Titanium alloy and its manufacturing method

Abstract

To provide a titanium alloy having an oxide film with excellent color development having less irregular color, and a production method thereof.SOLUTION: A titanium alloy includes a base material made of a titanium alloy, and an oxide film whose thickness is 20 nm or more and 200 nm or less on a surface of the base material, wherein an average content of each β-stabilizing element in the oxide film is 0.4 mass% or less; the total amount of β-stabilizing elements is 1.0 mass% or less, and a difference between the maximum content and the minimum content of each β-stabilizing element is 0.3 mass% or less. The titanium alloy can be produced under the condition that the content of β-stabilizing element in the base material is 3.0 mass% or less and a voltage application speed in anode oxidation is 240 V / min or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a titanium alloy having a coating and a method for producing the same. [Background technology]

[0002] Titanium alloys have been widely used, primarily in aircraft applications, taking advantage of their high specific strength. In recent years, titanium alloys have also been widely used in automobiles and consumer products. While commercially pure titanium, with its excellent workability, is used in building materials, high-strength titanium alloys such as alloy 64 (Ti-6Al-4V) and heat-resistant titanium alloys are used in fields requiring strength (e.g., mobile phone housings, zippers, mufflers, etc.). For example, there is a need for stainless steel and titanium in mobile phone housings. Titanium, in particular, has a biocompatible and lightweight, high-end image, and its use in high-end mobile phones (smartphones) is being considered. High strength is required for housings to prevent dents and cracks when dropped. Therefore, titanium alloys with higher strength than pure titanium are needed.

[0003] In recent years, there has been a demand for titanium alloys for these applications to be used in more ways than just high specific strength, with improved design. In automotive applications, titanium alloys with excellent high-temperature strength are also used in mufflers and other components. Improved design is also desired for mufflers to increase purchasing power.

[0004] To improve the design of titanium, a thin oxide film is formed on the surface by anodizing, which allows it to produce a variety of colors by utilizing the interference of light. Examples of titanium that is used after anodizing include consumer products such as tumblers and building materials. For these applications, pure titanium is mainly used due to its ease of processing.

[0005] On the other hand, titanium alloys are used in applications requiring strength at room temperature to high temperatures, such as mobile phone housings and mufflers. In recent years, there has been a need to anodize high-strength titanium alloys to improve their design. Furthermore, it is expected that titanium alloys will be anodized to produce various colors for improved design. Therefore, coloring by anodizing is being considered for titanium alloys as well.

[0006] As described above, there is a need for coloring titanium alloys by anodizing, but conventional anodizing has been developed mainly for pure titanium, and there has been little research into its application to alloys. For example, Patent Document 1 discloses a method for producing colored titanium and titanium alloy materials that have excellent oxide film adhesion and no color unevenness, but in the examples only pure titanium is used.

[0007] Titanium alloys that are in need of coloring by anodizing include Ti-5Al-1Fe (51AF) disclosed in Patent Document 2 and Ti-1Cu-1Sn-0.2Nb-0.2Si (10CSSN, for mufflers) disclosed in Patent Document 3. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 7-300697 [Patent Document 2] Japanese Patent Application Publication No. 7-70676 [Patent Document 3] International Publication No. WO2011 / 081077 Summary of the Invention [Problem to be solved by the invention]

[0009] The color development of titanium by anodizing occurs when a thin oxide film forms on the surface when a voltage is applied, and this film interferes with light. The color changes depending on the thickness of the oxide film formed. Therefore, to achieve a beautiful, single color, it is necessary to form an oxide film of uniform thickness. However, when the same method used to anodize conventional pure titanium is used on titanium alloys, it has been found that the color development due to the oxide film formed becomes uneven.

