U.S. patent application number 12/295640 was filed with the patent office on 2009-07-02 for material having ultrafine grained structure and method of fabricating thereof.
Invention is credited to Hiromi Miura.
Application Number | 20090165903 12/295640 |
Document ID | / |
Family ID | 38563703 |
Filed Date | 2009-07-02 |
United States Patent
Application |
20090165903 |
Kind Code |
A1 |
Miura; Hiromi |
July 2, 2009 |
Material Having Ultrafine Grained Structure and Method of
Fabricating Thereof
Abstract
In the present invention, there is provided a fabrication method
of a material having an ultrafine grained structure characterized
by including a step of providing a metal or an alloy having a
stacking fault energy no greater than 50 mJ/mm.sup.2 and a step of
introducing deformation twins having a twin interval no greater
than 200 nm into structures of the metal or the alloy by processing
the metal or the alloy. Further, according to this method, a
material having an ultrafine grained structure characterized in
that twins are included in a crystal structure and the twins have a
twin interval no greater than 200 nm.
Inventors: |
Miura; Hiromi; (Tokyo,
JP) |
Correspondence
Address: |
IPUSA, P.L.L.C
1054 31ST STREET, N.W., Suite 400
Washington
DC
20007
US
|
Family ID: |
38563703 |
Appl. No.: |
12/295640 |
Filed: |
April 3, 2007 |
PCT Filed: |
April 3, 2007 |
PCT NO: |
PCT/JP2007/057478 |
371 Date: |
October 1, 2008 |
Current U.S.
Class: |
148/559 ;
148/400; 148/434; 148/95; 72/252.5 |
Current CPC
Class: |
C22C 9/04 20130101; C22F
1/08 20130101; C22F 1/00 20130101 |
Class at
Publication: |
148/559 ;
148/400; 148/95; 148/434; 72/252.5 |
International
Class: |
C22F 1/08 20060101
C22F001/08; C22C 9/04 20060101 C22C009/04; B21B 3/00 20060101
B21B003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 3, 2006 |
JP |
2006-102216 |
Apr 25, 2006 |
JP |
2006-120942 |
Claims
1. A material comprising: an ultrafine grained structure including
a metal or an alloy that has a stacking fault energy no greater
than 50 mJ/mm.sup.2, wherein structures that predominantly exist in
the material have twins included in a crystal structure, wherein
the twins have a twin interval no greater than 100 nm.
2. A material comprising: an ultrafine grained structure including
a metal or an alloy that has a stacking fault energy no greater
than 50 mJ/mm.sup.2, wherein structures that are predominant in the
material have recrystallized grains, wherein twin boundaries having
a crystal grain size ranging from 20 nm to 600 nm predominantly
exist in the recrystallized grains.
3. A fabrication method of the material having an ultrafine grained
structure as claimed in claim 1 comprising the steps of: providing
a metal or an alloy having a stacking fault energy no greater than
50 mJ/mm.sup.2; and introducing deformation twins having a twin
interval no greater than 100 nm into structures of the metal or the
alloy by processing the metal or the alloy.
4. The method as claimed in claim 3, wherein the step of
introducing the deformation twins includes a step of performing a
multidirectional forging process on the metal or the alloy at a
temperature no greater than room temperature.
5. The method as claimed in claim 4, wherein the step of performing
the multidirectional forming process includes a step of performing
a forging process on the metal or the alloy at a strain rate no
less than 1.times.10.sup.-4/s.
6. The method as claimed in claim 4, wherein the temperature no
greater than room temperature is no greater than an absolute
temperature of 223 K.
7. The method as claimed in claim 4, further comprising a step of
performing an annealing process on the multidirectional-forged
metal or alloy.
8. The method as claimed in claim 3, wherein the step of
introducing the deformation twins includes a step of performing a
rolling process on the metal or the alloy at a temperature no
greater than room temperature.
9. The method as claimed in claim 8, further comprising a step of
performing an annealing process on the rolled metal or alloy.
10. (canceled)
11. The method as claimed in claim 8, wherein the step of
performing the rolling process includes a step of rolling the metal
or the alloy at a rolling rate no less than 5.times.10.sup.-1
cm/s.
12. The method as claimed in claim 8, wherein the step of
performing the rolling process includes a step of rolling the metal
or the alloy so that a final draft becomes no less than 20%.
13. The method as claimed in claim 8, wherein the step of
performing the rolling process includes a step of performing a
rolling process on the metal or the alloy at a temperature no
greater than an absolute temperature of 223 K.
14. A material having an ultrafine grained structure comprising:
structures predominantly existing in a single grain, the structures
including a first packet having a group of layered twins oriented
substantially in a first direction, and a second packet including
at least one of the twins inside the first packet that has a group
of layered twins oriented substantially in a second direction,
wherein the first and second directions form an angle other than 60
degrees.
15. The material having an ultrafine grained structure as claimed
in claim 14, wherein the structures include a first structure, the
first structure including the first packet having a group of
layered twins oriented substantially in the first direction and the
second packet including at least one of the twins inside the first
packet that has a group of layered twins oriented substantially in
the second direction, a second structure including recrystallized
grains of the first packet, and a third structure including
recrystallized grains formed of a plurality of layered twins
arranged substantially in the same direction, wherein the first,
second, and third structures predominantly exist in the single
crystal grain.
16. (canceled)
17. The method as claim as claimed in claim 7, wherein the step of
performing the annealing process includes a step of performing an
annealing process on the metal or the alloy at a temperature no
greater than 0.5.times.Tm, wherein Tm is a melting point of the
metal or the alloy.
18. The method as claim as claimed in claim 9, wherein the step of
performing the annealing process includes a step of performing an
annealing process on the metal or the alloy at a temperature no
greater than 0.5.times.Tm, wherein Tm is a melting point of the
metal or the alloy.
19. The material having an ultrafine grained structure as claimed
in claim 14, wherein the material is brass.
20. The material having an ultrafine grained structure as claimed
in claim 15, wherein the material is brass.
Description
TECHNICAL FIELD
[0001] The present invention relates to a material having an
ultrafine grained structure and a method of fabricating
thereof.
