JPS627839A - Manufacture of niti alloy - Google Patents

Abstract

PURPOSE:To provide functions such as shape memory effect, superelastic behavior, etc., by unidirectionally solidifying a molten metal of Ni-Ti alloy containing specific amounts of Ni or of the one in which >=1 element among Cu, Al, V, Zn, Mo, Cr, Fe, Co and rare earth elements is substituted for a part of Ni or Ti. CONSTITUTION:The molten metal 3 of the Ni-Ti alloy containing 50-60% Ni or of the one in which >=1 element among Cu, Al, V, Zn, Mo, Cr, Fe, Co and rare earth elements is substituted for a part of Ni or Ti s poured into a mold 2, where the molten metal 3 is cooled from the bottom of the mold 2 by means of a cooling chill 4, which the upper part is heated by means of a heater 5 outside the mold 2. Accordingly, the molten metal 3 is solidified from downward into solid phase 6, while the upper part is held in the molten state and formed into liquid phase 7. On slowly moving the mold 2 downward, the solid-liquid interface 8 is shifted to the liquid phase 7 side and grain bounda ries 9 grow unidirectionally, so that an ingot is formed into a unidirectionally solidified structure.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention is directed to the use of Ni, which exhibits shape memory effects, superelastic behavior, etc.
This invention relates to improvements in casting methods for Ti-based alloys.

BACKGROUND OF THE INVENTION FIG. 3 shows a conventional method for producing a Ni Ti alloy. In a conventional method for manufacturing a Ni Ti alloy, a method has been adopted in which a molten Ni Ti alloy is poured into a mold 1 from six directions indicated by arrows, and the molten metal is sequentially solidified.

Problems to be Solved by the Invention NiJi-based alloys contain a large amount of Tt, which has a strong affinity for oxygen, so they melt and! Even if you do the 8 construction carefully, N
400 to 5 o ppm in the ingot of T7i alloy
It was unavoidable that more oxygen was mixed in. The mixed oxygen may be dissolved in the NiTi matrix or dissolved in the Ti matrix.
4 becomes an oxide of Ni20 and precipitates at the grain boundaries of NiJi alloys.

If the content of oxides in the alloy becomes high, there is a risk that cracks may occur during hot working or cracks may occur during forming.

Furthermore, when a NiTi-based alloy containing a large amount of Ti5Ni20 oxide is used as a member for repeated shape recovery, the oxide becomes a stress concentration source and reduces the fatigue life.

Furthermore, due to the precipitation of 7 i 4 N i 20, the degree of Tim in the matrix decreases, so the martensitic phase transition start temperature (MS) decreases. Ti4NizO
The amount of precipitation of T! is not constant and varies slightly. n
The MS point also varies with the amount of Nf20 precipitated. Therefore, the production of a NiTi-based alloy having a desired MS point was the next step.

In addition, Ti 4 Ni 20 in the N+7+ matrix
If such substances are present, it becomes an obstacle to the movement of the martensite interface, resulting in temperature hysteresis (Af -Ms).
becomes larger. Therefore, an object of the present invention is to provide a method for manufacturing a NiTi alloy that has excellent formability, long fatigue life, high MS point, and small temperature hysteresis.

Means for Solving the Problems According to the present invention, the method for producing a NiTi alloy having functions such as shape memory effect and superelastic behavior is characterized in that N1 is reduced to 50 to 6
NiTi alloy containing 0% and the remainder consisting of Ti, or
Part of Ni or Ti is CLI, AfLSV, Zn1
It is characterized by solidifying a molten Ni Ti alloy substituted with one or more metals selected from the group consisting of M01Cr, Fe, QO, and rare earth elements in one direction.

Operation FIG. 1 is a schematic diagram showing an outline of the method for manufacturing a NiTi alloy according to the present invention. WIi, which is a NiT1-based alloy, is cooled from one direction, and solidification progresses in the opposite direction to the heat flow direction. Therefore, the molten metal is separated into a solidified solid phase and a molten liquid phase, and the solid-liquid interface between the solid phase and the liquid phase gradually moves toward the liquid phase.

Oxygen mixed into the molten metal dissolves more easily in the liquid phase than in the solid phase, so it moves from the solid phase to the liquid phase at the solid-liquid interface. Therefore, the oxygen concentration in the solid phase becomes lower than the oxygen concentration in the original molten metal.

Examples Example 1 FIG. 2 shows an example of a method for manufacturing a Ni 7i alloy according to the present invention. For mold 2, N1 heavy a55%
-Ti weight has increased by 4. The molten metal 3 is prepared in advance by a high frequency induction vacuum melting method and then transferred to the mold 2. The molten metal 3 is cooled from the bottom of the mold 2 by a cooling chiller 4, and the upper part is heated by a heater 5 located outside the mold 2. Therefore, the molten metal 3 solidifies from the lower part and becomes a solid phase 6, and the upper part remains in a molten state and becomes a liquid phase 7.

