Titanium alloy and method for manufacturing titanium alloy

Abstract

This titanium alloy has an increased oxygen content relative to the prior art, and comprises, with the whole thereof being 100 atomic % (at%), 15-27 at% tantalum (Ta), 1-8 at% tin (Sn), and 0.4-1.7 at% oxygen (O), the remainder being titanium (Ti) and unavoidable impurities. The average particle diameter of equiaxed α phases in this titanium alloy is preferably in the range of 0.01-1.0 µm. The area ratio occupied by equiaxed α phases per unit area in this titanium alloy is preferably in the range of 0.1-10%.

Description

Technical Field

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

Background Art

[0002] Conventionally, as a titanium alloy, a titanium alloy containing tantalum and tin has been proposed (see, for example, Japanese Patent No. 5855588). After cold working this titanium alloy and then heat treating it at a predetermined temperature, while removing the residual strain generated by the cold working, the α-phase and ω-phase were changed to the β-phase.

[0003] Also conventionally, as a method for producing a titanium alloy, for example, columnar titanium and wire-shaped or thin-plate-shaped additive metals are set in a crucible of a floating melting apparatus, melted under predetermined melting conditions, and then naturally cooled in the crucible to obtain a titanium alloy (see, for example, Japanese Patent Application Laid-Open No. 2002-012923). Also conventionally, as a method for producing a titanium alloy, for example, titanium powder and powder of a vanadium group element are mixed and set in a heating container, and these are pressurized and heated to be sintered to obtain a titanium alloy (see, for example, Japanese Patent No. 3375083).

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the research of the present inventors, when the titanium alloy described in Patent Document 1 is heat treated at 800°C or higher after cold working, it has characteristics such that a two-stage yield point occurs in the stress-strain diagram, and it has been clarified that the elastic limit strain decreases. For this reason, the titanium alloy has characteristics such that deformation easily occurs even with a small force. For example, when further performing lathe work on a bar (work) made of the titanium alloy after heat treatment, when a cutting tool is brought into contact with the titanium alloy bar, the bar easily bends by that force, making cutting difficult.

[0005] In the course of our previously undisclosed research, we attempted to resolve the above situation by performing an aging treatment after heat treatment on the titanium alloy described in Patent Document 1. However, the titanium alloy described in Patent Document 1 has a high content of tantalum, a β-stabilizing element, which presented the problem of requiring several days of aging treatment for the α-phase to precipitate.

[0006] In view of these circumstances, this first invention aims to provide a titanium alloy with a higher oxygen content than conventional alloys.

[0007] On the other hand, when attempting to actively incorporate oxygen into titanium alloys, the titanium alloy manufacturing method described in Patent Document 2 involves the mixing of atmospheric elements into the titanium alloy during the melting process, making it difficult to precisely control the composition of the titanium alloy. As a result, the manufacturing method described in Patent Document 2 has the problem of not being able to control the amount of oxygen at all.

[0008] Furthermore, according to the inventors' research, the titanium alloy manufacturing method described in Patent Document 3 had the problem that titanium and vanadium group elements diffused unevenly in the titanium alloy after sintering. As a result, the manufacturing method described in Patent Document 3 also had the problem that oxygen contained in the oxide film of the titanium powder diffused unevenly.

[0009] In view of these circumstances, the present invention aims to provide a method for manufacturing a titanium alloy that can homogenize the entire alloy while precisely controlling the composition ratio contained in the titanium alloy. [Means for solving the problem]

[0010] The titanium alloy of this first invention is characterized in that, when the total composition is 100 atomic percent (at%), it consists of 15 to 27 at% tantalum (Ta), 1 to 8 at% tin (Sn), 0.4 to 1.7 at% oxygen (O), and the remainder being titanium (Ti) and unavoidable impurities.

[0011] Furthermore, the titanium alloy of this first invention is characterized in that the average grain size of the equiaxed α phase is in the range of 0.01 μm to 1.0 μm.

[0012] Furthermore, the titanium alloy of this first invention is characterized in that the area occupancy rate of the equiaxed α phase per unit area is in the range of 0.1% to 10%.

[0013] Furthermore, in the titanium alloy of the first invention, when the stress at which the permanent strain reaches 0.5% in a tensile test is defined as the 0.5% strain stress, the 0.5% strain stress is characterized in that it is within the range of 400 MPa to 1200 MPa.

[0014] The method for producing a titanium alloy according to the second invention is characterized by comprising at least a mixing step of mixing titanium powder mainly composed of titanium (Ti) and vanadium group powder mainly composed of vanadium group elements to obtain a mixed powder; a solidification step of heating the mixed powder mixed in the mixing step to induce solid-phase diffusion bonding to obtain a solidified body; and a melting step of heating and melting the solidified body to produce a titanium alloy.

[0015] Furthermore, in the method for producing a titanium alloy according to the second invention, the solidification step is characterized by heating the mixed powder at a temperature between 900 and 1400°C to cause solid-phase diffusion bonding.

[0016] Furthermore, in the method for manufacturing a titanium alloy according to the second invention, the solidification step is characterized in that the mixed powder is subjected to a vacuum and solid-phase diffusion bonding by heating and pressurizing.

[0017] Furthermore, in the method for manufacturing a titanium alloy according to the second invention, the melting step is characterized in that the solidified material is dissolved by a vacuum arc remelting method or a cold crucible induction melting method.

[0018] Furthermore, the method for producing a titanium alloy according to the second invention is characterized in that the titanium alloy contains 0.4 to 1.7 at% oxygen (O) when the total composition is 100 atomic percent (at%).

[0019] Also, in the method for producing a titanium alloy of the second invention, the vanadium group powder is characterized by containing tantalum (Ta) or niobium (Nb) as a main component.

[0020] Also, in the method for producing a titanium alloy of the second invention, the titanium alloy is characterized by containing 1 to 8 atomic percent (at%) of tin (Sn) when the whole is 100 atomic percent (at%).

[0021] Further, the method for producing a titanium alloy of the second invention further includes a heat treatment step of performing heat treatment on the titanium alloy generated in the melting step, and an aging treatment step of performing aging treatment on the heat-treated titanium alloy. The titanium alloy that has undergone the melting step is composed of 15 to 27 atomic percent (at%) of tantalum (Ta), 1 to 8 atomic percent (at%) of tin (Sn), 0.4 to 1.7 atomic percent (at%) of oxygen (O), and the balance being titanium (Ti) and inevitable impurities when the whole is 100 atomic percent (at%).

[0022] Also, in the method for producing a titanium alloy of the second invention, in the aging treatment step, the titanium alloy is subjected to aging treatment for 24 hours or less to precipitate an α phase in the titanium alloy.

Effect of the Invention

[0023] According to the titanium alloy of the first invention, an excellent effect can be achieved in that the precipitation of an equiaxed α phase can be easily controlled. According to the method for producing a titanium alloy of the second invention, an excellent effect can be achieved in that each component of the titanium alloy can be homogenized while controlling the composition ratio contained in the titanium alloy with high precision. Hereinafter, the first invention and the second invention will be collectively referred to simply as the present invention.