BACKGROUND ART
[0002] As for methods of increasing mechanical strength of metals
or alloys, a method of forming crystal grains inside the structures
of a material into fine sizes is known. In addition, forming
crystal grains into fine sizes has an advantage of facilitating
processability of materials. Therefore, various studies on forming
crystal grains into fine sizes have been made in the past. The most
typical method of forming crystal grains into fine sizes is
referred to as a thermomechanical treatment. With this method,
crystal grains can be formed into fine sizes by thermally treating
a processed material under various conditions. For example, one
proposed method of forming crystal grains into fine sizes is
performed by recrystalling a hot-rolled copper alloy at a
temperature of approximately 300.degree. C. through 400.degree. C.
(see International Publication WO 2004/022805). Another proposed
method of forming crystal grains into fine sizes is performed by
annealing a hot-rolled or a cold-rolled iron type metal at a
temperature of approximately 750.degree. C. However, the smallest
crystal grain diameter which can be attained by using these methods
of thermally treating processed materials is at best approximately
1 .mu.m.
[0003] Recently, a method referred to as severe plastic deformation
has been drawing attention as a method for fabricating ultrafine
crystal grains A representative example of the severe plastic
deformation method is an ECAP (Equal Channel Angular Press) method
and an ARB (Accumulate Roll Bonding) method.
[0004] The ECAP method is performed by repeating the steps of
forcing a metal material (referred to as a "billet") through an
L-shaped die channel and pressing out the metal material from an
opening. This method enables crystal grains of the metal material
to be formed into ultra-fine sizes without changing the shape of
the metal material (see International Publication WO 2004/022805).
The ARB method is performed by repeating numerous times the steps
of rolling a sheet material to approximately 50%, cutting the sheet
in half, stacking the two halves, and rolling the stacked sheets.
By performing this series of steps, crystal grains of a material
can be formed into ultra-fine sizes.
Patent Document 1: International Publication No. WO 2004/022805
Patent Document 2: Japanese Laid-Open Patent Application No.
62-182219
Non-Patent Document 1: R. Z. Valiev, R. K. Islamgaliev, I. V.
Alexandrov, "Material Science" Vol. 45, p 103, 2000
Non-Patent Document 2: Nobuyasu Tsuji, "Iron and Steel", vol. 88,
p. 359-369, 2002
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0005] However, the ECAP method is unsuitable for industrial
fabrication of ultra-fine crystal grains since the method requires
many steps and is unable to manufacture very long materials.
Although the ARB method is able to obtain crystal grains having a
grain diameter of approximately 0.1 .mu.m in a sheet-thickness
direction the crystal grains are bulky with respect to an in-plane
direction of a rolled sheet material. Thus, a material having a
homogenous equiaxial ultra-fine grained structure cannot be
obtained. This leads to problems such as heterogeneous distribution
of strength of a material or inability to obtain a material having
a desired strength. This method also requires many steps and is
unsuitable for industrial production. Therefore, there is a demand
for an ultra-fine crystal grain fabrication method that can easily
form an equiaxial homogenous ultra-fine grained structure and
increase material strength.
[0006] In view of the problems described above, the present
invention provides a material having a high strength ultra-fine
grained structure and a method that can easily fabricate such
material.
Means of Solving the Problems
[0007] The present invention provides a material characterized by
having an ultrafine grained structure including a metal or an alloy
that has a stacking fault energy no greater than 50 mJ/mm.sup.2,
the material having twins included in a crystal structure, the
twins have a twin interval no greater than 200 nm.
[0008] By fabricating a material having such an ultrafine grained
structure, the maximum strength of the material can be
improved.
[0009] Throughout this application, it is to be noted that a
deformation twin is included in the term "twin".
[0010] Further, the present invention provides a material
characterized by having an ultrafine grained structure including a
metal or an alloy that has a stacking fault energy no greater than
50 mJ/mm.sup.2, the material including recrystallized grains having
a crystal grain size ranging from 20 nm to 600 nm. By fabricating
the structure of a material into such recrystallized structure, a
material having high strength and homogenous ultra-fine crystal
grains can be obtained.
[0011] Further, the present invention provides a fabrication method
of a material having an ultrafine grained structure characterized
by having: a step of providing a metal or an alloy having a
stacking fault energy no greater than 50 mJ/mm.sup.2; and a step of
introducing deformation twins having a twin interval no greater
than 200 nm into structures of the metal or the alloy by processing
the metal or the alloy. With this method, many deformation twins
can be introduced into the structures of the material, and crystal
grains can be formed into an ultrafine size by using the
intersection of the deformation twins.
[0012] Here, the step of introducing the deformation twins may
include a step of performing a multidirectional forging process
(hereinafter also referred to as "MDF process") on the metal or the
alloy at a temperature no greater than room temperature. In this
method, processing is simple. Thus, a material having an ultrafine
grained structure can be fabricated with this simple process.
[0013] Particularly, the multidirectional forging process may
include a step of performing a forging process on the metal or the
alloy at a strain rate no less than 1.times.10.sup.-4/s. An MDF
process at a high strain rate can increase deformation resistance
of the material. Thus, deformation twins can be easily introduced
into the material.
[0014] Alternatively or additionally, it is preferable for the
temperature no greater than room temperature to be no greater than
an absolute temperature of 223 K. An MDF process at such an
ultra-low temperature can easily increase deformation resistance of
a target process material. Thus, the same effect as increasing the
strain rate can be easily obtained. Therefore, a material having an
ultrafine grained structure can further easily be provided.
[0015] A step of performing an annealing process on the
multidirectional-forged metal or alloy may also be included.
Thereby, the structures of the material after forging can be made
homogenous.
[0016] In an alternative method, the step of introducing the
deformation twins may include a step of performing a rolling
process on the metal or the alloy at a temperature no greater than
room temperature. With this method, shear force can easily be
applied to the material, and high density deformation twins can be
relatively easily introduced into structures of the material.
Therefore, in a case of applying this method, crystal grains can be
formed into ultrafine size more easily than the MDF process.
[0017] A step of performing an annealing process on the rolled
metal or alloy may also be included. Thereby, the structures of the
material after forging can be made homogenous.
