JPS6263657A - Method for stabilizing shape memory characteristic of shape memory ti-ni alloy - Google Patents

Abstract

PURPOSE:To stabilize the shape memory characteristics of a shape memory alloy by selectively accumulating dislocation in the grains of the alloy so as to increase the dislocation density in the grains and to keep the dislocation density on the grain boundaries low. CONSTITUTION:Stock for a shape memory alloy is made to memorize a prescribed shape (a). The stock is so largely elongated as to cause plastic deformation (b). The load is relieved from the stock and the shape of the stock is restored by heating to the Md point or above at which restoration recrystallization is not caused or is hardly caused (c). The stock is rapidly cooled (d). The stages (b), (c), (d) are repeatedly carried out until the restored shape is stabilized.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention is directed to a Ti-Ni shape memory alloy which is used to stabilize the shape memory properties of a shape memory alloy when the alloy undergoes repeated deformation and shape recovery operations. -Regarding a method for stabilizing the shape memory properties of a Ni shape memory alloy.

Problems to be solved by conventional techniques and inventions The transformation of Ti-Ni shape memory alloys is complicated by problems such as martensitic transformation, precipitation, and oxidation, and it is difficult to detect each phase. The current situation is that different opinions are being advocated by different people.

Especially in the temperature range below 12H''K, there are still many points that are unclear, and therefore a complete heat treatment method for 1'-1-1'Ji shape memory alloys has not yet been established.

A conventional method for producing a Ti--Ni shape memory alloy and for memorizing a predetermined shape involves the following steps.

First, a shape memory alloy ingot of 100 to 1200
After hot working into a rough shape at a high temperature of 100°, cold working little by little and annealing at a temperature of 900 to 1000° are repeated to approximate the desired shape.

The reason why the annealing operation is repeated in this way is that the Ti--Ni shape memory alloy tends to undergo strong work hardening. Commercially available Ti-Ni shape memory alloy materials are processed through such a process, and are still in a very hard state after being cold-worked. The memory shape required for final use is
After annealing the Ti-44i shape memory alloy material obtained as described above, it is fixed while being deformed into a predetermined shape.
1511Iin~1h at a temperature of 600~1000'K
It is obtained by holding r.

Conventionally, when the memory-treated shape memory alloy obtained as described above is repeatedly subjected to the deformation-shape recovery operation, permanent strain occurs as the number of repetitions of the operation increases, and the Ti-Ni shape Problems have arisen in which the memory alloy loses its memorized shape (in other words, it does not return to its memorized shape) or breaks. Such problems of loss or breakage of the memorized shape are particularly likely to occur when the maximum heating temperature is high and the amount of deformation is large. Therefore, under heavy-duty usage conditions, for example, where shape recovery is performed by electrical heating, etc., which increases the amount of deformation and may also lead to partial heating, the memory associated with the repetitive operation may Problems such as loss of shape and breakage were likely to occur. For example, according to the inventor's experience, when a Ti-Ni alloy subjected to general memory treatment is used as it is in an electrically heated actuator, etc., the memorized shape disappears after the same number of repeated operations. , and it is necessary to geometrically adjust the installation state of the shape memory alloy (
For example, if a wire shape memory alloy is used, the length of the shape memory alloy must be readjusted).

For this reason, conventionally, when using shape memory alloys, the maximum heating temperature was kept low (for example, below A1 point + 60' K) and the amount of deformation was kept sufficiently small (for example, below 2%). ), many researchers and manufacturers recommended heating by conduction heating using a medium such as air or water, which is a heating method that facilitates uniform heating. However, when conducting conduction heating using air, water, or the like as a medium, there is a problem in that the responsiveness and controllability are lowered, and the device becomes larger, which tends to limit its use.

In this regard, energization heating, which heats the shape memory alloy with Joule heat by directly applying electricity to the shape memory alloy, has the advantages of good responsiveness, easy control, and the ability to miniaturize the device. . Therefore, it would be ideal if the shape memory alloy could be caused to recover its shape by heating with electricity without causing problems such as loss of the memorized shape or breakage.

