X-ray tube target and method for producing same

Abstract

The present invention provides an X-ray tube target for which it is possible to improve bonding strength while maintaining practical heat resistance and mechanical properties. Also provided is a method for producing such target.　One embodiment provides an X-ray tube target comprising a molybdenum alloy substrate, a carbon substrate, and an alloy layer disposed between the molybdenum alloy substrate and the carbon substrate. The alloy layer contains a solid solution formed from at least two elements selected from the group consisting of Ti, V, Zr, Nb, Mo, Ta, and W. The minimum hardness of the alloy layer is at least 70% of the maximum hardness.

Description

X-ray tube target and method for manufacturing the same

[0001] Embodiments of the present invention relate to an X-ray tube target and a method for manufacturing the same.

[0002] Medical CT devices aim for high definition, and many rotating anodes capable of obtaining high X-ray output are adopted. To achieve high output, it is necessary to increase the energy of the electron beam irradiated on the target. In a rotating anode, the target is rotated with respect to the electron beam and the focal point is constantly changed, thereby enabling high-output irradiation while suppressing the deterioration of the target. However, the target irradiated with a high-energy electron beam in a vacuum tube needs to have high heat resistance and a sufficient heat capacity for continuous operation.

[0003] Molybdenum alloy is used as a member satisfying the above heat resistance. In recent years, for improving the inspection speed, the diameter of the anode has been increasing so that a wide range can be imaged at once. On the other hand, with the increase in diameter, the weight of the target increases, and the burden on the rotor shaft becomes large. Therefore, it has been studied to replace a part of the molybdenum alloy with graphite having both heat resistance and light weight to reduce the weight while maintaining the size of the irradiation surface and the heat capacity.

[0004] The joining of molybdenum alloy and graphite requires a joining material and a joining method considering heat resistance and mechanical properties sufficient for the practical use of an X-ray tube.

[0005] Japanese Patent Laid-Open No. 4-228480, International Publication No. 2010-005001 of Japan

[0006] The problem to be solved is to provide an X-ray tube target and a method for manufacturing the same that can improve thermal fatigue characteristics while ensuring heat resistance.

[0007] According to an embodiment, there is provided an X-ray tube target including a carbon base material, a molybdenum alloy base material, and an alloy layer for joining the carbon base material and the molybdenum alloy base material. The alloy layer includes a solid solution composed of at least two elements selected from the group consisting of Ti, V, Zr, Nb, Mo, Ta, and W. The minimum hardness of the alloy layer is 70% or more of the maximum hardness.

[0008] Furthermore, according to the embodiment, a method for manufacturing an X-ray tube target is provided, which includes a step of joining a carbon substrate and a molybdenum alloy substrate using a material integrally molded from a material consisting of at least one element selected from the group consisting of Ti, V, Zr, and Mo, and a material consisting of at least one element selected from the group consisting of Nb, Ta, and W.

[0009] A schematic cross-sectional view showing an enlarged view of the joint portion of the cross-section along the stacking direction of the X-ray tube target according to the embodiment. A schematic cross-sectional view showing the Vickers hardness measurement points of the X-ray tube target shown in Figure 1. A schematic enlarged view of the area near the micro-Vickers indentation shown in Figure 2. A cross-sectional view showing an example of an X-ray tube target according to the embodiment.

[0010] For example, Patent Documents 1 and 2 describe joining by combining multiple high-melting-point metals and their alloys, and then solidifying a portion of these components. High-power X-ray tubes used for specific applications require that the joint layer maintain its structure without melting even when exposed to high temperatures of 1600°C. Zr is an example of a brazing material that wets well to both molybdenum alloy substrates and carbon substrates. Zr undergoes a eutectic reaction with Mo contained in the molybdenum alloy substrate at 1550°C and melts. Therefore, a joint layer made of Zr cannot ensure heat resistance. Patent Documents 1 and 2 propose a brazing material configuration in which a high-melting-point metal that does not undergo a eutectic reaction with Zr, such as Ta, Nb, or W, is inserted between Zr and the molybdenum alloy substrate, so that Zr and the molybdenum alloy do not come into direct contact.

[0011] In multilayer bonding layers containing elements such as Ta, Nb, and W, layers that melt and solidify at the bonding temperature coexist with layers that do not melt. The material structure differs between the melting and solidifying layers and the non-melting layers, often resulting in differences in mechanical properties and thermal expansion coefficients. Specifically, the MoNbTi diffusion phase, NbTi alloy phase, and ZrNb alloy phase described in Patent Document 2 are solid solutions and are characterized by high hardness and excellent mechanical properties due to solid solution strengthening. On the other hand, the Nb-rich phase has little effect from solid solution strengthening and reflects the properties of Nb alone, resulting in low hardness and inferior mechanical properties compared to alloys. Furthermore, the thermal expansion coefficient in the case of alloy phases changes proportionally to the alloy composition. Therefore, if the alloy composition has a gradual concentration gradient along the thickness direction of the bonding layer, the thermal expansion coefficient also changes gradually. Conversely, the concentration changes abruptly near the interface between the Nb-rich phase and other alloy phases, and therefore, a large difference in thermal expansion coefficient is likely to occur. A large difference in thermal expansion coefficient makes the material prone to becoming brittle due to repeated thermal fatigue.

