PROCESS FOR THERMOFORMING SEMI-FINISHED PRODUCTS MADE OF NITI ALLOYS TO IMPROVE THEIR PSEUDOELASTIC PROPERTIES

Abstract

 The present invention relates to a process for obtaining shape memory metal alloy articles with improved pseudoelastic properties. In particular, said method refers to the thermoforming of semi-finished products made of NiTi-based alloys with improved pseudoelasticity, which can also be modulated, if desired, depending on the technical problem to be solved, in the production of articles (in particular, jewellery) endowed with particular elasticity, but capable of withstanding considerable deformations without, however, giving rise to permanent plastic deformations.

Description

TECHNICAL FIELD

[0001] The present invention relates to a process for obtaining products made of shape memory metal alloys with improved pseudoelastic properties. In particular, said method refers to the thermoforming of semi-finished products made of NiTi alloys with improved pseudoelasticity, which can also be modulated, if desired, depending on the technical problem to be solved, in the production of articles (such as, for example, jewellery) endowed with particular elasticity and, however, capable of withstanding considerable deformations without giving rise to permanent plastic deformations.

STATE OF THE ART

[0002] In the art, the use of shape memory metal alloys, e.g. NiTi-based alloys, is well known in order to produce articles with good elasticity, such as spectacle frames.

[0003] In fact, these alloys have the property of withstanding considerable deformations (about ten times that of a traditional metal or alloy) without permanent plastic deformations remaining.

[0004] Without wishing to enter herein into the details of the complex mechanisms governing the solid state phase transformations in said alloys, generating the shape memory effect, and the pseudoelastic properties thereof (which have been extensively described by the present inventor in several patents, e.g., US 6,557,993, EP 1360540, which are incorporated herein by reference in their entirety, and others), the use of said alloys has proved to be particularly advantageous in the preparation of articles (e.g., spectacle frames) having a high degree of elasticity, with the possibility of deforming them over a wide range of temperatures, depending on individual needs.

[0005] However, the NiTi-based shape memory alloys known in the art still have a level of stiffness that is excessive for certain types of applications. By way of non-limiting example only, such alloys are unsuitable for making jewellery reinforcements (e.g. they require too much force to stretch a bracelet, for example, without a clasp to be worn, or to operate the opening/closing devices of a piece of jewellery such as a bracelet or necklace).

[0006] The excessive stiffness of these alloys is due to the work hardening of the material that occurs during processing; if this effect is not sufficiently removed by appropriate procedures, the material is less resistant to fatigue stress and during use in certain mechanisms it easily breaks after a few cycles of use or deformation.

[0007] Therefore, the need remains for a shape memory alloy that, in addition to the well-known advantageous properties of good elasticity and a wider temperature range within which material deformation can take place, also has low stiffness, sufficient to meet the application requirements of the material.

[0008] Again by way of non-limiting example, particularly in the field of jewellery production, there is a strong need for a bracelet that, when worn, only bends in certain preferred/desired areas and not in the more critical ones, where perhaps precious stones are set or where the precious metal coating is very thin.

OBJECTS AND SUMMARY OF THE INVENTION

[0009] The problem underlying the present invention is to overcome the drawbacks of the prior art by providing an adequate answer to the technical problem described above.

[0010] The Applicant has now unexpectedly found that a special combined heat treatment of a semi-finished product made of NiTi-based alloy can adequately respond to the technical problem described above.

[0011] It is, therefore, an object of the present invention to provide a process comprising at least a thermal pre-treatment of a semi-finished product made of NiTi-based alloy by passing through it a continuous electric current (which performs said thermal pre-treatment by heating due to the Joule effect) which unifies (i.e., homogenises) its microstructural state and reduces the work hardening which is at the basis of the shape memory effect and the pseudoelasticity or super-elasticity; said thermal pre-treatment is subsequently followed by a thermoforming process homogeneously applied to the entire semi-finished product or, optionally, also limited to different desired specific areas of the semi-finished product itself, as described in the appended claims.

[0012] Further subject matter of the present invention is an article, in particular jewellery, made of a shape memory NiTi alloy, having the improved pseudoelastic characteristics described herein, obtained by the above process, as described in the appended claims.

[0013] Other aspects of the present invention are described in the appended dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Further characteristics and advantages of the present invention will become clearer from the following detailed description of some preferred embodiments thereof, made with reference to the appended drawings.

