U.S. patent application number 11/169068 was filed with the patent office on 2006-09-28 for nanocrystalline titanium alloy, and method and apparatus for manufacturing the same.
This patent application is currently assigned to Postech Foundation. Invention is credited to Young-Gun Ko, Chong-Soo Lee, Dong-Hyuk Shin.
Application Number | 20060213592 11/169068 |
Document ID | / |
Family ID | 37033999 |
Filed Date | 2006-09-28 |
United States Patent
Application |
20060213592 |
Kind Code |
A1 |
Ko; Young-Gun ; et
al. |
September 28, 2006 |
Nanocrystalline titanium alloy, and method and apparatus for
manufacturing the same
Abstract
A method and apparatus for manufacturing a nanocrystalline
titanium alloy by performing an equal channel angular pressing
process to a titanium alloy material, and a nanocrystalline
titanium alloy manufactured using the method and apparatus. The
method for manufacturing the nanocrystalline titanium alloy
includes steps of preparing a titanium alloy material, and
performing an equal channel angular pressing process on the
titanium alloy material at an isothermal condition of 575.degree.
C. to 625.degree. C. The nanocrystalline titanium alloy according
to has a grain size of 300 nm.
Inventors: |
Ko; Young-Gun; (Gwangju,
KR) ; Lee; Chong-Soo; (Pohang-City, KR) ;
Shin; Dong-Hyuk; (Seoul, KR) |
Correspondence
Address: |
MARSHALL, GERSTEIN & BORUN LLP
233 S. WACKER DRIVE, SUITE 6300
SEARS TOWER
CHICAGO
IL
60606
US
|
Assignee: |
Postech Foundation
Pohang City
KR
790-784
|
Family ID: |
37033999 |
Appl. No.: |
11/169068 |
Filed: |
June 28, 2005 |
Current U.S.
Class: |
148/670 ;
148/421 |
Current CPC
Class: |
B22F 2998/00 20130101;
B22F 2998/00 20130101; C22C 14/00 20130101; C22C 1/0458 20130101;
B21C 23/001 20130101 |
Class at
Publication: |
148/670 ;
148/421 |
International
Class: |
C22C 14/00 20060101
C22C014/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 29, 2004 |
KR |
10-2004-0049406 |
Jan 28, 2005 |
KR |
10-2005-0007872 |
Claims
1. A method for manufacturing a nanocrystalline titanium alloy
comprising steps of: preparing a titanium alloy material; and
performing an ECAP (equal channel angular pressing) process to the
titanium alloy material at an isothermal condition of 575.degree.
C. to 625.degree. C.
2. The method of claim 1, wherein the ECAP process has a process
rate of 0.4 mm/s to 2 mm/s.
3. The method of claim 2, wherein the ECAP process has a process
rate of 1.3 mm/s to 2 mm/s.
4. The method of claim 1, wherein a total effective strain of the
ECAP process is 1 to 8.
5. The method of claim 1, comprising performing the ECAP process at
least twice.
6. The method of claim 5, comprising rotating the titanium alloy
material by a predetermined rotation angle with respect to a
previous ECAP process centering around a central axis passing
through a center of an inlet of the channel, from a second ECAP
process.
7. The method of claim 6, wherein the rotation angle is
substantially 180.degree..
8. The method of claim 7, comprising performing the ECAP process an
even number of times.
9. The method of claim 1, further comprising preheating the
titanium alloy material at a temperature of 575.degree. C. to
625.degree. C. for 7 minutes 30 seconds to 12 minutes 30 seconds,
between the preparing of the titanium alloy material and the
performing of the ECAP process.
10. The method of claim 1, wherein the titanium alloy material
comprises titanium as a main material and aluminum at 6 weight %,
vanadium at 4 weight %, and other impurities.
11. The method of claim 10, wherein an initial microstructure of
the titanium alloy material is an equiaxed crystal structure or a
lamellar structure.
12. A nanocrystalline titanium alloy manufactured by the method of
claim 1, wherein the nanocrystalline has a grain size of 300 nm or
less.
13. The nanocrystalline titanium alloy of claim 12, wherein the
nanocrystalline titanium alloy comprises a mixture of alpha phases
and beta phases and the beta phases are segmented and distributed
in the entire microstructure of the nanocrystalline titanium
alloy.
14. The nanocrystalline titanium alloy of claim 13, wherein the
nanocrystalline titanium alloy has a maximum elongation of 300% or
more.
15. The nanocrystalline titanium alloy of claim 13, wherein the
nanocrystalline titanium alloy has a strain-rate sensitivity
exponent of 0.4 or less.
16. The nanocrystalline titanium alloy of claim 13, wherein the
nanocrystalline titanium alloy has a superplastic forming
temperature of 575.degree. C. to 725.degree. C.
17. An apparatus for manufacturing a nanocrystalline titanium
alloy, comprising: an ECAP unit including a bent channel; a
temperature holding unit surrounding the ECAP unit and including at
least one heating member for heating the ECAP unit to a
predetermined temperature; and a temperature measuring unit for
measuring the temperature of the ECAP unit.
18. The apparatus of claim 17, further comprising an adiabatic unit
provided on a lower portion of the temperature holding unit.
19. The apparatus of claim 18, wherein the adiabatic unit comprises
asbestos.
20. The apparatus of claim 17, wherein an inner contact angle of
the bent portion of the channel is 90.degree. and the outer arc
angle thereof is 40.degree..
