EP0864664A1 - Process for producing a superelastic article from an alloy of nickel and titanium - Google Patents

Abstract

Method of manufacturing a superelastic piece of nickel and titanium alloy, characterized in that it consists in producing an ingot (1, 2, 3) from a mixture of nickel and titanium comprising 55.6% ± 0.4% by weight of nickel and carrying out a heat treatment (12) to generate martensite wafers by subjecting said ingot to a temperature between 480 and 520 ° C for a period of between 5 and 45 minutes. The process provides a truly superelastic material at room temperature. <IMAGE>

Description

The invention relates to a method for manufacturing a superelastic part made of nickel and titanium alloy.

It is known to baptize "superelastic materials" nickel and titanium alloys which have been treated thermal and / or work hardening. On a martensitic alloy with room temperature, work hardening can lead to formation of what is called "hardened martensite" which gives an elasticity of the order of 4%. This does not allow qualify the alloy obtained as "superelastic".

The method of the invention aims, on the contrary, to produce a truly superelastic part thanks to a reasoned choice of the composition of the titanium and nickel alloy and a martensite platelet generation heat treatment in this alloy.

This object is achieved by the process of the invention which is characterized in that it consists in producing an ingot from of a mixture of nickel and titanium comprising 55.6% ± 0.4% in weight of nickel and to carry out a heat treatment of generation of martensite wafers by subjecting said part at a temperature between 480 and 520 ° C during a duration between 1 and 60 minutes.

Thanks to the process of the invention, the ingot produced has a transition temperature at the end of the appearance of austenite A _{f of} less than 20 ° C. under stress, so that the material is really superelastic under normal temperature conditions. The martensite platelet generation heat treatment facilitates the movements of these platelets inside the material, which corresponds to the superelastic nature.

According to a first advantageous aspect of the invention, the process also includes a step of flash annealing, this annealing flash being carried out at a temperature between 600 ° and 800 ° C for a period of between 10 and 30 seconds, this duration being a function of the transverse dimensions of the part. This flash annealing partially anneals the surface of the part, which increases its ductility without harming its elasticity.

According to another advantageous aspect of the invention, the process includes a crystallization annealing step prior to platelet generation heat treatment of martensite, this crystallization annealing being carried out at a temperature between 700 and 800 ° C, preferably between 720 and 780 ° C for a period greater than two minutes. The temperature range chosen for crystallization annealing achieves true crystallization without precipitation phases of the alloy rich in nickel and without embrittlement of the alloy by grain enlargement.

According to another advantageous aspect of the invention, the process includes a work hardening step prior to treatment thermal generation of martensite wafers, this work hardening being between 15 and 28%, preferably between 20 and 27%. The purpose of work hardening is to break the structure of annealing and creating dislocations serving as sites of germination to deformation martensite.

Depending on the case, it can be provided that this work hardening is cold or lukewarm, i.e. at a temperature less than 500 ° C.

According to another advantageous aspect of the invention work hardening can be performed with intermediate annealing, at a temperature between 400 and 550 ° C.

• la figure 1 est une représentation schématique de la déformation obtenue en fonction de la température selon les phases présentes dans un alliage métallique ;
• la figure 2 est une représentation schématique de la valeur de la température de transition de fin d'apparition d'austénite en fonction du pourcentage de nickel dans un alliage de nickel et de titane et
• la figure 3 est un schéma blocs du procédé de l'invention.

The invention will be better understood and other advantages of it will appear more clearly in the light of the following description of a process for manufacturing a spectacle frame wire in accordance with its principle, given solely by way of example and made with reference to the accompanying drawings in which:

• Figure 1 is a schematic representation of the deformation obtained as a function of the temperature according to the phases present in a metal alloy;
• FIG. 2 is a schematic representation of the value of the transition temperature at the end of the appearance of austenite as a function of the percentage of nickel in a nickel and titanium alloy and
• FIG. 3 is a block diagram of the method of the invention.

• A_s est la température de début d'apparition d'austénite à partir de l'état martensitique ;
• A_f est la température de fin d'apparition d'austénite à partir de l'état martensitique ;
• M_s est la température de début d'apparition de martensite à partir de l'état austénitique et
• M_f est la température de fin d'apparition de martensite à partir de l'état austénitique.

For the manufacture of a superelastic piece such as a wire forming a spectacle frame, the choice of the composition of the alloy is made as a function of one of the four temperatures characteristic of the transition between the martensitic state and the state austenitic. These characteristic temperatures are as follows:

• A _s is the temperature at which austenite begins to appear from the martensitic state;
• A _f is the temperature at the end of the appearance of austenite from the martensitic state;
• M _s is the temperature at which martensite begins to appear from the austenitic state and
• M _f is the temperature at the end of the appearance of martensite from the austenitic state.

