EP0886195B1 - Auto-compensating spring for mechanical oscillatory spiral spring of clockwork movement and method of manufacturing the same - Google Patents

Description

The present invention relates to a self-compensating hairspring for mechanical balance-spring oscillator watch movement or other precision instrument, in paramagnetic alloy Nb-Zr containing between 5% and 25% in weight of Zr, obtained by cold rolling or drawing and having a Young's modulus thermal coefficient (CTE) adjustable by precipitation of the Zr-rich phases in the solid solution Nb-Zr, as well as a manufacturing process of a self-compensating hairspring for mechanical oscillator of timepiece.

We know that the precision of mechanical watches depends the stability of the natural frequency of the formed oscillator of the balance spring. When the temperature varies, the thermal expansion of the balance spring and balance wheel, as well as the variation of the Young's modulus of the hairspring modify the natural frequency of this oscillating assembly, disturbing the watch accuracy.

All the methods proposed to compensate for these variations in frequency are based on the consideration that this natural frequency depends exclusively on the ratio between the constant of the restoring torque exerted by the balance spring on the balance wheel and the moment of inertia of the latter, as indicated in the following relationship: F = 1 2π VS I F = natural frequency of the oscillator
with C = constant of the restoring torque exerted by the oscillating hairspring
I = moment of inertia of the oscillator pendulum

   avec:

E: module de Young du spiral de l'oscillateur

1 / E dE / dT= CTE = coefficient thermique du module de Young du spiral de l'oscillateur

α_s : coefficient de dilatation thermique du spiral de l'oscillateur

α_b ; coefficient de dilatation thermique du balancier de l'oscillateur

Since the discovery of Fe-Ni-based alloys with a positive Young's modulus thermal coefficient (hereinafter CTE), the thermal compensation of the mechanical oscillator is obtained by adjusting the CTE of the hairspring according to the coefficients of thermal expansion balance spring and balance wheel. Indeed, by expressing the torque and inertia from the characteristics of the balance spring and the pendulum, then by deriving equation (1) with respect to the temperature, we obtain the thermal variation of the natural frequency:

with:

E: Young's modulus of the oscillator hairspring

1 / E dE / dT = CTE = thermal coefficient of Young's modulus of the oscillator hairspring

α _s : coefficient of thermal expansion of the balance spring of the oscillator

α _b ; coefficient of thermal expansion of the oscillator pendulum

By adjusting the self-compensation term A = ½ ( CTE + 3α _s ) to the value of the thermal expansion coefficient of the pendulum, it is possible to cancel equation (2). Thus, the thermal variation of the natural frequency of the mechanical oscillator can be eliminated.

The coefficients of thermal expansion α _b of the most widely used balance wheel materials, such as copper, silver, gold, platinum or steel alloys are in the range of 10 to 20 ppm / ° C. To compensate for the effects of temperature variations on the natural frequency of the oscillators, the spiral alloys must therefore have a corresponding self-compensation term A. The precision desired for watches requires the ability to adjust the self-compensation term in manufacturing, in a controlled manner, with a tolerance of a few ppm / ° C around the value sought.

Ferromagnetic alloys based on iron, nickel or cobalt currently used for the production of hairsprings have an abnormally positive CTE in a range of approximately 30 ° C around room temperature, due to proximity of their Curie temperature. In the vicinity of this temperature, the magnetostrictive effects which decrease the Young's modulus of these alloys disappear, causing an increase in the module. Besides the fact that this range of temperature is relatively narrow, these alloys are sensitive to the effects of magnetic fields. These modify the elastic properties of hairsprings irreversibly and thereby change the natural frequency of the oscillator mechanical. In addition, the elastic properties of ferromagnetic alloys vary with the rate of work hardening cold, which requires controlling this parameter exactly during the production of the hairspring.

The CTE values sought for the balance springs produced with this family of alloys are adjusted by treatment thermal precipitation which also fixes the shape final of the hairspring by creep.

We have already proposed in CH-551 032 (D1), in CH-557 557 (D2) and DE-C3-15 58 816 (D3) alloys paramagnetic with high magnetic susceptibility and coefficient thermal of negative susceptibility, as an alternative to ferromagnetic alloys for the manufacture of self-compensating hairsprings and precision springs. These alloys have an abnormally positive CTE and have the advantage of having elastic properties insensitive to magnetic fields. Their elastic properties depend on the texture created during the drawing of the hairspring, but little of the work hardening rate, unlike ferromagnetic alloys. In addition, as mentioned in document D3, these alloys provide an area of thermal compensation for mechanical oscillators that extends over 100 ° C around of room temperature.

