U.S. patent application number 12/630128 was filed with the patent office on 2010-06-10 for two-way shape-recovery alloy.
This patent application is currently assigned to DAIDO TOKUSHUKO KABUSHIKI KAISHA. Invention is credited to Kozo OZAKI.
Application Number | 20100139813 12/630128 |
Document ID | / |
Family ID | 42045204 |
Filed Date | 2010-06-10 |
United States Patent
Application |
20100139813 |
Kind Code |
A1 |
OZAKI; Kozo |
June 10, 2010 |
TWO-WAY SHAPE-RECOVERY ALLOY
Abstract
The present invention provides a two-way shape-recovery alloy,
which contains less than 0.20 mass % of C, 13.00 to 30.00 mass % of
Mn, 0.10 to 6.00 mass % of Si, 0.05 to 12.00 mass % of Cr, 0.01 to
3.00 mass % of Ni, and less than 0.100 mass % of N, with the
remainder being Fe and unavoidable impurities, in which the
contents of Mn, Si, Cr and Ni satisfy the following expression (1):
600.ltoreq.33Mn+11Si+28Cr+17Ni.ltoreq.1050 (1).
Inventors: |
OZAKI; Kozo; (Nagoya-shi,
JP) |
Correspondence
Address: |
BACON & THOMAS, PLLC
625 SLATERS LANE, FOURTH FLOOR
ALEXANDRIA
VA
22314-1176
US
|
Assignee: |
DAIDO TOKUSHUKO KABUSHIKI
KAISHA
Nagoya
JP
|
Family ID: |
42045204 |
Appl. No.: |
12/630128 |
Filed: |
December 3, 2009 |
Current U.S.
Class: |
148/402 |
Current CPC
Class: |
C21D 2211/008 20130101;
C21D 2211/005 20130101; C21D 2211/004 20130101; C21D 2211/001
20130101; C22C 38/58 20130101; C21D 2201/01 20130101; C21D 6/004
20130101; C22C 38/02 20130101; C21D 6/005 20130101 |
Class at
Publication: |
148/402 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/02 20060101 C22C038/02; C22C 30/00 20060101
C22C030/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 4, 2008 |
JP |
2008-309262 |
Nov 24, 2009 |
JP |
2009-266700 |
Claims
1. A two-way shape-recovery alloy, which comprises: less than 0.20
mass % of C, 13.00 to 30.00 mass % of Mn, 0.10 to 6.00 mass % of
Si, 0.05 to 12.00 mass % of Cr, 0.01 to 3.00 mass % of Ni, and less
than 0.100 mass % of N, with the remainder being Fe and unavoidable
impurities, wherein the contents of Mn, Si, Cr and Ni satisfy the
following expression (1):
600.ltoreq.33Mn+11Si+28Cr+17Ni.ltoreq.1050 (1).
2. The two-way shape-recovery alloy according to claim 1, wherein
the difference (A.sub.f-M.sub.s) between a transformation finish
temperature in heating (A.sub.f point) and a transformation start
temperature in cooling (M.sub.s point) is 150.degree. C. or
smaller, and wherein the alloy has a transformation start
temperature in heating (A.sub.s point) of 100.degree. C. or
higher.
3. The two-way shape-recovery alloy according to claim 1, which
further comprises at least one of 0.10 to 2.00 mass % of Mo, 0.10
to 2.00 mass % of W, 0.05 to 1.00 mass % of V, and 0.10 to 5.00
mass % of Co.
4. The two-way shape-recovery alloy according to claim 2, which
further comprises at least one of: 0.10 to 2.00 mass % of Mo, 0.10
to 2.00 mass % of W, 0.05 to 1.00 mass % of V, and 0.10 to 5.00
mass % of Co.
5. The two-way shape-recovery alloy according to claim 1, which
further comprises 0.10 to 1.00 mass % of Cu+Al, wherein the content
of Ni and the total content of Cu+Al satisfies the following
relationship: Ni.gtoreq.(Cu+Al).
6. The two-way shape-recovery alloy according to claim 2, which
further comprises 0.10 to 1.00 mass % of Cu+Al, wherein the content
of Ni and the total content of Cu+Al satisfies the following
relationship: Ni.gtoreq.(Cu+Al).
7. The two-way shape-recovery alloy according to claim 3, which
further comprises 0.10 to 1.00 mass % of Cu+Al, wherein the content
of Ni and the total content of Cu+Al satisfies the following
relationship: Ni.gtoreq.(Cu+Al).
8. The two-way shape-recovery alloy according to claim 4, which
further comprises 0.10 to 1.00 mass % of Cu+Al, wherein the content
of Ni and the total content of Cu+Al satisfies the following
relationship: Ni.gtoreq.(Cu+Al).
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a two-way shape-recovery
alloy. More particularly, the invention relates to a two-way
shape-recovery alloy which can be caused to reversibly take a
low-temperature-state shape and a high-temperature-state shape by
utilizing the expansion and contraction which are accompanied phase
transformations, without substantially utilizing a plastic
deformation.
BACKGROUND OF THE INVENTION
[0002] When some kind of material is plastically deformed at a low
temperature and thereafter heated to a high temperature, then the
material returns to the shape which the material possessed before
the plastic deformation. This phenomenon is referred to as
shape-memory effect. Alloys showing the shape-memory effect are
called shape-memory alloys.
[0003] Shape-memory alloys are expected to be used in applications
such as
[0004] (1) a coil expander for changing the tension of a piston
ring according to temperature (see International Publication WO
2004/090318),
[0005] (2) a system for controlling oil flow rate according to
temperature (see JP-A-11-264425), and
[0006] (3) actuators and various switch parts which function also
as a temperature sensor.
[0007] Various materials have conventionally been known as
shape-memory alloys. Of these, Ti--Ni alloys are one of the most
well known classes of shape-memory alloys, The Ti--Ni alloys which
have undergone a shape-memory treatment at a high temperature are
used in various applications. The shape-memory effect of Ti--Ni
alloys is attributable to the following property: when a
low-temperature phase (martensite phase) which has undergone a twin
deformation with external force reversely transforms to a
high-temperature phase (austenite phase), this system returns to
the shape formed by a shape-memory treatment.
[0008] However, the Ti--Ni alloys have a problem that it is
difficult to use the alloys in a wide range of applications because
the material cost is high. There also is a problem that the alloys
have a transformation temperature around room temperature and,
hence, are not usable in applications where a shape-recovery
temperature of 100.degree. C. or higher is required.
[0009] In contrast, iron-based shape-memory alloys represented by
Fe--Mn--Si alloys are characterized by being inexpensive and having
a high shape-recovery temperature. The shape-memory effect of
iron-based alloys is attributable to the following property: when
the .epsilon. phase generated by a stress-induced epsilon
martensite transformation (transformation from the .gamma. (FCC)
phase to .epsilon. (HCP) phase induced by plastically deforming the
system at a temperature not lower than M.sub.s point and not higher
than M.sub.d point) reversely transforms to the .gamma. phase, this
system returns to the shape of the unprocessed system.
[0010] However, the iron-based shape-memory alloys have the
following and other problems:
[0011] (1) the iron-based alloys are inferior in shape-memory
effect to the Ti--Ni shape-memory alloys;
[0012] (2) the iron-based alloys are poor in corrosion resistance
and oxidation resistance because they contain iron; and
[0013] (3) the iron-based alloys are apt to crack when plastically
deformed in an annealed state.