[0018] It is preferable for the step of performing the annealing
process to include a step of performing an annealing process on the
metal or the alloy at a temperature no greater than 0.5.times.Tm,
wherein Tm is a melting point of the metal or the alloy. By
performing the annealing process at this temperature, the
structures can be made homogenous without causing the ultrafine
grains obtained after forging or rolling to become bulky.
[0019] The step of performing the rolling process may include a
step of rolling the metal or the alloy at a rolling rate no less
than 5.times.10.sup.-1 cm/s. Because deformation resistance can be
increased by increasing the rolling rate, many deformation twins
can be introduced into the structures of the material.
[0020] The step of performing the rolling process may include a
step of rolling the metal or the alloy so that a final draft
becomes no less than 20%. Because deformation resistance can be
increased by increasing the draft, many deformation twins can be
introduced into the structures of the material.
[0021] Particularly, it is preferable for the step of performing
the rolling process to include a step of performing a rolling
process on the metal or the alloy at a temperature no greater than
an absolute temperature of 223 K. By performing the rolling process
a such an ultra-low temperature, the deformation resistance of the
material can be increased. Therefore, many deformation twins can be
introduced into the structures of the material without increasing
the rolling rate and/or the draft during rolling. Thus, a material
having an ultrafine grained structure can be provided more
easily.
[0022] Further, the present invention provides a material having an
ultrafine grained structure characterized by including: a first
packet having a group of layered twins oriented substantially in a
first direction, and a second packet including at least one of the
twins inside the first packet that has a group of layered twins
oriented substantially in a second direction, the first and second
directions forming an angle other than 60 degrees. Here, a "packet"
refers to a group of layered twins arranged towards the same
crystal orientation as described below.
[0023] Further, the present invention provides a material having an
ultrafine grained structure characterized by having: a first
structure, the first structure including a first packet having a
plurality of groups of layered twins oriented substantially in a
first direction and a second packet including at least one of the
twins inside the first packet that has a group of layered twins
oriented substantially in a second direction; a second structure
including recrystallized grains of the first packet; and a third
structure including recrystallized grains formed of a plurality of
layered twins arranged substantially in the same direction that are
included in a single crystal grain.
[0024] The material having the ultrafine grained structure may be
brass.
EFFECTS OF THE INVENTION
[0025] According to the present invention, a high strength material
having an ultrafine grained structure is provided. Further, it is
possible to relatively easily obtain such a material having an
ultrafine grained structure.
BRIEF DESCRIPTION OF DRAWINGS
[0026] FIG. 1 is a schematic diagram showing an example of a
material having an ultrafine grained structure according to the
present invention;
[0027] FIG. 2 is a schematic diagram for describing an MDF process
method;
[0028] FIG. 3 illustrates a stress-strain curve for a copper-based
alloy material containing 30 mass % zinc fabricated by a method of
forming ultrafine grain crystals (MDF process method) of the
present invention according to the present invention;
[0029] FIG. 4 illustrates TEM photographs showing structures of a
copper-based alloy sample containing 30 mass % zinc after being
subject to an MDF process at 77 K (a) and 300 K (b), respectively,
and a schematic diagram showing an enlarged view of a packet
portion of the photograph (b);
[0030] FIG. 5 illustrates a structure diagram (OIM map diagram) of
a copper-based alloy sample containing 30 mass % zinc annealed for
8 hours at 503 K after an ultra-low temperature process according
to a crystal orientation distribution analyzing apparatus;
[0031] FIG. 6 is schematic diagram showing an example of a
configuration of an apparatus used for performing a method of
forming ultrafine grain crystals according to the present
invention;
[0032] FIG. 7 illustrates an TEN observation image of a structure
of a copper-based alloy containing 30 mass % zinc after being MDF
processed with a true strain rate of 1.times.10.sup.-3 at a
temperature of 77 K.;
[0033] FIG. 8 is a diagram showing changes of Vicker's hardness of
a copper-based alloy material containing 30 mass % zinc annealed at
temperatures of 503 K, 523 K, and 543 K after the material is
rolled 60% at a temperature of 77 K.;
[0034] FIG. 9 are photographs of a copper-based alloy containing 30
mass % zinc annealed for 1000 seconds at a temperature of 523 K
after being rolled 60% at an ultra-low temperature of 77 K, in
which (b) is a magnified view of (a);
[0035] FIG. 10 shows a relationship between anneal time and a mean
crystal grain size of a structure of a material in a case where a
copper-based alloy material containing 30 mass % zinc is annealed
at temperatures 503 K and 523 K after the material is rolled 60% at
a temperature of 77 K; and
[0036] FIG. 11 shows a stress-strain curve of copper at room
temperature after being MDF processed at an ultra-low temperature
of 77 K and at room temperature (300 K).
DESCRIPTION OF THE REFERENCE NUMERALS
[0037] Apparatus [0038] 20 Roller [0039] 30 Conveying base [0040]
40 Material [0041] 50 Ultra-low temperature tank [0042] 100
Ultrafine grained structure [0043] 110 Twin [0044] 120 Packet
BEST MODE FOR CARRYING OUT THE INVENTION
[0045] According to the present inventions a material having an
ultrafine grained structure is fabricated by a method characterized
by the steps of providing a metal or an alloy having a stacking
fault energy that is no greater than 50 mJ/mm.sup.2, and
introducing deformation twins having a twin interval no greater
than 200 nm into the structures of the metal or alloy.
[0046] Throughout this application, the term "ultrafine grained
structure" refers to a structure having a "mean crystal grain size"
less than 1 .mu.m. The mean crystal grain size of a deformed
structure is measured by using cross-sectional images obtained from
a Transmission Electron Microscope (TEM). The mean crystal grain
size of annealed structures is measured by calculating a mean
crystal grain area in a case where structures having a boundary
with a misorientation no less than 3 degrees is assumed as a
crystal grain based on a crystal orientation distribution image
obtained from Orientation Imaging Microscopy (OIM).