Brief Description of the Drawings

[0024] [Figure 1] It is a binary phase diagram of Ta-Ti. [Figure 2]This is a graph showing the results of a cold working property evaluation test for a titanium alloy (Ti-23Ta-xSn-0.26O) where the Ta content is 23 at% and the Sn content is x at% and the O content is 0.26 at% when the whole is 100 atomic % (at%). [Figure 3] This is a table showing the results of a cold working property evaluation test for a titanium alloy (Ti-23.4Ta-3.4Sn-xO) where the Ta content is 23.4 at% and the Sn content is 3.4 at% and the O content is x at% when the whole is 100 atomic % (at%). [Figure 4] (A) is a photograph of a STEM image when the surface of a comparative example titanium alloy (Ti-23.4Ta-3.4Sn-0.26O) as a comparative example was observed at a magnification of 2000 times using a scanning transmission electron microscope (STEM). (B) is a photograph of a STEM image when the surface of a first titanium alloy (Ti-23.4Ta-3.4Sn-0.75O) was observed at a magnification of 2000 times using a scanning transmission electron microscope (STEM). (C) is a photograph of a STEM image when the surface of a second titanium alloy (Ti-23.4Ta-3.4Sn-0.92O) was observed at a magnification of 2000 times using a scanning transmission electron microscope (STEM). Note that the numerical values attached to the elemental notations of Ta, Sn, and O in each parentheses corresponding to the comparative example titanium alloy and the first and second titanium alloys represent the atomic % (at%) numerical values of each element (Ta, Sn, O) when the whole of each of the comparative example titanium alloy and the first and second titanium alloys is 100 atomic % (at%). [Figure 5] (A) is a photograph of a STEM image when the surface of a third titanium alloy (Ti-23Ta-3Sn-0.65O) subjected to an aging treatment at 400 °C for 12 hours was observed at a magnification of 2000 times using a scanning transmission electron microscope (STEM). (B) is a photograph of a STEM image when the surface of a third titanium alloy (Ti-23Ta-3Sn-0.65O) subjected to an aging treatment at 500 °C for 12 hours was observed at a magnification of 2000 times using a scanning transmission electron microscope (STEM). Note that the numerical values attached to the elemental notations of Ta, Sn, and O in each parentheses corresponding to the third titanium alloy represent the atomic % (at%) numerical values of each element (Ta, Sn, O) when the whole of the third titanium alloy is 100 atomic % (at%). [Figure 6] This is an enlarged view of the dotted rectangular area in Figure 5(A). [Figure 7] (A) is a table showing the compositions of test specimens H1 and T1 prepared using the titanium alloy in the embodiment of the present invention. (B) is a stress-strain diagram showing the results of tensile tests on test specimens H1 and T1, respectively. [Figure 8] (A) is a stress-strain diagram obtained as a result of a tensile test in which repeated stress-strain was applied to specimen H1. (B) is a stress-strain diagram obtained as a result of a tensile test in which repeated stress-strain was applied to specimen T1. (C) is a superimposed diagram of the stress-strain diagrams of (A) and (B). [Figure 9] (A) is a table showing the compositions of test specimens H2 and T2 prepared using the titanium alloy in the embodiment of the present invention. (B) is a stress-strain diagram showing the results of tensile tests on test specimens H2 and T2, respectively. [Figure 10] (A) is a stress-strain diagram obtained as a result of a tensile test in which repeated stress-strain was applied to specimen H2. (B) is a stress-strain diagram obtained as a result of a tensile test in which repeated stress-strain was applied to specimen T2. ​​(C) is a superimposed diagram of the stress-strain diagrams of (A) and (B). [Figure 11] (A) is a superimposed diagram of the stress-strain diagrams obtained from tensile tests on specimen T1 and specimen T2, respectively. (B) is a superimposed diagram of the stress-strain diagrams obtained from tensile tests in which repeated stress-strain was applied to specimen T1 and specimen T2, respectively. [Figure 12] This graph shows the relationship between the stress σ at 0.5% strain and the oxygen content x when tensile tests were performed on specimens of titanium alloy with the composition Ti-23Ta-3Sn-xO (where 0.4 ≤ x ≤ 1.7) before and after aging treatment. [Figure 13](A) is a table showing the compositions of test specimens H3, T3, and T4 prepared using the titanium alloy in the embodiment of the present invention. (B) is a graph showing the results of Vickers hardness tests for each of the multiple test specimens H3, T3, and T4 with different heat treatment temperatures. [Figure 14] This flowchart shows the process for manufacturing a titanium alloy in an embodiment of the present invention. [Figure 15] (A) is a schematic diagram of a HIP apparatus used in the titanium alloy manufacturing method according to an embodiment of the present invention. (B) is a schematic diagram of an apparatus for performing vacuum arc remelting (VAR) used in the titanium alloy manufacturing method according to an embodiment of the present invention. [Figure 16] (A) is a SEM image of a solidified material produced by HIP treatment, observed at 400x magnification using a scanning electron microscope (SEM). (B) is a SEM image of elemental analysis of Ti using X-rays in the range of (A). (C) is a SEM image of elemental analysis of Ta using X-rays in the range of (A). (D) is a SEM image of elemental analysis of Sn using X-rays in the range of (A). [Modes for carrying out the invention]

[0025] The titanium alloys in embodiments of the present invention will be described below with reference to the attached drawings.

[0026] <Composition of Titanium Alloy> The titanium alloy in the embodiment of the present invention consists of 15-27 at% tantalum (Ta), 1-8 at% tin (Sn), 0.4-1.7 at% oxygen (O), and the remainder being titanium (Ti) and unavoidable impurities, when the total composition is 100 atomic percent (at%). The content of the remaining titanium (Ti) is not particularly limited; it is sufficient if titanium (Ti) is the most abundant element among the constituent elements when considered in terms of atomic ratio.

[0027] Furthermore, titanium alloys are broadly classified into three types: α-type titanium alloys, which have a close-packed hexagonal (HCP) α-phase as the matrix; β-type titanium alloys, which have a body-centered cubic (BCC) β-phase as the matrix; and α+β-type titanium alloys, in which both the close-packed hexagonal (HCP) α-phase and the body-centered cubic (BCC) β-phase coexist. However, the type of titanium alloy according to the present invention is not particularly limited.

[0028] <Tantalum (Ta)> Tantalum (Ta) is used to make the titanium alloy in this embodiment a titanium alloy that undergoes thermoelastic martensitic transformation. Ta has the function of lowering the transformation temperature from the β phase to the α phase to a lower temperature, stabilizing the β phase at room temperature, and making it less susceptible to slip deformation (plastic deformation).

[0029] The Ta content is preferably 15-27 at%, more preferably 19-25 at%, and most preferably 22-24 at%, when the total titanium alloy is considered to be 100 atomic percent (at%).

[0030] The upper limit for the Ta content is set based on the melting point of the titanium alloy. Figure 1 is a binary phase diagram of Ta-Ti. As shown in Figure 1, if the Ta content exceeds 27%, the melting point of the titanium alloy may exceed approximately 2000K, requiring a special melting furnace and increasing production costs. In addition, there is a possibility of incomplete melting of the Ta raw material, which can lead to a decrease in the quality of the titanium alloy.

[0031] The lower limit of the Ta content is set based on the β-phase stabilization function described above and the mechanical properties of titanium alloys as materials for medical devices and biomaterials. In other words, the β-phase stabilization function decreases as the Ta content decreases, and if the Ta content is less than 15 at%, it becomes difficult to maintain the β-phase at room temperature. For this reason, if the Ta content is less than 15 at%, it becomes difficult to obtain the mechanical properties (Young's modulus, tensile strength, and elastic deformation strain) required for medical devices and biomaterials, even if tin (Sn) is added. Accordingly, the Ta content when the total titanium alloy is considered as 100 at%, is preferably 15 at% or more, more preferably 19 at% or more, and most preferably 22 at% or more.

[0032] <Tin (Sn)> Tin (Sn) has an α-phase stabilization function that increases the transformation temperature and stabilizes the α-phase. In addition, Sn has the function of suppressing the precipitation of the ω-phase, which is a factor that increases Young's modulus, and thereby enhancing the superelastic effect of titanium alloys.

[0033] The Sn content is preferably 1 to 8 at%, and more preferably 2 to 6 at%, when the total titanium alloy is considered to be 100 atomic percent (at%).

[0034] The upper limit of the Sn content is set based on the workability (cold workability) of the titanium alloy. The graph shows the results of a cold workability evaluation test for a titanium alloy (Ti-23Ta-xSn-0.26O) with a Ta content of 23 at% when the total titanium alloy is set to 100 at%. Here, x is the Sn content (at%) when the total titanium alloy is set to 100 at%. In this cold workability evaluation test, first, multiple test pieces (thickness: 1 mm, no heat treatment) were prepared with varying Sn content x (when the total titanium alloy is set to 100 at%) of 0 at%, 1.5 at%, 3 at%, 6 at%, and 9 at%. These test pieces were then cold-rolled to a thickness of 0.1 mm (processing rate 86%), and the number of cracks with a length of 1 mm or more in each test piece after cold rolling was counted. This crack counting was performed within a range of 140 mm in the rolling direction for each test piece.

[0035] As shown in Figure 2, the results of this evaluation test confirmed that when the Sn content was 9 at%, the occurrence of cracks of 1 mm or more increased sharply, meaning that the workability deteriorated sharply. Similar trends were observed when the Ta content was different. Therefore, when the total titanium alloy is considered to be 100 at%, the Sn content is preferably 8 at% or less, and more preferably 6 at% or less, in order to obtain good workability.

[0036] The lower limit of the Sn content is not particularly limited, but in order to fully exhibit the ω-phase suppression function described above, it is preferable that the Sn content be 1 at% or more when the total titanium alloy is considered to be 100 at%.

[0037] <Oxygen (O)> Oxygen (O) has an α-phase stabilization function that increases the transformation temperature and stabilizes the α-phase. Furthermore, O has a stronger α-phase stabilization function than Sn. In addition, oxygen (O) has the function of restricting crystal deformation and preventing the development of shape memory and softening.

[0038] The content rate of O is preferably 0.4 to 1.7 at% and more preferably 0.6 to 1.0 at% when the entire titanium alloy is 100 atomic % (at%).