The physical causes that create the CTE abnormally positive of these paramagnetic alloys are explained in the above documents. According to them, these alloys have a high density of electronic states at the level of Fermi, as well as a strong electron-phonon coupling, which generates this abnormal behavior of the CTE.

Document D3 cites in particular the alloys of Nb-Zr, Nb-Ti and Nb-Hf as being suitable for the manufacture of balance springs for movement oscillators watchmaking. Document D2 gives an example of an Nb-Zr25% alloy. According to these documents, the springs with abnormally CTE positive are made from the annealed alloy high temperature and then quickly cooled so that obtain a supersaturated solid solution. In this state, the alloy is then more than 85% cold deformed. This strong deformation induces a texture favorable to a CTE positive. To adjust the CTE to the desired value, the alloy is finally heat treated in an interval of temperature which allows the precipitation of the solid solution supersaturated. The phases that precipitate from the solid solution have lower CTE, which results a decrease in the overall CTE and allows its adjustment.

DE-B-1 291 906 (D4) has also proposed binary alloys Nb-Zr containing between 15 and 35% by weight, more particularly 25% by weight of Zr, for manufacturing balance springs for watch movement oscillators.

The hairsprings produced using these binary alloys are manufactured by taking all necessary measures to minimize any oxygen pollution. To this end, thermal precipitation treatments used to adjust the CTE are run under vacuum conditions pushed, the alloys subjected to these treatments being more wrapped in titanium sheets that serve as a trap for oxygen.

We know that Nb-Zr alloys have a very great affinity for the oxygen which weakens them. This is how that the oxygen pollution of these alloys causes breaks during the work hardening operations necessary to the production of hairsprings or other precision springs.

These alloys having a coefficient of thermal expansion about 7 ppm / ° C, equation (2) shows that their CTE should be in the range of about 0 to 20 ppm / ° C to allow the compensation of the pendulums used commonly found in watches. However, as the document "Anomalien der Temperaturabhängigkeit des Elastizizätsmoduls von Niob-Zirkonium-Legierung und reinem Niob "by H. Albert, I. Pfeiffer, Z. Metallkde. 58, 311 (1967) (D5), binary alloys in solid solution containing around 10% to 30% of Zr have CTEs, at room temperature, higher than the values sought, as can be also see it on our measurements represented by the diagram of Figure 1 attached.

To lower the CTE, a precipitation heat treatment must be performed in the two-phase field of Binary phase diagram Nb-Zr. Various heat treatments were carried out at temperatures between 650 ° and 800 ° C in order to lower the CTE of the alloys containing 10% to 30% Zr.

The values obtained after treatments at 650 ° and 750 ° C are given in the diagram of figure 2. These treatments thermals lower the CTE of alloys containing more than 23% by weight of Zr. However, we can see that for Zr concentrations below 23%, the CTE cannot be lowered to the desired values for spirals, despite very long processing times.

This is confirmed by document D5, one of the authors is the inventor of document D4, where treatments from 64h at 600 ° C were made for alloys comprising from 19% to 33% by weight of Zr. Indeed, for concentrations greater than or equal to 25% by weight of Zr, the CTE at room temperature drops during heat treatment to very negative values, whereas, still according to this same document D4, for concentrations of 19% and 22%, of values close to 0 ppm / ° C are obtained. These values, after heat treatment, are lower than those measured during our trials and whose results are subject from the diagram in Figure 2. This difference is explained by the lower temperature chosen in document D5 for heat treatment.

The CTE measured for alloys with 19% and 22% in weight of Zr and treated for 64 h at 600 ° C would be suitable for the manufacture of hairsprings. However, the tests that we have shown that these processing conditions do not unfortunately do not allow a fixation of the creep spiral when the concentration of Zr is less than 20% by weight. For the rest, the duration of heat treatment necessary to obtain a CTE capable of getting self-compensating hairs is way too much long in the case of industrial production.

So the tests that we have done, which are found confirmed by document D5, show that binary alloys Nb-Zr containing less than 23% by weight of Zr (see Fig. 2) not suitable for the manufacture of self-compensating hairsprings for mechanical movement oscillators Contrary to what is claimed, on the basis of no practical test, by D4 (whose inventor is co-author of D5).

