases by about one order of magnitude, and the superplastic elongation of W-TiC that has been treated in this manner is actually significantly reduced. Normally, a small grain size is required for fine-grained superplasticity to occur. However, Patent Document 2 does not include any description of grain size within the above-mentioned range, and furthermore, it does not disclose or suggest any description regarding the elongation of superplastic deformation of the material after grain boundary sliding treatment.

[0011] Furthermore, the group of the present applicants has disclosed the tungstenalloy with high electrical resistivity and its manufacturing method in Japanese Patent Laid-Open No. 2023-45532 (Patent Document 3).First, Patent Document 3 describes that by dissolving 0.1-19 weight% of heterogeneous metal (especially ruthenium) in tungsten, electrical resistivity at room temperature becomes 4 times or more, and electrical resistivity at high temperature (>1000 ℃) becomes 2 times or more.In addition, Patent Document 3 describes that not only dissimilar metal such as ruthenium but also the constituent element (transition metal and carbon) of transition metal carbide (transition metal carbide) is further dissolved in tungsten.

[0012] However, since Patent Document 3 aims to increase the electrical resistivity, although tests have been conducted on tungsten-transition metal (particularly ruthenium) alloys to show an increase in electrical resistivity, no tests have been conducted to determine whether or not they exhibit a superplastic phenomenon, making it unclear whether or not they exhibit a superplastic phenomenon. Furthermore, Patent Document 3 only contains a simple description of tungsten-transition metal-transition metal carbides, and no tests have been conducted to show an increase in electrical resistivity, nor have any tests been conducted to determine whether or not they exhibit a superplastic phenomenon, as with tungsten-transition metal (particularly ruthenium) alloys, making it unclear whether or not they exhibit a superplastic phenomenon.

[0013] Japanese Patent Application Laid-Open No. 06-240395 International Publication No. 2013 / 018714 Japanese Patent Application Laid-Open No. 2023-45532

[0014] Eiichi Sato, "Clarification of organizational guidelines for reducing the superplastic forming temperature of titanium alloy thin plates," Amada Foundation Grant Research Report, Vol. 29, pp. 130-133 (2016).

[0015] In view of the above, the present invention provides a superplastic alloy comprising tungsten and a transition metal and a transition metal carbide.

[0016] The above-mentioned object of the superplastic alloy of the present invention can be effectively achieved by providing a superplastic alloy comprising tungsten and one or more dissimilar metal elements that are solid-dissolved and segregated at grain boundaries in a tungsten matrix, and one or more transition metal carbides that are finely precipitated at grain boundaries, intragranularly, or at grain boundaries and intragranularly, the superplastic alloy comprising 0.1 to 20 wt % of the dissimilar metal elements and 0.2 to 5.0 wt % of the transition metal carbides relative to the tungsten.

[0017] Furthermore, the above-mentioned object of the superplastic alloy according to the present invention is even more effectively achieved by the superplastic alloy having a total elongation of 50 to 1000%, or by the different metal element being selected from any one of ruthenium, osmium, iridium, rhodium, hafnium, and zirconium, or by the transition metal carbide being selected from carbides of a transition metal element of any one of Period 4, Period 5, and Period 6 and Groups 4 to 6.

[0018] In the present invention, tungsten, which is inherently poor in formability and has limitations on the shape of products, is subjected to the addition of a small amount of a transition metal element (such as ruthenium) that imparts large lattice strain, thereby causing solid solution and grain boundary segregation, and by providing high-density grain boundaries in the alloy in a thermally stable state, a superplastic alloy exhibiting superplastic deformation (phenomenon) has been created.

[0019] 1 is an image showing the surface of W-1Ru-1.2TiC, and 2 is an image showing the surface of pure tungsten (comparative example).

[0020] Hereinafter, embodiments of the present invention will be described.

[0021] First, the superplastic alloy according to the present invention basically contains 0.1 to 20 wt % of a different metal element and 0.2 to 5.0 wt % (corresponding to 0.6 to 14 mol %) of a transition metal carbide relative to tungsten, and the elongation of the superplastic alloy (referred to as "elongation rate" or "total elongation") is 50% to 1000%, and the strain rate is 5.0 × 10 －１ s －１ The alloy exhibits superplastic deformation at 1000 to 2000° C. The peak stress is 800 MPa or less. The grain size of the alloy is 20 μm or less, preferably 0.1 to 10 μm.

