JPH0665741B2 - Method for manufacturing superelastic NiTi alloy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a NiTi alloy that is easy to produce and exhibits excellent superelasticity.

NiTi alloys containing Ni and Ti in an atomic ratio of approximately 1: 1 show unique phenomena such as shape memory effect and superelasticity. In particular, superelasticity is caused by a strain of one digit or more compared with conventional elastic materials. Since it can return, it is expected to react as a spring material.

The following method is known to see such superelasticity near room temperature.

One of them is a method for producing a NiTi alloy having a martensite reverse transformation point, so-called Af point, which is about 30 ° C. lower than room temperature.
In this method, the alloy composition is usually Ni51at% and the balance is NiTi near Ti.
The alloy is heated to about 1000 ° C. and rapidly cooled to obtain a NiTi alloy having an Af point at a predetermined temperature. This method has an advantage that an alloy of any shape can be obtained by holding the NiTi alloy in a desired shape during heating and quenching. However, it is said that the Af point of NiTi alloy changes greatly depending on the alloy composition, and the temperature of the Af point shifts by as much as 10 ° C when the Ni content shifts by 0.1 at%. Therefore, it is extremely difficult to manufacture an alloy having an Af point at a predetermined temperature, and even if it is obtained, the yield thereof is extremely low. In addition, quenching from a temperature of about 1000 ° C
Since the NiTi alloy easily oxidizes at a temperature of 400 ° C. or higher, it must be heated and rapidly cooled in a vacuum or an inert gas, which is not suitable for industrial mass production.

The other is a method of obtaining superelasticity by cold-reducing a NiTi alloy having a Ni content of 50 to 51 at% and the balance of Ti being at least 10%. This method is advantageous in that it does not largely depend on the alloy composition and does not require a troublesome heating and quenching treatment, but the obtained superelasticity has a drawback that it is slightly inferior to the superelasticity obtained by the former method. Since the step of adding cold working comes to the end, it is difficult to obtain a superelastic NiTi alloy having a desired shape.

In view of this, the present invention has undergone various studies, is easy to produce, and has also developed a method for producing a NiTi alloy exhibiting good superelasticity,
An alloy consisting of 50 to 51 at% of Ni and the balance of Ti is annealed, cold-worked to reduce the surface area by 15 to 60%, fixed in a predetermined shape, and heat-treated at a temperature of 175 to 600 ° C. It is characterized by that.

That is, the present invention, Ni 50 ~ 51at%, the alloy consisting of the balance Ni is subjected to melt casting and processing in accordance with a conventional method, in the final step, by annealing, to remove the processing strain, 15-60% in the cold The surface-reducing process is applied, and then the molded product is fixed in a predetermined shape and heat-treated at a temperature of 170 to 600 ° C. The annealing treatment removes distortions such as working before the final step, and the temperature and holding time may be appropriately selected depending on the size of the NiTi alloy. For example, for a wire rod having a diameter of 1.0 mm, annealing is performed at a temperature of 700 ° C for 5 minutes. It is enough. Next, a surface reduction process of 15 to 60% is performed in the cold to finish it into a predetermined size, which is deformed into a desired shape and fixed, and then heat-treated at a temperature of 170 to 600 ° C.

The area reduction rate in cold working after annealing was limited to 15 to 60% because good superelasticity could not be obtained at surface area reduction of less than 15% and cold surface processing at surface area reduction of more than 60%. This is because it becomes difficult to carry out work, and troubles such as breakage often occur and efficient processing cannot be performed. Also, the heat treatment temperature is 170-600 ℃
The reason for this is that at temperatures below 170 ° C, the springback is so large that almost no molding can be performed.
This is because if it is within the temperature range of 00 ° C, molding can be performed by considering springback, and the heating temperature is
This is because if the temperature exceeds 600 ° C, good superelasticity cannot be obtained. The heating time varies depending on the size, shape, etc. of the superelastic NiTi alloy, and may be determined by experiments or the like. For example, for a wire having a diameter of 0.7 mm, it is sufficient to hold the wire at a temperature of 200 to 600 ° C. for about 1 to 120 minutes.

The present invention will be described below with reference to examples.

