Α+β type titanium alloy, titanium alloy rod, component, and method for producing α+β type titanium alloy

Abstract

This α+β type titanium alloy comprises 4.00-5.50% of Al, 1.40-4.50% of one or both of Fe and Cr, 1.50-5.00% of Mo, 0-4.00% of V, 0.05-0.25% of O, less than 0.10% of Si, less than 0.010% of C, and Ti and impurities as the remainder, wherein: included in the structure are a granular α phase having an aspect ratio of 8 or less and a mixed structure including a needle-like α phase and a β phase; the maximum width of the needle-like α phase is 0.01-5.00 μm; and, where the element having the higher content between Fe and Cr is element M, the concentration distribution of element M in the mixed structure satisfies formula (1).　Formula 1: (A - B) × C / D ≥ 0.160

Description

α+β titanium alloy, titanium alloy rod, part, and method for producing α+β titanium alloy

[0001] The present invention relates to an α+β titanium alloy, a titanium alloy rod, a part, and a method for manufacturing an α+β titanium alloy. This application claims priority to Japanese Patent Application No. 2024-064773, filed on April 12, 2024, the contents of which are incorporated herein by reference.

[0002] Titanium alloys are lightweight, high-strength materials used in fields such as aircraft, automobiles, and golf clubs. Among α+β titanium alloys composed of a close-packed hexagonal α phase and a cubic β phase, Ti-6Al-4V alloy, Ti-5Al-1Fe alloy, and Ti-5Al-2Fe-3Mo alloy are known to have particularly high strength. Among these α+β titanium alloys, some are used as parts for motors that operate at speeds of tens of thousands of rpm, specifically, motor sleeves to prevent rotor damage.

[0003] When a high-strength α+β titanium alloy is used for a motor sleeve, it is required to have high fatigue strength, which is the strength against repeated loads, and high creep properties against high stress conditions during high-speed rotation.

[0004] The Ti-5Al-2Fe-3Mo alloy mentioned above can achieve higher tensile strength and fatigue strength than the Ti-6Al-4V alloy by subjecting it to the solution treatment and aging treatment described in Patent Documents 1 and 2. However, because the proportional limit of the Ti-5Al-2Fe-3Mo alloy is relatively low, when it is used as a material for a motor sleeve, large creep strain may occur under conditions where high stress is applied due to the rotation of the motor, and there was room for improvement.

[0005] If a large creep strain occurs in a motor sleeve, the rotor's permanent magnets supported inside it may be damaged or shattered. Therefore, further improvements in the creep properties of α+β titanium alloys are required. To improve creep properties, a high proportional limit in the stress-strain curve is required. In particular, since the inside of a motor can reach a maximum temperature of 150°C, depending on the load on the motor, a high 0.05% yield strength at 150°C is required.

[0006] JP 2007-314834 A JP 2010-100943 A

[0007] An object of the present invention is to provide an α+β titanium alloy, a titanium alloy rod, and a method for manufacturing an α+β titanium alloy that have a high 0.05% yield strength at 150°C and excellent creep properties. Another object of the present invention is to provide a part that has excellent creep properties.

[0008] In order to solve the above problems, the present invention employs the following configuration: [1] An α+β type titanium alloy consisting of, in mass%, Al: 4.00% to 5.50%, one or both of Fe and Cr: 1.40% to 4.50%, Mo: 1.50% to 5.00%, V: 0% to 4.00%, O: 0.05% to 0.25%, Si: less than 0.10%, C: less than 0.010%, the balance: Ti and impurities, wherein the structure appearing in the cross section of the α+β type titanium alloy contains a granular α phase having an aspect ratio of 8 or less and a mixed structure containing an acicular α phase and a β phase, the maximum width of the acicular α phase being 0.01 μm to 5.00 μm, An α+β type titanium alloy, wherein the concentration distribution of element M in the mixed structure on the cross section satisfies formula (1), where element M is the element with the highest content of either Fe or Cr. (A-B) x C / D≧0.160 ... (1) where the concentration distribution of element M is the distribution of element M concentration on the line obtained by setting multiple measurement points along a line in the mixed structure on the cross section and irradiating each measurement point with an electron probe. In formula (1), A is the average concentration of the top 10 points in the concentration distribution of element M, B is the average concentration of the bottom 10 points in the concentration distribution of element M, C is the standard deviation in the concentration distribution of element M, and D is the arithmetic mean in the concentration distribution of element M. [2] The α+β type titanium alloy according to [1], further comprising one or two of Sn: 4.00% or less and Zr: 4.00% or less in place of a portion of Ti. [3] The alpha-beta titanium alloy according to [1] or [2], wherein the average equivalent circle diameter of the granular alpha phase is 20.0 μm or less. [4] The alpha-beta titanium alloy according to any one of [1] to [3], wherein the area ratio of the beta phase in the structure appearing in the cross section is 2.00% or more and less than 25.00%. [5] A titanium alloy rod made of the alpha-beta titanium alloy according to any one of [1] to [4]. [6] A part made of the alpha-beta titanium alloy according to any one of [1] to [4].[7] A method for producing an α+β type titanium alloy, the method comprising: (by mass%) Al: 4.00% or more and 5.50% or less, one or both of Fe and Cr: 1.40% or more and 4.50% or less, Mo: 1.50% or more and 5.00% or less, V: 0% or more and 4.00% or less, O: 0.05% or more and 0.25% or less, Si: less than 0.10%, C: less than 0.010%, and the balance: Ti and impurities; and (8) a method for producing an α+β type titanium alloy, the method comprising: subjecting an α+β type titanium alloy consisting of, in mass%, Al: 4.00% or more and 5.50% or less, one or both of Fe and Cr: 1.40% or more and 4.50% or less, Mo: 1.50% or more and 5.00% or less, V: 0% or more and 4.00% or less, O: 0.05% or more and 0.25% or less, Si: less than 0.10%, C: less than 0.010%, and the balance: Ti and impurities to a solution treatment and an aging treatment in this order, the aging treatment comprising a first treatment in which the alloy is held at a temperature T1 of 400°C or more and 650°C or less for 1 minute or more and 120 minutes or less, and a second treatment in which the alloy is held at a temperature of 350°C or more and (T1-20)°C or less for 60 minutes or more and then cooled. [8] A method for producing an α+β titanium alloy according to [7], which contains one or two of Sn: 4.00% or less and Zr: 4.00% or less instead of a portion of Ti. [9] A method for producing an α+β titanium alloy according to [7] or [8], wherein, after completion of the first treatment, the α+β titanium alloy is cooled to 100°C or less, and then the second treatment is carried out.

