β-type titanium alloy

Abstract

To provide a β titanium alloy with superior bendability.SOLUTION: A β titanium alloy contains a β phase in its tissue. When the crystal orientation of the β-phase is indicated by Euler angles g={φ1, Φ, φ2}, if the volume fraction of crystal grains having crystal orientations within the range of φ1: 5°-30°, Φ: 0°-60°, and φ2: 35°-55° is defined as X1, and the volume fraction of crystal grains having crystal orientations within the range of φ1: 0°-90°, Φ: 50°-60°, and φ2: 35°-55° is defined as X2, the following formula (1) is satisfied: X1 / X2≥0.30 (1).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a β-type titanium alloy that is excellent in workability. [Background technology]

[0002] Titanium has a high specific strength, which is one of the reasons it is used for weight reduction. Titanium also has excellent biocompatibility. Titanium alloys, in particular, have high strength and are expected to further contribute to weight reduction. For this reason, titanium and titanium alloys are used in transportation equipment such as aircraft, motorcycles, and automobiles, as well as consumer products such as eyeglasses. However, titanium generally has an α-phase hcp structure at room temperature, and due to this anisotropy, its mechanical properties are highly anisotropic. This makes it difficult to process, which can increase manufacturing costs.

[0003] In contrast, beta-type titanium alloys have a bcc-structured beta phase at room temperature and offer relatively excellent workability. They have a particularly low Young's modulus of approximately 80 GPa and are often used in components requiring spring properties. Representative beta-type titanium alloys include Ti-15-3 (Ti-15V-3Cr-3Sn-3Al), SSAT-2041CF (Ti-20V-4Al-1Sn), and Ti-13V-11Cr-3Al. However, with the recent trend toward lighter, smaller products and more precise shapes, there is often a demand for further improvements in the workability of beta-type titanium alloys, particularly their bending workability.

[0004] Patent Document 1 describes a method for producing a high-strength Ti alloy with excellent workability, which involves heating a β-type Ti alloy or near-β-type Ti alloy containing 1.0% or less Si, performing plastic working while maintaining a temperature range above the β transformation point to finely precipitate the Si compound, thereby suppressing recrystallization. Patent Document 2 also describes a β-type titanium alloy sheet containing, by mass, 14-25% V, 2.5-5% Al, 0.5-4% Sn, and 4% or less Cr, with the balance consisting of Ti and impurities, having a hard layer on the surface, whose hardness at a depth of 10 μm from the surface is 1.2 times or more the hardness at the center of the sheet thickness, and whose hard layer thickness is 2.0-10% of the sheet thickness on each side. However, Patent Documents 1 and 2 do not examine bending workability. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-343548 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-176115 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a β-type titanium alloy that is excellent in workability. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention employs the following configuration. [1] Contains β phase in the structure, When the crystal orientation of the β phase is expressed as Euler angles g = {φ1, Φ, φ2}, where X1 is the volume fraction of crystal grains having a crystal orientation in the ranges of φ1: 5° to 30°, Φ: 0° to 60°, and φ2: 35° to 55°, and X2 is the volume fraction of crystal grains having a crystal orientation in the ranges of φ1: 0° to 90°, Φ: 50° to 60°, and φ2: 35° to 55°, a β-type titanium alloy that satisfies the following formula (1): X1 / X2≧0.30 …(1) [2] In mass %, V: 12-25%, Al: 2.0-5.0% Sn: 0.5 to 4.0% O: 0.05 to 0.20%, Cr: 0-4.0% N: 0.050% or less, C: 0.080% or less, H: Contains 0.020 or less, The β-type titanium alloy according to [1], having a chemical composition consisting of the balance: Ti and impurities. [3] A β-type titanium alloy according to [1] or [2], in which the average grain size of the β phase is 100 μm or less. [Effects of the Invention]

[0008] According to the present invention, a β-type titanium alloy having superior bending workability compared to conventional alloys can be provided. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present inventors conducted extensive experiments and intensive research using a Ti-15-3 alloy as the base composition to develop a β-type titanium alloy with excellent bending workability. As a result, they discovered that the texture shown in (A) below has a significant effect on workability in β-type titanium alloys. In this specification, this texture will be referred to as {h11}<1 / h12> fiber.

