GB2238320A - Ni-Ti alloy production - Google Patents

Abstract

Ni-Ti intermetallic compounds are produced by subjecting a laminate of Ni foils and Ti foils to a diffusional heat treatment at a particular temperature partly producing liquid phase.

Description

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METHOD OF PRODUCING Ni-Ti INTERMETALLIC COMPOUNDS Background of the Invention Field of the Invention

This invention relates to a method of directly producing Ni-Ti intermetallic compounds from a lamination of Ni foils and Ti foils by using reactive diffusion.

Related Art Statement

Since Ni-Ti series alloys exhibit various performances in accordance with their chemical compositions, they are advanced to be put into practical use as a material for wide applications.

Heretofore, sheet or wire of Ni-Ti series alloy has been produced through steps of melting - hot rolling - cold rolling - intermediate annealing cold rolling final product likewise usual metallic materials.

However, the production by the above method is very difficult owing to the inherent properties of the Ni-Ti series alloy as mentioned below. For this end, a powder sintering method has been developed instead of the above method. According to the later method, Ni powder and Ti powder are mixed at a ratio corresponding to the final composition ratio and then the mixed powder is shaped into an objective final form or a form similar thereto by a shaping technique such as HIP, CIP, cold powder rolling or the like, which is subjected to a high temperature sintering to obtain a single phase Ni-Ti series alloy through reactive diffusion of Ni and Ti. In this method, the yields at the composition adjustment and the intermediate step are considerably improved as compared with the aforementioned method of from melting to cold working.

The above two methods are a general method used for the production of NiTi series alloy at the present. As another method solving the problems of the above methods, for example, Japanese Patent laid open No. 59-116340 proposes a method wherein Ni and Ti are closely adhered to each other through a film forming method such as pressing, plating, vapor der)osition or the like and heated and then reaction-diffused to obtain Ni-Ti phase.

In Japanese Patent laid open No. 62-120467, there has been proposed an improvement of the method described in the aforementioned Japanese Patent laid open No. 59-116340 on the wires of Ni-Ti series alloy, wherein plural composite wires each obtained by covering a surface of Ti core with Ni are bundled together, subjected to working for reducing the size and further to diffusion treatment to produce Ni-Ti phase. This method is sufficiently practical as a method of producing the wire. Furthermore, it is possible to produce a strip of about few mm to few cm by drawing the resulting wire.

k 1 As mentioned above, there are proposed various production methods on Ni- Ti series alloys, which have many problems to be solved as mentioned below. A main cause is a point that the atomic ratio of Ni to Ti in the Ni-Ti series alloy exhibiting useful properties is restricted to about 1:1 and the cold workability is very poor as compared with the usual metallic materials.

For example, in the production steps consisting of melting - hot rolling -cold rolling - intermediate annealing - cold rolling...... final product generally used for obtaining plate-like Ni-Ti series alloy, the combination of cold rolling and intermediate annealing steps should be repeated to a considerable extent for working to a given thickness.

Such a repetition of continuous working - softening annealing causes occurrence of edge tear in the rolling, decrease of yield due to oxidation and pickling loss or the like in the annealing pickling, degradation of properties due to the oxidation in the annealing and the like, so that the productivity of Ni-Ti is poor and the cost is too high. Particularly, the production of sheet product through cold working is industrially impossible on a composition of Ni-Ti series alloy containing not less than 50 at% of Ni required for development of super elasticity at low temperature.

Since such NiTi alloys are difficult in the working, the wires being relatively easy in the working have 1 mainly been produced in great amount, and the production quantity of sheet product is very small.

As a large factor obstructing the productivity to raise the cost, there is mentioned the difficulty of smelting into adequate composition. For example, in the shape-storing material, it is most important to control the actuating temperature to a given value, but in case of Ni-Ti alloy, the actuating temperature is varied to 10'C even by the change of Ni concentration of 0.1%. Therefore, the accurate composition adjustement is necessary, but since Ti has very high activity at high temperature and is lost through oxidation loss, reaction with mold or the like in the melt casting, it is very difficult to adjust the composition to the given value. As a result, special equipment is required for the melting, which obstructs a feature that the alloys having a constant quality are cheaply produced with a good yield.