While the state of the art in manufacturing of Nb-Zr alloys recommends minimizing by all means oxygen pollution to avoid fragile fractures during deformation operations, such as this emerges in particular from document D4 which recommends expressly perform heat treatment of alloys binaries Nb-Zr so as to maintain the concentration oxygen as low as the manufacturing process allows, we have chosen to boost Nb-Zr alloys with oxygen to facilitate precipitation of the phases rich in Zr. It is indeed known from "Natur, Grösse und Verteilung von Gitterstörungen und ihr Einfluss auf Hochfeldeigenschaften of Typ-III-Supraleiters Nb-Zr25 "from H. Hillmann, I. Pfeiffer, Z. Metallkde. 58, 129 (1967) (D6), than oxygen, even in low concentrations of around 1000 ppm by weight, modifies the phase diagram of the alloys binaries of Nb-Zr containing 25% by weight of Zr and accelerates precipitation of Zr-rich phases.

Contrary to what has been accepted for more than 25 years in the state of the art relating to the production of hairsprings auto compensators for mechanical parts oscillators Nb-Zr alloy watchmaking, the inventors of this invention discovered that doping these alloys containing between 5% and 25% by weight of Zr proves to be extremely beneficial insofar as it makes it possible to precipitate Zr-rich phases in these alloys, by treatments thermals carried out at compatible temperatures and durations with the manufacture of such hairsprings.

Therefore, the object of the present invention is to at least partially remedy the disadvantages of hairsprings self-compensators for mechanical oscillators, in particular for watch movements. More specifically, this invention aims to remedy the aforementioned drawbacks linked to self-compensating hairsprings in paramagnetic alloys and more specifically to Nb-Zr alloys.

To this end, this invention firstly relates to a self-compensating hairspring for mechanical oscillator watch movement or other precision instrument, in paramagnetic alloy Nb-Zr containing between 5% and 25% in weight of Zr, of the aforementioned type, as defined in claim 1.

This invention also relates to a manufacturing process of such a self-compensating balance spring for an oscillator mechanical clockwork according to claim 7.

Other features of this invention are the subject of claims dependent respectively on the two claims main above mentioned relating to a hairspring self-compensator and its manufacturing process.

The advantages of the present invention are considerable, to the extent it actually allows for the first times, to bring a truly industrial solution by which it becomes possible to adjust with precision and knowingly, the CTE of a paramagnetic alloy and therefore the self-compensating term of a self-compensating hairspring for mechanical clockwork oscillator made of such an alloy. Indeed, so far and in the absence of doping with an interstitial agent containing oxygen it was impossible to make such hairsprings binary alloy Nb-Zr below 20% by weight of Zr, for the reasons given above. Furthermore, as we will explain it later, it turns out that in the range of these alloys comprising between 20 and 25% by weight of Zr, the adjustment of the CTE by heat treatment is very dependent oxygen concentration. However, since with the solutions proposed in the state of the art, in particular with that of document D4, we did not control the concentration of oxygen which fluctuated according to the operating conditions between two sets of hairsprings, it was impossible, in the absence of knowledge of the oxygen content and of its role in adjusting the CTE, to precisely control this CTE and therefore the term self-compensation of the balance spring made.

In addition, the ferromagnetic alloys currently used are only self-compensating in a low temperature range and their Young's modulus undergoes variations irreversible, for example in the presence of fields magnetic, so that the natural frequency of the oscillator mechanical associated with such a hairspring is likely to change over time.

The solution proposed by the present invention provides therefore a decisive improvement compared to self-compensating hairsprings state of the art, since such hairsprings allow precise adjustment of their term autocompensation, Young's modulus of paramagnetic alloy being, moreover, insensitive to magnetic fields and at the rate of cold work hardening and finally, the range in which the CTE is abnormally positive and therefore allows self-compensation to manifest, goes from about 30 ° C around room temperature at around 100 ° C.

It is therefore no exaggeration to think that it is a very significant progress in the field of hairsprings paramagnetic alloy self-compensators for oscillators mechanical clockwork movements, since this invention makes it possible, for the first time, to manufacture such hairsprings with Zr levels of between 5% and 20%, area in which the precipitation of Zr-rich phases is easy to control and is not very sensitive to concentration an interstitial agent containing oxygen. It is also the first time that such alloys are offered with a Zr concentration between 20 and 25% by weight with the possibility of controlling the adjustment of the CTE by a control the content of oxygen-containing interstitial agent in the alloy.

