Titanium alloy and method for producing same

Abstract

A titanium alloy according to the present invention contains 15-25 at% of aluminum and 4.5-15 at% of a β-phase stabilizing element, with the balance being made up of titanium and unavoidable impurities. The titanium alloy has a single-phase structure in which only a β-phase is present, and has a crystal grain size of 200 µm or more.

Description

Titanium alloy and its manufacturing method

[0001] This application claims priority to Japanese Patent Application No. 2024-070611, filed April 24, 2024, the contents of which are incorporated herein by reference.

[0002] In recent years, active research has been conducted on β-type titanium alloys, aiming to develop low Young's modulus biomaterials and titanium materials for aerospace applications (Non-Patent Document 1). Since the discovery of Ti-Nb shape memory alloys (Non-Patent Document 2), shape memory and superelastic effects have been reported for various β-type titanium alloys. However, with the exception of a few alloys such as Ti-Zr-Nb-Sn, conventional β-type titanium alloys have a superelastic recoverable strain of approximately 3% at room temperature (Non-Patent Document 3), which is insufficient compared to the recoverable strain of the most widely used Ni-Ti alloys. As the applications of superelastic alloys continue to expand, there is a demand for even superior superelasticity over a given temperature range. Meanwhile, from the perspective of realizing a low-carbon, safe, and secure society, superelastic alloys are also required to have excellent processability and lightweight properties.

[0003] Mitsuo Shinie: Materia, 52(2013), 219-228.C. Baker: Metal. Sci. J., 5(1971), 92-100.HY Kim, Y. Ikehara, JI Kim, H. Hosoda, S. Miyazaki: Acta Mater., 54(2006), 2419-2429.

[0004] The present invention has been made in view of the above circumstances, and aims to provide a titanium alloy that has excellent superelasticity, workability, and light weight within a given temperature range, and a method for producing the same.

[0005] In order to solve the above problems, the present invention employs the following means.

[0006] (1) A titanium alloy according to one embodiment of the present invention contains 15 to 25 at% aluminum, 4.5 to 15 at% of a β-phase stabilizing element, and the remainder titanium and unavoidable impurities. The titanium alloy has a single-phase structure in which only the β phase exists, and the crystal grain size is 200 μm or more.

[0007] (2) In the titanium alloy described in (1) above, it is preferable that the β-phase stabilizing element contains at least one of chromium, manganese, and vanadium.

[0008] (3) In the titanium alloy described in either (1) or (2) above, it is preferable that chromium is contained as the β-phase stabilizing element in a ratio of 4.5 to 15 at %.

[0009] (4) In the titanium alloy described in either (1) or (2) above, it is preferable that manganese is contained as the β-phase stabilizing element in a ratio of 4.5 to 8 at %.

[0010] (5) In the titanium alloy described in either (1) or (2) above, it is preferable that vanadium is contained as the β-phase stabilizing element in a ratio of 5 to 15 at %.

[0011] (6) In the titanium alloy described in any one of (1) to (5) above, it is preferable that the specific gravity is 5 or less.

[0012] (7) In the titanium alloy described in any one of (1) to (6) above, it is preferable that the superelastic critical stress at room temperature is 400 MPa or more and 1000 MPa or less, the maximum strength is 850 MPa or more and 1500 MPa or less, and the maximum reversible strain is 3% or more and 12% or less.

[0013] (8) A method for producing a titanium alloy according to one embodiment of the present invention is a method for producing a titanium alloy as set forth in either (1) or (2) above, comprising the steps of: subjecting a mixture of the aluminum, the β-phase stabilizing element, and the titanium to a solution treatment at 1200°C or higher and 1600°C or lower for 1 hour or longer and 200 hours or shorter; and subjecting the mixture after the solution treatment to a cyclic heat treatment in which a thermal cycle is repeated at a rate of 0.1°C / min or higher and 20°C / min or lower between a high temperature range of 1100°C or higher and 1400°C or lower and a low temperature range of 500°C or higher and 1050°C or lower.

[0014] (9) The method for producing a titanium alloy described in (8) above may further include a step of subjecting the mixture after the cyclic heat treatment to an aging treatment at room temperature or higher and 400°C or lower for 0.5 hours or longer and 30 hours or shorter.