[0039] <Regarding the upper limit of the content rate of O (Cold working evaluation)> The upper limit value of the content rate of O is set based on the workability (cold workability) of the titanium alloy. If the content rate of O is made too high, the titanium alloy becomes too hard due to the function of preventing softening of O, and the workability deteriorates. Fig. 3 is a table showing the results of a cold working evaluation test for a titanium alloy (Ti-23.4Ta-3.4Sn―xO) in which the content rate of Ta is 23.4 at% and the content rate of Sn is 3.4 at% when the entire titanium alloy is 100 at%. Here, x is the content rate (at%) of O when the entire titanium alloy is 100 at%. Also, the titanium alloy used in the cold working evaluation test was created by the <Method for manufacturing a titanium alloy> described later.

[0040] In this cold working evaluation test, first, a plurality of test pieces (round wire shape with a diameter φ of 10 mm, without heat treatment) were prepared with the content rate x of O changed to 0.26 at%, 0.59 at%, 0.75 at%, 0.92 at%, 1.14 at%, 1.4 at% and 1.59 at% when the entire titanium alloy (Ti-23.4Ta-3.4Sn―xO) is 100 at%. Rotary forging was performed on these test pieces. At this time, each test piece was evaluated as to how much the working rate could be performed by rotary forging.

[0041] As shown in Fig. 3, as a result of this evaluation test, it was confirmed that there is no problem even if rotary forging is performed so that the processing rate is 75% or more for specimens with an O content of 0.26 to 1.14 at%. For specimens with an O content of 1.4 at%, there was no problem even when rotary forging was performed so that the processing rate was 50 to 75%, but cracks occurred when the processing rate exceeded 75%. For specimens with an O content of 1.59 at%, cracks occurred when rotary forging was performed so that the processing rate exceeded 50%. Therefore, in order to obtain good workability, the O content when the entire titanium alloy is 100 at% is preferably 1.7 at% or less, more preferably 1.4 at% or less, and even more preferably 1.2 at% or less.

[0042] <Regarding the lower limit of the O content (evaluation of the titanium alloy structure)> The lower limit of the O content is set based particularly on the mechanical properties. In other words, conventional titanium alloys have a large amount of β phase, which causes deformation with small forces, resulting in the problem of not being able to maintain the shape after forming. For this reason, in the titanium alloy of this embodiment, it is preferable that a certain amount of equiaxed α phase, which has the function of restraining crystal deformation and increasing the strength of the alloy and preventing softening, is precipitated. In the titanium alloy of this embodiment, as will be explained in the <Method for Manufacturing Titanium Alloy> described later, equiaxed α phase precipitates after cold working, heat treatment, and aging treatment. Incidentally, the heat treatment temperature is preferably in the range of 600°C to 1000°C, and more preferably in the range of 700°C to 900°C. Also, the aging treatment temperature is preferably in the range of 200°C to 550°C, and more preferably in the range of 300°C to 500°C. However, if the oxygen content in a titanium alloy is insufficient, Ta, as a β-stable element, functions more effectively than oxygen, as an α-stable element. As a result, even with aging treatment, the equiaxed α phase does not precipitate easily, and aging treatment may be required for several days to precipitate the equiaxed α phase. Furthermore, if the oxygen content is less than 0.4 at%, as shown in the comparative example titanium alloy (Ti-23.4Ta-3.4Sn-0.26O) in Figure 4(A) below, even after aging treatment for about 2 hours, the amount of equiaxed α phase precipitated is insufficient, making it difficult to maintain the shape after molding. Therefore, the oxygen content of the titanium alloy, when the total titanium alloy is considered as 100 at%, is preferably 0.4 at% or more, more preferably 0.6 at% or more, and most preferably 0.75 at% or more.

[0043] Figures 4(B) and 4(C) show STEM images of the surfaces of the first titanium alloy (Ti-23.4Ta-3.4Sn-0.75O) and the second titanium alloy (Ti-23.4Ta-3.4Sn-0.92O) in this embodiment, observed at a magnification of 2000x using a scanning transmission electron microscope (STEM). Figure 4(A) shows a STEM image of a comparative example titanium alloy (Ti-23.4Ta-3.4Sn-0.26O) under the same conditions as above. The first and second titanium alloys and the comparative titanium alloy were heat-treated at 700°C for 2 hours, followed by aging treatment at 300°C for 2 hours.

[0044] The STEM images of the first and second titanium alloys shown in Figures 4(B) and (C) show the deposition of equiaxed α phase (see the granular white areas in Figures 4(B) and (C)). The image in Figure 4(C), which has a higher O content, shows more equiaxed α phase deposition than the image in Figure 4(B). On the other hand, the STEM image of the comparative titanium alloy shown in Figure 4(A) does not show any white areas corresponding to the equiaxed α phase, indicating that the equiaxed α phase has hardly precipitated. It was confirmed that the equiaxed α phase precipitates even after aging treatment of about 2 hours for alloys with a high O content, as shown in Figures 4(B) and (C), but does not precipitate for alloys with a low O content, as shown in Figure 4(A).

[0045] Furthermore, STEM images of the surface of the tertiary titanium alloy (Ti-23Ta-3Sn-0.65O) observed at a magnification of 2000x using a scanning transmission electron microscope (STEM) are shown in Figures 5(A) and (B). The tertiary titanium alloy shown in Figure 5(A) was heat-treated at 900°C for 2 hours, followed by aging treatment at 400°C for 12 hours. The tertiary titanium alloy shown in Figure 5(B) was heat-treated at 900°C for 2 hours, followed by aging treatment at 500°C for 12 hours. The tertiary titanium alloy was produced using the <Titanium Alloy Manufacturing Method> described later.

[0046] The STEM images of the titanium tertiary alloy shown in Figures 5(A), (B), and 6 show the deposition of equiaxed α phase (see granular white areas). Incidentally, Figure 6 is a magnified view of the square region in Figure 5(A). The white areas in Figure 6 are the equiaxed α phase. The equiaxed α phase in Figure 5(A) can be confirmed by referring to Figure 6. Furthermore, the image shown in Figure 5(B) (titanium tertiary alloy aged at 500°C for 12 hours) shows more deposition of equiaxed α phase (see white areas in Figures 5(A) and (B)) than the image shown in Figure 5(A) (titanium tertiary alloy aged at 400°C for 12 hours). From these results, it was confirmed that a higher aging temperature results in a greater amount of equiaxed α phase deposition.

[0047] From these results, it was confirmed that the amount of equiaxed α-phase precipitation in titanium alloys can be controlled by the O content, aging treatment time, and aging treatment temperature. From this, it can be seen that the O content, when the total titanium alloy is considered to be 100 at%, is preferably 0.4 at% or more, more preferably 0.6 at% or more, and most preferably 0.75 at% or more.

[0048] Furthermore, when comparing the first and second titanium alloys with the third titanium alloy, the first and second titanium alloys show a greater amount of equiaxed α phase precipitation. This is because the first and second titanium alloys were heat-treated at a temperature corresponding to the α+β phase region of the first and second titanium alloys (700°C) before aging treatment, which promoted the precipitation of the α phase. However, the third titanium alloy was heat-treated at a temperature corresponding to the β phase region of the third titanium alloy (900°C) before aging treatment, which did not promote the precipitation of the α phase. Therefore, the amount of α phase precipitation can be adjusted by the heat treatment temperature before aging treatment. In other words, by controlling the phase state of the titanium alloy through heat treatment, the amount of α phase precipitation during aging treatment can be controlled. Incidentally, heat treatment before aging treatment is optional.

[0049] Incidentally, analysis of the STEM image of the first titanium alloy shown in Figure 4(B) revealed that the average grain size of the equiaxed α phase in the first titanium alloy (Ti-23.4Ta-3.4Sn-0.75O) was 0.13 μm. Furthermore, analysis of the STEM image of the second titanium alloy shown in Figure 4(C) revealed that the average grain size of the equiaxed α phase in the second titanium alloy (Ti-23.4Ta-3.4Sn-0.92O) was 0.17 μm. From these results, it can be inferred that as the O content increases, the average grain size of the equiaxed α phase precipitated by aging treatment tends to increase accordingly.

[0050] Furthermore, analysis of the STEM image of the titanium tertiary alloy shown in Figure 5(A) revealed that the average grain size of the equiaxed α phase in the titanium tertiary alloy (Ti-23Ta-3Sn-0.65O) aged at 400°C for 12 hours was 0.03 μm. Analysis of the STEM image of the titanium tertiary alloy shown in Figure 5(B) revealed that the average grain size of the equiaxed α phase in the titanium tertiary alloy (Ti-23Ta-3Sn-0.65O) aged at 500°C for 12 hours was 0.05 μm. From these results, it can be inferred that the average grain size of the precipitated equiaxed α phase tends to increase as the aging temperature increases.

