JPS62199757A - Manufacture of shape memory alloy material - Google Patents

Abstract

PURPOSE:To manufacture an NiTi shape memory alloy material having reversible shape change characteristics and small hysteresis by specifying the Ni content in an NiTi alloy as stock, conditions during heat treatment, the extent of strain produced during final shape memory heat treatment, and the cold working rate. CONSTITUTION:A hot worked material of an NiTi shape memory alloy consisting of 50.0-60.0wt% Ni and the balance essentially Ti is cold worked at <=60% working rate within the range defined by a line connecting point A (50.0% Ni content, 60.0% working rate), point B (50,0% Ni content, 15.0% working rate), point C (56.0% Ni content, 1.0% working rate) and point D (60.0% Ni content, 1.0% working rate). the cold worked material is subjected to soln. heat treatment at >=650 deg.C for >=1min and 0.2-5.0% strain is produced in the material by final shape memory heat treatment at 300-650 deg.C for >=1min. Thus, a shape memory alloy material having desired characteristics is stably manufactured in such simple treatment shapes.

Description

[Detailed Description of the Invention] <Industrial Application Field> This invention is applicable to temperature-sensitive actuators, thermostats,
Or NiT, which has reversible shape change characteristics and low hysteresis, is suitable for use as a fuse substitute electric circuit interrupting member, etc.
This invention relates to a method for manufacturing a shape memory alloy material.

<Prior art and its problems> It is well known that most alloys with thermoelastic martensitic transformation exhibit a shape memory effect, but among them, NiTi alloy is a polycrystalline material that is easy to manufacture. Moreover, since it does not contain precious metals, it is inexpensive and has a large amount of recovery strain, making it the shape memory alloy that is most commonly put into practical use.

Of course, the NiTi shape memory alloy, like other alloys with shape memory effects, also has a martensitic transformation start temperature (Ms
point), martensitic transformation end temperature (Mf point), reverse transformation start temperature (As point), and reverse transformation end temperature (Af point).
NiTi shape memory alloys have a similar characteristic of "recovering to their original shape when heated to a high temperature exceeding the f point," but NiTi shape memory alloys in particular repeat this deformed shape recovery behavior over several dozen cutting edges. Therefore, it is used in various temperature-sensitive actuators in combination with Paiska.

However, the temperature difference between the Ms point and Af point (i.e., Mf
It is also the temperature difference between the point and the As point, hereinafter referred to as “hysteresis”.
As shown in Figure 2, the temperature is as high as 30 to 40°C, and it only has the so-called ``unidirectional operating performance'' that operates only when there is a reverse transformation from the low-temperature martensite phase to the high-temperature parent phase. The problem was pointed out that there was no such thing.

For this reason, we have improved the above-mentioned drawbacks seen in NiTi shape memory alloys in the past, and have decided not to rely on Piasuka (
We attempted to equip the NiTi shape memory alloy itself with a reversible operating function as shown in Figure 3 [After solution heat treatment at 500 to 600°C or higher, final shape memory heat treatment at 600°C or lower with a constrained strain amount is performed. A processing method for NiTi shape memory alloy has also been proposed, but the reversible shape memory effect of NiTi shape memory tα alloy depends on comprehensive factors such as ``alloy composition,'' ``heat treatment temperature and time,'' and ``amount of restraint strain.'' Since the characteristics are subtly influenced by various factors, it has been extremely difficult to stably exhibit the desired characteristics even if the previously proposed methods described above were implemented as they are.

<Means for Solving the Problems> The present inventors have developed a NiTi shape memory alloy material that has a reversible shape memory effect and has small hysteresis based on the experimental research that has been accumulated so far from the viewpoints described above. He felt that there was some possibility for stable production of the alloy, and pointed to one way to realize it: ``If a reversible shape memory effect appears, it would be possible to remove marten from the matrix during cooling of the alloy.'' When transforming to the site phase, a phenomenon that is accompanied by ``intermediate phase transformation'' generally occurs, as shown in Figure 4 (a).
Taking this fact into account, we created NiTi alloys with Ni content varying over a wide range, and systematically varied the hot working conditions, cold working conditions, heat treatment conditions, etc. to develop the "intermediate phase". As we continued our basic research on appearance behavior and the "reversible shape memory effect," we came to the following findings. In other words, (alNi Ti shape memory alloy), the heat treatment temperature, heat treatment time, and amount of strain are the same for hot-worked materials, and the cold working rate, heat treatment temperature, heat treatment time, and amount of strain are for cold-worked materials. The amount of applied strain is closely intertwined with each other and affects the properties of the shape memory alloy, and if the comprehensive conditions in which these are delicately intertwined are ignored, it is impossible to stably produce a sufficient reversible shape memory effect. (b) NiT containing Ni in a specific proportion while
i Hot-worked shape memory alloy material is subjected to cold working at a specific working rate that also takes into account its Ni content, and if necessary, heat treatment at 300°C or higher, including solution heat treatment, for 1 minute or more. If the shape memory heat treatment is performed once or multiple times and the final shape memory heat treatment is performed while applying a strain of 0.2 to 5.0% to the final processed shaped material, the above-mentioned "intermediate phase" generation will be extremely easy. Moreover, since it is performed in a stable state, the reversible shape memory effect appears smoothly and stably, and also because "intermediate phase transformation" is closely related to the appearance of small hysteresis, the fifth As shown in the figure, "a material with a small temperature difference (ΔH) between the operating curves during the temperature rising process and the temperature cooling process" was unexpectedly realized.

