U.S. patent application number 12/708744 was filed with the patent office on 2010-06-10 for method of producing titanium alloy composite material.
This patent application is currently assigned to E&F CORPORATION. Invention is credited to Hidekazu Takizawa, Toshio Tanimoto.
Application Number | 20100143176 12/708744 |
Document ID | / |
Family ID | 37835611 |
Filed Date | 2010-06-10 |
United States Patent
Application |
20100143176 |
Kind Code |
A1 |
Tanimoto; Toshio ; et
al. |
June 10, 2010 |
METHOD OF PRODUCING TITANIUM ALLOY COMPOSITE MATERIAL
Abstract
A method of producing a titanium alloy composite material
comprises mixing carbon fibers and a powder of an element which
forms a carbide in reaction with carbon, subliming the element
under high temperature vacuum, and coating the carbon fibers with a
layer containing the element and the carbide to produce coated
carbon fibers. The method further comprises mixing the coated
carbon fibers and titanium alloy powder to form a mixture, and
applying a mechanical impact force to the mixture to fix the carbon
fibers on the surface of the titanium alloy powder to obtain a
carbon fiber-fixed titanium alloy powder. The method further
comprises sintering the carbon fiber-fixed titanium alloy powder to
form a sintered body and plastic working the sintered body to
disperse the carbon fibers in crystal grains of the titanium
alloy.
Inventors: |
Tanimoto; Toshio; (Tokyo,
JP) ; Takizawa; Hidekazu; (Nagano, JP) |
Correspondence
Address: |
LEYDIG VOIT & MAYER, LTD
700 THIRTEENTH ST. NW, SUITE 300
WASHINGTON
DC
20005-3960
US
|
Assignee: |
E&F CORPORATION
Tokyo
JP
NAGANO PREFECTURE
Nagano-shi
JP
|
Family ID: |
37835611 |
Appl. No.: |
12/708744 |
Filed: |
February 19, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11571434 |
Dec 29, 2006 |
|
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PCT/JP2006/316408 |
Aug 22, 2006 |
|
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12708744 |
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Current U.S.
Class: |
419/11 |
Current CPC
Class: |
B21B 3/02 20130101; Y10T
428/12139 20150115; C22C 47/04 20130101; Y10T 428/12812 20150115;
C22C 47/14 20130101; Y10T 428/12063 20150115; C22C 49/11 20130101;
Y10T 428/1234 20150115; Y10T 428/12806 20150115; B21B 3/00
20130101 |
Class at
Publication: |
419/11 |
International
Class: |
B22F 1/02 20060101
B22F001/02; B22F 3/10 20060101 B22F003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 7, 2005 |
JP |
2005-259797 |
Claims
1.-5. (canceled)
6. A method of producing a titanium alloy composite material,
comprising: mixing carbon fibers and a powder of an element which
forms a carbide in reaction with carbon, subliming the element
under high temperature vacuum, and coating the carbon fibers with a
layer containing the element and the carbide to produce coated
carbon fibers; mixing the coated carbon fibers and titanium alloy
powder to form a mixture, and applying a mechanical impact force to
the mixture to fix the carbon fibers on the surface of the titanium
alloy powder to obtain a carbon fiber-fixed titanium alloy powder;
sintering the carbon fiber-fixed titanium alloy powder to form a
sintered body; and plastic working the sintered body to disperse
the carbon fibers in crystal grains of the titanium alloy.
7. The method of producing a titanium alloy composite material
according to claim 6, further comprising aging the plastic-worked
titanium alloy composite material.
8. The method of producing a titanium alloy composite material
according to claim 6, including sintering is with pulsed electric
current.
9. The method of producing a titanium alloy composite material
according to claim 6, wherein the plastic working is at least one
process selected from hot rolling and isothermal forging.
10. The method of producing a titanium alloy composite material
according to claim 6, wherein the element comprises at least one
element selected from the group consisting of silicon (Si),
chromium (Cr), titanium (Ti), vanadium (V), tantalum (Ta),
molybdenum (Mo), zirconium (Zr), boron (B), and calcium (Ca).
11. The method of producing a titanium alloy composite material
according to claim 6, wherein the carbon fibers comprise carbon
nanotubes, vapor-grown carbon fibers, or a mixture thereof.
12. The method of producing a titanium alloy composite material
according to any claim 6, wherein the mixture of the carbon fibers
and the titanium alloy powder comprises 0.1% to 10% by mass of the
carbon fibers.
13.-16. (canceled)
Description
TECHNICAL FIELD
[0001] The present invention relates to a titanium alloy composite
material, a method of producing the titanium alloy composite
material, a titanium clad material using the titanium alloy
composite material, and a method of producing the titanium clad
material.
BACKGROUND ART
[0002] Titanium alloys have high relative strength and excellent
corrosion resistance, and have mainly been used in the fields of
aerospace, deep sea exploration, chemical plants, and the like.
Recently, titanium alloys have been widely used for consumer uses
such as heads or shafts of golf clubs, components of watches or
fishing goods, and eyeglass frames.
[0003] Recently, composite materials containing a titanium alloy
and carbon fiber combined for further improving mechanical
properties such as tensile strength and toughness have been
proposed. For example, Patent Documents 1 and 2 each disclose an
automobile component formed of a titanium alloy containing carbon
fibers such as carbon nanofibers. Patent Documents 1 and 2 each
further describe injecting ions of oxygen (O), nitrogen (N),
chlorine (Cl), chromium (Cr), carbon (C), boron (B), titanium (Ti),
molybdenum (Mo), phosphorus (P), aluminum (Al), and the like into
the carbon nanofibers, to thereby improve wetness and adhesiveness
between the carbon nanofibers and metal. Further, pure titanium has
also been cladded to a side surface of a core material made of a
titanium alloy, for example, for obtaining functions and properties
that cannot be obtained with a single substance (see Patent
Document 3, for example).
[0004] Patent Document 1: JP 2004-225084
[0005] Patent Document 2: JP 2004-225765
[0006] Patent Document 3: JP 2002-000971
DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
[0007] The inventors of the present invention, after diligent
study, have found that in the conventional techniques disclosed in
Patent Documents 1 and 2, titanium and carbon fibers react with
each other during formation of a composite. Thus, the inventors of
the present invention have found that the original properties of
the carbon fibers as a reinforcing material are significantly
degraded, and mechanical strength as expected cannot actually be
obtained. Further, as described in the above-mentioned Patent
Documents, it is also found that use of carbon nanofibers subjected
to ion injection treatment as a carbon fiber has improved
dispersibility of the carbon nanofibers in an alloy, however,
reactivity of the carbon nanofibers with titanium is rather
accelerated, and mechanical strength of the carbon nanofibers is
somewhat reduced. In the conventional technique disclosed in Patent
Document 3, mechanical properties of both a titanium alloy and pure
titanium are originally not sufficient, and thus cladding of the
titanium alloy and pure titanium provides no clad material having
remarkably improved mechanical properties.
[0008] Therefore, the present invention has been made in view of
solving the problems described above, and an object of the present
invention is to provide a titanium alloy composite material having
excellent mechanical strength such as tensile strength, Young's
modulus, toughness and hardness.