[0051] Furthermore, in the titanium alloy of the present invention, the average particle size P of the equiaxed α phase is preferably in the range of 0.01 μm to 1.00 μm (0.01 μm ≤ P ≤ 1.00 μm), more preferably in the range of 0.02 μm to 0.50 μm (0.02 μm ≤ P ≤ 0.50 μm), and even more preferably in the range of 0.03 μm to 0.30 μm (0.03 μm ≤ P ≤ 0.30 μm).

[0052] Furthermore, analysis of the STEM image of the first titanium alloy shown in Figure 4(B) revealed that the area ratio of the equiaxed α phase in the first titanium alloy (Ti-23.4Ta-3.4Sn-0.75O) was 1.89%. Similarly, the area ratio of the equiaxed α phase in the second titanium alloy (Ti-23.4Ta-3.4Sn-0.92O) shown in Figure 4(C) was 5.24%. The area ratio of the equiaxed α phase refers to the proportion of the area occupied by the equiaxed α phase per unit area in the STEM images of the cross-sections shown in Figures 4(B) and (C). From these results, it can be inferred that as the O content increases, the area ratio of the equiaxed α phase precipitated by aging treatment increases accordingly. Incidentally, in the STEM image of the comparative example titanium alloy (Ti-23.4Ta-3.4Sn-0.26O) shown in Figure 4(A), the equiaxed α phase could not be confirmed, so the area ratio of the equiaxed α phase in the comparative example titanium alloy is 0%.

[0053] Furthermore, analysis of the STEM images of the tertiary titanium alloy shown in Figure 5(A) revealed that the area percentage of the equiaxed α phase in the tertiary titanium alloy (Ti-23Ta-3Sn-0.65O) aged at 400°C for 12 hours was 0.21%. Similarly, the area percentage of the equiaxed α phase in the tertiary titanium alloy (Ti-23Ta-3Sn-0.65O) aged at 500°C for 12 hours, as shown in Figure 5(B), was 1.49%. From these results, it can be inferred that the area occupancy rate of the equiaxed α phase precipitated by aging increases with increasing aging temperature.

[0054] Based on the above results, the inventors analyzed the data and confirmed that the area occupancy Q of the equiaxed α phase in the titanium alloy of the present invention, which consists of 15-27 at% tantalum (Ta), 1-8 at% tin (Sn), 0.4-1.7 at% oxygen (O), and the remainder being titanium (Ti) and unavoidable impurities, is preferably in the range of 0.1% to 10% (0.1% ≤ Q ≤ 10%), more preferably in the range of 0.1% to 8% (0.1% ≤ Q ≤ 8%), and even more preferably in the range of 0.1% to 6% (0.1% ≤ Q ≤ 6%).

[0055] As described above, even with the same aging treatment time, a higher oxygen content in the titanium alloy results in a greater amount of equiaxed α phase precipitation, while a lower oxygen content results in a smaller amount of equiaxed α phase precipitation. Therefore, when precipitation of a specific amount of equiaxed α phase in a titanium alloy through aging treatment, a titanium alloy with a higher oxygen content requires a shorter aging treatment time than a titanium alloy with a lower oxygen content. Furthermore, even with titanium alloys of the same composition, a higher aging treatment temperature increases the amount of equiaxed α phase precipitation, while a lower aging treatment temperature decreases the amount of equiaxed α phase precipitation.

[0056] However, the equiaxed α phase increases the strength of titanium alloys. However, if the oxygen content becomes too high, it exceeds the solid solution limit of oxygen, leading to the formation of TiO and TiO2, which reduces workability. This is reflected in the cold workability evaluation test shown in Figure 3.

[0057] While general titanium alloys also contain small amounts of oxygen as an unavoidable impurity, the oxygen content in such cases is 0.2 at% or less when the total titanium alloy is considered to be 100 at%. In such titanium alloys, ta, as a β-stable element, functions more than oxygen, as an α-stable element, and equiaxed α phases do not precipitate easily even with aging treatment, requiring aging treatment for several days. On the other hand, in titanium alloys like the one in this embodiment, if the oxygen content is actively increased and reaches 0.4 at%, oxygen functions as an α-stable element, and a minimum amount of equiaxed α phase precipitates after 12 hours of aging treatment, and a sufficient amount of equiaxed α phase precipitates after 24 hours of aging treatment. Furthermore, when the oxygen content in the titanium alloy reaches 1.7 at%, oxygen functions sufficiently as an α-stable element, and a sufficient amount of equiaxed α phase precipitates even after 1 hour of aging treatment. For this reason, the aging treatment time is preferably within the range of 1 to 24 hours, and more preferably within the range of 1 to 4 hours.

[0058] The titanium alloy according to this embodiment exhibits extremely low elution of metal ions of its constituent elements, Ti, Ta, and Sn, as well as excellent corrosion resistance, low cytotoxicity, high biocompatibility, and is a non-magnetic material that is not easily magnetized by external magnetic fields, thus posing an extremely low risk of adverse effects on medical devices (such as MRI) that are sensitive to magnetism. It also possesses high elasticity, moderate rigidity, and high workability. In other words, the titanium alloy according to this embodiment has lower cytotoxicity and superior magnetic properties, corrosion resistance, mechanical properties, and workability compared to conventional titanium alloys. For this reason, it is suitable for medical devices such as guide wires, delivery wires, stents, aneurysm embolization coils, or venous filters in the catheter field, or for dental devices such as cleansers, reamers, files, or orthodontic wires, and for orthopedic devices such as artificial bones.

[0059] It should be noted that the titanium alloy according to the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the present invention. For example, conventionally known methods can be used to form medical guide wires, delivery wires, stents, aneurysm embolization coils or venous filters in the catheter field, or dental products such as cleansers, reamers, files or orthodontic wires, or artificial bones in the orthopedic field, from the titanium alloy according to the present invention. Examples include wire drawing, drawing, casting, forging, and press working. [Examples]

[0060] Next, examples of the titanium alloy according to the present invention will be described. The following test pieces T1 to T4 and H1 to H3 were prepared by the <Titanium Alloy Manufacturing Method> described later.

[0061] <Tensile Test 1> The inventors of the present invention performed tensile tests on test specimen T1 according to Example 1 and test specimen H1 according to Comparative Example 1, both prepared using titanium alloys according to embodiments of the present invention with varying oxygen content. Figure 7(A) is a table showing the compositions of test specimens T1 and H1 to be subjected to tensile testing. Test specimens T1 and H1 are round wires with a diameter of φ0.406 mm, formed by cold working (wire drawing) with a processing rate of 75%. Test specimen H1 according to Comparative Example 1 was prepared using a titanium alloy with the composition Ti-23Ta-3Sn-0.25O, and has a low oxygen content. Test specimen T1 according to Example 1 was prepared using a titanium alloy with the composition Ti-23Ta-3Sn-0.8O. The titanium alloys constituting test specimens T1 and H1 were heat-treated at 890°C for 1 minute, and no aging treatment was performed. Tensile tests were performed on the above test specimens T1 and H1 using a tensile testing machine. The measurement conditions were a distance of 50 mm between measurement points and a tensile speed of 2 mm / min.

[0062] Figure 7(B) shows the stress-strain diagrams obtained as a result of tensile tests performed on specimens T1 and H1. Up to a stress of approximately 200 MPa, specimens T1 and H1 deform in a manner that produces similar strains. At approximately 200 MPa, the graph has an inflection point K1. As shown in Figure 7(B), in specimen H1, the slope of the graph becomes considerably gentler after the inflection point K1 compared to the previous slope, suggesting that there is a yield region somewhere along the way (see the dotted circle area in Figure 7(B)). On the other hand, in specimen T1, as shown in Figure 7(B), the slope of the graph becomes gentler after the inflection point K1 compared to the previous slope, similar to specimen H1, but the slope of the graph is steeper compared to that of specimen H1 (see the dotted circle area in Figure 7(B)). In other words, regardless of whether or not aging treatment is performed, the test piece T1 of Example 1, which has a high O content, has harder (less yielding) material properties than the test piece H1 of Comparative Example 1, and can be said to stabilize the shape of the molded product.

[0063] <Tensile Test 2> Figure 8(A) is a stress-strain diagram obtained as a result of a tensile test in which repeated stress-strain was applied to specimen H1 of Comparative Example 1 under the same measurement conditions as Tensile Test 1. Figure 8(B) is a stress-strain diagram obtained as a result of a tensile test in which repeated stress-strain was applied to specimen T1 of Example 1 under the same measurement conditions as Tensile Test 1. Figure 8(C) is a superimposed view of the stress-strain diagrams of Figures 8(A) and (B). As is clear from Figures 8(A) and (C), when a tensile stress of 470 (MPa) is applied to specimen H1, and the load is removed after it has been deformed by approximately 0 to 2%, a permanent strain of approximately 0.35% remains. On the other hand, as is clear from Figures 8(B) and (C), when a tensile stress of 490 (MPa) is applied to specimen T1, and the load is removed after it has been deformed by approximately 0 to 2%, only a permanent strain of approximately 0.2% remains. As a result, it was confirmed that the test specimen T1 of Example 1, which had an increased O content, had a higher elastic limit strain than the test specimen H1 of Comparative Example 1.

