JP2010156041A - Two-way shape-recovery alloy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a two-way shape-recovery alloy which is inexpensive, has two-way operating 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. <P>SOLUTION: The two-way shape-recovery alloy comprises C<0.20 mass%, 13.0≤Mn≤30.00 mass%, 0.10≤Si≤6.00%, 0.05≤Cr≤12.00 mass%, 0.01≤Ni≤3.00%, and N<0.100 mass%, with the balance Fe with unavoidable impurities, wherein the contents of Mn, Si, Cr and Ni satisfy the following expression (1): 600≤33Mn+11Si+28Cr+17Ni≤1,050. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a bidirectional shape recovery alloy, and more particularly, reversibly transforms a shape at a low temperature and a shape at a high temperature by utilizing expansion and contraction accompanying a phase transformation without substantially using plastic deformation. It relates to a bidirectional shape recovery alloy that can be changed in an automatic manner.

When plastic deformation is applied to a certain material at a low temperature and then heated to a high temperature, the shape returns to the state before the plastic deformation. Such a phenomenon is called a shape memory effect, and an alloy showing the shape memory effect is called a shape memory alloy.
Shape memory alloys
(1) A coil expander for changing the tension of the piston ring according to the temperature (see Patent Document 1),
(2) System for controlling oil flow rate by temperature (see Patent Document 2),
(3) Actuator with various sensor functions for temperature and various switch parts,
Application to such as is expected.

Conventionally, various materials are known as shape memory alloys. Among these, Ti—Ni alloys are one of the most well-known shape memory alloys, and are used for various applications in a state where the shape memory treatment is performed at a high temperature. The shape memory effect of the TiNi-based alloy is derived from the characteristic that when a low temperature phase (martensite phase) twin-deformed by an external force reversely transforms to a high temperature phase (austenite phase), the shape memory treatment returns to the shape.
However, the TiNi-based alloy has a problem that it is difficult to develop a wide range of uses because of high material costs. Further, since the transformation temperature is near room temperature, there is a problem that it cannot be applied to applications requiring a shape recovery temperature of 100 ° C. or higher.

On the other hand, Fe-based shape memory alloys represented by Fe-Mn-Si alloys are characterized by low cost and high shape recovery temperature. The shape memory effect of Fe-based alloys is the stress-induced epsilon martensitic transformation (transformation from the γ (FCC) phase to the ε (HCP) phase induced by plastic deformation at a temperature between the M _s point and the M _d point) This is due to the characteristic that when the ε-phase generated by the process reversely transforms into the γ-phase, it returns to the shape before processing.
However, Fe-based shape memory alloys
(1) The shape memory effect is inferior to the TiNi-based shape memory alloy.
(2) Since it contains Fe, it is inferior in corrosion resistance and oxidation resistance.
(3) Cracks are likely to occur when plastically deformed in the annealed state.
There are problems such as.

In order to solve this problem, various proposals have heretofore been made.
For example, Patent Document 3 discloses a nitrogen-containing iron-based shape memory alloy containing Mn: 28.80%, Si: 5.24%, Cr: 0.20%, N: 0.11%, and the balance being Fe. It is disclosed.
This document describes that alloying nitrogen improves not only the shape memory characteristics of Fe—Mn alloys, but also mechanical characteristics including damping characteristics.

Patent Document 4 discloses an Fe-Mn-Si shape memory alloy that retains at least 15 minutes in a temperature range of more than 1000 ° C to less than 1200 ° C after forming an Fe-Mn-Si alloy having a predetermined composition. A manufacturing method is disclosed.
This document describes that by such a method, cracks during stress deformation caused by precipitation of fine intermetallic compounds enriched in Mn and Si at grain boundaries can be suppressed.

In Patent Document 5, an Fe—Mn—Si based alloy having a predetermined composition is processed at M _d ′ point (temperature at which ε martensite and α ′ martensite are not generated by processing induction) to 700 ° C. A method for producing an iron-based shape memory alloy that is annealed at a temperature of M _d 'point + 200 ° C. or higher is disclosed.
In the same document,
(1) By processing at a temperature equal to or higher than the M _d ′ point, it is possible to suppress the generation of ε martensite and α ′ martensite that impair the workability, and thus the processing limit can be greatly improved.
(2) M _d ′ point + annealing at 200 ° C. or higher causes recovery of strain in the γ phase caused by processing or recrystallization of the γ phase, thereby improving shape memory characteristics,
Is described.