As a method of avoiding various problems through the above smelting and cold working, powder sintering method has been developed. In this method, however, Ti powder which is hardly produced and is expensive should be used, so that the prodUct cost is too high. Therefore, the powder sintering method provides a somewhat advantage when being applied to the production of parts having a complicated shape or various kinds of parts in a small quantity, but is not suitable for the production of products such as plate and strip which should be stably and cheaply be supplied in a certain large amount. In addition, since the surface of powder used as a starting material for a powder sintered body is oxidized to a certain extent, a significant amount of oxide remains in the inside of the final product, which has a problem in the quality of the product.

Furthermore, Japanese Patent laid open No. 59116340 proposes a method wherein plate and strip are cheaply produced as compared with the powder sintering method by using the same reactive diffusion principle as in the powder sintering method. In this case, when this method is applied to the actual production of plate and strip, if it is intended to obtain NiTi alloy plate of single phase having a thickness of about O.lmm, it is required to conduct a diffusional heat treatment for a long time such as several hundred hours. In case that the thickness of each of Ni and Ti layers is thick, defects such as voids or the like are frequently generated in the inside of the plate during the diffusional heat treatment to injury the soundness of the structure, so that the thickness produdeable as a practical material by this method is only about several ten micronmeters at most. Thereofre, the latter method can not be said to be practical as an industrial production method.

Moreover, Japanese Patent laid open No. 6431938 discloses a method considered as an extension of the above production method. According to this method, the material is not particularly limited to Ni-Ti, but plural layers of foil-like metallic material are laminated and then subjected to a heat treatment to conduct diffusion.

In this method, however, the reactive diffusion is solid-phase diffusion, so that in case of the reactive diffusion between Ni and Ti, particularly solid-phase reactive diffusion of flatly laminated Ni-Ti plate as compared with the case of using powder as a starting material, there are caused peculiar problems as mentioned below, and consequently it is difficult to obtain members having a practically usable quality and also the treating time becomes long. Moreover, these problems have been conf. irmed from the experimental results of the inventors.

(1) A first point is a time required for the reactive di f f us ion. Considering the same weight, the interface area proceeding the diffusion (specific surface area: MM2/g) is less as compared with the powder, so that the long time is required for the progress of the diffusion. (2) A second point lies in that voids are frequently generated by Kirkendall effect as a phenomenon inherent to the interactive diffusion because the absolute number of atoms passing per unit interface area is increased by the same cause as mentioned above. Particularly, in case of interactive diffusion between Ni and Ti, the diffusion rate of Ni atom in Ti is larger by 1000 times or more than the diffusion rate of Ti atom in Ni, so that Ni atom largely tends to reduce in the vicinity of the interface, and consequently the occurrence of Kirkendall voids becomes conspicuous. The occurrence of voids not only injures the structure but also obstructs the subsequent reactive diffusion at the interface to interfer the homogenization of the composition, so that it is required to reduce the occurrence of the voids as far as possible. Moreover, the occurrence of voids is closely related to the heat treating temperature of reactive diffusion. In case of Ni-Ti, the occurrence of voids can be controlled to a certain extent at a relatively low temperature of about 7000C, but the diffusion rate becomes later and the homogenization of the composition takes a long time and it is unpractical.

On the other hand, the heat treatment is carried out near to an upper limit temperature for solid reaction of about 900C in order to shorten the reaction time, but in this case a large amount of voids is generated.

(3) A third point is a phenomenon resulting from a difference in interactive diffusion rate between Ni and Ti, in which the increase and decrease of volume is caused with the advance of diffusion between Ni and Ti layers and hence stress is produced at the interface to cause the mechanical peeling phenomenon. When this point is further explained in detial, the volume naturally tends to relatively reduce in the Ni layer preferentially discharging atom, and in this case such a tendency of volume reduction is developed thickness absorbing increases direction.

macroscopically as a decrease of layer thickness in the direction. On the other hand, in the Ti layer Ni atom, the layer thickness macroscopically and also the layer expands in the plane Therefore, the shearing force acts in the plane direction in the vicinity of the interface between Ni and T-4 layers, and consequently the mechanical peeling is caused at the interface.

From the above reasons, it is said that the production method described in Japanese Patent laid open No. 64-31938 is not practically applicable in industrial scale.

Although the production method described in Japanese Patent laid open No. 62-120467 can be said to be sufficiently applicable to the industrial production of wire and strip, the size of the resulting article is naturally critical because the size of original wire and strip is restricted, so that this method articles having various sizes such as thicker and wider products can not be produced.