[0015] According to the present invention, it is possible to provide a titanium alloy having excellent superelasticity, workability, and light weight within a given temperature range, and a method for producing the same.

[0016] 1 is a diagram showing the conditions of cyclic heat treatment in Examples 1 to 18 of the present invention. A photograph of the titanium alloy of Example 5. A graph showing the results of EBSD and XRD measurements performed on the titanium alloy of Example 5. An image obtained by TEM observation performed on the titanium alloy of Example 5. A mapping image of misorientation obtained by EBSD measurement performed on the titanium alloy of Example 5. A graph showing the results of a load-unload tensile test performed on the titanium alloy of Example 5, in which the tensile load was varied and the cycle of loading and unloading was performed. A graph showing the results of a load-unload tensile test performed on the titanium alloy of Example 5, in which the tensile load and unload cycle was repeated 200 times. A graph showing the results of a load-unload tensile test performed on the titanium alloy of Example 5, in which the tensile load and unload cycle was performed at different temperatures and in which the tensile load of 5% strain was performed. A diagram showing the positioning of the examples of the present invention in the relevant technical field in terms of the temperature range and specific gravity of the superelastic behavior of titanium alloys.

[0017] Hereinafter, the titanium alloy and its manufacturing method according to the embodiment of the present invention will be described in detail with reference to the drawings. Note that the drawings used in the following description may show characteristic parts enlarged for the sake of clarity, and the dimensional ratios of each component may not necessarily be the same as in reality. In addition, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto, and can be appropriately modified and implemented within the scope of the present invention.

[0018] [Titanium Alloy] A titanium alloy according to one embodiment of the present invention contains mainly aluminum (Al) in a ratio of 15 to 25 at% (atomic %), preferably 17 to 20 at%, and a β-phase stabilizing element in a ratio of 4.5 to 15 at% or 5 to 18 at%, with the balance being titanium (Ti) and unavoidable impurities.

[0019] The titanium alloy of this embodiment has a single-phase structure in which only the β phase exists as the matrix phase, and has high workability (hot workability, etc.). The β phase has a basic skeleton of the BCC-B2 structure of TiAl, with a portion of this structure substituted with β-phase stabilizing elements.

[0020] The β-phase stabilizing element is an element that acts to expand and stabilize the β-phase region, and is not particularly limited, but includes, for example, at least one of chromium (Cr), manganese (Mn), and vanadium (V). The chromium content is 4.5 to 15 at%, preferably 4.5 to 10 at%. The manganese content is 4.5 to 8 at%, preferably 5 to 6 at%. The vanadium content is 5 to 15 at% or 5 to 18 at%, preferably 6 to 15 at%.

[0021] Superelasticity is the property of returning to its original shape when a load is applied, even if the deformation exceeds elastic deformation. The titanium alloy of this embodiment has a coarse-grained structure composed of crystal grains with a grain size of 200 μm or more, due to a special manufacturing method described below. Since the larger the grain size, the less intergranular constraint there is, the superelasticity of the titanium alloy of this embodiment far exceeds that of commonly known alloys. Specifically, the superelastic critical stress is 400 MPa or more and 1000 MPa or less, the maximum strength is 850 MPa or more and 1500 MPa or less, and the maximum reversible strain is 3% or more and 12% or less.

[0022] [Method for Producing Titanium Alloy] The method for producing a titanium alloy according to this embodiment mainly includes a melting and casting step, a hot working step, a solution treatment step, a cyclic heat treatment step, and an aging treatment step.

[0023] (Melting and Casting Process) A mixture is prepared by melting and casting raw materials for aluminum, β-phase stabilizing elements, and titanium. That is, the various raw materials are melted and mixed by an arc melting method or the like, and then formed to prepare a mixture. The amounts of the various raw materials to be melted are adjusted so that the ratio of aluminum among the elements contained in the mixture is 15 to 25 at % and the ratio of β-phase stabilizing elements is 4.5 to 15 at %. Oxygen (O) may be added to the raw materials to be melted and cast. For example, titanium oxide (TiO ２ In this case, it is preferable to adjust the amounts of the various raw materials to be dissolved so that the amount of oxygen added is 0.5 to 1%.