Further, Patent Document 6 discloses an Fe—Mn—Si shape memory alloy obtained by heat-treating an Fe—Mn—Si alloy having a predetermined composition to form an α phase having a thickness of 10 μm or more on the surface. Yes.
In the same document,
(1) When the Fe—Mn—Si alloy is heat-treated in an appropriate atmosphere, an α phase having a body-centered cubic structure with a Mn concentration lower than that of the parent phase (γ phase) is generated on the surface; and
(2) The α phase has higher corrosion resistance than the γ phase and has good consistency with the γ phase, so that even if the matrix phase is deformed, peeling and cracking are less likely to occur, and sufficient corrosion resistance can be obtained. ,
Is described.

International Publication WO2004 / 090318 JP-A-11-264425 Special Table 2000-501778 JP 10-36943 A JP-A-2-213321 JP 7-292448 A

Generally, after a shape memory alloy is plastically deformed at a transformation temperature or lower and then heated to a temperature higher than the transformation temperature, the shape returns to the state before plastic deformation. However, even if it is cooled below the transformation temperature again, it usually does not return to its shape after being plastically deformed at a low temperature. Such a phenomenon of storing only the shape of the high temperature phase is particularly referred to as “one-way shape memory effect”.
On the other hand, when a certain shape memory alloy is strongly processed in the martensite state or deformed in the martensite state and then restrained and heated, a part of the shape of the low temperature phase can also be stored. Such a phenomenon of memorizing both the shape of the high temperature phase and the shape of the low temperature phase is particularly called a “two-way shape memory effect”. For example, it is known that a TiNi alloy in which a texture is partially formed exhibits a two-way shape memory effect.

In various applications such as the coil expander, oil flow control system, and actuator described above, the shape memory alloy often requires two-way operational characteristics. Therefore, in order to apply a shape memory alloy having a one-way shape memory effect to an element that requires two-way operation characteristics, the shape memory alloy and other parts are combined to give the element two-way operation characteristics. There is a need to. Two-way motion characteristics can be imparted in combination with a spring or weight, etc. (two-way motion characteristics) (bias type), two or more shape memory components (differential), etc. It has been known.
However, there is a limit to the miniaturization of the element in the method of imparting operational characteristics in two directions in combination with other components. Therefore, the application field is limited.

On the other hand, all of the conventionally known two-way shape memory alloys are expensive and poor in reproducibility, so that there are few practical examples. In addition, the conventional Fe-based shape memory alloy exhibits a characteristic (that is, a one-way shape memory effect) that recovers from a shape after plastic processing to a shape before plastic processing by reverse transformation (ε → γ). It does not indicate a memory effect.
Furthermore, in order to apply shape memory alloys to various applications, high shape recovery accuracy and strength that can withstand repeated shape recovery are required.
However, it is inexpensive, has bi-directional operating characteristics, has a higher shape recovery temperature than TiNi alloys (specifically, 90-100 ° C or higher), has high shape recovery accuracy, and can withstand repeated shape recovery. There has never been an example in which an alloy having a sufficient strength has been proposed.

  The problem to be solved by the present invention is low cost, bi-directional operating characteristics, higher shape recovery temperature than TiNi alloy, high shape recovery accuracy, and high strength that can withstand repeated shape recovery. The object is to provide a bidirectional shape recovery alloy.

In order to solve the above problems, the bidirectional shape recovery alloy according to the present invention is
C <0.20 mass%,
13.00 ≦ Mn ≦ 30.00 mass%,
0.10 ≦ Si ≦ 6.00 mass%,
0.05 ≦ Cr ≦ 12.00 mass%,
0.01 ≦ Ni ≦ 3.00 mass%,
N <0.100 mass%
And the balance consists of Fe and inevitable impurities,
The gist is to satisfy the following equation (1).
600 ≦ 33 Mn + 111Si + 28Cr + 17Ni ≦ 1050 (1)
The shape recovery alloy according to the present invention further includes:
(1) One or more elements selected from Mo, W, V, and Co, and / or
(2) A predetermined amount of Cu and / or Al + a predetermined amount of Ni
May further be included.