La figure 1 est un diagramme du CTE à température ambiante des alliages binaires Nb-Zr en solution solide à l'état écroui;

La figure 2 est un diagramme du CTE à température ambiante des alliages binaires Nb-Zr après revenu;

La figure 3 est un diagramme du CTE à température ambiante des alliages Nb-Zr-O dopés d'environ 1000 ppm en poids d'oxygène;

La figure 4 est un diagramme illustrant le domaine de l'espace Nb-Zr-O utilisable pour les spiraux;

La figure 5 est un diagramme illustrant le CTE à température ambiante de l'alliage Nb-Zr23%, revenu 3h à 750°C, en fonction du taux d'oxygène.

Other features and advantages will appear in the following description, as well as in the accompanying drawing which illustrates a series of explanatory diagrams relating to Nb-Zr alloys.

FIG. 1 is a diagram of the CTE at ambient temperature of the binary alloys Nb-Zr in solid solution in the hardened state;

FIG. 2 is a diagram of the CTE at ambient temperature of the binary alloys Nb-Zr after tempering;

FIG. 3 is a diagram of the CTE at ambient temperature of the Nb-Zr-O alloys doped with approximately 1000 ppm by weight of oxygen;

FIG. 4 is a diagram illustrating the domain of the space Nb-Zr-O usable for the balance springs;

FIG. 5 is a diagram illustrating the CTE at room temperature of the Nb-Zr23% alloy, returned 3 hours at 750 ° C., as a function of the oxygen rate.

Figure 3 shows the case of 10% -23% Zr alloys containing approximately 1000 ppm by weight of oxygen, subjected to a 3 hour tempering treatment at 750 ° C. We can see on this diagram that this income makes it possible to adjust the CTE to the desired values for self-compensating hairsprings (0 to 20 ppm / ° C), with alloys containing 10% -13% and 18% -22% of Zr. So general, by doping with more than 600 ppm by weight of oxygen, it is possible to adjust the CTE between 0 and 20 ppm / ° C for all Nb alloys containing 5% to 23% in weight of Zr. The recommended tempering temperatures are between 700 ° and 850 ° C. These temperatures and durations allow to simultaneously perform the creep fixing of the hairspring shape. Thanks to doping oxygen, the Zr concentrations necessary for manufacturing spirals can therefore be reduced and as we will see, checking the CTE is easier to do if the concentration of Zr is less than 20% by weight. Through elsewhere, the processing temperature that can be used to perform this CTE check, is high enough to allow creep fixing of the shape of the spiral, which was not previously possible with concentrations less than 23% by weight of Zr, which required treatment temperatures of the order of 600 ° C., that is to say below the fixing temperature of the spiral shape by creep.

a) Dans le premier domaine que l'on peut situer entre 25% et 35% en poids de Zr, la concentration d'oxygène doit être maintenue la plus faible possible, soit moins de 500 ppm en poids environ. Des concentrations plus élevées entraínent des ruptures de fil au tréfilage et des précipitations des phases riches en Zr beaucoup trop rapides pour permettre de bien contrôler la valeur du CTE désiré pour le spiral autocompensateur.

b) entre 25% et 20% en poids de Zr, la concentration d'oxygène doit être maintenue dans une bande étroite augmentant d'environ 500-800 ppm en poids pour l'alliage 25% à environ 600-2000 ppm en poids pour l'alliage 20% de Zr. En dessous de ces valeurs en agent de dopage, la précipitation des phases riches en Zr est trop lente. Au-dessus, elle est trop rapide pour permettre la fabrication de spiraux autocompensateurs avec un CTE contrôlable. Dans ce domaine de concentration de Zr, nous avons observé une grande dépendance du CTE vis-à-vis de la concentration d'oxygène. Pour exemple, le diagramme de la figure 5 illustre les CTE obtenus avec des alliages Nb-Zr23% en poids, après 3h à 750°C, pour différentes concentrations d'oxygène. On voit que le CTE passe de valeurs trop positives à des valeurs trop négatives sur quelques dizaines de ppm en poids d'oxygène. Cette sensibilité oblige de contrôler précisément la concentration d'oxygène pour garantir la reproductibilité des valeurs de CTE des spiraux autocompensateurs fabriqués avec ces alliages, ce qui est difficile à obtenir et à reproduire.