[0009] Another object of the present invention is to provide a
titanium clad material having remarkably improved mechanical
properties such as tensile strength, elongation and fracture
toughness.
Means for Solving the Problems
[0010] The inventors of the present invention, after conducting
intensive studies and development for solving the conventional
problems described above, have found that dispersion of carbon
fibers coated with a layer containing an element which forms
carbide in reaction with carbon and the carbide formed thereby in
crystal grains of titanium alloy is effective for solving the
problems, to complete the present invention. Further, the inventors
of the present invention have found that a clad material obtained
by cladding this titanium alloy composite material and a titanium
alloy having a high fracture toughness has remarkably improved
mechanical properties such as tensile strength, elongation and
fracture toughness.
[0011] That is, a titanium alloy composite material according to
the present invention is characterized by dispersing carbon fibers
coated with a layer containing an element which forms carbide in
reaction with carbon and the carbide formed thereby in crystal
grains of the titanium alloy.
[0012] It is preferable that the element which forms carbide in
reaction with carbon include at least one selected from the group
consisting of silicon (Si), chromium (Cr), titanium (Ti), vanadium
(V), tantalum (Ta), molybdenum (Mo), zirconium (Zr), boron (B) and
calcium (Ca).
[0013] It is preferable that the carbon fibers include carbon
nanotubes, vapor-grown carbon fibers or a mixture thereof. The
titanium alloy composite material preferably comprises 0.1% to 10%
by mass of the carbon fibers. The layer preferably has a thickness
of at least 0.5 nm.
[0014] A method of producing a titanium alloy composite material
according to the present invention is characterized by comprising:
a step of mixing carbon fibers and powder of an element which forms
carbide in reaction with carbon, and then sublimating the element
under high temperature vacuum to coat the carbon fibers with a
layer containing the element and the carbide; a step of mixing the
carbon fibers obtained in the former step and titanium alloy
powder, and applying mechanical impact force on the mixture to fix
the carbon fiber on a surface of the titanium alloy powder; a step
of sintering the carbon fiber-fixed titanium alloy powder obtained
in the former step; and a step of plastic working the sintered body
obtained in the former step to disperse the carbon fiber in crystal
grains of the titanium alloy.
[0015] It is preferable that a method of producing a titanium alloy
composite material further comprises a step of aging the
plastic-worked titanium alloy composite material. The sintering is
preferably conducted by a pulse electric current sintering method.
The plastic working is preferably conducted by at least one process
selected from a hot rolling process and an isothermal forging
process.
[0016] The titanium clad material according to the present
invention is characterized in that a titanium alloy composite
material with carbon fibers coated with a layer containing an
element which forms carbide in reaction with carbon and the carbide
formed thereby dispersed in crystal grains of the titanium alloy,
and a titanium alloy having a higher fracture toughness than that
of the titanium alloy composite material are sinter bonded to one
another. Further, it is preferable that the titanium clad material
comprise a pair of sheet materials made of the titanium alloy
having a higher fracture toughness than that of the above-mentioned
titanium alloy composite material, and a core material made of the
above-mentioned titanium alloy composite material arranged between
the sheet materials. The core material preferably has a honeycomb
structure.
[0017] A method of producing a titanium clad material according to
the present invention is characterized by comprising: laminating in
a mold a titanium alloy composite material with carbon fibers
coated with a layer containing an element which forms carbide in
reaction with carbon and the carbide formed thereby dispersed in
crystal grains of the titanium alloy, and a titanium alloy having a
higher fracture toughness than that of the titanium alloy composite
material; and sinter bonding the titanium alloy composite material
and the titanium alloy to one another by a pulse electric current
sintering method.
Effect of the Invention
[0018] According to the present invention, a titanium alloy
composite material having excellent mechanical strength such as
tensile strength, Young's modulus, toughness and hardness, and a
method of producing the same can be provided.
[0019] Further, according to the present invention, a titanium clad
material having remarkably improved mechanical properties such as
tensile strength, elongation and fracture toughness, and a method
of producing the same can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] [FIG. 1] A flow chart explaining a method of producing a
titanium alloy composite material of the present invention.
[0021] [FIG. 2] Diagrams showing examples of a laminate structure
of a titanium clad material of the present invention.
[0022] [FIG. 3] A diagram showing an example of the most preferred
laminate structure of the titanium clad material of the present
invention.
[0023] [FIG. 4] Graphs showing results of X-ray diffraction
measurement of carbon nanotubes coated with Si of Example 1.
[0024] [FIG. 5] An ultrahigh resolution FE-SEM image of titanium
alloy powder containing carbon fibers fixed thereon of Example
1.
[0025] [FIG. 6] A metallographic microscopic image of a
metallographic structure of a sintered body of Example 1.
[0026] [FIG. 7] A metallographic microscopic image of a
metallographic structure of a titanium alloy composite material
obtained in Example 1.
[0027] [FIG. 8] A graph showing results of strength measurement of
materials obtained in Examples 1 and 2 and Comparative Example 2
and 4.
[0028] [FIG. 9] Cutaway views of the titanium alloy composite
material obtained in Example 1 after material strength
measurement.
[0029] [FIG. 10] Graphs showing results of X-ray diffraction
measurement of carbon nanotubes coated with Cr of Example 4.
[0030] [FIG. 11] A metallographic microscopic image of a vicinity
of a sinter bonded interface of a titanium clad material of Example
6.
[0031] [FIG. 12] An enlarged image of part A of FIG. 11.
BEST MODE FOR CARRYING OUT THE INVENTION
[0032] Hereinafter, the present invention will be described in more
detail.
(Titanium Alloy Composite Material)
[0033] A titanium alloy composite material of the present invention
is obtained by dispersing carbon fibers coated with a layer
containing an element which forms carbide in reaction with carbon
and the carbide formed thereby in crystal grains of the titanium
alloy. That is, the layer coating the carbon fibers is formed of
the carbide formed through a partial reaction between the element
and the carbon fibers, and an unreacted element. This layer serves
as a layer for suppressing reactions between the carbon fibers and
titanium during formation of a composite and improves wetness with
the titanium alloy, and thus properties of the carbon fibers as a
reinforcing material are maintained after formation of the
composite. In the present invention, such coated carbon fibers are
dispersed in crystal grains, to thereby significantly improve
mechanical strength such as tensile strength, Young's modulus,
toughness and hardness. In the present invention, a state in which
the carbon fibers are dispersed in crystal grains of the titanium
alloy refers to a state in which the carbon fibers are at least
partly incorporated in fine crystal grains of the titanium alloy
while moderate dispersibility is maintained through plastic flow
during plastic working.
[0034] Meanwhile, the inventors of the present invention have
confirmed that, in the case where the coated carbon fibers are not
dispersed in the crystal grains, sufficient mechanical strength
cannot be obtained with a titanium alloy composite material
prepared by mixing coated carbon fibers and titanium alloy powder
and then sintering the mixture. The mechanical strength is
presumably reduced because the carbon fibers or TiC as carbide of
the carbon fibers forms a brittle layer having a high hardness at a
titanium alloy crystalline interface, and the brittle layer having
a high hardness serves as a defect causing cracks.