[0012] According to the embodiment, it is possible to provide an X-ray tube target and a method for manufacturing the same that can ensure heat resistance at 1600°C using a bonding layer composed of only two or more solid solution alloys. Examples of solid solution alloys include MoNbTi alloy, NbTi alloy, and ZrNb alloy.

[0013] The following describes an embodiment of an X-ray tube target and its manufacturing method. (X-ray tube target of the embodiment) The embodiment of the X-ray tube target includes a carbon substrate, a molybdenum alloy substrate, and an alloy layer disposed between the carbon substrate and the molybdenum alloy substrate. The alloy layer includes a solid solution of at least two elements selected from the group consisting of Ti, V, Zr, Nb, Mo, Ta, and W. The minimum hardness of the alloy layer is 70% or more of the maximum hardness. It is desirable that the minimum hardness of the alloy layer in the thickness direction is 70% or more of the maximum hardness. The embodiment of the X-ray tube target will be described with reference to Figures 1 to 3. Figure 1 is an example of an enlarged view of the joint portion of a cross-sectional view obtained when the X-ray tube target according to the embodiment is cut along the stacking direction. In Figure 1, it is assumed that the direction in which the substrates etc. constituting the target are stacked (stacking direction) is parallel to the z direction. It is assumed that one direction intersecting the stacking direction of the X-ray tube target is parallel to the x direction. Furthermore, it is assumed that other directions intersecting the stacking direction of the X-ray tube target are parallel to the y-direction.

[0014] The joint 1 of the X-ray tube target shown in Figure 1 includes a molybdenum alloy substrate 2, a carbon substrate 3, and an alloy layer 4 disposed between the molybdenum alloy substrate 2 and the carbon substrate 3. The alloy layer 4 is a joining layer that connects the molybdenum alloy substrate 2 and the carbon substrate 3.

[0015] The molybdenum alloy substrate 2 has an electron beam irradiation surface on at least a portion of its surface, and X-rays can be generated by irradiating the electron beam irradiation surface with an electron beam. The electron beam irradiation surface may be provided with a Re-containing W layer necessary for X-ray output. The molybdenum alloy is not particularly limited, but in addition to Mo, it may include at least one selected from the group consisting of, for example, oxygen, carbon, titanium, and zirconium.

[0016] The carbon substrate 3 contributes to weight reduction without impairing the mechanical properties of the target. Isotropic carbon can be used as an example of the material constituting the carbon substrate 3. It is preferable that the isotropic carbon is formed by cold isotactic pressing (CIP). CIP-formed isotropic carbon is dense and has excellent mechanical properties. The carbon substrate 3 has multiple grooves 5 on the surface to be joined to the alloy layer 4. Each groove 5 has a V-shape with depth in the lamination direction (z-direction). Multiple grooves 5 are arranged, for example, in the x-direction. The grooves 5 can suppress the delamination of the alloy layer 4 from the carbon substrate 3 due to thermal expansion and contraction of the carbon substrate 3. Note that there may be only one groove 5. Also, the shape of the groove 5 is not limited to a V-shape; for example, it may be U-shaped. There may be no grooves at all. For example, it is preferable that the depth of the groove 5 is 0.05 mm or more and less than or equal to the thickness of the second brazing layer described later. The thickness of the second brazing layer is, for example, 0.1 mm to 0.4 mm. The pitch is preferably 1.5 to less than 3 times the depth. The radius of the valleys is preferably 0.05 mm or more. The direction of the grooves is preferably perpendicular to the diameter. The depth and pitch do not all need to be the same shape, but it is preferable that they are symmetrical with respect to the center no matter where you take the diameter.

[0017] The alloy layer 4 is described below. The alloy layer 4 contains a solid solution composed of at least two elements selected from the group consisting of Ti, V, Zr, Nb, Mo, Ta, and W. The solid solution may also contain C. Whether or not it is a solid solution can be determined by comparing the results of elemental analysis of the cross-section of the alloy layer 4 with the equilibrium phase diagram. Elemental analysis is performed by SEM / EDS (Scanning Electron Microscope Energy Dispersive X-ray Spectroscopy). The detected composition is compared with the equilibrium phase diagram to determine whether the solid phase just below the bonding temperature or melting point is a solid solution phase. It is desirable that the alloy layer 4 does not contain elemental metals such as Nb. Not containing elemental metals means that in an SEM / EDS analysis specifying all the metal elements constituting the alloy layer, no more than two analysis points are found where the composition of a single metal element exceeds 99 mol%. It is desirable that the alloy layer 4 is a solid solution alloy layer. The solid solution alloy layer may be formed from one type of solid solution or from multiple types of solid solutions. Examples of solid solution alloys include MoNbTi alloys, NbTi alloys, ZrNb alloys, NbZrTiMoC alloys, TaZrTiVMoC alloys, and WZrTiMoC alloys. Solid solution alloy layers have high hardness and excellent mechanical properties due to solid solution strengthening.