[0022] As for superplasticity, as mentioned above in the Background Art section, if the strain rate sensitivity index (m value) is 0.3 or more and the elongation is 50% or more, the material exhibits superplasticity and can be used in many industrial applications. That is, if the m value is less than 0.3, the elongation does not elongate proportionally to a certain extent, and the material does not have so-called superplasticity. On the other hand, if the elongation amount (elongation rate) is more than 1000% (even if it is elongated), it is not of great industrial importance and becomes difficult to process.

[0023] The strain rate is more than this, i.e., 5.0 × 10 －１ s －１ If the rate of deformation is faster than 100%, deformation due to dislocation slip becomes dominant rather than deformation due to grain boundary sliding, and superplasticity is not exhibited.

[0024] Regarding the temperature, if it is less than 1000°C, the elongation is small and superplasticity is not exhibited. On the other hand, if it is more than 2000°C, even if superplasticity is exhibited at 2000°C or more, processing becomes difficult due to equipment limitations.

[0025] If the peak stress is greater than 800 MPa, even if superplasticity is exhibited, processing becomes difficult due to equipment limitations.

[0026] The term "different metal elements" as used herein refers to transition metal elements in the fourth period of the periodic table, including elements from groups 3 to 10 in the fourth period, as well as transition metal elements in the fifth and sixth periods of the periodic table, including elements from groups 3 to 5 and 7 to 10 in the sixth period (i.e., excluding molybdenum and tungsten). For example, usable metals in the fourth period range from scandium (atomic number 21) to nickel (atomic number 28); in the fifth period, from yttrium (atomic number 39) to palladium (atomic number 46); and in the sixth period, lanthanides, including hafnium (atomic number 72) to platinum (atomic number 78) can be used (molybdenum has atomic number 42 and tungsten has atomic number 74).

[0027] The different metal element can be selected from the elements listed above, but considering the improvement of the superplasticity and various brittleness of tungsten and molybdenum, elements of Groups 4, 5, 7, 8, and 9 of the fifth or sixth period, such as ruthenium (Ru), osmium (Os), tantalum (Ta), niobium (Nb), iridium (Ir), rhodium (Rh), hafnium (Hf), and zirconium (Zr), are preferred. Furthermore, ruthenium (Ru), osmium (Os), iridium (Ir), rhodium (Rh), hafnium (Hf), and zirconium (Zr) are more preferred. The reasons for this are that these dissimilar metal elements differ greatly from tungsten in atomic radius and electronegativity, and when they dissolve in tungsten, they cause large lattice distortion, which is thought to result in them tending to concentrate at grain boundaries; and when alloying with tungsten, the use of a forced solid solution / ultrafine grain introduction method such as mechanical alloying (MA) causes segregation of the dissimilar metal elements to the high-density grain boundaries introduced by MA during sintering (grain boundary segregation), which is thought to have an effect on superplastic deformation, its control, and various improvements in brittleness.

[0028] For a ternary superplastic alloy according to the present invention, such as W-A-B (W is tungsten, A is the dissimilar metal element described above, and B is a transition metal carbide), A is preferably 0.1 to 20 wt% relative to tungsten. If the A content is less than 0.1 wt%, the elongation (rate) increases compared to pure tungsten, i.e., superplasticity is exhibited, but the aforementioned 50 to 1000% elongation is not achieved. If the A content is greater than 20 wt%, the maximum solid solubility limit is exceeded, resulting in an elongation rate similar to that of pure tungsten, or there is a possibility that other embrittlement (low-temperature embrittlement, recrystallization embrittlement) may be promoted. Incidentally, the maximum solid solubility limits (in the equilibrium phase diagram) for tungsten are, for example, 19 wt% for osmium, 14 wt% for ruthenium, 10.3 wt% for iridium, 7 wt% for rhodium, 1.8 wt% for zirconium, and 9 wt% for hafnium. The solid solubility of the dissimilar metal elements in the above-mentioned temperature range of 1000 to 2000°C is less than the maximum solid solubility limit and decreases with decreasing temperature, but conversely, the degree of grain boundary segregation of the dissimilar metal elements (necessary for promoting grain boundary sliding) increases with decreasing temperature.

[0029] Here, B is described as a transition metal carbide, and the transition metal carbide contained in the superplastic alloy according to the present invention will be explained.

