JP3452335B2 - NiTi-based alloy - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the composition of NiTi-based alloys known as shape memory alloys and superelastic alloys. Shape memory alloys and super alloys having further improved fatigue life and hot / cold workability. The present invention relates to a NiTi-based alloy as an elastic alloy.

[0002]

2. Description of the Related Art Alloys called shape memory alloys such as NiTi-based alloys show an austenite structure that is a cubic structure in the parent phase of the high temperature phase, but monoclinic martensite below the transformation temperature (Ms point). Transform into tissue. When the structure of NiTi-based alloy is this martensite structure, external force
When (stress) is applied, it easily shows an apparent plastic deformation. Next, if this is heated above the transformation temperature (Af point),
Restores the original shape. Generally, this is called a shape memory effect, and is used in various industrial applications such as temperature sensitive valves and thermostats.

It is also well known that when stress is applied in the state of an austenite structure as a matrix, a large elastic deformation, which cannot be considered by ordinary metal materials, is exhibited.
This is called superelasticity or pseudoelasticity. When stress is applied to the austenite structure, stress-induced martensitic transformation (SIM transformation) occurs, and in the stress range where slip transformation does not occur, superelasticity is observed and a large elastic deformation is observed by supplementing the deformation strain with the crystal structure change called transformation. is there. This superelastic property has a great industrial value and is used in many applications as a material that is difficult to deform such as eyeglass parts, orthodontic wires, brassieres wires, and shoulder pad wires.

However, conventional methods having such excellent characteristics have
NiTi-based alloys also have some problems. One of them is
Cold workability is poor. For example, when reducing the diameter by cold drawing with a die wire, the area reduction rate is approximately 30%.
The cold working deteriorates the workability (extension and drawing are extremely reduced), and further processing cannot be performed. Therefore, it is necessary to perform heat treatment at a recrystallization temperature or higher, that is, strain relief annealing every time 30% cold working is performed, and it is necessary to repeat heat treatment a lot during cold working.

75 for metallic materials such as stainless steel
Since it is possible to perform cold working of at least%, it can be said that the NiTi-based alloy is an industrial material that has a very high ratio of processing cost to product cost. Therefore, NiTi-based alloy wires and thin plate materials become expensive, which is the biggest factor that hinders industrial application.

Shape memory alloys and superelastic alloys are
A major feature is that even if a large amount of deformation (maximum 8%) is applied, the shape returns to its original shape by raising the temperature or removing the stress. As described above, general metal materials are often used to be repeatedly subjected to large strain deformation that is not used, and the fatigue life thereof often becomes a problem. In particular, when used as a superelastic alloy such as an antenna core material for a mobile phone, a spectacle frame, a brassiere wire, etc., fatigue fracture may occur during use, which is a problem.

By the way, in order to improve the formability and impact resistance of the NiTi-based alloy, Y (yttrium) is set to 0.001 to
NiTi-based alloys containing 0.5 wt% or 50 to 800 ppm have been proposed. (JP-A-60-251241, Japanese Patent Publication No. 4-
No. 29727) In addition, in order to improve the workability of NiTi-based alloys, B (boron) is 0.001 to 0.5% by weight or 0.1 to 0.5% by weight.
A NiTi-based alloy containing 5 at% or a manufacturing method thereof has been proposed. (JP-A-61-295349, JP-B-5-33303
However, with these alloys or NiTi-based alloys produced by the manufacturing method, fatigue properties that can withstand repeated use are maintained while maintaining the shape recovery force required for currently used antenna core materials, eyeglass frames, brassier wires, etc. I was not satisfied with it.

[0008]

The object of the present invention is to solve the problems
In order to solve the above-mentioned drawbacks of NiTi-based alloys, it is an object of the present invention to provide a novel NiTi-based alloy that significantly improves cold workability, reduces manufacturing cost, and at the same time improves fatigue properties.

A more specific object of the present invention is to provide a NiTi-based alloy which simultaneously satisfies a hot drawing value of 90% or more at 700 ° C., a cold drawing value of 50% or more, and a fatigue fracture number of 3000 or more. Especially.

[0010]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present inventors have conducted various studies, and added a very small amount of Y (yttrium) and B (boron) to a conventional NiTi basic alloy at the same time. The present invention has been completed, knowing that doing is effective.

The gist of the present invention is that Ni is
In a Ti-based alloy, in order to improve the workability of the alloy and to maintain the shape memory effect and the functional characteristics such as superelasticity for a long time, a NiTi-based alloy having improved fatigue characteristics, comprising Y and B That is, their properties can be remarkably improved only by adding at the same time.

[0012]

Next, the reason why the alloy composition is limited as described above in the present invention will be explained together with its function. First, in the present invention, the base is a NiTi-based alloy, which has a NiTi atomic% ratio Ni / Ti of 0.9 to 1.1. When this ratio is less than 0.9, the shape memory effect and superelastic properties are improved. This is because the functional properties are deteriorated, while if it exceeds 1.1, the hot and cold workability is deteriorated and the molding process becomes difficult.