In the Fe—Mn—Si based alloy, when the content of the component elements is optimized, volume shrinkage occurs due to martensite transformation (γ → ε) during cooling, and volume expansion occurs due to reverse transformation (ε → γ) during heating. The shape change accompanying the expansion / contraction is reversible, and the amount of the shape change is relatively large. Further, the shape recovery temperature is higher than that of the TiNi-based alloy (specifically, 90 to 100 ° C. or higher), and the shape recovery accuracy is also high.
Furthermore, an Fe—Mn—Si alloy having a predetermined composition is inexpensive and has a strength that can withstand repeated shape recovery. In particular, when a substitutional solid solution strengthening element such as Mo or a precipitation strengthening element such as Cu is added, the strength is further improved.
Therefore, the bidirectional shape recovery alloy according to the present invention can be used for various functional parts that require two-way operation characteristics.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Bidirectional shape recovery alloy]
The bidirectional shape recovery alloy according to the present invention contains the following elements, the balance being made of Fe and inevitable impurities, and the balance of components satisfying a certain condition. The kind of additive element, its component range, and the reason for limitation are as follows.
In the present invention, “bidirectional shape recovery” means that the shape at low temperature and the shape at high temperature are mainly utilized by utilizing expansion and contraction accompanying phase transformation without substantially using plastic deformation. Reversible change.

[1.1. Main constituent elements]
(1) C <0.20 mass%.
C exists as an interstitial element in Fe and is a strong austenite forming element. Moreover, in normal steel, an α ′ (BCT) phase is formed by quenching, which leads to an improvement in strength. However, the FCC-BCT transformation is a transformation with volume expansion. Further, this transformation strongly depends on the cooling rate of the material, and when the cooling rate is changed, a bainite structure or a ferrite structure is formed and a stable volume expansion cannot be obtained. Furthermore, the bidirectional shape recovery effect cannot be obtained by this transformation.
Therefore, in order to exhibit the bidirectional shape recovery effect, it is necessary to prevent the α ′ phase from being generated by quenching. For that purpose, C content needs to be less than 0.20 mass%. The C content is more preferably less than 0.10 mass%.

(2) 13.00 ≦ Mn ≦ 30.00 mass%.
Mn is an additive element that is essential for stably realizing the bidirectional transformation of γ⇔ε. Mn works as an austenite forming element at high temperatures. At low temperatures, the greater the content, the more easily ε martensite is generated. In order to generate ε martensite, the Mn content needs to be 13.00 mass% or more. The Mn content is more preferably 15.00 mass% or more.
On the other hand, if the Mn content is excessive, the transformation temperature during cooling is greatly reduced, and the austenite phase may become a stable phase even at -50 ° C. Therefore, the Mn content needs to be 30.00 mass% or less. The Mn content is more preferably less than 25.00 mass%.

(3) 0.10 ≦ Si ≦ 6.00 mass%.
Si is an element that reduces stacking fault energy and promotes transformation from the γ phase to the ε phase. For that purpose, Si content needs to be 0.10 mass% or more. More preferably, Si content is 0.30 mass% or more.
On the other hand, when the Si content is excessive, the solid solution strengthening becomes remarkable and the ductility of the material is lowered. Therefore, the Si content needs to be 6.00 mass% or less. The Si content is more preferably 4.00 mass% or less.

(4) 0.05 ≦ Cr ≦ 12.00 mass%.
Cr has the effect of controlling the transformation temperature from the γ phase to the ε phase and the effect of improving the corrosion resistance of the material. In order to acquire such an effect, Cr content needs to be 0.05 mass% or more.
On the other hand, Cr acts as an α-stabilizing element at a high temperature. Therefore, if the Cr content is excessive, the structure after heat treatment tends to form an α ′ martensite structure. Therefore, the Cr content needs to be 12.00 mass% or less.

(5) 0.01 ≦ Ni ≦ 3.00 mass%.
Ni has the effect of adjusting the transformation temperature without causing a structural change during heat treatment by changing the amount of Ni added. In order to acquire such an effect, Ni content needs to be 0.01 mass% or more.
On the other hand, since Ni is a strong austenite forming element, when the Ni content is excessive, a structural change occurs. Therefore, Ni content needs to be 3.00 mass% or less.

(6) N <0.100 mass%.
N forms an N compound with Al or the like, and adversely affects hot workability and cold workability. Further, N dissolves in the form of interstitial elements in Fe and becomes a strong austenite forming element. When the N content becomes excessive, the transformation behavior changes like C, and an α ′ (BCT) phase is formed by quenching.
Therefore, in order to exhibit the bidirectional shape recovery effect, it is necessary to prevent the α ′ phase from being generated by quenching. For that purpose, N content needs to be less than 0.100 mass%. The N content is more preferably less than 0.050 mass%.

[1.2. Inevitable impurities]
Specific examples of inevitable impurities include the following.
(1) P <0.050 mass%.
P is inevitably mixed from the raw material. P is an element that segregates at the grain boundaries and reduces the hot workability of the material. Therefore, the P content is preferably less than 0.050 mass%. The P content is more preferably less than 0.010 mass%.