An alloy is really superelastic when the deformation martensite created under a stress Σ is not stable. To obtain a superelastic material in a temperature range corresponding to common use, for example above -20 ° C., the alloy should not be in the martensitic state in this temperature range. In other words, an alloy is sought whose temperature M _s is less than -20 ° C.

In practice, it has been demonstrated that the temperature difference separating M _s and A _f is of the order of 40 ° C for titanium and nickel alloys, so that, to comply with the condition previously stated, it is necessary to '' have an alloy whose temperature A _f under stress is less than 20 ° C. On the other hand, the effect of a stress on an alloy of titanium and nickel influences the temperatures identified above, to the point that these can be shifted upwards by approximately 30 ° C. This is why, when it is desired to have a temperature A _{f of} less than 20 ° C under stress, it is necessary to provide an alloy composition such that A _f is less than -10 ° C in the absence of stress.

It has been experimentally determined that the condition A _f below -10 ° C corresponds to an alloy of titanium and nickel rich in nickel, that is to say comprising nickel in a proportion of 55.6% ± 0.4% by weight with, optionally, conventional addition elements such as iron, copper or vanadium, these elements replacing nickel according to rules known to those skilled in the art. Particularly interesting results have been obtained in the case where the percentage of nickel is between 55.8 and 56% by weight.

On the other hand, tests on an alloy whose temperature A _f is equal to 20 ° C have shown that the alloy is very elastic at 20 ° C, but in a limiting manner. Indeed, at 15 ° C, the alloy has a "soft" component, a sign that the martensite is stable, while at 25 ° C, the alloy is perfectly elastic. This demonstrates that the limit set with A _f corresponds to a physical reality relating to the use of the material.

The process of the invention thus begins with a step 1 of mixture of nickel and titanium in the chosen proportions.

Stage 1 is followed by a fusion 2, at a temperature of the order of 1300 to 1500 ° C., leading to a first transformation of the ingot when hot, ie at a temperature of between 900 and 1000 ° C., represented by stage 3. Several stages of work hardening 4 and several stages of successive annealing 5 of the ingot can be provided, which is represented by the looping arrow F ₁ in FIG. 3.

The martensite platelet generation heat treatment included in the process of the invention, which is represented by step 12 in FIG. 3, must not have a negative influence on the transition temperature A _f, that is to say - say increase this temperature. However, in heat treatments carried out at a temperature below about 450 ° C., the heat treatment may have the effect of precipitating TiNi ₃ , Ti ₂ Ni ₃ or Ti ₂ Ni ₄ . Given the relative number of nickel atoms and titanium atoms in each molecule of TiNi ₃ , Ti ₂ Ni ₃ or Ti ₂ Ni ₄ , this precipitation corresponds to a decrease in the relative value of nickel in the alloy, so that the transition temperature is displaced according to arrow F in FIG. 2 and that it increases in the unwanted direction. It is therefore important to avoid as much as possible the formation of TiNi ₃ or other similar compounds during the heat treatment. It has been determined experimentally that little or no TiNi _{3 is formed} when the heat treatment is carried out at a temperature above 480 ° C.

In addition, treatment above 550 ° C begins annealing the alloy structure and leads to possibilities plasticity. Experience has shown that this phenomenon occurs produces from 530 ° C. In view of the above, we limit therefore the temperature of the heat treatment at 520 ° C. So the martensite platelet generation heat treatment does not disturb the composition and structure of the alloy the part being processed.

According to a first advantageous aspect of the invention, the process also includes a step 11 of flash annealing prior to platelet generation heat treatment martensite. This step 11 could also be later at treatment 12. This flash annealing is carried out at a temperature between 600 and 800 ° C for a period between 10 and 30 seconds. This duration depends on transverse dimensions of the wire, that is to say of its diameter. The purpose of the flash treatment is to improve the ductility, that is to say the fatigue resistance of the part, without harming with a superelastic effect. This is achieved if a fraction corresponding to approximately 10% of the mass of the part is annealed near the surface of it. In practice, and for be consistent with other steps of the process of the invention, this flash annealing can be carried out at a temperature included between 720 ° and 780 ° C.

Advantageously, a step 6 of annealing treatment of crystallization is also planned before the heat treatment generation of martensite platelets. This annealing of crystallization must bring a real recrystallization of the whole piece, that is to say that the grains elongated during rolling must be able to be broken to form grains smaller.