[0014] In order to overcome those problems, various proposals have
been made hitherto.
[0015] For example, JP-T-2000-501778 (the term "JP-T" as used
herein means a published Japanese translation of a PCT patent
application) discloses a nitrogen-containing iron-based
shape-memory alloy which contains 28.80% of Mn, 5.24% of Si, 0.20%
of Cr, and 0.11% of N, with the remainder of Fe.
[0016] This document includes a statement to the effect that not
only the shape-memory characteristics but also mechanical
properties, including damping characteristics, of an Fe--Mn alloy
are improved by alloying with nitrogen.
[0017] JP-A-10-36943 discloses a process for producing an
Fe--Mn--Si shape-memory alloy. In this process, an Fe--Mn--Si alloy
having a given composition is shaped and then held for 15 minutes
or more at a temperature higher than 1,000.degree. C. and lower
than 1,200.degree. C.
[0018] This document includes a statement to the effect that the
process is effective in inhibiting the cracking which occurs upon
stress deformation due to the intergranular precipitation of a fine
intermetallic compound rich in manganese and silicon.
[0019] JP-A-2-221321 discloses a process for producing an
iron-based shape-memory alloy. In this process, an Fe--Mn--Si alloy
having a given composition is processed at a temperature not lower
than the M.sub.d' point (the temperature at which neither a
martensite nor .alpha.' martensite is induced by processing) and
not higher than 700.degree. C., and is then annealed at a
temperature not lower than (M.sub.d' point+200.degree. C.).
[0020] This document includes statements to the effect that:
[0021] (1) because the alloy is processed at a temperature not
lower than the M.sub.d' point, the generation of .epsilon.
martensite and .alpha.' martensite, which adversely influence
processability, can be inhibited and, hence, a processing limit can
be greatly improved, and
[0022] (2) because annealing is conducted at a temperature not
lower than (M.sub.d' point+200.degree. C.), the strain generated in
the .gamma. phase by the processing is eliminated or the .gamma.
phase recrystallizes, resulting in an improvement in shape-memory
characteristics.
[0023] Furthermore, JP-A-7-292448 discloses an Fe--Mn--Si
shape-memory alloy produced by subjecting an Fe--Mn--Si alloy
having a given composition to a heat treatment to form the .alpha.
phase having a thickness of 10 .mu.m or larger in the surface
thereof.
[0024] This document includes statements to the effect that:
[0025] (1) by subjecting the Fe--Mn--Si alloy to a heat treatment
in a proper atmosphere, the .alpha. phase of the body-centered
cubic structure having a lower manganese concentration than the
matrix phase (.gamma. phase) is formed in the surface, and
[0026] (2) since the .alpha. phase has higher corrosion resistance
than the .gamma. phase and has satisfactory conformability with the
.gamma. phase, flaking or cracking is less apt to occur even when
the matrix phase deforms, whereby sufficient corrosion resistance
is obtained.
[0027] In general, when a shape-memory alloy is plastically
deformed at a temperature not higher than a transformation
temperature and thereafter heated to a temperature not lower than
the transformation temperature, then the shape thereof returns to
the state of the alloy which has not undergone the plastic
deformation. However, even when this alloy is cooled again to a
temperature not higher than the transformation temperature, this
alloy does not usually return to the shape imparted by the
low-temperature plastic deformation. This phenomenon, in which only
the shape of a high-temperature phase is memorized, is especially
called "one-way shape-memory effect".
[0028] On the other hand, when some kind of shape-memory alloy is
severely processed in the martensite state or is deformed in the
martensite state and then subjected to constraint heating, then
part of the low-temperature-phase shape can also be memorized. This
phenomenon, in which both a shape of a high-temperature phase and a
shape of a low-temperature phase are memorized, is especially
called "two-way shape-memory effect". For example, it is known that
a Ti--Ni alloy in which a texture has been partly formed shows the
two-way shape-memory effect.
[0029] In the various applications shown above, such as coil
expanders, oil flow rate control systems, and actuators, the
shape-memory alloys are frequently required to have two-way working
properties. Therefore, in order to apply a shape-memory alloy
having a one-way shape-memory effect to a device required to have
two-way working properties, it is necessary to combine this
shape-memory alloy with another part to impart two-way working
properties to the resultant device. Known methods for imparting
two-way working properties include a method in which a one-way
shape-memory alloy is combined with a spring, weight, or the like
to impart two-way working properties (bias method) and a method in
which two or more shape-memory parts are used (differential
method).
[0030] However, such methods in which a one-way shape-memory alloy
is combined with another part to impart two-way working properties
have limitations in device miniaturization. Those methods are hence
applicable to limited fields.
[0031] On the other hand, all the two-way shape-memory alloys which
have been known are expensive and are poor in reproducibility. Only
a limited number of such alloys have hence been put to practical
use. The conventional iron-based shape-memory alloys show the
property of returning from a shape formed by plastic processing to
the shape which was possessed before the plastic processing,
through a reverse transformation (.epsilon..fwdarw..gamma.) (i.e.,
one-way shape-memory effect). However, the iron-based shape-memory
alloys do not show a two-way shape-memory effect.
[0032] Furthermore, in order for a shape-memory alloy to be used in
various applications, the alloy is required to have high accuracy
of shape recovery and strength which enables the alloy to withstand
repetitions of shape recovery.
[0033] However, no proposal has been made on an alloy which is
inexpensive, has two-way working properties, has a higher
shape-recovery temperature than Ti--Ni alloys (specifically,
90-100.degree. C. or higher), has high accuracy of shape recovery,
and has strength which enables the alloy to withstand repetitions
of shape recovery.
SUMMARY OF THE INVENTION
[0034] An object of the invention is to provide a two-way
shape-recovery alloy which is inexpensive, has two-way working
properties, has a higher shape-recovery temperature than Ti--Ni
alloys, has high accuracy of shape recovery, and has strength which
enables the alloy to withstand repetitions of shape recovery.
[0035] Namely, the present invention relates to the following items
1 to 4.
[0036] 1. A two-way shape-recovery alloy, which comprises:
[0037] less than 0.20 mass % of C,
[0038] 13.00 to 30.00 mass % of Mn,
[0039] 0.10 to 6.00 mass % of Si,
[0040] 0.05 to 12.00 mass % of Cr,
[0041] 0.01 to 3.00 mass % of Ni, and
[0042] less than 0.100 mass % of N,
[0043] with the remainder being Fe and unavoidable impurities,
[0044] wherein the contents of Mn, Si, Cr and Ni satisfy the
following expression (1):
600.ltoreq.33Mn+11Si+28Cr+17Ni.ltoreq.1050 (1).
[0045] 2. The two-way shape-recovery alloy according to item 1,
[0046] wherein the difference (A.sub.f-M.sub.s) between a
transformation finish temperature in heating (A.sub.f point) and a
transformation start temperature in cooling (M.sub.s point) is
150.degree. C. or smaller, and
[0047] wherein the alloy has a transformation start temperature in
heating (A.sub.s point) of 100.degree. C. or higher.
[0048] 3. The two-way shape-recovery alloy according to item 1 or
2, which further comprises at least one of
[0049] 0.10 to 2.00 mass % of Mo,
[0050] 0.10 to 2.00 mass % of W,
[0051] 0.05 to 1.00 mass % of V, and
[0052] 0.10 to 5.00 mass % of Co.