On the other hand, it is well known that shape memory alloys have a phenomenon called a training effect.

Figure 4 shows the deformation-shape recovery behavior of Ti-1'4i shape memory alloy at a low heating temperature within a predetermined deformation or load (in this figure, the deformation is elongation deformation). ) shows the relationship between the number of motion repetitions and the elongation from the memorized shape due to plastic deformation of the shape memory alloy.

As is clear from this figure, in shape memory alloys,
If the heating temperature is kept low and the deformation-shape recovery operation is repeated within a predetermined deformation or load, the shape will gradually become stable and the movement will become smooth (the shaded area in Figure 4 shows the shape memory alloy). ), indicating a stable region where the shape is stable upon recovery. This is the phenomenon called the learning effect.

The present inventor previously disclosed in Japanese Patent Application Laid-open No. 60-70167,
We are proposing a ``shape memory alloy break-in method'' that uses the above-mentioned learning effect to stabilize the shape memory properties of shape memory alloys. However, there is no way to stabilize the shape memory properties of shape-memory alloys using such learning effects.
Under heavy-duty usage conditions such as the above-mentioned repeated application of relatively large deformation and heating by electrical heating that tends to cause local heating, the problem of loss or breakage of the memorized shape is still solved. I couldn't do that. Furthermore, when stabilizing the shape memory properties of a shape memory alloy using the inter-character effect as described above, the deformation and shape recovery operations must be repeated quite a number of times, which takes a long time. Moreover, there was also the problem that the fatigue life of shape memory alloys was shortened.

Purpose of the Invention The present invention has been made to solve the above problems.
Even under harsh usage conditions such as repeated relatively large deformations and electrical heating that tends to cause localized heating, it is unlikely to lose its memorized shape or break, and its shape memory properties are stable. The purpose of the present invention is to provide a treatment method for stabilizing the shape memory properties of a Ti-Ni shape memory alloy, which enables the production of a Ti-Ni shape memory alloy with a long fatigue life and a long fatigue life in a very short time. .

Means for Solving the Problems The method for stabilizing the shape memory properties of a Ti-Ni shape memory alloy according to the present invention includes the following steps.

(a) Memorize a predetermined shape in the shape memory alloy material.

(b) Applying large elongation deformation to the material to cause plastic deformation.

(c) Remove the load from the material and heat it to a temperature higher than the Md point and at a temperature at which no recovery recrystallization occurs or only a small amount of recovery recrystallization occurs to recover the shape. .

(d) rapidly cooling the material;

(e) The above (b) and (c) until the shape upon recovery is stabilized.
and repeating step (d).

At the current stage of operation, it has not yet been fully clarified theoretically why such an excellent effect can be achieved, but the method for stabilizing the shape memory properties of 7-i-Ni shape memory alloy according to the present invention According to, relatively large deformations are repeatedly applied,
In addition, even under severe usage conditions such as electrical heating that tends to cause localized heating, it is unlikely to lose its memorized shape or break, and its shape memory properties are stable.
Moreover, a Ti--Ni shape memory alloy with a long fatigue life can be obtained in a very short time.

The present inventor believes that the reason why the above-mentioned excellent effects can be obtained by the present invention is that, as will be explained in detail later, dislocations are selectively accumulated within the grains of the shape memory alloy, and the dislocation density within the grains is reduced. At present, we speculate that this is because the dislocation density at the grain boundaries remains low while the dislocation density increases.

Example Hereinafter, the present invention will be explained based on embodiments shown in the drawings.

In this example, commercially available Ti
-44i shape memory alloy wire (manufactured by Furukawa Electric Co., Ltd.) was subjected to shape memory property stabilization treatment.

(1) Anneal the wire using the usual method. This is to remove work hardening occurring in the wire.