[0030] The transition metal carbide in the W-A-B ternary superplastic alloy is selected from carbides of transition metal elements of either the 4th, 5th, or 6th period and any of the elements of Groups 4 to 6. The reason for selecting such carbides is that they have extremely high melting points and therefore exist thermally stable even at high temperatures, so that high-density grain boundaries introduced by, for example, mechanical alloying (hereinafter referred to as "MA") are pinned by the carbides, and the fine crystal grains necessary for exhibiting superplasticity are maintained without coarsening even at high temperatures. In addition, the transition metal carbides precipitated at the grain boundaries or within the grains often satisfy the Kurdjumov-Sach orientation relationship (K-S relationship), which provides good atomic matching with the tungsten matrix, and the solid solution segregation and precipitation of transition metal carbides, particularly at grain boundaries (which are inherently weak and prone to cracking), acts to significantly strengthen the grain boundaries, thereby improving superplastic deformation, recrystallization embrittlement, and the like, in addition to the desired superplasticity.

[0031] The transition metal carbide content is preferably 0.2 to 5.0 wt% (0.6 to 14 mol%) relative to tungsten. If the content is less than 0.2 wt%, the grain boundary pinning effect of the carbide is insufficient, causing the crystal grains to coarsen during sintering or heating after sintering, and preventing superplasticity. If the content is more than 5.0 wt%, the carbides distributed within the grains coarsen, causing fracture (cracks) within the carbides, which may result in elongation rates related to superplasticity remaining the same as those of pure tungsten or may even promote other embrittlement (low-temperature embrittlement, recrystallization embrittlement).

[0032] Furthermore, the superplastic alloy according to the present invention may be a quaternary system of, for example, tungsten-molybdenum-different metal element-transition metal carbide. In this case, tungsten and molybdenum are in the same group (group 6) and exhibit almost the same properties, and since they form a complete solid solution with each other, the ratios may be changed arbitrarily.

[0033] The superplastic alloy according to the present invention has been described above. Next, a method for producing the superplastic alloy according to the present invention will be explained, taking as an example a ternary superplastic alloy mainly consisting of tungsten, a different metal element, and a transition metal carbide.

[0034] This article describes a method for solid-solving and grain-boundary segregation of dissimilar metal elements in tungsten and for finely precipitating transition metal carbides at grain boundaries and intragranularly, i.e., a method for producing a superplastic alloy according to the present invention. First, the dissimilar metal elements and transition metal carbides, which are the starting powder materials, must be solid-solubilized in the tungsten matrix metal. (Note: "solid-solubilization of transition metal carbides" refers to the decomposition of transition metal carbides into their constituent transition metal elements and carbon, followed by the solid-solubilization of these constituent elements in the matrix metal.) The reason for solid-solubilizing transition metal carbides, unlike solid-solubilization of dissimilar metal elements, is that both dissolve and then finely precipitate at grain boundaries or intragranularly, allowing them to be utilized as fine precipitates. Therefore, solid-solubilization must be performed in a step prior to sintering (referred to as the "first solid-solubilization step"). Therefore, a method other than arc melting or other known methods must be employed for solid-solubilization. To solid-solubilize transition metal carbides, for example, in tungsten, mechanical alloying (MA) is preferred. Mechanical alloying is a method for alloying and ultrafine grain size (introduction of high-density grain boundaries) by repeatedly inducing severe plastic deformation, such as folding and rolling, of powder particles in an inert gas or hydrogen gas atmosphere and / or vacuum atmosphere, for example, by utilizing the collision energy of balls in a ball mill. However, in the present invention, vacuum conditions, which do not contain impurities such as oxygen and nitrogen that promote embrittlement, are preferred. However, in MA under vacuum, compared to MA under gas atmosphere, heat generated by MA accumulates inside the container, resulting in significant adhesion of the powder to the numerous balls and the inner wall of the container, making it difficult to recover the (valuable) mechanically alloyed powder. Therefore, during MA, cold air (below 0°C) is constantly blown into the container to forcibly cool it. When an inert gas is used, the inert gas itself becomes an impurity, forming nano-sized bubbles (Ar bubbles, etc.), which remain in the sintered body at a fairly high density, and these nano-bubbles grow and connect during high-temperature deformation, promoting the destruction of the sintered body, etc., which may make it difficult to control the superplastic deformation of the alloy itself or may hinder other improvements in embrittlement. Note that although the mechanical alloying (MA) method is said to be desirable in this first solid solution step, other methods may also be used.For example, the alternative method referred to here is a method in which the transition metal carbide is not dissolved in the matrix metal, and therefore, in order to maintain fine crystal grains at high temperatures, not as fine precipitates but as thermally stable fine dispersed particles (due to the grain boundary pinning effect of the dispersed particles), tungsten, the different metal element, and the transition metal carbide are each pulverized using a ball mill or the like and then mixed together, and this method also produces results similar to those of the MA method.