[0035] The fiber diameter, fiber length, shape, and the like of the
carbon fibers of the present invention are not particularly
limited, and a conventionally known carbon fiber generally used as
a reinforcing material can be used without limitation. Of those,
carbon nanotubes, a vapor-grown carbon fiber, or a mixture thereof
is preferably used from the viewpoint of further improving the
mechanical properties. Examples of carbon nanotubes include
monolayer carbon nanotubes and multilayer carbon nanotubes each
formed by a vapor phase growth method, an arc discharge method, a
laser vaporization method, or the like. Examples of vapor-grown
carbon fibers include discontinuous carbon fibers obtained through
crystal growth in a vapor phase by a vapor phase growth method, and
a graphite fiber. The vapor-grown carbon fibers may have any shape
such as acicular, coiled, tubular, or cup, and two or more kinds
thereof may be blended. From the viewpoint of further improving the
properties of a reinforcing material and the dispersibility in a
titanium alloy, the carbon nanotubes preferably have a fiber
diameter of 2 nm to 80 nm and a fiber length of 1 .mu.m to 100
.mu.m, and the vapor-grown carbon fibers preferably have a fiber
diameter of 80 nm to 200 nm and a fiber length of 5 .mu.m to 100
.mu.m.
[0036] The fiber diameter, fiber length, and shape of the carbon
fibers in the titanium composite material can be measured through
structural observation with an ultrahigh resolution FE-SEM or a
transmission electron microscope.
[0037] The content of the carbon fibers is preferably 0.1% to 10%
by mass, more preferably 0.2% to 5.0% by mass, and most preferably
0.4% to 3.0% by mass with respect to the titanium alloy composite
material. The content of the carbon fibers within the above ranges
allows further improvement in mechanical properties.
[0038] Note that the content of the carbon fibers in the titanium
composite material can be measured through structural observation
with an ultrahigh resolution FE-SEM or a transmission electron
microscope, and elemental analysis and analysis in accordance with
"JIS H1617 Methods for determination of carbon in titanium and
titanium alloys".
[0039] In the present invention, the element coating the carbon
fibers is not particularly limited as long as the element is
capable of forming carbide in reaction with carbon. The element is
preferably at least one selected from the group consisting of
silicon (Si), chromium (Cr), titanium (Ti), vanadium (V), tantalum
(Ta), molybdenum (Mo), zirconium (Zr), boron (B) and calcium (Ca).
The element is more preferably at least one selected from silicon
(Si) and chromium (Cr). The elements exemplified are capable of
further improving the mechanical properties because the carbide of
the elements has excellent compatibility with the titanium
alloy.
[0040] The thickness of the layer containing the above-mentioned
element and the carbide of the element is preferably at least 0.5
nm, more preferably 2 nm to 50 nm from the viewpoint of further
improving the mechanical strength by dispersion enhancement into
the titanium alloy, and particularly preferably 0.5 nm to 10 nm in
the case where carbon nanotubes are used as the carbon fiber.
[0041] Note that structural observation with an ultrahigh
resolution FE-SEM or a transmission electron microscope can confirm
whether or not the carbon fiber is coated with the layer containing
the element and the carbide of the element.
[0042] The titanium alloy to be used for preparation of the
titanium alloy composite material may have any crystal structure
such as: an .alpha.-structure (such as Ti--O or Ti-5Al-2.5Sn); a
near .alpha.-structure (such as Ti-6Al-5Zr-0.5Mo-0.2Si,
Ti-5.5Al-3.5Sn-3Zr-0.3Mo-1Nb-0.3Si, Ti-8Al-1Mo-1V, or
Ti-6Al-2Sn-4Zr-2Mo); an .alpha.+.beta.-structure (such as
Ti-6Al-4V, Ti-6Al-6V-2Sn, Ti-6Al-2Sn-4Zr-6Mo, or
Ti-4.5Al-3V-2Mo-2Fe); a near .beta.-structure (such as
Ti-5Al-2Sn-2Zr-4Mo-4Cr or Ti-10V-2Fe-3Al); or a .beta.-structure
(such as Ti-15Mo-5Zr-3Al, Ti-11.5Mo-6Zr-4.5Sn, Ti-15V-3Cr-3Al-3Sn,
Ti-15Mo-5Zr, or Ti-13V-11Cr-3Al). Further, a titanium alloy (e.g.,
a titanium alloy containing Ti-15V-6Cr-4Al as a base and TiB and/or
TiC added in a small amount, or a titanium alloy containing
Ti-22V-4Al as a base and TiB and/or TiC added in a small amount)
containing fine particles of TiB and/or TiC dispersed in a metal
structure and disclosed in JP-A-2005-76052 can preferably be used.
In consideration of the mechanical strength of the titanium alloy
composite material to be obtained eventually, preferred examples of
the titanium alloy include Ti-6Al-4V, Ti-15Mo-5Zr-3Al,
Ti-15V-3Cr-3Al-3Sn, Ti-10V-2Fe-3Al, Ti-4.5Al-3V-2Mo-2Fe, and a
titanium alloy disclosed in JP-A-2005-76052.
(Method of Producing Titanium Alloy Composite Material)
[0043] Next, a method of producing the titanium alloy composite
material of the present invention will be described.
[0044] FIG. 1 is a flow chart explaining a method of producing the
titanium alloy composite material of the present invention. This
method of producing the titanium alloy composite material of the
present invention is characterized by including: a carbon fiber
coating step of coating carbon fibers with a layer containing an
element which forms carbide in reaction with carbon and the carbide
formed thereby; a carbon fiber fixing step of fixing the carbon
fibers on a surface of titanium alloy powder; a sintering step of
sintering the carbon fiber-fixed titanium alloy powder; and a
carbon fiber dispersing step of dispersing the carbon fiber in
crystal grains of titanium alloy.
(1) Carbon Fiber Coating Step
[0045] The carbon fiber coating step of the present invention
refers to a step of coating the carbon fibers with the layer
containing an element which forms carbide in reaction with carbon
and the carbide formed thereby. In this step, the carbon fibers and
powder formed of the element which forms carbide in reaction with
carbon are charged into a mixing vessel provided with a stirring
mixer or the like, and the whole is mixed for about 15 to 30
minutes. The carbon fiber may employ the same carbon fiber as that
exemplified in the description of the titanium alloy composite
material. The powder to be used only needs to be formed of the
element which forms carbide in reaction with carbon, and is formed
of at least one selected from the group consisting of silicon (Si),
chromium (Cr), titanium (Ti), vanadium (V), tantalum (Ta),
molybdenum (Mo), zirconium (Zr), boron (B) and calcium (Ca). The
particle shape and average particle size of the powder are not
particularly limited, but use of powder having an average particle
size of 10 .mu.m to 50 .mu.m allows further improvement in
dispersibility of the carbon fiber.