In this NiTi-based alloy, Fe,
At least one alloying element selected from the group consisting of Co, V, Cr, Cu, and Nb may be contained in a total amount of 20 atomic% or less. All of these alloying elements are added to improve the above-mentioned functional characteristics, and if added in excess of 20 atomic%, not only the functional characteristics are deteriorated, but also the workability is deteriorated. Preferably, the total amount is limited to 20 atomic% or less. More preferably, it is blended at a ratio of 5 to 15 atom%.

According to the present invention, when yttrium is further added to the NiTi-based alloy to which a small amount of boron is added, the high temperature ductility value is significantly increased. It is considered that this is because the addition of yttrium has the effect of further refining the crystal grains and preventing coarsening of the crystal grains even at a high temperature of 700 ° C or higher. As a result, typically 900 ° C
The hot working temperature before and after was able to be raised to about 1100 ° C, and the number of heats during hot forging could be greatly reduced. Moreover, it was found that the addition of yttrium significantly reduced the surface oxidation during the forging heating, and it became possible to significantly reduce the surface yield and the yield due to the oxidation loss. This is probably because the addition of yttrium further enhanced the thermal oxidation resistance.

If the amount of yttrium added is 0.0005% or more in atomic ratio, the effect is exhibited, and even at 0.10%, the same effect is recognized. However, when the content exceeds 0.10%, the effect disappears, and segregation appears in the ingot, which causes variations in transformation temperature and the like, so the upper limit was made 0.10%. Preferably, Y: 0.005-0.08 atomic%.

When a small amount of boron is added to the above Y-containing NiTi-based alloy, the ductility values such as elongation and drawing are greatly improved and the cold workability is greatly improved. Equivalent effects have been demonstrated in cold working such as hole die wire drawing, swaging, press rolling and roll rolling.

If the amount of boron added is 0.0010% or more in terms of atomic ratio, the effect is exhibited, and even at 0.050%, the same effect is recognized. However, if the addition amount exceeds 0.050%, the workability decreases conversely, so the upper limit was made 0.050%. Preferably, it is 0.005 to 0.03 atomic%.

Further, the fatigue life is greatly improved by adding the trace amounts of boron and yttrium in the above-mentioned proportions.
NiTi-based alloys with simultaneous addition of yttrium and boron, and conventional NiTi without addition of yttrium and boron
Base alloy or Ni with yttrium or boron added alone
Comparing the number of rotational bending fatigue fractures in Ti-based alloys, the effect of extending the fatigue life by the simultaneous addition of yttrium and boron is recognized.

[0019]

EXAMPLE An example of the present invention will be described below. NiTi-based alloys with the chemical composition shown in Table 1 are melted in a vacuum induction melting furnace of 100 kg, and pure yttrium or nickel boron alloy is added individually by 0.0005 to 0.15% in yttrium and boron atomic% immediately after casting after melting through. , Or added simultaneously
After holding for 10 minutes, it was cast in a metal mold having a diameter of 250 mm in a vacuum.

The surface of the obtained ingot was mechanically cut, heated to 1000 ° C., and hot forged to obtain a billet having a diameter of 60 mm. Next, the surface of this forged billet was subjected to mechanical cutting, heated at 950 ° C. for 2 hours, and hot rolled to obtain a hot rolled wire having a diameter of 7.4 mm. Furthermore, cold hole die wire drawing and annealing heat treatment at 700 ° C for 20 minutes are repeated,
The wire rod has a diameter of 1 mm.

[0021] The cold-worked wire rod thus obtained is
After memorizing the linear shape by heat treatment at 0 ° C for 20 minutes, a fatigue test was conducted at room temperature (25 ° C) with a strain of 1.5% by a Hunter type rotary bending fatigue tester to confirm the fatigue life. Moreover, in order to compare cold workability and hot workability, the drawing value by a tensile test was measured.

The content of boron and yttrium in the NiTi alloy to which boron and yttrium were not added was 0.0001% for yttrium and 0.0003% for boron as a result of chemical analysis. Table 2 shows the reduction value in the tensile test and the number of times of Hunter type rotary bending fatigue fracture in each alloy.

The hot tensile test was carried out at 700 ° C., and the cold tensile test was carried out at 25 ° C. The hunter type rotary bending fatigue test is a test in which tensile / compressive strain is repeatedly applied so that the maximum bending strain on the surface of the wire is 1.5%, and it is carried out at room temperature of 25 ° C, and the number of breaks at that time was measured. .

Table 3 shows the Ni produced in the examples.
It shows evaluation of hot and cold workability, fatigue characteristics and thermal oxidation resistance of Ti-based alloys. In particular, the oxidation resistance was evaluated by the condition of scale formation and the condition of rough skin when heated at 950 ° C. for 2 hours (in air).

