JPH1036952A - Method for working nickel-titanium-zirconium (hafnium) shape memory alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To work the subject hardly workable alloy to a stock having a desired shape such as a sheet material or a wire rod by covering a lump of a shape memory alloy having a specified compsn. with a sheath of a soft steel and repeating hot working under the conditions of a specified temp. and draft. SOLUTION: A lump of a shape memory alloy contg., by atom, 45 to 50% Ni, one or more kinds of Zr and Hf by 9 to 16%, and te balance Ti is covered with a seath made of a soft steel, hot working is executed under the conditions of 850 to 950 deg.C working starting temp., >=770 deg.C working finishing temp. and <=10% draft per time, and this is repeated for required times. The reason that the lump of the alloy is covered with the sheath is that the generation of partial oxidation cracking caused by oxidation in the case Ti and Zr are exposed to the air is prevented and cracking is prevented by utilizing compressive restraint. In the case the working starting temp. is lower than 850 deg.C or the working finishing temp. is lower than 770 deg.C, the generation of cracking in the material can not be evaded, in the case the working starting temp. is higher than 950 deg.C, the sheath is oxidized and thinned, and the draft of 10% per time is the limit in the hot working.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a Ni—Ti based shape memory alloy having a reverse transformation start temperature (As point) of 100 ° C.
The present invention relates to a method for hot-working the raised material.

[0002]

2. Description of the Related Art A reverse transformation of a shape memory alloy, that is, a transformation from an M (martensite) phase to an A (austenite) phase occurs when an alloy is heated to increase its temperature from a lower temperature to a higher temperature. The temperature at which this reverse transformation starts is As
The point and the ending temperature are called Af point. The As point and the Af point are important as physical property values that characterize the behavior of the shape memory alloy, together with the Ms point and the Mf point, which are the temperatures at which the positive transformation (M transformation) starts and ends.

As is well known, the characteristics of shape memory alloys are superelasticity and shape memory effect, but the application of shape memory alloys performed so far mainly uses superelasticity. Applications using the shape memory effect have not been developed much. The use of the shape memory effect means that an alloy is deformed at a temperature below the Mf point, heated to a temperature above the Af point and returned to its pre-deformed shape, and the displacement and recovery force at that time are used. It is to be.

[0004] The Ni-Ti binary shape memory alloy is in a low temperature range of about 60 ° C even at a high Af point. Considering a device that operates in the event of a fire as an example of the use of the shape memory effect, it is desired that the device does not operate at a temperature of 100 ° C. or less, which can occur daily, but operates abnormally, that is, only at a higher temperature. This requires a material in which the inverse transformation of the shape memory alloy used occurs at a higher temperature than that of the known binary alloys.

[0005] In response to this demand, efforts have been made to increase the As point of the Ni-Ti based shape memory alloy.
It has been found that a ternary alloy to which d, Zr or Hf is added in an appropriate amount is useful. However, since Pd is an expensive metal, it is difficult to use Pd as a material for a part having a certain size such as a spring utilizing a shape memory effect. If the third component is Zr or Hf, it can be realized cost-effectively, but unfortunately, these third components M are added to the extent that the As point exceeds 100 ° C., specifically 9 atomic% or more. Then, Ni-Ti-M
The workability of the system ternary alloy is significantly reduced, and it becomes difficult to obtain an alloy material having a desired shape. Needless to say, to make a coil spring from a shape memory alloy, for example, it is necessary to first prepare a wire rod of the alloy.

[0006]

An object of the present invention is to process a Ni-Ti-Zr (Hf) -based ternary shape memory alloy, which is difficult to process as described above, to obtain a desired shape such as a plate or a wire. An object of the present invention is to provide a method for making a material having a specific quality.

[0007]

The Ni-Ti-Z of the present invention
The processing method of the r (Hf) -based shape memory alloy is Ni: 45 to
A mass of a shape memory alloy containing 50 atomic% and one or two of Zr and Hf (in the case of two types, the total amount): 9 to 16 atomic%, with the balance being substantially composed of Ti; Wrap in mild steel sheath, processing start temperature 850-95
The hot working at a rolling reduction of 10% or less is performed at a temperature condition of 0 ° C. and a working end temperature of 770 ° C. or more, and this is repeated a required number of times.