The mold 2 gradually moves downward, the molten metal 3 is gradually cooled to the upper part, and the solid-liquid interface 8 moves toward the liquid phase 7 side. Since the molten metal 3 is solidified in one direction, the grain boundaries 9 grow in one direction, and the ingot has a unidirectional solidification structure.

Gaseous impurities such as oxygen mixed in the molten metal 3 move from the solid phase 6 to the liquid phase 7 at the solid-liquid interface 8. Therefore, the solidified ingot contains oxygen and T! xNt20 oxide is less likely to be mixed in. However, since NiTi alloys are industrially usually melted in graphite crucibles, T1, N1, O (7) are included in the molten metal 3 as inclusions.
C exists. Since Tt (l) has a high melting point of 3250°C, it exists in the solid phase in the molten Ni Ti alloy. From Ni Ti alloy to TiC
cannot be removed. However, TiC has a specific gravity of 4
．． 2, and the specific gravity of the II Ti alloy is 6.5. Therefore, if unidirectional solidification is performed from bottom to top as in Example 1, TiC can be removed by floating.

Impurities in solution Ii3 are 1. As the solidification of the molten metal 3 progresses, it is concentrated in the remaining liquid phase 7, and is accumulated in the final solidified portion.

Therefore, 173 was cut off from the final solidified part of the resulting ingot, and the oxygen and carbon content concentration,
Tests were conducted to examine workability and fatigue life. Tests to investigate workability and fatigue life were conducted in the following manner. First, the ingot was formed into a wire rod with a diameter of 1 mm by hot working and cold working. This wire rod was tested by winding it around a wire having the same diameter of 1 mm, and the workability was tested by determining whether winding to the same diameter was possible. Furthermore, this wire is memorized into a coil shape,
A weight is hung on the tip of the coil, and τ=30k (1/mm2
Under a constant load, heating and cooling were performed alternately, and shape recovery and deformation were repeated. In this way, the fatigue life was measured by the number of times the wire rod recovered its shape until it broke.

For comparison, the above test was also conducted on a NiTi alloy cast by the conventional method shown in FIG. The results are shown in the table below.

Example 2 Using the same equipment as in Example 1, seeding was performed using a Ni weight of 5
A single crystal ingot of 5%-Ti weight ff145% gold was prepared. The oxygen content of this ingot was 100 ppm. Furthermore, when the same shape recovery test as in Example 1 was conducted, the number of shape recovery times before breakage was 60,000.

Practical Example 3 An In-45, 8% by weight D1 alloy was unidirectionally solidified in the same manner as in Example 1, and formed into a wire with a diameter of 0.5 mm by cold working.

When the transformation temperature of this line was measured by differential thermal analysis, the MS point was 95°C.

For comparison, Ni-4 of the same composition cast by the conventional method
5,81'! %T1 alloy was treated in the same manner as in Example 3 to form a wire with a diameter Q of 5 mm, and when measured, the Ms point was 80
It was ℃.

The difference between the reverse transformation start temperature and the transformation start temperature, that is, the temperature hysteresis, was 20°C for the alloy of Example 3 and 30°C for the alloy cast by the conventional method.

In the above embodiments, the NiTi-based alloy according to the present invention was
JIJ 3ti method is NiTi consisting only of Nt and T1.
It was used in the production of alloys, but some of the Ni or Ti was mixed with Cu, A, V, Zn, Mo, Cr, F8.
, Co, and rare earth elements.

Note that when the ingot is completely single crystallized, there is no precipitation of impurities in the ingot.

As described above, the incorporation of oxygen into the ingot can be significantly reduced by solidifying the melt field of the Ni Ti alloy in one direction. As a result, it is possible to create a NiTi-based alloy with superior formability, longer fatigue life, and a higher MS point and lower temperature hysteresis than before. The method can be advantageously used for manufacturing NiTi-based alloys that are used as materials for shape memory alloy actuators, superelastic members, and the like.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing an outline of the method for manufacturing a Ni Ti alloy according to the present invention. FIG. 2 is a sectional view showing an embodiment according to the present invention. Figure 3 shows the conventional Ni
FIG. 2 is a cross-sectional view showing a method for manufacturing a Ti-based alloy.

Claims (2)

[Claims] (1) Contains 50 to 60% Ni, the remainder consists of Ti,
A NiTi alloy having a shape memory effect or superelastic behavior, or a part of the Ni or Ti is Cu, Al,
Ni substituted with one or more metals selected from the group consisting of V, Zn, Mo, Cr, Fe, Co and rare earth elements
A method for producing a NiTi alloy, characterized by solidifying a molten Ti alloy in one direction. (2) Cooling the molten NiTi alloy from below,
The method for producing a NiTi-based alloy according to claim 1, wherein the molten metal is solidified from below to above.