[0046] Next, the mixture taken out of the mixing vessel is filled
in an unsealed vessel allowing air flow between inside and outside
of the unsealed vessel. The unsealed vessel is placed in a vacuum
furnace provided with a sealed furnace body, heating means for
heating inside the sealed furnace body and a vacuum pump for
creating vacuum inside the furnace body. Then, inside of the
furnace body is heated by heating means while the inside of the
furnace body is maintained in a vacuum state with the vacuum pump,
to thereby sublimate the powder of the element which forms carbide
in reaction with carbon. The vapor is brought into contact with the
carbon fibers to form a layer covering the surface of the carbon
fibers. This layer is made of the carbide formed in reaction
between a part of the sublimated element and the carbon fiber, and
an unreacted element. The conditions such as degree of vacuum,
heating temperature, and heating time may arbitrarily be set in
accordance with the kind of powder to be used. However, in
consideration of a balance between production cost and quality of
the layer covering the surface of the carbon fibers, the conditions
preferably include a degree of vacuum of 1.times.10.sup.-2 Pa to
1.times.10.sup.-3 Pa, a heating temperature of 1,200.degree. C. to
1,500.degree. C., and a heating time of 5 hours to 10 hours, for
example. Temperature increase rate and temperature decrease rate
are not particularly limited, but are each preferably 100.degree.
C./h to 200.degree. C./h.
[0047] In this way, the carbon fibers are coated with the element,
to thereby suppress a reaction between the carbon fibers and
titanium during formation of a composite of the carbon fibers and
the titanium alloy.
(2) Carbon Fiber Fixing Step
[0048] The carbon fiber fixing step of the present invention refers
to a step of fixing the carbon fibers obtained in the carbon fiber
coating step described above on the surface of titanium alloy
powder. In this step, the carbon fibers obtained in the carbon
fiber coating step are mixed with the titanium alloy powder. The
mixing ratio of the carbon fibers to the titanium alloy powder is
not particularly limited. However, from the viewpoint of further
improving the mechanical properties of the titanium alloy serving
as a base material, the mixture preferably includes 0.1% to 10% by
mass, more preferably 0.2% to 3.0% by mass, and most preferably
0.4% to 1.0% by mass of the carbon fibers. The particle shape and
average particle size of the titanium alloy powder are not
particularly limited, but use of powder having an average particle
size of 10 .mu.m to 50 .mu.m allows further improvement in
mechanical properties of a composite titanium alloy. In the case
where the carbon fiber is included in the mixture in an amount of
3% or more by mass, titanium alloy powder having a small average
particle size is preferably used from the viewpoint of suppressing
aggregation of the carbon fibers.
[0049] Next, mechanical impact force is applied to the mixture of
the carbon fibers and the titanium alloy powder to fix the carbon
fiber on the surface of the titanium alloy powder. In this way,
release of the carbon fibers from the surface of a titanium alloy
powder particle is prevented, and a homogeneous sintered body can
be obtained in the sintering step described below.
[0050] Specific examples of means for applying mechanical impact
force include: a stirring device such as a hybridization system
providing high mechanical impact force (manufactured by Nara
Machinery Co., Ltd.) or a mechanofusion system (manufactured by
Hosokawamicron Corporation); a dispersing device employing medium
particles; and a dry mixing and stirring device such as a Henschel
mixer or a V-type mixer. Of those, the hybridization system capable
of applying mechanical impact force including shear force between a
rotor and a stator, impact force between particles, and impact
force between a particle and a wall of the device in a high speed
flow is preferably employed for fixing the carbon fiber on the
surface of the titanium alloy powder particle uniformly and
rigidly.
(3) Sintering Step
[0051] The sintering step of the present invention refers to a step
of heating and sintering the carbon fiber-fixed titanium alloy
powder obtained in the carbon fiber fixing step described above. In
this step, the carbon fiber-fixed titanium alloy powder obtained in
the carbon fiber fixing step is formed into a molded product as
required, and sintering the molded product by a sintering method
conventionally known in the technical field such as a pulse
electric current sintering method, a hot press method, a gas
pressure sintering method, or a hot isotropic sintering method
preferably in vacuum or in an inert gas atmosphere. In the
conventional method, titanium and most of the carbon fibers react
with each other during sintering. Meanwhile, in the sintering step
of the present invention, the reaction between the carbon fibers
and titanium is suppressed by the layer covering the carbon fibers
(the carbon fibers partly reacts with titanium to form titanium
carbide), and the properties of the carbon fiber as a reinforcing
material are maintained.
[0052] Sintering conditions such as sintering temperature and
sintering time may arbitrarily be set in accordance with the
sintering method to be employed or the kind of titanium alloy to be
used, and the conditions preferably include a sintering temperature
of 800.degree. C. to 1,300.degree. C. and a sintering time of 5
minutes to 2 hours, for example.
[0053] Of the sintering methods exemplified above, the pulse
electric current sintering method is preferably employed from the
viewpoint of obtaining a homogeneous sintered body simply in a
short sintering time. In the case where sintering is conducted by
the pulse electric current sintering method, the carbon fiber-fixed
titanium alloy powder or the molded product thereof is filled in a
graphite die, and the whole is heated to a temperature of
850.degree. C. to 950.degree. C. with a temperature increase rate
of 50.degree. C./min to 100.degree. C./min, for example for,
sintering for 5 minutes to 10 minutes in a degree of vacuum of 4.0
Pa under a compression load of 20 MPa to 30 MPa. In the sintering
by the pulse electric current sintering method, neck growth between
particles alone is accelerated, and coarsening of particles due to
shrinkage between particles hardly occurs. Thus, particle size
before sintering is retained, and a sintered body having a fine
structure is obtained. In this way, the sintered body has a fine
structure, and thus the carbon fiber is easily dispersed in the
crystal grains uniformly in the carbon fiber dispersing step
described below. As a result, the mechanical strength of the
titanium alloy composite material to be obtained improves.
(4) Carbon Fiber Dispersing Step
[0054] The carbon fiber dispersing step of the present invention
refers to a step of plastic working the sintered body obtained in
the sintering step described above for dispersing the carbon fibers
in the crystal grains of the titanium alloy. The plastic working
may employ a method conventionally known in the technical field
without limitation, and examples thereof include a rolling process,
a forging process, and an extrusion process. Of those, the plastic
working preferably employs at least one process chosen from a hot
rolling process and an isothermal forging process. In particular,
the hot rolling process is preferred because the crystal grains are
drawn into a form of fiber for further improving the mechanical
strength of the titanium alloy composite material.
[0055] In the case where the sintered body is subjected to plastic
working through the hot rolling process, rolling conditions such as
rolling speed, rolling temperature, and draft are not particularly
limited. However, from the viewpoint of obtaining a titanium alloy
composite material having excellent mechanical strength, the
conditions preferably include a rolling strain/pass of 0.1 to 0.2,
a rolling temperature of 700.degree. C. to 850.degree. C., and a
draft of 65% or more. In particular, a draft of less than 65% may
undesirably cause insufficient dispersion of the carbon fiber in
the crystal grain, and thus the mechanical strength of the titanium
alloy composite material may degrade. Note that the term "draft" is
defined by (h.sub.1-h.sub.2) .times.100/h.sub.1 (wherein: h.sub.1
represents a sheet thickness before rolling; and h.sub.2 represents
a sheet thickness after rolling).
[0056] In the case where the titanium alloy composite material is
worked for producing a product having axial symmetry such as a
gear, working of a sheet material may provide insufficient product
precision due to in-plane anisotropy. Thus, it is preferred that a
cylindrical sintered body be produced in the sintering step and the
plastic working employ a hot extrusion process at preferably
1,000.degree. C. or more and preferably 1,000.degree. C. to
1,100.degree. C. or a swaging process.