[0047] Conventionally, it is considered that a twin does not
contribute to the mechanical strength of a material and regarded
that twins should be eliminated from crystal structures in order to
improve the mechanical strength of the material. In contrast,
according to an aspect of the present invention, crystal grains are
formed into an ultrafine size by affirmatively introducing twins
into structures of a material, to thereby improve the strength of
the material. More specifically, according to an embodiment of the
present invention, crystal grains are formed into ultrafine size by
introducing many deformation twins into the structures of a target
process material by performing a process accompanying a large
deformation on the target process material and intersecting the
deformation twins. According to studies by the inventor of the
present invention, with this process, mechanical strength of a
material significantly improves, particularly in a case % here an
ultrafine grained structure including deformation twins having twin
intervals no greater than 200 nm.
[0048] Here, the term "twin interval" refers to the distance "D"
indicated with arrows according to an ultrafine grained structure
100 having plural twins 110 as depicted in FIG. 1, that is, the
space between a twin line situated within one of the twins 110 and
another twin line adjacent to the twin line. This twin interval
cannot be measured by a normal optical microscope. Therefore,
according to an embodiment of the present invention, a twin
interval of a twin in an ultrafine grained structure of a material
is measured by a structural image obtained from a transmission
electron microscope at a magnification of approximately 8000-80000
times.
[0049] The processing method is not limited to that described
above. Other processing methods may be used for introducing a large
number of deformation twins into a material.
[0050] For example, as one processing method for introducing a
large number of deformation twins, there is a Multi-directional
forging (MDF) method. This method, which is also referred to as a
multi-axial forging method, is performed by repeating the steps of
applying compressive force with respect to a forging direction and
changing the forging direction so that the longitudinal axis of the
material is oriented in the compressing direction.
[0051] A detailed example of the MDF processing method is described
with reference to FIG. 2. FIG. 2 is a schematic diagram of a MDF
processing method. First, as depicted in (1) of FIG. 2, a bulk
material having an aspect ratio corresponding to rectangular shape
is prepared. The aspect ratio of the bulk material is determined
according to a compression rate obtained by a forging operation in
each axis direction (each forging operation is referred to as a
"pass") as illustrated in (2) through (4) of FIG. 2. In other
words, the aspect ratio of the bulk material can be changed
according to the compression rate of each pass that is used. In the
example shown in FIG. 2, the process strain of a single pass is
0.4. Deformation twins appear more easily when the process strain
of a single pass is increased since deformation resistance becomes
larger. For example, in a case where the strain that can be
introduced into the material is 0.8 (in this case, the aspect ratio
is 1.0:1.49:2.22), a process of three passes is theoretically
necessary in order to attain a total strain of 2.4.
[0052] With this MDF process, a large number of deformation twins
can be introduced into a material. Further, the crystal grains of
the material can be formed into ultrafine sizes by the intersection
among the introduced deformation twins. Particularly, since
compression of the material is performed from multiple directions
with the MDF process, an equiaxial ultrafine grained structure can
be obtained after performing the process. Further, this process
enables forming a material having an ultrafine grained structure by
a simple operation of repeating the steps of forging and changing
the compressing axis. Therefore, a material having an ultrafine
grained structure can be easily produced.
[0053] In general, in order to introduce a large number of
deformation twins into a structure of a material at a single time,
it is preferable to apply deformation to a material in a state
where the materials resistance against deformation (hereinafter
referred to as "deformation resistance") is large. Therefore, in
order to increase the deformation resistance, it is preferable to
make the temperature of the material during the MDF process as low
as possible and to make the strain to be applied to the material at
a single time as large as possible.
[0054] From these aspects, in a case of using the MDF process as a
method of introducing deformation twins, it is preferable to
perform the MDF process at a temperature no greater than room
temperature (300 K). This increases deformation resistance when
performing the MDF process and allows more deformation twins to be
introduced into a material at a single time. Further, in a case of
performing the MDF process at an "ultralow temperature" no greater
than 223 K (absolute temperature), it is preferable to select a
processing rate so that the strain rate becomes approximately no
less than 1.times.10.sup.-4/s. In a case other than such a case, it
is preferable to select a processing rate so that the strain rate
becomes approximately 5.times.10.sup.-4/s. By selecting a large
strain rate, the amount of deformation applied to a material for
each pass can be increased and deformation resistance can be
raised. Therefore, even in a case where the temperature at which
the process is performed is high (approximately room temperature),
a large number of deformation twins can be introduced into the
structures of a material.
[0055] Further, in a case industrially producing a material having
an ultrafine grained structure by performing the MDF processing
method, it is preferable to automatically rotate a target process
material each time a process is performed on the target process
material so that the longitudinal axis of the target process
material is oriented in the compressing direction. This operation
can easily be performed by using a motorized type or a mechanical
type target process material position (or orientation) controlling
part such as a manipulator. Thereby, the burden of changing the
orientation of the target process material for each pass can be
resolved.
[0056] Through these steps, a material having an ultrafine grained
structure including many twins 110 can be obtained as exemplarily
illustrated in FIG. 1. In the example of FIG. 1, the mean crystal
grain size ranges from a size of approximately 500 nm to a size at
most no greater than 1 .mu.m, and the twin interval ranges from
approximately 80 to 100 nm.
[0057] Although such deformation twins inside a single grain tend
to be arranged in parallel towards a single direction, deformation
twins oriented in another direction appear when the processing
direction of the sample is changed and the sample is forged again,
such that the deformation twins disconnect from each other, to
thereby create further ultrafine crystal grains. The deformation
twins exhibit misorientation of approximately 60 degrees formed by
two crystals that sandwich the twin surface. By the generation of
the deformation twins as well as the intersecting and disconnecting
of the deformation twins, an ultrafine grained structure having its
grain boundaries exhibiting high misorientation can be easily
fabricated.
[0058] It is to be noted that an annealing process may be performed
on the forged material. The ultrafine grained structure including
many deformation twins generated by the deformation can be made
homogeneous by performing the annealing process. It is preferable
to perform the annealing process at a temperature as low possible.
This is because there is a possibility that growth of ultrafine
crystal grains is accelerated when the process temperature becomes
high. Particularly, in a case where the melting point of the forged
material is expressed as Tm (K), it is preferable that the
temperature of the annealing process be no greater than 0.5 Tm. For
example, in a case where a copper-based alloy containing 30 mass %
zinc is used, the melting point of the alloy is 1223 K. Therefore,
the process temperature is set no greater than 611 K.