[0010]

number

[0011] Hereinafter, a β-type titanium alloy according to an embodiment of the present invention will be described. The β-type titanium alloy of this embodiment contains a β-phase in its structure, and when the crystal orientation of the β-phase is expressed as Euler angles g = {φ1, Φ, φ2}, where X1 is the volume fraction of crystal grains having crystal orientations in the ranges of φ1: 5° to 30°, Φ: 0° to 60°, and φ2: 35° to 55°, and X2 is the volume fraction of crystal grains having crystal orientations in the ranges of φ1: 0° to 90°, Φ: 50° to 60°, and φ2: 35° to 55°, the β-type titanium alloy satisfies the following formula (1): X1 / X2≧0.30 …(1)

[0012] In addition, in the β-type titanium alloy of this embodiment, the average crystal grain size of the β phase is preferably 100 μm or less.

[0013] First, the texture of the β-titanium alloy of this embodiment will be described. When a β-titanium alloy is cold-rolled into a plate, the texture formed when processed with fewer rolling passes has a φ1=20° structure and a texture with developed {h11}<1 / h12> fiber, compared to the texture formed when a plate of the same thickness is rolled with more rolling passes. β-titanium alloys with such textures are characterized by small r-values ​​and M-values. Generally, the lower the r-value and M-value, the better the bending workability, so the above-mentioned texture exhibits excellent bending workability. The r-value is the Lankford value, and the M-value is the Taylor factor.

[0014] In this embodiment, when the crystal orientation of the β phase is expressed by the Euler angles g = {φ1, Φ, φ2}, {h11}<1 / h12> fiber is defined as the volume fraction of crystal grains having crystal orientations in the ranges of φ1: 5° to 30°, Φ: 0° to 60°, and φ2: 35° to 55°, and the volume fraction of these crystal grains is designated as X1.

[0015] On the other hand, when a β-type titanium alloy is cold-rolled, γ-fiber develops instead of {h11}<1 / h12>fiber. This γ-fiber development increases the r-value and M-value, and deteriorates bending workability. Therefore, in this embodiment, γ-fiber is defined as the volume fraction of crystal grains with crystal orientations in the ranges of φ1: 0° to 90°, Φ: 50° to 60°, and φ2: 35° to 55°, and this volume fraction of crystal grains is designated as X2.

[0016] It has been found that good bending workability at room temperature can be obtained when the ratio X1 / X2 of the above X1 and X2 satisfies the following formula (1).

[0017] X1 / X2≧0.30 …(1)

[0018] X1 / X2 is preferably 0.35 or more, and more preferably 0.40 or more. The greater the bending workability, the better, but the upper limit is thought to be about 10.0. The upper limit in actual practice was 8.0.

[0019] The crystal orientation that represents the texture will be explained below. To express the crystal orientation in the texture three-dimensionally, a notation method using Euler angles is used. In the notation method using Euler angles (Bunge's notation method), three coordinate axes, RD, TD, and ND, which are orthogonal to each other, are considered as the sample coordinate system (coordinate system of the sheet material).

[0020] The crystal orientation distribution of a polycrystalline body is expressed by a function f(φ1, Φ, φ2) using Euler angles (φ1, Φ, φ2), and this function is called the crystal orientation distribution function (ODF).

[0021] The crystal orientation distribution function (ODF) can be determined using the electron backscattered diffraction pattern (EBSD) method. The EBSD pattern is measured and analyzed using the electron backscattered diffraction (EBSD) method while scanning the inspection surface with an electron beam using a scanning electron microscope (SEM). The Euler angles (φ1, Φ, φ2) of the crystal orientation at each measurement point are obtained by converting these into angles relative to the plate surface using computer calculations. The ODF (f(φ1, Φ, φ2)) can be calculated based on the data from the measurement points within the measured field of view.

[0022] Since the β-phase is uniformly distributed throughout the structure of a β-type titanium alloy, the crystal orientation distribution function can be measured at any cross section of the β-type titanium alloy. For example, a surface perpendicular to the width direction of a β-type titanium alloy plate at half the width (hereinafter also referred to as the "L-section") is polished to form a measurement surface, and the EBSD pattern is measured and analyzed using the electron backscatter diffraction (EBSD) method while scanning the electron beam with a scanning electron microscope (SEM) at a pitch of 10.0 μm (step of 10.0 μm) in a field of view of the entire plate thickness × 10 mm on this surface. The EBSD pattern is then converted into an angle relative to the plate surface by computer calculation to obtain the Euler angle of each measurement point for the titanium β-phase crystals.