As shown in Table 1, Ni and Ti were accurately weighed so that the total amount became 3Kg, and melted in a high frequency vacuum melting furnace (frequency 3KHz, vacuum degree 1 × 10 ^-4 Torr) using a carbon crucible, and made of cast iron. It was cast in a mold (inner diameter 50 mm). In melting, first, Ni and Ti were charged so that Ti did not touch the crucible wall, and the temperature of the molten metal was controlled so as not to exceed 1450 ° C.

Table 1 Alloy number Ni Ti at% (wt%) at% (wt%)
1 50.5 (55.06) 55.06 (44.94) 2 50.5 (49.5) 55.56 (44.44) 3 51.0 (49.0) 56.05 (43.95) After cutting the obtained ingot with a lathe, hot forging it into a round bar with a diameter of 20 mm. After cutting again, a wire rod having a diameter of 6 mm was obtained by hot rolling using a groove roll. This was drawn into a wire having a diameter of 1 mm by cold drawing and annealed at a temperature of 700 ° C for 5 minutes.

These wire materials were processed into a predetermined wire diameter by cold drawing. The wire diameter and the processing rate are shown in Table 2.

Table 2 Wire diameter (mm) Machining rate (%) 0.95 9.75 0.92 13.36 0.90 19.0 0.85 27.75 0.80 36.0 0.75 43.75 0.70 51.0 0.65 57.75 0.60 64.0 0.55 69.75 Table 2 Medium wire diameters 0.60mm and 0.55mm Breaks often occurred during processing.

As shown in FIG. 1 (a), the wire rods having the respective wire diameters shown in Table 2 are attached to the jig (a) having a U-shaped cross section by bending the wire rod (b) into a U-shape and directly attaching both legs of the wire rod (b). Fixed at 1 for each temperature
After heat treatment for ~ 120 minutes, it was removed from the jig. At that time, the opening angle θ shown in FIG. 1B was measured. As a result, all the alloys shown in Table 1 were the same, and the inner diameter was 0.75
FIG. 2 shows the opening angle θ when the wire of mm is heated at each temperature for 10 minutes as a function of the heating temperature. The U-shaped radius of curvature R was 4 times the line diameter.

As can be seen from Fig. 2, if the heat treatment temperature is 400 ° C or higher, it can be molded according to the shape of the jig, and between 175 ° C and 400 ° C, it can be molded by considering the springback, and it is less than 175 ° C. Then, it is understood that the molding cannot be performed by the heat treatment.

Next, the superelastic NiTi alloy wire molded in this way is equivalent to the U-shaped opening angle when the wire is deformed into a linear shape and then left without any deformation force, that is, θ shown in FIG. The difference ψ with the angle to be measured was measured. As a result, all the alloys shown in Table 1 are the same, and the alloy number No. 1 among them is as shown in FIG.

(1) in the figure is 0.92 mm in diameter (working rate 15.36%) to wire diameter 0.65 m
It shows the case of heating the wire of m (working rate 57.75 mm) to each temperature for 10 minutes, (2) shows the case of heating the same wire to each temperature for 5 minutes, and (3) shows the same wire to each temperature for 2 minutes. The case of heating for minutes is shown. In addition, (5) shows the case where a wire with a wire diameter of 0.95 mm (working rate 9.75%) is heated for 10 minutes. As can be seen from the figure, there is an upper limit to the heating temperature, and heating temperatures of 600 ° C or higher are not preferable. . If the processing rate exceeds 60%, fracture occurs during processing, resulting in inefficiency of 15%.
If it is less than this, good superelasticity cannot be obtained as can be seen from FIG.

As described above, according to the present invention, good superelasticity can be exhibited, and a desired shape can be easily obtained, which is a significant industrial effect.

[Brief description of drawings]

FIGS. 1 (a) and 1 (b) show an example of the shape fixing method in the heat treatment of the present invention. FIG. 1 (a) is a perspective view, and (b).
Is an explanatory view showing springback when removed, second
FIG. 3 is an explanatory diagram showing the relationship between the heat treatment temperature and the amount of springback in one embodiment of the present invention, and FIG. 3 is an explanatory diagram showing the relationship between the heat treatment temperature and superelasticity. a: Jig b ... TiNi alloy wire

Claims (1)

[Claims] 1. A NiTi alloy consisting of 50 to 51 at% of Ni and the balance of Ti is annealed, subjected to cold surface reduction of 15 to 60%, and then fixed in a predetermined shape at 175 to 600 ° C. A method for producing a superelastic NiTi alloy, characterized in that it is heat-treated at the temperature.