This invention was made based on the above findings, and contains 50.0 to 60.0% (hereinafter, % representing the component ratio is expressed as weight %) of Ni, and the remainder is substantially Ti. 60% or more on the line connecting points A, B, C, and D in FIG.
The following cold working is performed, or the cold working is followed by 6
After performing solution heat treatment for 1 minute or more at a temperature exceeding 50'C, further perform final shape memory heat treatment at 300 to 650 °C for 1 minute or more under strain of 0.2 to 5.0%. This feature makes it possible to easily and stably produce a NiTi shape memory alloy material that has reversible shape change characteristics and extremely small hysteresis.

Next, in the method of this invention, the target material old Ti
The reason why the Ni content of the alloy, the heat treatment conditions, the amount of strain imparted during the final shape memory heat treatment, and the cold working rate are limited as described above will be explained.

A) Ni content ratio of NiTi alloy Even if the Ni content ratio of the NiTi alloy as a raw material is less than 50.0% or exceeds 60.0%, its hot workability and cold workability deteriorate significantly, Since not only would molding become difficult, but also a shape memory alloy material with desired characteristics could not be stably realized, the Ni content of the NiTi alloy was determined to be 50.0 to 60.0%.

B) Final shape memory heat treatment temperature and holding time If the final shape memory heat treatment temperature is less than 300°C, it is not possible to obtain precipitates or crystal structures that are effective for the shape memory effect.
The "internal stress field" in the NiTi shape memory alloy, which is realized by the synergistic effect with the strain force applied during the final shape memory heat treatment, is not generated even by long heat treatment, and on the other hand, when the temperature exceeds 650 °C In this case, the amount of the precipitates or the crystal structure dissolves or disappears, and as in the case of less than 300"C, it is no longer possible to generate an "internal stress field" in a short heat treatment, and in any case, the desired result cannot be achieved. Shape memory effect cannot be obtained.

Further, even if the heat treatment time at this time is less than 1 minute, it is not possible to stably provide a sufficient shape memory effect, as in the case of the above-mentioned "heating temperature of less than 300°C".

Therefore, in the final shape memory heat treatment, the heating holding temperature was set at 300 to 650°C, and the heating holding time was set at 1 minute or more.

C) Amount of strain imparted during final shape memory heat treatment Amount of strain imparted during final shape memory heat treatment (constraint strain amount) is 0
．． If it is less than 2%, sufficient contribution will not be made to the generation of the "internal stress field", while if it exceeds 5.0%, the amount of deformation of the material will become too large and the restraint itself will become impossible.
Alternatively, an adaptive force field may occur and the intermediate phase may not be generated, resulting in inconveniences such as the reversible shape memory effect not being realized or hysteresis increasing.

Therefore, the amount of strain imparted during the final shape memory heat treatment is 0.2 to 5.
．． It was set as 0%.

D) Cold working rate The cold working rate is at point A in Figure 1 (Ni content: 50.
0%, processing rate: 60.0%), B (Ni amount: 5
0.0%, processing rate 715.0%), C (Ni amount: 5
6.0%, processing rate = 1.0%), D (Ni amount: 6
0.0%, processing rate: 1.0%), the effect of improving the formation of an intermediate phase and reducing the hysteresis during the final shape memory heat treatment will not be sufficiently noticeable.
On the other hand, if the cold working rate exceeds 60%, there is a risk that harmful surface cracks or fractures will occur. It was set as above the line connecting D and below 60%.

D) Processing conditions for solution heat treatment If the heating holding temperature during solution heat treatment is 650°C or lower or the holding time is less than 1 minute, sufficient solution heat treatment will not be achieved and the final shape memory heat treatment will not be possible. The amount of precipitates at the time of heating is not improved to an extent commensurate with the addition of the solution heat treatment step, and further improvement of the reversible shape memory effect cannot be expected.