[0023] The ODF of the β phase was calculated using the data measured under the above conditions using TSL Solutions' OIM Analysis software (version 8.1.0). Texture analysis using the spherical harmonic method of electron backscatter diffraction (EBSD) was used to calculate the ODF (expansion index = 16, Gaussian half-width = 5°). The calculation was performed taking into account the symmetry of the rolling deformation, ensuring line symmetry in the thickness direction, rolling direction, and width direction.

[0024] The present invention is directed to a β-type titanium alloy. That is, a titanium alloy in which the proportion of β-phase as determined by the electron backscatter diffraction method is greater than 50% by area. The proportion of β-phase is preferably 70% by area or more, and more preferably 90% by area or more.

[0025] In β-type titanium alloys, the formability improves when the average crystal grain size of the β phase becomes coarse. However, when the β-type titanium alloy is made into a thin plate material, if the average crystal grain size becomes too coarse, the number of crystal grains relative to the plate thickness decreases, which in turn reduces plastic deformability and bending workability. If the average crystal grain size is 100 μm or less, a sufficient number of crystal grains relative to the plate thickness can be ensured and workability will not decrease, so the average crystal grain size of the β phase is preferably 100 μm or less. More preferably, it is 50 μm or less. On the other hand, if the β phase becomes fine-grained, properties such as fatigue resistance improve, but the strength becomes high, resulting in a decrease in ductility. If the average crystal grain size is 10 μm or more, ductility will not decrease, so it is preferable to set it to 10 μm or more.

[0026] The grain size of the β phase is measured under the same conditions as when measuring the volume fraction of grains having a specified crystal orientation, and the average grain size of 100 or more grains is calculated. The grain size of the grains is the equivalent circle diameter.

[0027] The β-type titanium alloy of this embodiment contains a β-phase, and the ratio of the volume fraction of the crystal grains having the predetermined crystal orientation, X1 / X2, is sufficient to be 0.30 or more. There is no need to limit the chemical composition of the titanium alloy, but the following is an example of a chemical composition that can be applied to the β-type titanium alloy of this embodiment.

[0028] That is, the β-type titanium alloy of this embodiment may have a chemical composition containing, in mass%, V: 12-25%, Al: 2.0-5.0%, Sn: 0.5-4.0%, O: 0.05-0.20%, Cr: 0-4.0%, N: 0.050% or less, C: 0.080% or less, H: 0.020% or less, and the balance: Ti and impurities. In the following description, "mass%" will be abbreviated as "%".

[0029] [V content] V is an important element for stabilizing the β phase and maintaining the alloy structure as a single β phase at room temperature before aging treatment. In addition, because it is resistant to solidification segregation during melting, it can be easily incorporated in relatively large amounts. On the other hand, if the V content is low, even rapid cooling from the high-temperature β single-phase region, such as water cooling, may result in martensitic transformation without the β phase remaining, thereby degrading workability at room temperature. Therefore, the V content is preferably 12% or more, and more preferably 13% or more. On the other hand, if the V content exceeds 25%, the cold workability of the alloy may be significantly degraded due to a decrease in ductility caused by solid solution strengthening. Furthermore, the stability of the β phase becomes too high, making it difficult for the α phase to precipitate, which may lengthen the time required for aging treatment and further prevent sufficient strengthening from being achieved by aging treatment. Therefore, the V content should be 25% or less, preferably 23% or less, and more preferably 22% or less.

[0030] [Al content] β-type titanium alloys are strengthened by aging treatment in the final product, and it is preferable to include Al, an α-phase stabilizing element, to obtain a sufficient increase in strength during this process.Al also has the effect of suppressing the precipitation of brittle ω-phase during aging treatment and promoting the precipitation of α-phase.If the Al content is less than 2.0%, the effects of increasing strength and suppressing ω-phase are not sufficiently obtained.On the other hand, if the Al content exceeds 5.0%, the alloy becomes too hard in the β-phase single phase state, reducing cold workability.Therefore, the Al content is set to 2.0 to 5.0%, and preferably 4.0% or less.