(5) Aging Treatment Step
[0057] The method of producing a titanium alloy composite material
of the present invention preferably further includes a step of
subjecting the titanium alloy composite material obtained in the
carbon fiber dispersing step described above to aging treatment.
Conditions for the aging treatment may arbitrarily be set in
accordance with the kind of titanium alloy serving as a base
material, and the aging treatment may be conducted at 400.degree.
C. to 600.degree. C. for 4 h to 24 h, for example. The titanium
alloy composite material is subjected to the aging treatment, to
thereby further improve the mechanical strength of the titanium
alloy composite material.
(Titanium Clad Material)
[0058] A titanium clad material of the present invention is
characterized in that the titanium alloy composite material
described above, that is, the titanium alloy composite material
dispersing carbon fibers coated with a layer containing an element
which forms carbide in reaction with carbon and the carbide formed
thereby in crystal grains of the titanium alloy, and a titanium
alloy having a higher fracture toughness than that of the titanium
alloy composite material (hereinafter, abbreviated as high
toughness titanium alloy) are sinter bonded to one another.
[0059] FIG. 2 shows examples of a laminate structure of the
titanium clad material of the present invention. Examples of the
laminate structure of the titanium clad material include: a
structure (FIG. 2(a)) in which a sheet material 2 formed of the
high toughness titanium alloy is stacked on a sheet material 1
formed of the titanium alloy composite material to form a laminate,
and the laminate is sinter bonded together; a structure (FIG. 2(b))
in which a sheet material 1 formed of the titanium alloy composite
material and a sheet material 2 formed of the high toughness
titanium alloy are stacked alternatively to form a laminate, and
the laminate is sinter bonded together; a sandwich structure (FIG.
2(c)) in which a sheet core material 3 formed of the titanium alloy
composite material is provided between a pair of sheet materials 2
each formed of the high toughness titanium alloy, that is, a sheet
core material 3 formed of the titanium alloy composite material is
sandwiched by a pair of sheet materials 2 each faulted of the high
toughness titanium alloy to form a laminate, and the laminate is
sinter bonded together; a sandwich structure (FIG. 2(d)) in which a
sheet core material 4 formed of the high toughness titanium alloy
is provided between a pair of sheet materials 1 each formed of the
titanium alloy composite material, that is, a sheet core material 4
formed of the high toughness titanium alloy is sandwiched by a pair
of sheet materials 2 each formed of the titanium alloy composite
material to form a laminate, and the laminate is sinter bonded
together; and a cylindrical structure (FIG. 2(e)) in which a
cylindrical core material 6 formed of the titanium alloy composite
material is inserted into a cylindrical material 6 formed of the
high toughness titanium alloy to form a laminate, and the laminate
is sinter bonded together. In the structure of FIGS. 2(c) or (d),
the sheet core material 3 formed of the titanium alloy composite
material or the sheet core material 4 formed of the high toughness
titanium alloy may have a honeycomb structure for reduction in
weight of the titanium clad material. In consideration of a balance
between the mechanical properties and the reduction in weight of
the titanium clad material to be obtained eventually, the most
preferred structure is the structure shown in FIG. 3 in which: a
plurality of sheet materials 7 each formed of the titanium alloy
composite material and having a honeycomb structure are stacked
together, and the whole is sandwiched by a pair of sheet materials
1 each formed of the titanium alloy composite material to form a
honeycomb core material; the honeycomb core material is sandwiched
by a pair of sheet materials 2 each formed of the high toughness
titanium alloy, and the whole is sinter bonded together. Note that
in the structure described above employing the sheet materials 7
each having a honeycomb structure, the pair of sheet materials 1
each formed of the titanium alloy composite material may be
omitted. In the laminate structures described above, the size and
thickness of the sheet material, core material, and the like may
arbitrarily be set in accordance with a product. ps (High Toughness
Titanium Alloy)
[0060] The high toughness titanium alloy to be used in the present
invention is not particularly limited as long as the high toughness
titanium alloy has a higher fracture toughness than that of the
titanium alloy composite material described above. To be specific,
a high toughness titanium alloy having a higher fracture toughness
than that of the titanium alloy composite material may arbitrarily
be selected from titanium alloys such as: an .alpha.-structure
titanium alloy (such as Ti--O or Ti-5Al-2.5Sn); a near
.alpha.-structure titanium alloy (such as Ti-6Al-5Zr-0.5Mo-0.2Si,
Ti-5.5Al-3.5Sn-3Zr-0.3Mo-1Nb-0.3Si, Ti-8Al-1Mo-1V, or
Ti-6Al-2Sn-4Zr-2Mo); an .alpha.+.beta.-structure titanium alloy
(such as Ti-6Al-4V, Ti-6Al-6V-2Sn, Ti-6Al-2Sn-4Zr-6Mo, or
Ti-4.5Al-3V-2Mo-2Fe); a near .beta.-structure titanium alloy (such
as Ti-5Al-2Sn-2Zr-4Mo-4Cr or Ti-10V-2Fe-3Al); a .beta.-structure
titanium alloy (such as Ti-15Mo-5Zr-3Al, Ti-11.5Mo-6Zr-4.5Sn,
Ti-15V-3Cr-3Al-3Sn, Ti-15Mo-5Zr, or Ti-13V-11Cr-3Al); and a
titanium alloy (e. g., a titanium alloy containing Ti-15V-6Cr-4Al
as a base and TiB and/or TiC added in a small amount, or a titanium
alloy containing Ti-22V-4Al as a base and TiB and/or TiC added in a
small amount) containing fine particles of TiB and/or TiC dispersed
in a metal structure and disclosed in JP-A-2005-76052. In
consideration of the mechanical strength of the titanium clad
material to be obtained eventually, Ti-6Al-4V, Ti-15Mo-5Zr-3Al,
Ti-15V-3Cr-3Al-3Sn, Ti-10V-2Fe-3Al, Ti-4.5Al-3V-2Mo-2Fe, and a
titanium alloy disclosed in JP-A-2005-76052 are preferred for
excellent mechanical properties such as elongation and tensile
strength. The high toughness titanium alloy may be subjected to
known solution aging treatment (e.g., subjecting the high toughness
titanium alloy to solution treatment at 780.degree. C. to
800.degree. C. for 1 h, and then to aging treatment at 400 to
500.degree. C. for 10 to 30 h). The high toughness titanium alloy
is subjected to the solution aging treatment, to thereby enhance
tensile strength of the high toughness titanium alloy.
[0061] Note that the fracture toughness in the present invention is
measured by a K.sub.IC testing method in accordance with ASTM
E399-90 or ISO 12737.
(Method of Producing Titanium Clad Material)
[0062] Next, a method of producing the titanium clad material of
the present invention will be described.
[0063] A method of producing the clad material according to the
present invention is characterized by laminating the titanium alloy
composite material and high toughness titanium alloy described
above in a mold, and sinter bonding the whole by a pulse electric
current sintering method.
[0064] To be specific, a sheet material (or core material) formed
of the titanium alloy composite material and a sheet material (or a
core material) formed of the high toughness titanium alloy are
arbitrarily laminated in a die (graphite die), and the whole is
heated to a temperature of 950.degree. C. to 1,100.degree. C. with
a temperature increase rate of 50.degree. C./min to 100.degree.
