Non-magnetic member, titanium alloy and manufacturing method thereof

Abstract

To provide a titanium alloy that can achieve both of electrical characteristics (resistivity) and mechanical properties (Young's modulus, strength or the like) at high levels.SOLUTION: A titanium alloy comprises, considering the total mass as 100 mass% (simply referred to as "%"), Al: 5-9.5%, Fe: 1-4%, Mo: 1-5%, Mn: 0.3-4%, C: 0.01-0.15%, and Ti: the balance, and may also comprise O: 0.1-0.6%. The metallographic structure of the titanium alloy may comprise crystal grains composed of (α+β) phase and grain boundaries composed of network-like α phase surrounding the crystal grains. The crystal grains form needle crystals and at the grain boundaries, Al and C become enriched. The crystal grains have an average particle size of, for example, 50-200 μm. The grain boundaries have an average thickness of, for example, 1-20 μm. The inventive titanium alloy is suitable, for example, for a non-magnetic member demanding properties such as high specific resistance and high strength.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to titanium alloys and the like. [Background technology]

[0002] Titanium alloys (including titanium matrix composites), which are lightweight and have excellent strength, are used in various fields such as aviation, space, and automobiles. For this reason, various titanium alloys have been proposed depending on the required specifications (properties) and applications, and for example, the following documents contain relevant descriptions: [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2005-320618 [Patent Document 2] WO2014 / 27677 [Non-patent literature]

[0004] [Non-Patent Document 1] S. Abkowitz et al US patent 4.731.115 [Non-patent document 2] T. Furuta, et al., "Effects of C and Al on the High Resistivity and Stiffness of High-Strength TiC(1-X) Particle-Reinforced Ti-Based Composites Fabricated by Reaction Sintering," Journal of the Japan Institute of Metals, Vol. 83, No. 3 (2019), pp. 97-106. Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 proposes Ti-Al-Fe-Mo-Mn-Si-C alloys (Nos. 13 and 16 in Table 1 of Patent Document 1). This titanium alloy contains 0.04% Si as an essential element and 0.007% or 0.003% C as an impurity.

[0006] Patent Document 2 proposes a Ti-Al-Fe-Si-O alloy (Table 1 of Patent Document 2). The microstructure of this titanium alloy consists of fine acicular α-phase. Note that the titanium alloys of Patent Document 1 and Patent Document 2 are both made of ingots obtained by plasma melting.

[0007] Non-Patent Document 1 proposes a Ti-based composite material in which TiC (reinforcing particles) is dispersed in a titanium alloy (base material). Such a Ti-based composite material is high-strength and high-rigidity.

[0008] Non-Patent Document 2 proposes a Ti-based composite material in which TiC 1-c (0 < c < 1) is dispersed. This Ti-based composite material not only has high strength and high rigidity but also has a high specific resistance. Note that the Ti-based composite materials of Non-Patent Document 1 and Non-Patent Document 2 are both made of sintered materials obtained by firing powder compacts.

[0009] The present invention has been made in view of such circumstances, and an object thereof is to provide a new titanium alloy or the like different from the prior art.

Means for Solving the Problems

[0010] As a result of intensive studies to solve this problem, the present inventors have newly found that a titanium alloy of a predetermined composition system can be excellent not only in mechanical properties (strength, rigidity, etc.) but also in electrical properties (specific resistance, etc.). By developing this result, the present invention described below has been completed.

[0011] 《Titanium Alloy》 (1) The present invention is a titanium alloy having the following component composition in terms of mass ratio to the whole (simply referred to as “%”). Al: 5 to 9.5%, Fe: 1 to 4%, Mo: 1 to 5%, Mn: 0.3 to 4%, C: 0.01 to 0.15%, Ti: the balance

[0012] (2) The titanium alloy of the present invention can exhibit, for example, high strength, high rigidity, or high resistivity. The reason for this is currently thought to be as follows: The titanium alloy of the present invention can form crystal grains consisting of the (α + β) phase and a network of grain boundaries consisting of the α phase surrounding each crystal grain. The crystal grains can become, for example, needle-like crystals, which can contribute to increasing the strength and rigidity of the titanium alloy. The grain boundaries can, for example, promote electron scattering through the concentration of Al and C, which can contribute to increasing the resistivity of the titanium alloy. It is believed that the formation of such a metal structure allows the titanium alloy of the present invention to exhibit excellent mechanical and electrical properties.