[0035] On the other hand, for dissimilar metal elements, solid solution by heating after sintering or during sintering is effective because the dissimilar metal elements will be dissolved in the amount shown in the equilibrium phase diagram if heated for a sufficient time to allow the interdiffusion required for solid solution to proceed. However, heating must be performed within a range where the grain boundary pinning effect of transition metal carbides is effective and grain size coarsening does not occur. Furthermore, if the atomic radius or electronegativity of the dissimilar metal elements differs significantly from that of tungsten, the dissimilar metal elements' solid solubility limit (shown in the equilibrium phase diagram) is limited, and so only limited solid solubility can be achieved by heating after sintering or during sintering. Even in such cases, MA can achieve solid solubility above the solubility limit (forced solid solution). However, if heating is performed for a long period of time until thermal equilibrium is reached during sintering or after sintering, dissimilar metal elements in an amount exceeding the solid solubility limit at that temperature will precipitate within the grains or at the grain boundaries. Therefore, short heating times and rapid cooling are required to ensure solid solubility above the solubility limit. Furthermore, even when it is difficult to promote solid solution by heating as described above, it is effective to achieve solid solution before sintering by using MA.

[0036] Here, in the dissolution step (first dissolution step) of dissolving tungsten, different metal elements, and transition metal carbides, the dissolution temperature in the dissolution step before sintering is preferably -20 to 100°C. If it is below -20°C, the container and balls used for MA may be damaged (low-temperature embrittlement), and if it is above 100°C, the MA powder will be significantly coagulated on the balls and the inner wall of the container. －８ ~10 ０ Pa vacuum condition or pure hydrogen atmosphere (up to atmospheric pressure = 1.013 × 10 ５Pa), an inert gas atmosphere (up to atmospheric pressure), or a mixed gas atmosphere of hydrogen and an inert gas (up to atmospheric pressure) is preferred. －８ If the pressure is less than 10 Pa, it is difficult to maintain such an ultra-vacuum. ０ Above Pa, oxidation and nitridation of the powder cannot be ignored. Next, the dissolution temperature in the dissolution process (referred to as the second dissolution process) during or after sintering is preferably between 1200°C and the temperature (approximately 3000°C) showing the maximum solid solubility limit (in the equilibrium phase diagram). Below 1200°C, the solid solubility is insufficient. Above the temperature showing the maximum solid solubility limit, i.e., 3000°C, further increases in temperature do not increase the solid solubility. Incidentally, in the present application, the "temperature showing the maximum solid solubility limit (in the equilibrium phase diagram)" refers to the temperature showing the solid solubility limit when the maximum amount of tungsten and dissimilar metal elements are dissolved. For example, for osmium, it is 2945°C, for ruthenium, 2300°C, for iridium, 2545°C, for rhodium, 2240°C, for zirconium, and 2512°C for hafnium, all of which are below 3000°C.

[0037] Next, the superplastic alloy powder containing the dissimilar metal elements and transition metal carbides is sintered. Regarding the sintering method, if the objective is simply sintering, atmospheric pressure sintering or the previously mentioned hot isostatic pressing (HIP) method can be used. In contrast, for example, spark plasma sintering methods using a plasma sintering apparatus (such as the Ed-Pas method, pulse current sintering, pulse current pressure sintering, plasma activated sintering, and current heating sintering) allow sintering (to obtain a high-density sintered body) at a fairly low heating temperature in a short time (approximately 10 minutes) that does not reach thermal equilibrium at that temperature. The cooling time can also be varied over a wide range, from rapid cooling that suppresses precipitation. For these reasons, spark plasma sintering can simultaneously perform sintering and solid solution formation. Furthermore, even for alloy powders containing dissimilar metal elements forcibly dissolved by MA or the like, it is possible to obtain a high-density sintered body while maintaining its high solid solubility (before precipitation occurs). In any case, the above-mentioned methods can be appropriately adopted in consideration of the conditions of use, etc., regarding sintering.

