Patent Application
20150064046
The present invention relates to a Ti/TIC composite with improved tensile strength and a production method thereof.
Titanium is light, having an atomic weight of 47.9, and has excellent properties of high tensile strength and corrosion resistance.
Titanium is used in, e.g., the field of aerospace, which requires to be light in weight and high in tensile strength, and used for electric and chemical plants requiring corrosion resistance, and outer hull of submersible vessels or submarines, and preserves because of its seawater-resistant characteristics. Specifically, Ti-6Al-4V alloy accounts for half of consumption of titanium alloys as essential materials.
Typically, a sponge titanium is subjected to pressing and melting to produce an ingot to be subjected to plastic forming.
However, in a pure titanium used when corrosion resistance is required, generally, the tensile strength is 300 to 700 MPa and is required to be higher in strength. To solve the problem, a composite using reinforcing materials is being considered.
Since Ti-6Al-4V alloy that is currently widely being used includes rare earth elements in the composition, for the strategy of elements and cost, a titanium composite with improved tensile strength is desired.
For example, Patent Document 1 discloses a particle dispersion type titanium alloy in which TiC and/or TiB are dispersed. In the invention, titanium powder, mother alloy powder, and carbon powder (or Boron powder) are mixed and subjected to sintering to react titanium and carbon (or Boron) to thereby obtain a particle dispersion type composite.
For example, Patent Document 2 proposes a method in which TiC is mixed with Ti powder and sintered to obtain materials. Also, a method of obtaining TiC using carbon powder is proposed.
In the production method of a particle dispersion type titanium alloy in which TiC and/or TiB are dispersed as disclosed in Patent Document 1, whether or not the carbon has completely become TiC affects the mechanical performance, and the crystal grain grows in the sintering process, which causes deterioration of the mechanical properties. Although the method disclosed in Patent Document 2 does not make a reference to carbon that does not react with Ti, but the remaining carbon that does not react not only has no effect on the movement of dislocations at the time of recrystallization, but also can cause embrittlement. Therefore, a further improvement in the mechanical property is desired.
The present invention has the structure as recited in the following items [1] to [20].
1. A Ti/TiC composite,
wherein no simple substance of carbon essentially exists in a TiC, and
wherein 0.3 mass % or more of oxygen is solidified in the composite.
2. The Ti/TiC composite as recited in item 1, wherein 0.04 mass % or more of nitrogen is solidified in the composite.
3. The Ti/TiC composite as recited in item 1 or 2, wherein TiC is dispersed in a Ti matrix and no carbon is precipitated in the Ti matrix.
4. The Ti/TiC composite as recited in any one of items 1 to 3, wherein a size of the TiC is 0.5 to 5 μm.
5. The Ti/TiC composite as recited in any one of items 1 to 4, wherein a crystal grain size of the Ti is 3 to 10 μm.
6. The Ti/TiC composite as recited in any one of items 1 to 5, wherein a crystal grain texture has no preferred orientation and an orientation difference of each crystal grain is 15° or more.
7. The Ti/TiC composite as recited in any one of items 1 to 6, wherein the Ti/TiC composite has an upper yield point in a relation between a tensile strength and an elongation.
8. The Ti/TiC composite as recited in any one of items 1 to 7, wherein the Ti/TiC composite contains one or more metals selected from the group consisting of Fe, Co, Ni, Sc, V, Cr, Mn, Cu, Y, Zr, Nb, W, Mo, Mg, Al, and Si.
9. A Ti/TiC composite having an upper yield point in a relation between a tensile strength and an elongation.
10. A production method of a Ti/TiC composite, comprising:
alloying by giving mechanical energy and/or thermal energy to a mixture of pure titanium powder and carbon and solidifying oxygen and nitrogen; and thereafter sintering it.
11. The production method of a Ti/TiC composite as recited in item 10, wherein the carbon is a carbon fiber.
12. The production method of a Ti/TiC composite as recited in item 11, wherein the carbon fiber is a carbon fiber obtained by a vapor phase growth method.
13. The production method of a Ti/TiC composite as recited in item 11 or 12, wherein a fiber diameter of the carbon fiber of 90% or more is 50 to 300 nm in a fiber diameter distribution of a number standard of the carbon fiber.
14. The production method of a Ti/TiC composite as recited in any one of items 10 to 13, wherein the Ti/TiC composite obtained by sintering reaction is hot extruded.