[0053] 4. The two-way shape-recovery alloy according to any one of
items 1 to 3, which further comprises
[0054] 0.10 to 1.00 mass % of Cu+Al,
[0055] wherein the content of Ni and the total content of Cu+Al
satisfies the following relationship:
Ni.gtoreq.(Cu+Al).
[0056] In the Fe--Mn--Si alloy, optimizing the contents of
constituent elements results in volume contraction which occurs
through a martensite transformation (.gamma..fwdarw..epsilon.) upon
cooling and in volume expansion which occurs through the reverse
transformation (.epsilon..fwdarw..gamma.) upon heating. The shape
changes accompanied by the expansion/contraction are reversible and
the amounts of the shape changes are relatively large. Furthermore,
the shape-recovery temperature thereof is higher than those of
Ti--Ni alloys (specifically, 90-100.degree. C. or higher), and the
accuracy of shape recovery thereof is high.
[0057] In addition, the Fe--Mn--Si alloy having the given
composition is inexpensive and has strength which enables the alloy
to withstand repetitions of shape recovery. In particular, the
strength is further improved by adding a substitutional
solid-solution strengthening element such as Mo, or a precipitation
strengthening element such as Cu.
[0058] Consequently, the two-way shape-recovery alloy of the
invention can be used in various functional parts required to have
two-way working properties.
[0059] The two-way shape-recovery alloy of the invention can be
used as, e.g., a switch or actuator which works based on
temperature changes, an expander for a piston ring, and a
temperature-sensitive member for use in the oil supply mechanism of
a viscous-fluid coupling.
BRIEF DESCRIPTION OF THE DRAWINGS
[0060] FIG. 1 is a presentation showing the changes in length of a
eutectoid steel (0.77 mass % carbon) with changing temperature and
with phase transformations.
[0061] FIG. 2 is a presentation showing a heating-cooling
transformation curve for the alloy of Example 7.
[0062] FIG. 3 is a presentation showing the relationship between
A.sub.f-M.sub.s and A.sub.s in the alloys of the Examples and
Comparative Examples.
[0063] FIG. 4 shows the results of a thermal fatigue test of the
alloy obtained in Example 2.
BEST MODE FOR CARRYING OUT THE INVENTION
[0064] One embodiment of the invention is explained below in
detail.
1. Two-Way Shape-Recovery Alloy
[0065] The two-way shape-recovery alloy of the invention contains
the elements shown below, with the remainder being iron and
unavoidable impurities, and has a component balance which satisfies
a given requirement. The kinds of the additive elements, ranges of
the contents thereof, and reasons for the limitations are as
follows. Herein, in the present specification, all the percentages
defined by mass are the same as those defined by weight,
respectively.
[0066] In the invention, the term "two-way shape recovery" means
that an alloy is caused to reversibly take a low-temperature-state
shape and a high-temperature-state shape by mainly utilizing the
expansion and contraction which are accompanied by phase
transformations, without substantially utilizing a plastic
deformation.
1.1. Main Constituent Elements
[0067] (1) C<0.20 mass %
[0068] Carbon is present as an interstitial element in the iron and
is a potent austenite-forming element. In ordinary steel, carbon
forms the .alpha.' (BCT) phase upon quench-hardening and this leads
to an improvement in strength. However, the FCC-BCT transformation
is a transformation which accompanies volume expansion.
Furthermore, since this transformation highly depends on the
cooling rate of material, a change in cooling rate results in the
formation of a bainite structure or ferrite structure, thereby
making it impossible to obtain stable volume expansion. Moreover,
this transformation does not produce a two-way shape-recovery
effect.
[0069] Consequently, in order for an alloy to exert a two-way
shape-recovery effect, the alloy should be prevented from
generating the .alpha.' phase upon quench-hardening. Accordingly,
the alloy must have a carbon content lower than 0.20 mass %. The
carbon content thereof is more preferably lower than 0.10 mass
%.
(2) 13.00.ltoreq.Mn.ltoreq.30.00 mass %
[0070] Manganese is an additive element which is essential for
stably attaining the two-way transformations between .gamma. and
.epsilon.. At high temperatures, manganese functions as an
austenite-forming element. The higher the content of manganese is,
the more the .epsilon. martensite is apt to generate at low
temperatures. From the standpoint of generating .epsilon.
martensite, the content of manganese must be 13.00 mass % or
higher. The content of manganese is more preferably 15.00 mass % or
higher.
[0071] On the other hand, in case where the manganese content is
excessively high, the result is a considerably lowered
transformation temperature in cooling and there is a possibility
that the austenite phase might be a stable phase even at
-50.degree. C. Consequently, the content of manganese must be 30.00
mass % or lower. The content of manganese is more preferably lower
than 25.00 mass %.
(3) 0.10.ltoreq.Si.ltoreq.6.00 mass %
[0072] Silicon is an element which reduces stacking-fault energy to
accelerate the transformation from the .gamma. phase to the
.epsilon. phase. From this standpoint, the content of silicon must
be 0.10 mass % or higher. The content of silicon is more preferably
0.30 mass % or higher.
[0073] On the other hand, in case where the silicon content is
excessively high, the strengthening by solid-solution formation is
significant and this leads to a decrease in material ductility.
Consequently, the content of silicon must be 6.00 mass % or lower.
The content of silicon is more preferably 4.00 mass % or lower.
(4) 0.05.ltoreq.Cr.ltoreq.12.00 mass %
[0074] Chromium has the function of controlling the temperature at
which the transformation from the .gamma. phase to the .epsilon.
phase occurs, and further has the function of improving the
corrosion resistance of the material. From the standpoint of
obtaining such effects, the content of chromium must be 0.05 mass %
or higher.
[0075] On the other hand, chromium functions as an
.alpha.-stabilizing element at high temperatures. Therefore, an
excessively high chromium content tends to convert a heat-treated
structure into an .alpha.' martensite structure. Consequently, the
content of chromium must be 12.00 mass % or lower.
(5) 0.01.ltoreq.Ni.ltoreq.3.00 mass %
[0076] Nickel has the function of regulating transformation
temperatures without causing a structural change in a heat
treatment. From the standpoint of obtaining this effect, the
content of nickel must be 0.01 mass % or higher.
[0077] On the other hand, nickel is a potent austenite-forming
element. Therefore, an excessively high nickel content results in a
structural change. Consequently, the content of nickel must be 3.00
mass % or lower.
(6) N<0.100 mass %
[0078] Nitrogen combines with aluminum and other elements to form
nitrogen compounds and thereby adversely-influences hot workability
or cold workability. Furthermore, nitrogen functions as an
interstitial element to form a solid solution in the iron and
serves as a potent austenite-forming element. As in the case of
carbon, an excessively high nitrogen content changes transformation
behavior and results in the formation of the .alpha.' (BCT) phase
in quench-hardening.
[0079] Consequently, in order for exerting a two-way shape-recovery
effect, it is necessary to prevent the alloy from generating the
.alpha.' phase upon quench-hardening. From this standpoint, the
content of nitrogen must be lower than 0.100 mass %. The content of
nitrogen is more preferably lower than 0.050 mass %.
1.2. Unavoidable Impurities
[0080] The unavoidable impurities specifically include the
followings.