[0018] The alloy layer 4 may contain metal carbide particles 6. The shape of the metal carbide particles 6 is not particularly limited, but can be granular, fibrous, etc. The form of the metal carbide is not limited to particles, and may be layered, for example. In the target 1 shown in Figure 1, the layered metal carbide 7 is located at the interface between the groove 5 forming surface of the carbon substrate 3 and the alloy layer 4. Since the metal carbide is harder than the alloy layer 4 or the molybdenum alloy substrate 2 and is stable up to high temperatures, it can be expected to improve the mechanical strength of the X-ray tube target at room temperature and high temperatures. Examples of metal carbides include ZrC and metal carbides in which a portion of the Zr in ZrC is replaced with at least one element selected from the group consisting of Ti, V, Nb, Mo, Ta, and W. The type of metal carbide used may be one type or two or more types. The type of metal carbide can be determined from the molar percentage ratio of the metal element to carbon when quantitative analysis is performed in SEM / EDS analysis.

[0019] The hardness of the alloy layer 4 has a distribution, for example, along the thickness direction (z-axis direction). The minimum hardness is 70% or more of the maximum hardness. Such an alloy layer 4 can reduce the variation in hardness in the thickness direction. As a result, the alloy layer 4 can also reduce the variation in the coefficient of thermal expansion in the thickness direction. Consequently, the X-ray tube target 1 can improve its thermal fatigue characteristics while ensuring heat resistance at 1600°C. The minimum hardness may be 100% of the maximum hardness, that is, the minimum hardness may be the same value as the maximum hardness.

[0020] The hardness of alloy layer 4 is measured in Vickers hardness. For measuring Vickers hardness, for example, a Shimadzu micro-Vickers tester can be used. The method for measuring Vickers hardness will be explained with reference to Figures 2 and 3. Figure 2 schematically shows the measurement points when measuring the Vickers hardness of the target cross-section shown in Figure 1. In Figure 2, only the alloy layer of the target cross-section shown in Figure 1 is magnified to make the position of the measurement points easier to see. Figure 3 is a magnified cross-sectional view of the area near the micro-Vickers indentation in the cross-section shown in Figure 2.

[0021] Vickers hardness is measured according to JIS Z 2244 (2020), with a load of 50g and a pressurizing time of 20 seconds. The diagonal dimensions of the indentation are read in both directions using the attached measuring instrument, and the average value is converted to Vickers hardness. The measurement sample is made by cutting a jointed body (X-ray tube target) to an appropriate size and embedding it in resin so that the cross-section of the alloy layer (joint layer) is visible parallel to one side. The cross-section of the joint layer is wet polished up to #4000 and then polished to a mirror finish with a 0.2 μm diamond buff. In addition, the parallelism between the mirror surface of the sample and the bottom surface opposite it is adjusted to be 0.01 mm or less.

[0022] As shown in Figure 2, the measurement point starts at position P1, which includes at least one carbon substrate 3, and is scanned 0.05 mm increments along a single axis (parallel to the z-axis in Figure 1) toward the molybdenum alloy substrate 2 from the carbon substrate 3 in the thickness direction of the alloy layer 4 (bonding layer) (z-axis direction in Figure 1). The endpoint P2 is a position that includes at least one molybdenum alloy substrate 2. The measured Vickers hardness is calculated by determining the ratio of the minimum hardness to the maximum hardness, using only the measurements of the alloy layer (bonding layer). The number of measurements depends on the thickness of the alloy layer, but the minimum number of measurements is four. The above measurement is performed at any five locations, and if four or more of the five measurement locations show a minimum hardness of 70% or more of the maximum hardness, the alloy layer in question is judged to have a minimum hardness of 70% or more of the maximum hardness. To determine whether a micro-Vickers indentation is made on the bonding layer, the measurement sample can be examined using SEM-EDS (Scanning Electron Microscope Energy Dispersive X-ray Spectroscopy). An arbitrary micro-Vickers indentation 8 located in the alloy layer 4 in the cross-section shown in Figure 2 will be used as an example of analysis. Figure 3 shows a schematic magnified view of the area around the micro-Vickers indentation 8 shown in Figure 2. As shown in Figure 3, eight analysis points 9 are defined as points perpendicular to the diagonal edges and midpoints of the indentation 8, at a distance D. In Figure 3, the eight analysis points 9 are indicated by black circles. The distance D can be, for example, 3 μm. For each analysis point 9, the atomic concentrations of C and Mo are analyzed. For all eight points, the measured values ​​in the portion where C is less than 40 at% and Mo is less than 80 at% are taken as the hardness of the alloy layer (bonding layer).

[0023] The thickness of the alloy layer (bonding layer) is preferably 0.2 mm or more and 2.0 mm or less.

