METASTABLE ß TITANIUM ALLOY, TIMEPIECE SPRING MADE FROM SUCH AN ALLOY AND METHOD FOR PRODUCTION THEREOF

Abstract

A metastable titanium alloy is provided, which includes, by weight percent, between 24 and 45% niobium, between 0 and 20% zirconium, between 0 and 10% tantalum and/or between 0 and 1.5% silicon and/or less than 2% oxygen, said alloy having a crystallographic structure containing: a mix of austenitic phase and alpha phase; and a presence of omega phase precipitates the volume fraction of which is less than 10%. Also provided is a timepiece spring made from such an alloy and a method for producing such a spring.

Claims

1. A metastable titanium alloy comprising: as a percentage by weight, between 24 and 45% niobium, between 0 and 20% zirconium, between 0 and 10% tantalum and/or between 0 and 1.5% silicon and/or less than 2% oxygen, said alloy having a crystallographic structure comprising: a mixture of austenitic phase and alpha phase; and the presence of omega-phase precipitates the volumetric concentration of which is less than 10%, and said alloy alpha phase has a volumetric concentration comprised between 1 and 40%.

2. The alloy according to claim 1, characterized in that the alpha phase and the omega phase are present in the form of precipitates within a matrix constituted by austenitic grains.

3. The alloy according to claim 1, in which a grain size is less than 1 m.

4. The alloy according to claim 1, in which: an alpha-phase precipitates size is less than 500 nm; and an omega-phase precipitates size is less than 100 nm.

5. A timepiece spring produced from metastable titanium alloy, said metastable titanium alloy comprising, as a percentage by weight, between 24 and 45% niobium, between 0 and 20% zirconium, between 0 and 10% tantalum and/or between 0 and 1.5% silicon and/or less than 2% oxygen, said alloy having a crystallographic structure comprising: a mixture of austenitic phase and alpha phase; and a presence of omega-phase precipitates the volumetric concentration of which is less than 10%.

6. The timepiece spring according to claim 5, characterized in that the alpha phase of the metastable titanium alloy has a volumetric concentration comprised between 1 and 40%, preferably between 2 and 35%, preferably between 5 and 30%.

7. The timepiece spring produced from metastable titanium alloy according to claim 2.

8. The spring according to claim 5, in which the spring is a hairspring.

9. The spring according to claim 5, in which the spring is a mainspring.

10. A balance-wheel and hairspring combination comprising: the hairspring according to claim 8, a balance-wheel made from metastable titanium alloy, said metastable titanium alloy comprising, as a percentage by weight, between 24 and 45% niobium, between 0 and 20% zirconium, between 0 and 10% tantalum and/or between 0 and 1.5% silicon and/or less than 2% oxygen, said alloy having a crystallographic structure comprising: a mixture of austenitic phase and alpha phase; and a presence of omega-phase precipitates the volumetric concentration of which is less than 10%.

11. The balance-wheel and hairspring combination according to claim 10, in which the metastable titanium alloy is characterized in that the alpha phase has a volumetric concentration comprised between 1 and 40%.

12. The balance-wheel and hairspring combination comprising: the hairspring produced from metastable titanium alloy, said metastable titanium alloy comprising, as a percentage by weight, between 24 and 45% niobium, between 0 and 20% zirconium, between 0 and 10% tantalum and/or between 0 and 1.5% silicon and/or less than 2% oxygen, said alloy having a crystallographic structure comprising: a mixture of austenitic phase and alpha phase; a presence of omega-phase precipitates the volumetric concentration of which is less than 10%; and a balance-wheel made from metastable titanium alloy according to claim 2.

13. spring-barrel combination comprising: the mainspring according to claim 9; a barrel made from metastable titanium alloy, said metastable titanium alloy comprising, as a percentage by weight, between 24 and 45% niobium, between 0 and 20% zirconium, between 0 and 10% tantalum and/or between 0 and 1.5% silicon and/or less than 2% oxygen, said alloy having a crystallographic structure comprising: a mixture of austenitic phase and alpha phase; and a presence of omega-phase precipitates the volumetric concentration of which is less than 10%.

14. The spring-barrel combination according to claim 13, in which the metastable titanium alloy is characterized in that the alpha phase has a volumetric concentration comprised between 1 and 40%.