(2) S <0.100 mass%.
S is inevitably mixed from the raw material. S segregates at the grain boundaries and inhibits hot workability. In the present invention, since the Mn content is large, even if S is mixed, MnS is formed and the influence on hot workability is small, but the smaller the S, the better. Therefore, the S content is preferably less than 0.100 mass%. The S content is more preferably less than 0.050 mass%.

(3) O <0.050 mass%.
O is inevitably mixed in the steel. O forms an oxide with Al and Si and adversely affects hot workability and cold workability. Therefore, the O content is preferably less than 0.050 mass%. The O content is more preferably less than 0.020 mass%.

(4) Mo <0.10 mass%.
(5) W <0.10 mass%.
(6) V <0.05 mass%.
(7) Co <0.10 mass%.
All of Mo, W, V, and Co may inevitably be mixed into the steel. These elements are not elements that greatly affect the transformation temperature and the structure morphology, but are preferably less than the above values.
All of these elements function as substitutional solid solution strengthening elements. In such a case, more than the above value may be added. This point will be described later.

(8) Cu <0.10 mass%.
Cu is an element inevitably mixed from the raw material. When the Cu content is excessive, red hot brittleness is exhibited and the workability is remarkably deteriorated. In order to maintain workability, the Cu content is preferably less than 0.10 mass%. The Cu content is more preferably less than 0.05 mass%.
It is also possible to positively add Cu on the condition that a predetermined amount of Ni is added, and to perform precipitation strengthening by secondary Cu precipitation. In such a case, the Cu content is allowed up to 1.00 mass%. This point will be described later.

(9) Al <0.10 mass%.
Since Al is used as a deoxidizing material like Si, it is inevitably mixed in. Al forms an oxide with O and adversely affects hot workability and cold workability. Therefore, the Al content is preferably less than 0.10 mass%.
In addition, it is also possible to positively add Al on condition that a predetermined amount of Ni is added, and to improve the strength by secondary precipitation of the AlNi intermetallic compound. In such a case, the Al content is allowed up to 1.00 mass%. This point will be described later.

[1.3. Ingredient balance]
The bidirectional shape recovery alloy according to the present invention needs to satisfy the following formula (1) in addition to the component elements being in the above-described range.
600 ≦ 33 Mn + 111Si + 28Cr + 17Ni ≦ 1050 (1)

The value obtained from the formula (1) has a correlation with the transformation temperature of the alloy and is obtained empirically. By optimizing the component balance of Mn, Si, Cr, and Ni, it is possible to stably secure the γ phase at a high temperature (300 ° C. or higher) and the ε phase at a low temperature (−50 ° C. or lower).
As described above, Mn is mainly an austenite forming element and an element that forms an ε phase after cooling. Si promotes the formation of the ε phase from the γ phase at low temperatures, but acts as an α stabilizing element at high temperatures. Cr acts as an α-stabilizing element at high temperatures, but is an element that controls the transformation temperature from the γ phase to the ε phase. Ni is an element that controls the transformation temperature from the γ phase to the ε phase.

The smaller the value of equation (1), the higher the heat transformation end temperature ( _Af point). _{If the Af} point is too high, creep deformation may occur during reverse transformation (ε → γ), and the shape recovery accuracy may decrease. In order to obtain high shape recovery accuracy, the _Af point needs to be 400 ° C. or lower. For that purpose, the value of the formula (1) needs to be 600 or more. The value of the formula (1) is more preferably 700 or more.
On the other hand, the higher the value of equation (1), the lower the heat transformation start temperature (A _s point). When (1) the value is too high, A _s point becomes room temperature or less, the difficult to shape recovery at a temperature higher than the shape recovery temperature of TiNi based alloy. The A _s point higher than the shape recovery temperature of TiNi based alloy, in order to shape recovery at 90 to 100 ° C. or higher temperatures, (1) the value of the expression should be 1050 or less. The value of the formula (1) is more preferably 900 or less.

[1.4. Transformation temperature]
The martensitic transformation (γ → ε) starts at the cooling transformation start temperature (M _s point) and ends at the cooling transformation end temperature (M _f point). On the other hand, reverse transformation (ε → γ) starts at the heating transformation start temperature (A _s point) and ends at the heating transformation end temperature (A _f point).
As described above, (1) to optimize the value of the expression, the A _s point can be 90 ° C. or higher, or 100 ° C. or higher.