[0008]

In the present invention, the reason why the composition of the shape memory alloy to be processed is determined as described above is as follows.

Ni: 45 to 50 atomic% In order to secure the shape memory effect in the Ni-Ti alloy, it is necessary that the Ni content is at least 45 atomic%.
It is preferable to add Zr or Hf as the third component in such a manner as to replace Ti belonging to the same group in the periodic table from the viewpoint of the stability of the metallographic structure. From that viewpoint, the upper limit of the Ni content is 50 atomic%. Become. If the amount of Ni exceeds 50 atomic%, the effect of increasing the As point by adding Zr or Hf sharply decreases.

Zr or Hf (in the case of two kinds, in total): 9 to 16 atomic% In order to raise the As point to 100 ° C. or more by adding these elements, it is necessary to add at least 9 atomic%.
On the other hand, the addition of a large amount exceeding 16% makes the alloy extremely brittle and hinders the improvement of workability aimed at by the present invention.

One of the reasons for enclosing a lump of alloy in a sheath during processing is that Ti and Zr are highly active metals among the constituents of the alloy, and when exposed to the atmosphere at a hot working temperature, oxidation proceeds. This is because partial oxidative cracking occurs accordingly. Another is to prevent cracking by using the compressive restraining force of the sheath. As long as it has appropriate strength and high workability for this purpose, the sheath material is arbitrary, and carbon steel called mild steel is the most suitable because it is inexpensive and easy to process. It was mentioned as a sheath material. Not only carbon steel but also stainless steel that can be easily processed can be naturally used as a sheath material, and the term “mild steel” is used in the present application to include them.

The reason why the temperature condition of the hot working is determined as described above is that when the working start temperature is lower than 850 ° C. or the working end temperature is lower than 770 ° C., the draft is reduced to 10% or less using a sheath. This is also because the occurrence of cracks in the material is inevitable. If the processing start temperature exceeds 950 ° C., the sheath itself is oxidized and thinned, and sometimes the work material may be exposed. is there.

A reduction of 10% is the limit of the hot working according to the present invention.

[0014]

The above description will be made by way of examples. First, three kinds of Ni-Ti-Zr ternary shape memory alloys having the alloy compositions shown in Table 1 were melted in a vacuum induction furnace. . The alloy melt was cast into a cylindrical ingot having a diameter of 20 cm, and the ingot was sliced into a disk-shaped sample having a height of 5 cm.

1) Each sample was heated to 900 ° C., 1000 ° C. or 1050 ° C., and was subjected to upsetting forging at a draft of 5%. The results were as shown in Table 1. Cracks occurred without exception, and further processing was not possible.

Table 1 No. alloy composition (atomic%) processing start temperature Ni Ti Zr 900 ° C 1000 ° C 1050 ° C 1 48.5 40.5 11.0 Cracking Cracking Cracking 2 49.0 40.0 11.0 Cracking Cracking Cracking 3 49.5 39.5 11.0 Cracking Occurrence of cracks Occurrence of cracks It is not impossible to avoid cracks if the processing rate is lower, but at a low processing rate of less than 5%, the number of times of processing increases, productivity is low, and it cannot withstand industrial implementation.

2) From a cylinder and a plate of mild steel (S45C) having a thickness of 5 mm, a set of a bottomed cylindrical body having a size such that one can just fit into the other is manufactured, and this is used as a sheath. As shown in FIGS. 1A and 1B, the disk-shaped sample (1) was wrapped in an inner can (2) and an outer can (3). This sheathed sample No. 1 to 3 at a temperature of 900 ° C, 1
Heating was performed at 000 ° C. or 1050 ° C., and forging was performed at a draft of 8%. In each sample, cracking occurred at a heating temperature of 900 ° C, and forging was possible at 1000 ° C and 1050 ° C. However, in the case of 1050 ° C., the sheath itself is strongly oxidized, and the sheath gradually becomes thinner as the forging is repeated, and the sheath and the material alloy as the contents are fused together, and the forged material is subjected to forging. There is a problem in that the removal of the sheath is hindered.