15. A spring-barrel combination comprising: the mainspring produced from metastable titanium alloy, said metastable titanium alloy comprising, as a percentage by weight, between 24 and 45% niobium, between 0 and 20% zirconium, between 0 and 10% tantalum and/or between 0 and 1.5% silicon and/or less than 2% oxygen, said alloy having a crystallographic structure comprising: a mixture of austenitic phase and alpha phase; a presence of omega-phase precipitates the volumetric concentration of which is less than 10%; and a barrel made from metastable titanium alloy according to claim 2.

16. A method for the manufacture of a timepiece spring according to claim 5, said method comprising: work hardening of the alloy at a work-hardening rate greater than or equal to 50%; forming the spring based on the work-hardened alloy; and heat treatment of the formed alloy at a temperature comprised between 300 C. and 600 C. during a time comprised between 2 and 30 min; said work-hardening step comprises: introducing the alloy into a tooling used for work hardening said alloy, said alloy having a temperature of less than 500 C. when it is introduced into the tooling used for the work hardening; and heating the tooling used for work hardening said alloy at a temperature comprised between 150 C. and 500 C.

17. The method according to claim 16, in which forming the spring comprises: cold rolling of the alloy at a rate of reduction of a cross section of the alloy less than or equal to 50%; coiling of said rolled alloy; and heat treatment at a temperature comprised between 300 C. and 900 C.

18. The method according to claim 16, comprising a step of preparation for work hardening, said step of preparation for work hardening comprising: heating the alloy to a deposition temperature; graphite-based deposition on a surface of said alloy; and drying said alloy at a temperature comprised between 100 C. and 500 C.

19. The method according to claim 18, in which the temperature of deposition is comprised between 100 C. and 500 C.

20. The method according to claim 16, in which the work hardening is implemented by wire drawing.

Description

DESCRIPTION OF THE FIGURES AND EMBODIMENTS

[0170] Other advantages and features of the invention will become apparent on reading the detailed description of embodiments and modes of realization which are in no way limitative, and from the following drawings:

[0171] FIG. 1 shows a diffractogram of an alloy A1 according to the invention having undergone a step of wire drawing E1 according to the invention and a diffractogram of an alloy A2 corresponding to the alloy A1 having undergone a step of heat treatment T1 according to the invention,

[0172] FIG. 2 shows an image of the alloy A2 obtained by atomic force microscopy (AFM),

[0173] FIGS. 3, 4 and 5 show images of the alloy A2 obtained by transmission electron microscopy (TEM) and X-ray diffraction,

[0174] FIG. 6 shows the linear expansion coefficient of the alloy A2 and of an alloy sold under the trade name of Nispan C, mainly used for the manufacture of hairsprings,

[0175] FIG. 7 shows the stress-strain curves of an alloy, sold under the trade name of Nivaflex, mainly used for the manufacture of mainsprings, and of the alloy A2,

[0176] FIG. 8 shows the elastic modulus and the breaking strength as a function of temperature of the alloy A2,

[0177] FIG. 9 shows the diameter of a wire made from alloy A2, obtained by the method E1 according to the invention, as a function of the drawn length,

[0178] FIG. 10 shows magnetometric measurements carried out on the alloy Nispan C and on the alloy A2.

[0179] As the embodiments described hereinafter are in no way limitative, variants of the invention can be considered comprising only a selection of the characteristics described, in isolation from the other characteristics described (even if this selection is isolated within a phrase comprising these other characteristics), if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art. This selection comprises at least one, preferably functional, characteristic without structural details, or with only a part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art.

[0180] An embodiment of a timepiece spring according to the invention is now described. The timepiece spring is obtained from a wire of 2 to 3 mm diameter made from metastable titanium alloy comprising 40.5% niobium as a percentage by weight.

[0181] The method for the production of the spring comprises heating the wire to a temperature of 350 C., followed by dipping the wire in an aqueous solution comprising graphite in suspension. The wire is then dried at a temperature of 400 C. for 5 to 30 seconds. The wire is then drawn through a tungsten carbide or diamond die at a temperature of 400 C. The wire is introduced into the die without being heated. The wire is passed through the die several times. The deformation applied reduces progressively from one pass to another and varies from 25 to 8% in variation of the cross section of the wire. When the cross section of the wire is comprised between 2 and 1 mm, the rate of reduction of the cross section of the wire is 15% per pass, when the cross section of the wire is comprised between 1 and 0.5 mm, the rate of reduction of the cross section of the wire is 10% per pass and when the cross section of the wire is less than 0.5 mm, the rate of reduction of the cross section of the wire is 8% per pass. The wire is always drawn in the same direction. The set of steps described above constitute the wire drawing step E1 and the alloy according to the embodiment having undergone the step E1 is denoted A1.