In addition, when the bidirectional shape recovery effect is applied to an element that requires two-way operation characteristics, it is desirable that the reversible shape change occurs in a narrow temperature range. That is, the smaller the difference (A _f −M _s ) between the heating transformation end temperature (A _f ) and the cooling transformation start temperature (M _s ), the better. Generally, A _f -M _s in a low alloy steel is 200 to 300 ° C. or more. However, in the bidirectional shape recovery alloy according to the present invention, component elements that affect the transformation temperature such as Mn and Si described above are included. By optimizing, A _f −M _s can be made 200 to 300 ° C. or less. In order to reduce the hysteresis during heating / cooling, A _f −M _s is preferably 150 ° C. or less. A _f -A _s is more preferably 100 ° C. or lower.
The transformation temperature can be obtained from the intersection point by drawing a tangent line before and after the slope of the expansion / contraction curve changes.

[1.5. Sub-constituent elements]
The bidirectional shape recovery alloy according to the present invention may further include any one or more of the following elements in addition to the elements described above.

[1.5.1. Substitution-type solid solution strengthening element]
(1) 0.10 ≦ Mo ≦ 2.00 mass%.
(2) 0.10 ≦ W ≦ 2.00 mass%.
(3) 0.05 ≦ V ≦ 1.00 mass%.
(4) 0.10 ≦ Co ≦ 5.00 mass%.
When strength improvement is desired in the bidirectional shape recovery alloy according to the present invention, a substitutional solid solution strengthening element can be added as long as the transformation behavior due to heating and cooling is not affected. Examples of substitutional solid solution strengthening elements include Mo, W, V, and Co. Any one of these may be added, or two or more may be added.
In order to achieve solid solution strengthening, it is preferable that the contents of Mo, W, V, and Co are each not less than the above lower limit value.
On the other hand, if the content of these elements is excessive, the effect is saturated or the cost is increased, and the transformation behavior may be affected. Therefore, the content of these elements is preferably set to the upper limit value or less.

[1.5.2. Precipitation strengthening element]
(5) 0.10 ≦ (Cu + Al) ≦ 1.00 mass%.
(6) Ni ≧ (Cu + Al).
When Cu is added alone, Cu precipitates at the grain boundary, and the hot workability decreases. However, when a predetermined amount of Ni is added at the same time as Cu is added, Ni suppresses precipitation of Cu at the grain boundaries. As a result, Cu is secondarily precipitated in the grains, and the strength is improved.
In order to obtain such an effect, the Cu content is preferably set to 0.10 mass% or more. On the other hand, when Cu content becomes excessive, hot workability will fall. Therefore, the Cu content is preferably 1.00 mass% or less.
In order to enhance precipitation strengthening without deteriorating hot workability, Ni is preferably added in an amount equal to or greater than that of Cu, and more preferably at least twice that of Cu.

Similarly, when Al is added alone, a large amount of oxide is generated, and hot workability and cold workability are reduced. However, when a predetermined amount of Ni is added at the same time as Al is added, NiAl-based intermetallic compounds are secondarily precipitated and the strength is improved.
In order to obtain such an effect, the Al content is preferably 0.10 mass% or more. On the other hand, when the Al content is excessive, hot workability and cold workability are lowered. Accordingly, the Al content is preferably 1.00 mass% or less.
In order to enhance precipitation strengthening without reducing hot workability or cold workability, Ni is preferably added in an amount equal to or greater than that of Al, and more preferably twice that of Al. That's it.

Furthermore, on the condition that a predetermined amount of Ni is added, Cu and Al can be added simultaneously, and precipitation strengthening can be achieved by both Cu and Al. In order to obtain such an effect, the total content of Cu and Al is preferably 0.1% by mass or more.
On the other hand, in order to suppress a decrease in hot workability and cold workability, the total content of Cu and Al is preferably 1.00 mass% or less.
Even when Cu and Al are added simultaneously, Ni is preferably added in an amount equal to or greater than the total amount of Cu and Al, and more preferably twice or more the total amount of Cu and Al.

[2. Functional parts using bidirectional shape recovery alloy]
The bidirectional shape recovery alloy according to the present invention reversibly changes the shape at a low temperature and the shape at a high temperature by utilizing expansion / contraction associated with the transformation of γ⇔ε substantially without using plastic deformation. There is an action to change.
Therefore, such a bidirectional shape recovery alloy
(1) Switches and actuators using changes in shape at high temperatures and shapes at low temperatures,
(2) an actuator having a mechanism for amplifying a shape recovery amount associated with a temperature change by using a spring or lever principle;
(3) Switches and actuators that require a shape recovery temperature of 100 ° C or higher,
(4) Piston rig expander (see, for example, Patent Document 1),
(5) a temperature sensitive member used in the oil supply mechanism of the viscous fluid coupling device (see, for example, Patent Document 2),
It can be applied to functional parts such as.