[10] A method for producing an α+β titanium alloy according to [7] or [8], wherein, after completion of the first treatment, the α+β titanium alloy is cooled to a temperature of 350°C or more and (T1-20)°C or less, and then the second treatment is carried out.

[11] A method for producing an α+β titanium alloy according to any one of [7] to

[10] , wherein the solution treatment is a step of heating the α+β titanium alloy to a temperature of (β transformation point - 100)°C or more and less than the β transformation point, and then quenching it to 300°C or less.

[0009] According to the present invention, it is possible to provide an α+β type titanium alloy, a titanium alloy rod, and a method for manufacturing an α+β type titanium alloy, which have a high 0.05% proof stress at 150°C and excellent creep properties. Furthermore, according to the present invention, it is possible to provide a part having excellent creep properties.

[0010] 1 is a schematic diagram of the metal structure of an α+β type titanium alloy according to one embodiment of the present invention. It is a diagram showing an example of a backscattered electron image of an α+β type titanium alloy according to the same embodiment. It is a diagram showing the granular α and mixed structure, and fields of view 1 and 2, which are examples of analysis areas by EDS, in the backscattered electron image of FIG. 2. It is a schematic diagram of the metal structure of an α+β type titanium alloy for explaining the measurement position of the concentration distribution of element M in the same embodiment.

[0011] For example, Ti-5Al-2Fe-3Mo alloy, an α+β titanium alloy, can be strengthened by retaining the β phase through water quenching after solution treatment, and then precipitating fine α phases within the β phase through subsequent aging treatment. However, the behavior and element partitioning during aging treatment vary depending on the solution treatment temperature and aging temperature. For example, if the aging temperature is low and the progress of element partitioning and the removal of residual strain is slow, both the α and β phases become unstable.

[0012] The inventors' investigations revealed that even in an α+β titanium alloy that exhibits high tensile strength and fatigue strength, if an unstable β phase is present in the structure, the unstable β phase will undergo plastic deformation first under low stress, resulting in large creep strain. Therefore, it was expected that if the unstable β phase could be stabilized and the 0.05% proof stress improved while maintaining high tensile strength and fatigue strength, it would be possible to reduce the creep strain of the α+β titanium alloy and improve its creep properties.

[0013] Further, the inventors conducted extensive research and discovered that excellent creep properties can be obtained by forming a structure having a mixed structure consisting of a granular α phase and an acicular α phase and a β phase, further refining the acicular α phase in the structure, and further controlling the element distribution in the acicular α phase and the β phase to a preferred state.

[0014] Next, a heat treatment method for obtaining a structure suitable for the α+β type titanium alloy of the present invention will be described.

[0015] When a titanium alloy processed into a desired shape is subjected to a general manufacturing method in which solution treatment and aging treatment are sequentially performed, a mixed structure containing a granular α phase as well as fine acicular α and β phases is obtained. However, it was found that in this mixed structure, the elemental distribution of Fe in the β phase is insufficient, and the β phase is in an unstable state. It was presumed that the presence of such an unstable β phase is the cause of the low 0.05% proof stress.

[0016] Therefore, instead of performing a typical single aging treatment, a second aging treatment was performed at a low temperature for a long period of time after the first aging treatment. This resulted in the morphology of extremely fine acicular α grains being maintained, while the elemental partitioning of Fe was promoted, stabilizing the β phase together with the acicular α phase, and it was found that this made it possible to provide a titanium alloy with excellent creep properties in an industrially stable manner. Since Fe is an effective β-stabilizing element and is thought to diffuse and partition relatively easily, the present invention focused on the concentration distribution of Fe. It was also found that when Fe is not contained, it can be replaced by Cr, a eutectoid element that similarly has the effect of stabilizing the β phase and diffuses relatively easily.

[0017] Hereinafter, an α+β type titanium alloy and a method for producing the same according to an embodiment of the present invention will be described.

[0018] The α+β titanium alloy of this embodiment is an α+β titanium alloy consisting of, in mass%, Al: 4.00% to 5.50%, one or both of Fe and Cr: 1.40% to 4.50%, Mo: 1.50% to 5.00%, V: 0% to 4.00%, O: 0.05% to 0.25%, Si: less than 0.10%, C: less than 0.010%, and the balance: Ti and impurities. The structure appearing in the cross section of the α+β titanium alloy contains a granular α phase with an aspect ratio of 8 or less and a mixed structure containing an acicular α phase and a β phase, and the maximum width of the acicular α phase is 0.01 μm to 5.00 μm. When the element M is the element with the highest content of Fe or Cr, the concentration distribution of element M in the mixed structure on the cross section satisfies formula (1). (A-B) x C / D ≥ 0.160 ... (1) Here, the concentration distribution of element M is the distribution of element M concentration on a line obtained by setting multiple measurement points along a line in the mixed structure on a cross section and irradiating each measurement point with an electron probe. Furthermore, in the above formula (1), A is the average concentration of the top 10 points in the concentration distribution of element M, B is the average concentration of the bottom 10 points in the concentration distribution of element M, C is the standard deviation in the concentration distribution of element M, and D is the arithmetic mean in the concentration distribution of element M. Furthermore, the α+β titanium alloy of this embodiment may contain one or two of Sn: 4.00% or less and Zr: 4.00% or less in place of a portion of Ti. Furthermore, in the α+β titanium alloy of this embodiment, the average equivalent circle diameter of the granular α phase is preferably 20.0 μm or less. Furthermore, in the α+β titanium alloy of this embodiment, the lattice constant of the β phase is preferably 0.3220 nm or less. Furthermore, the α+β type titanium alloy of this embodiment preferably has a 0.05% yield strength of 900 MPa or more at 150° C. Furthermore, the α+β type titanium alloy of this embodiment preferably has an area ratio of β phase of 2.00% or more and less than 25.00%.

[0019] In this specification, 0.05% proof stress is the stress at which a permanent strain of 0.05% occurs. Specifically, it refers to the stress at the intersection of the stress-strain curve and a line obtained by shifting the elastic region approximation line by 0.05%.

[0020] First, the chemical composition of the α+β titanium alloy will be explained. In the following explanation, the percentages indicating the content of each element mean mass percent.

[0021] The α+β type titanium alloy of this embodiment preferably has a metal structure in which, for example, at 25°C, the α phase is the main phase and the β phase is the secondary phase. However, in order to improve fatigue properties and creep properties, it is necessary to increase the material strength, and a chemical composition that results in a tensile strength of 1000 MPa or more is desirable.

