JPH05255815A - Iron-based shape memory alloy - Google Patents

Abstract

PURPOSE:To provide an iron-based shape memory alloy having excellent shape memory property as well as improved ductility by preventing the grain boundary precipitation of eta-Ni3Ti when an Fe-Ni-Co-Ti alloy is ausaged. CONSTITUTION:This iron-based shape memory alloy consists of, by weight, 31-35% Ni, 8-15% Co, 2.5-6.5% Ti, at least one among 1.5-10.0& Al, 0.5-5.0% Mo, 1.0-5.0% W, 0.5-5.0% Nb and 0.002-0.010% B and the balance Fe with inevitable impurities.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to an iron-based shape memory alloy.

[0002]

2. Description of the Related Art Shape memory alloys are metallic materials that are expected to be applied to various fields such as industry, energy and medicine by utilizing their unique functions, and some of them have already been put to practical use. Being tried. Shape memory phenomenon and pseudoelasticity phenomenon appear in alloys that undergo thermoelastic martensitic transformation, and metal materials exhibiting such phenomenon are mainly found in non-ferrous alloys, but also in iron-based alloys, It is known that Fe-25 atomic% Pt and Fe-30 atomic% Pd become thermoelastic martensite and show a complete shape memory phenomenon.

Furthermore, recently, when a Fe-Ni-Co-Ti alloy is aged in the austenite region, that is, ausaged and then cooled to a low temperature, a thin plate is used.
e) ・ Martensite structure is formed (General Lecture of Autumn Meeting of the Japan Institute of Metals, p. 216 (September 1982)), and it is further found that this alloy exhibits a shape memory phenomenon. Since this alloy is an iron-based alloy, it is easy to manufacture and relatively inexpensive, and it is a highly practical shape memory alloy.On the other hand, in this alloy, when ausaging, the grain boundaries are austenite. It is already known that Ni ₃ Ti in the η phase (hereinafter, referred to as η-Ni ₃ Ti) is precipitated as a field reaction type precipitate (Summary of Spring Meeting of the Japan Institute of Metals, pages 198 and 306). Page (April 1984)).

The inventors of the present invention have conducted further studies from the viewpoint of the influence of the precipitates on the mechanical properties of the alloy. As a result, the ductility of the alloy is lowered when the precipitates are present at the grain boundaries. Found. The low ductility of the shape memory alloy leads to the weakness of the old grain boundaries against repeated deformation, and thus the grain boundary fracture is likely to occur.

[0005]

DISCLOSURE OF THE INVENTION The present inventors have made Fe-Ni-Co
In order to solve the above-mentioned problems in -Ti-based shape memory alloys, the grain boundary precipitation of η-Ni ₃ Ti that occurs during ausaging is prevented without impairing the shape memory properties that appear when ausaging this alloy. Or, as a result of extensive research on additive elements to be suppressed, the proper addition amount is not necessarily the same depending on the Ti amount in the alloy,
Generally, by adding one kind selected from the group consisting of Al, Mo, W, Nb and B alone or by adding two or more kinds effectively, the grain boundary precipitation of the η-Ni ₃ Ti can be effectively performed. The present invention is based on the finding that an iron-based shape memory alloy that can be prevented and thus improved in ductility and excellent in shape memory can be obtained.

[0006]

The iron-based shape memory alloy according to the present invention comprises, in weight percent, (a) Ni 31-35%, Co 8-15%, and Ti 2.5-6.5%. , (B) Al 1.5 to 10.0%, Mo 0.5 to 5.0%, W 1.0 to 5.0%, Nb 0.5 to 5.0%, and B 0.002 to 0. It is characterized by comprising at least one element selected from the group consisting of 010%, and the balance iron and unavoidable impurities.

Thin plate martensite is characterized in that it is completely twinned martensite, and that stress due to transformation strain is relaxed by elastic deformation in the austenite matrix and plastic deformation does not occur. In order to generate such thin plate martensite, it is advantageous that the base material strength (yield strength) is large or the rigidity is small. In such a case, plastic deformation of the matrix phase due to transformation strain is advantageous. Is unlikely to occur. Further, even when the volume change at the time of transformation and the amount of transformation shear are small, the strain to the parent phase due to transformation becomes small, so that plastic deformation hardly occurs. Furthermore, the large tetragonality of martensite is also advantageous for the production of thin plate martensite. As this tetragonal ratio increases, the shear amount of (112) twin deformation of martensite decreases, and the twin interface energy decreases. These have the function of facilitating the formation of twin crystals in the martensite crystal and increasing the density. Further, the larger the tetragonal crystal ratio, the smaller the amount of transformation shear, and the less likely plastic deformation of the matrix phase occurs.

Another factor that favors the formation of thin plate martensite is the temperature at which martensite is formed, ie
That is, the Ms point is low. This is because as the Ms point is lower, twinning deformation in martensite is more likely to occur than slip deformation. In addition, the strength of the base material also increases, making it difficult for plastic deformation. In the shape memory alloy according to the present invention, Ni,
Co and Ti are required to be in the above range in order to form thin plate martensite in the alloy.
It does not produce plate martensite and therefore does not show shape memory. In particular, Ni is effective in lowering the Ms point. Ti has an effect of uniformly and finely depositing ordered γ′-Ni ₃ Ti in the matrix austenite by ausage, strengthening the matrix, or appearance of tetragonal martensite. Further, Co is effective in raising the Kuriy point of the base material and increasing the difference from the Ms point to reduce the change in transformation volume and further reduce the rigidity of the base phase.