Summary of the Invention

The invention is to advantageously solve the above problems and to provide a method of advantageously producing Ni-Ti intermetallic compounds which can cheaply produce thicker or wider articles in industry.

The inventors have made various studies in order to solve the above problems and found that the given object can very effectively be achieved when the form of reactive diffusion is not solid diffusion but is diffusion utilizing liquid phase.

That is, when heat treatment is"carried out by selecting a temperature range producing liquid phase in a particular composition range at Ni-Ti binary phase diagram, the liquid phase is partly produced at the interface between Ni and Ti, and consequently not only the reaction is completed in a short time but also materials having less defects can be obtained.

Moreover, a liquid phase sintering method in powdery metallurgy is known as a method for including liquid phase in reaction between solid phases. This method utilizes only a component forming a low melting point binder with a relatively small volume percentage as a liquid phase. On the contrary, the method according to the invention is not a concept of using the binder in the liquid sintering method and has a characterisitc that the liquid phase forming interface moves together with the advance of diffusion and finally approximately 100% of the material is rendered into a liquid phase state at least at once in accordance with -g- It the heat treating conditions, which is entirely different from the liquid phase sintering method.

Since there is no example of attempting the use of liquid phase in the conventional production method of Ni-Ti intermetallic compound using the reactive diffusion, such a utilization of liquid phase is first attained in the invention.

The invention is based on the above knowledge.

That is, the invention lies in a method of producing Ni-Ti intermetallic compounds by alternately laminating plural Ni toils and Ti foils one upon the other and subjecting the laminate to a diffusional heat treatment to form Ni-Ti intermetallic compound having 48-55 at% of Ni, characterized in that said diffusional heat treatment is carried out within a temperature range partly forming liquid phase (first invention).

Furthermore, the composition of the intermetallic compound (atomic %) is determined by the adjustment of thickness in the Ni foil and Ti foil to be laminated (second invention), and the thickness of the foil is limited to not more than 20 g m (third invention). Moreover, in the production method of Ni-Ti intermetallic compound according to the invention, the diffusion heat treatment is carried out under vacuum or in an inert gas atmosphere at a temperature of 9551015"C for 1 second to 10 hours (fourth invention), or i at a temperature of higher than 1015"C but not higher than 11100C for 1 second to 1 hour (fifth invention), or at a temperature of higher than 1110'C but not higher than 1240'C for I second to 10 minutes (sixth invention), or at a temperature of 955-1015'C and/or higher than 10150C but not higher than 1110'C for 1 second to 1 minute and at a temperature of higher than 11100C but not higher than 1240C for 1 second to 10 minutes. (seventh invention).

Brief Description of the Drawing

The invention will be described with reference to the accompanying drawing, wherein:

Fig. 1 is an Ni-Ti binary phase diagram. Description of the Preferred Embodiments

As mentioned in the first invention, the reactive diffusion at a state partly producing the liquid phase is fundamental, the concrete temperature range of which will be described with reference to the fourth to seventh inventions below.

Fourth invention In the fourth invention, the diffusion heat treatment is carried out within a temperature range coexisting.8-Ti-Ni solid solution, Ti2Ni and liquid phase (region I) in Ni-Ti binary phase diagram as shown in Fig. 1. In this temperature range, the liquid phase is produced in a phase of Ti rich side, and when the reaction is ideally proceeded, 83 vol% of total of the material is finally rendered into the liquid phase and the liquid phase reaction is completed at a time that all of Ti rich side phase is rendered into Ti2Ni.

In fact, Ti somewhat diffuses into Ni side as a solid phase through solid diffusion, so that the liquid phase forming ratio is less than 83 vol%. After the completion of liquid phase reaction, the reaction migrates into solid diffusion reaction at this temperature range, and the reactive diffusion progresses till the whole of the material is rendered into homogeneous composition. In this case, the solid phase diffusion between Ti2Ni and Ni is rate-determining, so that the reaction time for forming Ni-Ti single phase becomes somewhat long (not less than 1 second, particularly about 10 hours in case of laminating Ni and Ti of 20 g m) as compared with the cases of fifth to seventh inventions conducting reactive diffusion at a temperature region more increasing the liquid phase forming volume and higher than the above temperature range as mentioned later, while the solid phase of Ni rich side supporting the liquid phase portion in form of sandwich is relatively thick, so that the second invention is advantageous for suppressing strain and deformation of the material usually produced with the reactive diffusion to a low level.