[0038] Incidentally, the sintering temperature in the sintering step is preferably 1200 to 2000°C. The alloy powder that has been subjected to MA treatment and is in a clean state has extremely high sinterability, and sintering progresses rapidly around 1200°C. However, if the temperature is less than 1200°C, sintering is insufficient, and if the temperature is higher than 2000°C, the crystal grains become coarse, and the fine crystal grains and fine precipitates (nanostructure) necessary for improving embrittlement cannot be obtained. Furthermore, the cooling time from 2000°C to 600°C is preferably 1 to 60 minutes. If it is less than 1 minute, it is difficult to achieve with this manufacturing method, and if it is more than 60 minutes, it is inefficient. Furthermore, when sintering, it is preferable to use a 10 －８ ~10 －２ Preferably, the reaction is carried out under vacuum conditions of 10 Pa. －８ If the pressure is less than 10 Pa, it is difficult to maintain such an ultra-vacuum. －２ Above 100 Pa, oxidation of the superplastic alloy may become a problem.

[0039] Although an embodiment of the present invention has been described above, it goes without saying that the present invention is not limited to this embodiment, and various aspects may be adopted within the scope of the matters described in the specification, claims and / or drawings of this application.

[0040] An example of the embodiment described above will be described. Note that this example is merely an example, and the present invention is not limited to the following example, and various embodiments may be adopted within the scope of the matters described in the specification, claims, and / or drawings of the present application.

[0041] [Production Example] Production of Superplastic Alloy for High-Temperature Tensile Deformation Behavior Test First, as a sample for testing the tensile deformation behavior of the ternary superplastic alloy of the present invention at high temperatures, three types (other than pure tungsten) were produced: tungsten-1.2 wt. % titanium carbide (hereinafter referred to as "W-1.2TiC") containing only transition metal carbide, tungsten-1 wt. % ruthenium (hereinafter referred to as "W-1Ru") containing only the dissimilar metal element ruthenium, and tungsten-1 wt. % ruthenium-1.2 wt. % titanium carbide (hereinafter referred to as "W-1Ru-1.2TiC") containing both transition metal carbide and ruthenium. The production conditions are shown in Table 1 below. Among the three alloys produced, the average grain size of W-1.2TiC and W-1Ru-1.2TiC, which contain transition metal carbides with grain boundary pinning effect, was relatively fine at 1.2 μm and 2.5 μm, respectively, even after heating at 1650 ° C for about 5 hours by GSMM. On the other hand, W-1Ru, which does not contain transition metal carbides, had an average grain size of 8.3 μm after heating at 1800 ° C for 2 hours under vacuum. Here, the reason for using ITER-grade pure tungsten as a comparative example is that, in general, to exhibit superplasticity, fine grains are required, but tungsten alloys are used with severe plastic processing such as hot rolling. Therefore, the structure is elongated and enlarged, and the appearance of superplasticity cannot be expected. Here, the ITER-grade tungsten used in the International Thermonuclear Experimental Reactor (ITER), an international thermonuclear fusion experimental reactor planned in Europe, is well known as available tungsten with a fine structure, and therefore this material was adopted as the material for the comparative example.

[0042]

[0043] In Table 1, "MA" stands for mechanical alloying, and "GSMM" stands for grain boundary sliding-based microstructural modification for strengthening weak random grain boundaries. The pure tungsten used as a comparative example was obtained by sintering high-purity powder at high temperature for a long period of time in a hydrogen gas flow, followed by hot rolling and stress-relief treatment, but was not subjected to mechanical alloying, GSMM treatment, or vacuum no-load heat treatment.

[0044] [Example 1] High-Temperature Tensile Deformation Behavior Test Part 1 In Example 1, a high-temperature tensile deformation behavior test was performed on four samples: three types of samples (W-1.2TiC, W-1Ru, and W-1Ru-1.2TiC) produced in the above production examples, and pure tungsten (comparison example). The dimensions of the micro-test specimens used were a total length of 16 mm, a gripping width of 4 mm, and a thickness of 0.5 mm (gauge section: width 1.2 mm, thickness 0.5 mm, length 5 mm). To prevent oxidation of the test specimens during the test, Ta foil (thickness 20 μm) was applied to both sides of the test specimens. The tensile test jig was made of high-strength isotropic graphite and used a compression load type jig that allows one-touch attachment of the tensile test specimens.