[0010] The present invention aims to provide a titanium alloy having an oxide film with excellent color development and minimal color unevenness, and a method for producing the same. In particular, the present invention provides a high-strength titanium alloy with strength and excellent workability equivalent to that of Ti64 alloy, and a titanium alloy used in mufflers, etc., that has better color development than Ti64 alloy. [Means for solving the problem]

[0011] That is, the gist of the present invention is as follows. [1] A titanium alloy having a base material made of a titanium alloy and an oxide film on its surface with a thickness of 20 nm to 200 nm, The base metal contains, by mass%, Al: 4.5 to 6.4%, Fe: 0 to 2.3%, Si: 0 to 0.40%, C: less than 0.08%, N: 0.05% or less, O: 0.4% or less, and the balance being Ti and impurities; A titanium alloy characterized in that the average content of each β-stabilizing element in the oxide film is 0.4 mass% or less, the total amount of all β-stabilizing elements is 1.0 mass% or less, and further, the difference between the maximum and minimum contents of each β-stabilizing element within the oxide film surface is 0.3 mass% or less. [2] The base material has a total content of β-stabilizing elements of 0.2~ 3.0 mass %in The titanium alloy according to [1], [3] The titanium alloy according to [1] or [2], characterized in that the base material has a β phase fraction of 20% or less. 。 [4 ] In place of a part of the Ti, the alloy further contains, by mass%, 1% or less of one or more of Ni, Cr, and Mn, each of which is 1% or less. One of [1] to [3] The titanium alloy according to claim 1. [5]A titanium alloy having a base material made of a titanium alloy and an oxide film having a thickness of 20 nm or more and 200 nm or less on the surface thereof, The base material contains, in mass%, Cu: 0.5 to 1.5%, Sn: 0.5 to 1.5%, Si: more than 0.1% but 0.6% or less, Nb: 0.1 to 0.4%, and O: 0.1% or less, with the remainder being Ti and impurities; A titanium alloy characterized in that the average content of each β-stabilizing element in the oxide film is 0.4 mass% or less, the total amount of all β-stabilizing elements is 1.0 mass% or less, and further, the difference between the maximum and minimum contents of each β-stabilizing element within the oxide film surface is 0.3 mass% or less. [6] The base material has a total content of β stabilizing elements of 0.2~ 3.0 mass %in It is characterized by the fact that 5 ] A titanium alloy described in. [ 7 ] The base material is characterized in that the β phase fraction is 20% or less, [ 5 ] or [ 6 Titanium alloys according to the 。

[0012] [ 8 ][1] to [ 7 A method for producing a titanium alloy according to any one of the preceding claims, A method for producing a titanium alloy, characterized in that the voltage application rate during anodization is 350 V / min or less. [Effects of the Invention]

[0013] The present invention makes it possible to obtain a titanium alloy having an oxide film with excellent color development properties. DETAILED DESCRIPTION OF THE INVENTION

[0014] The color of titanium produced by anodizing can be freely controlled by adjusting the thickness of the oxide film. Since the thickness of this film is generally determined by the voltage, the thickness of the oxide film can be controlled by adjusting the voltage. However, titanium alloys contain a large amount of alloying elements, and it has been found that some elements remain in the oxide film, changing the electrical resistance of the oxide film and resulting in a change in the color after coloring. Depending on the alloy type, alloying elements may become mixed into the oxide film during the anodizing process, requiring ingenuity in the method, which can pose challenges in terms of work and cost. Hereinafter, the oxide film may be simply referred to as the film.

[0015] This invention focuses on titanium alloys that maintain stable color during anodizing and their surface control technology. Specifically, by controlling the alloy composition of the base material and adjusting the voltage rise rate (voltage application rate) during anodizing, it is possible to achieve stable, beautiful anodizing color development even with high-strength titanium alloys containing a large amount of elements.

[0016] <Coating thickness> The color produced by anodizing titanium is caused by the interference of light, so the color can be controlled by controlling the film thickness. In the present invention, the thickness is set to 50 nm or more and 500 nm or less, which allows various colors to be produced.

[0017] 《β stabilizing element》 In the present invention, the β-stabilizing element is Completely soluble element V, Mo, Nb, Eutectoid element Fe, Cr, Ni, Mn, Cu, Si is .

[0018] <<Content and content difference of β-stabilizing elements in coating>> In anodizing, the thickness of the coating formed is determined by the electrical resistance of the coating and the applied voltage. It has been discovered that the presence of β-stabilizing elements in the coating, contained in the titanium alloy base material, changes the electrical resistance, resulting in changes in coating thickness and color. Depending on the base material's composition or the anodizing conditions, β-stabilizing elements can be present in large amounts in the coating, causing a color change. The upper limit was set at 0.4% by mass or less for each β-stabilizing element in the coating, and the total average content of all β-stabilizing elements (also referred to as the "total amount of all β-stabilizing elements") at 1.0% by mass or less, since the color change is small. The lower limit is set at 0% by mass, since the color change is minimal if the β-stabilizing elements are not present.