15. A production equipment using the Ti/TiC composition as recited in any one of items 1 to 9.
16. An engine equipment using the Ti/TiC composition as recited in any one of items 1 to 9.
17. A heat exchange facility using the Ti/TiC composition as recited in any one of items 1 to 9.
18. A building using the Ti/TiC composition as recited in any one of items 1 to 9.
19. An automotive using the Ti/TiC composition as recited in any one of items 1 to 9.
20. An airplane using the Ti/TiC composition as recited in any one of items 1 to 9.
A Ti material extremely high in mechanical strength can be obtained.
A Ti/TiC composite according to a preferred embodiment of the present invention is a composite in which TiC is dispersed in a Ti matrix, and a stand-alone carbon does not essentially exist in the TiC structure. The state in which “a stand-along carbon does not essentially exist” in the present invention means that unreacted carbon C not bonded with Ti is not observed when, in an arbitrary cross-section, ten points in a field of vision in an area of 100 μm2 enlarged by 3000 magnifications were measured by WDS. Therefore, in the TiC, carbon exists as TiC and carbon exists evenly without segregated.
Because carbon exists evenly inside the TiC, it acts as a particle of dispersion reinforcement mechanism in the composite, in the heat input process of hot extrusion step and during the usage and works as a pinning effect for the grain boundary growth when the Ti crystal grain in the matrix recrystallizes, preventing the movement of the grain boundary, which suppresses recrystallization. As for the tensile strength, the tensile strength also improves since the movement of dislocation is suppressed. Furthermore, the tensile strength is improved along with the solid solution strengthening of carbon, effects of grain refining, and stress-induced transformation.
Also, when oxygen is entered into solid solution at a high density in the Ti/TiC composite, oxygen enters into the c axis of the hcp structure (α-Ti) as an interstitial solid solution, increasing the misfit strain and improving the tensile strength, and since oxygen gathers at the grain boundary due to the dragging effect at the time of processing, the growth of the crystal grain is prevented. To exert the effects, it is preferable that the density of oxygen entering in solid solution is 0.3 mass % or more. It is preferable that the oxygen density is 0.3 to 1.5 mass % and further preferable that the oxygen density is 0.35 to 0.5 mass %.
Furthermore, when nitrogen is entered into solid solution at a high density, the nitrogen enters into the c axis of the hcp as an interstitial solid solution, increasing the misfit strain and improving the tensile strength. To exert the effect, it is preferable that the nitrogen density is 0.04 mass % or more. It is preferable that the oxygen density is 0.04 to 1.5 mass % and more preferable that the oxygen density is 0.04 to 0.3 mass %.
Also, it is preferable that carbon is not precipitated in the Ti matrix.
It is preferable that the TiC exists in a size of 0.5 to 5 μm. When the TiC in the size of the aforementioned range exists, the size of the crystal grain of the Ti matrix is small, preventing the grain boundary movement in the heat input process, as well as helping the improvement of the tensile strength as the dispersion-strength mechanism. An especially preferable size of the TiC is 0.5 to 1 μm.
In the Ti matrix, it is preferable that Ti exists in a crystal size of 3 to 10 μm. When the size of the Ti crystal is within the aforementioned range, the tensile strength improves based on the Hall Petch rule. An especially preferable size of the Ti crystal grain is 3 to 5 μm.
Furthermore, the aggregate texture of the crystal grain does not have a preferred orientation, and it is preferable that the direction of each crystal grain is 15° or more in a random direction. According to the aggregate texture, recrystallization is less likely to occur for the heat input to the materials, lessening the decrease of strength during production.
Regarding the crystal orientation,
Also, the Ti/TiC composite according to the preferred embodiment, due to the effect of the dispersion reinforcement mechanism, as shown in
Also, in the Ti/TiC composite, one or more metals selected from the group consisting of Fe, Co, Ni, Sc, V, Cr, Mn, Cu, Y, Zr, Nb, W, Mo, Mg, Al and Si can be included. Since these metals form intermetallic compounds with Ti and provide dispersion reinforcement, the mechanical strength improves. Also, since the metal remaining in the carbon fiber is dispersed in a delicate state, it is considered that intermetallic compounds can be easily formed and the contribution to dispersion reinforcement can be large.
[Production Method of Ti/TiC Composite]
The Ti/TiC composite having the aforementioned structure can be obtained by providing mechanical energy and/or thermal energy to the mixture of pure titanium powder and carbon and then subjecting it to sintering reaction.
The preferred conditions of the pure titanium powder and the carbon are as follows.
It is preferable that a pure titanium powder having a purity of 99 mass % or more and an average particle diameter of 10 to 30 μm is used.
The carbon is not especially limited and various known carbon materials can be used, but it is preferable that carbon black or carbon fiber is used.