[0013] <Non-magnetic material> The present invention can also be understood as a non-magnetic member (electromagnetic member) made of, for example, the above-mentioned titanium alloy. The non-magnetic member of the present invention can reduce eddy current loss that occurs when used in an alternating magnetic field in a high-frequency (e.g., high rotation speed) range. Furthermore, even when large forces (centrifugal forces, inertial forces, etc.) act due to high-speed motion (rotation, reciprocation, etc.), damage and deformation of the non-magnetic member can be prevented, and the non-magnetic member can be made thinner, lighter, and smaller.

[0014] The "non-magnetic" (magnetic permeability) is sufficient as long as it does not short-circuit the magnetic circuit of the electromagnetic device. The non-magnetic member of the present invention does not have to be entirely made of a titanium alloy, and the entire member does not necessarily have to be non-magnetic. In short, it is sufficient that at least a portion of the non-magnetic member of the present invention is made of a non-magnetic titanium alloy.

[0015] 《Manufacturing method》 The present invention can also be understood as a method for manufacturing the above-mentioned titanium alloy or non-magnetic member. The titanium alloy may be a sintered material or a melt-cast material, and it is preferable that the structure be controlled to obtain the desired high properties. Therefore, the present invention may be, for example, a method for manufacturing a titanium alloy (non-magnetic member) that includes a structure control step in which a titanium base material made of a sintered body or a melt-cast body is heated and recrystallized. Note that the structure control step may also serve as, for example, the molding or heat treatment of the non-magnetic member, etc.

[0016] "others" Unless otherwise specified, "x to y" in this specification includes a lower limit value x and an upper limit value y. Any numerical value included in the various numerical values ​​or numerical ranges described in this specification may be used as a new lower limit or upper limit value to create a new range such as "a to b." Furthermore, "x to y μΩm" in this specification means x μΩm to y μΩm. The same applies to other unit systems (MPa, GPa, etc.). [Brief explanation of the drawings]

[0017] [Figure 1] Photographs (SEM images) of the structure of the titanium alloy of each sample. [Figure 2A] This is an element mapping image of the titanium alloy sample 1 analyzed by EPMA. [Figure 2B] This is an enlarged image. [Figure 3] This is an element mapping image obtained by EPMA analysis of the titanium alloy sample C4. [Figure 4A] 1 is a graph showing the element distribution obtained by line analysis using an EPMA near the grain boundary of the titanium alloy of Sample 1. [Figure 4B] 1 is a graph showing the element distribution obtained by line analysis using an EPMA near the grain boundary of the titanium alloy sample C4. [Figure 5] 1 is a graph showing the element distribution obtained by EDX analysis of the processed titanium alloy material of Sample 1. [Figure 6] FIG. 2 is an explanatory diagram showing a method for measuring resistivity. DETAILED DESCRIPTION OF THE INVENTION

[0018] One or more components selected arbitrarily from this specification may be added to the above-described components of the present invention. The contents described in this specification apply not only to titanium alloys and non-magnetic members, but also to their manufacturing methods. Furthermore, the components may be method-related or product-related. Which embodiment is best depends on the target, required performance, etc.

[0019] Titanium alloy (1) Composition The titanium alloy contains at least Al, Fe, Mo, Mn and C, and may further contain O. The composition of the titanium alloy is detailed as follows:

[0020] Al, C, and O are α-phase stabilizing elements; if they are too little, the resistivity will be insufficient, and if they are too much, the elongation will be small. Furthermore, Al and C are dissolved in large amounts not only in the crystal grains but also in the grain boundary α phase (grain boundaries consisting of the α phase). It is thought that the concentration of Al and C in the grain boundary α phase contributes greatly to the improvement of the resistivity of titanium alloys.