(1) P<0.050 mass %
[0081] Phosphorus unavoidably comes into the alloy from raw
materials. Phosphorus is an element which segregates at grain
boundaries to reduce the hot workability of the material. It is
therefore preferred to reduce the content of phosphorus to be lower
than 0.050 mass %. The content of phosphorus is more preferably
lower than 0.010 mass %.
(2) S<0.100 mass %
[0082] Sulfur unavoidably comes into the alloy from raw materials.
Sulfur segregates at grain boundaries to impair hot workability. In
the invention, since the alloy has a high manganese content, the
sulfur which has come into the alloy forms MnS and hence exerts a
limited influence on hot workability. However, the smaller the
sulfur amount is, the more the alloy is preferred. It is therefore
preferred to reduce the content of sulfur to be lower than 0.100
mass %. The content of sulfur is more preferably lower than 0.050
mass %.
(3) O<0.050 mass %
[0083] Oxygen unavoidably comes into the steel. Oxygen combines
with aluminum and silicon to form oxides and thereby adversely
influences hot workability or cold workability. It is therefore
preferred to reduce the content of oxygen to be lower than 0.050
mass %. The content of oxygen is more preferably lower than 0.020
mass %.
(4) Mo<0.10 mass % (5) W<0.10 mass % (6) V<0.05 mass % (7)
Co<0.10 mass %
[0084] Molybdenum, tungsten, vanadium, and cobalt each may
unavoidably come into the steel. Although these elements do not
exert a considerable influence on transformation temperatures or
the type of structure, it is preferred to reduce the contents
thereof to be lower than the values shown above.
[0085] Incidentally, these elements each function as a
substitutional solid-solution strengthening element. In such a
case, the elements may be added in amounts not smaller than the
values shown above. This respect will be described later.
(8) Cu<0.10 mass %
[0086] Copper is an element which unavoidably comes into the alloy
from raw materials. Excessively high copper contents cause the
alloy to show red shortness, and considerably impair the
processability thereof. From the standpoint of maintaining
processability, it is preferred to reduce the content of copper to
be lower than 0.10 mass %. The content of copper is more preferably
lower than 0.05 mass %.
[0087] Incidentally, it is possible to positively add copper, on
condition that a given amount nickel should be added, to thereby
conduct precipitation strengthening based on the secondary
precipitation of copper. In such a case, a copper content of up to
1.00 mass % is allowable. This respect will be described later.
(9) Al<0.10 mass %
[0088] Aluminum unavoidably comes into the alloy because it is used
as a deoxidizes like silicon. Aluminum combines with oxygen to form
an oxide and thereby adversely influences hot workability or cold
workability. It is therefore preferred to reduce the content of
aluminum to be lower than 0.10 mass %.
[0089] Incidentally, it is possible to positively add aluminum, on
condition that a given amount of nickel should be added, to thereby
improve strength based on the secondary precipitation of an Al--Ni
intermetallic compound. In such a case, an aluminum content of up
to 1.00 mass % is allowable. This respect will be described
later.
1.3. Component Balance
[0090] The two-way shape-recovery alloy of the invention must
satisfy the following expression (1) besides the requirement that
the contents of component elements should be the respective ranges
shown above.
600.ltoreq.33Mn+11Si+28Cr+17Ni.ltoreq.1050 (1)
[0091] The value determined from expression (1) correlates with the
transformation temperatures of the alloy, and is an experiential
value. By optimizing the component balance among manganese,
silicon, chromium, and nickel, the .gamma. phase can be stably
ensured at a high temperature (300.degree. C. or higher) and the
.epsilon. phase can be stably ensured at a low temperature
(-50.degree. C. or lower), respectively.
[0092] As stated above, manganese mainly serves as an
austenite-forming element and also functions as an element which
forms the .epsilon. phase upon cooling. Silicon accelerates the
conversion of the .gamma. phase to the .epsilon. phase at low
temperatures but functions as an .alpha.-stabilizing element at
high temperatures. Although chromium functions as an
.alpha.-stabilizing element at high temperatures, it is an element
effective in controlling the temperatures at which the .gamma.
phase transforms to the .epsilon. phase. Nickel is an element
effective in controlling the temperatures at which the .gamma.
phase transforms to the .epsilon. phase.
[0093] The smaller the value of expression (1) is, the higher the
transformation finish temperature in heating (A.sub.f point) is. In
case where the A.sub.f point is too high, there is a possibility
that a creep deformation might occur during the reverse
transformation (.epsilon..fwdarw..gamma.), resulting in reduced
accuracy of shape recovery. In order for obtaining high accuracy of
shape recovery, it is necessary that the A.sub.f point should be
400.degree. C. or lower. From the standpoint of attaining this, the
value of expression (1) must be 600 or larger. The value of
expression (1) is more preferably 700 or larger.
[0094] On the other hand, the larger the value of expression (1)
is, the lower the transformation start temperature in heating
(A.sub.s point) is. In case where the value of expression (1)
becomes too large, the A.sub.s point becomes room temperature or
lower and it becomes difficult to cause this alloy to undergo shape
recovery at a temperature higher than the shape-recovery
temperatures of Ti--Ni alloys. From the standpoints of attaining an
A.sub.s point which is higher than the shape-recovery temperatures
of Ti--Ni alloys and thereby enabling the alloy of the invention to
undergo shape recovery at a temperature of 90-100.degree. C. or
higher, the value of expression (1) must be 1,050 or smaller. The
value of expression (1) is more preferably 900 or smaller.
1.4. Transformation Temperatures
[0095] A martensite transformation (.gamma..fwdarw..epsilon.)
starts at a transformation start temperature in cooling (M.sub.s
point) and is finished at a transformation finish temperature is
cooling (M.sub.f point). On the other hand, the reverse
transformation (.epsilon..fwdarw..gamma.) starts at a
transformation start temperature in heating (A.sub.s point) and is
finished at a transformation finish temperature in heating (A.sub.f
point).
[0096] As stated above, the A.sub.s point can be elevated to
90.degree. C. or higher, or 100.degree. C. or higher, by optimizing
the value of expression (1).
[0097] In the case where a two-way shape-recovery effect is applied
to a device required to have two-way working properties, it is
desirable that reversible shape changes should occur in a narrow
temperature range. Namely, the smaller the difference
(A.sub.f-M.sub.s) between the transformation finish temperature in
heating (A.sub.f) and the transformation start temperature in
cooling (M.sub.s) is, the better the alloy is. In general, the
value of A.sub.f-M.sub.s in low-alloy steels is 200-300.degree. C.
or larger. In contrast, in the two-way shape-recovery alloy of the
invention, the value of A.sub.f-M.sub.s can be reduced to
200-300.degree. C. or smaller by optimizing the contents of the
component elements, such as Mn and Si, which influence the
transformation temperature. From the standpoint of reducing the
size of the hysteresis loop accompanying heating/cooling, the value
of A.sub.f-M.sub.s is preferably 150.degree. C. or smaller. The
value of A.sub.f-M.sub.s is more preferably 100.degree. C. or
smaller.
[0098] Incidentally, each transformation temperature can be
determined by drawing a tangent to an expansion-contraction curve
at each of points respectively located before and after the area
where the inclination of the curve changes and taking the
temperature corresponding to the point of intersection of these
tangents as the transformation temperature.
1.5. Minor Constituent Elements
[0099] The two-way shape-recovery alloy of the invention may
further contain one or more of the following elements besides the
elements described above.
