JP2008050660A - Shape memory alloy - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an iron-manganese-based shape memory alloy having such an improved ductility as to be capable of being plastic-worked by warm working without needing hot working. <P>SOLUTION: The alloy having high ductility and improved plastic working properties without decreasing its shape memory characteristics is the iron-manganese-based shape memory alloy which has a composition containing iron, manganese (Mn) solely, or further chromium (Cr), and/or nickel (Ni) and silicon (Si), has an Ms point of room temperature or lower, and further includes 1 mass% aluminum (Al). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention is an iron (Fe) -A (A: manganese (Mn) alone or a chromium (Cr) or / and nickel (Ni) element) -silicon (Si) alloy having a Ms point of room temperature or less. More particularly, the memory alloy relates to the improvement of ductility.

Although the shape memory alloy is a shape memory alloy that has been expected to be practically known as shown in Non-Patent Document 1 to Non-Patent Document 9, it must be rolled, drawn, or the like by hot working. The molding process was difficult.
When hot working is applied, there is a risk that unpredictable property changes may occur, and as a result, processing for eliminating the changed properties in the melting process is required.
Such a melting treatment reduces processing accuracy, and as a result, it has been extremely difficult to obtain a high-precision processed product having stable characteristics.
The optimum temperature range for hot working is as narrow as 800 ° C. to 1000 ° C. When the temperature is higher than this, the workability is drastically lowered and cracking is difficult, which is another factor that deteriorates productivity.

Acta Metallurgica, 30, 1177-1183, 1982, A. et al. Sato, E .; Shishima, K .; Soma, T .; Mori. Plasticity and processing, 45, 697-701, 2004, Hisashi Naoi, Tadakatsu Maruyama. Materials Science and Technology, 14, 1213-1217, 1998, O.M. Grassel, G.M. Frommyer. ISIJ International, 30, 674-679, 1990, H.I. Otsuka, H. et al. Yamada, T .; Maruyama, H.M. Tanahashi, S .; Matsuda, M .; Murakami. Metal, 66, 63-72, 1996, Tadakatsu Maruyama, Hiroaki Otsuka. Materials Science and Engineering A, 315, 174-179, 2001, D.A. F. Wang, D.W. Z. Liu, Z .; Z. Dong, W.D. X. Liu, J .; M.M. Chen. Scripta Materialia, 44, 2809-2814, 2001, S.A. Kajiwara, D.H. Liu, T .; Kikuchi, N.A. Shinya. Materials Transactions, 43, 585-588, 2002, A.A. Baruj, T .; Kikuchi, S .; Kajiwara, N .; Shinya. Journal of the Japan Institute of Metals, 69, 659-662, 2005, Takahiro Sawaguchi, Takeshi Kikuchi, Kazuyuki Ogawa, Nobuo Sugawara, Yosaku Ikeo, Masao Kojima, Takatoshi Ogawa.

  The present invention aims to improve the ductility to such an extent that it can be processed without requiring hot working, based on the recognition that the above problem is caused by the extremely poor ductility of the alloy. It was.

  The shape memory alloy of the present invention employs a configuration characterized by containing 1% by mass of aluminum (Al).

With the above configuration, the shape memory characteristics are not inferior to those of conventional ones, but the ductility exceeds 50% and can be sufficiently processed by warm processing.
In addition, although it is known that the alloy which consists of said element and whose aluminum is 2 mass% or more has favorable ductility, these also obtain practical shape memory characteristics also by a training process. It was something that could not be done.
Compared to this, although it is a difference of only 1%, it has an unpredictable characteristic that it has a high ductility while having a shape memory characteristic equal to or higher than that not containing aluminum. This is the shape memory alloy of the present invention.

Using a high-frequency vacuum induction furnace, four types of alloys shown in Table 1 were melted. Example 1 is an alloy containing 1% by mass of Al. For reference, an Al-free shape memory alloy (Comparative Example 1) and alloys containing 2 and 3% by mass of Al (Comparative Examples 2 and 3) were also produced.
Thereafter, various test specimens were prepared according to the manufacturing process flow of FIG. 1 and subjected to measurement of shape recovery rate and ductility.
In hot forging / rolling, heating at 1000 ° C. and processing were alternately repeated so that the processing temperature would not be 800 ° C. or lower. In the homogenization and solution heat treatment, the sample was vacuum sealed in a stainless steel foil, and heat treatment was performed in an argon atmosphere to prevent surface oxidation and manganese deficiency from the surface.
In this way, the test piece was processed with the same processing conditions and with as little influence as possible from the processing environment.

The shape recovery rate was measured by deforming the test piece along the side of a cylindrical jig having a diameter of 13 mm and then heating it at 600 ° C. for 10 minutes. Some samples were bent due to the shape memory effect. It tries to open to the original linear shape (Fig. 2).
Table 2 shows the shape recovery rate and ductility of the four alloys before and after heating.

It was found that Example 1 to which 1% by mass of Al was added exhibited substantially the same shape memory effect as Comparative Example 1 to which Al was not added, and a much higher ductility than Comparative Example 1.
When Al is further added, the ductility is further increased, but when the added amount of Al is 2% by mass or more, the shape memory effect is hardly exhibited (Comparative Examples 2 and 3).
From Table 2, it can be seen that the ductility increases almost in proportion to the amount of Al added. As the Al content increases by 1%, the ductility increases by as much as 20%. Therefore, it can be easily inferred that the addition amount of Al is effective even with a small amount. In the present invention, from the viewpoint of improving ductility, the lower limit of the amount of Al added is set to 0.5 mass% at which ductility improvement of 10% is expected.