Since carbon fiber does not obstruct the dispersion of titanium at the TiC forming step at the time of sintering, it is preferable because non-reaction carbon is less likely to remain. Also, since TiC is formed by dispersing Ti in the carbon fiber, the interface with the matrix is matched, making it preferable since it functions as reinforcement. Furthermore, the effects of the stress-induced transformation contribute to the improvement of the tensile strength.
It is preferable that the carbon fiber is a carbon fiber in which 90% or more of the fiber diameter distribution (based on number) is in the range of 50 to 300 nm in the fiber diameter and it is more preferable that 90% or more of the fiber diameter distribution (based on number) of the carbon fiber is within the range to 70 to 200 nm in the fiber diameter. When there are many carbon fibers in which the fiber diameter exceeds 300 nm, carbon is more likely to be segregated. When there are many carbon fibers in which the fiber diameter is less than 50 nm, the cohesion of carbon fiber more likely occurs, causing a decrease in the strength.
The type of the carbon fiber is not especially limited, but e.g., a vapor phase growth carbon fiber can be used. For the production method of a vapor phase growth carbon fiber, there are roughly two types: a substrate method and a floating catalyst method. The substrate method is a method in which the metal catalyst is carried by a substrate or a carrier and brought into contact with hydrocarbon gas to grow the carbon fiber. The floating catalyst method is a method of obtaining a carbon fiber by introducing a raw material liquid or the gasification thereof, in which ferrocene as a catalytic source and a sulfur compound are dissolved in benzene, a carbon source, to a pro-circulation reaction furnace heated to 1,000° C. or over using a carrier gas such as hydrogen. Generally, a carbon fiber is formed from the catalytic metal in an early period of reaction and the approximate length of the carbon fiber is determined. Afterward, thermolysis carbon accumulates on the surface of the carbon fiber, thereby advancing the growth in the radial direction and forming a carbon structure in the form of an annual ring. Therefore, the adjustment of the fiber diameter is made possible by controlling the quantity of sedimentation of the thermolysis carbon on the carbon fiber during reaction, that is, the reaction time, the raw material density in the atmosphere, and the reaction temperature. The carbon fiber obtained by the aforementioned reaction is covered by thermolysis carbon having low crystallinity. The carbon fiber can be used as it is, but it can be heat-treated at 800 to 1500° C. and used. To raise the crystallinity of the carbon fiber, graphitization treatment can be further performed at 2,000 to 3,500° C.
Generally, as methods for evaluating the crystallinity of the carbon materials, the spacing (d002) of the carbon hexagon net plane (002) surfaces measured by x-ray diffraction measurement and the ratio (Id/Ig) of the peak height (Id) of the 1,300 to 1,400 cm−1 band and the peak height (Ip) of the 1,530 to 1,650 cm−1 band of the Raman scattering spectrum are known. In the carbon fiber subjected to a heat-treatment at 800 to 1, 500° C., since the graphite structure is not developed very much, a clear X-ray diffraction peak belonging to d002 is not detected, and therefore, Id/Ig of the Raman scattering spectrum is in the range of 0.9 to 1.1. For the carbon fiber subjected to a graphitization treatment at 2,000 to 3,500° C., d002 is 0.34 nm or less and Id/Ig is 0.30 or less.
For the carbon fiber, the length of the fibers can be adjusted by a crusher, and for a branched carbon fiber, the branches of the fiber can be broken. When the ratio of fibers thinner than 50 nm increases, aggregates over 100 μm are formed from the high cohesiveness of the thin fibers, thereby making the dispersion of carbon fibers difficult. It is preferable that the BET specific surface area of the carbon fiber is 6 to 40 m2/g, or more preferable to be 8 to 25 m2/g, and further preferable to be 10 to 20 m2/g. It is preferable that the aspect ratio of the carbon fiber is 2 to 150 or more preferably 5 to 100. The aspect ratio is calculated by dividing the fiber length of the carbon fiber by the fiber diameter of the carbon fiber. To measure the length of the fibers, for a fiber in which the fiber diameter is measured by a scanning electron microscope, the magnification is changed to 5,000 times, then the fiber is photographed panoramically and the length of both ends of the fiber is measured. When the aspect ratio is larger than 150, it is more likely to be entangled, making it difficult to be dispersed.
As a carbon fiber, from the crystal structure, mainly three types, the platelet type, the herringbone type, and the tubular type, are known. Other carbon fibers having, e.g., a cup stack structure is known, but any crystal structure can be used.