C./min, for example, for sintering for 5 min to 10 min in a degree
of vacuum of 1.0 Pa to 4.0 Pa under a compression load of 15 MPa to
30 MPa, to thereby bond together the sheet material found of the
titanium alloy composite material and the sheet material formed of
the high toughness titanium alloy. In the sintering by the pulse
electric current sintering method, neck growth between particles
alone is accelerated, and coarsening of particles due to shrinkage
between particles barely occurs. Thus, particle size before
sintering is retained, and a sinter bonded body having a fine
structure is obtained. Thus, a titanium clad material having
remarkably improved mechanical properties such as tensile strength,
elongation, and fracture toughness can be obtained. Note that for
enhancing bonding strength between the sheet material formed of the
titanium alloy composite material and the sheet material formed of
the high toughness titanium alloy, surfaces to be bonded together
are preferably subjected to surface treatment conventionally known
in the technical field such as degreasing treatment (e.g., washing
with an organic solvent) or surface polishing treatment (e.g.,
polishing with #600 to #1000 sand paper) in advance.
[0065] The honeycomb core material formed of the titanium alloy
composite material may be produced by: punching out hexagonal
pieces from a sheet material formed of the titanium alloy composite
material with a laser punch or the like, and removing flash
obtained after punching as required to produce a sheet material
formed of the titanium alloy composite material and having a
honeycomb structure; and subjecting surfaces of the sheet materials
formed of the titanium alloy composite material with a honeycomb
structure that are to be bonded together to surface treatment and
degreasing treatment, stacking together the sheet materials with
good precision by using an alignment jig or the like, and
sandwiching the whole by a pair of sheet materials each formed of
the titanium alloy composite material. However, in the case where
the titanium clad material is produced by using the honeycomb core
material formed of the titanium alloy composite material, inside of
the honeycomb core material is bonded in a state (e.g., a state of
reduced pres sure) in accordance with conditions for pulse electric
current sintering. Thus, in the case where the inside of the
honeycomb core material must be adjusted to the same pressure as
that of a use environment of the titanium clad material, a minute
vent hole may be provided on the honeycomb core material.
(Application of Titanium Alloy Composite Material and Titanium Clad
Material)
[0066] The titanium alloy composite material and titanium clad
material of the present invention have excellent mechanical
properties such as tensile strength, elongation, Young's modulus,
fracture toughness, and hardness, and can be widely used for
products requiring such properties including industrial machinery,
automobiles, motorcycles, bicycles, household appliances, aerospace
equipment, ships and vessels, sports and leisure equipment, and
medical equipment. To be specific, the titanium alloy composite
material and titanium clad material of the present invention may
preferably be used: for connecting rods, engine valves, valve
springs, retainers, suspensions, body frames, or the like in
applications for automobiles and motorcycles; for fan blades,
compressor blades, discs, frames, body panels, fasteners, flags,
spoilers, main gears, exhaust air ducts, fuel tanks, or the like in
applications for aerospace equipment; and for artificial bones,
artificial joints, implant screws, surgical instruments, or the
like in applications for medical equipment.
[0067] As sports and leisure equipment, in the case where the
titanium alloy composite material of the present invention is used
for a face part of a golf club, for example, thickness reduction
can be realized due to relative strength improvement compared with
a conventional titanium alloy, to thereby increase the coefficient
of rebound. The thickness reduction allows surplus weight, to
thereby enhance the degree of freedom in design and allow setting
of unprecedented centers of gravity. As described above, a golf
club provided with a head employing the titanium alloy composite
material of the present invention can extend the carrying distance
and enlarge the sweet spot. Thus, a golfer can hit a ball straight
with little bend.
EXAMPLES
[0068] Hereinafter, the present invention will be described in more
detail by way of examples and comparative examples, but the present
invention is not limited thereto.
[0069] Evaluation of mechanical properties of the titanium alloy
composite material was conducted following the methods described
below.
<Material Strength Measurement>
[0070] The titanium alloy composite material was cut out into a
dumbbell-shaped test piece having a length of 30 mm in a rolling
direction, and parallel and perpendicular directions, the length of
the parallel part being 15 mm, and the width of the parallel part
being 5 mm with a carbon dioxide gas laser. A strain gauge was
attached to the parallel part, and strength measurement was
conducted at a crosshead speed of 1 mm/min by using a material
testing machine (manufactured by Shimadzu Corporation, Autograph
AG-1, 100 kN).
<Young's Modulus Measurement>
[0071] Young's modulus measurement was conducted by using a modulus
measuring device (manufactured by Toshiba Tungaloy Corporation,
UMS-R).
<Hardness Measurement>
[0072] Hardness measurement was conducted by using a Rockwell
hardness testing machine (manufactured by Akashi Corporation,
ATK-F3000).
Example 1
[0073] 20 g of multilayer carbon nanotubes having an average fiber
diameter of 10 to 25 nm and an average fiber length of 10 to 50
.mu.m and 2 g of Si powder having an average particle size of 40
.mu.m were weighed with an electrical balance, and then were mixed
in a mortar for about 30 min. The obtained mixture was charged into
a 1-L tantalum vessel. A tantalum cap was placed over the
container, and then the container was placed in a vacuum furnace.
The vacuum furnace was heated from room temperature to 300.degree.
C. in 4 hours under vacuum to a degree of vacuum of
2.times.10.sup.-3 Pa, heated to 1,400.degree. C. in 7 hours, and
maintained at 1,400.degree. C. for 5 hours for sublimation of Si,
to thereby coat a surface of the carbon nanotubes with Si. The
degree of vacuum while the temperature was maintained at
1,400.degree. C. was maintained at about 3.times.10.sup.-3 Pa by Si
sublimation. Then, the furnace was cooled under vacuum, to thereby
obtain carbon nanotubes coated with Si. FIG. 4 shows results of
X-ray diffraction measurement of the obtained carbon nanotubes. The
results of X-ray diffraction measurement, and EDX analysis and
observation with an ultrahigh resolution field emission scanning
electron microscope revealed that a surface modified layer (layer
containing Si and SiC) having a thickness of 0.5 nm in a thin
position and about 5 nm in a thick position was formed on the
surface of the carbon nanotubes.
[0074] A Ti-6Al-4V alloy produced as titanium alloy powder by a
powder atomization method and having a particle size distribution
including 2.3% by mass of +45 .mu.m, 20.2% by mass of 38 to 45
.mu.m, 27.8% by mass of 25 to 38 .mu.m, and 49.7% by mass of -25
.mu.m was prepared. Carbon nanotubes were weighed such that they
were included in an amount of 0.5% by mass in a mixture of this
titanium alloy powder and the Si-coated carbon nanotubes obtained
above. Mechanical impact force was applied to the mixture in an
argon gas by using a hybridizer (manufactured by Nara Machinery
Co., Ltd.) which is a kind of powder stirring and mixing device. As
shown in FIG. 5, the carbon nanotubes were attached to the surface
of the titanium alloy powder after the treatment. The carbon
nanotubes attached to the surface of the titanium alloy powder were
beaten by collision of the titanium alloy powder and was embedded
(i.e., fixed) directly below the surface of the titanium alloy
powder.