[0059] FIG. 3 illustrates an example of a stress-strain curve for
materials having an ultrafine grained structure fabricated by a
method of the present invention at room temperature. The samples
used in this example are fabricated by introducing a strain of 0.4
(1 pass), 2.4 (6 passes), and 6.0 (15 passes) to a copper-based
alloy containing 30 mass % zinc. FIG. 3 shows results of the
samples fabricated by performing an MDF process at room temperature
(300 K). The maximum strength of a typical material which is not
subject to a process of forming ultrafine crystal grains is
approximately 500 MPa. On the other hand, a sample subject to the
MDF process at a temperature of 77 K exhibits an increased maximum
strength ranging from 600 Mpa to 900 MPa. Even a sample subjected
to the MDF process at a temperature of 300 K exhibits an increased
maximum strength ranging from 700 MPa to 800 MPa except for a
sample to which a strain of 0.4 (1 pass) is introduced.
[0060] FIG. 4 shows photographs of the structures of samples after
an MDF process. The samples are fabricated by performing the MDF
process where a strain of 6.0 (15 passes) is introduced to a
copper-based alloy containing 30 mass % zinc. The photograph (a) on
the left side of FIG. 4 shows a structure after performing the MDF
process at 77 K and the photograph (b) at the center of FIG. 4
shows a structure after performing the MDF process at 300 K.
Further, the schematic diagram on the right side of FIG. 4 is an
enlarged view showing a part of the photograph at the center of
FIG. 4.
[0061] Particularly, as shown in (c) of the drawing, a group of
layered twins arranged in the same crystal orientation is formed in
a crystal grain of a sample to which the MDF process is performed.
In the present application, this group of layered twins is referred
to as a packet (packet 120). When observing each twin in the packet
120 in further details a second packet including a group of smaller
layered twins is formed in a single twin. It can be understood that
each of the twins included in the second packet is arranged in the
same crystal orientation. It is believed that this structure is
formed by dividing the twins in the packet 120 into a group of
smaller twins by repeating a deformation process using the MDF
process. According to this observation, whenever a deformation
process is performed, the twins included in a group of twins in a
single packet are divided into a group of finer layered twins.
Accordingly, packets as well as their crystal grains can be formed
into a fine size, to thereby form an ultrafine grained
structure.
[0062] Typically, in a case of introducing plural twin groups into
a structure by using, for example, thermomechanical treatment, the
misorientation formed by the group of twins is 60 degrees. However,
in a case of twins introduced by the MDF process, a packet, which
exists before a deformation process is performed, becomes affected
by repetition of the deformation process, to thereby cause crystal
rotation of a group of twins in the packet. Therefore, the
misorientation between a group of twins of a first packet generated
by a first deformation process and a group of twins of a second
packet generated by a second deformation process form an angle
other than 60 degrees.
[0063] Therefore, by applying a method of fabricating a material
having an ultrafine grained structure according to an embodiment of
the present invention to a target process material, its crystal
grains can be formed into ultrafine size and strength of the
material can be improved.
[0064] Further, as another method of introducing many deformation
twins, there is a roll process method. In order to introduce many
deformation twins into a material, it is preferable to deform the
material with shear. With the roll process method, shear strain can
be easily applied to a material and high density deformed crystals
can be relatively easily introduced in the material structure.
Therefore, in a case of applying this method, crystal grains can be
formed in an ultrafine size with a method easier than the MDF
process method. Further, the roll process method is not confined by
the size of the target process material. For example, the method
can be applied to a large size material such as a board material
having a large area.
[0065] As for process conditions when performing the roll process
on a material, it is preferable to use one of an "ultra low
temperature process", a "low temperature-high rate process", a "low
temperature-high pressure process" or a "high rate-high pressure
process". Here, the "ultra low temperature process" refers to a
method of rolling a target process material while maintaining the
temperature of the target process material at an "ultra low
temperature" no greater than 223 K (absolute temperature). As
described above, in order to introduce many deformation twins in a
single time, it is preferable to make the temperature of the
material as low as possible and to make the strain to be applied to
the material at a single time as large as possible, so that
deformation resistance can be increased. In the "ultra low
temperature process", deformation resistance can be increased
regardless of other process parameters, because a roll process is
performed on a material having its temperature lowered to an ultra
low temperature no greater than 223 K. In other words, with this
condition of maintaining a material at an ultra low temperature, a
deformation resistance sufficient for forming many deformation
twins can be obtained. Therefore, unlike the other process
conditions described below, precise control of other parameters
(e.g., deformation rate) is unnecessary.
[0066] Further, the "low temperature-high rate process" and the
"low temperature-high pressure" are methods of rolling a target
roll material while maintaining the temperature of the target roll
material at approximately 223-300K (room temperature). Unlike the
"ultra low temperature process", it is difficult to introduce high
density deformation twins merely by performing a rolling process at
this temperature. Therefore, with these conditions, high density
deformation twins are introduced in a target roll material by
combining with a parameter that increases the amount of deformation
that can be applied at a single time. For example, in a case of the
"low temperature-high rate process", deformation resistance is
increased by applying strain to a target roll material at a high
rate, to thereby introduce many deformation twins in the target
roll material. In order to apply strain to a material at a high
rate, the rolling rate is to be greater than that of the "ultra low
temperature process" and is preferred to be at least
5.times.10.sup.-1 cm/s. In a case of the "low temperature-high
pressure process", since deformation twins can be generated more
easily as the amount of strain increases, draft of the target roll
material is increased by processing the target roll material under
a high pressure rolling condition, to thereby introduce high
density deformation twins. In this case, it is preferable for the
final draft of the material to be no less than 20%. Further, from
the aspect of evenly distributing the deformation twins, it is
preferable for the final draft of the material to be no less than
60%.
[0067] Further, the "high rate-high pressure process" is a method
of rolling a target roll material in a non-low temperature range
such as room temperature. The deformation resistance of the target
roll material is increased by combining a high rate process and a
high pressure process, to thereby introduce high density
deformation twins. For example, in a typical case of rolling at
room temperature, deformation twins appear by setting the rolling
rate to 5.times.10.sup.-1 cm/s and the draft to 70% or more.