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of
Korean Patent Application Nos. 10-2004-0049406 filed on Jun. 29,
2004, and 10-2005-0007872 filed on Jan. 28, 2005, both applications
filed in the Korean Intellectual Property Office, the entire
content of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] (a) Field of the Invention
[0003] The present invention relates to a nanocrystalline titanium
alloy and a method and apparatus for manufacturing the same, and
more particularly to a method and apparatus for manufacturing a
nanocrystalline titanium alloy by performing an equal channel
angular pressing process to a titanium alloy material, and a
nanocrystalline titanium alloy manufactured by using the
method.
[0004] (b) Description of the Related Art
[0005] Since a titanium alloy has a high specific strength and
excellent corrosion resistance, it can be widely used in various
fields such as in the aerospace industry, the chemical industry, as
an implant material, and as a sports product material. Since the
titanium alloy has an improved superplastic property, weight and
manufacturing cost thereof can be reduced by a superplastic forming
process. Accordingly, if the titanium alloy is applied to various
industries, significant benefits can be obtained.
[0006] It is known that a titanium alloy must be subjected to the
superplastic forming process at a high process temperature of
850.degree. C. or more and a slow process rate of 10.sup.-3/sec or
less. However, since the superplastic property is significantly
affected by microstructure, a titanium alloy consisting of fine
grains can be subjected to the superplastic forming process at a
lower process temperature and a quicker process rate. Thereby, as
the nano-technology is developed, research into a method for
manufacturing a titanium alloy having fine grains has been actively
progressed.
[0007] On the other hand, a method for manufacturing a material
having fine grains includes a powder metallurgy method, a
mechanical alloying method, a rapid solidifying method, a
recrystallization method, a forging method, a rolling method, and a
drawing method. However, it is difficult to manufacture a material
having a desired size using these methods, and internal pores may
be formed in the material. Since, the size of the recrystallization
grain is limited or the cross section is reduced by the increment
of the deformation amount. Thus, a large amount of deformation
cannot be applied to the material. Accordingly, there is a limit to
refining the grain size of the material. Thus, these methods for
refining the grain cannot be actually applied.
[0008] Recently, rigid-plastic working methods for performing
plastic working with separate heat treatment and refining a grain
in which pores are not formed have been suggested. The
rigid-plastic working methods include a high pressure torsion (HPT)
method, an equal channel angular pressing (ECAP) method, and so
on.
[0009] The HPT method shear-deforms a material at a high pressure
and can be performed at a rapid rate even at room temperature.
However, there is a limit in the size of the material, and the
microstructure and the thickness of the material are
inhomogeneous.
[0010] The ECAP method is a method for introducing a material into
an L-shaped channel and shear-deforming the material. The ECAP
method can be performed using existing press equipment. Also, the
ECAP method can be scaled up and thus is economical. In addition,
although the deformation amount increases, the cross section of the
material does not decrease and thus a large amount of deformation
can be applied to the material.
[0011] In case of titanium alloys, however, the process temperature
of the titanium alloy is very high and the flow stress thereof is
reduced as the deformation amount increases. Thus, extreme cracks
may be generated in the surface of the titanium alloy when
performing the ECAP method. Accordingly, it is difficult to
manufacture a titanium alloy having nanometer-size grains by the
ECAP method.
SUMMARY OF THE INVENTION
[0012] An object of the present invention is to provide a
nanocrystalline titanium alloy which consists of fine grains
without cracks and that can be subjected to a superplastic forming
process, and a method for manufacturing the same.
[0013] Moreover, another object of the present invention is to
provide an apparatus for manufacturing a nanocrystalline titanium
alloy by an adequate process condition.
[0014] The present invention provides a method for manufacturing a
nanocrystalline titanium alloy including steps of preparing a
titanium alloy material and performing an ECAP (equal channel
angular pressing) process to the titanium alloy material at an
isothermal condition of 575.degree. C. to 625.degree. C. The
nanocrystalline titanium alloy is manufactured by the method for
manufacturing the nanocrystalline titanium alloy according to the
present invention and may have a grain size of 300 nm or less.
[0015] The present invention also provides an apparatus for
manufacturing a nanocrystalline titanium alloy including an ECAP
unit having a bent channel, a temperature holding unit which
surrounds the ECAP unit and includes at least one heating member
for heating the ECAP unit to a predetermined temperature, and a
temperature measuring unit for measuring the temperature of the
ECAP unit.
[0016] As mentioned above, according to the method for
manufacturing the nanocrystalline titanium alloy of the present
invention, the ECAP process is performed in an optimal range. Thus
cracks are not generated and the nanocrystalline titanium alloy
having the nanometer-size grain can be manufactured. At this time,
since a secondary process or coating agent which was conventionally
required is not used, the process can be simplified and a
nanocrystalline titanium alloy having a larger volume can be easily
manufactured.
[0017] Further, the nanocrystalline titanium alloy according to the
present invention has nanometer-size grains, and it has extremely
excellent elongation because the segmented beta phases are
uniformly distributed in the entire microstructure of the alloy.
Particularly, a nanocrystalline titanium alloy of a lamellar
structure which has low elongation and thus cannot be
conventionally used can have the highest elongation. Thereby, the
titanium alloy material used in the ECAP process can be variously
selected, and thus the application fields of the nanocrystalline
titanium alloy become various.