[0024] In Figures 1 and 2, the alloy layer 4 and each layer constituting the alloy layer 4 have a layer shape with a surface parallel to the xy plane, but the shape is not limited to this, and for example, the surface may have irregularities. Also, in Figures 1 and 2, the boundaries of the components constituting the target (molybdenum alloy substrate, carbon substrate, alloy layer) are shown, but the boundaries may not be clear. (Method for manufacturing an X-ray tube target of the embodiment) The X-ray tube target of the embodiment is manufactured, for example, by joining a molybdenum alloy substrate and a carbon substrate via a bonding layer in which multiple high-melting-point metals are laminated. According to one example, the method for manufacturing an X-ray tube target of the embodiment includes a step of joining the carbon substrate and the molybdenum alloy substrate using a material integrally molded from a material consisting of at least one element selected from the group consisting of Ti, V, Zr, and Mo, and a material consisting of at least one element selected from the group consisting of Nb, Ta, and W.

[0025] The joining is preferably performed using a material formed by integrally molding multiple members and a brazing material. The brazing material preferably includes a first brazing layer located on the joining surface of the molybdenum alloy substrate and a second brazing layer located on the joining surface of the carbon substrate. The first brazing layer can be formed from, for example, Ti or a Ti alloy. The Ti alloy preferably contains, for example, at least one element selected from the group consisting of Nb, V, and Mo as an element other than Ti. The second brazing layer can be formed from, for example, Zr or a Zr alloy. The Zr alloy preferably contains, for example, at least one element selected from the group consisting of Nb and Mo as an element other than Zr.

[0026] The thicknesses of the first brazing layer and the second brazing layer may be the same or different. Preferably, the thicknesses of the first and second brazing layers are 0.1 mm or more and 0.4 mm or less. Preferably, the ratio (α2 / α1) of the thickness of the second brazing layer (let's call it α2) to the thickness of the first brazing layer (let's call it α1) is 0.5 or more and 4 or less. If the amount of the second brazing material is less than the amount of the first brazing material, or if the amount of the first brazing material is less than the amount of the second brazing material, bonding defects such as voids and peeling are more likely to occur in the brazing layer with the less abundant material. By setting the ratio of the thickness of the second brazing layer to the thickness of the first brazing layer to 0.5 or more and 4 or less, it is possible to avoid a shortage in the amount of either the first or second brazing material, and thus it is possible to suppress the occurrence of bonding defects such as voids and peeling in the amount of first brazing material and the second brazing layer.

[0027] It is desirable that the integrally molded material is integrally molded before joining. By integrally molding before joining, alloying of the joining layer can be promoted. As a result, the compositional unevenness of the joining layer can be reduced, and thus the uniformity of the hardness of the joining layer can be improved. The integrally molded material may be in the form of a plate or foil. Alternatively, the integrally molded material may be produced by film deposition such as cladding, paste, or sputtering. It is desirable that the integrally molded material is an intermediate layer located between the first brazing layer and the second brazing layer.

[0028] The intermediate layer preferably includes a core material and surface materials laminated on both sides of the core material. The core material preferably is made of a material that does not melt at the joining temperature. An example of such a material is a material containing at least one element selected from the group consisting of Nb, Ta, and W. The surface material preferably has a first surface material formed on one side of the core material and a second surface material formed on the other side of the core material. The first surface material preferably comes into contact with the first brazing layer. The first surface material can be made of, for example, Ti or a Ti alloy. The second surface material preferably comes into contact with the second brazing layer. The second surface material can be made of, for example, Zr or a Zr alloy. By the first surface material coming into contact with the first brazing layer and the second surface material coming into contact with the second brazing layer, the decrease in heat resistance due to the eutectic reaction between Mo and Zr can be suppressed. It is preferable that the first surface material and the first brazing layer share one or more constituent elements. It is preferable that the second surface material and the second brazing material share one or more constituent elements.

[0029] In the intermediate layer, it is desirable that the first and second surface materials are integrated with the core material by lamination or other means. The intermediate layer can be formed, for example, by cladding (cladding a plate). A cladding plate can be manufactured by the following method: Prepare foils of the respective metals or alloy foils containing the respective metals as the core material and the first and second surface materials. Laminate the prepared foils in the order of the first surface material, core material, and second surface material, perform hot working in a hot rolling mill, and then heat in a vacuum at 1400°C to 1600°C to bond them together and obtain a cladding plate. Since cladding plates exhibit significant surface oxidation, it is desirable to perform surface polishing, acid treatment, or both before using them as an intermediate layer. Alternatively, the intermediate layer may be manufactured using a film deposition method such as chemical vapor deposition (CVD). Alternatively, the intermediate layer may be manufactured by screen printing metal pastes that will be the first and second surface materials onto a metal plate or alloy plate that will be the core material, and then baking them in a vacuum.