[0021] The content of Al is, for example, 5 to 9.5%, 5.5 to 9%, or even 6 to 8.5%. The content of C is, for example, 0.01 to 0.15%, 0.02 to 0.12%, or even 0.04 to 0.1%. The content of O is, for example, 0.1 to 0.5%, 0.15 to 0.45%, or even 0.2% to 0.4%. Note that the component composition (%) in this example refers to the mass percentage (mass%) of the entire titanium alloy unless otherwise specified.

[0022] Fe, Mo, and Mn are β-phase stabilizers, and if they are insufficient, strength will be insufficient, while if they are excessive, elongation will be reduced. These β-phase stabilizers (especially Mo) are thought to concentrate near the interface (β / α interface) between crystal grains (β phase) and grain boundaries (α phase), contributing to the formation and growth of the grain boundary α phase mentioned above.

[0023] Mo is contained in an amount of, for example, 1 to 5%, 1.5 to 4.5%, or even 2 to 4%. Fe is contained in an amount of, for example, 1 to 4%, 1.5 to 3.5%, or even 2 to 3%. Mn is contained in an amount of, for example, 0.3 to 4%, 1.5 to 3.5%, or even 2 to 3%.

[0024] In addition to the elements mentioned above, titanium alloys may contain elements (impurity elements) that are technically and economically difficult or unavoidable to remove, or modifier elements that improve the properties of the alloy, in amounts of, for example, 1% or less, 0.5% or less, or even 0.3% or less. Examples of such elements include N, S, and neutral elements (completely solute elements) such as Sn and Zr.

[0025] (2) Organization The metal structure of titanium alloys can change depending on the composition, manufacturing process, heat treatment, etc. The metal structure has, for example, crystal grains and mesh-like grain boundaries surrounding the crystal grains, with the crystal grains consisting of the (α + β) phase and the grain boundaries consisting of the α phase. Titanium alloys with such a metal structure have excellent electrical and mechanical properties. More specifically, these are as follows.

[0026] The crystal grains are, for example, needle-like crystals made of thin α-phase or β-phase. The size of the crystal grains (referred to as "grain size" regardless of shape) is, for example, 50 to 200 μm, or even 75 to 150 μm in average. Furthermore, in the titanium alloy of the present invention, thick layered grain boundaries can be formed in a mesh-like pattern. The average thickness of the grain boundaries is, for example, 1 to 20 μm, or even 5 to 15 μm. The average grain size and average thickness of the crystal grains can be determined, for example, by analyzing (calculating) an image (field of view: 300 μm × 400 μm) obtained by observing the metal structure with a microscope (e.g., SEM) using analysis software: ImageJ (an open source program).

[0027] (3)Characteristics Titanium alloys can exhibit excellent electrical and mechanical properties. As electrical properties, they can exhibit high resistivity of, for example, 2 to 5 μΩm, 2.1 μΩm to 4 μΩm, or even 2.2 μΩm to 3 μΩm. The resistivity value is measured, for example, by a DC four-terminal method (see FIG. 6).

[0028] As for mechanical properties, for example, the tensile strength (breaking strength) can be as high as 1200 to 1700 MPa, 1250 to 1650 MPa, or even 1350 to 1550 MPa, and the 0.2% proof stress can be as high as 1150 to 1600 MPa, 1200 to 1500 MPa, or even 1250 to 1450 MPa. Also, the Young's modulus can be as high as 115 to 135 GPa, or even 120 to 130 GPa. Furthermore, the ductility can be as high as 0.3 to 5%, or even 0.7 to 4%.

[0029] 《Manufacturing method》 Titanium alloys can be manufactured by, for example, sintering, melting, powder additive manufacturing (so-called 3D printers), etc. Sintering, in which a powder compact is heated to obtain a sintered body, can reduce post-processing by achieving near-net shaping, allowing parts to be manufactured with a high yield.

[0030] Titanium alloys may be plastically processed (forged, pressed, etc.) in a cold or hot state, or may be heat-treated. For example, if a titanium base material (raw material) made of a sintered or ingot body is hot-processed and then cooled at a predetermined rate, the metal structure described above can be obtained (structure control process). Specifically, a hot processing step in which processing strain is applied at 950 to 1350°C, or even 1200 to 1300°C, and a cooling step in which cooling is performed at 0.1 to 13°C / s, or even 1 to 10°C / s, after the hot processing step, are preferably performed. This is thought to cause dynamic recrystallization, resulting in the formation of the unique metal structure described above.