[0045] In the high temperature tensile test, the initial strain rate was 5.0 × 10 －４ s －１ and placed under vacuum (2-5×10 －２ The flow stress (the stress at which the work hardening rate first becomes nearly zero, which nearly coincides with the peak stress) and elongation (total elongation) of each sample were calculated for the specimens at temperatures (tensile test temperatures) of 1400, 1600, and 1800°C. The results are shown in Table 2 below. In Table 2, the W-1.2 wt% TiC, W-1 wt% Ru, and W-1 wt% Ru-1.2 wt% TiC samples are referred to as "W-1.2TiC," "W-1Ru," and "W-1Ru-1.2TiC," respectively.

[0046]

[0047] In Table 2, for W-1Ru at a temperature of 1400°C, the test results at 1600°C and 1800°C indicated that the test specimens would fracture due to brittleness before reaching peak stress at 1400°C, so a high-temperature tensile test was not performed. For W-1.2TiC and W-1Ru-1.2TiC, the temperature exceeded the operating range of the test equipment and no fracture was observed at 1800°C. That is, as described in the Background Art (page 2, paragraph 3), it was predicted that GSMM-treated W-1.2TiC would not be able to elongate by 100% or more within the test temperature range, and therefore a tensile test jig with a short stroke (approximately 5 mm, corresponding to a total elongation of approximately 100%) was used, making it impossible to stretch the specimen to fracture. On the other hand, for W-1Ru-1.2TiC, a test jig with a fairly long stroke (approximately 18 mm, corresponding to a total elongation of approximately 360%) was used, but since no fracture was observed even after reaching a total elongation of approximately 350% at 1800°C, the test was stopped.

[0048] As a result, the peak stress monotonically decreased with increasing temperature for all samples. The flow stress of W-1Ru-1.2TiC was significantly lower than that of W-1.2TiC across the entire test temperature range. Meanwhile, the elongation (total elongation) either decreased (pure tungsten) or showed only a slight increase (W-1Ru) with increasing temperature. However, for both titanium carbide-containing systems (W-1.2TiC and W-1Ru-1.2TiC), the elongation increased with increasing temperature. Furthermore, W-1Ru-1.2TiC exhibited a significant increase in elongation, demonstrating clear superplasticity (giant elongation) at 1800°C and 1600°C. Based on the observations of the test specimen surface after fracture (see Example 3) and the observations of the stress drop behavior at the time of fracture (brittle fracture), the mechanism of this superplastic deformation is believed to be due to grain boundary sliding. Furthermore, the reason why W-1Ru-1.2TiC had a lower flow stress and a larger total elongation than W-1.2TiC is thought to be due to the promotion of grain boundary sliding due to the grain boundary segregation of Ru. On the other hand, the total elongation of W-1Ru was significantly smaller, even far smaller than that of pure tungsten. The decrease in ductility (high-temperature embrittlement) of W-1Ru is thought to be due to the promotion of grain boundary sliding due to the grain boundary segregation of Ru, and the fact that the accommodation mechanism for stress concentration that occurs at irregular or discontinuous parts such as grain boundary triple junctions due to grain boundary sliding cannot function effectively due to the large crystal grain size, resulting in the generation, growth, and connection of cavities, leading to grain boundary fracture. It is considered that the solid solution of Ru within the grains creates an atmosphere of solute elements (Ru) around the moving dislocations, which prevents the dislocations from sliding (solute atmosphere drag resistance), making grain boundary sliding relatively more likely to occur.

[0049] [Example 2] High-Temperature Tensile Deformation Behavior Test Part 2 Next, a high-temperature tensile test was further carried out on each test piece in Example 1, where the total elongation of the test piece did not exceed the operating range of the apparatus, when the tensile temperatures were fixed at 1600°C and 1350°C. 1350°C is equal to the temperature of the HIP treatment.

[0050] In Example 2, the test pieces were W-1 wt % Ru-1.2 wt % TiC (hereinafter referred to as "W-1Ru-1.2TiC(a)") that had been subjected to Ed-Pas (spark plasma) treatment under conditions of 1530°C, 40 MPa, and 0.5 hours, and W-1 wt % Ru-1.2 wt % TiC (hereinafter referred to as "W-1Ru-1.2TiC(b)") that had been subjected to GSMM treatment at 1650°C for approximately 5 hours. Four test pieces were prepared from alloys of W-1.2 wt% TiC (hereinafter referred to as "W-1.2TiC(a)") that had been subjected to HIP treatment under conditions of 1350°C, 196 MPa, and 3 hours, and W-1.2 wt% TiC (hereinafter referred to as "W-1.2TiC(b)") that had been subjected to GSMM treatment at 1650°C for approximately 5 hours, as well as one pure tungsten (ITER grade) as a comparative example. Of these, W-1.2TiC(a) has basically the same composition as the tungsten alloy that exhibits superplasticity described in Patent Document 2. Furthermore, pure tungsten was not subjected to the same treatment as the other test pieces after purchase.