[0019] Furthermore, even if a β-stabilizing element is contained, if the content variation within the coating surface is small, the color is less likely to change, which is preferable. After extensive research, it was found that a more beautiful color can be obtained if the difference between the maximum and minimum content of each β-stabilizing element within the coating surface is 0.3 mass% or less for each β-stabilizing element. Therefore, in this patent, the difference between the maximum and minimum content of each β-stabilizing element within the oxide coating surface is set to 0.3 mass% or less. Since it is better to have no change, the lower limit is 0 mass%.

[0020] <<Method for measuring content in coating>> The elemental content of the coating can be measured by GD-OES (Glow Discharge Optical Emission Spectroscopy) or AES (Auger Electron Spectroscopy) to measure the distribution of the content in the depth direction of the coating. With AES, the coating is drilled by sputtering and the elemental content is analyzed at each depth. The depth distribution of the components was measured from the surface to 1 μm at five arbitrary points on the colored titanium surface. At each measurement point on the coating surface, the region with an O content of 60% or more in the depth direction was considered to be the oxide layer. The average content of each β-stabilizing element in that region was measured and used as the average content of each β-stabilizing element at that measurement point. Hereafter, this value is referred to as the "average content of each β-stabilizing element" at each measurement point. The maximum value among these values ​​is referred to as the "maximum average content of each β-stabilizing element." Furthermore, the average value of all measurement points for each β-stabilizing element was calculated to determine the total amount of each β-stabilizing element. The "total amount of all β-stabilizing elements" was calculated by adding up the total amount of each β-stabilizing element for all β-stabilizing elements. Furthermore, for each β-stabilizing element, the maximum and minimum values ​​in the above region at each measurement point were extracted, and the difference between the maximum and minimum content of each β-stabilizing element was calculated from these results to represent the variation. GD-OES results were given priority for the measurement method, and if measurement was difficult due to issues such as the shape of the test specimen, AES results were used instead. Because the surface is contaminated by various sources (such as dust in the atmosphere), it was thoroughly cleaned with alcohol or other cleaning agents before analysis.

[0021] <<Content of β-stabilizing elements and β-phase fraction in base material>> As described above, it has been found that there are suitable ranges for the content of the β-stabilizing element and the β-phase fraction of the base material in order to optimize the content and content difference of the β-stabilizing element in the coating, as will be explained below.

[0022] Generally, to increase the strength of titanium alloys, a few percent of β-stabilizing elements are added depending on the application. As the content of β-stabilizing elements increases, the β-phase fraction increases even at room temperature. However, it has been found that, in the color development of titanium alloys by anodizing, as the total amount of β-stabilizing elements in the base material increases and the β-phase fraction in the base material increases, the behavior of the β-stabilizing elements in the coating may deviate from the preferred range of the present invention, resulting in color mixing. The higher the total amount of β-stabilizing elements in the base material, the higher the β-phase fraction in the base material tends to be. Therefore, the total amount of β-stabilizing elements in the base material is preferably 3.0% by mass or less. When the total amount of β-stabilizing elements in the base material is 3.0% by mass or less, the behavior of the β-stabilizing elements in the coating can be adjusted to the preferred conditions described above, in conjunction with optimizing the anodizing conditions described below. The total amount of β-stabilizing elements in the base material is preferably 2.5% or less. On the other hand, if the amount is too small, strength cannot be obtained at room temperature to high temperatures. Therefore, the lower limit is preferably 0.2% by mass or more.

[0023] It has been found that the color development during anodizing of titanium is influenced by the phase structure of the crystal grains of the base material. In titanium alloys, if the content of beta-stabilizing elements is increased to increase strength, the beta-phase fraction increases. However, it has been found that as the beta-phase fraction increases, the color after anodizing changes between the alpha and beta phases, which can affect the external color as well. The smaller the beta-phase fraction, the less of an effect this has. After extensive research, it has been determined that a beta-phase fraction of 20% or less has little effect on the external color, so the preferred upper limit in area fraction is 20%. On the other hand, the lower the lower limit, the better, so 0% is the limit.