[0015] The different features in the individual configurations can be combined with each other as desired according to the following description, if the advantages resulting specifically from a particular combination are to be availed of.

[0016] In these drawings,

• Figure 1 shows a series of graphs illustrating the known mechanical behaviour at different temperatures of a semi-finished NiTi alloy material (a wire) of the prior art. In particular, the graph (e) in Fig. 1 shows the typical pseudoelastic curve that can be obtained from a work hardened material, where a characteristic shape, which can be defined as "flag-shaped", rather reduced and sloping, can be identified. In it, the plateau is scarcely visible and the slope is similar to that of the graph of the pure super-elastic behaviour illustrated in graph (c) of Fig. 1 with slightly broader hysteresis;
• Figure 2 is entirely analogous to Figure 1 and also shows a series of graphs illustrating the known mechanical behaviour at different temperatures of a wire made of NiTi alloy of the prior art also word hardened by processing. In particular, Figure 2 also presents the same type of graph (e) illustrating the typical pseudoelastic curve obtainable from a material of the prior art in which the characterising "flag-shape" is equally rather reduced and sloping;
• Figure 3 in turn shows a graph representing a bending response curve of a first preferred embodiment of an article obtained by the process according to the present invention;
• Figure 4 is a perspective view of a second preferred embodiment of an article according to the present invention; and
• Figure 5 is a bending response curve of a portion of the article in Fig. 4.

DETAILED DESCRIPTION OF THE INVENTION

[0017] For the illustration of the drawings, use is made in the following description of identical numerals or symbols to indicate construction elements with the same function. Moreover, for clarity of illustration, certain references may not be repeated in all drawings.

[0018] While the invention is susceptible to various modifications and alternative constructions, certain preferred embodiments are shown in the drawings and are described hereinbelow in detail. It must in any case be understood that there is no intention to limit the invention to the specific embodiments illustrated, but, on the contrary, the invention intends to cover all the modifications, alternative and equivalent constructions that fall within the scope of the invention as defined in the claims.

[0019] The use of "for example", "etc.", "or" indicates non-exclusive alternatives without limitation unless otherwise indicated. The use of "comprises" and "includes" means "comprises or includes, but not limited to", unless otherwise indicated.

[0020] The process according to the present invention has been devised and implemented with the aim of obtaining articles, preferably jewellery, based on a shape memory NiTi alloy with improved pseudoelasticity properties, which are homogeneous and controlled, but which can also be modulated within the same geometry, should this be necessary or desired, and with precise process and result-related repeatability. Thanks to appropriate treatments, if necessary or desired also confined only to specific points/areas of the semi-finished product, it is in fact possible to modulate the microstructure and the degree of work hardening thereof, thus also conferring different flexibility properties to the aforementioned points/areas with respect to the rest of the article. Everything is then stabilised in such a way that it is repeatable and the final mechanical performance is always at the same level.

[0021] It is well known that the mechanical properties of alloys, such as, preferably, NiTi-based alloys, are closely linked to their microstructural conditions, which are, as a rule, modified by means of thermo-mechanical treatments, i.e. by different combinations of mechanical forming and heat treatment, in order to make them structurally more homogeneous. The present invention relates to a combined set of procedures for obtaining semi-finished products made of NiTi alloys with the desired improved pseudoelastic properties, possibly also modulated differently from zone to zone when necessary or desired. The present invention thus relates to a process for thermoforming shape memory metal articles/semi-finished products, preferably, selected from NiTi alloys, having improved pseudoelastic properties, as described above, at least comprising the following steps:

• a first step, consisting of an appropriate first heat treatment (or "heat pre-treatment") step to homogenise the microstructure of the starting article, for the preparation of the desired semi-finished product, said heat treatment being essentially carried out by heating due to the Joule effect generated by a continuous electric current that is passed through the part under certain appropriate conditions; said first step being followed by
• a second step, comprising a further appropriate second heat treatment (or "thermoforming step") to create the final shape (or "shape setting") of the desired part, said second heat treatment being achieved by appropriate treatment in a furnace under appropriate temperature conditions.

[0022] Said second heat treatment in the furnace is possibly followed by a further third heat treatment, which is carried out by heating due to the Joule effect generated by a continuous electric current that is applied, under certain conditions, to the entire part previously obtained from the previous thermoforming process, or only to specific points/areas of the article itself.