1.5.1 Substitutional Solid-Solution Strengthening Elements
[0100] (1) 0.10.ltoreq.Mo.ltoreq.2.00 mass % (2)
0.10.ltoreq.W.ltoreq.2.00 mass % (3) 0.05.ltoreq.V.ltoreq.1.00 mass
% (4) 0.10.ltoreq.Co.ltoreq.5.00 mass %
[0101] In the case where the two-way shape-recovery alloy of the
invention is desired to be improved in strength, a substitutional
solid-solution strengthening element can be added thereto so long
as this exerts no influence on the transformation behavior
exhibited by the alloy upon heating/cooling. Examples of the
substitutional solid-solution strengthening element include
molybdenum, tungsten, vanadium, and cobalt. Any one of these
elements may be added, or two or more thereof may be added.
[0102] From the standpoint of attaining the solid solution
strengthening, it is preferred that the contents of molybdenum,
tungsten, vanadium, and cobalt should be not lower than the
respective lower limits shown above, respectively.
[0103] On the other hand, when the contents of these elements are
excessively high, not only the effect is not enhanced any more or
an increased cost results but also there are cases where such high
contents thereof influence transformation behavior. It is therefore
preferred that the contents of these elements should be not higher
than the respective upper limits shown above, respectively.
1.5.2. Precipitation Strengthening Elements
[0104] (5) 0.10.ltoreq.(Cu+Al).ltoreq.1.00 mass %
(6) Ni.gtoreq.(Cu+Al)
[0105] In case where copper is added alone, the copper precipitates
at grain boundaries to reduce hot workability. However, when a
given amount of nickel is added simultaneously with the addition of
copper, the nickel inhibits the copper from precipitating at grain
boundaries. As a result, the copper undergoes secondary
precipitation within the grains to improve strength.
[0106] From the standpoint of obtaining this effect, it is
preferred to regulate the content of copper to 0.10 mass % or
higher. On the other hand, an excessively high copper content
results in a decrease in hot workability. It is therefore preferred
to regulate the content of copper to 1.00 mass % or lower.
[0107] From the standpoint of attaining precipitation strengthening
without reducing hot workability, it is preferred to add nickel in
an amount equal to or larger than the copper amount. More
preferably, the nickel amount is at least two times the copper
amount.
[0108] Likewise, in case where aluminum is added alone, an oxide
generates in a large amount to reduce hot workability or cold
workability. However, when a given amount of nickel is added
simultaneously with the addition of aluminum, the secondary
precipitation of an Ni--Al intermetallic compound occurs to improve
strength.
[0109] From the standpoint of obtaining this effect, it is
preferred to regulate the content of aluminum to 0.10 mass % or
higher. On the other hand, an excessively high aluminum content
results in a decrease in hot workability on cold workability. It is
therefore preferred to regulate the content of aluminum to 1.00
mass % or lower.
[0110] From the standpoint of attaining precipitation strengthening
without reducing hot workability or cold workability, it is
preferred to add nickel in an amount equal to or larger than the
aluminum amount. More preferably, the nickel amount is at least two
times the aluminum amount.
[0111] Furthermore, it is possible to simultaneously add copper and
aluminum, on condition that a given amount of nickel should be
added, to thereby attain precipitation strengthening with both the
copper and the aluminum. From the standpoint of obtaining this
effect, it is preferred to regulate the total content of copper and
aluminum to 0.1 mass % or higher.
[0112] On the other hand, from the standpoint of inhibiting hot
workability and cold workability from decreasing, it is preferred
to regulate the total content of copper and aluminum to 1.00 mass %
or lower.
[0113] Also in the case of simultaneously adding copper and
aluminum, it is preferred to add nickel in an amount equal to or
larger than the total amount of the copper and the aluminum. More
preferably, the nickel amount is at least two times the sum of the
copper and the aluminum.
[0114] In this regard, with regard to each element contained in the
alloy of the invention, according to an embodiment, the minimal
amount thereof present in the alloy is the smallest non-zero amount
used in the Examples of the developed alloys as summarized in
Tables 1 and 2. According to a further embodiment, the maximum
amount thereof present in the alloy is the maximum amount used in
the Examples of the developed alloys as summarized in Tables 1 and
2.
2. Functional Parts Employing the Two-Way Shape-Recovery Alloy
[0115] The two-way shape-recovery alloy of the invention has the
function of reversibly taking a low-temperature-state shape and a
high-temperature-state shape based on the expansion/contraction
which are accompanied by the transformation between .gamma. and
.epsilon., without substantially using a plastic deformation.
[0116] Therefore, the two-way shape-recovery alloy having such
function can be applied to functional parts such as:
[0117] (1) a switch or actuator which takes advantage of changes
between a high-temperature-state shape and a low-temperature-state
shape,
[0118] (2) an actuator having a mechanism in which the shape
recovery deflection accompanying a temperature change is amplified
on the principle of a sprig or lever,
[0119] (3) a switch or actuator required to have a shape-recovery
temperature of 100.degree. C. or higher,
[0120] (4) an expander for a piston ring (see, for example,
International Publication WO 2004/090318), and
[0121] (5) a temperature-sensitive member for use in the oil supply
mechanism of a viscous-fluid coupling device (see, for example,
JP-A-11-264425).
[0122] Although the two-way shape-recovery alloy of the invention
can be used as it is, the alloy may be used after the surface
thereof is subjected to any of various surface treatments. Examples
of the surface treatments include nitriding, PVD, and CVD. By such
surface treatments, oxidation resistance and wearing resistance can
be imparted.
[0123] The two-way shape-recovery alloy to which wearing resistance
has been imparted by a surface treatment can be applied to a
mechanical part (e.g., a coil spring, piston ring, or the like)
which is used in the state of being in contact with a mating
material.
3. Process for Producing the Two-Way Shape-Recovery Alloy
[0124] The two-way shape-recovery alloy of the invention can be
produced by melting raw materials which have been mixed together in
a given proportion and then casting the melt. It is preferred that,
after the cast is forged to impart a given shape thereto, the
forged alloy is subjected to a solution heat treatment (ST
treatment) and subsequent air cooling in order to eliminate the
influence of the forging. The temperature for the solution heat
treatment is preferably 700-1,200.degree. C.
[0125] In the case where a precipitation strengthening element has
been added, it is preferred to conduct an aging treatment after a
solution heat treatment and subsequent air cooling. It is preferred
that the aging treatment is conducted at a temperature of from
400.degree. C. to 600.degree. C. for a period of from 0.5 hours to
less than 5 hours.
4. Functions of the Two-Way Shape-Recovery Alloy
[0126] In FIG. 1 is shown the changes in length of a eutectoid
steel (0.77 mass % carbon) with changing temperature and with phase
transformations.
[0127] At a temperature around room temperature (point A), the
eutectoid steel has a ferrite (.alpha.) phase structure. When
heated to an austenite (.gamma.) phase region, this eutectoid steel
undergoes expansion.fwdarw.contraction.fwdarw.expansion along the
curve A.fwdarw.B.fwdarw.C.fwdarw.D as shown in FIG. 1. Further,
when this eutectoid steel is gradually cooled from the
.gamma.-phase region to room temperature, the eutectoid steel
undergoes contraction.fwdarw.expansion.fwdarw.contraction along the
curve D.fwdarw.E.fwdarw.F.fwdarw.A and returns to the shape which
the steel possessed before the heating. The reason why the
eutectoid steel contracts along the curve B.fwdarw.C during heating
is that an .alpha..fwdarw..gamma. transformation occurs. The reason
why the eutectoid steel expands along the curve E.fwdarw.F during
cooling in that a .gamma..fwdarw..alpha. transformation occurs.