[0022] Al: 4.00% or more and 5.50% or less Al acts as an α-stabilizing element and has the effect of increasing strength through solid solution strengthening. In order to obtain sufficient strength of 1000 MPa or more, it is necessary to contain 4.00% or more of Al. However, if the Al content exceeds 5.50%, ductility and cold workability at high temperatures and room temperature may decrease. Therefore, the Al content is set to 4.00% or more and 5.50% or less. The lower limit of Al is preferably 4.25% or more, more preferably 4.50% or more, and even more preferably 4.70% or more.

[0023] One or both of Fe and Cr: 1.40% or more, 4.50% or less. Fe and Cr act as β-stabilizing elements and have the effect of increasing strength through solid solution strengthening. To obtain sufficient strength of 1000 MPa or more, it is necessary to contain one or both of Fe and Cr in a total amount of 1.40% or more. On the other hand, as the Fe and Cr contents increase, the amount of β phase increases and workability improves. However, since Fe and Cr are elements that tend to segregate during solidification, the segregation of these elements becomes more pronounced as the Fe and Cr contents increase. Therefore, one or both of Fe and Cr are set to 4.50% or less. When both Fe and Cr are contained, the total amount is set to 1.40% or more, 4.50% or less.

[0024] Mo: 1.50% or more, 5.00% or less Like Fe, the strength of Mo increases with increasing content. Furthermore, the β-phase fraction at high temperatures increases, improving hot workability. Therefore, Mo is contained in an amount of 1.50% or more. Preferably, it is 1.70% or more, and more preferably, 2.00% or more. On the other hand, Mo, like Fe, is an element prone to segregation. If Mo is contained in excess, segregation becomes significant in large ingots weighing several hundred kg, and segregation cannot be eliminated even if measures to alleviate segregation are taken in subsequent processes. Therefore, the upper limit of the Mo content is set to 5.00% or less.

[0025] V: 0% or more, 4.00% or less Like Fe, the strength increases as the V content increases. Furthermore, the β-phase ratio at high temperatures increases, improving hot workability. Therefore, V may be added. On the other hand, like Fe, V is an element that is prone to segregation. If V is added in excess, segregation becomes significant in large ingots weighing several hundred kg, and segregation cannot be eliminated even if measures to mitigate segregation are taken in subsequent processes. Therefore, the upper limit of the V content is set to 4.00% or less.

[0026] O: 0.05% or more, 0.25% or less O increases strength but reduces ductility. Therefore, the O content is set to 0.25% or less. There is no particular lower limit, but in order to ensure strength, 0.05% or more is preferable.

[0027] Si: less than 0.10% C: less than 0.010% Si and C are impurities, and if they are contained in large amounts, they may reduce room temperature ductility and cold workability. Therefore, it is desirable that Si be less than 0.10% and C be less than 0.010%. Note that, since the inclusion of Si and C is unavoidable as inevitable impurities, the lower limit of their substantial content is usually 0.005% or more for Si and 0.0005% or more for C.

[0028] Balance: Ti and impurities Impurities other than Si and C include N and H. The upper limit of N is preferably 0.08% or less, and the upper limit of H is preferably 0.015% or less. Furthermore, impurity elements include Ni, Mn, Nb, and Cu, and the total content of these elements is preferably less than 0.3%, and each element is preferably 0.1% or less.

[0029] Furthermore, the α+β type titanium alloy of this embodiment may contain one or both of Sn: 4.00% or less and Zr: 4.00% or less in place of a portion of Ti.

[0030] Sn and Zr are elements that dissolve in the α phase and the β phase to strengthen both phases. When dissolved in the α phase, they can strengthen the steel without impairing ductility and toughness. If the Sn content and Zr content each exceed 4.00%, toughness decreases. Therefore, the Sn content and Zr content are each set to 4.00% or less. When one or both of Sn and Zr are contained, they may each be 0.20% or more, 0.50% or more, or 1.00% or more.

[0031] Next, the structure of the α+β titanium alloy of this embodiment will be described. As shown in Figure 1, the α+β titanium alloy of this embodiment contains, in its cross-section, a granular α phase 1 with an aspect ratio of 8 or less, and a mixed structure 2 containing an acicular α phase 21 and an acicular β phase 22. Note that the cross section refers to, for example, a cross section perpendicular to the longitudinal direction of a bar (titanium alloy bar) made of an α+β titanium alloy. Alternatively, in a plate made of an α+β titanium alloy, the cross section refers to a cross section perpendicular to the surface of the plate. The cross section perpendicular to the surface of the plate includes a "cross section parallel to the plate width and plate thickness" and a "cross section parallel to the plate thickness and rolling direction," but either cross section is acceptable.

[0032] In this embodiment, the inclusion of phases or structures other than the granular α phase and the mixed structure in the structure is not excluded, but it is not necessary for phases or structures other than the granular α phase and the mixed structure to be contained.

[0033] Granular α-phases with an aspect ratio of 8 or less are considered to be the α-phase formed during solution treatment that remains after aging treatment. The shape is equiaxed or elongated, and the cross section shows a shape similar to that of grains. In this embodiment, α-phases with an aspect ratio of 8 or less are referred to as granular α-phases. The aspect ratio is the ratio of the major axis length to the minor axis length of the α-phase (major axis length / minor axis length).

[0034] The average equivalent circle diameter of the granular α phase is preferably 20.0 μm or less. If the average equivalent circle diameter of the granular α phase is too large, the tensile strength will be reduced. Furthermore, if the average equivalent circle diameter is too large, it will easily become the starting point for crack initiation in normal fatigue, and the fatigue strength will also be reduced. Therefore, the average equivalent circle diameter of the granular α grains is preferably 20.0 μm or less.

[0035] The mixed structure is a region other than the granular α phase, and includes acicular α phase and β phase. The mixed structure is formed by aging treatment.

[0036] The acicular α phase contained in the mixed structure has an aspect ratio of more than 8. Its shape is needle-like. In this embodiment, an α phase with an aspect ratio of more than 8 is called an acicular α phase. It is believed that the α phase takes on a unique needle-like shape as a result of being strongly influenced by the crystal orientation relationship during shear transformation during aging treatment. Since the aspect ratio of the granular α phase is 8 or less, it is possible to distinguish between the granular α phase and the acicular α phase by observing the shape of each phase in a cross section.

[0037] The maximum width of the acicular α phase is preferably in the range of 0.01 μm or more and 5.00 μm or less. If the maximum width of the acicular α phase is large, the tensile strength decreases. On the other hand, if the maximum width is too small, fatigue cracks tend to propagate more easily, resulting in a decrease in fatigue strength. Therefore, the maximum width of the acicular α phase is preferably 0.01 μm or more and 5.00 μm or less. The lower limit of the maximum width of the acicular α phase is preferably 0.03 μm or more, more preferably 0.05 μm or more, and even more preferably 0.08 μm or more. The upper limit of the maximum width of the acicular α phase is preferably 4.90 μm or less, more preferably 4.85 μm or less.