Therefore, the solution heat treatment conditions are: heating holding temperature: 650°C
and retention time: 1 minute or more, respectively.

Next, the present invention will be specifically explained with reference to Examples.

<Example> First, a NiTi alloy having a composition as shown in Table 1 was melted in a reduced pressure inert gas atmosphere, and hot rolled at a temperature slightly below 950°C to obtain a plate material of Q, 3x thickness. After obtaining the material, it was cold rolled at a rolling reduction of 1 to 55%, and a part of it was also subjected to solution heat treatment at a temperature exceeding 650°C.

Next, the plate material obtained in this way is formed into the final target shape, and then 0°2 to 4.5
Heating and holding temperature with % strain added: 300-70
A final shape memory heat treatment was performed at 0° C. for a holding time of 1 to 6000 minutes, followed by rapid cooling.

Added Strain M (%) Next, the existence of the mesophase transformation peak shown in Figure 4(a) was investigated using a differential scanning calorimeter, and those having the mesophase transformation peak were examined as shown in Figure 4(b). The Af point and mesophase transformation start temperature (Ms
' ) was measured.

In addition, we investigated whether the shape changes spontaneously due to temperature changes as shown in Figure 3, and from this we determine whether there is a reversible shape memory effect. ΔH was determined from the temperature-lowering process operating curve.

These results are shown in Table 1.

The results shown in Table 1 also show that the NiTi shape memory alloy material produced according to the conditions specified in the present invention has an "intermediate phase" and a "reversible shape memory function", and has a Ms' point and an Af point. The temperature difference (hysteresis) is small at 4 to 6℃, and Δ
It is clear that H is also very small at 1 to 2°C, whereas materials obtained under manufacturing conditions that deviate from the specifications of the present invention have "intermediate phase" and "reversible shape memory function". It can be seen that the temperature difference between the Ms point and the Af point is large.

In this example, only the results regarding the plate sample are shown, but it goes without saying that similar effects can be obtained even when the material is shaped like a wire rod or any other shape.

<Overall Effects> As explained above, according to the present invention, it is possible to realize an N1Tt shape memory alloy material having reversible shape change characteristics and low hysteresis through a simple processing process, and it is possible to realize it by itself. Regardless of whether it is used in conjunction with a auxiliary PIASUKA, it will be possible to further expand the field of use of shape memory alloys and further improve the performance of various devices such as temperature-sensitive actuators and thermostats. This brings about extremely useful effects industrially.

[Brief explanation of drawings]

Fig. 1 is a graph showing the range of cold working rate necessary for producing the desired excellent reversible shape memory effect in NfTi shape memory alloy in relation to the Nt content. A graph showing the transformation point measurement results of a conventional N1Tj shape memory alloy using a differential scanning calorimeter, Figure 3 shows the sample shape during the final shape memory heat treatment carried out under constraint strain and has a reversible shape memory function. FIG. 3(a) is a schematic diagram showing an example of shape change of a sample; FIG. 3(a) shows the shape of the sample during final shape memory heat treatment, and FIG. 3(b) shows shape change of a sample with reversible shape memory function. FIG. 4, which shows examples, shows a transformation point measurement curve using a differential scanning calorimeter where an intermediate phase has appeared, and a transformation point measurement curve using a differential scanning calorimeter for measuring the temperature difference between the Ms' point and the Af point. It is a curve, as shown in Figure 4 (a
)° is the transformation point measured curve using a differential scanning calorimeter where an intermediate phase has appeared, and Fig. 4(b) shows the transformation curve measured using a differential scanning calorimeter to measure the temperature difference between the Ms' point and the Af point. FIG. 5, which shows point measurement curves, is an operating curve of the NiTi shape memory alloy material in the temperature rising and cooling process. Applicant Nippon Stainless Co., Ltd. Agent Patent Attorney Takeshi Imai I I Figure 12 Figure 3 (b) χ4 diagram

Claims (1)

[Claims] (1) Points A, B in FIG. 1,
Perform cold working on the line connecting C and D to 60% or less,
Subsequently, a final shape memory heat treatment at 300-650°C was performed to a temperature of 0.
A method for producing a NiTi shape memory alloy material having reversible shape change characteristics and small hysteresis, the method comprising applying a strain of 2 to 5.0% for 1 minute or more. (2) Points A, B in FIG. Cold working is performed on the line connecting C and D to 60% or less, and then 650%
After performing solution heat treatment for 1 minute or more at a temperature exceeding ℃,
Furthermore, the final shape memory heat treatment at 300-650℃ was performed by 0.2
A method for producing a NiTi shape memory alloy material having reversible shape change characteristics and having small hysteresis, the method comprising applying a strain of ~5.0% for 1 minute or more.