[0018] Moreover, by using the superplastic forming temperature
lower than the conventional temperature, the manufacturing cost of
the superplastic forming process can be remarkably reduced, and
problems such as abrasion of the process apparatus at a high
temperature and a reduction of the life span can be reduced, and
the advent of impurities can be solved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The above and other features and advantages of the present
invention will become more apparent by describing in detail
exemplary embodiments thereof with reference to the attached
drawings, in which:
[0020] FIG. 1 is a schematic diagram of an apparatus for
manufacturing a nanocrystalline titanium alloy according to an
embodiment of the present invention;
[0021] FIG. 2 is a schematic perspective view of an ECAP unit shown
in FIG. 1;
[0022] FIG. 3 is a plan view of the apparatus shown in FIG. 1;
[0023] FIG. 4 is a cross-sectional view schematically showing an
operating principle of the apparatus shown in FIG. 1;
[0024] FIG. 5 is a flowchart showing a method for manufacturing a
nanocrystalline titanium alloy according to an embodiment of the
present invention;
[0025] FIG. 6 is a perspective view schematically showing an ECAP
processing step of the present invention;
[0026] FIGS. 7A to 7D are photographs of nanocrystalline titanium
alloys of Embodiment 1 to Embodiment 4 of the present
invention;
[0027] FIG. 8 is a photograph of a titanium alloy of Comparative
Example 1;
[0028] FIG. 9 is a photograph of a nanocrystalline titanium alloy
of Embodiment 5 of the present invention;
[0029] FIGS. 10A to 10C are photographs of initial microstructures
of titanium alloy materials used in Embodiment 6 to Embodiment 8 of
the present invention taken with an optical microscope;
[0030] FIGS. 11A to 11C are photographs of nanocrystalline titanium
alloys manufactured in Embodiment 6 to Embodiment 8 of the present
invention taken with an optical microscope and a scanning electron
microscope;
[0031] FIGS. 12A to 12C are photographs of nanocrystalline titanium
alloys manufactured in Embodiment 6 to Embodiment 8 of the present
invention taken with a transmission electron microscope;
[0032] FIG. 13A is a photograph of the nanocrystalline titanium
alloy manufactured in Embodiment 6 after heat treatment at
600.degree. C. taken with a transmission electron microscope;
[0033] FIG. 13B is a photograph of the nanocrystalline titanium
alloy manufactured in Embodiment 6 after heat treatment of
650.degree. C. taken with a transmission electron microscope;
[0034] FIG. 13C is a photograph of the nanocrystalline titanium
alloy manufactured in Embodiment 6 after heat treatment of
700.degree. C. taken with a transmission electron microscope;
[0035] FIG. 14 is a graph showing flow stress curves of
nanocrystalline titanium alloys manufactured in Embodiment 6 to
Embodiment 8 of the present invention;
[0036] FIG. 15 is a photograph of initial sample pieces of the
nanocrystalline titanium alloys manufactured in one of Embodiment 6
to Embodiment 8 and sample pieces thereof after a tensile test;
[0037] FIG. 16 is a photograph of an initial sample piece of a
titanium alloy manufactured by a conventional high pressure torsion
(HPT) method and a sample piece thereof after a tensile test;
and
[0038] FIG. 17 is a photograph of an initial sample piece of a
titanium alloy manufactured by a conventional thermomechanical
treatment and a sample piece thereof after a tensile test.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0039] Hereinafter, a nanocrystalline titanium alloy and an
apparatus and method for manufacturing a nanocrystalline titanium
alloy according to the present invention will be described in
detail by explaining exemplary embodiments of the invention with
reference to the attached drawings.
[0040] The apparatus for manufacturing the nanocrystalline titanium
alloy according to the present invention will first be described in
detail, and then the method for manufacturing the nanocrystalline
titanium alloy and the nanocrystalline titanium alloy will be
described in detail.
[0041] FIG. 1 is a schematic diagram of an apparatus for
manufacturing a nanocrystalline titanium alloy according to an
embodiment of the present invention, FIG. 2 is a schematic
perspective view of an ECAP unit shown in FIG. 1, and FIG. 3 is a
plan view of the apparatus shown in FIG. 1.
[0042] Referring to FIGS. 1 to 3, the apparatus according to the
embodiment of the present invention is fixed to a hydraulic press
device (not shown) by a fastening member 50, and includes an ECAP
unit 10, a temperature measuring unit 20, a temperature holding
unit 30, and an adiabatic unit 40.
[0043] The ECAP unit 10 can be made by adhering two blocks, and
includes an L-shaped channel 12 having a uniform cross section. An
effective strain of a one-time ECAP process can be adjusted
according the bending degree of the channel 12. By repeatedly
performing the ECAP process, the total effective strain increases
in multiple proportions. In the present embodiment, an effective
strain of 1 can be applied to a material by a one-time ECAP process
using an ECAP unit 10 of which the inner contact angle (.theta. of
FIG. 4) of the bent portion of the channel 12 is 90.degree. and the
outer arc angle (.psi. of FIG. 4) is 40.degree..
[0044] The temperature measuring unit 20 is located on one side of
the ECAP unit 10 and measures a temperature of the ECAP unit 10. In
the present embodiment, the temperature measuring unit 20 may be,
for example, a thermoelectric couple.
[0045] The temperature holding unit 30 is located at the periphery
of the ECAP unit 10 and holds the temperature of the ECAP unit 10
to be uniform. In the present embodiment, the temperature holding
unit 30 may be formed in a cylindrical shape, and the diameter of
the lower portion 30b may be greater than that of the upper portion
30a thereof.