[0030] In the intermediate layer, the thickness of the core material (let's call it thickness T), the thickness of the first surface material (let's call it thickness T1), and the thickness of the second surface material (let's call it thickness T2) may be equal or different from each other. Preferably, the sum of the thicknesses of the first surface material T1 and the second surface material T2 (let's call it total thickness T0) is less than or equal to the thickness of the core material T. If the total thickness T0 is greater than the thickness of the core material T, excessive interdiffusion of elements during heat treatment may occur, potentially leading to the formation of a low-melting-point phase. This low-melting-point phase may not only impair heat resistance, but may also reduce the processability of the intermediate layer when it is manufactured by rolling. By making the total thickness T0 less than or equal to the thickness of the core material T, the formation of the low-melting-point phase can be suppressed. As a result, the heat resistance and rollability of the intermediate layer can be ensured. Furthermore, it is preferable that the ratio of the thickness of the first surface material T1 to the thickness of the core material T (T1 / T) and the ratio of the thickness of the second surface material T2 to the thickness of the core material T (T2 / T) are both 1 / 100 or greater. The cladding effect is easily obtained when the ratio of the thickness T1 of the first outer layer to the thickness T of the core material (T1 / T) and the ratio of the thickness T2 of the second outer layer to the thickness T of the core material (T2 / T) are 1 / 100 or greater. Note that the thickness ratio (T1 / T) and the thickness ratio (T2 / T) may be the same value or different values. Preferably, the ratio of the thickness T2 of the second outer layer to the thickness T1 of the first outer layer (T2 / T1) is between 0.3 and 2. If the ratio of the thickness T2 of the second outer layer to the thickness T1 of the first outer layer (T2 / T1) is less than 0.3 or greater than 2, the difference in thickness will be large, making the cladding material prone to warping. If the cladding material is warped, handling and processing during the creation of the intermediate layer becomes difficult, making the creation of the intermediate layer difficult. In addition, it becomes difficult to accurately mount a warped intermediate layer before the joining process. By setting the ratio (T2 / T1) to between 0.3 and 2, warping of the clad material can be suppressed.

[0031] The thickness of the intermediate layer is preferably 0.1 mm or more and 1.2 mm or less. The thickness of the intermediate layer can be expressed as the sum of T and T0, or the sum of T, T1, and T2.

[0032] The joining process can be carried out, for example, using a vacuum furnace. The joining temperature is preferably between 1800°C and 2000°C. By setting the joining temperature within this range, the eutectic reaction between Mo and Zr can be suppressed by the shielding effect of the intermediate layer, thereby preventing the formation of voids in the alloy layer. As a result, an alloy layer with highly uniform hardness can be formed. The holding time at the joining temperature is preferably between 10 minutes and 60 minutes. The joining may be carried out while maintaining a constant temperature, or by gradually or continuously changing the temperature. Furthermore, to promote the diffusion of the joined layer, it is desirable to hold the mixture at 1600°C for between 60 minutes and 360 minutes during cooling. This holding step may be performed as a separate post-process from the joining treatment.

[0033] The thickness of the bonding layer is preferably 0.2 mm to 2.0 mm. The thickness of the bonding layer is obtained by using a quantitative line analysis tool of SEM / EDS to perform a uniaxial scan in the thickness direction of the bonding layer and identify the region where C is less than 40 at% and Mo is less than 80 at%. If the above boundary composition is not obtained within the scanning range, it is necessary to expand the scanning range until the boundary composition is obtained.

[0034] The method of the embodiment may include steps other than joining, cooling, and holding. Furthermore, after the holding step, polishing of the target surface may be performed as needed. Additionally, the method may include a step of providing a rotation axis for the X-ray tube target.

[0035] An example of applying the X-ray tube target of the embodiment to an X-ray tube anode target will be described with reference to Figure 4. The X-ray tube anode target 40 shown in Figure 4 can generate X-rays by irradiating the electron beam irradiation surface 42 with an electron beam. The X-ray tube anode target 40 comprises a molybdenum alloy substrate 41, an electron beam irradiation surface 42, a carbon substrate 43, and an alloy layer (bonding layer) 44. The molybdenum alloy substrate 41 has a substantially frustoconical shape. The molybdenum alloy substrate 41 is provided with a cylindrical through hole 41a that is concentric with the rotation center 45. The electron beam irradiation surface 42 has a ring shape. The electron beam irradiation surface 42 is provided on the inclined surface of the outer circumference of the molybdenum alloy substrate 41. The electron beam irradiation surface 42 can mitigate the impact received during electron beam irradiation. The electron beam irradiation surface 42 is formed from, for example, a ReW alloy.

[0036] The carbon substrate 43 has a cylindrical shape. The carbon substrate 43 is provided with a cylindrical through hole 43a that is concentric with the rotation center 45. In addition, a V-shaped groove (not shown) is provided on the ring-shaped upper surface of the carbon substrate 43 at a position concentric with the rotation center 45.

[0037] The alloy layer 44 is positioned between the molybdenum alloy substrate 41 and the carbon substrate 43, joining them together. The alloy layer 44 is similar to the alloy layer 4 described with reference to Figure 1. The alloy layer 44 is provided with a cylindrical through-hole 44a that is concentric with the rotation center 45. The through-hole 44a is connected to the through-holes 41a and 43a, respectively.

[0038] A rotating shaft (not shown) is inserted into a cylindrical space consisting of through holes 41a, 44a, and 43a.

[0039] The X-ray tube target of this embodiment can be applied to CT devices such as medical CT (computed tomography) scanners.