    On the other hand, the shape memory effect is equal to or higher than that without addition even if about 1% of Al is added, but when it becomes 2%, it rapidly decreases and shows only a very small shape recovery. It is estimated that there is an upper limit on whether or not a significant shape memory effect is exhibited between the amount of 1% and 2%, and approximately in the vicinity of 1.5%.

As introduced in various references such as Reference 1, page 1183, and Reference 2, page 697, section 2.1, the shape memory effect in this alloy system (Comparative Example 1) is the austenite of the fcc structure. This is because the crystal undergoes stress-induced transformation to a martensite crystal having an hcp structure due to deformation, and then reverse transformation to the original austenite crystal by heating to a temperature higher than the reverse transformation temperature.
On the other hand, as shown in the introduction on pages 3 and 1213, the structure before deformation of the TWIP steel (Comparative Examples 2 and 3) is the same fcc austenite crystal as that of the shape memory alloy. Instead of transformation, twin deformation occurs.
In Example 1, it is considered that at the time of deformation, both the stress-induced transformation and twin deformation occur simultaneously, with the former responsible for shape memory characteristics and the latter responsible for ductility. This can be confirmed from Table 2 that Example 1 shows intermediate characteristics between Comparative Example 1 and Comparative Examples 2 and 3.

According to Reference 2, page 697, section 2.2, the conditions for obtaining the shape memory effect are: the martensitic transformation start temperature (Ms point) is below room temperature, and the antiferromagnetic magnetic transformation temperature ( _TN point) of γ-austenite. ) Is below the Ms point.
According to Literature 2, Table 2, etc., the component ranges for satisfying such conditions and exhibiting the shape memory effect are Mn: 29 mass% to 33 mass% and Si: 6 mass%. Further, according to Document 4, page 675, Table 1, a part of Mn may be replaced with 5 to 12% by mass of Cr or 4 to 7% of Ni. In this case, the corrosion resistance is improved. . As the optimum component composition, for example, Fe-28Mn-6Si-5Cr, Fe-14Mn-6Si-9Cr-5Ni, and the like have been proposed.

Based on the above-mentioned model, the results of Examples bear the shape memory effect as a deformation mode by adding Al not exceeding 2% to Fe-30Mn-6Si, which is a typical component of the shape memory alloy. It can be interpreted that the stress-induced transformation and the twin deformation responsible for ductility can be generated simultaneously, but it can be easily analogized that the same Al addition effect can be applied to all the above shape memory alloys.
The same effects can be achieved with respect to various manufacturing processes proposed for improving the shape memory characteristics of Fe—Mn—Si based shape memory alloys and the addition of trace components other than Al.

According to Reference 5, page 64, and the like, the shape memory characteristics are improved by repeating several percent deformation and heating above the reverse transformation temperature. This thermomechanical heat treatment, known as the training effect, strengthens the matrix austenite to suppress slip deformation (inhibits the shape memory effect) and promotes the shape memory effect as a nucleation site for ε martensite. ) To lower the transformation-induced stress. It can be easily inferred that such a change in internal structure is also effective in a new alloy containing Al.
In addition, various processing heat treatment methods for improving the shape memory effect, such as texture control (reference 5, pages 64) and ausforming (reference 6, FIG. 1 and FIG. 2), can also be applied to the first embodiment of the present invention. it is conceivable that.

  Documents 7 and 8 suggest that nano-level fine NbC carbide generation by addition of a small amount of Nb and C and aging heat treatment or thermomechanical treatment can improve the shape memory effect by a simpler process than the training process. . This is because NbC simultaneously achieves the strengthening of the parent phase and the nucleation site of ε martensite, and the same effect can be inferred for the Al-added alloy.

The effect of such fine precipitates is considered to be applicable not only to NbC but also to all compounds that precipitate in steel materials such as various carbides, nitrides, and intermetallic compounds.
That is, this invention can add the ductility improvement effect by addition of Al, utilizing the effect of all the component compositions and manufacturing processes of the Fe-Mn-Si type alloy proposed so far.
As applications of the Fe—Mn—Si-based shape memory alloy, various fastening applications such as pipe fastening members (References 2 and 5), prestressing reinforcement of concrete (Reference 9), and the like have been proposed. The alloy of the present invention is applicable to all these uses.

As applications of the Fe—Mn—Si-based shape memory alloy, various fastening applications such as pipe fastening members (References 2 and 5), prestressing reinforcement of concrete (Reference 9), and the like have been proposed. The alloy of the present invention is applicable to all these uses.

Manufacturing process flow of test piece Photograph showing shape change of Fe-Mn-Si alloy and Fe-Mn-Si-Al alloy by heating Model showing how to calculate the shape recovery rate

Claims (1)

Iron (Fe) -A (A: Manganese (Mn) alone or this and chromium (Cr) or / and nickel (Ni) element) -silicon (Si) alloy, which is a shape memory alloy having an Ms point of room temperature or less. A shape memory alloy containing 1% by mass of aluminum (Al)