[0046] Also, the temperature holding unit 30 includes a cylindrical
heating member 33 for heating the ECAP unit 10 to hold a uniform
temperature. The heating member 33 is connected to an external
power source (not shown) and receives power for holding the
temperature of the ECAP unit 10. In the present embodiment, the
heating member 33 includes eight inner heating members 33a which
are located adjacent to the ECAP unit 10, and four outer heating
members 33b which are located at the outer portion of the
temperature holding unit 30. Here, the inner heating members 33a
control the temperature of the ECAP unit 10 and the outer heating
members 33b insulate it.
[0047] Although the temperature, holding unit 30 includes twelve
cylindrical heating members 33 in the present embodiment, the
present invention is not limited to this. Various structures for
uniformly holding the temperature of the ECAP unit may be applied
and may be included in the scope of the present invention.
[0048] Also, the adiabatic unit 40 is formed on the lower surface
of the temperature holding unit 30 and minimizes the heat transfer
to the outside. The adiabatic unit 40 may be composed of
asbestos.
[0049] FIG. 4 is a cross-sectional view schematically showing an
operating principle of the apparatus shown in FIG. 1.
[0050] Referring to FIG. 4, the apparatus for manufacturing the
nanocrystalline titanium alloy according to the present embodiment
is fixed to the hydraulic press device (not shown) having a plunger
14 by a fastening member 50, for example, a fixing screw. At this
time, by changing the rate of the plunger 14, the process rate can
be controlled.
[0051] In the present embodiment, a titanium alloy material 16 is
injected into the channel 12 of the ECAP unit 10 and is then
pressed by the plunger 14 so that the titanium alloy material 16
passes through the bent portion of the channel 12, thereby
performing the ECAP process. In the present specification, the
titanium alloy material 16 is a titanium alloy which is not
subjected to all ECAP processes, and the nanocrystalline titanium
alloy is a titanium alloy which is subjected to all ECAP
processes.
[0052] The titanium alloy material 16 is shear-deformed when
passing through the bent portion of the channel 12 by the ECAP
process. In the ECAP apparatus, since the cross section is not
changed when performing the process, a very large amount of
deformation can be applied to the material. Accordingly, large
amount of deformation energy can be accumulated in the material and
the accumulated deformation energy serves as a driving power for
refining the grain.
[0053] The apparatus for manufacturing the nanocrystalline titanium
alloy according to the present embodiment can uniformly hold the
temperature of the ECAP unit by including the temperature measuring
unit 20, the temperature holding unit 30, and the adiabatic unit
40. Thus, an adequate process condition can be applied to the
material when performing the ECAP process.
[0054] FIG. 5 is a flowchart showing a method for manufacturing a
nanocrystalline titanium alloy according to an embodiment of the
present invention, and FIG. 6 is a perspective view schematically
showing an ECAP processing step of the present invention.
[0055] Referring to FIG. 5, the method for manufacturing the
nanocrystalline titanium alloy according to the present embodiment
includes a preparing step S11 for a titanium alloy material, a
preheating step S12 for preheating the titanium alloy material, and
an ECAP processing step S13 for performing the ECAP process to the
titanium alloy material at an isothermal condition of 575.degree.
C. to 625.degree. C.
[0056] Further describing the method in detail, first, a titanium
alloy material is prepared (S11). At this time, a titanium alloy
material having various compositions and shapes may be prepared in
consideration of an application field of the titanium alloy.
[0057] In the present embodiment, an example of a titanium alloy
material comprises titanium as a main material and aluminum at 6
weight %, vanadium at 4 weight %, and other impurities. The
aluminum and vanadium are added to increase the strength and
ductility. Here, the amount of the aluminum is determined to
prevent Ti.sub.3Al from being formed, which may weaken the material
upon the shear deformation, and the amount of the vanadium is
determined to prevent a flaking phenomenon which may be caused upon
cooling. The titanium alloy material having these compositions has
excellent strength at a high temperature and good formability and
thus can be applied to various fields.
[0058] Also, the titanium alloy material is formed of mixtures of
alpha(.alpha.) phases and beta(.beta.) phases. And, an initial
microstructure of the titanium alloy is an equiaxed crystal
structure or a lamellar structure in which a beta phase is formed
between the alpha phases in a thin band shape. The equiaxed crystal
structure may be formed by heating at the region in which the alpha
phase and the beta phase are mixed and then cooling. The lamellar
structure may be formed by heating at a temperature greater than a
transformation temperature of the beta phase and then cooling by
using nucleation and an auto-catalytic growth mechanism. The size
of colony of the lamellar structure can be controlled by the time
of the heating treatment, and the interlayer interval of the
lamellar structure can be controlled by the cooling rate.
[0059] At this time, in order to minimize problems caused due to
friction of the titanium alloy material and the channel of the ECAP
unit during the ECAP process and more equalize a process rate,
graphite may be coated on the titanium alloy material.
[0060] Next, the titanium alloy material is preheated to the
isothermal condition of 575.degree. C. to 625.degree. C. for 7
minutes 30 seconds to 12 minutes 30 seconds (S12). This temperature
is equal to the ECAP process temperature. That is, this temperature
allows the temperature of the inside of the titanium alloy material
to be uniform during performance of the ECAP process to more
improve the effect due to the isothermal condition.
[0061] The apparatus according to the present invention can control
the process temperature and hold the isothermal condition.
Accordingly, in the present embodiment, the preheating step can be
performed by the apparatus according to the present invention.