[0075] 50 g of the raw material powder subjected to fixing
treatment was weighed and charged into a graphite die of a pulse
electric current sintering device. The raw material powder was
pressurized at 30 MPa with a graphite cylinder, depressurized to a
degree of vacuum on the order of 4 Pa, heated from room temperature
to 900.degree. C. with a temperature increase rate of 100 .degree.
C./min, and maintained at 900.degree. C. for 5 min for sintering.
The obtained sintered body (i.e., intermediate) was observed with a
metallographic microscope. As shown in FIG. 6, the sintered body
had a structure in which the carbon nanotubes and titanium carbide
formed through a partial reaction between the carbon nanotubes and
titanium surrounded the titanium alloy fine particles.
[0076] Next, the sintered body was cut into a size of 35
mm.times.35 mm.times.5 mm, and subjected to pack welding with a
stainless steel SUS 304 sheet material for preventing oxidation
during hot rolling. The cut-out piece was heated to about
800.degree. C. by burner heating, and subjected to hot rolling in a
longitudinal direction as a sheet material at a rolling strain/pass
of 0.1 and a draft of 68%, to thereby obtain a titanium alloy
composite material of Example 1. The obtained titanium alloy
composite material was observed with a metallographic microscope.
As shown in FIG. 7, the titanium alloy composite material had a
structure in which the carbon nanotubes and titanium carbide were
dispersed in the crystal grains of the titanium alloy.
[0077] FIG. 8 shows the results of material strength measurement of
the titanium alloy composite material of Example 1. FIG. 9 show
results of observation of a broken-out section of the titanium
alloy composite material after material strength measurement by
using an ultrahigh resolution field emission scanning electron
microscope (manufactured by Hitachi High-Technologies Corporation,
S-5200) and an energy dispersive X-ray analyzer (manufactured by
EDAX Japan Co., Ltd.). In FIGS. 9(b) to (f), a light-colored part
refers to a part containing a large amount of a target element.
FIG. 9 revealed that the shape of the carbon nanotubes remained and
the carbon nanotubes near the surface was changed to titanium
carbide through a reaction with titanium. Aluminum and vanadium are
components of the titanium alloy, but did not react with the carbon
nanotubes. Coated Si was partly observed.
[0078] Table 1 collectively shows the results of measurement of
tensile strength, Young's modulus, and hardness.
TABLE-US-00001 TABLE 1 Tensile Young's strength modulus Hardness
(MPa) (GPa) (HRC) Example 1 1500 126 47.8 Example 2 1614 127 49.0
Example 3 1522 125 46.0 Example 4 1556 125 45.1 Example 5 1607 125
45.6 Comparative example 1 1074 109 39.3 Comparative example 2 963
110 37.9 Comparative example 3 672 124 45.6 Comparative example 4
493 121 44.7
Example 2
[0079] The titanium alloy composite material was prepared in the
same manner as in Example 1, and then a pack material was removed.
The titanium alloy composite material was charged into a vacuum
furnace, subjected to a vacuum, and subjected to an aging treatment
at 500.degree. C. for 8 hours under an argon gas (133 Pa)
replacement, to thereby obtain the titanium alloy composite
material of Example 2. FIG. 8 shows the results of material
strength measurement of the titanium alloy composite material of
Example 2. Table 1 collectively shows the results of measurement of
tensile strength, Young's modulus and hardness.
Example 3
[0080] The titanium alloy composite material of Example 3 was
obtained in the same manner as in Example 2 except that: the amount
of the carbon nanotubes in the mixture of the titanium alloy powder
and the Si-coated carbon nanotubes was changed to 0.4% by mass; and
the draft of hot rolling was changed to 77%. Table 1 collectively
shows the results of measurement of tensile strength, Young's
modulus, and hardness.
Example 4
[0081] 20 g of multilayer carbon nanotubes having an average fiber
diameter of 10 to 25 nm and an average fiber length of 10 to 50
.mu.m and 6 g of Cr powder having an average particle size of 10
.mu.m were weighed with an electrical balance, and then were mixed
in a mortar for about 30 min. The obtained mixture was charged into
a 1-L tantalum vessel. A tantalum cap was placed over the
container, and then the container was placed in a vacuum furnace.
The vacuum furnace was heated from room temperature to 300.degree.
C. in 7 hours under vacuum to a degree of vacuum of
2.times.10.sup.-3 Pa, heated to 1,273.degree. C. in 4 hours, and
maintained at 1,273.degree. C. for 5 hours for sublimation of Cr,
to thereby coat the surface of the carbon nanotubes with Cr. The
degree of vacuum while the temperature was maintained at
1,273.degree. C. was maintained at about 3.times.10.sup.-3 Pa by Cr
sublimation. Then, the furnace was cooled under vacuum, to thereby
obtain carbon nanotubes coated with Cr. FIG. 10 shows results of
X-ray diffraction measurement of the obtained carbon nanotubes. The
results of X-ray diffraction measurement, and EDX analysis and
observation with an ultrahigh resolution field emission scanning
micros cope revealed that a surf ace modified layer, which contains
Cr, Cr.sub.3C.sub.2, and Cr.sub.7C.sub.3 and has a thickness of 1
to 2 nm in a thin position and about 3 nm in a thick position, was
formed on the surface of the carbon nanotubes.
[0082] A Ti-6Al-4V alloy produced as titanium alloy powder by a
powder atomization method and having a particle size distribution
including 2.3% by mass of +45 .mu.m, 20.2% by mass of 38 to 45
.mu.m, 27.8% by mass of 25 to 38 .mu.m, and 49.7% by mass of -25
.mu.m was prepared. Carbon nanotubes were weighed such that the
carbon nanotubes were included in an amount of 0.4% by mass in a
mixture of this titanium alloy powder and the Cr-coated carbon
nanotubes obtained above. Mechanical impact force was applied to
the mixture in an argon gas by using a hybridizer (manufactured by
Nara Machinery Co., Ltd.) which is a kind of powder stirring and
mixing device, and the Cr-coated carbon nanotubes were fixed
directly below the surface of the titanium alloy powder.
[0083] 50 g of the above-mentioned raw material powder subjected to
fixing treatment was weighed and charged into a graphite die of the
pulse electric current sintering device. The raw material powder
was pressurized at 30 MPa with a graphite cylinder, depressurized
to a degree of vacuum on the order of 4 Pa, heated from room
temperature to 900.degree. C. with a temperature increase rate of
100.degree. C./min, and maintained at 900.degree. C. for 5 minutes
for sintering.
[0084] Next, the sintered body was cut into a size of 35
mm.times.35 mm.times.5 mm, and subjected to pack welding with a
stainless steel SUS 304 sheet material for preventing oxidation
during hot rolling. The cut-out piece was heated to about
800.degree. C. by burner heating and subjected to hot rolling in a
longitudinal direction as a sheet material at a rolling strain/pass
of 0.1 and a draft of 82%, and the pack material was removed. The
titanium alloy composite material was charged into a vacuum
furnace, subjected to vacuuming, and subjected to aging treatment
at 500.degree. C. for 8 hours under an argon gas (133 Pa)
replacement, to thereby obtain the titanium alloy composite
material of Example 4. Table 1 collectively shows the results of
measurement of tensile strength, Young's modulus and hardness.