[0024] <Anodic oxidation conditions> The thickness of the film formed by anodization is generally determined by the anodization voltage, and the thickness of the oxide film can be controlled by controlling the voltage. For example, anodization using a 1% by mass solution of sulfuric acid or phosphoric acid at 15 to 100 V with a retention time in the solution of 10 seconds to 5 minutes can form a film with a thickness of 50 nm to 500 nm.

[0025] In the present invention, it has been found that by adjusting the voltage application rate when applying a voltage during anodization, the behavior of the β-stabilizing element in the coating falls within the preferred range of the present invention. Specifically, the slower the voltage application rate, the more the behavior of the β-stabilizing element in the coating falls within the preferred range of the present invention, resulting in a titanium alloy having an oxide coating with little color unevenness and excellent color development. A voltage application rate of 350 V / min or less is preferable, and 300 V / min or less is even more preferable. Furthermore, in order to obtain a titanium alloy having an oxide film with little color unevenness and excellent color development, it is preferable to carry out anodization after degreasing and pickling of the surface of the base material.

[0026] <Content of each element in the base material> The present invention can be applied to any titanium alloy composition as long as the total amount of β-stabilizing elements is controlled within the above-mentioned preferred range as the base material. In particular, the present invention can be suitably used in titanium alloys with the following two types of composition. Hereinafter, in the composition, % means mass %.

[0027] <Composition of the first base material> The chemical composition of the first base metal contains Al: 4.5 to 6.4%, Fe: 0 to 2.3%, Si: 0 to 0.40%, C: less than 0.08%, N: 0.05% or less, O: 0.4% or less, and the balance being Ti and impurities.

[0028] [Al: 4.5-6.4%] Al is an α-stabilizing element with high solid-solution strengthening ability, and should be contained at 4.5% or more. Increasing the content can increase strength. On the other hand, if the content exceeds 6.4%, cold rollability is significantly reduced, and solidification segregation and other factors can cause excessive solid-solution strengthening of certain α phases, creating locally hard regions and reducing impact toughness. Therefore, the upper limit was set at 6.4%.

[0029] [Fe: 0-2.3%] Fe is an inexpensive additive element among the β-stabilizing elements, and because it has a high solid-solution strengthening ability, increasing its content increases strength at room temperature. On the other hand, because Fe is an additive element that is very susceptible to solidification segregation, excessive Fe content increases performance variation and reduces fatigue strength in some locations, so the upper limit is set at 2.3%. It is preferably 2.1% or less. The lower limit can be adjusted depending on the application, so it can be 0%, but the above functions can be exhibited if the content is 0.5% or more.

[0030] [Si: 0-0.60%] Although Si is a β-stabilizing element, it also dissolves in the α-phase, providing high solid-solution strengthening. As mentioned above, it is difficult to exceed 2.3% Fe due to segregation issues, so if necessary, strength can be increased by solid-solution strengthening with Si. However, too much Si forms intermetallic compounds called silicides, which reduce workability. If the Si content exceeds 0.60%, coarse silicides are formed during the manufacturing process, reducing fatigue strength, so the upper limit is set at 0.60%. The lower limit can be adjusted depending on the application, so 0% is acceptable, but the above functions can be achieved if the content is 0.05% or more.

[0031] [O, N, C content] Since the inclusion of large amounts of O, N, and C can reduce ductility and workability, the O content is limited to 0.40% or less, preferably 0.3% by mass or less. Furthermore, the C content is limited to less than 0.08%, and the N content is limited to 0.05% or less. Since the inclusion of O, N, and C is unavoidable as they are unavoidable impurities, the effective contents are usually 0.01% or more for O, 0.0001% or more for C, and 0.0001% or more for N.