3) From the above-mentioned experiments, he experienced that forging requires heating to a certain temperature or higher.
It was also found that the actual processing start temperature and actual processing end temperature were more important than the heating temperature. Therefore, the sample No. Samples 1 to 3 were placed in a mild steel can sheath in the same manner as described above, and were forged under the conditions listed in Table 2. The workability is shown in Table 2.

Table 2 Processing start temperature Processing end temperature Processing rate Workability 790-810 ° C 630-690 ° C 10% Crack generation 890-920 ° C 770-850 ° C 10% No crack 880-920 ° C 770-860 ° C 15% Crack Occurrence 4) Since the second condition gave a good result, the sheath material was changed to S
The forging test was repeated, changing to US 304 stainless steel. It has been experienced that if the forging is repeated many times, the sheath is cracked, and if the forging is continued as it is, the sheathed alloy is also cracked. From these facts, it can be seen that the sheath material should be as tough as possible.

5) Using the above-mentioned mild steel can sheath, a processing start temperature of 890 to 920 ° C. and a processing end temperature of 770 to 8
Upsetting at 50 ° C. and a rolling reduction of 10% was repeated four times to obtain a flat forged product. This was cut as shown in FIG. 2 and the sheath was removed to obtain a prismatic material having a width of 5 cm, a length of 15 cm and a thickness of 2.5 cm. The samples were partially wrapped and partially wrapped again in a mild steel sheath. The sheath was made of a mild steel (S45C) plate having a thickness of 3 mm. Both are heated and the processing start temperature is about 800 ° C, 900
C., 1000.degree. C., 1050.degree. C. or 1100.degree. C., and forging with a draft of 10% was added.

A sample without a sheath has a processing start temperature of 8
Cracks occurred at 00 ° C and 900 ° C. At 1000 ° C. or higher, no cracks were observed, but oxidation was observed. Oxidation is quite pronounced at 1050 ° C., leading to material failure,
It progressed violently at 100 ° C., and it was found that it was impossible to work at a high temperature without a sheath.

On the other hand, the sample wrapped in the sheath cracked at a processing start temperature of 800 ° C., but could be forged at 900 ° C. or higher. However, in the case of 1100 ° C., wear due to oxidation of the sheath progressed, and the sheath was damaged during repeated heating and forging, exposing the alloy therein.

[0023]

According to the present invention, it has been hoped that Ni-Ti, which is low in workability and gives a desired shape, has been hopeless.
Hot working of a -Zr (Hf) -based ternary shape memory alloy has become possible, and a plate or a wire can be obtained therefrom. Accordingly, an alloy having a higher reverse transformation temperature (As point) than a conventional Ni—Ti-based binary shape memory alloy, for example, A
Those having an s point exceeding 100 ° C. can be used as operating elements of various devices utilizing the shape memory effect of this alloy.

[Brief description of the drawings]

FIG. 1 is a view showing a workpiece in which a shape memory alloy material is wrapped in a sheath used in an embodiment of the present invention, wherein A is a longitudinal sectional view and B is a plan view.

FIG. 2 is a perspective view showing a state in which a sample is further cut out from a forged workpiece of FIG. 1;

[Explanation of symbols]

 1 Disc-shaped sample of shape memory alloy 2 Inner can 3 Outer can

Claims (1)

[Claims] 1. Ni: 45 to 50 atomic%, and Zr
And one or two of Hf (in the case of two, the total amount):
A lump of a shape memory alloy containing 9 to 16 at% and having an alloy composition substantially consisting of Ti is wrapped in a mild steel sheath, and a working start temperature of 850 to 950 ° C and a working end temperature of 77.
A method of processing a Ni-Ti-Zr (Hf) -based shape memory alloy, comprising performing hot working at a rolling reduction of 10% or less once under a temperature condition of 0 ° C or more and repeating the necessary number of times.