[0051] First, at 1600°C, each test piece was subjected to an initial strain rate of 5.0 × 10 －４ s －１ and 5.0 × 10 －３ s －１ The total elongation (elongation rate), flow stress (peak stress), and strain rate sensitivity index (m value) were determined for the two types of W-1.2TiC and W-1Ru-1.2TiC. Next, for W-1.2TiC(a) and W-1Ru-1.2TiC(a), which have small crystal grain sizes and are expected to have the greatest elongation, the total elongation (elongation rate), flow stress (peak stress), and strain rate sensitivity index (m value) were determined for the two types of W-1.2TiC and W-1Ru-1.2TiC. －４ s －１ The total elongation and flow stress at the initial strain rate of 1600°C and 1 × 10 ―２ s －１ The total elongation and flow stress at the initial strain rate were determined. The results are shown in Table 3 below, along with the grain size of the test specimen.

[0052]

[0053] First, regarding the crystal grain size, the pure tungsten used as a comparative example had a crystal grain size of about 10 μm, whereas the four test pieces related to the alloys had a crystal grain size of 2.5 μm at most, ranging from approximately 0.2 to 2.5 μm.

[0054] Next, regarding the total elongation (elongation rate) at 1600°C, the W-1Ru-1.2TiC ternary superplastic alloy according to the present invention exhibited an elongation rate of approximately 300% compared to the comparative example or the W-1.2TiC system. In particular, W-1Ru-1.2TiC(a) exhibited an elongation rate of approximately 1 × 10 ―２ s －１ At a fairly high strain rate of 1 × 10 , the total elongation did not break even after reaching 347%. ―２ s －１ The superplasticity that appears at an initial strain rate above this is called low-temperature, high-speed superplasticity. Since 1600°C corresponds to 0.5 Tm of tungsten, it can be seen that W-1Ru-1.2TiC(a) exhibits low-temperature, high-speed superplasticity. This alloy also exhibits low-temperature, high-speed superplasticity at an even lower temperature of 1350°C (0.44 Tm) and 1.5 x 10 －４ s －１ At a typical strain rate of 1000 sq. m, the specimen did not break even after reaching a total elongation of 320%.

[0055] Next, the deformation stress (peak stress) was measured at 1600°C and a strain rate of 5.0 × 10 －４ s －１ and 5.0 × 10 －３ s －１ In both cases, the results were W-1.2TiC > Comparative Example > W-1Ru-1.2TiC. W-1Ru-1.2TiC (a), which showed the greatest elongation among the W-1Ru-1.2TiCs, had a particularly low flow stress, 1 / 2 to 1 / 5 of that of W-1Ru-1.2TiC (b). Furthermore, even at a low temperature of 1350°C (0.44Tm), the flow stress of W-1Ru-1.2TiC (a) was low at 64.5 MPa, about 1 / 4 of that of W-1.2TiC (a).

[0056] Finally, m value is approximately proportional to total elongation (elongation rate), and the order of largest is W-1Ru-1.2TiC>W-1.2TiC>comparative example.One of the characteristics of superplasticity is that m value is large, m>0.3, and in this test result, the m value of the tungsten alloy that shows superplasticity or is considered to show superplasticity is 0.5~0.6.In addition, generally, the proportional relationship between total elongation and m value is not recognized.

[0057] These results demonstrate that at least the ternary superplastic alloys according to the present invention exhibit superplasticity, and that the fine-grained W-1Ru-1.2TiC(a) in particular exhibits particularly large elongation and low flow stress, demonstrating low-temperature, high-speed superplasticity. Depending on the selection and compounding ratio of the additive elements and transition metal carbides, even greater effects may be achieved.

[0058] [Example 3] Confirmation of the surface after high-temperature tensile test Next, in Example 3, the surface of the test piece after the high-temperature tensile test in Example 1 was confirmed. －４ s －１ 2 is an image showing the surface of a W-1Ru-1.2TiC test piece when a tensile test was performed at 1800°C and a strain rate of 2.0 × 10 －３ s －１ 1 is an image showing the surface of a test piece of pure tungsten (comparative example) when a tensile test was performed at 1000 kJ / cm.