Evaluation of hot workability: "○" has a reduction value of 90%
The above cases are shown, and “x” shows the case of less than 90%. Evaluation of cold workability: "○" indicates a case where the drawing value is 50% or more, and "x" indicates a case where it is less than 50%.

Evaluation of fatigue characteristics: "○" indicates the number of fatigue fractures
Indicates a case of 3000 times or more, and "x" indicates a case of less than 3000 times. Evaluation of thermal oxidation resistance: “◯” indicates that the scale thickness was thin, and “x” indicates that the scale thickness was thick and that the scale was peeled to cause rough skin.

As is clear from Table 2, the present invention is 0.00.
In alloy Nos. 1 to 20 added with 05 to 0.10% yttrium and 0.001 to 0.005% boron, alloy No. 28, 31 to 33 without addition and alloy No. B or B and one other addition A large increase in hot drawing value was observed at 700 ° C. as compared with .25 to 27 and 30, and as a result, hot workability was improved as shown in Table 3. Further, it exceeds the scope of the present invention
It can be seen that in alloy No. 29 containing 0.15% yttrium added, conversely, the aperture value is lower than in the alloy without addition.

Further, as is clear from Table 3, 950 in the atmosphere.
The growth of a high temperature heated oxide film when heated at 0 ° C. for 2 hours was suppressed by the addition of a small amount of yttrium, and it was confirmed that the effect of preventing oxidation loss and rough skin was obtained.

The range specified in the present invention is 0.0005 to 0.10.
% Yttrium and 0.001 to 0.05% boron alloy No. 1 to 20 and non-added alloy No. 28 and 31 to
33, 1 of alloy No. 30 and Y with 0.075% boron added
Room temperature (25 ℃) in alloy Nos. 21-24 and 29 with seeds added
Comparing the cold drawing values in Table 1, it is clear from Table 2 that the alloy containing 0.001 to 0.05% of boron has room temperature (25
The squeeze value at (℃) is higher than that of the alloy without addition and 0.075% addition alloy. Further, it can be seen that the alloy of the present invention in which 0.0005 to 0.10% of Y is added has a further improved cold drawing value as compared with the alloy in which Y is not added simultaneously. As a result, in the hole die wire drawing, the conventional NiTi alloy had a limit of working at a surface reduction rate of 50%, but it was possible to reach 75% by adding boron. The same effect was observed in cold working such as swaging, press rolling and roll rolling.

Also in the comparison of the rotational bending fatigue rupture life, as is clear from Table 2, the simultaneous addition of yttrium and boron has the effect of extending the fatigue life, and the simultaneous addition of both makes the effect more remarkable. You can see. Further, even in a NiTi alloy containing at least one of Fe, Co, V, Cr, Cu and Nb, the same effect was confirmed by the simultaneous addition of yttrium and boron.

From the above, it was confirmed that the simultaneous addition of a small amount of yttrium and boron is effective in improving hot workability and thermal oxidation resistance, and further effective in improving cold workability. Further, it was confirmed that the simultaneous addition of yttrium and boron improves the fatigue characteristics, and it was verified that the effect becomes more remarkable.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

According to the present invention, by simultaneously trace yttrium and boron NiTi based alloy, 700
Excellent hot workability with hot drawing value of 90% or more at ℃
And excellent cold workability with a cold drawing value of 50% or more.
It can be obtained, and the reduction of manufacturing cost can be realized. In addition, it has an excellent fatigue life of over 3000 fatigue breaks.
It is also possible to obtain . Moreover, the present invention can be applied to almost all NiTi-based alloy products, and its industrial value is extremely large.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor ▲ Taka ▼ Masaki Shima               1985 Nogi, Nogi-cho, Shimotsuga-gun, Tochigi Prefecture               In ceremony company Sanyo Special Alloy                (56) References JP-A-63-223137 (JP, A)                 JP-A-61-210142 (JP, A)                 JP-A-60-251241 (JP, A)                 JP-A-61-295349 (JP, A)                 JP 62-10261 (JP, A)                 Japanese Patent Publication 4-29727 (JP, B2)                 Japanese Patent Publication 5-33303 (JP, B2)

Claims (2)

(57) [Claims] 1. Y of 0.0005% or more and 0.10% or less in atomic%
(Yttrium) and B (boron) of 0.0010% to 0.050% at the same time, the balance being Ni and Ti, and the Ni / Ti atomic% ratio being 0.9 to 1.1. Yes, and hot at 700 ° C. aperture 90% Hiyakanshibo
A NiTi-based alloy characterized by a tensile strength of 50% or more and a fatigue fracture count of 3000 or more . 2. Further, Fe, Co, V 2, Cr, Cu, and Nb.
The NiTi-based alloy according to claim 1, containing at least one element selected from the group consisting of 20 atomic% or less in total.