Thereby, the process can be simplified and the preheating effect
can be more efficiently realized. However, the present invention is
not limited to this. The titanium alloy material may be preheated
using a separate device or process and this is also included in the
scope of the present invention.
[0062] Subsequently, the ECAP process is performed to the titanium
alloy material in the isothermal condition of 575.degree. C. to
625.degree. C. (S13). If the process temperature of the ECAP
process is greater than 625.degree. C., the ECAP apparatus may be
deformed or the grain of the titanium alloy material may be grown.
Also, if the process temperature of the ECAP process is less than
575.degree. C., it is difficult to process because the titanium
alloy material is a high-temperature material and cracks may be
generated therein. That is, the temperature condition of the ECAP
process is determined to an optimal temperature to efficiently
refine the grain of the titanium alloy and to not generate
cracks.
[0063] At this time, the process rate of the ECAP process may be
0.4 mm/s to 2 mm/s. If the process rate of the ECAP process is
greater than 2 mm/s, cracks may be generated by the repetitive ECAP
process, and if the process rate of the ECAP process is less than
0.4 mm/s, the ECAP processing time is very long and thus the
process efficiency may be deteriorated. That is, the process rate
of the present invention is determined to a condition that does not
generate cracks in the titanium alloy material and can optimize the
process time of the ECAP process. At this time, it is preferable
that the process rate of the ECAP process is in the range of 1.3
mm/s to 2 mm/s.
[0064] Furthermore, the total effective strain of the ECAP process
may be in the range of 1 to 8. In order to apply an adequate total
effective strain, the ECAP process may be repeatedly performed one
or more times.
[0065] Referring to FIG. 6, if the ECAP process is repeatedly
performed twice or more times, from the second ECAP process the
titanium alloy material 16 rotates by a predetermined angle
centering around a central axis L passing through a center C of an
inlet 121 of the channel 12.
[0066] In the present embodiment, the rotation angle of the
titanium alloy material 16 may be substantially 180.degree. in each
ECAP process.
[0067] That is, in a first ECAP process, the titanium alloy
material 16 is injected into the channel 12 so that any virtual
point A which exists in the titanium alloy material 16 passes
through an outer bent portion 12b, as shown in (a) of FIG. 6. In a
second ECAP process, the titanium alloy material 16 is injected
into the channel 12 so that the virtual point A passes through an
inner bent portion 12a, as shown in (b) of FIG. 6.
[0068] By this process, the titanium alloy material 16 deformed by
the first ECAP process is deformed to an original shape after the
second ECAP process. At this time, the deformation is concentrated
to one shear surface by the first ECAP process and then the
deformation is performed again by the second ECAP process. Thereby,
a nanocrystalline titanium alloy of the equiaxed crystal is
obtained by the even number of ECAP processes.
[0069] In the present invention, the rotation angle of the titanium
alloy material 16 and the number of the ECAP processes can be
variously adjusted.
[0070] In the method for manufacturing the nanocrystalline titanium
alloy according to the present invention, a separate upset forging
step or the usage of a coating agent is unnecessary by holding the
factors of the ECAP process in an adequate range. Thus, the
effective of the process can be improved.
[0071] In the nanocrystalline titanium alloy according to the
present invention manufactured using this method, the grain size is
in range of 300 nm or less and few cracks are formed by performing
the ECAP process at the isothermal condition of an adequate
temperature.
[0072] Also, by rotation of the titanium alloy material when
performing the ECAP process, the nanometer-size grain can be formed
even in the case that the initial microstructure of the titanium
alloy material is the lamellar structure. Also, the nanocrystalline
titanium alloy according to the present invention can have a high
dislocation density associated with the grain refining.
[0073] At this time, in the nanocrystalline titanium alloy
according to the present invention, the beta phases are segmented
and are uniformly formed in the entire microstructure to increase
the boundary between the alpha phase and the beta phase in the
alloy. Generally, since the boundary sliding of the boundary
between the alpha phase and the beta phase is superior to that of
the boundary between the alpha phases and the boundary between the
beta phases, the nanocrystalline titanium alloy according to the
present invention can have greater elongation.
[0074] The segmented beta phases have different characteristics
according to the initial microstructure of the titanium alloy
material. That is, the beta phases may be more segmented in the
titanium alloy material of a lamellar structure having a
predetermined lamellar spacing, so the lamellar structure having a
predetermined lamellar spacing may have a very high elongation
characteristic because of low stress. That is, in the present
invention, a nanocrystalline titanium alloy having much better
elongation can be manufactured using the titanium alloy material of
the lamellar structure, which cannot be conventionally used because
it has low elongation. Thereby, in the ECAP process, the titanium
alloy material may be variously selected.
[0075] Generally, if the strain-rate sensitivity exponent is equal
to or less than 0.45, it is known that the nanocrystalline titanium
alloy cannot have the superplastic property. However, the
nanocrystalline titanium alloy according to the present invention
has excellent elongation of at least 300% although the strain-rate
sensitivity exponent is equal to or less than 0.4. Accordingly, the
nanocrystalline titanium alloy according to the present invention
has an excellent superplastic property, which is improved by three
times to eight times with respect to the titanium alloy having the
micrometer-size grain. This is because the growth of the neck is
deteriorated due to the work-hardening phenomenon according to the
grain refining, and the boundary between the alpha phase and the
beta phase is well-formed and thus the boundary sliding is easy.
Thereby, the application field of the nanocrystalline titanium
alloy can become various.