[0040] In the X-ray tube target of the embodiment described above, the alloy layer for joining the molybdenum alloy substrate and the carbon substrate contains a solid solution composed of at least two elements selected from the group consisting of Ti, V, Zr, Nb, Mo, Ta, and W. Furthermore, the alloy layer has multiple hardness levels, and the minimum hardness among the multiple hardness levels is 70% or more of the maximum hardness. As a result, the variation in the hardness of the alloy layer can be reduced, making it possible to realize an X-ray tube target that can improve thermal fatigue characteristics while ensuring heat resistance.

[0041] Furthermore, the method for manufacturing an X-ray tube target according to the embodiment includes a step of joining a carbon substrate and a molybdenum alloy substrate using a material integrally molded from a material consisting of at least one element selected from the group consisting of Ti, V, Zr, and Mo, and a material consisting of at least one element selected from the group consisting of Nb, Ta, and W. This method makes it possible to form an alloy layer (joining layer) with improved hardness uniformity. As a result, it is possible to realize an X-ray tube target that can improve thermal fatigue characteristics while ensuring heat resistance.

[0042] The above embodiments will be described in detail below with reference to examples, but the present invention is not limited to the following embodiments without departing from the spirit of the invention.

[0043] (Examples and Comparative Examples) X-ray tube targets for Examples 1-6 and Comparative Examples 1-3 were manufactured by joining a molybdenum alloy substrate having a Re-containing W layer on the irradiation surface with a carbon substrate made of CIP-molded isotropic carbon. Table 1 shows the composition of the alloy layers (joining layers) related to Examples 1-6 and Comparative Examples 1-3. Each joining layer has a first brazing layer, a second brazing layer, and an intermediate layer located between the first and second brazing layers. The thickness of the joining layer is the sum of the thicknesses of the first brazing layer, the second brazing layer, and the intermediate layer. The molybdenum alloy substrate, joining layer, and carbon substrate were laminated such that the first brazing layer was in contact with the molybdenum alloy substrate and the second brazing layer was in contact with the carbon substrate.

[0044] For the first brazing material layer, a single Ti (denoted as Ti in Table 1), a binary alloy sheet of Ti-Nb system, V-Ti system or Ti-Mo system was used. Each alloy sheet was produced by skull melting to form an alloy ingot, which was processed into a slag of appropriate size and then processed into a sheet by hot rolling and intermediate heat treatment. The thickness (mm) of the first brazing material layer in each example is shown in the parentheses in Table 1.

[0045] For the second brazing material layer, a single Zr or a rolled plate of Zr-Nb alloy was used. The thickness (mm) of the second brazing material layer in each example is shown in the parentheses in Table 1.

[0046] For the intermediate layers of Examples 1 to 7 and Comparative Example 3, a composite material in which a sheet of Nb, Ta, or W was used as the core material, Ti was arranged on one side, and Zr was arranged on the other side was used. In the column of the intermediate layer in Table 1, the types of metals of the first facing material contacting the first brazing material layer, the type of metal of the core material, and the types of metals of the second facing material contacting the second brazing material layer are shown in the order of the first facing material / core material / second facing material. For the intermediate layers of Comparative Example 1 and Comparative Example 2, a single Nb rolled plate was used. The thickness (mm) of the intermediate layer in each example is shown in the parentheses in Table 1.

[0047] The bonding conditions were maintained at a constant temperature within the range of 1850 °C or higher and 1900 °C or lower for a constant time within the range of 30 minutes or longer and 60 minutes or shorter in a vacuum. The bonding temperature and holding time for each example are shown in Table 1.

[0048]

[0049] Table 2 shows the thickness ratio of the constituent elements of the intermediate layer and the processing method of the cladding treatment.

[0050]

[0051] As shown in Table 2, in Examples 1-4 and 7, clad materials were produced by hot rolling and applied as an intermediate layer. In Example 5, a metal paste was screen printed onto an Nb plate by CVD, and in Example 6, a metal paste was screen printed onto the Nb plate. For the intermediate layers of Examples 1, 2 and 7, a bulk material with Nb as the core material, Ti on one side and Zr on the other, was processed into a clad material by hot rolling. In Example 3, a clad plate was produced in the same manner as in Example 1, except that Ta was used as the core material. In Example 4, a clad plate was produced in the same manner as in Example 1, except that W was used as the core material. In Example 5, Ti and Zr were deposited onto the surface of the Nb plate by CVD. In Example 6, a metal paste of Ti and Zr was applied to the surface of the Nb plate by screen printing. To ensure sufficient diffusion between elements, the composite plate materials of Examples 1-7 were placed in a vacuum furnace after each lamination process (after hot working) and heat-treated at 1500°C for 6 hours. The heat-treated material was wet-polished with #1200 grit sandpaper to remove the oxide film. After polishing, it was thoroughly dried to prevent further oxidation.

[0052] In Comparative Examples 1 and 2, the intermediate layers were composed of Nb plates alone and therefore no cladding treatment was applied. In Comparative Example 3, the intermediate layer used a cladding material prepared in the same manner as in Example 1, with Nb as the core material. In Comparative Example 3, the molybdenum alloy substrate, the bonding layer, and the carbon substrate were laminated and bonded so that the Zr surface of the intermediate layer was in contact with the first brazing layer and the Ti surface was in contact with the second brazing layer.