[0064] Furthermore, as is clear from Figures 8(A) and (C), when a tensile stress of 503 MPa is applied to test specimen H1, causing it to deform by approximately 0 to 2.2%, and then the load is removed, a permanent strain of approximately 0.5% remains. On the other hand, as is clear from Figures 8(B) and (C), in order to leave a permanent strain of approximately 0.5% in test specimen T1, it is necessary to apply a tensile stress of 625 MPa, causing it to deform by approximately 0 to 2.6%, and then remove the load. As a result, it was confirmed that test specimen T1 of Example 1, which had an increased O content, had a higher elastic limit strain than test specimen H1 of Comparative Example 1. [Examples]

[0065] <Tensile Test 3> Furthermore, the inventors of the present invention performed tensile tests on test specimen T2 according to Example 2 and test specimen H2 according to Comparative Example 2, both prepared using titanium alloys according to embodiments of the present invention with varying oxygen content. Figure 9(A) is a table showing the composition of test specimens T2 and H2 that were subjected to tensile testing. The composition of the titanium alloys constituting test specimens T2 and H2 is obtained by adding an additional 1 hour of aging treatment at 300°C to the titanium alloys constituting test specimen T1 of Example 1 and test specimen H2 of Comparative Example 1. Tensile tests were performed on the above test specimens T2 and H2 using a tensile testing machine. The measurement conditions were a distance between scoring points of 50 mm and a tensile speed of 2 mm / min.

[0066] Figure 9(B) shows the stress-strain diagrams obtained as a result of tensile tests performed on specimens T2 and H2. Up to a stress of approximately 200 MPa, specimens T2 and H2 deform in a manner that produces similar strains. At approximately 200 MPa, the graph has an inflection point K2. As shown in Figure 9(B), for specimen H2, the slope of the graph becomes considerably gentler after the inflection point K2 compared to the previous slope (see the dotted circle area in Figure 9(B)). On the other hand, for specimen T2, as shown in Figure 9(B), the slope of the graph becomes gentler after the inflection point K2 compared to the previous slope, similar to specimen H2, but the slope of the graph is steeper compared to that of specimen H2 (see the dotted circle area in Figure 9(B)). In other words, even after aging treatment, specimen T2 has material properties that make it harder (less prone to yielding) than specimen H2, and it can be said that it can stabilize the shape of the molded product.

[0067] Furthermore, comparing test specimens T2,H2 with test specimens T1,H1, the slope of the graph after the inflection point is steeper for test specimens T2,H2 than for test specimens T1,H1. This is because the titanium alloy that has undergone aging treatment contains more equiaxed α phases compared to the titanium alloy that has not undergone aging treatment, and therefore possesses the properties of a harder metal. As a result, the titanium alloy of this embodiment that has undergone aging treatment is easier to form after cold working.

[0068] Furthermore, comparing the results of tensile test 1 of specimen T1 in Example 1 (see Figure 7(B)) with the results of tensile test 3 of specimen T2 in Example 2 (see Figure 9(B)), as shown in Figure 11(A), up to the stress interval where the graphs intersect, which is slightly less than 800 MPa, specimen T2 exhibits less strain than specimen T1 even when the same stress is applied. Also, after the stress in specimens T1 and T2 reaches their respective maximum values ​​(tensile strengths), the strain increases at a nearly constant stress (tensile strength) for both specimens T1 and T2. Incidentally, the maximum stress (tensile strength) of specimen T1 is approximately 870 MPa, and the maximum stress (tensile strength) of specimen T2 is approximately 840 MPa. The strain leading to fracture in specimen T2 is approximately 1.2 times greater than that of specimen T1. These results suggest that titanium alloys subjected to aging treatment exhibit greater equiaxed α-phase precipitation and harder properties, as well as improved ductility, compared to titanium alloys that have not undergone aging treatment.

[0069] <Tensile Test 4> Figure 10(A) is a stress-strain diagram obtained as a result of a tensile test in which repeated stress-strain was applied to specimen H2 under the same measurement conditions as Tensile Test 3. Figure 10(B) is a stress-strain diagram obtained as a result of a tensile test in which repeated stress-strain was applied to specimen T2 under the same measurement conditions as Tensile Test 3. Figure 10(C) is a superimposed view of the stress-strain diagrams of Figures 10(A) and (B). As is clear from Figures 10(A) and (C), when a tensile stress of slightly less than 500 (MPa) is applied to specimen H2, causing it to deform by approximately 0 to 2%, and then the load is removed, a permanent strain of approximately 0.2% remains. On the other hand, as is clear from Figures 10(B) and (C), when a tensile stress of 580 (MPa) is applied to specimen T2, causing it to deform by approximately 0 to 2%, and then the load is removed, only a permanent strain of slightly less than 0.1% remains. As a result, it was confirmed that specimen T2, which had an increased O content, had a higher elastic limit strain than specimen H2.

[0070] Furthermore, as is clear from Figures 10(A) and (C), when a tensile stress of 616 MPa is applied to test specimen H2, causing it to deform by approximately 0 to 2.6%, and then the load is removed, a permanent strain of approximately 0.5% remains. On the other hand, as is clear from Figures 10(B) and (C), in order to leave a permanent strain of approximately 0.5% in test specimen T2, it is necessary to apply a tensile stress of 768 MPa, causing it to deform by approximately 0 to 3.0%, and then remove the load. As a result, it was confirmed that test specimen T2 of Example 2, which had an increased O content, had a higher elastic limit strain than test specimen H2 of Comparative Example 2.

[0071] Furthermore, comparing the results of tensile test 2 of specimen T1 in Example 1 (see Figure 8(B)) with the results of tensile test 4 of specimen T2 in Example 2 (see Figure 10(B)), as shown in Figure 11(B), for example, specimen T1, which has not undergone aging treatment, shows a strain of 0.2% when a tensile stress of approximately 490 (MPa) is applied and it is deformed by approximately 2%, and then unloaded. However, specimen T2, which has undergone aging treatment, shows a strain of slightly less than 0.1% when a tensile stress of approximately 580 (MPa) is applied and it is deformed by approximately 2%, and then unloaded. In other words, the titanium alloy that has undergone aging treatment has a higher elastic limit strain than the titanium alloy that has not undergone aging treatment.

[0072] Similarly, as shown in Figure 11(B), for example, when a specimen T1 that has not undergone aging treatment is subjected to a tensile stress of approximately 625 MPa and deformed by approximately 2.6%, and then unloaded, it exhibits a strain of 0.5%. However, when a specimen T2 that has undergone aging treatment is subjected to a tensile stress of approximately 768 MPa and deformed by approximately 3%, and then unloaded, it exhibits a strain of 0.5%. In other words, titanium alloys that have undergone aging treatment have a higher elastic limit strain than titanium alloys that have not undergone aging treatment.

[0073] Here, in a tensile test, the stress at which the permanent strain reaches 0.5% is defined as the 0.5% strain stress σ (MPa). As is clear from Figure 8(B), the 0.5% strain stress σ in specimen T1 is 625 (MPa). Also, as is clear from Figure 10(B), the 0.5% strain stress σ in specimen T2 is 768 (MPa). In other words, for a titanium alloy with the composition Ti-23Ta-3Sn-0.8O, the 0.5% strain stress σ before and after aging treatment will be in the range of 625 to 768 (MPa).

[0074] On the other hand, as is clear from Figure 8(A), the stress σ at 0.5% strain in specimen H1 is 503 (MPa). Also, as is clear from Figure 10(A), the stress σ at 0.5% strain in specimen H2 is 616 (MPa). In other words, for titanium alloys with the composition Ti-23Ta-3Sn-0.25O, the stress σ at 0.5% strain before and after aging treatment will be in the range of 503 to 616 (MPa).