[0059] In Figure 1, the gauge section stretched uniformly and broke without forming a neck, no slip lines indicating the slippage of numerous dislocations were observed, and a cavity formed by grain boundary sliding was visible near the fractured section. From these facts, it is believed that the giant elongation in W-1Ru-1.2TiC was caused by grain boundary sliding, which is the deformation mechanism of fine-grained superplastic deformation.

[0060] In FIG. 2, necks are formed and fracture occurs, and slip lines are observed in each crystal grain. This suggests that the primary deformation of pure tungsten is dislocation slip, not grain boundary slip, and this corresponds well to the results for pure tungsten in Table 3 (i.e., the total elongation is small, and the m value is 0.15 (<0.3), indicating that the deformation of pure tungsten is not superplastic).

[0061] The superplastic (tungsten-containing) alloy of the present invention overcomes the difficult workability of conventional tungsten and can be applied to a variety of industries. In fact, aluminum alloys and titanium alloys are used as near-net-shape aerospace materials. Conventional superplastic alloys have a heat resistance temperature of around 1000°C, so the impact of superplastic tungsten alloys with heat resistance exceeding 2000°C can be said to be revolutionary. Furthermore, this technology is expected to be easily applied to molybdenum, which is a related metal. The W-1Ru-1.2TiC(a) shown in the examples is thought to exhibit superplasticity around 1200°C (0.4Tm), where grain boundary diffusion necessary for grain boundary sliding is thought to occur.

[0062] It has extremely low deformation resistance and large elongation, making it highly formable. It can be used to manufacture ultra-high temperature heaters and reflectors in any shape. It can also be used for ultra-high temperature HIP cans.

[0063] Because the crystal grain size is small, a smooth surface, a characteristic of grain boundary sliding, can be obtained even after large plastic deformation. This is an important property for reflectors, etc., and will remain advantageous even if additive manufacturing of tungsten becomes a reality in the future.

[0064] Various molding techniques for industrial use of superplasticity can be applied, such as "vacuum blow molding," "hot press," "deep drawing," "bulge forming," "dieless drawing," and "extrusion." Furthermore, the combined use of Super Plastic Forming and Diffusion Bonding can be applied to the manufacture of composite materials. There is also a high possibility of realizing a technology in which the powder after MA is directly sprinkled on the bonding interface and bonded.

[0065] By using the superplastic alloy of the present invention, large plastic deformation activates the interface, promoting diffusion bonding and facilitating bonding to other metals. It also easily conforms to fine irregularities. Therefore, it can be used to manufacture composite materials. By bonding it to the surface of graphite, high-temperature materials with little outgassing can be manufactured. Composite materials that can be called lightweight tungsten can be manufactured. Tantalum composite materials that are highly tough but not active at ultra-high temperatures can be manufactured by coating them on tantalum, which is active at high temperatures. Direct bonding to heat-resistant ceramics can realize ultra-heat-resistant ceramic-metal composite materials. While tungsten itself cannot be manufactured into large materials and is difficult to bond using HIP or other methods, superplastic tungsten can be used as a bonding material for tungsten-to-tungsten joints.

[0066] Currently, we use W-1Ru-1.2TiC with a grain size of 2-5 μm after grain boundary sliding treatment. However, since the grain size before grain boundary sliding treatment is 100-200 nm, if the as-sintered material (i.e., W-1Ru-1.2TiC(a)) is used, it can also be applied to the field of microelectronics.

Claims

1. A superplastic alloy comprising tungsten and one or more dissimilar metal elements dissolved in a tungsten matrix and segregated at grain boundaries, and one or more transition metal carbides finely precipitated at grain boundaries, intragranularly, or at grain boundaries and intragranularly, said superplastic alloy comprising, relative to said tungsten, 0.1 to 20% by weight of said dissimilar metal elements and 0.2 to 5.0% by weight of said transition metal carbides.

2. The superplastic alloy according to claim 1, wherein said superplastic alloy has a total elongation of 50 to 1000%.

3. A superplastic alloy according to claim 1 or 2, wherein the dissimilar metal element is selected from the group consisting of ruthenium, osmium, iridium, rhodium, hafnium and zirconium.

4. A superplastic alloy according to claim 1, wherein the transition metal carbide is selected from the group consisting of carbides of transition metal elements of the 4th, 5th or 6th period and of elements of groups 4 to 6.

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