[0038] The β phase contained in the mixed structure is a β phase formed by the elemental partitioning of Fe or Cr, which is a β stabilizing element, during aging treatment. As a result of the elemental partitioning, the concentration of Fe or Cr contained in the β phase is thought to approach the equilibrium concentration.

[0039] The lattice constant of the β phase is thought to correspond to the stability of the β phase. The more β-stabilizing elements with atomic radii smaller than titanium are dissolved in the solid solution, the smaller the lattice constant of the β phase becomes, and the more stable the state becomes. Therefore, the lattice constant of the β phase is preferably 0.3220 nm or less. There is no particular lower limit for the lattice constant of the β phase, but it may be 0.3180 nm or more from the viewpoint of ensuring strength.

[0040] In addition, the α+β type titanium alloy of this embodiment preferably has a β phase area ratio in the structure appearing in the cross section of 2.00% or more and less than 25.00%. If the β phase area ratio is 2.00% or more, the necessary tensile strength can be ensured. Furthermore, if the β phase area ratio is less than 25.00%, unstable β phase is less likely to form, and the decrease in the proportional limit can be suppressed.

[0041] Next, the element distribution state in the mixed structure of this embodiment will be described. In the α+β type titanium alloy of this embodiment, when the element M is the element with the higher content of Fe or Cr, the concentration distribution of element M in the mixed structure on the cross section must satisfy formula (1). Note that when the contents of Fe and Cr are the same, Fe is the element.

[0042] (A-B)×C / D≧0.160… (1)

[0043] Here, the concentration distribution of element M is a distribution of the concentration of element M on a line obtained by setting a plurality of measurement points along a line in a mixed structure appearing in a cross section and irradiating each measurement point with an electron probe. Also, in the above formula (1), A is the average concentration of the top 10 points in the concentration distribution of element M, B is the average concentration of the bottom 10 points in the concentration distribution of element M, C is the standard deviation in the concentration distribution of element M, and D is the arithmetic mean in the concentration distribution of element M.

[0044] The element M, Fe and Cr, are eutectoid β-stabilizing elements that diffuse faster during aging than other elements and are more likely to distribute during aging. Therefore, the concentration distribution of element M is suitable as an indicator of the stability of the β-phase in a mixed structure. The larger the left side of formula (1) ((A-B) x C / D), the greater the concentration fluctuation of element M on the straight line. Conversely, the smaller the left side of formula (1), the smaller the concentration fluctuation of element M. In a mixed structure, the aging treatment described below causes element distribution of element M, which concentrates element M in the β-phase and stabilizes the β-phase. However, the concentration of element M in the acicular α-grains decreases, resulting in a large concentration fluctuation of element M. Furthermore, satisfying formula (1) reduces the proportion of unstable β-phase and improves the 0.05% proof stress at 150°C. Furthermore, formula (1) is, for example, 10 or less.

[0045] For example, the α+β type titanium alloy of this embodiment preferably has a 0.05% yield strength of 900 MPa or more at 150° C. There is no particular upper limit to the 0.05% yield strength at 150° C., but it may be set to 1400 MPa or less from the viewpoint of ensuring workability.

[0046] The method for evaluating the structure of the α+β titanium alloy of this embodiment will be described below. First, because the mixed structure containing the acicular α phase and the β phase and the granular α phase have different element concentrations, they can be easily distinguished by observation using backscattered electron images from an SEM. After distinguishing between the mixed structure and the granular α phase by observation using backscattered electron images, the aspect ratio of the α phase and the average circular equivalent grain size of the granular α phase are measured using EBSD. Details of these measurement methods will be described later. Furthermore, by analyzing the region identified as a mixed structure through structure observation using EDS (Energy Dispersive X-ray Spectroscopy), the amounts of elements contained in the acicular α phase and the β phase can be easily measured.

[0047] The concentration distribution of element M is measured as follows. First, as described above, a backscattered electron image of a cross section of an α+β titanium alloy is observed using a scanning electron microscope (SEM) to identify the granular α phase and the mixed structure. The observation magnification is 20,000x. The field size at 20,000x observation magnification is approximately 6.4 μm × 4.8 μm. FIG. 2 shows an example of a backscattered electron image of an α+β titanium alloy according to this embodiment. FIG. 3 shows the backscattered electron image of FIG. 2 , showing the granular α (granular α phase) and mixed structure, as well as fields 1 and 2, which are examples of EDS analysis areas. The granular α phase contains large amounts of Al and O, which are lighter than Ti, while the mixed structure contains large amounts of Fe, Cr, and Mo, which are heavier than Ti. Lighter elements have a lower backscattered electron emission rate, so the granular α phase appears darker in the backscattered electron image. For example, as shown in Figure 2, the black region is the granular α phase, and the gray region is the mixed structure. Therefore, dark regions in the backscattered electron image are identified as the granular α phase. The mixed structure is defined as the region other than the granular α phase. However, if a phase or structure that clearly cannot be considered a mixed structure containing acicular α phase and β phase is included in the region other than the granular α phase, it is excluded from the mixed structure. Phases or structures excluded from the mixed structure include intermetallic compounds such as TiFe compounds and granular ω phase. Phases or structures excluded from the mixed structure are observed as having a white (bright) contrast in the region other than the granular α phase. When the α+β titanium alloy is in the form of a rod, a test piece is taken from the longitudinal center of the rod, with the cross section perpendicular to the longitudinal direction as the observation surface. The measurement point on the observation surface is located at a depth of 1 / 2 the radius r from the surface. The observation magnification is 20,000x, and the SEM acceleration voltage is 15 kV.

[0048] FIG. 4 is a schematic diagram of the metallographic structure of an α+β-type titanium alloy to explain the measurement positions of the concentration distribution of element M. FIG. 4 is a schematic diagram of the metallographic structure in fields 1 and 2 in FIG. 3. The schematic diagrams of fields 1 and 2 in FIG. 4 are also schematic diagrams of images acquired at a magnification of 20,000 times. On the identified mixed structure, for example, as shown in FIG. 4, two straight lines, each 4 μm, 5 μm, or 6 μm in length, are drawn. However, the two straight lines are made to be the same length. By setting the length of the lines to 4 μm, 5 μm, or 6 μm, the lines are drawn so as to span multiple acicular α phases and multiple β phases, as shown in FIG. 4. Then, multiple measurement positions are set for each line, and point analysis of the mixed structure is performed at each measurement position to measure the concentration of element M at each measurement position. The measurement positions are set at equal intervals of 0.005 μm along the analysis line. Point analysis is performed by irradiating each measurement position with an electron probe and detecting the resulting characteristic X-rays. For example, an EDS (energy dispersive X-ray elemental analyzer) attached to an SEM can be used. The probe diameter of the electron probe is 0.010 μm or less, preferably 0.005 μm or less, and more preferably 0.003 μm or less. This allows the concentration distribution of element M on two straight lines to be obtained.