[0032] [Ni, Cr, Mn content] The chemical composition of the first base metal contains the above-mentioned elements, with the remainder being Ti and impurities. Furthermore, one or more of Ni, Cr, and Mn may be included in the range of 1% or less each, instead of a portion of Ti. Ni, Cr, and Mn, like Fe and Cu, are eutectoid β-stabilizing elements, and like other β-stabilizing elements, improve strength at room temperature. However, excessive content can lead to segregation during solidification and the formation of equilibrium intermetallic compounds (TiNi, TiCr, TiMn), degrading fatigue strength and room-temperature ductility. Therefore, the upper limit of the content of these elements is set to 1%. When these optional elements are included in a titanium alloy base metal, the total content of the β-stabilizing elements in the base metal is preferably 3.0% by mass or less, in order to ensure that the behavior of each β-stabilizing element in the oxide film falls within the scope of the present invention.

[0033] <<Composition of the second base material>> The second base metal has a chemical composition containing Cu: 0.5 to 1.5%, Sn: 0.5 to 1.5%, Si: more than 0.1% but not more than 0.6%, Nb: 0.1 to 0.4%, and O: not more than 0.1%, with the balance being Ti and impurities.

[0034] [Cu: 0.5-1.5%] Like Fe, Cu is relatively inexpensive among the β-stabilizing elements and has a high solid-solution strengthening ability, so it is contained in an amount of 0.5% or more. It also has excellent solid-solution strengthening ability in the medium to high temperature range. In addition, unlike Al, it is an extremely effective element that does not impair workability. On the other hand, if the amount of Cu contained is much greater than the solid solubility limit of Cu in the α phase, the β-phase fraction will increase, reducing strength from room temperature to high temperatures and causing precipitation of Ti2Cu, which may significantly reduce workability at room temperature. For this reason, the upper limit is set at 1.5%.

[0035] [Sn: 0.5-1.5%] Sn is a neutral element that improves strength at room temperature to high temperatures through solid solution strengthening, so it is contained in an amount of 0.5% or more. However, if the content of these elements is too high, twin deformation of titanium may be suppressed, which may deteriorate cold workability. Therefore, the upper limit is set at 1.5%.

[0036] [Si: more than 0.1%, less than 0.6%] Although Si is a β-stabilizing element, it also dissolves in the α-phase, exhibiting high solid-solution strengthening ability, so it is contained in an amount of more than 0.1%. High strength can be achieved by solid-solution strengthening with Si. However, excessive Si content forms intermetallic compounds called silicides, which reduce workability. Since a Si content of more than 0.6% generates coarse silicides during the manufacturing process, reducing fatigue strength, the upper limit is set at 0.6%. A content of 0.5% or less is preferable.

[0037] [Nb: 0.1 to 0.6%] Nb is a beta-stabilizing element that forms a complete solid solution and improves strength at room temperature, so it is contained at 0.1% or more. However, it has been found that if it is contained in excess, a large amount of this element remains in the coating after anodization, which can have a negative effect on color development. If it is contained at 0.6% or less, the effect on color development is small, so it is set at 0.6% or less.

[0038] [O: 0.1% or less] Since a large amount of O can reduce ductility and workability, the O content is set to 0.1% or less. However, since O is an unavoidable impurity, its inclusion is unavoidable, so the actual O content is usually 0.01% or more.

[0039] Other common component compositions of the base material titanium alloy: In the component compositions of the first and second base materials, the following components may be contained in place of a portion of the remaining Ti.

[0040] [Zr content] Zr is a neutral element that improves strength at room temperature to high temperatures through solid solution strengthening. However, if the Zr content is too high, twin deformation of titanium may be suppressed, which may deteriorate cold workability. Therefore, the upper limit is set to 2% by mass. Preferably, it is 1.5% by mass or less.

[0041] [V,Mo] V and Mo are β-stabilizing elements that form a complete solid solution and improve strength at room temperature. However, it has been found that excessive content of these elements can result in a large amount of these elements remaining in the coating after anodization, which can lead to deterioration of color development. Since a content of 1% by mass or less has little effect on color development, the content is set to 1% by mass or less. A content of 0.5% by mass or less is preferred. The lower limit can be adjusted depending on the application, so it can be as low as 0%. When these optional elements are contained in the titanium alloy base material, the total content of the β-stabilizing elements in the base material is preferably 3.0% by mass or less in order to ensure that the behavior of each β-stabilizing element in the oxide coating falls within the range of the present invention.