[0023] It is well known that during the traditional drawing process and/or mechanical deformation of an alloy, e.g. NiTi-based, spots/areas characterised by an increased degree of work hardening and/or accumulation of processing defects can form in the material subjected to the above treatment. Work hardening is defined as the final degree of section reduction of the semi-finished product after processing and without intermediate heat treatment of the material. Work hardening is indicated by a percentage value and for a NiTi-based alloy can be at most about 50%. This value corresponds proportionally to the presence of processing defects, such as dislocations and inclusions. These defects have as their first effect the stiffening of the material obtained and are normally distributed in the material in an uncontrolled, therefore, non-homogeneous manner. This effect can cause the formation of a non-uniform microstructural state throughout the semi-finished product; a state that can be the cause of non-uniform responses in the degree of elasticity of the final product after thermoforming or, in some cases, even of breakage during stress.

[0024] As previously described, it has now been unexpectedly found that it is possible to advantageously carry out a so-called "stress-relieving" procedure, i.e. the redistribution and/or uniform reorganisation of the microstructure of an article, with a substantial reduction of the aforementioned defects generated in the material during traditional processing, by passing a continuous electric current through it, with subsequent heat treatment by means of the Joule effect generated by the current itself. This heat treatment by means of the Joule effect homogenises the degree of work hardening throughout the material by equalising its microstructural properties prior to the desired final thermoforming processes.

[0025] The experimental conditions of this treatment (voltage, current amperage and time of application of the heat treatment) vary as the size of the semi-finished products (e.g. wires) subjected to the treatment changes.

[0026] By way of non-limiting example, in a preferred embodiment of the invention, the starting NiTi alloy article is a wire, preferably selected from those commonly commercially available, which is strongly work-hardened (e.g., between 40% and 50%) by conventional cold working (by way of non-limiting example, a rolling or drawing process), i.e., without intermediate heat treatment for tempering. This wire can in any case also be purchased already in the desired work-hardened state (Cold Worked or As Drawn) (or, as mentioned above, it can also be obtained in the desired work-hardened state after a short heat treatment (Straight Annealed)).

[0027] In a particularly preferred form of the invention, the wire is a round wire with a thickness (diameter) comprised between 0.10 mm and 30 mm; between 0.10 mm and 10 mm; between 0.20 mm and 9 mm; between 0.30 mm and 8 mm; between 0.40 mm and 7 mm; between 0.50 mm and 6 mm; between 0.60 mm and 5 mm; between 0.70 mm and 4 mm; between 0.80 mm and 3 mm; other values not specifically included in the above list are however freely possible depending on processing requirements and, of course, also form part of the present invention. Or, in another particularly preferred form of the invention, the wire is a square section wire with sides comprised between 0.10 mm x 0.10 mm and 5 mm x 5 mm; preferably, between 0.20 mm x 0.20 mm and 2 mm x 2 mm. Or, in another particularly preferred form of the invention, the wire is a rectangular wire with sides comprised between 0.10 mm x 0.15 mm and 0.15 mm x 30 mm. As for the possible lengths, they are freely variable and/or selectable depending on the purpose and end product desired.

[0028] A direct electric current is then passed through this wire (first heat pre-treatment step) applying a voltage averaging between 3 volts and 180 volts; between 10 volts and 150 volts; between 20 volts and 100 volts; preferably, between 30 volts and 90 volts; more preferably, between 40 volts and 80 volts. The intensity of the applied electric current will depend substantially on the size of the article, e.g. preferably the length of the wire.

[0029] For example, for application processes in the jewellery industry, it is preferable to use a direct current comprised between 0.2 amperes and 15 amperes; between 0.3 amperes and 10 amperes; between 0.4 amperes and 5 amperes; between 0.4 amperes and 4 amperes; preferably between 0.4 amperes and 3 amperes; even more preferably between 0.5 amperes and 2 amperes, applying it for a time comprised between 3 and 10 seconds, preferably between 4 and 9 seconds, more preferably between 5 and 8 seconds, or between 5 and 10 seconds, depending on the thickness and length of the wire. If necessary, this first heat treatment step can also be carried out alternating it with traditional treatment steps, such as rolling and/or drawing, in order to optimise the microstructure by eliminating work hardening defects and the possibility of breaking the material.

[0030] This first heat treatment (or heat pre-treatment) step is followed by a second heat treatment (or thermoforming) step to achieve the desired final "shape-setting".