[0068] It is to be noted that, among these rolling process methods,
the "ultra low temperature process" is preferred. This is because
the other methods need to apply a large strain or pressure
instantaneously to the material and require a specialized large
size apparatus, which thereby leads to a problem where a process
cannot be performed with a standard apparatus. This is also due to
difficulty in applying a large strain instantaneously depending on
the material such as a material having high ductility.
Nevertheless, other than the constraints of the apparatus or the
material, deformation twins can be introduced with any of the
process conditions.
[0069] It is to be noted that an annealing process may be performed
on the forged material. The ultrafine grained structure including
many deformation twins generated by the deformation can be made
homogenous by performing the annealing process. It is preferable to
perform the annealing process at a temperature as low possible.
This is because there is a possibility that growth in the size of
ultrafine crystal grains is accelerated when the process
temperature becomes high. Particularly, in a case where the melting
point of the rolled material is expressed as Tm (K), it is
preferable that the temperature of the annealing process be no
greater than 0.5 Tm. For example, in a case where a copper-based
alloy containing 30 mass % zinc is used, the melting point of the
alloy is 1223 K. Therefore, the process temperature is set no
greater than 611 K.
[0070] By this annealing process, a substantially homogenous
ultrafine grained structure having a mean grain size ranging from
approximately 20 nm to 600 nm can be obtained as shown in FIG. 5.
FIG. 5 is a map diagram according to Orientation Imaging Microscopy
(OIM) showing a sample in which a copper-based alloy containing 30
mass % zinc is rolled at an ultra low temperature of 77K (60%
drift) and then annealed for 8 hours at a temperature of 503 K.
According to the diagram, it is to be noted that the ultrafine
grained structure fabricated by a method according to an embodiment
of the present invention exhibits little progression of grain
growth even after the annealing process. It is believed that the
deformation twins oriented in different directions inside a
structure confine the crystal grains and prevent the grain size
growth. Therefore, the material having the ultrafine grained
structure fabricated by the method according to an embodiment of
the present invention has a significant characteristic of having
satisfactory thermal stability.
[0071] It is to be noted that, according to research results of the
inventor of the present invention, there is a tendency that the
ultrafine grained structure containing many deformation twins
having a twin interval no greater than 200 nm (as shown in FIG. 1)
is easier to obtain as the stacking fault energy of a metal or an
alloy is smaller. This is because, with a metal or an alloy having
a large stacking fault energy, it is difficult to apply stress to a
material beyond its critical stress for generating deformation
twins since dislocation density of the material does not easily
increase even if a process such as an MDF process is performed.
Therefore, it is preferable to apply the present invention to a
metal or an alloy having a stacking fault energy no greater than 50
mJ/mm.sup.2. The metal or the alloy having a low stacking fault
energy may be, for example, silver (stacking fault energy of
approximately 22 mJ/mm.sup.2), copper (78 mJ/mm.sup.2), cobalt (15
mJ/mm.sup.2), nickel (128 mJ/mm.sup.2), brass (approximately 20
mJ/mm.sup.2), and stainless steel (211 mJ/mm.sup.2). It is to be
noted that "brass" is a copper based alloy containing 20 mass %
zinc (the stacking fault energy described above is a value of a
copper-based alloy containing 30 mass % zinc). Even in a case where
the stacking fault energy of the metal or the alloy itself is
greater than 50 mJ/mm.sup.2, the stacking fault energy can be
significantly reduced by adding one or more impure elements to the
metal or the alloy. For example, even with the above-described
metal or the alloy as well as other alloys having a stacking fault
energy no less than 50 mJ/mm.sup.2, it may fall within the range of
the present invention by adding an impure element thereto.
[0072] Next, an example of a method of fabricating a material
having an ultrafine grained structure according to an embodiment of
the present invention is described. The following example is
described where the processing method for introducing deformation
twins in the structure of a material is a method using a rolling
process under the condition "ultra low temperature process".
[0073] FIG. 6 schematically shows an example of a rolling apparatus
for performing the method (rolling process under the condition of
ultra low temperature) according to an embodiment of the present
invention. With the method according to this embodiments a rolling
apparatus 10 includes an ultra-low temperature tank 50, a conveying
apparatus 30, and a pair of rollers 20. The conveying apparatus 30
is used for guiding a target roll material 40 towards the rollers
20. The ultra-low temperature tank 50 is used for cooling the
target roll material 40 beforehand. The temperature of the
ultra-low temperature tank 50 is no greater than 223 K. It is,
however, preferable for it to be no greater than liquid-nitrogen
temperature (77 K). In an alternative rolling apparatus 10, the
ultra-low temperature tank 50 may be removed or have its position
changed. What is important is that the target roll material 40 is
to be cooled at the above-described temperature immediately before
traveling through the rollers 20. For example, the apparatus may be
configured having a cooling tank provided at the middle of a
conveying path, so that the target roll material 40 can pass
through the cooling tank before being conveyed to the rollers 20
and have its corresponding roll part cooled to the above-described
temperature.
[0074] By performing the following steps, the rolling apparatus 10
can introduce deformation twins into the target roll material 40
such as a copper-based metal containing 30 mass % zinc. First, the
target roll material 40, which is cooled beforehand in the
ultra-low temperature tank 50, is placed on the conveying apparatus
30. Then, the conveying apparatus 30 is activated to convey the
target roll material 40 in the direction of the rollers 20. The
target roll material 40, when conveyed to the position of the
rollers 20, is rolled by the rollers 20. Although it is preferable
that the conveying rate (rolling rate) of the target roll material
40 to be no less than approximately 1.times.10.sup.-1 cm/s., it is
not so limited. Further, although it is preferable that the drift
for a single pass be approximately 10-20%, it is not so limited. As
described above, under the condition of the ultra-low temperature
process, the draft and the conveying rate does not have a large
effect to the density of the generated deformation twins.