[0024] (Hot Working Step) The melted and cast mixture is subjected to hot working (rolling, forging, etc.) as necessary to form a predetermined shape. The temperature range of the hot working is preferably 1000° C. or higher and 1600° C. or lower. The reduction ratio of the hot working can be 90% or higher.

[0025] (Solution Treatment Step) The hot-worked mixture is subjected to solution treatment (homogenization treatment) by heating at a temperature (solution temperature) of 1200°C or higher and 1600°C or lower, preferably 1300°C or higher and 1500°C or lower. The holding time for solution treatment is 1 hour to 200 hours, more preferably 2 hours to 24 hours. After heating, the mixture is rapidly cooled at a rate of 200°C / s or higher by water cooling or the like. This solution treatment removes strain introduced during processing and makes the structure more homogenous.

[0026] (Cyclic Heat Treatment Step) The solution-treated mixture is subjected to cyclic heat treatment, which involves repeating a cycle of high-temperature treatment and low-temperature treatment with a temperature difference of 1100° C. or more and 1050° C. or less, multiple times.

[0027] The high-temperature treatment in each cycle is a treatment to form a single-phase structure consisting of only the β phase. The temperature of the high-temperature treatment is 1100°C or higher and 1400°C or lower, preferably 1100°C or higher and 1200°C or lower. The time of the high-temperature treatment is 40 minutes or higher and 10 hours or lower, preferably 60 minutes or higher and 5 hours or lower.

[0028] The low-temperature treatment in each cycle is a treatment to form a two-phase structure of β phase and α phase. The temperature of the low-temperature treatment is 500°C or higher and 1050°C or lower, preferably 600°C or higher and 900°C or lower. The time of the low-temperature treatment is 5 minutes or higher and 20 minutes or lower, preferably 5 minutes or higher and 15 minutes or lower. The temperature increase rate and temperature decrease rate are 20°C / min or lower, preferably 10°C / min or lower.

[0029] This cyclic heat treatment can induce abnormal growth of crystal grains within the mixture. By adjusting the various conditions of the cyclic heat treatment, it is possible to obtain a coarse-grained structure consisting of crystal grains with a diameter of 200 μm or more. The crystal grains of the resulting alloy contain subgrains with a 2° tilt in the crystal axis. Depending on their size, the resulting alloy may be composed of a single crystal. Grain growth is a phenomenon that reduces the area of ​​high-energy grain boundaries. In normal grain growth, each crystal grain grows at a uniform rate, but in abnormal grain growth, some crystal grains coarsen faster than the surrounding crystal grains. The more cycles of cyclic heat treatment are performed, the more this coarsening progresses, resulting in the growth of larger crystal grains. For example, one cycle can grow crystal grains with a diameter of 200 μm or more. Eight cycles can grow crystal grains with a diameter of 5 cm or more. Eighteen cycles can grow crystal grains with a diameter of 20 cm or more.

[0030] (Aging Treatment Step) The mixture that has undergone the cyclic heat treatment may be subjected to aging treatment at a temperature of room temperature or higher and 400°C or lower, preferably 200°C or higher and 400°C or lower. The holding time for the aging treatment is 30 hours or shorter, preferably 24 hours or shorter. This aging treatment promotes ordering of the β phase, and allows the titanium alloy of this embodiment to be obtained, having an ordered β single-phase structure. Whether or not to perform aging treatment may be determined, for example, depending on the composition of the titanium alloy to be produced.

[0031] As described above, the titanium alloy of this embodiment is composed of crystal grains coarsened by cyclic heat treatment, and therefore has excellent superelasticity with a recoverable strain of 5% or more. This superelasticity is maintained even after tensile tests are repeated 200 times or more, and is expressed over a wide temperature range from about 4 K to about 400 K. Furthermore, since the titanium alloy of this embodiment has a single-phase structure containing only the β phase, it has excellent workability and, although it has a small Young's modulus, it also has excellent mechanical strength. Furthermore, since the titanium alloy of this embodiment contains aluminum in the basic skeleton of its crystal structure, it can have a lower specific gravity and is lightweight compared to conventional titanium alloys containing other metal elements.