Example 5
[0085] The titanium alloy composite material of Example 5 was
obtained in the same manner as in Example 4 except that: the amount
of the carbon nanotubes in the mixture of the titanium alloy powder
and the Cr-coated carbon nanotubes was changed to 0.5% by mass; and
the draft of hot rolling was changed to 81%. Table 1 collectively
shows the results of measurement of tensile strength, Young's
modulus and hardness.
Comparative Example 1
[0086] 50 g of the titanium alloy powder used in Example 1 was
weighed and charged into a graphite die of the pulse electric
current sintering device. The raw material powder was pressurized
at 30 MPa with a graphite cylinder, depressurized to a degree of
vacuum on the order of 4 Pa, heated from room temperature to
900.degree. C. with a temperature increase rate of 100.degree.
C./min, and maintained at 900.degree. C. for 5 min for sintering.
Next, the sintered body was cut into a size of 35 mm.times.35
mm.times.5 mm, and subjected to pack welding with a stainless steel
SUS 304 sheet material for preventing oxidation during hot rolling.
The cut-out piece was heated to about 800.degree. C. by burner
heating, and subjected to hot rolling in a longitudinal direction
as a sheet material at a rolling strain/pass of 0.1 and a draft of
68%, to thereby obtain a titanium alloy composite material of
Comparative Example 1. Table 1 collectively shows the results of
measurement of tensile strength, Young's modulus and hardness.
Comparative Example 2
[0087] The titanium alloy composite material of Comparative Example
2 was obtained in the same manner as in Comparative Example 1
except that the hot rolling was omitted. FIG. 8 shows the results
of material strength measurement of the titanium alloy composite
material of Comparative Example 2. Table 1 collectively shows the
results of measurement of tensile strength, Young's modulus and
hardness.
Comparative Example 3
[0088] The titanium alloy composite material of Comparative Example
3 was obtained in the same manner as in Example 2 except that the
multilayer carbon nanotubes were directly used without Si coating.
Table 1 collectively shows the results of measurement of tensile
strength, Young's modulus and hardness.
Comparative Example 4
[0089] The titanium alloy composite material of Comparative Example
4 was obtained in the same manner as in Example 1 except that the
hot rolling was omitted. FIG. 8 shows the results of material
strength measurement of the titanium alloy composite material of
Comparative Example 4. Table 1 collectively shows the results of
measurement of tensile strength, Young's modulus and hardness.
[0090] The results revealed that the titanium alloy composite
material of each of Examples 1 to 5 had a tensile strength of 1,500
MPa or more and a Young's modulus of more than 120 GPa, and thus
had significantly improved mechanical strength than that of
conventional titanium alloys (Comparative Examples 1 and 2).
[0091] Meanwhile, the titanium alloy composite material of
Comparative Example 4 produced by omitting the hot rolling, which
means no carbon nanotubes were dispersed in the crystal grains of
the titanium alloy, had a low tensile strength of 493 MPa, and thus
had a mechanical strength more significantly degraded than those of
the conventional titanium alloys (Comparative Examples 1 and 2). In
the titanium alloy composite material of Comparative Example 4, the
carbon nanotubes or titanium carbonate was present on a periphery
of titanium alloy fine particles like a shell of a boiled egg, and
served as the origins of cracks. Thus, sufficient mechanical
strength presumably cannot be obtained.
[0092] The titanium alloy composite material of Comparative Example
3 employing the carbon nanotubes without Si coating had mechanical
strength more degraded than those of the conventional titanium
alloys (Comparative Examples 1 and 2). In the titanium alloy
composite material of Comparative Example 3, bonding between the
titanium alloy and the carbon nanotubes was insufficient, and thus
sufficient mechanical strength presumably cannot be obtained.
[0093] Evaluation of the mechanical properties of the titanium clad
material was conducted following the procedure described below.
<Material Strength Measurement>
[0094] A target material was cut out into a dumbbell-shaped test
piece having a length of 30 mm in a rolling direction, and parallel
and perpendicular directions, a length of a parallel part of 15 mm,
and a width of the parallel part of 5 mm with a carbon dioxide gas
laser. A strain gauge was attached to the parallel part, and
strength measurement was conducted at a crosshead speed of 1 mm/min
by using a material testing machine (manufactured by Shimadzu
Corporation, Autograph AG-1, 100 kN).
<Elongation Measurement>
[0095] A strain gauge was attached to the parallel part of the test
piece of the target material through an adhesive, and a lead wire
of the strain gauge was connected to a bridge. Then, the whole was
set in a material testing machine through a strain meter for
elongation measurement.
<Fracture Toughness Measurement>
[0096] The fracture toughness measurement was conducted by a
K.sub.IC testing method in accordance with ASTM E399-90 or ISO
12737. Introduction of a fatigue precrack and measurement of
fracture toughness were conducted with an electrohydraulic servo
fatigue testing machine (MTS 810 Test Start II).
Example 6
[0097] The elongation and fracture toughness K.sub.IC of the
titanium alloy composite material (thickness of 1.6 mm) obtained in
Example 2 were measured. The elongation was 6%, and the fracture
toughness K.sub.IC was 45.1 MPaM.sup.1/2.
[0098] Next, the titanium alloy composite material of Example 2 and
a Ti-4.5Al-3V-2Mo-2Fe sheet material (available from JFE Steel
Corporation, SP-700, thickness of 1.0 mm, subjected to solution
aging treatment at 510.degree. C. for 1 hour) as a high toughness
titanium alloy were laminated into a graphite die of the pulse
electric current sintering device. The whole was pressurized at 30
MPa with a graphite cylinder, depressurized to a degree of vacuum
on the order of 4 Pa, heated from room temperature to 950.degree.
C. with a temperature increase rate of 100.degree. C./min, and
maintained at 950.degree. C. for 5 min for sintering, to thereby
obtain the clad material of Example 6 containing the titanium alloy
composite material and the high toughness titanium alloy bonded
together. This titanium alloy composite material had a tensile
strength of 1,425 MPa, an elongation of 9.7%, and a fracture
toughness K.sub.IC of 50.4 MPam.sup.1/2. Meanwhile, the high
toughness titanium alloy (i.e., conventional titanium alloy) used
above had a tensile strength of 1,213 MPa, an elongation of 14.4%,
and a fracture toughness K.sub.IC of 55.8 MPam.sup.1/2.
[0099] FIG. 11 shows a metallographic microscopic image of the
vicinity of a sinter bonded interface of the titanium clad material
of Example 6, and FIG. 12 shows an enlarged image of an A part of
FIG. 11. The metallographic microscopic images suggest that in the
titanium clad material of Example 6, the titanium alloy composite
material and the high toughness titanium alloy are favorably sinter
bonded together.
[0100] The results revealed that the titanium clad material of
Example 6 contained the titanium alloy composite material and the
high toughness titanium alloy favorably sinter bonded together, and
thus had a tensile strength of more than 1,400 MPa, an elongation
of more than 9%, and a fracture toughness of more than
MPam.sup.1/2, which are mechanical properties more remarkably
improved than those of the conventional titanium alloys.
* * * * *