[0031] <Non-magnetic parts / electric devices> The titanium alloy of the present invention can provide a non-magnetic member that satisfies, for example, high resistivity, high strength, high rigidity, and low magnetic permeability. The specific use of the non-magnetic member is not important, but it can be used, for example, as a protective member (protective tube, protective case) for a permanent magnet (field source) incorporated into an electric motor (electromagnetic device, electric device) (see, for example, JP 2020-43746 A). One example of an electric motor is a centrifugal compressor that requires high rotational speed. Compressors are used, for example, in engine superchargers and air compressors for fuel cells. [Example]

[0032] The sintered material was subjected to hot working to produce various samples (titanium alloys), and their electrical properties (resistivity) and mechanical properties (Young's modulus, tensile strength, 0.2% yield strength, elongation) were evaluated. The present invention will be described in more detail below with reference to these specific examples.

[0033] <<Sample Preparation>> (1) Raw material powder The Ti powder used was a commercially available hydrogenated and dehydrogenated powder (manufactured by Toho Tech Co., Ltd.) that had been classified using a sieve (#350) (particle size 160 μm or less).

[0034] As the alloy powders serving as the alloying element sources, one or more of the following powders were used. (a) Ti-36%Al powder (manufactured by Daido Steel Co., Ltd. / particle size 160 μm or less) (b) Fe-60%Mo powder (manufactured by Taiyo Koko Co., Ltd. / particle size 45 μm or less) (c) Fe-78%Mn-1.74C powder (manufactured by Fukuda Metals Co., Ltd. / particle size 45 μm or less) (d) Al-40% V powder (Kinseimatec Co., Ltd. / average particle size: 9 μm) (e) Mo powder (manufactured by Nippon Shinkinzoku Co., Ltd. / average particle size: 9 μm) (f) TiC powder (manufactured by Nippon Shinkinzoku Co., Ltd. / average particle size: 3 μm)

[0035] Unless otherwise specified, the compositions shown in the examples are the mass percentages (mass%) of each raw material powder or mixed powder relative to the total mass, and are simply indicated by "%." The average particle size of the powder was determined using a laser diffraction / scattering particle size analyzer (MT3300EX, manufactured by Nikkiso Co., Ltd.). The oxygen source was adsorbed on the particle surface of the powder or the treatment atmosphere.

[0036] (2)Mixing process The raw material powders were weighed and blended to obtain the compositions shown in Table 1, and mixed for 1 hour in a V-type mixer. In this way, mixed powders were prepared for each sample. For samples C1 and C2, the composition (mass%) of the entire base material excluding TiC and the volume fraction of TiC relative to the total (total including the base material) are shown in Table 1.

[0037] (3) Molding process For samples 1 to 4, the mixed powder was molded into a round bar-shaped compact (approximately φ16 mm × 150 mm) under a molding pressure of 4 t / cm. 2 (392 MPa).

[0038] For samples C1 to C4, the mixed powder was placed in a polyvinyl chloride tube (PVC) and subjected to CIP molding to form a round rod-shaped compact (approximately φ16 mm × 150 mm). At this time, the molding pressure was also 4 t / cm. 2 (392 MPa).

[0039] (4) Sintering process Each compact was placed in a vacuum (1 × 10 -5 The samples were heated to 1300°C at a pressure of 1000 torr and sintered. The heating rate up to the sintering temperature was approximately 5°C / min, and the cooling rate after the sintering time had elapsed was 10°C / s. The holding time at 1300°C (sintering time) was 4 hours for samples 1 to 4 and 16 hours for samples C1 to C4.

[0040] (5) Processing process Each sintered body was hot worked (forged) in an air atmosphere. The heating temperature was 1200°C and the working ratio was 56%. The working ratio was calculated as the cross-sectional area reduction ratio (Aw / Ao). Aw is the cross-sectional area after working, and Ao is the cross-sectional area before working.