[0128] On the other hand, when the eutectoid steel is rapidly
cooled from the .gamma.-phase region, this steel undergoes
contraction.fwdarw.expansion along the broken-line curve (curve
D-H) as shown in FIG. 1 and comes to have a shape different from
the shape of steel before the heating. When the eutectoid steel
which has been rapidly cooled is heated again, this eutectoid steel
repeatedly undergoes expansion and contraction along the curve
H.fwdarw.J.fwdarw.K.fwdarw.L.fwdarw.M.fwdarw.N.fwdarw.O and finally
reaches point D.
[0129] The reason why the length of the steel as measured after the
rapid cooling (point H) is larger than the length of the steel as
measured before the heating (point A) is that the rapid cooling of
the eutectoid steel from the .gamma.-phase region to a temperature
not higher than the M.sub.s point results in a martensite
transformation (.gamma.(FCC).fwdarw..alpha.' (BCT) transformation)
which accompanies volume expansion. Furthermore, the reason why the
expansion or contraction occurring at temperatures of 400.degree.
C. and lower is larger than the change in length resulting from
thermal expansion is that .epsilon.-carbide formation,
residual-.gamma. decomposition, and .theta.-carbide formation occur
with the increase of the temperature.
[0130] In iron-based alloys for general use, the martensite
transformation which is caused by such a heat treatment and the
reverse transformation are positively used for structure
control.
[0131] However, since the .gamma..fwdarw..alpha.' transformation,
which occurs upon cooling, accompanies volume expansion, general
iron-based alloys cannot be used as shape-recovery alloys required
to contract upon cooling.
[0132] The .gamma..fwdarw..alpha.' transformation highly depends on
the cooling rate of the material. Therefore, a change in cooling
rate may result in the formation of a bainite structure or ferrite
structure and stable volume expansion (i.e., reproducibility of
shape recovery) cannot be obtained.
[0133] Furthermore, the .alpha..fwdarw..gamma. transformation
finish temperature in heating (A.sub.f point) is as high as
700.degree. C. or above. Moreover, the difference between the
A.sub.f point and the .gamma..fwdarw..alpha.' transformation start
temperature in cooling (M.sub.s point) is as large as
200-300.degree. C. or more. Namely, the hysteresis loop
accompanying heating/cooling is large.
[0134] In contrast, the two-way shape-recovery alloy of the
invention comprises an Fe--Mn--Si alloy as the base, and the
contents of the component elements therein are optimized.
Therefore, when this alloy is cooled from a high temperature
(300.degree. C. or higher) to a low temperature (-50.degree. C. or
lower), a transformation occurs from the .gamma. (FCC) phase to the
.epsilon. (HCP) phase and neither the .alpha. (BCC) phase nor the
.alpha.' (BCT) phase generates. Since the .gamma..fwdarw..epsilon.
transformation causes volume contraction, the cooling results in
contraction to a degree higher than the shape change accompanied by
thermal contraction.
[0135] On the other hand, when this alloy is heated, the
.epsilon..fwdarw..gamma. transformation occurs. The heating hence
results in expansion to a degree higher than the shape change
accompanied by thermal expansion. In addition, the changes in shape
accompanied by the expansion/contraction are reversible. No plastic
deformation is hence necessary for shape recovery.
[0136] Furthermore, the two-way shape-recovery alloy of the
invention shows a relatively large shape change amount.
Specifically, by optimizing the component elements, the degree of
change in length (.DELTA.L/L.sub.0.times.100) in heating becomes
0.3% or higher, preferably 0.5% or higher, more preferably 0.7% or
higher. By optimizing the shape of this two-way shape-recovery
alloy (e.g., shaping the alloy into a spring), that shape change
amount can be further increased.
[0137] On the other hand, the degree of change in length in cooling
is the same as the degree of change in length in heating.
Specifically, the degree of change in length per heating/cooling
cycle is 0.1% or lower, and the degree of shape recovery is
exceedingly high. Even when a heating/cooling cycle is repeated
several hundred times, the rate of shape recovery hardly
deteriorates with the lapse of time.
[0138] Moreover, since the two-way shape-recovery alloy of the
invention comprises an Fe--Mn--Si alloy as the base, the
shape-recovery temperature (A.sub.s point) is higher than those of
conventional Ti--Ni alloys. Since the component elements have been
optimized, the hysteresis loop accompanying heating/cooling
(A.sub.f-M.sub.s) is smaller than those of general iron-based
alloy.
[0139] Specifically, when the component elements are optimized so
that expression (1) is satisfied, the A.sub.s point becomes
90.degree. C. or higher, preferably 100.degree. C. or higher.
Likewise, when the component elements are optimized so that
expression (1) is satisfied, the value of A.sub.f-M.sub.s becomes
200.degree. C. or smaller, preferably 150.degree. C. or smaller,
more preferably 100.degree. C. or smaller.
[0140] In addition, since the two-way shape-recovery alloy of the
invention comprises an Fe--Mn--Si alloy as the base, it is
inexpensive and has strength which enables the alloy to withstand
repetitions of shape recovery. In particular, the strength is
further improved by adding a substitutional solid-solution
strengthening element such as Mo or a precipitation strengthening
element such as Cu.
[0141] Consequently, the two-way shape-recovery alloy of the
invention can be used in various functional parts required to have
two-way working properties.
EXAMPLES
Examples 1 to 28 and Comparative Examples 1 to 10
1. Production of Samples
[0142] Each of the materials respectively having the chemical
compositions shown in Table 1 and Table 2 (50 kg each) was melted
in a high-frequency-heating melting furnace, followed by casting.
The casts obtained were respectively subjected to soaking at
1,200.degree. C. for 24 hours, subsequently forged to .phi.30 mm at
a temperature of 800.degree. C. or higher, and then gradually
cooled. In order to eliminate the influence of the forging
conditions, etc., the resultant forged alloys were respectively
subjected to a solution heat treatment at 800.degree. C. for 30
minutes and then air-cooled.
[0143] Furthermore, with respect to Examples 10 to 13, in which 0.1
mass % or more copper had been added, and Examples 14 to 18, in
which 0.1 mass % or more aluminum had been added, an aging
treatment was conducted after the solution heat treatment and the
air cooling. The aging treatment was conducted at a temperature of
500.degree. C. for a period of 1.5 hours.