Moreover, although the bidirectional | two-way shape recovery alloy which concerns on this invention can be used as it is, you may use it in the state which gave the surface various surface treatments. Examples of the surface treatment method include nitriding treatment, PVD treatment, and CVD treatment. Such surface treatment can impart oxidation resistance and wear resistance.
The bidirectional shape recovery alloy imparted with wear resistance by the surface treatment can be used for functional parts (for example, a coil spring, a piston ring, etc.) used in a contact environment with a counterpart material.

[3. Production method of bidirectional shape recovery alloy]
The twin-method shape recovery alloy according to the present invention can be manufactured by melting and casting raw materials blended at a predetermined ratio. After forging the ingot into a predetermined shape, it is preferable to perform solution treatment (ST treatment) + air cooling in order to eliminate the influence of forging. The solution treatment temperature is preferably 700 to 1200 ° C.
Moreover, when the precipitation strengthening type element is added, it is preferable to perform an aging treatment after solution treatment + air cooling. The aging treatment is preferably performed at a temperature of 400 ° C. or higher and 600 ° C. or lower for 0.5 hour or more and less than 5 hours.

[4. Action of bidirectional shape recovery alloy]
FIG. 1 shows the temperature change of eutectoid steel (0.77 mass% C) and the change in length accompanying the phase transformation.
The eutectoid steel has a ferrite (α) phase structure near room temperature (point A). When this eutectoid steel is heated to the austenite (γ) phase region, as shown in FIG. 1, the eutectoid steel expands, contracts, and expands along the line A → B → C → D. Further, when the eutectoid steel is gradually cooled from the γ phase region to room temperature, the eutectoid steel contracts, expands and contracts along the line D → E → F → A, and returns to the shape before heating. The reason for shrinkage along the B → C line during heating is that the α → γ transformation occurs. Further, the reason for expansion along the E → F line during cooling is that the γ → α transformation occurs.

On the other hand, when the eutectoid steel is rapidly cooled from the γ-phase region, as shown in FIG. 1, the eutectoid steel contracts and expands along the broken line (DH line), and the shape changes compared to before heating. Further, when the rapidly cooled eutectoid steel is heated again, the eutectoid steel repeatedly expands and contracts along the lines H → J → K → L → M → N → O, and finally reaches the D point.
The length after quenching (H point) is longer than the length before heating (A point) because the eutectoid steel is rapidly cooled from the γ phase region to a temperature below the M _s point, thereby causing martensite with volume expansion. This is because site transformation (γ (FCC) → α ′ (BCT) transformation) occurs. In addition, the expansion or contraction exceeding the length change accompanying thermal expansion occurs at a temperature of 400 ° C. or less because the generation of ε-carbide, decomposition of residual γ, and the formation of θ-carbide occur with increasing temperature. is there.

Generally used Fe-based alloys actively use martensitic transformation and its reverse transformation caused by such heat treatment for structure control.
However, since the γ → α ′ transformation is accompanied by volume expansion during cooling, it cannot be used as a shape recovery alloy that is required to shrink during cooling.
Further, the γ → α ′ transformation strongly depends on the cooling rate of the material. Therefore, when the cooling rate is changed, a bainite structure or a ferrite structure is formed, and stable volume expansion (that is, reproducibility of shape recovery) cannot be obtained.
Furthermore, the α → γ transformation end temperature (A _f point) during heating is as high as 700 ° C. or higher. Further, the difference between the _Af point and the γ → α ′ transformation start temperature (M _s point) during cooling is 200 to 300 ° C. or higher, and the hysteresis during heating and cooling is large.

On the other hand, the bidirectional shape recovery alloy according to the present invention is based on the Fe—Mn—Si alloy and the content of the component elements is optimized, so that the temperature is high (300 ° C. or higher) to low (−50 ° C. or lower). ), The transformation from the γ (FCC) phase to the ε (HCP) phase occurs, and neither the α (BCC) phase nor the α ′ (BCT) phase is generated. Since the γ → ε transformation causes volume shrinkage, it involves shrinkage more than the shape change accompanying heat shrinkage during cooling.
On the other hand, since the ε → γ transformation occurs during heating, the expansion is more than the shape change accompanying thermal expansion. Moreover, the shape change accompanying the expansion / contraction is reversible, and plastic deformation is not required to recover the shape.