When producing a carbon fiber, Fe, Co, Ni, Sc, Ti, V, Cr, Mn, Cu, Y, Zr, Nb, W and Mo can be used as a catalyst, or a catalyst carrier in which alumina, zirconia, titania, magnesia, calcium carbonate, calcium oxide, calcium hydroxide or silica is in a state of a simple substance or a compound oxide can also be used, and when graphitization treatment is not performed, the metal contained in these catalysts and catalyst carriers remain in the carbon fiber, but these metals are rather effective in the present invention. For example, when Fe remained as impurities, it forms an intermetallic compound with Ti, thereby contributing to the dispersion reinforcement.
The pure titanium powder and carbon are mixed. The mixture ratio is not limited.
The mixture of the pure titanium powder and carbon provides mechanical energy and/or thermal energy prior to sintering. As TiC is formed from the imparted energy, oxygen and nitrogen are solutionized.
The means for imparting energy is not limited as long as TiC can be formed, and oxygen, and nitrogen can be solutionized, but as the means of imparting mechanical energy, a planetary ball mill in which revolution and rotation are combined can be used. Since the planetary ball mill imparts a high energy shear force to the material, it is effective for mechanical alloy, solid solution, and crystallization. However, if more than a certain amount of energy can be imparted, it is not limited to the planetary ball mill. Also, as the means for imparting thermal energy, there are processes such as heat-treatment, heat at the time of sintering, and heat processing at the time of plasticity processing.
Although carbon is an interstitial type solution element, an oversaturated carbon forms TiC and does not contribute to the reinforcement mechanism above the solid solubility limit. Also, according to the Labusch model, oxygen and nitrogen have the effect of improving the tensile strength. Although oxygen and nitrogen exists in the Ti ingredient, not all is solid solutionized in the crystal lattice and solid solution is further promoted by the imparting of energy.
In addition, in the mixture of the pure Ti powder and carbon, a processing oil is preferably used as a lubricant, preventing the aggregation of carbon to allow even dispersion.
The aforementioned material in which energy was imparted is a powder, which encourages the formation of TiC by sintering reaction, dispersing evenly in the matrix. Also, the material is subjected to solidification by the sintering reaction. A preferable temperature for the sintering reaction is TiC to 1,100° C., and more preferably 800 to 1,000° C.
The Ti/TiC composite to which energy imparting and sintering reaction were subjected is further subjected to hot plasticity processing such as hot extrusion, thereby preventing the grain boundary growth by the TiC, providing texture in random directions. The preferred temperature for hot extrusion is 900 to 1,100° C., and the preferable extrusion ratio is 15 or more.
Since the Ti/TiC composite of the present invention is strengthened to a high degree, it can be used as a member for various plants, facility equipments, and heat exchange facilities. For example, it can be used as a member of production facility such as a condenser and various pipes for thermal and nuclear power generation and piping, tower tank, and heat exchanger for oil and chemical factories. Furthermore, it can be used for various members for a building such as, e.g., exterior of a building. Also, it can be used as an automotive part such as an automotive muffler, or a part for a plane. It can also be used in the field of aerospace, electric and chemical plants, and midget submarines, outer hull of submarines, and crawls.
As the materials to produce the Ti/TiC composite, commercial pure titanium powder (CP-450, made by Toho Technical Center Company, Average particle size: 21.9 μm, oxygen: 0.27 mass %, nitrogen: 0.03 mass %) and vapor phase growth carbon fiber (VGCF (Registered trademark), made by Showa Denko Kabushiki Kaisha, having a mean diameter of 150 nm, and average length of 8 μm) were used. The details for the pure titanium powder are shown in Table 1 and the details for the vapor phase growth carbon fiber are shown in Table 2.
The pure titanium powder and the vapor phase growth carbon fiber were mixed with a planetary ball mill. At that time, the ratio of each ingredient was weight ratio of 0.2% (Example 1), 0.4% (Example 2), 0.8% (Example 3) and 1.0% (Example 4) for 200 g of Ti powder. Each powder was sealed in a ZrO2 pot having a 10 mm diameter with a ZrO2 ball in argon gas to prevent oxidation. The milling processing was performed by a Pulwerisette 5 made by Fritsch. The weight ratio of the ball and the powder was set to 4:1 and processed for 24 hours at 100 rpm. The milling processing was stopped for 5 minutes every 15 minutes to prevent overheating.
After mixing with the ball mill, the mixed powder was subjected to sintering using a SPS (Spark Plasma Sintering) device (SPS-1030S, made by SPS Syntex Inc.) using a graphite die. The temperature rise was performed at 20 K/min and maintained a: 1.8×103 s at 1073 K. At that time, the pressure was 30 MPa and the vacuum degree was 5 Pa. The diameter of the billet obtained by sintering was 42 mm and the height was 32 mm. It was thereafter subjected to heat extrusion. After maintaining the billet for 180 at 1,273 K in argon gas, it was subjected to extrusion by an extruder with thrust of 2,000 kN. The extrusion ratio was 37 and the ram speed was 3.0 mm/s.