[0076] Also, the nanocrystalline titanium alloy according to the
present invention is thermally stable at the temperature of
575.degree. C. to 725.degree. C., and the superplastic forming can
be performed at this temperature. That is, in the nanocrystalline
titanium alloy according to the present invention, the grain is
suppressed from being coarsened by the recrystallization and the
grain growth as the temperature increases.
[0077] In the nanocrystalline titanium alloy according to the
present invention, the superplastic forming temperature is in the
range of 575.degree. C. to 725.degree. C., which is less than the
conventional superplastic forming temperature by 150.degree. C. to
300.degree. C. Thereby, in the nanocrystalline titanium alloy
according to the present invention, problems such as abrasion of
the apparatus at a high temperature and reduction of the life span
can be reduced. As the result, the cost of the superplastic forming
can be reduced.
[0078] Hereinafter, the nanocrystalline titanium alloy according to
the present invention will be described in detail through
experiments. The below-mentioned embodiments are exemplary and the
present invention is not limited to these. Here, as an alloy
material, a titanium alloy containing aluminum at 6 weight % and
vanadium at 4 weight % was used.
<Experiment 1>
Embodiment 1
[0079] A titanium alloy material having a diameter of 9.5 mm and a
length of 80 mm was subjected to the ECAP process once at an
isothermal condition of 600.degree. C. and a process rate of 7.3
mm/s to 10 mm/s to manufacture the nanocrystalline titanium alloy
according to Embodiment 1.
Embodiment 2
[0080] A titanium alloy material having a diameter of 9.5 mm and a
length of 80 mm was subjected to the ECAP process once at an
isothermal condition of 600.degree. C. and a process rate of 3.2
mm/s to 4.2 mm/s to manufacture the nanocrystalline titanium alloy
according to Embodiment 2.
Embodiment 3
[0081] A titanium alloy material having a diameter of 9.5 mm and a
length of 80 mm was subjected to the ECAP process once at an
isothermal condition of 600.degree. C. and a process rate of 1.3
mm/s to 2 mm/s to manufacture the nanocrystalline titanium alloy
according to Embodiment 3.
Embodiment 4
[0082] A titanium alloy material having a diameter of 9.5 mm and a
length of 80 mm was subjected to the ECAP process once at an
isothermal condition of 600.degree. C. and a process rate of 0.4
mm/s to 0.44 mm/s to manufacture the nanocrystalline titanium alloy
according to Embodiment 4.
COMPARATIVE EXAMPLE 1
[0083] A titanium alloy material having rectangular of 23.75 mm and
23.01 mm, and length of 127 mm was subjected to the ECAP process at
a non-isothermal condition to manufacture a titanium alloy
according to Comparative Example 1. The non-isothermal condition
means that the titanium alloy material is held at 900.degree. C.
for 45 minutes to reduce the flow stress and is then subjected to
the ECAP process at 300.degree. C. for a short time of 2
seconds.
[0084] The photographs of the surfaces of the nanocrystalline
titanium alloys according to Embodiment 1 to Embodiment 4 are shown
in FIGS. 7A to 7D, respectively, and the effective strain, the
surface crack depth, and the crack fraction of the nanocrystalline
titanium alloys according to Embodiment 1 to Embodiment 4 are shown
in Table 1. Additionally, a photograph of the surface of the
nanocrystalline titanium alloy according to Comparative Example 1
is shown in FIG. 8. TABLE-US-00001 TABLE 1 Shear Surface Process
defor- crack Crack rate Process mation Effective depth fraction
[mm/s] time [s] time [s] strain [mm] [%] Embodiment 7.3-10 11-8 0.7
1.5 3.0 32.6 1 Embodiment 3.2-4.2 25-19 1.3 0.75 1.5 16.5 2
Embodiment 1.3-2 54-40 3.3 0.3 0.5 5.4 3 Embodiment 0.4- 200- 16.6
0.06 0.4 4 4 0.44 179
[0085] Comparing FIGS. 7A to 7D with FIG. 8, the nanocrystalline
titanium alloys according to Embodiment 1 to Embodiment 4 have a
surface crack depth of 3 mm or less, but the titanium alloy of
Comparative Example 1 has a surface crack depth of 10 mm or more.
That is, it can be seen that excessive cracks are generated in the
nanocrystalline titanium alloy of Comparative Example 1. Also, the
nanocrystalline titanium alloys according to Embodiment 1 to
Embodiment 4 have crack fractions of at most 32.6%, but the
titanium alloy of Comparative Example 1 has at least 50%.
[0086] That is, it can be seen that an excellent nanocrystalline
titanium alloy can be manufactured by performing the ECAP process
at an isothermal condition of 575.degree. C. to 625.degree. C.
according to the present invention. Thereby, the production rate of
the nanocrystalline titanium alloys according to Embodiment 1 to
Embodiment 4 increases to more than that of Comparative Example 1
by at least 70% in consideration of the size thereof. That is, it
can be seen that the nanocrystalline titanium alloy according to
the present invention can improve productivity.
[0087] If the crack fraction is greater than 10%, fine cracks
generated in the first process may provide the location of the
cracks which may be generated in the second process. Accordingly,
it is preferable that the crack fraction is 10% or less.
Accordingly, it is preferable that the process rate is equal to or
less than 2 mm/s.
[0088] Also, if the process rate is less than 0.4 mm/s, the process
time exceeds 200 seconds. Accordingly, it is preferable that the
process rate is 0.4 mm/s or more in view of process efficiency.