Fifth invib7ntion In the fifth invention, the heat treatment is carried out within a temperature range coexisting 6 -Ti- 1 Ni solid solution, TiNi and liquid phase but not producing liquid phase at Ni rich side rather than TiNi in the phase diagram of Fig. 1 (region II). In this temperature region, liquid phase is formed in the phase of Ti rich side, so that when the reaction is ideally proceeded, about 99 vol% of total of the material finally becomes liquid phase and the liquid phase reaction is completed at a time that all of Ti rich side phase is rendered into a composition of '48 at% Ni - 52 at% Ti (composition on border line of coexisting region of TiNi and TiNi+liquid phase in the phase diagram). The actual liquid phase forming ratio changes in accordance with ratio of Ni to Ti in the laminated state and temperature, while the border between NiTi region and region of NiTi+ liquid phase is not completely decided, so that it can not be clearly defined at this -1.ime.

At the above temperature range, approximately the whole of the material passes the liquid phase state at once, so that the reaction forming the NiTi single phase is completed for a short time (about 1 second to 1 hour). The rate-determining of the reaction is considered to be a diffusion of Ni into solid solution for producing the melting of 6 -Ti-Ni solid solution or a melting reaction between B-Ti-Ni solid solution and liquid phase interface. In any case, the reaction is not slow, which does not come into question. Moreover, 1 X the holding of liquid phase portion in the progress course of the reaction is mainly Ni phase of solid phase state and further TiNi3 produced outside Ni phase through diffusion reaction as well as TiNi phase.

Sixth invention In the sixth invention, the heat treatment is carried out within a temperature range coexisting j6 -TiNi solid solution, TiNi and liquid phase at Ti rich side rather than NiTi and coexisting TiNi3, TiNi and liquid phase at Ni rich side rather than NiTi in the phase diagram of Fig. 1 (region III) for short time. In this temperature range, the whole amount of the material finally passes through liquid phase state at once with the advance of the reaction- Therefore, the reaction time till the whole of the material is rendered into TiNi single phase is very short (about 1 second to 10 minutes). The material is actually rendered into TiNi single phase for few seconds to few minutes in accordance with the thickness of each layer of the laminate- In the reactive diffusion at this temperature range, it should be taken a care of existing a course of passing all portion of the material through liquid phase or solid-liquid coexisting phase with the advance of the reaction.

In the fourth and fifth inventions, a part of Ni phase and TiNi3 and TiNi produced by the reaction are 11 always existent at a solid phase state in the progress of the reaction, so that they serve to hold the liquid phase or the solid-liquid coexisting phase. On the contrary, in the sixth invention, there is existent no portion always holding the solid phase state before and after the reaction, so that the material itself has not substantially an ability of strongly holding the shape of the material during the reaction. Therefore, in order to conduct the reactive diffusion at the above temperature range, it is required to use a support means for holding the shape of the material from exterior such as a mold of ceramic or heat- resistant alloy or the like. Even in this case, however, all of the material is not simultaneously rendered into liquid phase state but is rendered into coexisting state of TiNi3, a -Ti-Ni solid solution and liquid phase, so that there is existent an ability of holding the shape through interfacial tension between liquid phase and solid phase, capillary tube phenomenon and the like. Therefore, there is no fear of indefinitely destroying the shape as in the complete liquid.

Seventh invention The seventh invention is a heat treating method capable of maintaining the shape holding ability of the material itself while passing the whoie amount of the material through liquid phase state at once and always holding a part thereof at solid phase state during the reactive diffusion. In this method, the reaction first progresses likewise the fourth and fifth inventions (heat treatment is carried out at 955-1050"C or 1015-1110C for 1 second to 1 minute), and thereafter the liquid phase reaction is completed at a state that a small amount of Ni phase and/or Ni3Ti phase produced by solid phase diffusion reaction are left. Then, the heat treatment is carried out at the same temperature range and time (1110-1240'C, 1 second to 10 minutes) as in the fourth invention, whereby the remaining Ni phase held at the solid phase state and a part of TiNi3 and TiNi produced by the reaction are rendered into the liquid phase state and the reaction is completed at a time that the whole is rendered into TiNi single phase.