[0042] [O, N, C content] Since the inclusion of large amounts of O, N, and C can reduce ductility and workability, the O content is limited to 0.40% or less, preferably 0.3% by mass or less. Furthermore, the C content is limited to 0.08% or less, and the N content is limited to 0.05% or less. Since the inclusion of O, N, and C is unavoidable as they are unavoidable impurities, the effective contents are usually 0.01% or more for O, 0.0001% or more for C, and 0.0001% or more for N.

[0043] Titanium products may be produced, for example, in the form of plates, as rolled and vacuum annealed, or after pickling. Bars may be pickled or machined to remove surface scale. In the present invention, the surface may be in either state. [Example]

[0044] An oxide film was formed on the surface of the titanium alloy by anodizing. The chemical composition of the base titanium alloy is shown in Table 1. In each example in Table 2, a titanium alloy having the chemical composition of the base material No. in Table 1 was used as the base material. After rolling, the base material was pickled, and then an oxide film was formed by anodizing. The pickling was carried out using an aqueous solution of 10% nitric acid and 5% hydrofluoric acid. In addition, in Examples 14 and 15, vacuum annealing was carried out after rolling, and then anodizing was carried out without pickling.

[0045] The anodization conditions were a 1% by mass sulfuric acid solution, and the holding time after reaching the maximum voltage in the solution was 60 seconds or less. The voltage was varied in the range of 20 to 100 V for each example to adjust the film thickness. Inventive Examples Nos. 5 and 7, a 1% by mass phosphoric acid solution was used instead of a sulfuric acid solution.

[0046] As for the voltage application rate for anodization, as shown in the anodization conditions in Table 2, two conditions were used: slow condition (80 V / min) and fast condition (400 V / min).

[0047] The content of β-stabilizing elements in the coating was measured using the GD-OES described above. At five random points on the colored titanium surface, the depth distribution of the components was measured from the surface to 1 μm. At each measurement point on the coating surface, the region with an O content of 60% or more in the depth direction was considered to be an oxide layer. The average content of each β-stabilizing element in that region (oxide layer region) was measured and recorded as the average content of each β-stabilizing element at each measurement point. The maximum value among these values ​​is listed in the "Maximum Average Content of Each β-stabilizing Element" column in Table 2. The average value of the average content of each β-stabilizing element across all measurement points was calculated to represent the total amount of each β-stabilizing element. The total amount of each β-stabilizing element was then added up to represent the "Total Amount of All β-stabilizing Elements" in Table 2. Furthermore, the maximum and minimum values ​​for each β-stabilizing element were extracted from all measurement points, and the difference between the maximum and minimum contents of each β-stabilizing element was calculated from these results. Furthermore, the largest difference between the maximum and minimum content of each β-stabilizing element is shown in the "Variation / max-min" column in Table 2. The samples were thoroughly cleaned with alcohol before analysis. Note that because GD-OES analysis detects dirt adhering to the surface, the surface layer down to a depth of 10 nm was excluded.

[0048] The thickness of the oxide layer was measured by the GD-OES analysis described above, where the region with an O content of 60% or more was considered to be the oxide layer. As with the β-stabilizing element content, the GD-OES analysis detects dirt adhering to the surface, so the depth of the surface layer down to 10 nm was excluded.

[0049] The "Total amount of β-stabilizing elements" in the "Mother phase" column of Table 2 indicates the total amount of β-stabilizing elements in the chemical composition of Table 1. The "β-phase fraction" in the "Mother phase" column was measured using SEM (scanning electron microscopy) / EPMA (Electron Probe Micro Analyzer). In EPMA, the region where the β-stabilizing elements were 1 mass% or more was defined as the β-phase. 500 × 500 μm 2 Measurements were taken at five points on any plane parallel to the rolling direction and the thickness direction of the material, and the average value was calculated and evaluated as the β phase fraction.