This crystallization annealing must be long enough to bring the whole room to the desired temperature. For a small diameter wire, this is done after two about minutes. For a large piece or a coil complete with wire, crystallization annealing may take longer of one hour.

As before, it is necessary to avoid precipitating nickel-rich phases, of the TiNi ₃ type, which would have the consequence of depleting the nickel alloy and would lead to an increase in the value of the transition temperature A _f according to arrow F in FIG. 2. In practice, it has been found that the precipitation of TiNi ₃ can be avoided when the temperature of the recrystallization annealing 6 is maintained above 700 ° C. Indeed, tests have been carried out at around 680 ° C and it has been shown that the temperature A _f is offset by around 10 ° C, which is not acceptable. For more security, one can choose to carry out the annealing at a temperature higher than 720 ° C.

It should be noted that the limit at 700 ° C. envisaged above is not significant for flash annealing since its very short duration does not allow effective precipitation of TiNi ₃ or other titanium compounds and nickel. This is why flash annealing can take place at a temperature above 600 ° C only.

Furthermore, above about 850 ° C, the treatment of annealing tends to weaken the alloy by magnifying the grains or even burns when the annealing temperature reaches 900 ° C. This is why, for safety, we limit the annealing temperature at about 800 ° C.

Particularly satisfactory results have been obtained when the annealing temperature is maintained between 720 and 780 ° C.

One can in particular provide that at the end of the annealing treatment, a sample is taken in a step 7 to verify that the temperature A _f remains below -10 ° C.

According to an advantageous variant of the invention, a step 8 work hardening is also provided in the process in order to break the annealing structure and create dislocations serving as a germination site for deformation martensite. Practical tests have shown that when this work hardening is limited to 15%, "annealed" austenite remains inside material and the superelastic effect is not optimal. We speak in this case of "soft" component of the material.

In addition, one could demonstrate that a higher work hardening at 30% does not bring a better result compared to a 28% work hardening. In addition, too much work hardening negatively affects hardness and fatigue life of the material, so that in practice, we limit the work hardening at values between 15 and 28% and, for more than security, we generally work between 20 and 27% work hardening.

This work hardening can be carried out in one or more stages, which is represented by the looping arrow F ₂ in FIG. 3, cold or lukewarm, that is to say at a temperature below 500 ° C.

Optionally, an intermediate annealing step 9 can be provided at a temperature between 400 and 550 ° C so not to generate a new recrystallization of the alloy. This intermediate annealing allows, in particular, a setting easier workpiece shape while improving elongation elastic obtainable.

Before the heat treatment 11, the wire is calibrated in a shaping step 10.

A superelastic part obtained by the process of the invention finds many applications. In particular, the process can be used for the manufacture of threads, the diameter is between 0.5 and 5 mm and which can serve as glasses frame, but also bra frame, cell phone antenna, needle, prosthetic piece or for ancillary equipment intended for the installation of prostheses in the medical field. The cross section of these wires can be round, square, rectangular or other, as desired the user.

Claims (8)

Method for manufacturing a superelastic part in alloy of nickel and titanium, characterized in that it consists in producing an ingot (1, 2, 3) from a mixture of nickel and titanium comprising 55.6% ± 0.4% by weight of nickel and to carry out a generation heat treatment (12) martensite wafers by subjecting said part to a temperature between 480 and 520 ° C for a period between 5 and 60 minutes. Method according to claim 1, characterized in that that it comprises a flash annealing step (11), said annealing flash being carried out at a temperature between 600 and 800 ° C, for a period of between 10 and 30 seconds, said duration being a function of the transverse dimensions of said piece. Method according to claim 1 or 2, characterized in what it includes a crystallization annealing step (6) prior to said heat treatment (12) for generating martensite wafers, said crystallization annealing being carried out at a temperature between 700 and 800 ° C, preferably between 720 and 780 ° C, for a period greater than two minutes. Method according to one of claims 1 to 3, characterized in that it comprises a step (8) of previous work hardening said heat treatment (12) for generating martensite plates, said work hardening being between 15 and 28%, preferably between 20 and 27%. Method according to claim 3 and 4, characterized in that said step (8) of work hardening is subsequent to said crystallization annealing step (6). Method according to claim 4, characterized in that said work hardening (8) is carried out cold. Method according to claim 4, characterized in that said work hardening (8) is carried out lukewarm, that is to say at a temperature below 500 ° C. Method according to claim 6 or 7, characterized in that said work hardening (8) is carried out with annealing intermediate (9) at a temperature between 400 and 550 ° C.

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