[0075] As these steps are repeated for a necessary number of times
(passes), many deformation twins are introduced inside the
material. It is preferable to cool the target roll material 40
again, each time a single pass of the rolling process is performed
on the target roll material 40. This is to keep the target roll
material 40 at a temperature suitable for generating deformation
twins when repeating the rolling on the target roll material 40
since the rolling process causes the temperature of the target roll
material 40 to increase. However, in a case where the temperature
of the target roll material 40 can be maintained at a suitable
ultra-low temperature range (e.g., a case where the entire rolling
apparatus 10 is kept in a low temperature atmosphere), it is
possible to repeat two or a few passes of the rolling process when
the temperature of the target roll material 40 is within a
predetermined range.
[0076] With the roll process method using the above-described
apparatus, many deformation twins can be easily introduced because
the target roll material 40 is maintained at an ultra-low
temperature to have a sufficiently large deformation resistance.
After the process, a material having an ultrafine grained structure
can be obtained.
[0077] It is, however, possible to introduce deformation twins into
the target roll material 40 at a non-ultra-low temperature such as
room temperature (high rate-high pressure process). In this case,
there is an advantage that the ultra-low temperature tank 50 of the
above-described apparatus is unnecessary. Nevertheless, there is a
need to devise a method of increasing deformation resistance such
as by increasing the conveying rate of the target roll material 40
or increasing the draft during the rolling process. For example,
with a high rate-high pressure process, the conveying rate of the
target roll material is 5.times.10.sup.-1 cm/s., and the final
draft after the rolling process is no less than 70%. Even with this
rolling process, an ultrafine grained structure having homogenous
and high density deformation twins can be obtained.
[0078] It is to be noted that an annealing process may be performed
on the rolled material. As described above, it is preferable to
perform the annealing process at a temperature as low possible. For
example, in a case where the melting point of the material is
expressed as Tm, with an annealing process performed at a
temperature no greater than 0.5.times.Tm, an ultrafine grained
structure having recrystallized grains with size ranging from 20 nm
to 600 nm can be realized.
[0079] Next, evaluation experiment results of a material having an
ultrafine grained structure obtained by applying the method of the
present invention are described.
EXPERIMENT 1
[0080] In experiment 1, the MDF process was performed on a sample
material and the strength of the processed material was evaluated.
A copper based alloy containing 30 mass % zinc and having a
stacking fault energy of 20 mJ/mm.sup.2 was used as the material of
experiment 1.
[0081] FIG. 7 shows an image of a structure of a copper based alloy
containing 30 mass % zinc after being MDF processed with a true
strain rate of 1.times.10.sup.-3 at a temperature of 77 K. This
process performs the MDF process on the material for 6 passes and
the amount of cumulative strain introduced to the material is 2.4.
From this image, it can be understood that many deformation twins
are introduced into the material by performing the MDF process and
that an ultrafine grained structure having a mean grain size no
greater than 1 .mu.m is formed by having the deformation twins
intersecting each other. In the selected area diffraction regarding
the 1 .mu.m area encompassed by a circle of the image, a hollow
ring (a phenomenon in which diffraction spots are connected to form
a ring-like shape) appears. Normally, in a case where many crystals
are oriented in different directions within a structure,
diffraction spots corresponding to each of its grains are arranged
extremely close to each other, to thereby create the hollow ring.
Thus, from these results, it can be understood that many ultrafine
crystal grains are contained inside the structure of the material
obtained by the method of the present invention.
[0082] FIG. 3 illustrates a stress-strain curve of samples obtained
at room temperature after the samples are subjected to the MDF
process at temperatures 77 K and 300 K (room temperature). Strains
of 0.4, 2.4, and 6.0 are introduced to the materials of the samples
by performing the MDF process for 1 pass, 6 passes, and 15 passes,
respectively. In a case of a sample obtained by performing the MDF
process at an ultra-low temperature of 77 K, the maximum strength
increases from 600 MPa to 900 MPa when the amount of strain is
increased from 0.4 to 6.0 as shown in the upper part of FIG. 3.
Each of the samples exhibited an elongation of approximately 20%.
The maximum strength of the same material but using a conventional
thermal mechanical treatment method is normally approximately 500
MPa. Therefore, from these results, it can be understood that
strength of a material significantly improves by applying the MDF
process method. Further, the elongation of the same material but
using a conventional method of forming crystals having ultrafine
grain size (e.g., ECAP method) is normally approximately 10%.
Therefore, it can be understood that elongation can be improved by
applying a method of forming crystals having an ultrafine grain
size according to an embodiment of the present invention.
[0083] In a case of a sample obtained by performing the MDF process
at 300 K (room temperature), the maximum strength increases from
500 MPa to 800 MPa when the amount of strain is increased from 0.4
to 6.0 as shown in the lower part of FIG. 3. This shows a tendency
of strength improvement by increasing the amount of strain even in
a case where the MDF process is performed at room temperature of
300 K. However, in performing the MDF process at room temperatures
it is regarded to be more difficult to increase deformation
resistance of the material compared to performing the MDF process
at the ultra-low temperature process of 77 K because the number of
deformation twins contained in a structure is smaller when compared
with the same number of passes. However, according to the results
of TEM observation with respect to a sample being MDF processed
until the cumulative strain is 6.0 (maximum strength of
approximately 800 MPa), homogenous ultrafine crystal grains can be
formed even with a process performed at room temperature. Each of
the three samples, which were fabricated with the above-described
process conditions, exhibited an expansion of approximately 20%.
This is an improvement compared to that of a conventional material
(10%).
EXPERIMENT 2
[0084] In experiment 2, the status and stability of a process
structure is evaluated in a case of using the same material
(copper-based alloy containing 30 mass % zinc) as experiment 1 and
performing an ultra-low temperature rolling process on the
material. Firsts an ultra-low temperature rolling process using the
apparatus shown in FIG. 6 is performed on this material. The
temperature of the material during rolling is 77 K. The draft
corresponding to each pass of the rolling process ranges from 10%
to 20%. The structure of the material after a 60% rolling was
observed by using a Transmission Electron Microscope (TEM). As a
result, it is revealed that the same structure as that of FIG. 1
having ultrafine crystal grains having a grain size ranging from
approximately 500 nm but less than 1 .mu.m can be obtained. It is
also revealed that many deformation twins are included in a crystal
grain and that the twin interval between the deformation twins
ranges from approximately 80-100 .mu.m.
