KR910000008B1 - Making process for ni-ti memory alloy - Google Patents

Abstract

A two-way shape memorized alloy of a nickel-titanium shape memory alloy is mfd. by melting 50 wt.% nickel and 50 wt.% titanium to make an ingot, heat-treating the ingot in the vacuum furnace at 1050 deg.C for 10hr, treating the surface of the ingot to make the Ni-Ti shape memory alloy material, heating the alloy material at 600 deg.C for 100 min to eliminate the allowed memory, vacuum- treating the material at 750 deg.C for 1hr, furnace-cooling it to 400 deg.C, and water-cooling or air-cooling it. The memorized alloy has a two-way shape memory effect.

Description

Manufacturing method for granting two-way shape memory of nickel-titanium shape memory alloy

1 is a two-way memory heat treatment temperature curve.

2 is a transformation temperature and hysteresis curve.

3 is a transformation state or operation of the finished product.

The present invention relates to a manufacturing method for imparting a two-way shape memory effect in a nickel-titanium shape memory alloy.

In addition to nickel-titanium alloys, shape memory alloys are known to include 10 types of copper zinc, gold-cadmium and silver-cadmium alloys, but Ni-Ti shape memory alloys have excellent thermal, mechanical fatigue life and shape recovery rate. It is applied to various industrial fields such as spacecraft antenna, medical device, glasses frame, driving device, pipe coupling, etc., because it has excellent characteristics such as 30kg / mm of large force when recovering the shape into a circular shape.

In general, the memory method of the shape memory alloy is a one-way method, and the lattice arrangement of the shape memory alloy has a B2 lattice arrangement on the austenite where the shape is remembered.

When cooled below the shape memory alloy martensite transformation temperature, it becomes a martensite phase, and the appearance of the product does not change, but has a B19 gap array in which atoms are distorted.

In this way, the shape memory alloy whose shape is memorized at a temperature above the austenite transformation temperature is deformed on the martensite to form a shape that is different from the stored shape, and when it is heated above the austenite transformation temperature, the shape memory alloy is transformed into the shape austenite phase. The original shape is recovered from the deformed shape, and when cooled below the martensite transformation temperature, it is transformed into the martensite phase, but is not restored to the deformed shape in the state of the martensite phase.

The method mentioned above is called a one-way memory.

Shape memory alloy by this method can do only one thing with one part.

The shape memory alloy to be provided in the present invention can be two-piece (two-way) shape memory alloy in one component.

Two-Way memory is used to increase the temperature of shape memory alloy to form austenite phase on martensite, recover to shape of austenite phase, and to form martensite when deformed on martensite phase. Say it is being recovered.

The transformation of the austenitic phase from the shape memory alloy into the martensite phase is called "martensite transformation," and the temperature at which the transformation starts is called Martensite Transformation Starting Temperature (Ms). The ending temperature is called Martensite Transformation Fini shing temperature (Mf).

In addition, the shape memory alloy is transformed into an austenite phase by heating on the martensite phase is called austenite transformation, wherein the start temperature is austenite transformation starting temperature (As), the end temperature is the austenite transformation end temperature ( Af: Austenite Tranformation finishing temperature temperature.

An object of the present invention is to provide a method for storing a two-way shape by heat-treating the Ni-Ti-based shape memory alloy in the 550-750 ℃ section.

The two-way memory heat treatment used in the present invention is not performed in the existing Wenway shape memory heat treatment temperature of 200 ° C.-500 ° C., but in 550-750 ° C.

If the shape memory alloy is thermally treated at 500 ° C. or lower, it is a Winway memory because the lattice structure of the Ni-Ti type memory alloy has regularity.

However, when the heat treatment is performed in the 550-750 ℃ section as described above, the lattice becomes a regular-irregular lattice, so even if the shape memory alloy is deformed below Mf and heated above Af, all martensite phases are not transformed into austenite phases. Some retain the martensite phase, but are subjected to metamorphic stress by the transformed austenite phase, so that the martensite phase is kept in an extended state.

On the contrary, if the shape memory alloy is cooled below Mf above Af, the two-way recovery displacement occurs because the transformation stress acting on the stretched martensite (ie, under stress) is removed.

At this time, the memory heat treatment time of the shape memory alloy is different depending on the thickness of the wire, but in general, it is good to perform 30 minutes to 60 minutes to maintain the transformation of the sufficient internal structure.

Therefore, the selection of the appropriate heat treatment time according to the wire thickness can be calculated by the following equation. In other words, the diffusion distance depends on the diffusion coefficient and time.