[0031] [Sn content] Although Sn is a neutral element, it has the same effect as Al described above. Therefore, by substituting Sn for Al, strengthening can be achieved without impairing cold workability. If the Sn content is less than 0.5%, the effect of substituting Al is small, resulting in poor strengthening. On the other hand, if the Sn content exceeds 4.0%, the material becomes too hard in the single β phase state, reducing cold workability. Therefore, the Sn content is set to 0.5 to 4.0%.

[0032] [Cr content] Cr is a eutectoid reaction element and has a higher β-stabilizing ability than V. Therefore, even a small amount of Cr is effective in stabilizing the β-phase and ensuring cold workability. However, Cr is an element that easily segregates during melting, and adding a large amount of Cr increases the strength variation within the ingot. In addition, the β-phase becomes too stable, making it difficult for the α-phase to precipitate, which lengthens the time required for aging treatment and further hinders the strengthening achieved by aging treatment. Therefore, the above problems are unlikely to occur if the Cr content is up to 4.0%, so this may be the upper limit. Preferably, it is 3.5% or less. Cr need not be added. Therefore, the lower limit of the Cr content is 0% or more, preferably greater than 0%.

[0033] [O content] O is an element with high solid solution strengthening ability. If the O content is less than 0.05%, the strength is insufficient. Therefore, the O content is 0.05% or more, preferably 0.06% or more. On the other hand, if the O content exceeds 0.20%, the strength becomes too high, significantly reducing the formability. Therefore, the O content is 0.20% or less, preferably 0.18% or less.

[0034] [N content, C content, H content] If the N content, C content, and H content are excessive, the ductility of the β-type titanium alloy will decrease. Therefore, the N content is set to 0.050% or less, the C content to 0.080% or less, and the H content to 0.020% or less. The C content is preferably less than 0.050%. The contents of these elements may be 0%, but setting the N content to less than 0.0001%, the C content to less than 0.0001%, and the H content to less than 0.00001% will significantly increase the refining cost. Therefore, the N content may be set to 0.0001% or more, the C content to 0.0001% or more, and the H content to 0.00001% or more. The N content may be set to 0.001% or more, the C content to 0.001% or more, and the H content to 0.001% or more.

[0035] The balance is Ti and impurities.

[0036] Furthermore, when the β-titanium alloy of this embodiment is made into a plate material, the plate thickness is preferably 4 mm or less. The form of the β-titanium alloy of this embodiment is not limited to a plate material, and it may also be a rod material or a tube material.

[0037] Next, a method for producing the β-type titanium alloy of this embodiment will be described. First, a titanium alloy slab is produced. The method for producing the titanium alloy slab is not particularly limited, but it can be produced by, for example, vacuum arc melting (VAR) or electron beam melting (EBR). The surface of the obtained titanium alloy slab may be cut if necessary.

[0038] Next, in the hot rolling step, the titanium alloy slab is heated to 800°C to 1100°C and then hot rolled. The hot rolling is preferably performed under conditions where the total rolling reduction is 80% or more, preferably 90% or more. By setting the total rolling reduction to 80% or more, it is possible to impart strain necessary for recrystallization during annealing.

[0039] The titanium material after the hot rolling process may or may not be annealed at a soaking temperature of 600°C to 950°C for a soaking time of 1 minute to 30 minutes (60 seconds to 1800 seconds). Cooling after annealing may be natural cooling. Annealing may be performed in an air atmosphere, an inert atmosphere, or a vacuum atmosphere. When hot-rolled sheet annealing is performed, the unrecrystallized grains in the hot-rolled sheet can be precipitated as fine recrystallized grains, resulting in more uniform and finer crystals in the metal structure of the titanium alloy sheet obtained as a final product.

[0040] The annealed titanium material may be subjected to pickling or cutting to remove oxide scale and the like, and further to cleaning treatment, if necessary.