[0182] The wire is then cold rolled; the reduction in the cross section applied is 10% so as to obtain a resilient metal ribbon having a rectangular cross section.

[0183] The ribbon is then wound on a mandrel so as to form an Archimedes spiral comprising 15 turns.

[0184] The ribbon is then immobilized, then heat treated at a temperature of 475 C. for 600 seconds. The heat treatment step constitutes the step denoted T1. The alloy A2 corresponds to the alloy A1 having subsequently undergone the step T1.

[0185] With reference to FIG. 1, the diffractograms A1 and A2 show the effect of the heat treatment step T1 on the crystalline structure of the alloy according to the invention. The diffractogram A1 shows only the peaks characteristic of the (austenitic) phase. After the step T1, the diffractogram of A2 shows the peaks characteristic of the and a phases. The significant width of the base of the peaks indicates the presence of considerable work hardening of the alloy.

[0186] The inventors noted an optimum temperature range, comprised between 200 et 450 C., for work hardening of the alloy A1 for which there is (i) absence of generalized precipitation of phases and (ii) effective work hardening of the alloy.

[0187] The inventors also noted an optimum volumetric concentration range of alpha phase of the alloy A1. This range corresponds to an alpha-phase volumetric concentration comprised between 5 and 30%, making it possible, after implementation of steps E1 and T1, (i) to obtain super-elastic properties, (ii) to increase the mechanical strength of the alloy, (iii) to have a low elastic modulus and (iv) to obtain negligible sensitivity of the elastic modulus to temperature variations.

[0188] With reference to FIG. 2, an AFM image can be seen of the microstructure of an alloy wire A2 of 285 m diameter. FIG. 2 shows the presence of recrystallized equiaxed grains the size of which is comprised between 150 and 200 nm. The inventors noted that when heat treatment is carried out under the conditions described above, i.e. at moderate temperatures and for a short time, it allows recrystallization of grains of very small diameters, typically of grains less than 150 nm.

[0189] With reference to FIGS. 3, 4 and 5, MET images are shown of the microstructure of an alloy wire A2, of 285 m diameter. FIG. 3 shows the presence of grains 1 of an alpha phase within a matrix of grains of beta phase. These alpha-phase grains 1 are present in the form of equiaxed grains of 100 to 200 nm within -phase grains. Under the conditions of the method according to the invention, the alpha-phase grains 1 are few and distributed homogeneously among the -phase grains. The inventors noted that the heat treatment allows precipitation of an alpha phase and homogeneous germination of the alpha phase within the -phase precipitates. These alpha-phase grains 1 have an average size less than 150 nm. An electronic diffraction diagram of the selected area is shown in the insert I1 situated at the top right in FIG. 3. It can be seen that the diffraction of the beta-phase grains tends to form rings, indicating a randomization of the crystallographic orientations of the beta-phase grains. This randomization of crystallographic orientations of the beta-phase grains confirms a recrystallization induced by the step T1.

[0190] FIG. 4 confirms the presence of omega-phase grains 2 within the matrix of beta-phase grains. These omega-phase grains 2 have an average size less than 50 nm. Under the conditions of the method according to the invention, the omega-phase grains, which are deleterious for the mechanical properties of the alloy but necessary in order to initiate the precipitation of the alpha-phase grains, (i) are dispersed within the beta-phase grains, (ii) have a low volumetric concentration, typically less than 5% and (iii) have a low average grain size.

[0191] FIG. 5 confirms the joint presence of the alpha, beta and omega phases within the alloy A2. An electronic diffraction diagram of the selected area is shown in the insert I1 situated at the top right in FIG. 3. The diffractogram indicates the presence of alpha- and omega-phase grains within the matrix of beta-phase grains.

[0192] The inventors noted that the precipitation of the alpha-phase grains is initiated by the presence of the omega-phase grains.