<Experiment 2>
Embodiment 5
[0089] A titanium alloy material having a diameter of 9.5 mm and a
length of 80 mm was subjected to the ECAP process four times at an
isothermal condition of 600.degree. C. and a process rate of 1.3
mm/s to 2 mm/s to manufacture the nanocrystalline titanium alloy
according to Embodiment 5. A photograph of the surface of the
nanocrystalline titanium alloy according to Embodiment 5 is shown
in FIG. 9.
[0090] Referring to FIG. 9, it can be seen that the nanocrystalline
titanium alloy according to Embodiment 5 is composed of uniform
grains having sizes of 300 mm or less. This is because the grain is
refined by the ECAP process for applying the adequate process rate
and the strain at the isothermal condition of an adequate
temperature. That is, the nanocrystalline titanium alloy having
uniform grains having sizes of 300 mm or less can be
manufactured.
<Experiment 3>
Embodiment 6
[0091] A titanium alloy material was subjected to a heating
treatment at a temperature of 950.degree. C. for 2 hours and then a
furnace cooling treatment to prepare a titanium alloy material of
an equiaxed crystal structure having a grain size of 11 .mu.m. An
optical microscopic photograph of this titanium alloy material is
shown in FIG. 10A.
[0092] This titanium alloy material was subjected to the ECAP
process four times at a temperature of 600.degree. C. to
manufacture the nanocrystalline titanium alloy according to
Embodiment 6. At this time, the rotating angle of each ECAP process
was 180.degree..
Embodiment 7
[0093] A titanium alloy material was subjected to a heating
treatment at a temperature of 1050.degree. C. for 1 hour and then a
furnace cooling treatment to prepare a titanium alloy material of a
lamellar structure having a colony size of 310 .mu.m and an
interlayer interval of 4.1 .mu.m. An optical microscopic photograph
of this titanium alloy material is shown in FIG. 10B.
[0094] This titanium alloy material was subjected to the ECAP
process four times at a temperature of 600.degree. C. to
manufacture the nanocrystalline titanium alloy according to
Embodiment 6. At this time, the rotating angle of each ECAP process
was 180.degree..
Embodiment 8
[0095] A titanium alloy material was subjected to a heating
treatment at a temperature of 1050.degree. C. for 1 hour and then a
furnace cooling treatment to prepare a titanium alloy material of a
lamellar structure having a colony size of 310 .mu.m and an
interlayer interval of 1 .mu.m. An optical microscopic photograph
of this titanium alloy material is shown in FIG. 10C.
[0096] This titanium alloy material was subjected to the ECAP
process four times at a temperature of 600.degree. C. to
manufacture the nanocrystalline titanium alloy according to
Embodiment 8. At this time, the rotating angle of each ECAP process
was 180.degree..
[0097] The nanocrystalline titanium alloys according to Embodiment
6 to Embodiment 8 were photographed with an optical microscope and
a scanning electron microscope and the photographs are shown in
FIGS. 11A to 11C, respectively, and were photographed with a
transmission electron microscope and are shown in FIGS. 12A to 12C.
In FIGS. 12A to 12C, zone axis is .sup.[{overscore (1)}2{overscore
(1)}3].
[0098] Referring to FIGS. 11A to 11C, the grain sizes of the alpha
phase and the beta phase of the nanocrystalline titanium alloy
according to the present invention are smaller than those of the
titanium alloy materials shown in FIGS. 10A to 10C. At this time,
although the equiaxed crystal and the lamellar structure have
different strengths and deformation behavior, the grain can be
refined by the manufacturing method according to the present
invention.
[0099] Also, in the nanocrystalline titanium alloys according to
Embodiment 6 to Embodiment 8, the beta phases are extremely
deformed and segmented and thus are uniformly distributed in the
entire microstructure. Particularly, it can be seen that the beta
phases are most extremely segmented in the nanocrystalline titanium
alloy according to Embodiment 8.
[0100] Accordingly, the nanocrystalline titanium alloys according
to Embodiment 6 to Embodiment 8 have high elongation by the
boundary sliding of the boundary between the alpha phase and the
beta phase. It can be seen that the nanocrystalline titanium alloy
according to Embodiment 8 in which the beta phases are most
extremely segmented has the highest elongation.
[0101] Referring to FIGS. 12A to 12C, it can be seen that the grain
sizes of the nanocrystalline titanium alloys according to
Embodiment 6 to Embodiment 8 are in the range of 200 nm to 300 nm,
and the dislocation density is very high and the grain boundary is
not obvious. That is, according to the nanocrystalline titanium
alloy according to the present invention, the nanocrystalline
titanium alloy having fine grains and high dislocation density can
be manufactured.
<Experiment 4>
[0102] Photographs of the nanocrystalline titanium alloy according
to Embodiment 6 which was subjected to the heating treatment at
600.degree. C., 650.degree. C., and 700.degree. C., respectively,
are shown in FIGS. 13A, 13B, and 13C, respectively.
[0103] As shown in FIGS. 13A to 13C, it can be seen that the grain
is fine and the dislocation density is high after the
nanocrystalline titanium alloy according to Embodiment 6 was
subjected to the heating treatment of 600.degree. C., 650.degree.
C., and 700.degree. C., respectively. That is, it can be seen that
the nanocrystalline titanium alloy according to the present
invention is thermally stable at the above temperature. This
tendency is shown in Embodiment 7 and Embodiment 8. That is, it can
be seen that the nanocrystalline titanium alloy according to the
present invention is not coarsened even at the temperature of
575.degree. C. to 725.degree. C. and has a fine grain.