Where X is the diffusion distance, D is the diffusion coefficient, t is the time, Do is the material-specific constant, R is the gas constant, T is the heat treatment temperature, and ▲ H is the enthalpy.

In the present invention, the amount of nickel in the nickel-titanium-based shape memory alloy is 48-52% in atomic%.

The intermetallic compound phase is exhibited by such a component composition ratio, and there exists a shape effect which has a stress of 8% with respect to a cross-sectional area.

Hereinafter, the present invention will be described in detail with reference to Examples.

[Formation step of mold suppression alloy]

Ingot is melted with 50at% nickel and 50at% titanium.

After the ingot is charged into a vacuum heat treatment furnace maintained at 1050 ° C., the homogenization heat treatment is maintained for 10 hours.

The surface of the homogenized ingot is cut and polished to remove surface oxides and nitrides generated during ingot heat treatment.

The material thus obtained is processed to produce wire rods with a diameter of 5 meters.

The wire is drawn to produce various nickel-titanium-based shape memory alloy wires of 0.1, 0.5, 0.75, and 1.0 mm in diameter.

[Memory Step of Two-Way Shape]

In order to eliminate the memory of the shape and temperature given above, the alloy wire is heated to 600 ° C. and heat-treated for 100 minutes.

The wire rod is molded into the shape of the required product.

The molded product is put in a vacuum heat treatment maintained at 750 ° C, and after 1 hour, it is cooled as it is, and then cooled to 400 ° C rapidly by water quenching or air cooling to two-way. Obtain the finished shape memory alloy.

In addition, when ingot is made of 48at% of nickel, 52at% of titanium, 52at% of nickel, and 48at% of titanium, and the same two-way shape memory step is performed, the shape memory alloy having the alloy composition of 50at% of nickel and 50at% of titanium is performed. It shows the same two-way shape memory effect.

However, as the austenite transformation temperature changes depending on the composition of the alloy, Af is 117 ° C for nickel 48at% and 52at% titanium, and Af is -120 ° C for 52at% nickel and 48at% titanium.

Figure 1 shows the temperature change process that the product undergoes in the two-way memory storage step.

In FIG. 1, the molded product placed in the vacuum heat treatment furnace is rapidly rising from room temperature to the temperature of the heat treatment furnace at 750 ° C. After 1 hour in this state, the molded product is gradually cooled in the heat treatment furnace and rapidly reaches a temperature of 400 ° C. Is getting lower.

The two-way shape is memorized in the product by the heat treatment process as described above.

2 is a diagram showing the characteristics of the wire rod.

Referring to FIG. 2, the Af point was 65 ° C. when the first heat treatment (100 minutes at 600 ° C.) of the two-way shape memory step was performed.

When this was subjected to the second heat treatment (vacuum heat treatment at 750 ° C. for 1 hour, followed by furnace cooling and water cooling), the Af point was increased by 15 ° C. to 80 ° C., and the temperature hysteresis was about 1.7 ° C.

As the cause of the As point rises, it can be pointed out that by maintaining at 750 ° C. for 1 hour after cooling for 1 hour, the amount of nickel is formed in the alloy such as Ti ₃ Ni ₄ precipitates, so that the amount of nickel is relatively reduced. have.

3 is an example of a spring produced by the present invention, showing a transformation step of a two-way shape memory alloy.

(a) is a form memorized on austenite.

(b) is the strain state given in the martensite state.

(c) Two-Wat transformation operation: When the temperature is cooled below Mf, the shape is restored in the state of deformation in the martensite state, that is, in the figure of (b).

At this time, the As point is 80 ℃.

Thus, the two-way shape memory alloy, and a method of manufacturing the same.

By storing the two-way memory in the shape memory alloy, two parts can be replaced by one or used in a complex control device. Thus, the use of the shape memory alloy can be expanded. Its practicality is increased, and when used as a machine element, it can be used in many fields such as diversification of functions and reliability of products.

Claims (2)

The nickel-titanium shape memory alloy is heated at 600 ° C. for 100 minutes to eliminate the memory already given, and then maintained at 750 ° C. for 1 hour, and then cooled to 400 ° C., followed by water quenching. And a two-way shape memory of the nickel-titanium shape memory alloy, characterized by comprising a memory heat treatment step. The nickel-titanium shape memory alloy was heated at 600 ° C. for 100 minutes to eliminate the memory already given, and then maintained at 750 ° C. for 1 hour, and then cooled to 400 ° C., followed by air cooling. And a two-way shape memory of the nickel-titanium shape memory alloy, characterized by comprising a memory heat treatment step.

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