[0075] Based on the experimental results described above, our inventors' analysis revealed that the stress σ at 0.5% strain for a titanium alloy with the composition Ti-23Ta-3Sn-xO (where the O content x is 0.4 (at%) ≤ x ≤ 1.7 (at%)) can be approximately expressed as a linear function with x as the variable. This graph is shown in Figure 12. For titanium alloys before aging treatment (without aging treatment), the stress σ at 0.5% strain can be expressed as a function σ = 221.82x + 447.55. After aging treatment, the stress σ at 0.5% strain can be expressed as a function σ = 276.36x + 546.91. As is clear from Figure 12, before aging treatment, when the O content x = 0.4 (at%), the stress σ at 0.5% strain is 536 (MPa), and when the O content x = 1.7 (at%), the stress σ at 0.5% strain is 824 (MPa). In other words, before aging treatment, in the range 0.4 ≤ x ≤ 1.7, 536 ≤ σ ≤ 824 is satisfied. Also, as is clear from Figure 12, after aging treatment, when x = 0.4, the stress σ at 0.5% strain is 657 (MPa), and when x = 1.7, the stress σ at 0.5% strain is 10¹⁶ (MPa). In other words, after aging treatment, in the range 0.4 ≤ x ≤ 1.7, 657 ≤ σ ≤ 10¹⁶ is satisfied. From the above, it was verified that, in the range of 0.4 ≤ x ≤ 1.7, the stress σ at 0.5% strain satisfies at least 536 (MPa) ≤ σ ≤ 10¹⁶ (MPa) before and after aging treatment.

[0076] Considering the above results, when a tensile test is performed on a titanium alloy specimen with the composition Ti-23Ta-3Sn-xO (where 0.4 ≤ x ≤ 1.7), the stress σ at 0.5% strain is preferably 400 MPa or higher and 1200 MPa or lower, i.e., within the range of 400 MPa to 1200 MPa (400 MPa ≤ σ ≤ 1200 MPa). Furthermore, the stress σ at 0.5% strain is preferably 500 MPa or higher, and more preferably 530 MPa or higher. In addition, the stress σ at 0.5% strain is preferably 1000 MPa or lower, more preferably 900 MPa or lower, and more preferably 830 MPa or lower.

[0077] For reference, considering the stress at which permanent strain reaches 0.2% in a tensile test (stress at 0.2% strain), as is clear from Figure 8(B), the stress at 0.2% strain in specimen T1 is 490 (MPa). Also, as is clear from Figure 10(B), the stress at 0.2% strain in specimen T2 is approximately 700 (MPa). In other words, for titanium alloys with the composition Ti-23Ta-3Sn-0.8O, the stress at 0.2% strain before and after aging treatment will be in the range of 490 to 700 (MPa). Furthermore, as is clear from Figure 10(A), the stress at 0.2% strain in specimen H2 is 500 (MPa), so for titanium alloys with the composition Ti-23Ta-3Sn-0.25O, the stress at 0.2% strain σ will be 500 (MPa) or less.

[0078] Since the stress at a permanent strain of 0.2% is in a range where it is prone to variation depending on the specimen, analysis should ideally be performed using the stress at 0.5% strain, as mentioned above. However, if we were to perform the analysis using the stress at 0.2% permanent strain, it can be estimated that, before aging treatment, the stress at 0.2% strain in a titanium alloy with the composition Ti-23Ta-3Sn-xO (where 0.4≦x≦1.7) would generally satisfy the range of 400≦σ≦700 (MPa). Furthermore, after aging treatment, it can be estimated that the stress at 0.2% strain in a titanium alloy with the composition Ti-23Ta-3Sn-xO (where 0.4≦x≦1.7) would generally satisfy the range of 600≦σ≦900. Therefore, it can be inferred that the stress at 0.2% strain in a titanium alloy with the composition Ti-23Ta-3Sn-xO (where 0.4 ≤ x ≤ 1.7) generally satisfies the relationship 400 ≤ σ ≤ 900 before and after aging treatment. [Examples]

[0079] <Vickers hardness test> Furthermore, the inventors of the present invention performed Vickers hardness tests on test specimen T3 of Example 3-1 and test specimen T4 of Example 3-2, which were prepared using titanium alloys according to embodiments of the present invention with varying oxygen content. In this Vickers hardness test, 10 measurement points were used, and a load of 0.1 (N) was applied to each measurement point. A test specimen H3 of Comparative Example 3 was also prepared and subjected to a similar Vickers hardness test.

[0080] Figure 13(A) is a table showing the compositions of the test specimens T3, T4, and H3 used in the Vickers hardness test. Test specimens T3, T4, and H3 are round wires with a diameter of φ0.517 mm. Test specimen H3 is made from a titanium alloy with the composition Ti-23Ta-3Sn-0.25O, and has a low oxygen content. Test specimen T3 is made from a titanium alloy with the composition Ti-23Ta-3Sn-0.6O, and test specimen T4 is made from a titanium alloy with the composition Ti-23Ta-3Sn-0.8O. Test specimens T3, T4, and H3 were prepared both without heat treatment and after heat treatment at 450°C, 500°C, 550°C, 650°C, 700°C, and 750°C for 30 minutes.

[0081] The results of this Vickers hardness test are shown in Figure 13(B). As shown in Figure 13(B), up to a heat treatment temperature of 550°C, there was no significant change in test specimens T3, T4, and H3 due to heat treatment. On the other hand, at heat treatment temperatures of 650°C or higher, it was confirmed that the hardness of the titanium alloy tended to increase as the oxygen content increased. In other words, when the oxygen content is increased, the heat treatment temperature is preferably in the range of 600°C to 1000°C, and more preferably in the range of 700°C to 900°C.

[0082] The above tensile tests 1 through 4 and the Vickers hardness test confirmed that increasing the oxygen content improves both the elastic limit strain and the material's strength. As a result, it was found that moderately increasing the oxygen content results in a titanium alloy with appropriate hardness and elastic limit strain that facilitates post-processing.

[0083] <Manufacturing method for titanium alloys> The method for manufacturing a titanium alloy according to embodiments of the present invention will be described below with reference to the attached drawings. The method for manufacturing a titanium alloy according to embodiments of the present invention will be described below with reference to Figures 14 and 15.

[0084] <Mixing process> First, as shown in Figure 14, a mixing step is performed in which Ti powder, mainly composed of titanium (Ti), Ta powder, mainly composed of tantalum (Ta), and Sn powder, mainly composed of tin (Sn), are prepared and mixed in a predetermined mixing ratio (step S100). In this embodiment, the Ti powder, Ta powder, and Sn powder are assumed to have a particle size that can pass through a sieve with a mesh size of 325 (mesh / inch) or less. Preferably, the Ti powder contains 90% or more titanium (Ti), more preferably 95% or more titanium (Ti), and even more preferably 99% or more titanium (Ti). In this case, the remaining components of the Ti powder include components other than titanium (Ti). Furthermore, the titanium (Ti) in the Ti powder may include pure titanium (Ti) or titanium (Ti) having an oxide film. The above Ta powder preferably contains 90% or more tantalum (Ta), more preferably 95% or more tantalum (Ta), and even more preferably 99% or more tantalum (Ta). In this case, the remaining components of the Ta powder include components other than tantalum (Ta). Furthermore, the tantalum (Ta) in the above Ta powder may be pure tantalum (Ta) or tantalum (Ta) having an oxide film. The above Sn powder preferably contains 90% or more tin (Sn), more preferably 95% or more tin (Sn), and even more preferably 99% or more tin (Sn). In this case, the remaining components of the Sn powder include components other than tin (Sn). Furthermore, the tin (Sn) in the above Sn powder may be pure tin (Sn) or tin (Sn) having an oxide film.

[0085] For the same weight, powders with smaller particle sizes have a greater number of particles than powders with larger particle sizes. As a result, when compared by the same weight, powders with smaller particle sizes have a larger surface area that reacts with oxygen than powders with larger particle sizes. Titanium powder (Ti) and tantalum powder (Ta) are stable in the atmosphere due to their oxide film. As a result, when compared by the same weight, mixing in titanium powder (Ti) or tantalum powder (Ta) with smaller particle sizes results in a higher oxygen content in the final titanium alloy product than mixing in powders with larger particle sizes. Therefore, the particle size of titanium powder (Ti) and tantalum powder (Ta) affects the oxygen content in the titanium alloy, and the oxygen content in the titanium alloy can be adjusted by appropriately selecting the particle size of each powder. For this reason, to create multiple titanium alloys with different oxygen content, it is sufficient to change the particle size of at least one of the Ti or Ta powders. Furthermore, if the desired oxygen content cannot be achieved by simply reducing the particle size of the Ti powder and / or Ta powder, titania powder may be added as it contains oxygen itself. Although it is possible to change the O content by changing the particle size of the Sn powder, in this embodiment the Sn powder content is small, so changing the particle size of the Sn powder does not significantly affect the O content in the titanium alloy.