[0032] The effects of the present invention will be more clearly understood from the following examples. Note that the present invention is not limited to the following examples and can be practiced with appropriate modifications within the scope of the present invention.

[0033] (Examples 1 to 8) Titanium alloys of Examples 1 to 8 of the present invention were manufactured according to the above embodiment. The Al content was set to 17 to 24 at%. Cr was selected as a β-phase stabilizing element, and its content was set to 4.5 to 10 at%. The specific gravity of each of the manufactured titanium alloys at room temperature was 5 or less. The solution treatment temperature and time were set to 1473 K and 2 hours, respectively. The aging treatment temperature and time were set to 473 K and 0.5 to 24 hours, respectively.

[0034] Figure 1 shows the conditions for the cyclic heat treatment. As shown in Figure 1, the cyclic heat treatment was performed by repeating 18 cycles of high-temperature treatment at 1200°C for 60 minutes and low-temperature treatment at 700°C for 10 minutes. During the cyclic heat treatment, the titanium alloy became a single-phase structure consisting of only the β phase in the temperature range above 1030°C, and a two-phase structure consisting of the β phase and the α phase in the temperature range below 1030°C.

[0035] Specific values ​​of various manufacturing conditions for the titanium alloys of Examples 1 to 8 are shown in Table 1.

[0036]

[0037] (Examples 9 to 14) Titanium alloys of Examples 9 to 14 of the present invention were manufactured in accordance with the above embodiment. The Al content was set to 15 to 20 at%. Mn was selected as a β-phase stabilizing element, and its content was set to 5 to 6 at%. Other conditions were the same as in Examples 1 to 8. Specific values ​​of the various manufacturing conditions for the titanium alloys of Examples 9 to 14 are shown in Table 2.

[0038]

[0039] (Examples 15 to 18) Titanium alloys of Examples 15 to 18 of the present invention were manufactured in accordance with the above embodiment. The Al content was set to 15 to 20 at%. V was selected as a β-phase stabilizing element, and its content was set to 6 to 15 at%. Other conditions were the same as those of Examples 1 to 8. Specific values ​​of the various manufacturing conditions for the titanium alloys of Examples 15 to 18 are shown in Table 3.

[0040]

[0041] Figure 2 is a photograph of the titanium alloy of Example 5, which has been hot-worked into a plate shape. This titanium alloy consists of a large single crystal measuring approximately 20 cm x approximately 1.2 cm x approximately 0.3 cm. EBSD and XRD measurements were performed on this titanium alloy. Figure 3 shows the results. The results of these two measurements confirm that the titanium alloy of Example 5 is a single crystal with a single β phase. The titanium alloys of Examples 1 to 4 and 6 to 18, in which the Al content and the β-phase stabilizing element content were varied within the scope of the present invention, also form single crystals.

[0042] TEM observation was performed on the titanium alloy of Example 5. Figure 4 shows the image obtained by TEM observation and the electron beam diffraction pattern (upper right of the image). From this image and electron beam diffraction pattern, it can be seen that the titanium alloy of Example 5 has a regular BCC structure (B2 structure) composed of two elements (Ti and Al). The titanium alloys of Examples 1 to 4 and 6 to 18, in which the Al content and the β-phase stabilizing element content were changed within the range of the present invention, are also thought to have a similar BCC structure.

[0043] EBSD measurement was performed on the titanium alloy of Example 5. Figure 5 shows a mapping image of the crystal orientation misalignment obtained from this measurement. It can be seen that subgrains with a crystal orientation misalignment of approximately 2° are formed. The grain size of the subgrains is 20 μm to 100 μm, indicating that they are giant single crystals.

[0044] A load-unload tensile test was conducted on the titanium alloy of Example 5, in which cycles of loading and unloading were repeated while varying the tensile load within a strain range of 2 to 13%. Figure 6 shows the results. The superelastic critical stress was 795 MPa, the recoverable strain was 7.3%, the fracture strain was approximately 12%, and the maximum strength was 906 MPa. These results demonstrate that the titanium alloy of Example 5 exhibits excellent superelasticity. The titanium alloys of Examples 1 to 4 and 6 to 18, in which the Al content and the β-phase stabilizing element content were varied within the range of the present invention, also exhibited similar superelasticity.