[0053] Table 3 shows the ratio of the thickness α2 of the second brazing layer to the thickness α1 of the first brazing layer (α2 / α1), the relationship between the total thickness T0 of the first surface material T1 and the second surface material T2 and the thickness T of the core material, the ratio of the thickness T1 of the first surface material to the thickness T of the core material (T1 / T), the ratio of the thickness T2 of the second surface material to the thickness T of the core material (T2 / T), and the ratio of the thickness T2 of the second surface material to the thickness T1 of the first surface material (T2 / T1).

[0054]

[0055] Table 4 shows the variation in Vickers hardness in the solid solution alloy layer region within the bonded layer. A Shimadzu micro-Vickers tester was used to measure Vickers hardness. The test was conducted in accordance with JIS Z 2244. The load was 50 g, the pressurization time was 20 seconds, and the diagonal dimensions of the indentation were read in both directions using the attached measuring instrument. The average value was then converted to Vickers hardness. The measurement sample was an X-ray tube target, which is the bonded body, cut to an appropriate size and embedded in resin so that the cross-section of the bonded layer appeared parallel to one side. The cross-section of the bonded layer was wet-polished up to #4000, and then polished to a mirror finish with a 0.2 μm diamond buff. The sample was also adjusted so that the parallelism between the mirror surface and the bottom surface opposite it was 0.01 mm or less.

[0056] Measurement points were started from positions that included at least one carbon substrate, and the measurement was performed uniaxially in 0.05 mm increments from the carbon substrate toward the molybdenum alloy substrate in the thickness direction of the bonding layer. The above measurements were performed at five arbitrary locations. Of the five measurement points, if four or more had a minimum hardness of 70% or more of the maximum hardness, it was considered "uniform," and if three or fewer had a minimum hardness, it was considered "non-uniform." A schematic diagram of how the measurement points were selected is shown in Figure 2.

[0057] However, measurement points must be selected to be at least 2 mm apart in the direction of the joint surface. Also, indentations made on carbides must be excluded. To determine whether an indentation was made on a carbide, the measurement sample can be examined by SEM-EDS analysis. Point analysis was performed at a total of eight points: the vertices of the diagonals of the indentation (diagonal edges) and 3 μm perpendicularly from the midpoint of the side connecting the vertices. Details of the analysis points around the indentation are shown in Figure 3.

[0058] For the analysis, a JEOL (JEOL Ltd.) FE-SEM / EDS was used, with an acceleration voltage of 15 kV and a detection time of 50 seconds. The specified elements were the constituent elements of the brazing material, as well as Mo and C. When the composition was obtained in atomic percent for each analysis point, if even one of the eight points had a C content of 40 at% or more, or a Mo content of 80 at% or more, that indentation was excluded from the hardness evaluation.

[0059] Examples 1 to 7 all showed no variation in hardness, indicating that the bonding layer was formed by a uniform solid solution alloy layer. On the other hand, Comparative Examples 1 to 3 all showed large variations in hardness, and tended to form soft areas in the intermediate regions of the bonding layer.

[0060] Table 4 shows the composition of the solid solutions contained in the alloy layers of the examples and comparative examples. Furthermore, the alloy layers of Examples 1 to 7 contained ZrC as a metal carbide. ZrC was present in granular or layered form near the boundary between the alloy layer and the carbon substrate. The values ​​in parentheses in the solid solution alloy composition indicate the at% of each element. Since the composition of the solid solution alloy varies depending on the location in the alloy layer, the at% of each element represents the range from the minimum to the maximum value in the alloy layer.

[0061]

[0062] Table 5 shows the bonding ratio, heat resistance at 1600°C, and thermal fatigue characteristics of the examples and comparative examples. The bonding ratio was measured using ultrasonic testing with a Hitachi Fine-SAT. The bonding ratio was defined as the area ratio of sound areas without defects at the interface relative to the design bonding area. Bonding performance was evaluated using the bonding area ratio after bonding as an indicator; a ratio of 90% or more was considered good bonding performance. Heat resistance and thermal fatigue characteristics were evaluated as the change in the bonding area ratio when the bonded body was repeatedly heated. Heating was performed by holding at 1600°C for 30 minutes in an argon atmosphere. The heating rate and cooling rate were set to 20°C / min. Heat resistance was evaluated using the change in the bonding area ratio after one heating cycle as an indicator. If the change in the bonding area ratio was less than 2%, the heat resistance was classified as good; if it was 2% or more, the heat resistance was classified as poor. Furthermore, the change in the bonding area ratio after repeating the above heat treatment 10 times was compared with the value before the start of the test, and the amount of change was used as an indicator to evaluate the thermal fatigue characteristics. If the change in the bonding area ratio was less than 3%, the thermal fatigue characteristics were considered good; if it was 3% or more, the thermal fatigue characteristics were considered poor.

[0063] Examples 1 to 7 showed good bonding properties, heat resistance, and thermal fatigue characteristics. On the other hand, Comparative Example 1 had poor bonding properties and thermal fatigue characteristics. Comparative Example 2 had relatively good bonding properties, but poor thermal fatigue characteristics. Comparative Example 3 was poor in bonding properties, heat resistance, and thermal fatigue characteristics.