[0086] For example, when the total is considered to be 100 atomic percent (at%), the mixture consists of 15-27 at% tantalum (Ta), 1-8 at% tin (Sn), 0.4-1.7 at% oxygen (O), with the remainder being titanium (Ti) and unavoidable impurities. The Ti powder or titania powder, Ta powder, and Sn powder are uniformly mixed in this ratio. Converting this to weight percentage, when the total is considered to be 100 wt%, Ta will be 40-56 wt%, Sn will be 2-10 wt%, O will be 0.1-0.3 wt%, and the remainder will be titanium (Ti) and unavoidable impurities. For example, in the case of a titanium alloy Ti-23.4Ta-3.4Sn-xO, when the total is considered to be 100 wt%, the mixture will consist of 52 wt% Ta powder, 5 wt% Sn powder, and the remainder being Ti powder or titania powder. The O content is adjusted by controlling the particle size of Ti powder or Ta powder having an oxide film on its surface, or by using titania powder. In this embodiment, since the Sn powder content is low, it is presumed that the influence of Sn powder on the O content is small. The notation at% represents atomic percentage, and below, the notation at% is used to represent the atomic percentage of the corresponding element when the entire titanium alloy is considered to be 100 atomic percent (at%). In addition, below, the numerical values ​​attached to the elemental notations of Ta, Sn, and O representing the composition of the titanium alloy (see the values ​​in parentheses immediately following "titanium alloy" below) represent the atomic percentage (at%) of each element (Ta, Sn, O) when the entire titanium alloy is considered to be 100 atomic percent (at%).

[0087] <Solidification process> Next, as shown in Figure 14, the mixed powder of Ti powder or titania powder, Ta powder, and Sn powder (hereinafter simply referred to as "mixed powder"), which has been uniformly mixed in the above mixing step, is subjected to a solidification step (step S101) under vacuum, in which the mixed powder is solidified by solid-phase diffusion bonding. The mixed powder becomes a solidified body through the solidification step. Since the mixed powder is solidified by solid-phase diffusion bonding under vacuum, it is possible to limit the entry of unintended oxygen (O) from the outside air into the solidified body. In this embodiment, the solidified body refers to a mass formed by solid-phase diffusion bonding of the mixed powder. In the solidification step, for example, a normal heat treatment (e.g., sintering treatment by heating), or a pressurized and heated treatment performed simultaneously with pressurization is employed. For pressurization and heating, one of the following methods is used: Hot Isostatic Pressing (HIP), Spark Isostatic Pressing (SIP), or Spark Plasma Sintering (SPS). The mixed powder is solidified by one of these methods. Incidentally, HIP and SIP are isotropic (hydrostatic) pressurization methods, while SPS uses a press machine and mold, so it is directional pressurization (axial pressurization). In this embodiment, the pressure under vacuum in which the mixed powder is placed is, for example, 1.0 × 10⁻⁶. -1 (Pa) or less is preferred, and 1.0 × 10 -2 ~1.0×10 -5 A range within (Pa) is more preferable, and 1.0 × 10 -2 ~1.0×10 -3 A range within (Pa) is even more preferable.

[0088] Here, referring to Figure 15(A), we will explain using the case where the solidification process uses the hot isostatic pressing method (hereinafter referred to as HIP treatment) as an example. First, the mixed powder 5 is pressurized and filled into the HIP container 2. The HIP container 2 is composed of, for example, a cylindrical container with one end open and the other end closed, and a lid. The mixed powder 5 is filled into the cylindrical container while being compressed, and the cylindrical container is placed in the vacuum chamber of an electron beam apparatus (not shown). Then, the pressure inside the vacuum chamber is set to, for example, 1.0 × 10⁻⁶ -2 ~1.0×10 -3 Electron beam welding is performed under a vacuum within the range of (Pa), and a lid is welded to the opening of the HIP container 2 to seal it. As a result, the mixed powder is placed under vacuum inside the HIP container 2.

[0089] While it is preferable to use a material other than Ta for the HIP container 2, Ta is extremely expensive and not practical for mass production. Therefore, for example, it is preferable to use a material mainly composed of Ti or iron as the material for the HIP container 2, and more preferably a material mainly composed of iron. Incidentally, in a typical HIP treatment, the HIP container 2 is made from the same material as the material with the highest melting point among the mixed powders 5 (in this case, Ta).

[0090] Then, the HIP container 2 is placed inside the heat-insulating section 3A of the HIP furnace 3 of the HIP device 1. The HIP device 1 is configured to create a high-temperature, high-pressure atmosphere inside the heat-insulating section 3A of the HIP furnace 3 by heating with a substantially inert gas such as argon and a heater 4. The gas is supplied from the outside to the inside of the HIP furnace 3 through the gas introduction passage 3B of the HIP furnace 3. When high temperature and high pressure are applied to the HIP container 2 for a predetermined time, the mixed powder 5 is pressurized and heated through the HIP container 2. As a result, the mixed powder 5 undergoes solid-phase diffusion bonding and becomes a solidified body. Since the mixed powder 5 is sealed inside the HIP container 2 under vacuum, even if the mixed powder 5 is pressurized and heated through the HIP container 2, it is possible to limit the entry of unintended oxygen (O) from the outside air into the solidified body. Incidentally, immediately after the HIP treatment, the HIP container 2 and the solidified body are firmly bonded together. Therefore, in order to separate the HIP container 2 from the solidified body, the HIP container 2 and the layer in which the HIP container 2 and the solidified body are mixed are cut using a machine tool. As a result, only the solidified body remains. This forms a cylindrical solidified body.

[0091] In the HIP treatment, the internal temperature of the HIP furnace 3 of the HIP apparatus is set to, for example, 1000°C, the pressure to 98 MPa, and the HIP container is left under these conditions for a predetermined time to form a solidified body. The internal temperature of the HIP furnace 3 can be any range as long as the HIP container 2 is not damaged or melted. For example, the internal temperature of the HIP furnace 3 is preferably 700°C to 1600°C, more preferably 900°C to 1400°C, and even more preferably 1000°C to 1200°C. The internal pressure of the HIP furnace 3 is preferably 50 to 200 MPa, more preferably 70 to 180 MPa, and even more preferably 90 to 120 MPa.

[0092] Note that the melting point of Ta is quite high at 3017°C. Even if the internal temperature of the HIP furnace 3 is set to, for example, 900°C to 1400°C, if the proportion of Ta is high, such as in the titanium alloy of this embodiment (15 at% or more), the possibility of each powder diffusing completely uniformly is low. In other words, the solidified body produced by the HIP treatment may contain regions where Ta, Sn, Ti, and O are not uniformly diffused, and where one of Ta, Sn, Ti, or O is unevenly distributed. To confirm this, the inventors of this application set the internal temperature of the HIP furnace 3 to 1000°C and the internal pressure of the HIP furnace 3 to 98 MPa, and solidified the mixed powder (with a Ta powder content of 23.4 at%, a Sn powder content of 3.4 at%, and the remainder being Ti powder, with a particle size of 10 to 45 μm for each powder) as described above. Using a scanning electron microscope (SEM) and X-rays, they investigated how Ta, Sn, and Ti were distributed in the solidified body. The results are shown in Figures 16(A) to (D). Figure 16(A) is a scanning electron microscope (SEM) image of the solidified material observed at a magnification of 400x. As shown in Figures 16(B) to (D), the distribution of Ti, Ta, and Sn within the range of the SEM image in Figure 16(A) is not uniform, indicating that the Ti powder, Ta powder, and Sn powder are not uniformly diffused. From this, it was confirmed that, at an internal temperature of HIP furnace 3 of approximately 900°C to 1400°C, the possibility of complete alloying of the Ti powder, Ta powder, and Sn powder with this composition, where the proportion of Ta is very high, is low.

[0093] On the other hand, raising the internal temperature of the HIP furnace 3 to a temperature higher than the above temperature can lead to damage to the HIP container 2 itself, and consequently, a malfunction of the HIP furnace 3 itself. Furthermore, in order to achieve heating at temperatures higher than 1400°C, it is conceivable to form the HIP container 2 from a material that can withstand high temperatures (for example, Ta), but as already mentioned, using Ta material for the HIP container 2 would make it extremely expensive and therefore impractical.

[0094] <Dissolution process> Next, as shown in Figure 14, a dissolution step is performed to dissolve the solidified mixed powder solidified in the above solidification step (step S102). The solidified body is dissolved in the dissolution step, becoming a titanium alloy ingot. As described above, the solidified body does not have uniform diffusion of each component, but by performing the dissolution step, each component is uniformly melted (dissolved), and a titanium alloy ingot in which each component is uniformly dispersed can be obtained. In the dissolution step, it is preferable to dissolve the solidified body at a temperature that can simultaneously dissolve Ta, Sn, and Ti, which have different melting points, in order to simultaneously dissolve and uniformly diffuse them. In the melting process, one of the following methods is used, for example: Vacuum Arc Remelting (VAR), ElectroSlag Remelting (ESR), Vacuum Induction Melting (VIM), Cold Crucible Induction Melting (CCIM), Plasma Arc Melting (PAM), or Electron Beam Melting (EBM), and the solidified material is melted by one of these methods.