(II) General shape memory processing as described above (e.g.
The predetermined shape (predetermined length) is memorized by fixing the shape of the wire and holding it at a temperature of about 673° for about one hour.

(Apply a large elongation deformation that causes Ill deformation (for example, 7.5 to 10% or more in tensile strain from the shape recovery state). In this case, as a guideline for the above-mentioned "large deformation that causes plastic deformation", As shown in FIG. 1, the point where the stress-strain line suddenly rises during deformation may be used as a guide.

(IV) When the load is removed, at a high temperature above the Md point (A 1 point + 60°K to 100″ or higher), and recovery recrystallization does not occur, or even if recovery recrystallization occurs, it is only slight. The shape is restored by heating it by pulsed current heating to a temperature that only causes a change in shape.

(V-) Cool rapidly. If the wire of the shape memory alloy is thin, this rapid cooling is sufficient due to natural air cooling (even with such natural air cooling, if the wire is thin,
00''/second or more), forced air cooling may be performed in which air is blown onto the shape memory alloy.

Note that when slow cooling is performed at a rate of about 1° C./min, the effects of the present invention cannot be obtained.

(Vl) The above (III) until the shape upon recovery is stabilized.
, (IV)' and (V) are repeated. In this case, if the treatment is appropriate, the length will stabilize at an extremely fast pitch as the number of repetitions increases, and the shape at the time of recovery will become stable within a few or several repetitions, as shown in Figure 2. It can be confirmed that it is stable. Note that the shaded area in FIG. 2 indicates a stable region where the shape during recovery is stable.

(VI) Immediately after the completion of step (VI), the shape memory alloy is repeatedly deformed and restored by applying a stress 50 to 100% higher than the expected stress during actual use. However, in this case, the heating temperature for shape recovery is set to be slightly higher than the Md point.

Shape memory alloys that have been subjected to this shape memory property stabilization treatment are stable in shape from the beginning and can be subjected to relatively large deformations (4 to 7% in tension), as well as to severe conditions such as heating by electrical heating. It was confirmed that even if repeated operations were performed, the memorized shape was unlikely to disappear or break. For example, by using the pulse current heating method previously proposed by the present inventor in Japanese Patent Application No. 59-256547, a stress smaller than the force applied when deforming in step (III) can be applied without raising the temperature greatly exceeding the Af point. If used in this condition, 2
It was experimentally found that there was almost no change in length even after X105 repetitions.

Shape memory alloys subjected to this treatment have different mechanical properties other than operating life compared to those subjected to general shape memory treatment. The solid line in FIG. 3 is a stress-strain diagram of such a shape memory alloy.

On the other hand, the broken line in the figure shows a stress-strain diagram of a shape memory alloy that has been subjected to a general shape memory treatment. Although it does not necessarily change during heating shape recovery, it differs markedly during low temperature deformation.

Products subjected to this treatment can be deformed with a very small stress, and therefore can be deformed with a small piascus, and can be expected to improve efficiency when used as an actuator.

This is thought to be because the reversible shape memory effect caused by the stress remaining in the material due to the large plastic deformation imparted in step (1) effectively works in the same direction as the deformation caused by the Pyaska.

In addition, even when manufacturing a shape memory alloy in a special shape that is difficult to process, such as a coil shape, first the stabilized wire is wound into a coil shape and fixed at a temperature at which recovery recrystallization barely occurs. By holding it for a short time and rapidly cooling it, you can create something with similar durability.

In addition, although detailed research has not yet been conducted, shape memory alloys subjected to this treatment can undergo deformation and shape recovery operations extremely stably many times under certain heat input conditions, so it is possible to simply It appears that not only the memory shape is stabilized, but also the transformation point.

Note that if step (VI[) is performed, the above-mentioned effect can be further improved by 1q, but even if step (VII) is omitted, a certain degree of effect can be obtained.