According to this two-stage heat treatment, the whole amount of the material passes through liquid phase state at once, in which a part of Ni phase is always existent as a solid phase state at the first half stage, while a part of TiNi phase produced by the reaction is always existent as a solid phase state at the last half stage, so that there is no fear that the material is melted to injury the holding of 'Lhe shape.

According to the invention, the reason why the composition of Ni in the Ni-Ti intermetallic compound to be produced is limited to 48-55 at% is due to the following fact.

That is, when the lower limit of Ni content is 7 less than 48 at%, compounds having properties useful for shape-storing alloy or super-elastic alloy can not be obtained. While, when it exceeds 55 at%, the material becomes brittle and it can not be put into practical use from a viewpoint of fatigue strength and the like.

In the invention, any conventionally wellknown methods such one upon the other deposition method as method of alternately piling foils sputtering method, CVD method, vapor and the like are suitable as the method of forming Ni-Ti alternate laminated layers. In this case, the thickness of each phase is preferable to become thinner considering the treating time, but there is practically no problem when the thickness is not more than about 20 g m. In brief, the total thickness of each of Ni and Ti layers should strictly be controlled to the target composition. Moreover, the composition of the intermetallic compound is dependent upon the lamination number.

After the lamination, it is advantageous to apply a light pressure or conduct a preliminary heat treatment, if necessary. Because, when air bubbles are existent in the lamination interface, they can be removed by the application of light pressure to avoid the degradation of quality in the product.

Moreover, since the reaction producing NiTi from Ni and Ti is exothermic, when the laminate is rapidly heated, the reaction excessively proceeds to fuse the laminate due to self-heat generation exceeding the melting point, so that such a fusion is previously prevented by the preliminary heat treatment. By the preliminary heat treatment, TiNis, TiNi, Ti2Ni and the like are produced between Ni layer and Ti layer, which serve as a barrier for preventing the direct reaction between Ni and Ti to advantageously prevent the fusion due to the self-heat generation reaction.

According to the invention, the treating time in the dif f usional heat treatment -is wide-range because it changes in accordance with the thickness of each layer in the laminate. Moreover, when the liquid phase reaction is completed, it is particularly observed in the fourth invention that Ti3Ni4, Ti2N13, TiNi3 phase or the like is precipitated in the matrix through aging at Ni > 50 at% in addition to TiNi single phase. Since the composition corresponding to the composition range of TiNi in the phase diagram becomes ununiform even under the other conditions, in order to render whole of the material into a uniform composition, it is desirable to conduct a normalized annealing for several hours to few ten hours, or a treatment for making the scattering of temperature in the resulting NiTi phase apart from the diffusional heat treatment for the formation of NiTi single phase after the completion of liquid phase reaction.

Moreover, the temperature range according to X i 1 1 the invention is introduced from the presently known NiTi binary phase diagram (Fig. 1). Although it is considered that the absolute value of the temperature changes from further detailed accurate studies of the phase diagram in future, the essential feature of the invention lies in the region of phase change in the NiTi binary phase diagram, so that if the absolute value of the temperature is corrected, the acceptable temperature range according to the invention is changed by taking the corrected value as a standard. The similar change of the absolute temperature value is considered even through third element (Cu < 20% or the like) included inevitably or added for the purpose of improving properties in the alloy system according to the invention.

The following examples are given in illustration of the invention and are not intended as limitations thereof.

In the production of Ni-Ti intermetallic compound according to the invention, the starting material was prepared as follows.

In order to obtain an atomic ratio of Ni to Ti of 50.5:49.5, pure Ni foils and pure Ti foils adjusted to a thickness ratio of 38.8:61.2 were alternately laminated one upon the other at the toil number of 25, which was rolled to a total thickness of 0.2 mm and then subjected to preliminary heat treatment at 75WC for 4 1 hours and at 90TC for II hour to prepare a starting material. Example 1 The above starting material was heated from room temperature to 1000C at a temperature rising rate of 60 C/min under vacuum and maintained at this temperature for 1 hour' and then cooled in a furnace. Moreover, the heat treatment was carried out at a state that the starting material was placed on a flat plate of zirconia ceramic.

When the thus obtained test specimen was deformed at room temperature and then immersed in warm water of 9TC, it was immediately turned to the original shape. Furthermore, when the section of the specimen was observed by means of a microscope, the lamination structure existing before the diffusional heat treatment was disappeared to form a complete NiTi single phase state.