Further, the bidirectional shape recovery alloy according to the present invention has a relatively large shape change amount. Specifically, by optimizing the component elements, the length change rate during heating (ΔL / L ₀ × 100) is 0.3% or more, 0.5% or more, or 0.7% or more. Become. When the shape of the bidirectional shape recovery alloy is optimized (for example, a spring shape), the amount of change in shape can be further amplified.
Moreover, the length change rate at the time of cooling shows a value equivalent to the length change rate at the time of heating. Specifically, the rate of change in length per heating / cooling cycle is 0.1% or less, and the shape recovery rate is extremely high. Furthermore, even if the heating / cooling cycle is repeated several hundred times, the shape recovery rate does not deteriorate with time.

Moreover, since the bidirectional shape recovery alloy according to the present invention is based on an Fe—Mn—Si alloy, its shape recovery temperature (A _s point) is higher than that of a conventional TiNi alloy. Further, by optimizing the component elements, the hysteresis (A _f −M _s ) during heating and cooling becomes smaller than that of a general Fe-based alloy.
Specifically, optimizing the component elements so as to satisfy the expression (1), A _s point becomes 90 ° C. or higher, or 100 ° C. or higher. Similarly, when the component elements are optimized so as to satisfy the expression (1), A _f −M _s becomes 200 ° C. or lower, 150 ° C. or lower, or 100 ° C. or lower.

Furthermore, since the bidirectional shape recovery alloy according to the present invention is based on an Fe—Mn—Si alloy, it is inexpensive and has strength that can withstand repeated shape recovery. In particular, when a substitutional solid solution strengthening element such as Mo or a precipitation strengthening element such as Cu is added, the strength is further improved.
Therefore, the bidirectional shape recovery alloy according to the present invention can be used for various functional parts that require bidirectional operation characteristics.

(Examples 1-28, Comparative Examples 1-10)
[1. Preparation of sample]
Materials having chemical compositions shown in Tables 1 and 2 (each 50 kg) were melted in a high-frequency heating melting furnace and cast. The obtained ingot was kept soaked at 1200 ° C. for 24 hours, then forged to φ30 at a temperature of 800 ° C. or higher and gradually cooled. In order to eliminate the influence of forging conditions, etc., a solution treatment at 800 ° C. for 30 minutes + air cooling was performed.
Furthermore, about Examples 10-13 which added 0.1 mass% or more of Cu, and Examples 14-18 which added Al of 0.1 mass% or more, the aging treatment was performed after solution treatment + air cooling. The aging temperature was 500 ° C., and the aging time was 1.5 hours.

[2. Test method]
[2.1. Transformation temperature and length change rate]
Using a differential thermal expansion coefficient measuring apparatus, the transformation temperature (A _s , A _f , M _s , M _f ) during heating / cooling and the length change rate (expansion coefficient) during heating transformation were measured. The specimen shape was φ5 × 20 mm, the heating rate during heating was 10 ° C./min, and the cooling rate during cooling was 10 ° C./min.

[2.2. Organization]
The sample held at −50 ° C. was subjected to X-ray diffraction to identify the phase. For the X-ray, Co _Kα ray was used.

[2.3. Thermal fatigue test]
A thermal fatigue test was performed using a test piece having a parallel part length of 40 mm. Of the parallel part of the test piece, the strain measurement part (region having a length of 15 mm) was heated, and both ends of the test piece were restrained when the maximum temperature was reached. From this state, cooling and heating were repeated for 300 cycles, and the relationship between the temperature change and the stress generated in the test piece was investigated. The maximum temperature was 300 ° C and the minimum temperature was 50 ° C. The heating rate was 250 ° C./min on average, and the cooling rate was 83 ° C./min on average.
[2.4. Tensile test]
A tensile test was performed using a JIS 14A (M18) test piece. The tensile test conditions conformed to JIS Z2241.

[3. result]
[3.1. Transformation temperature, length change rate, and structure]
Table 3 shows the rate of change in length (ΔL / L ₀ × 100) during heating transformation, A _f −M _s , A _s , the value of equation (1), and the structure at −50 ° C.