According to the method of the present invention, it is possible to stabilize the extremely excellent shape memory properties as described above, but as mentioned above, at this stage, it is unclear why such excellent effects can be obtained. Theoretically, this has not yet been fully elucidated. Here, the inventor's reasoning will be described below.

First, the meaning of step (III) is to introduce dislocations into the shape memory alloy, increase the dislocation density of the shape memory alloy, and suppress the appearance of internal shear deformation in the shape memory alloy even under high stress. be.

Furthermore, the meaning of the steps (IV) and (V) is presumed as follows.

When the dislocation density increases in step (III), work hardening occurs in the shape memory alloy. Here, this work hardening appears to be due to an increase in dislocation density at the grain boundaries rather than an increase in dislocation density within the grains of the shape memory alloy. Then, when steps (IV) and (V) are performed, a phenomenon similar to stress relief annealing occurs selectively at the grain boundaries because the energy state at the grain boundaries is higher than that inside the grains. While the dislocations are eliminated, the dislocations remain as they are within the grains, and the dislocation density within the grains is maintained in a high state.

Here, it is considered preferable that the heating operation in (I[I) is performed by electrical heating as in the above embodiment. This is because the grain boundaries have higher electrical resistance between the grain boundaries and inside the grains of shape memory alloys, so when heated by electrical current heating, the grain boundaries are heated to a higher temperature than the grain boundaries, and dislocations at the grain boundaries are more easily suppressed. This is necessary because it is thought that it can be efficiently and selectively eliminated.

By repeating the operations (III), (IV), and (V) several times, the shape memory alloy becomes in a state where the dislocation density becomes higher and higher within the grains, and the dislocation density becomes smaller at the grain boundaries. As a result, it is possible to suppress the appearance of internal slip deformation even under high stress without causing work hardening, and it is possible to obtain excellent stabilization of memory properties such as stabilization of the memory shape as described above. It is assumed that it is possible.

Incidentally, if the temperature is necessarily raised too high in the step (IV), recovery recrystallization will occur within the grains to an extent exceeding the allowable level, resulting in a decrease in dislocation density within the grains as well. Also, if we only heat it to a temperature lower than the Md point,
A disadvantage arises in that residual stress remains in the form of a martensitic phase.

In addition, the process (■) above seems to produce its effect through the same mechanism as the conventional general learning effect, but it requires much fewer repetitions than the conventional learning effect to shape the shape. Memory properties become stable. Therefore, there is no effect on fatigue life.

Effects of the Invention As described above, the present invention solves the problems of loss of memorized shape and breakage even under harsh usage conditions such as repeated relatively large deformations and heating by electrical heating that tends to cause localized heating. It is possible to obtain an l'-1-Ni shape memory alloy in a very short period of time, which is less likely to cause oxidation, has stable shape memory properties, and has a long fatigue life. It is.

[Brief explanation of the drawing]

Figure 1 is a stress-strain diagram of Ti-Ni shape memory alloy,
FIG. 2 is a graph showing the relationship between the number of repetitions and the elongation of the shape memory alloy, which does not show that the shape of the shape memory alloy becomes stable each time a predetermined process is repeated in one embodiment of the present invention.
Figure 3 is a stress-strain diagram of the shape memory alloy treated according to the above example and the shape memory alloy subjected to ordinary heat treatment, and Figure 4 is the learning effect of the conventionally known Ti-Ni shape memory alloy. FIG.

Claims (1)

[Scope of Claim] A method for stabilizing the shape memory properties of a Ti-Ni shape memory alloy, comprising the following steps. (a) Memorize a predetermined shape in the shape memory alloy material. (b) Applying large elongation deformation to the material to cause plastic deformation. (c) Remove the load from the material and heat it to a high temperature above the M_d point, at which no recovery recrystallization occurs or only a small amount of recovery recrystallization occurs, to recover the shape. . (d) rapidly cooling the material; (e) The above (b) and (c) until the shape upon recovery is stabilized.
and repeating step (d).