[0049] Then, the average concentration A of the top 10 points in the concentration distribution of element M, the average concentration B of the bottom 10 points in the concentration distribution, the standard deviation C in the concentration distribution, and the arithmetic mean D in the concentration distribution are calculated, and (A-B) x C / D, which is the left side of equation (1), is calculated.

[0050] Next, we will describe methods for measuring the aspect ratio of the α phase, the average circular equivalent particle diameter of the granular α phase, the maximum width of the acicular α phase, and the area fraction of the β phase. The aspect ratio of the α phase, the average circular equivalent particle diameter of the granular α phase, and the maximum width of the acicular α phase are measured using an EBSD (Electron Backscatter Diffraction) device attached to a scanning electron microscope (SEM), and the area fraction of the β phase is measured using XRD. Note that the width (minor axis length) of the acicular β phase in the mixed phase structure is too small, on the order of several nanometers to several tens of nanometers, so EBSD cannot identify the β phase and calculate its area fraction.

[0051] For example, if the α+β titanium alloy is in the form of a round bar, a test piece is taken from the longitudinal center of the round bar, with the cross section perpendicular to the longitudinal direction as the observation surface. The observation surface is mechanically polished to a mirror finish, and then colloidal silica polishing is performed to remove the strain layer created by mechanical polishing. The measurement point on the observation surface is located at a depth of half the radius r of the round bar from the surface. The field of view at the measurement point on the observation surface of the test piece is a rectangular area measuring 250 μm long x 250 μm wide and 10 μm long x 10 μm wide. The former area is used to measure granular α grains, and the latter area is used to measure acicular α phases. The measurement interval is 0.5 μm for the former area and 0.02 μm for the latter area, and the measurement is performed using EBSD at an accelerating voltage of 15 kV. The obtained measurement results are analyzed using OIM (crystal orientation analysis software manufactured by TSL Solutions Co., Ltd.). First, a Partition is created that only covers the α phase, and the aspect ratio of each α phase is calculated from the ratio of its major axis length to its minor axis length.

[0052] Next, the boundary of the α phase is determined by setting the difference in the angle of the crystal orientation (misorientation angle) between adjacent EBSD measurement points to 15° or less, and the circle equivalent diameter is calculated from the number of measurement points of the α phase (granular α phase) with an aspect ratio of 8 or less to obtain the average circle equivalent diameter.

[0053] For α phases with an aspect ratio of more than 8 (acicular α phases), the maximum value of the minor axis diameter calculated from the ellipse approximation is defined as the maximum width.

[0054] The area ratio of the β phase is evaluated using an XRD (X-ray Diffraction) device. A test piece is taken from a titanium alloy bar, with the cross section parallel to the longitudinal direction as the observation surface. After mechanical polishing of the observation surface, the surface is ground 50 μm with a solution of nitric acid: hydrofluoric acid: lactic acid = 3:1:1 to remove the strain layer caused by the mechanical polishing. The measurement point on the observation surface is located at a depth of 1 / 2 the radius r from the surface. X-ray diffraction results are obtained using a Cu tube, a tube voltage of 40 kV, Kα1 characteristic X-rays, a measurement range of 30 to 90°, a slit width of 2 mm, and a measurement interval of 0.02°. The area fraction of the β phase is then calculated using the five diffraction peaks of the α phase (10-10), (0002), (10-11), (10-12), and (11-20) and the three diffraction peaks of the β phase (110), (200), and (211). The integrated intensity of the diffraction peaks is determined by fitting a background based on the signals before and after the peak and subtracting that signal. Furthermore, the diffraction peaks due to the Kα2 ray are removed from the diffraction pattern after fitting. The area fraction of the β phase is calculated from the diffraction peaks after these operations. Note that, among the notations for the plane orientation of the α phase, "-1" and "-2" refer to the notation of 1 or 2 with a bar above it. The volume fraction measured by XRD is considered to be the area fraction.

[0055] The lattice constant of the β phase is evaluated using an XRD (X-ray diffraction) device. A test piece is taken from a titanium alloy bar, with a cross section parallel to the longitudinal direction as the observation surface. The measurement point on the observation surface is located at a depth of 1 / 2 the radius r from the surface. Using a Cu tube, the tube voltage is 40 kV, the characteristic X-ray used is Kα1 ray, the measurement range is 30 to 90°, the slit width is 2 mm, and the measurement interval is 0.02° to obtain X-ray diffraction results. Then, using the three diffraction peaks of the β phase, (110), (200), and (211), the lattice constant of the β phase is calculated by extrapolation in sin2θ (2θ: diffraction peak position).

[0056] The α+β type titanium alloy of this embodiment can be applied to rods (titanium alloy rods), plates, wires, pipes, etc. From the rods, for example, motor sleeves, shafts, disks, blades, or fasteners (bolts, nuts, etc.) can be manufactured. From the plates, for example, covers can be manufactured. Motor sleeves can also be manufactured from plates.

[0057] Furthermore, the α+β titanium alloy of this embodiment is suitable for motor parts, specifically as a material for motor sleeves, which are reinforcing materials for motor rotors. The α+β titanium alloy of this embodiment has a higher 0.05% yield strength at 150°C than conventional titanium alloys and has excellent creep properties, so even if stress is continuously applied by the centrifugal force when the motor rotates, the motor sleeve will not break, and the motor's lifespan will be extended.

[0058] In fact, when the motor sleeve made of the α+β type titanium alloy of this embodiment is fitted into the rotor of a motor and used, it will not break during use for a cumulative driving time of 10 hours or more under conditions of an operating temperature of 15°C or higher and 150°C or lower, and a load stress of 900 MPa or lower, and will have excellent creep properties.

[0059] Furthermore, the use of the α+β type titanium alloy of this embodiment is not limited to motor sleeves, but can also include other automobile parts, compressors, aircraft parts, etc. Applications of the α+β type titanium alloy of this embodiment include, for example, aircraft-related shafts, disks, blades, covers, or fasteners. That is, parts according to embodiments of the present invention include various parts, such as parts that slide or rotate, fixed parts, or parts that may come into contact with such parts, specifically shafts, disks, blades, covers, or fasteners (bolts, nuts, etc.).