[0031] This second heat treatment can be carried out, for example, by applying a combination of the following steps, i.e. applying the following steps in sequence, or applying only one, depending on the purpose and characteristics of the article to be obtained:

• heat treatment, e.g. in an electric furnace, with the use of suitable templates or moulds, to achieve shaping/forming of the semi-finished product obtained in the first heat pre-treatment step mentioned above; this heat treatment in a furnace being, if necessary, followed by
• the use of an electric current applied to the entire article obtained from the previous heat treatment in the furnace, or only to specific selected/desired areas of said article, heat treating it by the Joule effect in a selective manner, limited to said areas.

[0032] The precise modulation of this combined treatment is made possible thanks to the expertise developed in the technique of heat treatment in furnaces, the variation of applied direct electric current and the thermal effect (Joule) produced by this. That is, to the knowledge underlying the company's know-how on the use of combined treatments and with different parameters to appropriately modulate the mechanical properties of the material and its flexibility, which are directly related to the microstructural state.

[0033] With this combined method, it was advantageously possible to perform homogeneous thermoforming in the furnace and/or also to specifically choose any particular areas to which the treatment should be applied in order to obtain a product that had a particular pseudoelasticity and softness of deformation even only in certain desired specific points/areas and that had better structural strength in the other areas depending on the desired purpose of use.

[0034] The NiTi alloy article obtained by the combined process of the present invention described above can express variable mechanical properties depending on the geometry, but may also be modulated within the geometry itself, providing the necessary mechanical response depending on the use to be made of each part of the article in its application.

[0035] The advantageous effect of this process of the present invention is, on the one hand, to maintain the advantages already obtained with the methods of the prior art (for example, the widening of the temperature range at which the material can be deformed) and, on the other hand, to obtain a material that is less rigid, possibly even only in certain preferred areas, and thus more suitable for applications such as reinforcing jewellery.

[0036] This combined process of the present invention advantageously makes it possible to obtain an object, e.g. a bracelet, which has a very soft but at the same time very firm embrace on the arm, unlike those obtained according to the prior art.

Experimental example section of preferred embodiments of the invention

[0037] A wire of a NiTi alloy of commercial origin (wherein the percentage weight composition (%) of the alloy is comprised between Ni(46%)Ti(54%) (p:p) and Ni(55%)Ti(45%); preferably, Ni(51%)Ti(49%) (p:p) and Ni(50.8%)Ti(49.2%) (pp)) with a round cross-section, having a thickness of 1 mm and a length of 50-100 cm and work-hardened by at least 10%, preferably by at least 20%, more preferably by at least 30%, even more preferably by at least 40% or 50%, is subjected to the following processing steps:

• rendering the microstructural state of the wire uniform/homogeneous (at the basis of the shape memory effect) by passing a continuous current varying from 40 to 80 volts through the wire (e.g. with the aid of an electrical transformer) (current intensity of approx. 0.5-3 amperes) for a period of 5-10 seconds; this "stress relieving" procedure being applied alternating, or not, with traditional mechanical processing steps, such as, for example, rolling and/or drawing, until the desired final size is achieved; then
• wrapping/applying the pre-treated wire onto a suitable mould reproducing the desired final shape, e.g. a 50 cm diameter round tube to obtain a round bracelet with the same internal diameter; then
• firing the mould in an electric metal-forming furnace with a refractory mouth at a temperature comprised between 200 °C and 650 °C, preferably comprised between 470 °C and 530 °C, depending on the size of the mould and the thickness and length of the wire, for a period of 5 to 120 minutes, preferably comprised between 15 and 40 minutes.

[0038] In addition to this furnace treatment step, it is possible, if desired or necessary, to further improve the overall elasticity of the previously obtained product by following up with a further heat treatment step, by means of the Joule effect, said further heat treatment phase comprising:

• passing a direct current with a voltage of 40 V to 80 V (current intensity of 0.5 to 3 amperes) for times of 5 to 10 sec into the article; or,
  if desired, or necessary, obtaining points or zones with differentiated elasticity on the same article previously obtained,
• passing a direct current, with a voltage of 40 V to 80 V (intensity of 0.5 to 3 amperes), into specific selected portions of the wire, or of the article, by applying the electrodes to said zones (on which the elasticity is to be modulated) for times of 5-10 sec. In this case, the application is then carried out only at one or more specific points/areas of the already thermoformed wire (e.g., to provide the above-mentioned round bracelet) so that the final article presents different degrees of elasticity at different points, i.e. a modular elasticity.