[0045] The titanium alloy of Example 5 was subjected to a load-unload tensile test in which 200 cycles of loading and unloading under 5% strain tension were repeated. Figure 7 shows the results. Even after 200 cycles, the initial maximum strength was nearly maintained, and the original shape was restored after each unloading cycle. This result demonstrates that the titanium alloy of Example 5 has excellent superelastic fatigue resistance properties. The titanium alloys of Examples 1 to 4 and 6 to 18, in which the Al content and the β-phase stabilizing element content were varied within the ranges of the present invention, are also expected to exhibit similarly excellent superelastic fatigue resistance properties.

[0046] The titanium alloy of Example 5 was subjected to a 5% strain tensile loading and unloading cycle once at each of four different temperatures: 30 K, 160 K, 298 K, and 400 K. Figure 8 shows the results. Recovery to the original shape after unloading was confirmed at all temperatures. These results demonstrate that the titanium alloy of Example 5 exhibits excellent superelasticity over a wide temperature range, at least from 30 K to 400 K. The titanium alloys of Examples 1 to 4 and 6 to 18, which vary the Al content and β-phase stabilizing element content within the scope of the present invention, have also been confirmed to exhibit excellent superelasticity over a similarly wide temperature range. Depending on the manufacturing conditions, some alloys exhibited superelasticity over a temperature range of 4.2 K to 403 K.

[0047] The mechanical properties and temperature ranges of superelastic behavior of the titanium alloys of Examples 1 to 18 are shown in Table 4.

[0048]

[0049] (Examples 19 to 26) Titanium alloys of Examples 19 to 26 of the present invention were manufactured in accordance with the above-described embodiment. The Al content was set to 17 to 22 at%. Cr was selected as a β-phase stabilizing element, and its content was set to 4 to 4.75 at%. In addition, titanium oxide (TiO ２ Oxygen was added by mixing powder of 2-(2-methyl-2,4-dioxane)-2-one. The amounts of the various raw materials to be dissolved were adjusted so that the amount of oxygen added was 0.5 to 1%. Other conditions were the same as in Examples 1 to 8. Specific values ​​of the various manufacturing conditions for the titanium alloys of Examples 19 to 26 are shown in Table 5.

[0050]

[0051] The mechanical properties and temperature ranges of superelastic behavior of the titanium alloys of Examples 19 to 26 are shown in Table 6.

[0052]

[0053] (Examples 27 to 35) Titanium alloys of Examples 27 to 35 of the present invention were manufactured in accordance with the above embodiment. The Al content was set to 15 to 25 at%. V was selected as a β-phase stabilizing element, and its content was set to 8 to 18 at%. Other conditions were the same as in Examples 1 to 8. Specific values ​​of the various manufacturing conditions for the titanium alloys of Examples 27 to 35 are shown in Table 7.

[0054]

[0055] The mechanical properties and temperature ranges of superelastic behavior of the titanium alloys of Examples 27 to 35 are shown in Table 8.

[0056]

[0057] Figure 9 shows the positioning of the present invention in the relevant technical field in terms of the temperature range and specific gravity of the superelastic behavior of titanium alloys. As can be seen from this figure, the titanium alloy of the present invention has properties not available with conventional technology. That is, the titanium alloy of the present invention exhibits superelasticity over a temperature range approximately five times that of Ni-Ti alloys. Furthermore, the titanium alloy of the present invention exhibits superelasticity over a temperature range similar to that of Fe alloys (Fe-Mn-Al-Ni), while exhibiting a specific gravity approximately half that of Fe alloys (Fe-Mn-Al-Ni).

[0058] (Comparative Examples 1 to 5) Titanium alloys of Comparative Examples 1 to 5 were manufactured for comparison with the above Examples. The Al content was 13 to 30 at%. Cr was selected as a β-stabilizing element, and its content was 4.5 to 4.75 at%. The solution treatment temperature was 1273 to 1473 K. The aging treatment time was 0 to 24 hours. No cyclic heat treatment was performed. The other conditions were the same as those of Examples 1 to 8.