c) Dans le domaine compris entre 5% et 20% en poids de Zr, il faut introduire au moins 600 ppm en poids d'oxygène pour permettre une précipitation des phases riches en Zr et donc un ajustement contrôlable de la valeur du CTE. Pour ces concentrations en Zr, on observe une très faible sensibilité de la valeur du CTE par rapport à la concentration de l'alliage en oxygène. Aucune concentration supérieure d'oxygène n'a été mise en évidence dans les alliages réalisés au cours de nos essais. Cette limite doit certainement exister, ne serait-ce que pour des raisons de fragilité des alliages lorsque la concentration d'oxygène augmente trop, mais elle n'a pas affecté nos expériences. Compte tenu de ces constatations nous n'avons pas jugé utile de définir une limite supérieure qui ne présente en pratique aucun intérêt pour le résultat recherché, puisque ce résultat peut être obtenu de manière parfaitement reproductible sans connaítre cette limite supérieure et compte tenu du fait que c'est de toute façon dans ce domaine de l'alliage Nb-Zr que la concentration d'oxygène est le moins critique pour autant que l'on prenne soin d'atteindre au moins la limite inférieure susmentionnée. Typiquement nous pouvons dire qu'il est possible, dans tous les cas, d'atteindre l'objet de la présente invention en dopant l'alliage Nb-Zr de ce domaine (5%-20% de Zr) entre 600 et 1500 ppm en poids d'oxygène.

The optimal concentration of oxygen to be introduced into the alloy depends on the concentration of Zr. We can distinguish three areas of Zr concentration which are schematically illustrated in the diagram of FIG. 4.

a) In the first range which can be situated between 25% and 35% by weight of Zr, the oxygen concentration must be kept as low as possible, ie less than 500 ppm by weight approximately. Higher concentrations cause wire breaks in the drawing and precipitation of the Zr-rich phases that are far too rapid to allow the desired CTE value for the self-compensating hairspring to be properly controlled.

b) between 25% and 20% by weight of Zr, the oxygen concentration must be maintained in a narrow band increasing by approximately 500-800 ppm by weight for the alloy 25% to approximately 600-2000 ppm by weight for the alloy 20% of Zr. Below these doping agent values, the precipitation of the Zr-rich phases is too slow. Above, it is too fast to allow the production of self-compensating hairsprings with a controllable CTE. In this Zr concentration range, we have observed a great dependence of the CTE on the oxygen concentration. For example, the diagram in FIG. 5 illustrates the CTEs obtained with Nb-Zr23% by weight alloys, after 3 h at 750 ° C., for different concentrations of oxygen. It can be seen that the CTE goes from too positive values to too negative values over a few tens of ppm by weight of oxygen. This sensitivity means that the oxygen concentration must be precisely controlled to guarantee the reproducibility of the CTE values of the self-compensating hairsprings manufactured with these alloys, which is difficult to obtain and to reproduce.

c) In the range between 5% and 20% by weight of Zr, it is necessary to introduce at least 600 ppm by weight of oxygen to allow precipitation of the phases rich in Zr and therefore a controllable adjustment of the value of the CTE. For these Zr concentrations, a very low sensitivity of the CTE value is observed relative to the concentration of the oxygen alloy. No higher oxygen concentration was demonstrated in the alloys produced during our tests. This limit must certainly exist, if only for reasons of brittleness of the alloys when the oxygen concentration increases too much, but it has not affected our experiments. Given these observations, we did not consider it useful to define an upper limit which in practice is of no interest for the desired result, since this result can be obtained in a perfectly reproducible manner without knowing this upper limit and taking into account that in any case, it is in this area of the Nb-Zr alloy that the oxygen concentration is the least critical, provided that care is taken to reach at least the aforementioned lower limit. Typically we can say that it is possible, in all cases, to achieve the object of the present invention by doping the Nb-Zr alloy of this domain (5% -20% of Zr) between 600 and 1500 ppm by weight of oxygen.

Above 25% by weight of Zr, on the one hand, the alloy is difficult to work and, on the other hand, given the faster precipitation speeds it's very difficult to control the CTE in a reproducible manner. On the contrary, we could see how much easier it is to work with Nb-Zr alloys comprising less than 25%, preferably, less than 20% by weight of Zr.

It has indeed been found that the resistance to deformation decreases and ductility increases when the concentration in Zr decreases. However, the mechanical properties of the hairspring completed decrease. It is possible to improve these mechanical properties by adding at least one element likely to harden, chosen from among the elements following in proportions of between 0.01% and 5% in weight: Be, Al, Si, Ge, Sc, Y, La, Ti, Hf, V, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au.