[0039] A first embodiment of an article obtained by the process according to the present invention shows a quasi-linear bending response with hysteresis characterised by a trend of its bending response curve wherein at least one section has a plateau with a non-zero slope and in any case less than a characteristic slope of an elastic deformation of the steel, hereinafter also only "characteristic slope", for example greater than at least 10% and less than 85% of the characteristic slope, as shown in illustrative terms in Figure 3.

[0040] In general terms, a NiTi alloy can exhibit either a mechanical bending response behaviour characterised by a sharply flag-shaped bending response curve, indicative of substantially pure pseudoelastic mechanical behaviour, or quasi-linear behaviour with hysteresis, i.e. intermediate or combined behaviour between perfectly linear behaviour, typical for example of steel, and substantially pure pseudoelastic behaviour.

[0041] In this description and in the appended claims, "mechanical bending response properties with quasi-linear behaviour with hysteresis" is intended to denote mechanical properties characterised by a bending response curve having a non-zero slope plateau and in any case less than a characteristic slope of elastic deformation of the steel, hereafter also only "characteristic slope", e.g. greater than at least 10% and less than 85% of the characteristic slope.

[0042] In the context of the present description and the appended claims, "mechanical bending response properties with substantially pure pseudoelastic behaviour" is intended to denote mechanical properties characterised by a bending response curve with an essentially zero slope plateau, e.g. below 10% of the characteristic slope.

[0043] In this description and in the appended claims, "plateau" is understood to mean a section of a curve with a lower gradient than a previous section, for example a first section with a gradient reduced by at least 10% compared to a second previous section.

[0044] The bending response curve shown in Figure 3 is representative of a part obtained by the process according to the invention which has a residual work hardening made homogeneous by means of a thermal pre-treatment by means of a Joule effect caused by an electric current and then subjected to a final thermoforming heat treatment in accordance with the present invention.

[0045] This results in a flag-shaped curve with reduced hysteresis and a non-negligible slope, which is a sign of a mechanical response in the material that is deformed in an overall manner at different stress levels, thus with a plateau slope of loading and unloading, but still with significant hysteresis and complete recovery of deformation, indicative of high flexibility and soft, adaptable elasticity.

[0046] With reference to Figure 4, a second preferred embodiment of an article according to the present invention is illustrated, specifically a jewellery article, such as a bracelet, collectively referred to as 14.

[0047] The bracelet 14 comprises at least a portion of the bracelet made of a nickel-titanium alloy, hereinafter also referred to as NiTi or Nitinol alloy, having a percentage weight composition (%) of the alloy comprised between Ni(46%)Ti(54%) (w:w) and Ni(55%)Ti(45%); preferably, Ni(51%)Ti(49%) (w:w) and Ni(50.8%)Ti(49.2%) (w:w).

[0048] In the second preferred embodiment according to the present invention, the at least one portion of the bracelet comprises at least two zones 14a,14b with different mechanical properties from each other, in particular different mechanical bending response properties from each other.

[0049] By way of example, the bracelet 14 in Figure 4 is made entirely of NiTi alloy and comprises at least two zones 14a,14b with different mechanical bending response properties from each other.

[0050] Alternatively, the bracelet 14 may either comprise at least one portion made of a NiTi alloy with at least two zones 14a,14b with different mechanical bending response properties from each other, or comprise a core (not shown) at least one portion of which is made of such a NiTi alloy with at least two zones with different mechanical bending response properties from each other.

[0051] According to a first variant of the at least one portion of the bracelet, the at least two zones 14a,14b with different mechanical bending response properties from each other both exhibit quasi-linear behaviour with hysteresis, albeit different from one zone to another.

[0052] Figure 5 shows the bending response curves 20 of a portion of bracelet 14 comprising two zones 14a,14b with different mechanical properties according to the first variant, characterised through a temperature-controlled single cantilever bending test with reference to the ASTM F2516 | Metals | Tension Testing standard, in which the characteristic slope of the elastic deformation corresponds to a modulus of approximately 70GPa .

[0053] In particular, the portion of bracelet made of NiTi alloy comprises at least a first zone 14a with mechanical bending response properties with quasi-linear behaviour with hysteresis more proximal to linear behaviour, so as to guarantee the tightness of the bracelet worn, and at least a second zone 14b having mechanical bending response properties with quasi-linear behaviour with hysteresis most proximal to pseudoelastic behaviour, so as to give the second zone sufficient deformation softness to improve the wearability of the bracelet and not hinder the movement of the hand and wrist.