Comparative Example 1 (JST) and Comparative Example 2 (NSC), to exceed the (1) the value of the expression is 1050, A _s is low. In Comparative Example 3 (JST-2), since Cr is excessive and the value of the formula (1) exceeds 1050, A _f -M _s exceeds 600 ° C., and an α phase is generated during cooling. .
Comparative Example 4 (equivalent to SUS304) was a γ single phase even at −50 ° C. due to excessive Ni. In Comparative Example 5 (SUS420), Comparative Example 6, and Comparative Example 7, the component balance was not appropriate, so an α phase was generated.
Comparative Example 8, since it exceeds the (1) the value of the expression is 1050, A _s is low. In Comparative Example 9, since the Cr was excessive, an α ′ phase was generated. Further, Comparative Example 10 was a γ single phase even at −50 ° C. because N was excessive.

On the other hand, since the components of Examples 1 to 28 are optimized, each includes an ε phase at −50 ° C., and does not include an α phase and an α ′ phase. The rate of change in length during heating was 0.3% or more. A _f -M _s was 300 ° C. or lower, and A _s was 90 ° C. or higher.

In FIG. 2, the transformation curve at the time of the heating-cooling of the alloy of Example 7 is shown. From FIG. 2, it can be seen that the γ⇔ε transformation occurs during heating and cooling, thereby causing a reversible shape change.
Figure 3 shows the relationship between A _f -M _s and A _s of the alloy of Examples and Comparative Examples. Alloy tissue is epsilon phase or epsilon + gamma phase embodiment, A _s is relatively located on the low temperature side, A _f -M _s also relatively small. On the other hand, the alloy of the comparative example including the alpha phase or alpha 'phase and, at the same time A _s is equal to or higher than 600 ℃, A _f -M _s also tended to increase.

[3.2. Thermal fatigue test]
FIG. 4 shows the relationship between the temperature change in the first, 100th and 300th cycles of the alloy obtained in Example 2 and the stress generated in the test piece.
From FIG.
(1) During the thermal fatigue test, the heating transformation temperature (A _s , A _f ) and the cooling transformation temperature (M _s ) are substantially constant,
(2) The generated stress is almost constant regardless of the number of repetitions.
I understand.
From the above results, it has been found that the alloy according to the present invention exhibits stable characteristics as a bidirectional shape recovery alloy.

[3.3. Tensile test]
Table 4 shows the results of the tensile test. From Table 4,
(1) While the comparative example contains a low-strength material, the materials of Examples 1 to 28 all have a tensile strength exceeding 800 MPa.
(2) In addition to the main constituent elements, when a predetermined amount of Al and / or Cu is added and an aging treatment is performed, the tensile strength is further improved.
(2) In addition to the main constituent elements, when a predetermined amount of Mo, W, V, and / or Co is added, the tensile strength is further improved.
I understand that.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

  The bidirectional shape recovery alloy according to the present invention can be used for switches and actuators utilizing temperature changes, piston ring expanders, temperature sensitive members used in oil supply mechanisms for viscous fluid joints, and the like.

It is a figure which shows the change of the length accompanying the temperature change and phase transformation of eutectoid steel (0.77mass% C). It is a figure which shows the transformation curve at the time of the heating-cooling of the alloy of Example 7. FIG. It is a diagram showing the relationship between A _f -M _s and A _s of the alloy of Examples and Comparative Examples. 3 is a result of a thermal fatigue test of the alloy obtained in Example 2. FIG.

Claims (4)

C <0.20 mass%,
13.00 ≦ Mn ≦ 30.00 mass%,
0.10 ≦ Si ≦ 6.00 mass%,
0.05 ≦ Cr ≦ 12.00 mass%,
0.01 ≦ Ni ≦ 3.00 mass%,
N <0.100 mass%
And the balance consists of Fe and inevitable impurities,
A bidirectional shape recovery alloy that satisfies the following formula (1).
600 ≦ 33 Mn + 111Si + 28Cr + 17Ni ≦ 1050 (1) The difference (A _f −M _s ) between the heating transformation end temperature (A _f point) and the cooling transformation start temperature (M _s point) is 150 ° C. or less,
The bidirectional shape recovery alloy according to claim 1, wherein the heat transformation start temperature (A _s point) is 100 ° C or higher. 0.10 ≦ Mo ≦ 2.00 mass%,
0.10 ≦ W ≦ 2.00 mass%,
0.05 ≦ V ≦ 1.00 mass%, and
0.10 ≦ Co ≦ 5.00 mass%
The bidirectional shape recovery alloy according to claim 1 or 2, further comprising any one or more of: Further including 0.10 ≦ (Cu + Al) ≦ 1.00 mass%,
The bidirectional shape recovery alloy according to claim 1, wherein Ni ≧ (Cu + Al).

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