[0060] Next, a method for producing the α+β type titanium alloy of this embodiment will be described.

[0061] The method for producing an α+β titanium alloy according to this embodiment involves subjecting an α+β titanium alloy having the above-described chemical composition to a solution treatment and an aging treatment, in that order. The aging treatment involves a first treatment in which the alloy is held at a temperature T1 of 400°C to 650°C for 1 minute to 120 minutes, followed by a second treatment in which the alloy is held at a temperature of 350°C to (T1-20)°C for 60 minutes or more and then cooled. After the first treatment, the α+β titanium alloy may be cooled to 100°C or less before the second treatment. Alternatively, after the first treatment, the α+β titanium alloy may be cooled to 350°C to (T1-20)°C before the second treatment. Furthermore, the solution treatment is preferably a process in which the α+β titanium alloy is heated to a temperature of (β transformation point - 100)°C or more but less than the β transformation point, and then rapidly cooled to 300°C or less. The solution treatment is a heat treatment carried out in the α+β region as described above for the purpose of controlling the structure of the titanium alloy. The manufacturing method of this embodiment will be described in detail below.

[0062] The raw material for producing the α+β type titanium alloy of this embodiment includes titanium ingots melted by vacuum arc remelting (VAR) or electron beam remelting (EBR), as well as rods, plates, tubes, etc. made of α+β type titanium alloys produced using these as raw materials. Materials processed into the shapes of various parts, such as motor sleeves, may also be used. The dimensions of the raw material are not particularly limited, as long as they can be handled in the subsequent heat treatment.

[0063] Next, in the solution treatment, the material made of α+β type titanium alloy is heated to a temperature equal to or higher than (β transformation point - 100)°C but lower than the β transformation point, and then rapidly cooled to 300°C or lower. The β transformation point Tβ (°C) can be expressed as the following formula for the contents of the main elements and O, N, and C in the titanium alloy. The element symbols represent the mass % values ​​of the alloy elements.

[0064] Tβ(℃)=900+20×Al+120×(O+N)+100×C-10×Mo-18×(Fe+Cr)-15×V-2Sn-5Zr

[0065] If the solution treatment temperature is lower than (β transformation point - 100) °C, the effect of two-step aging cannot be obtained, and unstable β phase remains, resulting in a decrease in 0.05% proof stress. Furthermore, if the solution treatment temperature is higher than the β transformation point (Tβ) °C, coarse acicular α grains are formed, resulting in a significant decrease in fatigue strength and ductility. Therefore, the solution treatment temperature is preferably in the range of (β transformation point - 100) °C or higher and lower than the β transformation point.

[0066] Furthermore, in the solution treatment, rapid cooling is performed after heating to induce age hardening. Rapid cooling is generally performed by immersing the titanium alloy material in a sufficient amount of water to obtain a sufficient cooling rate, but other means that can obtain a cooling rate equivalent to or greater than that of water cooling may also be used. Rapid cooling is preferably continued until the surface temperature of the titanium alloy material reaches 300°C or less.

[0067] The holding time for the solution treatment is sufficient as long as the material to be heat treated is kept uniformly heated, and may be as short as about one minute depending on the shape of the material to be heat treated and the heat capacity of the furnace.

[0068] When heated to the solution temperature, the α phase decreases with a decrease in aspect ratio. Furthermore, while the solution temperature is maintained, the α phase proportion equivalent to the equilibrium state is maintained. Then, when rapidly cooled, the α phase remains. This remaining α phase becomes granular, with a shape similar to that of grains. Furthermore, the higher the solution temperature, the smaller the average equivalent circle diameter of the granular α phase.

[0069] Next, the aging treatment is a first treatment in which the material made of the titanium alloy after the solution treatment is held at a temperature T1 of 400°C or higher and 650°C or lower for 1 minute or longer and 120 minutes or shorter, and then a second treatment in which the material is held at a temperature of 350°C or higher and (T1-20)°C or lower for 60 minutes or longer, followed by cooling.

[0070] In the first treatment, if the aging temperature T1 is less than 400°C, the progress of age hardening and residual strain removal is slow, and the β phase becomes unstable. On the other hand, if the aging temperature T1 exceeds 650°C, the acicular α phase in the mixed structure becomes coarse, and the tensile strength and fatigue strength decrease. Therefore, the aging temperature T1 is preferably in the range of 400°C or higher and 650°C or lower.

[0071] Next, if the aging time of the first treatment is less than 1 minute, the progress of age hardening and residual strain removal is insufficient, and the β phase becomes unstable. On the other hand, if the aging time exceeds 120 minutes, the acicular α phase in the mixed structure becomes coarse, and the tensile strength and fatigue strength decrease. Therefore, the aging time of the first treatment is preferably in the range of 1 minute or more and 120 minutes or less.

[0072] Next, in the second treatment, if the aging temperature is less than 350°C, the element distribution progresses slowly, and the β phase becomes unstable. Furthermore, if the aging temperature exceeds (T1-20)°C, the acicular α phase with an aspect ratio of more than 8 becomes coarse, resulting in a decrease in tensile strength and fatigue strength. Furthermore, the number and magnitude of concentration fluctuations of Fe or Cr increase, resulting in a decrease in 0.05% proof stress. Therefore, the aging temperature in the second treatment is preferably in the range of 350°C or higher and (T1-20)°C or lower.

[0073] Next, if the aging time in the second treatment is less than 60 minutes, the element distribution and the residual strain removal will not proceed sufficiently, and the β phase will become unstable. Therefore, the aging time in the second treatment is preferably 60 minutes or more. There is no particular upper limit to the aging time in the second treatment, but it may be set to, for example, 6000 minutes or less from the viewpoint of production efficiency.

[0074] The α+β titanium alloy after the first treatment may be cooled to 100°C or less before the second treatment. By cooling the α+β titanium alloy after the first treatment to 100°C or less, the composition distribution ratio at equilibrium increases. In other words, the upper limit at which Fe and Cr can be concentrated increases. In other words, by cooling the α+β titanium alloy after the first treatment to 100°C or less, the composition distribution is likely to be promoted.

[0075] Alternatively, the α+β type titanium alloy after the first treatment may be cooled to a temperature of 350°C or higher and (T1-20)°C or lower before the second treatment. By cooling to a temperature of (T1-20)°C or lower, the composition distribution ratio at equilibrium increases slightly, and diffusion is more likely to occur, facilitating distribution. Furthermore, by cooling to a temperature of 350°C or higher, it is possible to prevent the diffusion rate from decreasing significantly.