[0064]

[0065] According to at least one embodiment or example of the X-ray tube target, the alloy layer joining the molybdenum alloy substrate and the carbon substrate contains a solid solution composed of at least two elements selected from the group consisting of Ti, V, Zr, Nb, Mo, Ta, and W. Furthermore, the minimum hardness of the alloy layer is 70% or more of the maximum hardness. As a result, it is possible to provide an X-ray tube target that can improve thermal fatigue characteristics while ensuring heat resistance.

[0066] While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents.

[0067] The invention of the embodiment is described below.

[0068] (1) An X-ray tube target comprising a carbon substrate, a molybdenum alloy substrate, and an alloy layer containing a solid solution of at least two elements selected from the group consisting of Ti, V, Zr, Nb, Mo, Ta, and W for joining the carbon substrate and the molybdenum alloy substrate, wherein the minimum hardness of the alloy layer is 70% or more of the maximum hardness.

[0069] (2) The target for an X-ray tube according to (1), wherein the solid solution is at least one selected from the group consisting of MoNbTi alloy, NbTi alloy, and ZrNb alloy.

[0070] (3) The X-ray tube target according to (1) or (2), wherein the hardness is Vickers hardness.

[0071] (4) The X-ray tube target according to any one of (1) to (3), wherein the alloy layer contains a metal carbide.

[0072] (5) A method for manufacturing an X-ray tube target, comprising the step of joining a carbon substrate and a molybdenum alloy substrate using a material integrally molded from a material consisting of at least one element selected from the group consisting of Ti, V, Zr, and Mo, and a material consisting of at least one element selected from the group consisting of Nb, Ta, and W.

[0073] (6) The method for manufacturing an X-ray tube target according to (5), further comprising the step of preparing the integrally molded material by cladding, chemical vapor deposition, or screen printing before joining.

[0074] (7) The method for manufacturing an X-ray tube target according to (5) or (6), wherein, before joining, a first brazing layer is placed between the integrally molded material and the molybdenum alloy substrate, and a second brazing layer is placed between the integrally molded material and the carbon substrate.

[0075] (8) The method for manufacturing an X-ray tube target according to any one of (5) to (7), wherein the integrally molded material comprises a core material made of at least one element selected from the group consisting of Nb, Ta, and W; a first surface material formed on one surface of the core material and in contact with the first brazing layer; and a second surface material formed on the other surface of the core material and in contact with the second brazing layer, wherein the first surface material and the second surface material each contain at least one element selected from the group consisting of Ti, V, Zr, and Mo.

[0076] 1... Joint of the X-ray tube target, 2... Molybdenum alloy substrate, 3... Carbon substrate, 4... Alloy layer, 5... Groove, 6... Metal carbide particles, 7... Metal carbide layer, 8... Micro Vickers indentation (indenter mark), 9... Analysis point, 40... X-ray tube anode target, 41... Molybdenum alloy substrate, 41a... Through hole, 42... Electron beam irradiation surface, 43... Carbon substrate, 43a... Through hole, 44... Alloy layer, 44a... Through hole, 45... Center of rotation.

Claims

1. An X-ray tube target comprising a carbon substrate, a molybdenum alloy substrate, and an alloy layer containing a solid solution of at least two elements selected from the group consisting of Ti, V, Zr, Nb, Mo, Ta, and W for joining the carbon substrate and the molybdenum alloy substrate, wherein the minimum hardness of the alloy layer is 70% or more of the maximum hardness.

2. The target for an X-ray tube according to claim 1, wherein the solid solution is at least one selected from the group consisting of MoNbTi alloy, NbTi alloy, and ZrNb alloy.

3. The target for an X-ray tube according to claim 1, wherein the hardness of the alloy layer is measured in Vickers hardness.

4. The target for an X-ray tube according to claim 1, wherein the alloy layer contains a metal carbide.

5. A method for manufacturing an X-ray tube target, comprising the step of joining a carbon substrate and a molybdenum alloy substrate using a material integrally molded from a material consisting of at least one element selected from the group consisting of Ti, V, Zr, and Mo, and a material consisting of at least one element selected from the group consisting of Nb, Ta, and W.

6. The method for manufacturing an X-ray tube target according to claim 5, further comprising the step of preparing the integrally molded material by cladding, chemical vapor deposition, or screen printing before joining.

7. The method for manufacturing an X-ray tube target according to claim 5, wherein, before joining, a first brazing layer is placed between the integrally molded material and the molybdenum alloy substrate, and a second brazing layer is placed between the integrally molded material and the carbon substrate.

8. The method for manufacturing an X-ray tube target according to claim 7, wherein the integrally molded material comprises a core material made of at least one element selected from the group consisting of Nb, Ta, and W; a first surface material formed on one surface of the core material and in contact with the first brazing layer; and a second surface material formed on the other surface of the core material and in contact with the second brazing layer, wherein the first surface material and the second surface material each contain at least one element selected from the group consisting of Ti, V, Zr, and Mo.

Images