[0041] The sintered body (processed product) after hot working was cooled in the air at a cooling rate of 5°C / s. No subsequent heat treatment was performed. The samples (titanium alloys) obtained in this manner were used for the subsequent measurements and observations.

[0042] "measurement" (1) Electrical properties (resistivity) The resistivity was measured using a system source meter (KEITHLEY) by the DC four-terminal method, as shown in Figure 6. Specifically, electrodes were first formed on a rectangular prism (3 mm (t) × 4 mm (w) × 20 mm) fabricated from each sample as follows: The center of the prism (voltage electrode spacing (L): 10 mm) was masked with masking tape. Terminal wires (silver wires: φ0.20 mm) were wrapped around the masked end portions and the two outer edges (see Figure 6). Silver paste (Dotite D-550, Fujikura Kasei Co., Ltd.) was applied to the wound portions and both end faces of the prism. After application, the prism was heated in air at 100°C for 12 hours to dry. In this way, test specimens equipped with current and voltage electrodes were prepared.

[0043] The resistivity (electrical resistivity) of each sample was calculated from the voltage (V) and current (I) measured by the DC four-terminal method at room temperature and the cross-sectional shape (S = t × w) of the test piece (see formula (1) in Figure 6). The resistivity (measured value) of each sample thus obtained is also shown in Table 1.

[0044] (2) Mechanical properties (Young's modulus, tensile strength, elongation) A tensile test was carried out using a round bar tensile test piece (parallel part diameter: φ2.4 mm, gauge length: 14 mm) made from each sample, using an autograph (AUTOGRAPH AG-1 50 kN, manufactured by Shimadzu Corporation).

[0045] The tensile test was performed at room temperature in air at a strain rate of 5 × 10 -4 The stress-strain relationship was determined based on the load-stroke diagram obtained from the load cell and video extensometer, and the mechanical properties were identified (see JIS Z 2241:2011). The results are also shown in Table 1. The tensile strength was calculated based on the load at break and the initial shape of the test piece. The elongation is the strain of the test piece at break.

[0046] "observation" (1) The metal structure of each sample before the tensile test was observed using a scanning electron microscope (SEM). The observed images (SEM images) of Sample 1, Sample C1, and Sample C4 are shown in Figure 1.

[0047] (2) The metal structure was subjected to elemental analysis using an EPMA (Electron Probe Micro Analyzer). Observation images (EPMA images) of Sample 1 are shown in Figures 2A and 2B (collectively referred to as "Figure 2"), and an EPMA image of Sample C4 is shown in Figure 3.

[0048] (3) Linear analysis was performed using an EPMA across the vicinity of the grain boundaries of the metal structure. The analysis results for Sample 1 and Sample C4 are shown in Figure 4A and Figure 4B, respectively (both figures are collectively referred to as "Figure 4").

[0049] (4) Using a test piece obtained by backward piercing extrusion of the titanium alloy of Sample 1, the vicinity of the grain boundary was analyzed by EDX (Energy Dispersive X-ray Spectroscopy). The diffusion distribution of each element obtained in this way is shown in Figure 5.

[0050] (5) Furthermore, X-ray diffraction analysis (XRD / Cu-Kα) of the metal structure confirmed that the crystal grains consisted mainly of the (α+β) phase, and the grain boundaries consisted mainly of the α phase.

[0051] "evaluation" (1)Characteristics As is clear from Table 1, the titanium alloys having the specified compositions had high resistivity as well as high rigidity, strength, and ductility. Specifically, the resistivity was 2.3 μΩm or more, Young's modulus was 115 GPa or more, tensile strength was 1400 MPa or more, 0.2% yield strength was 1300 MPa or more, and elongation was 1% or more. On the other hand, the titanium alloys of samples C1 to C4 did not simultaneously satisfy all of these properties at a high level.

[0052] (2) Organization As is clear from Figure 1, a titanium alloy with a specific composition (e.g., sample 1) had a metallographic structure consisting of needle-like (α + β) phases within the crystal grains and a network of grain boundaries consisting of broad α phases. This metallographic structure was unique and not observed in other samples such as C1 and C4.