Other doping elements than oxygen, such as nitrogen, carbon, boron or phosphorus can be added, either at the same time or after the doping treatment by the oxygen used to allow the adjustment of the CTE by precipitation of Zr-rich phases. As will be seen thereafter, we almost always find some proportion of nitrogen in addition to oxygen in the alloy.

Once the hairspring is completely finished, it is possible to carry out a doping operation additional with a gas containing at least one of the elements aforementioned dopants used to harden the hairspring. This treatment will obviously make the hairspring more brittle, which which no longer has the same importance once it is placed in shape is finished. It can therefore be interesting increase the hardness and mechanical properties of hairspring completed, although oxygen doping to adjust the CTE already contributes to the hardening of the hairspring. Well understood, this treatment must be carried out at a temperature which does not reach the CTE setting temperature, i.e. at a temperature not exceeding 650 ° C.

Examples

We will now describe a series of relative examples to the process of manufacturing self-compensating hairsprings according to the present invention. We will give everything first the general operating conditions applicable to the set of examples and then we will give an array relating to different alloys produced from these operating conditions.

The Nb-Zr alloy is poured under high vacuum in an oven with electronic bombardment. The bars obtained are then sheathed, for example by a sheath of copper alloy, nickel or stainless steel, according to usual procedure for this type of Nb-Zr alloy, to keep it safe from oxygen. These bars are then rolled or drawn to cold to a diameter between 0.05 and 1.5mm, in intercalating, if necessary, intermediate anneals.

The wire obtained is then taken out of its protective sheath to be subjected to an oxygen doping operation according to a known technique, either by anodic oxidation, either by thermal oxidation. In the case of oxidation anodic, the oxygen concentration introduced is controlled by the choice of wire diameter, anodizing voltage, duration of voltage application, temperature and the composition of the electrolyte.

For thermal oxidation, the oxygen concentration introduced is controlled by the choice of wire diameter, the temperature, pressure and type of oxidizing gas, as well only by the duration of the treatment.

After the oxygen doping operation, the wire is cold deformed until a corresponding section is obtained to that of the hairspring. This wire is then wound into shape spiral, then it is heat treated to fix its form by creep and adjust the CTE to the value sought in depending on the type of alloy, as indicated above.

We give in table I which follows, some examples relating to thermal doping with oxygen for different alloys and different wire diameters.

It is obvious that when a second doping treatment is carried out on the completed self-compensating hairspring, as the possibility was mentioned previously, the quantities of oxygen, of nitrogen, depending on the case, may be significantly greater than the quantities appearing in this table I. However, the quantities indicated in this table are those which serve to allow the CTE of the hairspring to be adjusted, generally between 0 and 20 ppm / ° C, by controlled precipitation of the Zr-rich phases. As indicated above, in the alloy range between 5% and 20%, the higher proportion of interstitial doping agent is not critical provided that it is at least above a lower limit situated around 600-800 ppm by weight. Zr (%) weight ⊘ (mm) Temp. (° C) Duration (min.) Gas Pressure (Pa) Oxygen (ppm) Nitrogen (ppm) 23 1 1080 120 N _{2 /} H ₂ 10 ⁵ 1100 1200 20 0.9 1100 60 - 10 ^-4 1200 150 20 0.15 450 2 air 10 ⁵ 900 70 15 0.25 450 3 air 10 ⁵ 800 50 10 0.25 450 3 air 10 ⁵ 950 50

However, once the CTE is adjusted and whatever the alloy, it is possible to add at least one of the aforementioned interstitial agents in a second operation doping intended to improve mechanical properties hairspring finished. During this second operation, other elements likely to diffuse in the hairspring, like carbon, boron or phosphorus, could also be added to harden.

Other means of improving the mechanical properties of the hairspring could consist, as already mentioned, of incorporating into the alloy a certain amount of one of the elements listed in table II, in proportions which can vary between 0.01%. and 5% by weight. Element N ° column Hardens the Nb according to the literature Be IIa al IIIa * Yes IVa Ge IVa * sc IIIb Y IIIb The IIIb Ti IVb * Hf IVb * V Vb * Your Vb * Cr VIb * MB VIb * W VIb * mn VIIb Re VIIb Fe VIIIb * Ru VIIIb Bone VIIIb Co VIIIb Rh VIIIb Ir VIIIb Or VIIIb * Pd VIIIb Pt VIIIb Cu Ib * Ag Ib At Ib Some of the elements in Table II are mentioned in the literature as allowing hardening, others of these elements have been selected according to their phase diagram with Nb.