TABLE-US-00001 TABLE 1 Composition (mass %) C Si Mn P S Cu Ni Cr Mo
W V Co Al O N Example 1 0.10 1.24 19.12 0.011 0.042 0.06 0.01 1.45
0.08 0.10 0.02 0.07 0.039 0.028 0.092 Example 2 0.08 5.24 15.37
0.034 0.033 0.02 1.34 11.81 0.07 0.01 0.05 0.08 0.064 0.010 0.063
Example 3 0.11 0.72 19.55 0.023 0.004 0.03 0.64 3.11 0.04 0.06 0.00
0.04 0.021 0.027 0.090 Example 4 0.01 0.32 15.99 0.044 0.010 0.03
1.98 6.33 0.05 0.00 0.00 0.06 0.090 0.019 0.013 Example 5 0.04 5.24
22.12 0.031 0.017 0.09 1.66 5.81 0.05 0.03 0.05 0.02 0.078 0.044
0.060 Example 6 0.05 4.01 19.02 0.024 0.042 0.05 0.77 3.42 0.06
0.06 0.01 0.01 0.018 0.041 0.036 Example 7 0.15 5.09 14.42 0.015
0.042 0.06 1.87 8.99 0.00 0.07 0.02 0.01 0.006 0.027 0.049 Example
8 0.01 1.09 18.33 0.009 0.041 0.01 0.78 11.18 0.03 0.05 0.04 0.08
0.023 0.030 0.093 Example 9 0.08 0.90 14.51 0.024 0.042 0.05 1.50
5.41 0.01 0.05 0.03 0.09 0.037 0.040 0.043 Example 10 0.02 3.84
20.28 0.010 0.022 0.71 0.98 3.20 0.25 0.09 0.02 0.09 0.001 0.019
0.078 Example 11 0.11 1.11 20.68 0.007 0.050 0.46 0.74 8.90 0.02
0.04 0.02 0.07 0.027 0.009 0.013 Example 12 0.17 3.80 16.06 0.014
0.003 0.63 1.46 11.06 0.04 0.01 0.01 0.07 0.045 0.004 0.098 Example
13 0.13 3.79 15.57 0.020 0.032 0.69 2.38 7.82 0.01 0.04 0.02 0.02
0.038 0.006 0.098 Example 14 0.10 2.08 18.01 0.021 0.033 0.08 2.95
11.87 0.08 0.03 0.03 0.06 0.639 0.033 0.036 Example 15 0.05 2.67
19.02 0.028 0.045 0.02 3.00 0.93 0.03 0.06 0.00 0.00 0.604 0.032
0.040 Example 16 0.03 1.34 21.39 0.020 0.044 0.01 1.95 3.98 0.00
0.06 0.00 0.03 0.966 0.027 0.026 Example 17 0.17 5.42 24.97 0.007
0.007 0.32 0.98 4.55 0.04 0.09 0.01 0.05 0.263 0.027 0.066 Example
18 0.17 3.03 16.78 0.034 0.041 0.05 2.69 4.21 0.05 0.08 0.01 0.07
0.433 0.032 0.069 Example 19 0.14 1.22 14.84 0.027 0.018 0.00 1.53
6.39 0.59 0.77 0.02 0.06 0.002 0.019 0.063 Example 20 0.16 1.11
17.75 0.035 0.039 0.05 1.94 3.81 0.08 0.06 0.25 1.15 0.044 0.035
0.084 Example 21 0.16 0.77 21.00 0.036 0.030 0.05 0.33 2.94 0.10
1.99 0.05 0.05 0.041 0.011 0.013 Example 22 0.13 3.15 24.07 0.039
0.034 0.03 1.48 1.68 0.02 0.04 0.02 4.86 0.080 0.014 0.016 Example
23 0.16 5.83 19.44 0.019 0.008 0.01 1.42 0.34 0.01 0.07 0.96 0.09
0.070 0.038 0.038 Example 24 0.10 5.45 27.06 0.047 0.001 0.06 1.36
1.25 1.88 0.03 0.01 3.22 0.072 0.047 0.090 Example 25 0.19 4.86
13.64 0.013 0.047 0.04 1.99 7.33 0.09 1.33 0.52 0.00 0.070 0.018
0.006
TABLE-US-00002 TABLE 2 Composition (mass %) C Si Mn P S Cu Ni Cr Mo
W V Co Al O N Example 26 0.03 2.11 27.21 0.010 0.046 0.01 1.22 3.02
1.12 0.03 0.02 2.33 0.063 0.034 0.041 Example 27 0.09 1.64 18.84
0.024 0.017 0.09 1.90 2.80 0.34 0.50 0.22 0.54 0.012 0.004 0.064
Example 28 0.06 0.95 16.26 0.049 0.045 0.04 0.25 4.21 0.08 1.68
0.04 0.07 0.020 0.033 0.070 Comparative 0.02 0.31 22.11 0.011 0.034
0.02 0.02 12.10 0.03 0.01 0.02 0.01 0.021 0.011 0.004 Example 1
Comparative 0.02 6.17 28.09 0.021 0.023 0.08 0.15 5.01 0.01 0.02
0.01 0.02 0.033 0.022 0.018 Example 2 Comparative 0.02 4.43 22.11
0.011 0.034 0.02 0.02 12.10 0.03 0.01 0.02 0.01 0.021 0.011 0.004
Example 3 Comparative 0.05 0.49 0.71 0.022 0.021 0.04 9.22 18.02
0.06 0.08 0.03 0.02 0.024 0.017 0.022 Example 4 Comparative 0.33
0.38 0.62 0.016 0.017 0.09 0.02 13.33 1.01 0.01 0.02 0.03 0.033
0.043 0.033 Example 5 Comparative 0.05 3.98 8.12 0.024 0.042 0.05
0.22 3.11 0.06 0.06 0.01 0.01 0.018 0.041 0.036 Example 6
Comparative 0.05 4.01 11.11 0.024 0.042 0.05 0.02 3.42 0.06 0.06
0.01 0.01 0.018 0.041 0.036 Example 7 Comparative 0.05 4.41 33.22
0.024 0.042 0.05 0.06 3.23 0.06 0.06 0.01 0.01 0.018 0.041 0.036
Example 8 Comparative 0.05 3.97 13.55 0.024 0.042 0.05 0.23 22.14
0.06 0.06 0.01 0.01 0.018 0.041 0.036 Example 9 Comparative 0.10
0.81 13.55 0.023 0.043 1.01 2.23 18.33 1.01 0.01 0.01 0.01 0.043
0.063 0.410 Example 10
2. Test Methods
2.1. Transformation Temperatures and Degree of Change in Length
[0144] A differential dilatometer was used to determine
transformation temperatures in heating/cooling (A.sub.s, A.sub.f,
M.sub.s, and M.sub.f) and the degree of the change in length
occurring with the transformation in heating (coefficient of
expansion). The size of each test piece was .phi.5 mm.times.20 mm,
the rate of heating was 10.degree. C./min, and the rate of cooling
was 10.degree. C./min.
2.2. Structure
[0145] A sample which had been held at -50.degree. C. was subjected
to X-ray diffractometry to identify the phase. As the X-ray was
used the K.sub..alpha. line of cobalt.
2.3. Thermal Fatigue Test
[0146] A test piece having a parallel-part length of 40 mm was
subjected to a thermal fatigue test. A strain measurement part
(region having a length of 15 mm) in the parallel part of the test
piece was heated and, at the time when a maximum temperature was
reached, both ends of the test piece was fixed. The test piece in
this state was subjected to 300 repetitions of a cooling/heating
cycle to examine the relationship between the temperature change
and the stress generated in the test piece. The maximum temperature
and minimum temperature were set at 300.degree. C. and 50.degree.
C., respectively. The rate of heating was 250.degree. C./min on
average, and the rate of cooling was 83.degree. C./min on
average.
2.4. Tensile Test
[0147] Tensile test was carried out using a JIS14A (M18) sample.
Conditions of the tensile test were in accordance with JIS
Z2241.
3. Results
3.1. Transformation Temperatures, Degree of Change in Length, and
Structure
[0148] In Table 3 are shown the degree of change in length with
transformation in heating (.DELTA.L/L.sub.0.times.100),
A.sub.f-M.sub.s, A.sub.s the value of expression (1), and the
structure observed at -50.degree. C.