[0053] As is clear from Figure 2, in the metallographic structure of Sample 1, the α-phase stabilizing elements Al and C were concentrated almost uniformly in the grain boundary α-phase, which was about a few micrometers wide. Looking more closely, Mo, Mn, and Fe were concentrated almost uniformly at the interface between the β-phase of the crystal grains and the grain boundary α-phase. This can also be seen in Figure 4A. Furthermore, as can be seen from Figure 5, it was confirmed that such metallographic structure and element distribution were reproduced even after extrusion processing.

[0054] On the other hand, as is clear from a comparison of Figures 2, 3, and 4, the element distribution near the grain boundaries as seen in Sample 1 was not observed in Sample C4 etc. When the observed image of Sample 1 was analyzed using the method described above, the average grain size was found to be 120 μm and the average thickness of the grain boundary α phase was 10 μm.

[0055] 《Consideration》 (1) Specific resistance The reason why the titanium alloy of the present invention exhibits high resistivity is thought to be as follows. It is thought that the mesh-like grain boundaries formed by the solid solution of Al and C in the α phase promote non-uniform deformation of the lattice, making it easier for electrons to scatter. It is presumed that the high resistivity is due to the crystal grains being covered by the grain boundaries made of such a high resistivity phase.

[0056] (2) Enrichment phenomenon at grain boundaries at the interface between different phases Prior β grain boundaries formed by hot working become preferential nucleation sites for the α phase. In the case of a diffusion-controlled phase transformation from the β phase to the two-phase (α and β) ((α + β) phase), the distribution and diffusion behavior of elements present at the interface between the β and α phases (referred to as the "β / α interface") govern the growth behavior of the β phase to the grain boundary α phase.

[0057] The β-phase stabilizing elements such as Mo and Mn (especially Mo), which have a low diffusion rate in the β-phase, are also released slowly near the β / α interface, and tend to concentrate near the β / α interface. As a result, it is thought that the preferential growth of the grain boundary α-phase is promoted near the prior β grain boundary.

[0058] In addition, it is thought that C, an interstitial solute element, concentrates at the grain boundaries because it dissolves in the grain boundaries more than within the crystal grains, resulting in a lower free energy and greater stability.

[0059] [Table 1]

Claims

1. A titanium alloy consisting of the following component composition, expressed as a mass percentage (simply referred to as "%) of the whole: Al: 5 to 9.5% Fe: 1 to 4% Mo: 1 to 5% Mn: 0.3 to 4% C: 0.01~0.15% O: 0.1-0.6% Remainder: Ti and impurity elements

2. It consists of a metal structure having crystal grains and mesh-like grain boundaries surrounding the crystal grains, The crystal grains are composed of an (α+β) phase, 2. The titanium alloy of claim 1, wherein the grain boundaries are comprised of the alpha phase.

3. 3. The titanium alloy according to claim 2, wherein the crystal grains are needle-like crystals.

4. The titanium alloy according to claim 2, wherein the crystal grains have an average grain size of 50 to 200 μm.

5. 3. The titanium alloy according to claim 2, wherein Al and C are more concentrated at the grain boundaries than at the crystal grains.

6. The titanium alloy according to claim 2, wherein the grain boundaries have an average thickness of 1 to 20 μm.

7. A non-magnetic member made of the titanium alloy according to claim 1 and used in an alternating magnetic field.

8. 8. The non-magnetic member according to claim 7, wherein the titanium alloy has a resistivity of 2 μΩm or more.

9. 9. The non-magnetic member according to claim 7, wherein the titanium alloy has a 0.2% yield strength of 1200 MPa or more and a Young's modulus of 115 GPa or more.

10. a structure control step of heating and recrystallizing a titanium base material made of a sintered body or a melted body, A manufacturing method for obtaining the titanium alloy according to claim 2.

11. 11. The method for producing a titanium alloy according to claim 10, wherein the structure control step comprises a hot working step of applying working strain at 950 to 1350°C, and a cooling step of cooling at 0.1 to 13°C / second after the hot working step.

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