[0193] In addition, the precipitation of omega and alpha phase during the step T1 is accelerated by the prior step of work hardening during warm wire drawing in the step E1.

[0194] With reference to FIG. 6, the evolution of the linear expansion coefficients of the alloy A2 and of an alloy sold under the trade name of Nispan are shown. The curve 3 shows the evolution of the expansion of the alloy A2 as a function of temperature and the curve 4 shows the evolution of the expansion coefficient of Nispan as a function of temperature. The value of the linear expansion coefficient is 9.10.sup.6 for the alloy A2 and 8.10.sup.6 for Nispan. The value of the expansion coefficient of a material reflects the influence of temperature on the dimensions of the spring by the effects of contraction and expansion of the material. The value of the expansion coefficient of a material thus reflects the influence of temperature on the mechanical properties of the spring and therefore the influence of temperature on the torque delivered by a spring composed of this material. It is noted here that the coefficient of the alloy A2 is low, and identical to that of Nispan.

[0195] With reference to FIG. 7, the stress-strain curves 5, 6 are shown of an alloy sold under the trade name of Nivaflex, 5 and of the alloy A2, 6. The breaking strength is 1000 MPa for the alloy A2 and 2000 MPa for the Nivaflex; the elastic modulus is 40 GPa for the alloy A2 and 270 GPa for the Nivaflex, and the recoverable deformation is 3% for the alloy A2 and 0.7% for the Nivaflex. The area below the stress-strain curve on release allows the potentially restorable elastic energy to be calculated, this elastic energy being 10 Kj/mm.sup.3 for the Nivaflex and 16 Kj/mm.sup.3 for the alloy A2. This characteristic indicates that a mainspring made from the alloy A2 allows a greater quantity of energy to be stored than the mainsprings made from Nivaflex.

[0196] With reference to FIG. 8, the elastic modulus and the elastic strength of the alloy A2 are shown as a function of temperature. The elastic modulus is almost constant between 200 and 50 C., reducing by a value of 54 GPa for a temperature of 200 C. to a value of 53 GPa for a temperature of 50 C. This characteristic indicates that the torque of a spring made from alloy A2 has high stability over a temperature range comprised between 200 and 50 C. The breaking strength increases by a value of approximately 800 MPa for a temperature of 200 C. to a value of 1350 MPa for a temperature of 50 C.

[0197] With reference to FIG. 9, the evolution of the diameter of the alloy wire A2 is shown as a function of the length of the drawn wire. It is noted that for a wire having a final diameter of 85 microns and a drawn length of 15 m, the maximum variation in the diameter over the entire length of the wire is comprised between 0.1 and 0.2 m.

[0198] The regularity and the surface condition of the wires obtained by the wire drawing method according to the invention are compatible with the expected requirements for horological applications.

[0199] With reference to FIG. 10, the evolution of the induced moment is shown as a function of the applied magnetic field, for temperatures of 10 C. (references 6 and 9), 20 C. (references 7 and 10) and 45 C. (references 9 and 11), for Nispan 6, 7, 8 and alloy A2 9, 10, 11. As a result of the negligible value of the induced moment in the alloy A2, an enlargement 12 of the curves 9, 10, 11 is given. It is also noted that despite the enlargement 12, the curves 9, 10, 11 remain superimposed. For Nispan, the induced moment saturates from 550 mT and shows values comprised between 60 and 80 emu/g, depending on temperature. As a comparison, for the alloy A2, the induced moment in the material for an applied magnetic field of 3 T is approximately 0.15 emu/g. At 550 mT, the induced moment in the alloy A2 is 1000 times less than the induced moment in Nispan.

[0200] The main drawback of the commercial alloys currently used for producing timepiece springs arises from the sensitivity of these alloys to the neighbouring magnetic fields. This sensitivity introduces a perpetual, cumulative drift in the torque of the spring. The very low magnetic susceptibility of the alloy A2 makes it possible to increase significantly the constancy of the torque of the timepiece springs made from alloy according to the invention, as the effect on said springs of the neighbouring magnetic fields is infinitesimal.

[0201] Of course, the invention is not limited to the examples which have just been described, and numerous adjustments can be made to these examples without exceeding the scope of the invention.

[0202] In addition, the different characteristics, forms, variants and embodiments of the invention can be combined together in various combinations provided they are not incompatible or mutually exclusive.