[0041] Next, the annealed β-type titanium alloy is subjected to one or more cold rolling processes in its longitudinal direction. The number of cold rolling processes may be three or less. The rolling reduction rate per cold rolling process is 65% or more, preferably 80% or more. By setting the rolling reduction rate per cold rolling process to 65% or more, it is possible to develop {h11}<1 / h12> fiber and increase its volume fraction X1. As a result, by setting X1 / X2 to 0.30 or more, it is possible to improve the bending workability of the β-type titanium alloy.

[0042] When cold rolling is performed multiple times, intermediate annealing may be performed. The intermediate annealing conditions are preferably a soaking temperature of 750°C or higher and 950°C or lower. The holding time t (seconds) at the soaking temperature T (°C) is preferably set to satisfy the following formula (2). Formula (2) is the Larson-Miller parameter.

[0043] 22000≦(T+273.15)×(Log 10 (t)+20)≦28000…(2)

[0044] The intermediate annealing may be performed in an air atmosphere, an inert atmosphere, or a vacuum atmosphere.

[0045] Furthermore, the β-type titanium alloy after cold rolling may be subjected to final annealing. The conditions for final annealing are preferably a soaking temperature of 750°C or higher and 950°C or lower. Furthermore, the holding time t (seconds) at the soaking temperature T (°C) is preferably set so as to satisfy the above formula (2). Cooling after final annealing may be natural cooling. Final annealing may be performed in any of an air atmosphere, an inert atmosphere, or a vacuum atmosphere. By performing final annealing, it is possible to remove strain during cold rolling and adjust the material properties to be suitable for bending. [Example]

[0046] Next, the present invention will be described in more detail with reference to examples. However, the examples described below are merely examples of embodiments of the present invention and are not intended to limit the present invention.

[0047] Slabs made of β-type titanium alloys were produced by vacuum arc melting (VAR). Specifically, metal raw materials whose compositions were adjusted to obtain the chemical compositions shown in Table 1 were vacuum arc melted to produce molten metal, and this molten metal was then charged into a water-cooled copper mold to produce slabs made of β-type titanium alloys with a thickness of 200 mm. Blank cells in Table 1 indicate that no alloying elements were intentionally added.

[0048] Next, the slab was heated to 950°C to 1100°C and then hot-rolled. The hot-rolling conditions were such that the total rolling reduction was 80% or more. As a result, a hot-rolled sheet made of a β-type titanium alloy with a thickness of 3 mm to 6 mm was obtained.

[0049] Next, the hot-rolled sheet was annealed under the conditions shown in Table 2A. The annealing atmosphere was a vacuum atmosphere. After annealing, the hot-rolled sheet was pickled to remove oxide scale.

[0050] Next, the β-type titanium alloy plate after the hot-rolled sheet annealing was subjected to cold rolling in its longitudinal direction at least once, and intermediate annealing was performed as necessary. Tables 2A and 2B show the cold rolling conditions and intermediate annealing conditions. The annealing atmosphere was air.

[0051] Next, a part of the cold-rolled β-type titanium alloy plate was subjected to final annealing under the conditions shown in Table 2B. The annealing atmosphere was a vacuum atmosphere. In this way, β-type titanium alloy plates No. 1 to 21 were obtained. The thickness of the obtained titanium alloy plates was all 1.0 mm or less.

[0052] The evaluation method for texture is explained below. To express the crystal orientation in the texture three-dimensionally, a notation method using Euler angles was used. In the notation method using Euler angles (Bunge's notation method), three coordinate axes, RD, TD, and ND, which are orthogonal to each other, are considered as the sample coordinate system (coordinate system of the plate material).

[0053] The crystal orientation distribution of a polycrystalline body is expressed by a function f(φ1, Φ, φ2) using Euler angles (φ1, Φ, φ2), and this function is called the crystal orientation distribution function (ODF).

[0054] The crystal orientation distribution function (ODF) can be determined using the electron backscattered diffraction pattern (EBSD) method. The EBSD pattern is measured and analyzed using the electron backscattered diffraction (EBSD) method while scanning the inspection surface with an electron beam using a scanning electron microscope (SEM). The Euler angles (φ1, Φ, φ2) of the crystal orientation at each measurement point are obtained by converting the angle relative to the plate surface using computer calculations. The ODF (f(φ1, Φ, φ2)) is calculated based on the data of the measurement points within the measured field of view.