Except that the planetary ball mill of the milling device was changed to a table mill, the composite was produced by subjecting the same materials as Example 4 to the same processes in the milling processing, sintering and hot extrusion.
Except that only the pure titanium powder was used as the material and no carbon fiber was used, the composite was produced by subjecting it to the same process in the milling processing, sintering and heat extrusion as Example 1.
Except that 200 g of pure titanium powder and weight ratio 1% of TiC powder (ceramic powder) to the pure Ti powder was used as the ingredient, the composite was produced by subjecting it to the same process in the milling processing, sintering and heat extrusion in Example 1.
Except that 200 g of pure titanium powder and weight ratio 0.1% (Comparative Example 4), 0.2% (Comparative Example 5), and 0.4% (Comparative Example 6) of graphite powder to the pure Ti powder was used as the material, and a rocking mill was used as the milling device, the composite was produced by subjecting it to the same process in the milling processing, sintering and heat extrusion in Example 1.
Table 3 shows the summary of the production method.
The composite produced by the aforementioned production method (simple material of titanium only for Comparative Example 2) was cut into 3 mm diameter and 20 mm Gauge length to be used as an evaluation sample. For the evaluation sample, evaluation was performed for the following items.
After the cross section of the evaluation sample cut along the extrusion direction was subjected to physical abrasion, it was observed with FE-SEM.
Examination was performed by EDS (EX-64175JMV made by JEOL) attached to the FE-SEM.
The evaluation sample was put inside the graphite furnace in TC-300 made by LECO and heated to 3,273 K to measure the oxygen density and the nitrogen density.
The tensile strength of the evaluation sample was measured under the condition of strain rate of 5×10−4/s in a tensile testing machine. The measurement result is shown in Table 3 and
In the EBSD (Electron Backscatter Diffraction, Electron beam backscattering diffractometry (scanning electron microscope-crystal orientation analysis)) attached to the FE-SEM, the area of 25 μm×25 μm was observed in the 20 kV voltage and 0.2 μm step, then the equivalent circle diameter was calculated from the area of the measured crystal grain, and the average value was used as the crystal particle size. Using the measured particle size of the TiC particle size, the Ti crystal particle size was evaluated in three levels: 15 μm or more as “1”, over 10 μm and under 15 nm as “2”, and 5 to under 10 μm as “3”.
The measurement result is shown in Table 3.
Furthermore, the stretch was measured for Examples 2, 3, 4, and Comparative Examples 1 and 2 to examine the relationship between the tensile strength and stretch. The relationship between the tensile strength and the stretch is shown in
Furthermore, for Examples 2, 3, 4, and Comparative Examples 1 and 2, the crystal particle size of the Ti matrix by EBSD, the TiC particle size (reproduction of Table 3), and the volume fraction of TiC are shown in Table 4, the mechanical property is shown in Table 5, and the crystal lattice parameters, etc., are shown in Table 6. Also,
From Table 3, although the vapor phase growth carbon fiber amount was increased to 1 mass % in Examples 1 to 4, there were no non-reaction carbons, indicating a big improvement in the tensile strength. Also, since oxygen and the quantity of nitrogen was solid solutionized to the c axis, the strength was improved. Furthermore, since the crystal orientation of the crystals was random, the strength was improved. Also, since the crystal grains were small, the strength was improved.
In Comparative Example 1, when a table mill was used in place of the planet type ball mill, the dispersion of carbon fiber was affected and since the crystal particle size was large, the improvement effect of the strength was not seen. In Comparative Example 2, when only the pure titanium powder was used for processing, the crystal particle size was not affected and the directions were not random, so there were no improvement effects in the strength. Comparative Example 3 shows a case in which TiC as ceramics is added, and although the size of the crystal grain itself is small, the tensile strength does not reach that of Examples only with the addition of TiC. This is thought to be because a gap is formed between the interfaces of TiC and Ti thereby causing the stress transmission to be poor. Comparative Examples 4 to 6 show cases in which graphite was added in place of carbon fiber, but since non-reaction carbons exist, the contributions to the tensile strength is small.
Also, according to
The Ti/TiC composite of the present invention can be used in equipment for thermal and nuclear power generation and equipment for oil and chemical factories which require high strength.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-178951 | Aug 2013 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| 61872158 | Aug 2013 | US |