TABLE-US-00003 TABLE 3 .DELTA.L/L.sub.0 A.sub.f-M.sub.s A.sub.s
Structure (%) (.degree. C.) (.degree. C.) Expression (1) (at
-50.degree. C.) Example 1 0.88 168 234 685 .epsilon. Example 2 0.55
145 154 918 .epsilon. Example 3 0.79 180 234 751 .epsilon. Example
4 0.80 132 233 742 .epsilon. Example 5 0.47 103 121 979 .epsilon. +
.gamma. Example 6 0.75 189 198 781 .epsilon. Example 7 0.70 195 207
815 .epsilon. Example 8 0.52 134 145 943 .epsilon. + .gamma.
Example 9 0.91 230 251 666 .epsilon. Example 10 0.70 141 195 818
.epsilon. Example 11 0.50 127 138 956 .epsilon. + .gamma. Example
12 0.57 152 161 906 .epsilon. + .gamma. Example 13 0.70 149 193 815
.epsilon. Example 14 0.44 92 98 1000 .epsilon. + .gamma. Example 15
0.81 189 232 734 .epsilon. Example 16 0.66 166 185 865 .epsilon.
Example 17 0.42 98 104 1028 .epsilon. + .gamma. Example 18 0.79 202
223 751 .epsilon. Example 19 0.85 211 233 708 .epsilon. Example 20
0.81 207 243 738 .epsilon. Example 21 0.74 189 201 789 .epsilon.
Example 22 0.58 149 155 901 .epsilon. + .gamma. Example 23 0.80 214
234 739 .epsilon. Example 24 0.42 103 119 1011 .epsilon. + .gamma.
Example 25 0.80 203 221 743 .epsilon. Example 26 0.40 89 108 1026
.epsilon. + .gamma. Example 27 0.79 211 231 750 .epsilon. Example
28 0.90 246 257 669 .epsilon. Comparative 0.34 183 56 1072 .gamma.
+ .epsilon. Example 1 Comparative 0.25 135 45 1138 .gamma. +
.epsilon. Example 2 Comparative 0.28 699 674 1118 .alpha. +
.epsilon. Example 3 Comparative 0.87 690 .gamma. Example 4
Comparative 1.28 320 665 398 .alpha. Example 5 Comparative 1.28 469
654 403 .alpha. Example 6 Comparative 1.13 354 333 507 .epsilon. +
.alpha. Example 7 Comparative 0.11 228 32 1236 .epsilon. Example 8
Comparative 0.28 397 632 1115 .alpha.' martensite Example 9
Comparative 0.43 1007 .gamma. Example 10
[0149] Comparative Example 1 (JST) and Comparative Example 2 (NSC)
were low in A.sub.s because the values of expression (1) exceeded
1,050. Comparative Example 3 (JST-2) had a value of A.sub.f-M.sub.s
exceeding 600.degree. C. and generated the .alpha. phase upon
cooling, because the chromium content was excessively high and the
value of expression (1) exceeded 1,050.
[0150] Comparative Example 4 (corresponding to SUS304) contained
only the .gamma. phase even at -50.degree. C. because the nickel
content was excessively high. Comparative Example 5 (SUS420),
Comparative Example 6, and Comparative Example 7 generated the
.alpha. phase because each alloy had an improper component
balance.
[0151] Comparative Example 8 was low in A.sub.s because the value
of expression (1) exceeded 1,050. Comparative Example 9 generated
the .alpha.' phase because the chromium content was excessively
high. Furthermore, Comparative Example 10 contained only the
.gamma. phase even at -50.degree. C. because the nitrogen content
was excessively high.
[0152] In contrast, Examples 1 to 28 at -50.degree. C. each
contained the .epsilon. phase and contained neither the .alpha.
phase nor the .alpha.' phase, because the components had been
optimized. The degree of change in length during heating was 0.3%
or higher in each Example. The value of A.sub.f-M.sub.s was
300.degree. C. or smaller in each Example, and A.sub.s was
90.degree. C. or higher in each Example.
[0153] In FIG. 2 is shown a heating-cooling transformation curve
for the alloy of Example 7. It can be seen from FIG. 2 that
transformations between .gamma. and .epsilon. occur during
heating/cooling and this results in reversible changes in
shape.
[0154] In FIG. 3 is shown the relationship between A.sub.f-M.sub.s
and A.sub.s in the alloys of the Examples and Comparative Examples.
In each of the alloys of the Examples, in which the structure is
the .epsilon. phase or is constituted of the .epsilon. phase and
the .gamma. phase, the A.sub.s is on the relatively low-temperature
side and the A.sub.f-M.sub.s is relatively small. In contrast, the
alloys of the Comparative Examples including the .alpha. phase or
.alpha.' phase tend to have an A.sub.s of 600.degree. C. or higher
and a large value of A.sub.f-M.sub.s.
3.2. Thermal Fatigue Test
[0155] In FIG. 4 is shown the relationship between the temperature
change and the stress generated in the test piece in the first
cycle, 100th cycle, and 300th cycle in the alloy obtained in
Example 2.
[0156] It can be seen from FIG. 4 that
[0157] (1) throughout the thermal fatigue test, the transformation
temperatures in heating (A.sub.s and A.sub.f) and the
transformation temperature in cooling (M.sub.s) were almost
constant, and
[0158] (2) the stress generated was almost constant regardless of
the number of repetitions.
[0159] It was found from the results given above that the alloys
according to the invention exhibit stable characteristics when used
as two-way shape-recovery alloys.
3.3. Tensile Test
[0160] Table 4 shows the results of the tensile test. As shown in
Table 4, the followings can be seen.
[0161] (1) Some of the Comparative Examples were low in strength,
while all the Examples 1 to 28 had a strength higher than 800
MPa.
[0162] (2) When a certain amount(s) of Al and/or Cu is/are added in
addition to main constituent elements and the aging treatment is
then carried out, tensile strength is further improved.
[0163] (3) When a certain amount(s) of Mo, W, V and/or Co is/are
added, tensile strength is further improved.
TABLE-US-00004 Tensile strength (MPa) Example 1 820 Example 2 873
Example 3 855 Example 4 863 Example 5 903 Example 6 835 Example 7
842 Example 8 863 Example 9 837 Example 10 867 Example 11 887
Example 12 989 Example 13 997 Example 14 1065 Example 15 1013
Example 16 899 Example 17 964 Example 18 997 Example 19 1124
Example 20 946 Example 21 955 Example 22 997 Example 23 948 Example
24 1015 Example 25 976 Example 26 996 Example 27 1004 Example 28
896 Comparative Example 1 834 Comparative Example 2 842 Comparative
Example 3 863 Comparative Example 4 630 Comparative Example 5 753
Comparative Example 6 793 Comparative Example 7 673 Comparative
Example 8 621 Comparative Example 9 1134 Comparative Example 10
593
[0164] While the invention has been described above in detail with
reference to embodiments thereof, the invention should not be
construed as being limited to the embodiments in any way. Various
modifications can be made in the invention without departing from
the spirit of the invention.
[0165] The present application is based on Japanese Application No.
2008-309262 filed on Dec. 4, 2008 and Japanese Application No.
2009-266700 filed on Nov. 24, 2009, the contents thereof being
incorporated herein by reference.
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