[0055] Since the β phase is uniformly distributed in the structure of β-type titanium alloy, a surface perpendicular to the plate width direction at 1 / 2 the plate width of the β-type titanium alloy plate (hereinafter also referred to as the "L cross section") was polished to form the measurement surface, and the EBSD pattern was measured and analyzed using the electron backscatter diffraction (EBSD) method while scanning the electron beam with a scanning electron microscope (SEM) at a 10.0 μm pitch (step 10.0 μm) in a field of view of the entire plate thickness x 10 mm on this surface.The EBSD pattern was then converted into an angle relative to the plate surface by computer calculation, and the Euler angle of each measurement point was obtained.

[0056] The ODF of the β phase was calculated using the data measured under the above conditions using TSL Solutions' OIM Analysis software (version 8.1.0). Texture analysis using the spherical harmonics method of electron backscatter diffraction (EBSD) was used to calculate the ODF (expansion index = 16, Gaussian half-width = 5°). Taking into account the symmetry of the rolling deformation, calculations were performed so that the data was symmetrical with respect to the thickness, rolling, and width directions. The results are shown in Table 3.

[0057] The bending workability was evaluated as follows. A three-point bending test was carried out on the obtained titanium alloy plate (size: length in the rolling direction 60 mm, length in the rolling width direction 30 mm). The test conditions were as follows: the bending axis was set in the rolling direction, the bending radius was 0.4 mm, and the clearance CL (mm) was set to satisfy the following formula (3) for a plate thickness A of 1 mm, and the bending test was carried out at a pressing speed of 10 min / mm.

[0058] CL=2.0×(A1+1) …(3)

[0059] To evaluate workability, the surface of the bent section at a plunge angle of 80° was visually inspected and observed under an optical microscope at magnifications of 20x to 100x in 2 to 10 fields of view. The presence of cracks was marked "x", and the absence of cracks was marked "o". Furthermore, cases where there were no cracks but wrinkles or large irregularities occurred on the surface were marked "△". Cases with cracks were deemed to be unacceptable, and cases with no cracks marked "○" or "△" were deemed to be acceptable. The results are shown in Table 3.

[0060] As shown in Tables 1 to 3, when cold rolling was performed 1 to 3 times, the cold rolling reduction ratio was less than 65% in at least one cold rolling for Nos. 5, 6, 8, 20, and 21. As a result, the X1 / X2 ratio was less than 0.30, and bending workability was reduced.

[0061] In addition, Nos. 16 and 19 had Larson-Miller parameters outside the specified range, which deviated from the preferred manufacturing conditions. As a result, X1 / X2 was less than 0.30, and bending workability was reduced.

[0062] On the other hand, as shown in Tables 1 to 3, Nos. 1 to 4, 7, 9 to 15, 17, and 18 had X1 / X2 of 0.30 or more and good bending workability because the hot-rolled sheet annealing conditions, cold rolling conditions, intermediate annealing conditions, and final annealing conditions were within the preferred ranges. Furthermore, Nos. 1 to 4, 7, 9 to 15, 17, and 18 had β-phase area fractions of more than 50%, confirming that they were β-type alloys.

[0063] [Table 1]

[0064] Table 2A

[0065] Table 2B

[0066]

Table 3

Claims

1. Contains β phase in the structure, The crystal orientation of the β phase is defined as the Euler angle g = {φ １ , Φ, φ ２ }, then φ １ :5°~30°, Φ:0°~60°, φ ２ : The volume fraction of crystal grains having a crystal orientation in the range of 35° to 55° is X1, φ １ :0°~90°, Φ:50°~60°, φ ２ : A β-type titanium alloy that satisfies the following formula (1) when the volume fraction of crystal grains having a crystal orientation in the range of 35° to 55° is X2. X1 / X2≧0.30…(1)

2. In mass%, V: 12-25%, Al: 2.0-5.0%, Sn: 0.5-4.0%, O: 0.05-0.20%, Cr: 0-4.0%, N: 0.050% or less, C: 0.080% or less, H: 0.020 or less, The β-type titanium alloy according to claim 1, having a chemical composition consisting of the balance: Ti and impurities.

3. 3. The β-type titanium alloy according to claim 1, wherein the average grain size of the β-phase is 100 μm or less.

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