Claims (12)

1. A self-compensating balance-spring for a balance-spring/balance assembly of a mechanical oscillator of a precision instrument in particular a horological movement, made of a paramagnetic Nb-Zr alloy containing between 5% and 25% by weight of Zr and having a Young's modulus whose temperature coefficient (TCY) is such that it can substantially nullify the expression : 1 E dE dT +3α s -2α b , where

   E : Young's modulus of the oscillator spring;

   1 / E dE / dT= TCY = temperature coefficient of the oscillator spring's Young's modulus;

   α _s : coefficient of thermal expansion of the oscillator's spring; and

   α_b: coefficient of thermal expansion of the oscillator's balance,

   characterized by the fact that it contains at least 500 ppm by weight of an interstitial doping agent at least partly formed of oxygen.
2. The balance-spring according to claim 1, characterised in that it comprises between 5% and 20% by weight of Zr and at least 600 ppm by weight of said interstitial doping agent.
3. The balance-spring according to claim 1, characterised in that, to control precipitation of Zr rich phases in the Nb-Zr solid-solution when said Nb-Zr alloy comprises between 20% and 25% by weight of Zr, the amount of said interstitial doping agent varies from 600 to 2000 ppm by weight for a concentration of 20% by weight of Zr to 500 to 800 ppm by weight for a concentration of 25% by weight of Zr.
4. The balance-spring according to any preceding claim, characterised in that the proportion of oxygen in said interstitial doping agent is comprised between 20% and 100% by weight.
5. The balance-spring according to any preceding claim, characterised in that, in addition to said doping agent for controlling the precipitation of Zr rich phases in the Nb-Zr solid-solution, it further comprises an amount of at least one hardening doping agent selected from the following elements : oxygen, nitrogen, carbon, boron and phosphorous.
6. The balance-spring according to any preceding claim, characterised in that it further comprises between 0.01% and 5% by weight of at least one element selected from : Be, Al, Si, Ge, Sc, Y, La, Ti, Hf, V, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au.
7. Process for manufacturing a self-compensating balance-spring made of a Nb-Zr alloy comprising 5% to 25% of Zr for a mechanical balance/balance-spring oscillator of a precision instrument in particular a horological movement, wherein a bar is formed from said alloy, this bar is transformed into a wire having a diameter comprised between 0.05 and 1.5 mm by cold rolling or cold drawing in the absence of oxygen, the diameter of this wire is reduced by cold rolling or cold drawing and shaped into a ribbon suitable for the balance-spring, this ribbon is wound into the shape of a spiral and submitted to at least one heat treatment under a controlled pressure and/or controlled atmosphere to reduce the temperature coefficient of the Young's modulus (TCY) by the controlled precipitation of Zr rich phases and to define the shape of the balance-spring, characterised in that the wire contains an interstitial agent in an amount producing the controlled precipitation of Zr rich phases, and the wire thus obtained is heated between 650°C and 880°C for 1h to 24h, to adjust the TCY to the desired value.
8. The process according to claim 7, characterised in that a Nb-Zr alloy comprising between 5% and 20% by weight of Zr is formed and the amount of said interstitial agent in said wire is adjusted by doping with at least 600 ppm in an oxygen-containing atmosphere.
9. The process according to claim 7, characterised in that a Nb-Zr alloy comprising between 20% and 25% by weight of Zr is formed and the amount of said interstitial agent in said wire is adjusted by doping from 600 to 2000 ppm by weight for a concentration of 20% by weight of Zr to 500 to 800 ppm by weight for a concentration of 25% by weight of Zr.
10. The process according to any one of claims 7 to 9, characterised in that said ribbon wound into a spiral shape is placed under vacuum to carry out said heat treatment.
11. The process according to any one of claims 7 to 9, characterised in that after heat treatment to adjust the TCY and define the shape of the self-compensating balance-spring, said spring undergoes a hardening heat treatment at a temperature below 650°C in an atmosphere containing a partial pressure of a gas containing at least one element capable of diffusing into the balance-spring.
12. The process according to claim 11, characterised in that said elements are selected from : oxygen, nitrogen, carbon, boron and phosphorous.

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