<Experiment 5>
[0104] The nanocrystalline titanium alloys according to Embodiment
6 to Embodiment 8 were subjected to a tensile test at a temperature
of 700.degree. C. for 10.sup.-3/sec. The measured flow stress curve
is shown in FIG. 14. Also, the maximum elongations of the titanium
alloy materials of Embodiment 6 to Embodiment 8, and the maximum
elongation, the strain-rate sensitivity exponent, and the
work-hardening exponent of the nanocrystalline titanium alloy
according to Embodiment 6 to Embodiment 8 are shown in Table 2.
[0105] Initial sample pieces (i) of the nanocrystalline titanium
alloys according to one of Embodiment 6 to Embodiment 8 and sample
pieces (a) of the nanocrystalline titanium alloys which were
subjected to a tensile test at a temperature of 700.degree. C. for
10.sup.-3/sec, and sample pieces (b) of the nanocrystalline
titanium alloys which were subjected to a tensile test at a
temperature of 700.degree. C. for 10.sup.-4/sec are shown in FIG.
15.
[0106] Further, an initial sample piece (i) of the titanium alloy
manufactured by a conventional HPT process, a sample piece (a) of
the alloy which is subjected to a tensile test at a temperature of
650.degree. C. for 10.sup.-2/sec, a sample piece (b) of the alloy
which was subjected to a tensile test at a temperature of
650.degree. C. for 10.sup.-4/sec, and a sample piece (c) of the
alloy which was subjected to a tensile test at a temperature of
725.degree. C. for 10.sup.-2/sec are shown in FIG. 16. Moreover, an
initial sample piece (i) of the titanium alloy manufactured by a
conventional thermomechanical treatment and a sample piece (a) of
the titanium alloy which was subjected to a tensile test at a
temperature of 800.degree. C. for 10.sup.-2/sec, a sample piece (b)
of the titanium alloy which was subjected to a tensile test at a
temperature of 800.degree. C. for 10.sup.-3/sec, and a sample piece
(c) of the titanium alloy which was subjected to the tensile test
at a temperature of 800.degree. C. for 2.times.10.sup.-2/sec are
shown in FIG. 17. TABLE-US-00002 TABLE 2 Embodiment Embodiment
Embodiment 6 7 8 Maximum elongation of 163 96 78 titanium alloy
material [%] Maximum elongation of 476 330 610 nanocrystalline
titanium alloy [%] Strain-rate sensitivity 0.33 0.30 0.36 exponent
of nanocrystalline titanium alloy Work-hardening exponent 0.62 0.65
0.80 of nanocrystalline titanium alloy
[0107] Referring to FIG. 14, it can be seen that the work-hardening
phenomenon in which stress increases as the strain increases to a
certain strain occurs in the nanocrystalline titanium alloys
according to Embodiment 6 to Embodiment 8. Also, it can be seen
that the nanocrystalline titanium alloys according to Embodiment 6
to Embodiment 8 have very excellent superplastic properties. The
flow stress increases in the order of Embodiment 7, Embodiment 8,
and Embodiment 6.
[0108] Referring to Table 2 and FIG. 15, it can be seen that the
nanocrystalline titanium alloys according to Embodiment 6 to
Embodiment 8 have high elongation of 300% or more although the
strain-rate sensitivity exponents are 0.33, 0.30, and 0.36,
respectively. This is because local neck growth is suppressed by
the work-hardening phenomenon. It can be seen that the
work-hardening exponent increases in the order of Embodiment 6,
Embodiment 7, and Embodiment 8.
[0109] Particularly, it can be seen that an elongation of 610% can
be obtained by performing the ECAP process according to the present
invention to the titanium alloy material of the lamellar structure
having the elongation of 78% in Embodiment 8. Conventionally, the
titanium alloy material having the lamellar structure cannot be
used, because it has low elongation. However, the nanocrystalline
titanium alloy manufactured by the manufacturing method according
to the present invention can have higher elongation compared with
the nanocrystalline titanium alloy having the equiaxed structure.
This is because the nanocrystalline titanium alloy according to
Embodiment 8 has a highest work-hardening exponent and the beta
phases are uniformly segmented in the entire microstructure by the
ECAP process as in FIG. 11C of experiment 3 and thus the boundary
between the alpha phase and the beta phase is much formed. On the
other hand, referring to FIG. 16, the titanium alloy manufactured
by the conventional HPT process may have high elongation. However,
it cannot be seen that the elongation is accurately measured,
because the titanium alloy was manufactured from a very small
titanium alloy material. Further, the HPT process cannot be applied
to actual industry, because the microstructure according to the
diameter direction of the sample piece is very inhomogeneous and
the size of the titanium alloy which can be processed is very
small. Also, the elongation result according to the
thermomechanical treatment shown in FIG. 17 is measured at a high
temperature of 800.degree. C., and this result cannot be obtained
at a low temperature as in the present invention.
[0110] That is, the titanium alloy according to the present
invention has an excellent superplastic property at a process
temperature lower than that of the conventional titanium alloy, and
a nanocrystalline titanium alloy with a size that can be used in
the actual industry can be manufactured.
[0111] Although the exemplary embodiments of the present invention
have been described, the present invention is not limited to the
exemplary embodiments, but may be modified in various forms without
departing from the scope of the appended claims, the detailed
description, and the accompanying drawings of the present
invention. Therefore, it is natural that such modifications belong
to the scope of the present invention.
* * * * *