Example 2

The starting material was heated from room temperature to 105TC at a temperature rising rate of 6TC/min under vacuum, maintained at this temperature for 30 minutes and then cooled in a furnace.. Moreover, the placing state of the starting material in the heat treatment was the same as in Example 1.

When the thus obtained test specimen was deformed at room temperature and immersed in warm water 1 9 of 90, C, it was immediately turned to the original shape. Furthermore, the section of the specimen was a complete NiTi single phase state.

Example 3

The starting material was heated from room temperature to 1150'C at a temperature rising rate of 6TC/min under vacuum, maintained at this temperature for 5 minutes and then cooled in a furnace. Moreover, the placing state of the material in the heat treatment was the same as in Example 1.

When the thus obtained test specimen was deformed at room temperature and then immersed in warm water of 9TC, it was immediately turned to the original shape. The structure was a complete NiTi single phase state.

Example 4

The starting material was heated from room temperature to 11500C at a temperature rising rate of 600C/min under vacuum, maintained at this temperature for 10 minutes, further heated to 115TC at a rate of 6TC/min, maintained at this temperature for 5 minutes and then cooled in a furnace. Moreover, the placing state of the material in the heat treatment was the same as in Example 1.

When the thus obtained test specimen was deformed at room temperature andthen immersed in warm water of 9TC, it was immediately turned to the original shape. Further, the structure single phase state. Example 5 was a complete NiTi The same heat treatment as in Example was carried out by rendering the starting material into a cylindrical shape and placing in a furnace at a standing state.

After the heat treatment, the test specimen was somewhat deformed but substantially held at the original shape.

For the comparison, when the above cylindrical starting material was subjected to the heat treatment under the same conditions as in Example 3, the deformation of the test specimen became conspicuous, so that the specimen could not be opened into a flat platelike shape.

Comparative Example 1 Ni foils and Ti foils each having a thickness of 20 a m were alternately laminated one upon the other to a total thickness of 0.5 mm, which was cold rolled at a reduction of 10% and then subjected to a diffusion annealing at 95TC for 5 hours.

When the section of the thus obtained specimen was observed by means of a microscope, many layer-like voids were generated, so that it was difficult to manufacture wide and thick products in industry.

As mentioned above, according to the t n k invention, thick and wide Ni-Ti intermetallic compounds, which have never been manufactured by the conventional method in industrial scale, can cheaply be produced in a high quality for a short time.

1 1

Claims (9)

1. A method of producing a Ni-Ti intermetallic compound, which comprises subjecting a laminate comprising one or more Ni foils and one or more Ti 5 folls to a diffuslonal heat treatment to form a NI-TI intermetallic compound containing 48-55 at% of Ni, said diffusional heat treatment being carried out within a temperature range at which a liquid phase is at least partly formed.

2. A method according to claim 1, wherein the composition of the intermetallic compound is determined by adjusting the thickness of the Ni foil and/or of the Ti foil.

3. A method according to claim 1 or 2, wherein the thickness of the Ni foil and/or of the Ti foil is not more than 20 microns.

4. A method according to any of claims 1 to 3, wherein the diffusional heat treatment is carried out under vacuum or in an inert gas atmosphere at a temperature of 955-10150C for 1 second to 10 hours.

5. A method according to any of claims 1 to 3, wherein the diffusional heat treatment is carried out at a temperature of higher than 10150C but not higher than 11100C for 1 second to 1 hour.

6. A method according to any of claims 1 to 3, wherein the diffusional heat treatment is carried out at a temperature of higher than 11100C but not higher than 12400C for 1 second to 10 minutes.

7. A method according to any of claims I to 3, wherein the diffusional heat treatment is carried out at a temperature of 955-10150C and/or higher than 1015'C but not higher than 11100C for 1 second,to 1 minute and at a temperature of higher than 11100C but not higher than 12400C for 1 second to 10 minutes after the completion of the liquid phase forming reaction.

8. A method according to claim 1, substantially t k as described in any of the foregoing Examples.

9. A Ni-Ti intermetallic compound produced by a method according to any of claims 1 to 8.

Published 1991 at 77he Patent Office. State House, 66/71 High Holborn, London WCIR 47p. Further copies may be obtained from Sales Branch. Unit 6, Nine Mile Point. Cwmfelinfach, Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques ltd, St Mary Cray, Kent.