When the Fe-Ni-Co-Ti alloy is ausaged as described above, Ni ₃ Ti in the γ'phase is finely precipitated in the austenite grains, but when the precipitation in the grains is saturated,
Originally, since a stable phase of Ni ₃ Ti is _{η-Ni 3 Ti, γ'-} Ni 3 T
i changes to η-Ni ₃ Ti. The change in this case occurs in separate nucleation, and the nucleation position is at the grain boundary. That is, Ni and Ti precipitated as γ'-Ni ₃ Ti again form a solid solution in the matrix,
It moves to the grain boundary and reprecipitates as the final stable phase, η-Ni ₃ Ti.

The shape memory alloy according to the present invention has the above-mentioned N content.
In addition to i, Co and Ti, it contains at least one element selected from the group consisting of Al, Mo, W, Nb and B. All of these elements Al, Mo, W, Nb and B prevent η-N from diffusing Ni and Ti.
Prevents grain boundary precipitation of i ₃ Ti. Furthermore, Al and Nb are γ'-
Stabilizes Ni ₃ Ti, and B suppresses the generation of η-Ni ₃ Ti grain boundary precipitation nuclei. Further, Mo and Nb not only enhance the shape memory property of the alloy but also enhance the austenite strength, and as a result, have the effect of enhancing the shape recovery force of the alloy.

Appropriate amounts of addition of these elements for effectively exhibiting the above-mentioned effects depend on the amount of Ti in the alloy. The alloy according to the present invention contains Ti in the range of 2.5 to 6.5%. In order to effectively obtain the above effects,
In the present invention, Al is added in an amount of 1.5 to
It is in the range of 1.0%, preferably in the range of 1.5 to 6.5%, Mo is in the range of 0.5 to 5.0%, W is in the range of 1.0 to 5.0%, and Nb is in the range of 0. 5 to 5.0% range, B is 0.002 to 0.01
It is in the range of 0%.

In the iron-based shape memory alloy according to the present invention, the alloy having the above-mentioned predetermined composition is heated to 900 to 1200 ° C. for solution treatment, and then subjected to ausage treatment for 100 hours or less at a temperature of 500 to 800 ° C. It can be manufactured by using a thin plate
Generate martensite. That is, the alloy according to the present invention, after being deformed by any method below a certain temperature,
By heating to a temperature at or above the end temperature Af of the reverse transformation where martensite returns to the parent phase during heating, the shape exhibits a shape memory property in which the shape is restored before the deformation.

[0013]

EXAMPLES As shown in Tables 1 and 2, Fe-Ni-Co-Ti alloy was used as a basic alloy, and Al, Mo, W, Nb and /
Alternatively, an alloy containing B is manufactured by a vacuum melting method, forged,
A plate having a thickness of 5 mm, a width of 70 mm and a length of 1000 mm was manufactured by rolling and used as a test material. The test material was heated at 1150 ° C. for 1 hour for solution treatment and then air-cooled.
After being aged at 4 ° C. for 4 hours, the precipitation of η-Ni ₃ Ti was observed. After the ausage treatment, it is cut into a flat plate having a thickness of 1 mm, a width of 5 mm, and a length of 50 mm, and -1
Bending deformation was performed at a temperature of 96 ° C. into a V-shape having a bending angle of 100 °, and then the shape recovery rate and the shape recovery force were examined by the degree of returning to a flat plate after being taken out at room temperature.
Furthermore, after the above-mentioned ausage treatment, a tensile test piece was prepared, and a tensile test was performed at room temperature to measure elongation. The results are shown in Tables 1 and 2.

The conventional alloys shown in Table 1 are Fe-Ni-
It is a Co-Ti-based basic alloy with a large amount of η-Ni ₃ Ti precipitated at the austenite grain boundaries, and its elongation is also extremely low. In Comparative Alloys 3 and 4, a small amount of η-Ni ₃ Ti was precipitated at the grain boundaries because the added amount of Al was insufficient. In contrast to such conventional alloys and comparative alloys, according to the alloy of the present invention, no precipitation of η-Ni ₃ Ti is observed at the grain boundaries,
Elongation is more than 20%, shape memory is almost 100%
Indicates.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

As described above, according to the alloy of the present invention, η
To -Ni ₃ Ti grain boundary precipitates is prevented, the ductility is remarkably improved, elongation increases, is highly practical as shape memory alloys.

Claims (1)

[Claims] 1. In addition to (a) Ni 31-35%, Co 8-15%, and Ti 2.5-6.5% by weight, (b) Al 1.5-10.0%, Mo 0.5 to 5.0%, W 1.0 to 5.0%, Nb 0.5 to 5.0%, and B 0.002 to 0.010%, at least one element selected from the group consisting of: And an iron-based shape memory alloy comprising the balance iron and inevitable impurities.