[0076] After solution treatment or aging treatment, a cutting process may be performed to adjust the surface texture or to achieve a desired shape. For example, when manufacturing a motor sleeve from a bar, the center of the bar is hollowed out to form a cylindrical shape, and the hollow portion is expanded in diameter. Furthermore, when manufacturing a large motor sleeve, a large-diameter bar may be ring-forged into a cylindrical shape, followed by heat treatment and cutting to produce a cylindrical α+β titanium alloy. When manufacturing a motor sleeve from a plate, the cylindrical titanium plate is butted and welded at both ends. However, when identifying the metal structure described above, heavily processed portions and welded sections are excluded from the analysis. A structure with up to 10% strain may also be analyzed. Furthermore, plate materials may be subjected to temper rolling under conventional conditions. Applications of the α+β titanium alloy of this embodiment include, for example, aircraft-related shafts, disks, blades, covers, and fasteners. That is, the parts according to the embodiments of the present invention include various parts, such as parts that slide or rotate, fixed parts, or parts that may come into contact with the parts, and specific examples thereof include shafts, disks, blades, covers, or fasteners (bolts, nuts, etc.).

[0077] The 0.05% proof stress at 150°C, which is one of the effects of the present invention, is measured by a measurement method appropriate for the part, and the test piece used for the measurement is taken from the center position of the wall thickness of the part.

[0078] The present invention will now be described in more detail with reference to examples.

[0079] Titanium ingots Nos. 1 to 12 and Nos. 17 to 26 having the chemical compositions shown in Table 1 were produced by hot forging using the vacuum arc remelting (VAR) method, and then heated to 900°C and hot rolled into round bars with a diameter of 30 mm.

[0080] In addition, Nos. 13 to 16 were produced by hot forging titanium ingots having the chemical compositions shown in Table 1, and then heated to 900°C and hot forged into round bars having a diameter of 230 mm.

[0081] Next, solution treatment and aging treatment were performed according to the conditions shown in Table 2A. The "WQ" in the cooling step for solution treatment refers to water cooling. Note that in No. 10, "cooling after first treatment: yes" means that the alloy was cooled to 100°C or below after the first treatment (first aging treatment) and then heated to the treatment temperature for the second treatment (second aging treatment). For the other titanium alloys, the alloy was cooled to the temperature for the second treatment after the first treatment. In this manner, round bars made of α+β titanium alloys were produced.

[0082] The obtained round bar material made of α+β type titanium alloy was evaluated for the average circular equivalent diameter of the granular α phase, the maximum width of the acicular α phase in the mixed structure of acicular α phase and β phase, the lattice constant of the β phase, the area ratio of the β phase, and the 0.05% yield strength using the following procedures.

[0083] (Average Equivalent Circular Diameter and Maximum Width of the α Phase) The metallographic structure was observed using an EBSD device attached to an SEM. A test specimen was taken with a cross section parallel to the longitudinal direction of the titanium material as the observation surface. The measurement point on the observation surface was located at a depth of 1 / 2 the radius r from the surface. A rectangular region measuring 250 μm long and 250 μm wide or 10 μm long and 10 μm wide at the measurement point on the observation surface of the test specimen was used as the field of view. The measurement interval was 0.5 μm for the former region and 0.02 μm for the latter region. Measurement was performed using EBSD at an acceleration voltage of 15 kV. The former region was used to measure the granular α phase, and the latter region was used to measure the acicular α phase. The obtained measurement results were analyzed using OIM (crystal orientation analysis software manufactured by TSL Solutions Co., Ltd.). First, a Partition targeting only the α phase was created and used as the subject of analysis.

[0084] Next, the boundary of the α phase was determined by setting the difference in the crystal orientation angle (misorientation angle) between adjacent EBSD measurement points to 15° or less, and the circle-equivalent diameter was calculated from the number of measurement points of the granular α phase to determine the average circle-equivalent diameter of the granular α phase. For the acicular α phase, the maximum value of the minor axis diameter calculated from the ellipse approximation was used as the maximum width.

[0085] The area ratio of the β phase was evaluated using an X-ray diffraction (XRD) device. A test piece was taken from a titanium alloy rod, with a cross section parallel to the longitudinal direction as the observation surface. The measurement point on the observation surface was located at a depth of 1 / 2 the radius r from the surface. Using a Cu tube, the tube voltage was 40 kV, the characteristic X-ray used was Kα1 ray, the measurement range was 30 to 90°, the slit width was 2 mm, and the measurement interval was 0.02° to obtain X-ray diffraction results. The area ratio of the β phase was calculated using the five diffraction peaks of the α phase (10-10), (0002), (10-11), (10-12), and (11-20) and the three diffraction peaks of the β phase (110), (200), and (211). In addition, among the notations of the plane orientation of the α phase, "-1" and "-2" mean that a bar is placed above 1 or 2.

[0086] (Concentration distribution of element M) First, as described above, a cross section of the α+β type titanium alloy was observed using a scanning electron microscope (SEM) to observe the backscattered electron image, and the granular α phase and the mixed structure were identified. The observation magnification was 20,000 times. The acceleration voltage of the SEM was 15 kV. The mixed structure was determined as the region excluding the region where the contrast was observed as white in the region other than the granular α phase.

[0087] A total of two straight lines, each 5 μm long, were drawn on the identified mixed structure. Then, multiple measurement positions were set for each line, and point analysis was performed on the mixed structure at each measurement position to measure the concentration of element M at each measurement position. The measurement positions were set at equal intervals of 0.005 μm along the analysis line. The point analysis was performed by irradiating each measurement position with an electron probe and detecting the resulting characteristic X-rays. Specifically, an EDS (energy dispersive X-ray elemental analyzer) attached to the SEM was used. The probe diameter of the electron probe was 0.003 μm or less. This resulted in the concentration distribution of element M along the two straight lines.

[0088] Then, the average concentration A of the top 10 points in the concentration distribution of element M, the average concentration B of the bottom 10 points in the concentration distribution, the standard deviation C in the concentration distribution, and the arithmetic mean D in the concentration distribution were calculated, and (A-B) x C / D, which is the left side of equation (1), was calculated.

[0089] (β-Phase Lattice Constant) The β-phase lattice constant was evaluated using an XRD (X-ray Diffraction) device. A test piece was taken from a titanium alloy rod with a cross section parallel to the longitudinal direction as the observation surface. The measurement point on the observation surface was located at a depth of 1 / 2 the radius r from the surface. X-ray diffraction results were obtained using a Cu tube, a tube voltage of 40 kV, Kα1 characteristic X-rays, a measurement range of 30 to 90°, a slit width of 2 mm, and a measurement interval of 0.02°. Then, using the three diffraction peaks of the β-phase, (110), (200), and (211), the β-phase lattice constant was calculated by extrapolation at sin2θ (2θ: diffraction peak position).