[0059] (Comparative Examples 6 to 8) Titanium alloys of Comparative Examples 6 to 8 were produced for comparison with the above Examples. The Al content was 10 to 15 at%. Mn was selected as a β-stabilizing element, and its content was 5 to 10 at%. The solution treatment temperature was 1273 to 1473 K. The aging treatment time was 0 to 24 hours. No cyclic heat treatment was performed. The other conditions were the same as those of Examples 9 to 14.

[0060] (Comparative Examples 9 and 10) Titanium alloys for Comparative Examples 9 and 10 were manufactured for comparison with the above Examples. The Al content was set to 10-15 at%. V was selected as a β-stabilizing element, and its content was set to 10 at%. The solution treatment temperature was set to 1273-1473 K. The aging treatment time was set to 24 hours. No cyclic heat treatment was performed. The other conditions were the same as those of Examples 15-18.

[0061] Table 5 shows the manufacturing conditions of the titanium alloys of Comparative Examples 1 to 10 and whether or not they had superelastic properties.

[0062]

[0063] None of the titanium alloys of Comparative Examples 1 to 10 possess superelastic properties. This is believed to be because the manufacturing conditions of any of the titanium alloys of Comparative Examples 1 to 10 are outside the range of the present invention. Specifically, in Comparative Examples 1, 2, 5, and 8, the Al content is outside the range of 15 to 25 at% specified in the present invention. Furthermore, in Comparative Examples 3, 4, 7, and 9, the solution treatment temperature is outside the range of 1200°C or higher and 1600°C or lower specified in the present invention. In Comparative Example 6, the Mn content is outside the range of 4.5 to 8 at% specified in the present invention.

[0064] The fact that none of the titanium alloys of Comparative Examples 1 to 9 had superelastic properties is thought to be due in part to the fact that they were not subjected to cyclic heat treatment during their manufacturing process. The crystal grains in the titanium alloys of Comparative Examples 1 to 9 all grew normally, and are thought not to have reached a size large enough to exhibit superelasticity.

Claims

1. A titanium alloy characterized by containing 15 to 25 at% aluminum and 4.5 to 15 at% of a β-phase stabilizing element, with the remainder consisting of titanium and unavoidable impurities, having a single-phase structure in which only the β phase exists, and having a crystal grain size of 200 μm or more.

2. The titanium alloy according to claim 1, characterized in that the β-phase stabilizing element contains at least one of chromium, manganese, and vanadium.

3. A titanium alloy according to claim 1 or 2, characterized in that the β-phase stabilizing element contains chromium in a ratio of 4.5 to 15 at %.

4. A titanium alloy according to claim 1 or 2, characterized in that manganese is contained as the β-phase stabilizing element in a ratio of 4.5 to 8 at %.

5. A titanium alloy according to claim 1 or 2, characterized in that vanadium is contained as the β-phase stabilizing element in a ratio of 5 to 15 at %.

6. A titanium alloy according to claim 1 or 2, characterized in that the specific gravity is 5 or less.

7. A titanium alloy according to claim 1 or 2, characterized in that the superelastic critical stress is 400 MPa or more and 1000 MPa or less, the ultimate strength is 850 MPa or more and 1500 MPa or less, and the maximum reversible strain is 3% or more and 12% or less.

8. A method for producing a titanium alloy according to claim 1 or 2, comprising the steps of: subjecting the mixture of aluminum, the β-phase stabilizing element, and the titanium to solution treatment at 1200°C or higher and 1600°C or lower for 1 hour or longer and 200 hours or shorter; and subjecting the mixture after the solution treatment to cyclic heat treatment, which involves repeating a thermal cycle between a high temperature range of 1100°C or higher and 1400°C or lower and a low temperature range of 500°C or higher and 1050°C or lower at a rate of 0.1°C / min or higher and 20°C / min or lower.

9. The method for producing a titanium alloy according to claim 8, further comprising the step of subjecting the mixture after the cyclic heat treatment to an aging treatment at a temperature of from room temperature to 400°C for 0.5 to 30 hours.

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