[0050] The color quality of the oxide coating was evaluated using a color microscope. For color microscope observation, a random area (five or more points) on the plate surface was observed at a magnification of 50 to 200 times. First, 100 to 5,000 points other than the randomly selected color were extracted from the image. Then, for each extracted point, the Lab color space values ​​a and b were extracted (a and b are indicators of color direction, with a representing the color intensity from green to red, and b representing the color intensity from blue to yellow). Next, a and b were each divided into 10-step intervals from -90 to 90 (-90 to -80, -80 to -70, 80 to 90), and the number corresponding to each range was calculated for each of the 18 x 18 = 324 combinations. The center value of the range with the most number of these 324 combinations (for example, if the maximum value is a: 10 to 20, b: -10 to -20, then a = 15, b = -15) is taken as the reference point. The difference in color difference between this reference point and each extracted point was evaluated. Specifically, the difference in color difference between the coordinates in the Lab color space for each extracted point and the reference point: √(Δa 2 +Δb 2 ) (Δa, Δb are the differences between the a and b values ​​at an arbitrary point and the a and b values ​​at a reference point.) When the area ratio for which the color difference was 30 or less was 60% or more, it was marked "Good", when it was less than 60% and 50% or more, it was marked "Good", and when it was less than 50%, it was marked "Poor". Here, the area ratio was calculated by dividing the number of extracted points with a color difference of 30 or less by the total number of extracted points. The evaluations are shown in the "Quality" and "Color Uniformity" columns of Table 2, with Good and Good representing pass and Bad representing fail. In Table 2, values ​​outside the range of the present invention are underlined.

[0051] [Table 1]

[0052] [Table 2]

[0053] For Inventive Examples 1 to 17 in Table 2, the behavior of the β-stabilizing elements in the oxide film was within the preferred range of the present invention, resulting in good color uniformity in the oxide film. This was because the total amount of β-stabilizing elements and the β-phase fraction in the base material were within the preferred range of the present invention, and the voltage application rate under the anodizing conditions was suitable. Inventive Example 7, however, the total amount of β-stabilizing elements in the base material was somewhat high at 2.6 mass%, and the β-phase fraction was somewhat high at 21%, so the color uniformity was only rated fair.

[0054] In Comparative Examples 1 to 3 in Table 2, the behavior of the β-stabilizing elements in the oxide film was outside the preferred range of the present invention, resulting in poor color uniformity in the oxide film. This was because the total amount of β-stabilizing elements in the base material was outside the preferred range of the present invention in Comparative Example 1, and the voltage application rate during anodizing was outside the preferred conditions in Comparative Examples 2 and 3.

Claims

1. A titanium alloy base material and a titanium alloy having an oxide film on the surface thereof with a thickness of 20 nm to 200 nm, The base material is, in mass %, Al: 4.5-6.4%, Fe: 0 to 2.3%, Si: 0-0.60% and C: less than 0.08% N: 0.05% or less, O: 0.4% or less and the remainder being Ti and impurities, A titanium alloy characterized in that the average content of each β-stabilizing element in the oxide film is 0.4 mass% or less, the total amount of all β-stabilizing elements is 1.0 mass% or less, and further, the difference between the maximum and minimum contents of each β-stabilizing element within the oxide film surface is 0.3 mass% or less.

2. The titanium alloy according to claim 1, wherein the base material has a total content of β stabilizing elements of 0.2 to 3.0 mass%.

3. 3. The titanium alloy according to claim 1, wherein the base material has a β phase fraction of 20% or less.

4. 4. The titanium alloy according to claim 1, further comprising, in mass%, 1% or less of one or more of Ni, Cr, and Mn, each of which is 1% or less, in place of a portion of the Ti.

5. A titanium alloy base material and a titanium alloy having an oxide film on the surface thereof with a thickness of 20 nm to 200 nm, The base material is in mass % Cu: 0.5-1.5%, Sn: 0.5-1.5%, Si: more than 0.1%, less than 0.6%, Nb: 0.1 to 0.6%, and O: 0.1% or less and the remainder being Ti and impurities, A titanium alloy characterized in that the average content of each β-stabilizing element in the oxide film is 0.4 mass% or less, the total amount of all β-stabilizing elements is 1.0 mass% or less, and further, the difference between the maximum and minimum contents of each β-stabilizing element within the oxide film surface is 0.3 mass% or less.

6. The titanium alloy according to claim 5, wherein the base material has a total content of β stabilizing elements of 0.2 to 3.0 mass%.

7. 7. The titanium alloy according to claim 5, wherein the base material has a β phase fraction of 20% or less.

8. A method for producing a titanium alloy according to any one of claims 1 to 7, A method for producing a titanium alloy, characterized in that the voltage application rate during anodization is 350 V / min or less.

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