[0090] (0.05% Proof Stress) The 0.05% proof stress was measured by a tensile test using a test piece with a parallel portion having a diameter of 6.0 mm and a length of 30 mm. The tensile test piece was taken from a position at a depth of 1 / 2 of the radius r from the surface, with the longitudinal direction of the titanium material aligned with the longitudinal direction of the test piece. The tensile test was carried out at 150°C with a strain rate of 8.3 × 10 －５ s －１ In the obtained nominal stress-nominal strain curve, the intersection of the line obtained by offsetting (translating) only 0.05% strain from the approximate line between 200 MPa and 500 MPa and the curve was taken as the 0.05% proof stress. A value of 900 MPa or more was considered to be acceptable.

[0091] As shown in Tables 1 and 2A-2C, in Examples Nos. 1-3, 6-16, 21-26, and 29-32 of the present invention, the cross-sectional structure contained a granular α phase with an aspect ratio of 8 or less and a mixed region structure containing acicular α and β phases, the maximum width of the acicular α phase was 0.01 μm or more and 5.00 μm or less, and the concentration distribution of the M element (Fe or Cr) within the mixed structure satisfied formula (1). As a result, the 0.05% yield strength at 150°C was 900 MPa or more. Therefore, the titanium alloys of the present invention exhibit excellent creep properties when used in motor sleeves.

[0092] Hollow cylindrical motor sleeves were actually manufactured from titanium alloys of the present invention, Nos. 1 to 3, 6 to 16, 21 to 26, and 29 to 33. Specifically, the center of each titanium alloy rod was hollowed out to form a cylindrical shape, and the diameter of the hollow portion was expanded to manufacture the motor sleeves. These were fitted into the rotor of a motor and used under conditions of an operating temperature of 15°C to 150°C and a load stress of 900 MPa or less, for a cumulative operating time of 100 hours or more, without breakage.

[0093] On the other hand, the aging conditions of Comparative Examples 4, 5, 17 to 19, 27, and 28 were outside the range of the present invention. As a result, the Fe or Cr concentration distribution in the mixed region mixed structure did not satisfy formula (1). Furthermore, in Nos. 17 and 18, the maximum width of the acicular α phase was outside the range of 0.01 μm or more and 5.00 μm or less. As a result, the 0.05% proof stress of Comparative Examples 4, 5, 17 to 19, 27, and 28 was less than 900 MPa.

[0094] In addition, in Comparative Example 20, the Fe content was outside the range of the present invention, and therefore formula (1) was not satisfied, resulting in a 0.05% yield strength of less than 900 MPa.

[0095]

[0096]

[0097]

[0098]

[0099] 1 α phase 2 Mixed structure 21 Acicular α phase 22 Acicular β phase

Claims

1. An α+β type titanium alloy consisting of, in mass%, Al: 4.00% or more and 5.50% or less, one or both of Fe and Cr: 1.40% or more and 4.50% or less, Mo: 1.50% or more and 5.00% or less, V: 0% or more and 4.00% or less, O: 0.05% or more and 0.25% or less, Si: less than 0.10%, C: less than 0.010%, and the balance: Ti and impurities, wherein the structure appearing in the cross section of the α+β type titanium alloy contains a granular α phase having an aspect ratio of 8 or less and a mixed structure containing an acicular α phase and a β phase, and the maximum width of the acicular α phase is 0.01 μm or more and 5.00 μm or less, An α+β-type titanium alloy, wherein, when element M is an element having a higher content of either Fe or Cr, the concentration distribution of element M in the mixed structure on the cross section satisfies formula (1): (A-B) x C / D≧0.160 ... (1), where the concentration distribution of element M is a distribution of the concentration of element M on the line obtained by setting a plurality of measurement points along a line in the mixed structure on the cross section and irradiating each measurement point with an electron probe. Also, in formula (1), A is the average concentration of the top 10 points in the concentration distribution of element M, B is the average concentration of the bottom 10 points in the concentration distribution of element M, C is the standard deviation in the concentration distribution of element M, and D is the arithmetic mean in the concentration distribution of element M.

2. An α+β type titanium alloy as described in claim 1, containing one or both of Sn: 4.00% or less and Zr: 4.00% or less in place of a portion of Ti.

3. The α+β type titanium alloy according to claim 1, wherein the average equivalent circle diameter of the granular α phase is 20.0 μm or less.

4. An α+β type titanium alloy as set forth in claim 1, wherein the area ratio of the β phase in the structure appearing in the cross section is 2.00% or more but less than 25.00%.

5. A titanium alloy rod made of the α+β type titanium alloy according to any one of claims 1 to 4.

6. A part made of the α+β type titanium alloy according to any one of claims 1 to 4.

7. A method for producing an α+β type titanium alloy, comprising: Al: 4.00% or more and 5.50% or less; Fe or Cr, or both: 1.40% or more and 4.50% or less; Mo: 1.50% or more and 5.00% or less; V: 0% or more and 4.00% or less; O: 0.05% or more and 0.25% or less; Si: less than 0.10%; C: less than 0.010%; and the balance: Ti and impurities; and a solution treatment and an aging treatment are carried out in this order on an α+β type titanium alloy, the aging treatment comprising a first treatment in which the alloy is held at a temperature T1 of 400°C or more and 650°C or less for 1 minute or more and 120 minutes or less, and a second treatment in which the alloy is held at a temperature of 350°C or more and (T1-20)°C or less for 60 minutes or more and then cooled.

8. A method for producing an α+β type titanium alloy according to claim 7, which contains one or two of Sn: 4.00% or less and Zr: 4.00% or less in place of a portion of Ti.

9. A method for producing an α+β type titanium alloy as described in claim 7 or claim 8, wherein after the first treatment is completed, the α+β type titanium alloy is cooled to 100°C or less and then the second treatment is carried out.

10. A method for producing an α+β type titanium alloy as described in claim 7 or claim 8, wherein after completion of the first treatment, the α+β type titanium alloy is cooled to a temperature of 350°C or higher (T1-20)°C or lower, and then the second treatment is carried out.

11. A method for producing an α+β type titanium alloy as set forth in claim 7 or claim 8, wherein the solution treatment is a process in which the α+β type titanium alloy is heated to a temperature of (β transformation point -100)°C or higher and lower than the β transformation point, and then rapidly cooled to 300°C or lower.

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