[0095] Here, referring to Figure 15(B), we will explain using the vacuum arc remelting method (VAR) as an example in the melting process. First, the cylindrical solidified body prepared in the solidification process is used as a consumable electrode 6 and is connected to a rod 9 suspended inside the arc melting furnace 8. As a result, the consumable electrode 6 is suspended and supported by the rod 9 inside the arc melting furnace 8 with the molten metal pool 10 located directly below it. In this state, when an electric current is passed through the rod 9 to the consumable electrode 6, an arc discharge occurs between the consumable electrode 6 and the molten metal pool 10. The high heat generated by the arc discharge heats and melts the consumable electrode 6, which then accumulates below to form a titanium alloy ingot 11.

[0096] Alternatively, the titanium alloy ingot 11 may be used as a consumable electrode 6, connected to the rod 9, and then placed in the arc melting furnace 8 as described above, and melted again by applying current. This process can be repeated multiple times to increase the reliability of homogenization of each component, and therefore may be repeated even further. In this manner, the titanium alloy is prepared.

[0097] <Cold working process> Next, as shown in Figure 14, cold working is performed on the titanium alloy ingot prepared in the melting process (step S103). Cold working of the titanium alloy ingot forms titanium alloy workpieces such as wires and rods.

[0098] <Heat treatment process> Next, as shown in Figure 14, a heat treatment step is performed on the titanium alloy workpiece formed in the cold working step (step S104). The heat treatment temperature is preferably 600°C to 1000°C, and more preferably 700°C to 900°C. Note that the heat treatment step may be omitted.

[0099] <Statute of Limitations Treatment Process> Next, as shown in Figure 14, an aging treatment is performed on the titanium alloy that has undergone the heat treatment step described above, or on the titanium alloy that has undergone the cold working step but has not undergone the heat treatment step (step S105). The aging treatment temperature is preferably 200°C to 550°C, and more preferably 300°C to 500°C. When the titanium alloy in this embodiment is subjected to the above aging treatment for a predetermined time, equiaxed α phases and the like precipitate in the titanium alloy. Note that the α phase is not limited to equiaxed structures, but may include other forms.

[0100] As mentioned in the explanation of the third titanium alloy shown in Figures 5(A) and (B) produced by this manufacturing method, for titanium alloys of the same composition, a higher aging treatment temperature results in a greater amount of equiaxed α-phase precipitation. Consequently, the average grain size and area occupancy rate of the precipitated equiaxed α-phase increase.

[0101] As mentioned before, while general titanium alloys also contain small amounts of oxygen as an unavoidable impurity, the oxygen content in such cases is 0.2 at% or less when the total titanium alloy is considered to be 100 at%. In such titanium alloys, ta, as a β-stable element, functions more than oxygen, as an α-stable element, and even with aging treatment, the equiaxed α phase does not precipitate easily, requiring aging treatment for several days. On the other hand, by actively increasing the oxygen content using this manufacturing method, when the oxygen content reaches 0.4 at%, oxygen functions as an α-stable element, and a minimum amount of equiaxed α phase precipitates after 12 hours of aging treatment, and a sufficient amount of equiaxed α phase precipitates after 24 hours of aging treatment. Furthermore, when the oxygen content in the titanium alloy reaches 1.7 at%, oxygen functions sufficiently as an α-stable element, and a sufficient amount of equiaxed α phase precipitates even after 1 hour of aging treatment. For this reason, the aging treatment time is preferably within the range of 1 to 24 hours, and more preferably within the range of 1 to 4 hours.

[0102] Furthermore, the method for manufacturing titanium alloys according to the present invention is not limited to the embodiments described above. For example, a powder composed of vanadium (V) or niobium (Nb), which are other elements of the vanadium group (group 5a elements: vanadium group elements), may be used together with or in place of tantalum (Ta) powder. In other words, the composition of the titanium alloy may include vanadium (V) or niobium (Nb), which are other elements of the vanadium group, together with or in place of tantalum (Ta). The titanium alloy obtained by this manufacturing method has extremely stable quality. Incidentally, since vanadium (V) or niobium (Nb), like tantalum, have high melting points, solid-phase diffusion bonding alone tends to result in an uneven internal composition of the titanium alloy, making it difficult to guarantee the quality of metal parts after cold working.

[0103] However, the melting points of V, Nb, and Ta are 1910°C, 2477°C, and 3017°C, respectively, while the melting point of titanium is 1668°C. In other words, the melting point of V is only slightly higher than that of titanium, but the melting points of Nb and Ta are about 1.5 times higher than that of titanium. For this reason, it is presumed that Nb and Ta will not diffuse uniformly as much as V during the solidification process. As a result, Nb, like Ta, may end up with a distribution similar to that shown in Figure 3 during the solidification process. Therefore, it can be said that homogenization during the dissolution process is particularly important for Nb and Ta compared to V.

[0104] It should be noted that the titanium alloy and method for manufacturing the titanium alloy according to the present invention are not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the present invention. For example, conventionally known methods can be used to form medical guide wires, delivery wires, stents, clips, aneurysm embolization coils or venous filters, or dental cleansers, reamers, files or orthodontic wires, etc., from the titanium alloy according to the present invention, such as wire drawing, drawing, casting, forging, and press working. [Industrial applicability]

[0105] The titanium alloy of the present invention can be used in the catheter field, such as in medical guide wires, delivery wires, stents, clips, aneurysm embolization coils, or venous filters; in the dental field, such as in dental cleansers, reamers, files, or orthodontic wires; and in the orthopedic field, such as in artificial bone. [Explanation of Symbols]

[0106] 1 HIP device 2 HIP container 3 HIP furnace 3A Insulation Section 3B Gas introduction passage 4 Heater 5 Mixed powder 6 Consumable electrode 8. Arc melting furnace 9 rods 10 molten metal pool 11 ingots

Claims

1. When the total is assumed to be 100 atomic percent (at%), 15-27 at% tantalum (Ta), 1-8 at% tin (Sn), 0.4 to 1.7 at% oxygen (O), The remainder consists of titanium (Ti) and unavoidable impurities. It has an equiaxial α phase, The above is characterized in that the average particle size of the equiaxed α phase is in the range of 0.01 μm to 1.0 μm. Titanium alloy.

2. When the total is 100 atomic percent (at%), 15-27 at% tantalum (Ta), 1-8 at% tin (Sn), 0.4 to 1.7 at% oxygen (O), The remainder consists of titanium (Ti) and unavoidable impurities. It has an equiaxial α phase, The area occupancy rate of the equiaxed α phase per unit area in the cross-section is in the range of 0.1% to 10%. Titanium alloy.

3. When the total is 100 atomic percent (at%), 15-27 at% tantalum (Ta), 1-8 at% tin (Sn), 0.4 to 1.7 at% oxygen (O), The remainder consists of titanium (Ti) and unavoidable impurities. In a tensile test, if the stress at which the permanent strain reaches 0.5% is defined as the stress at 0.5% strain, The stress at 0.5% strain is characterized in that it is within the range of 400 MPa to 1200 MPa. Titanium alloy.

4. At a minimum, a mixing step of mixing titanium powder containing titanium (Ti) and vanadium group powder containing vanadium group elements to obtain a mixed powder, A solidification step is to obtain a solidified body by heating the mixed powder mixed in the mixing step and causing solid-phase diffusion bonding, A melting step in which the solidified body is heated and melted to produce a titanium alloy, A heat treatment step involves heat treatment of the titanium alloy produced in the melting step, A aging treatment step in which the heat-treated titanium alloy is subjected to aging treatment, Equipped with, The titanium alloy obtained through the melting process is characterized in that, when the total volume is 100 atomic percent (at%), it consists of 15 to 27 at% tantalum (Ta), 1 to 8 at% tin (Sn), 0.4 to 1.7 at% oxygen (O), and the remainder being titanium (Ti) and unavoidable impurities. A method for manufacturing titanium alloys.

5. The solidification step is characterized by heating the mixed powder at a temperature between 900 and 1400°C to cause solid-phase diffusion bonding. A method for producing a titanium alloy according to claim 4.

6. The solidification step is characterized by placing the mixed powder under vacuum and causing solid-phase diffusion bonding by heating and pressurizing. A method for producing a titanium alloy according to claim 4 or 5.

7. The dissolution step is characterized by dissolving the solidified body by a vacuum arc remelting method or a cold crucible induction dissolution method. A method for producing a titanium alloy according to claim 4 or 5.

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