[0085] Next, changes of structure are examined in a case where a
material rolled 60% at a temperature of 77 K is annealed with
respect to each temperature. FIG. 8 is a diagram showing changes of
Vicker's hardness of a material annealed at temperatures of 503 K,
523 K, and 543 K after the material is rolled 60% at a temperature
of 77 K. From this diagram, although hardness may also depend on
annealing temperature, it is understood that hardness steeply drops
after 10.sup.3 through 10.sup.4 seconds and that static
recrystallization occurs after this period.
[0086] In FIG. 9 are TEM photographs of a sample of a copper-based
alloy containing 30 mass % zinc annealed for 1000 seconds at a
temperature of 523 K after being rolled (60% draft) at an ultra-low
temperature of 77 K. The photograph on the right side is a
magnified photograph of the photograph on the left side. At the
stage where the material is annealed for 1000 seconds at 523 K, it
is revealed that, although some "packets" generated by the
deformation still remain, static recrystallization is already
started in which a part of the "packets" is recrystallized.
Further, it is revealed that recrystallized grains at a portion
inside a crystal grain, which are generated by plural layered
deformation twins, are arranged in the same direction. The grain
size of the recrystallized grains is approximately 20 nm.
Therefore, it is believed that the deformation twins contained in
the recrystallization grains have a grain size significantly
smaller than 20 nm.
[0087] Accordingly, an ultrafine grained structure containing
"packets", recrystallized grains of the packets, and other
recrystallized grains having plural layered deformation twins
arranged in the same direction are fabricated by rolling a
copper-based alloy containing 30 mass % zinc at an ultra-low
temperature and annealing the rolled material.
[0088] FIG. 10 shows a relationship between anneal time and a mean
crystal grain size of a structure of a material in a case where the
material is annealed at temperatures 503 K and 523 K after the
material is rolled 60% at a temperature of 77 K. The mean crystal
grain size corresponding to data indicated with a bracket in FIG.
10 (anneal time=100 s) is calculated by obtaining the simple
average of the interval and the length of the deformation twins (as
shown in "D" and "L" of FIG. 1) because the grains are too fine to
identify. The mean crystal grain size corresponding to other data
is calculated by using photographs of a transmission electron
microscope and performing a cross-sectional method assuming that
structures having boundaries are crystal grains. Because
deformation twins contained in the structure have a high
misorientation angle no less than 60 degrees, it is evident that
these structures are crystal grains surrounded by high
misorientation boundaries. Therefore, structures having boundaries
are assumed as crystal grains. It is to be noted that fine
deformation twins developed inside the crystal grains are used for
calculation. The actual crystal grain sizes are smaller than those
shown in FIG. 10. From FIG. 10, it can be understood that, even
after the annealing process, the crystal grains hardly become bulky
and are approximately 0.6 .mu.m at most.
[0089] FIG. 5 shows an example of a structure in a case where a
copper-based alloy containing 30 mass % zinc is rolled at a
temperature of 77 K and then annealed for 8 hours at a temperature
of 503 K. Observation of this example is performed using an
Orientation Imaging Microscopy (OIM) apparatus. From FIG. 5, it is
understood that a substantially homogenous ultrafine crystal grains
having a mean grain size of approximately 500 nm by the annealing
process. The mean crystal grain size was calculated from mean
crystal grain area where structures having boundaries with a
misorientation angle no less than 3 degrees are assumed as crystal
grains. Almost all of the structures had boundaries of high
misorientation angles no less than 15 degrees. From this diagram,
it is understood that crystal grains do not become bulky even after
a long annealing time of 8 hours. From these results, it is
understood that a homogenous ultrafine grained structure can be
obtained by performing the annealing process on structures into
which a large amount of deformation twins are introduced. It is
also understood that the obtained structure is extremely thermally
stable and difficult to become bulky. This is because growth of
grains are restrained by forming crystal grains into an ultrafine
size by introducing many twins into the structures of a material
and generating many twins having different orientations.
EXPERIMENT EXAMPLE 3
[0090] Next, a similar experiment was performed on copper (stacking
fault energy of 78 mJ/mm.sup.2) for comparison. FIG. 11 shows a
stress-strain curve of copper at room temperature after being MDF
processed at an ultra-low temperature of 77 K and at room
temperature (300 K). The horizontal axis indicates cumulative
strain. In a case where the MDF process was performed on copper at
300 K, the maximum stress becomes 380 Mpa when accumulative stress
strain becomes greater than 2. After that, the maximum stress did
not change where severe plastic deformation was performed until a
cumulative strain of 6. According to the results of observing the
structure of the experimented sample, hardly any deformation twins
were generated. On the other hand, with a sample on which the MDF
process was performed at a temperature of 77 K, the maximum stress
was 590 Mpa where the cumulative strain was 2. It had been found
that a few deformation twins were generated at some portions by
observing the structure of the sample to which the cumulative
strain of 2 was applied. However, the deformation twins were
heterogenous and could not homogenously attain an ultrafine grained
structure as the above-described copper-based alloy containing 30
mass % zinc. From this result, it can be understood that, even if
deformation stress is large, deformation twins are difficult to
obtain in a case of a material having a large stacking fault
energy. This is because materials have their own unique critical
stress for generating deformation twins with a high percentage.
That is, in order to generate many deformation twins in a structure
of a material by processing the material, the stress applied to the
material is to surpass its critical stress (a critical stress of
approximately 400-600 Mpa is estimated in a case of copper). In a
case of copper having high stacking fault energy, the stress
applied to the material cannot surpass its critical stress even it
a process such as the MDF process is performed on the material due
to difficulty in increasing its dislocation density. Thus, it is
anticipated that deformation twins as many as the copper-based
alloy containing 30 mass % zinc cannnot be generated with the same
process conditions.
[0091] The present application is based on Japanese Priority
Application Nos. 2006-102216 and 2006-120942 filed on Apr. 3, 2006
and Apr. 25, 2006, respectively, with the Japanese Patent Office,
the entire contents of which are hereby incorporated by
reference.
* * * * *