[0054] Within the scope of the present description and the appended claims, the relative expressions "most proximal" or "more proximal" are intended to mean a comparison of the zones with different mechanical bending response properties identifiable within the portion of bracelet.

[0055] The portion of the bracelet comprising at least two zones 14a, 14b with different mechanical bending response properties from each other is characterised by a bending response represented by a curve 20 with at least two differently sloping curve sections 21,22, each representative of the points at which bending is concentrated.

[0056] According to the first variant of the at least one portion of bracelet, each section 21,22 of the bending response curve provided by the portion of bracelet 14 exhibits quasi linear behaviour with hysteresis, each characterised by a different plateau slope.

[0057] In any case, the plateau slope of each section of the curve 21,22 is comprised between 85%-10% of the characteristic slope of an elastic deformation, such as the characteristic deformation of steel, which always presents a linear trend, without any kind of plateau. In other words, the trend of each section 21,22 of the bending response curve given by the portion of bracelet 14 according to the first variant corresponds to a deformation with martensite induction.

[0058] In particular, two different responses are clear in Figure 5, both with pseudoelasticity combined with linear elasticity, but at different loads and deformations corresponding to a differentiated response in two zones 14a,14b of the material. Furthermore, there is hysteresis, characteristic of pseudoelasticity, with complete recovery after a few settling cycles of a linear deformation.

[0059] In detail, a first curve 21 representative of the first zone 14a with mechanical bending response properties with quasi-linear behaviour with hysteresis proximal to linear behaviour has a plateau slope less than the characteristic slope, but still more than 50% of the characteristic slope, preferably comprised between 60%-85% of the characteristic slope, more preferably comprised between 65%-80% of the characteristic slope.

[0060] In addition, a second curve 22 representative of the second zone 14b with mechanical bending response properties with quasi-linear behaviour with hysteresis close to pseudoelastic behaviour has a plateau slope comprised between 20%-50% of the characteristic slope, more preferably comprised between 25%-45% of the characteristic slope, more preferably comprised between 30%-40% of the characteristic slope.

[0061] In the embodiment shown in Figure 4, the bracelet 14 is made entirely of a NiTi alloy in accordance with the first variant described above, presenting a first zone 14a with mechanical bending response properties with quasi-linear behaviour with hysteresis more proximal to linear behaviour, and a second zone 14b configured to rest against a part of the user's body, with mechanical bending response properties with quasi-linear behaviour with hysteresis closer to pseudoelastic behaviour.

[0062] In accordance with a second variant of the at least one portion of bracelet 14 made of NiTi alloy, there are three zones 14a,14b,14c with different mechanical bending response properties from one another. This variant is also illustrated schematically in Figure 4 by the identification of a third zone 14c with mechanical bending response properties different from the first two zones 14a,14b, indicated in brackets.

[0063] This third zone 14c can be obtained by carrying out the additional heat treatment (posttreatment) step outlined above. Such further heat treatment can be performed at one or more specific zones of the already thermoformed semi-finished product to form the bracelet 14 or at a portion thereof so that the portion of the bracelet presents a plurality of zones 14a, 14b, 14c having different mechanical bending response properties from one another, i.e. exhibiting modular elasticity.

[0064] In this way, it is advantageously possible to obtain a bracelet that has at least locally a pseudoelasticity and softness of deformation particularly suitable for the specific use, e.g., hugging the wrist or arm of a user without restricting their movement, while at the same time ensuring that the shape of the bracelet is maintained. The article of which at least one part is made of NiTi alloy obtained through the combined process described above can thus express mechanical properties that vary depending on the geometry, but can also be modulated within the geometry itself, providing the mechanical response required for the specific use.

[0065] In the second variant, the portion of bracelet made of NiTi alloy comprises a first zone 14a with mechanical bending response properties with quasi-linear behaviour with hysteresis more proximal to linear behaviour, a second zone 14b with mechanical bending response properties with quasi-linear behaviour with hysteresis more proximal to pseudoelastic behaviour, and a third zone 14c with mechanical bending response properties with substantially pure pseudoelastic behaviour.

[0066] In terms of the bending response curve, the third zone 14c is represented by a third curve section (not illustrated) with a plateau slope less than 20% of the characteristic slope, preferably less than 15% of the characteristic slope, more preferably less than 10% of the characteristic slope. This range is therefore representative of pseudoelasticity characteristics that allow it to adhere well to the wrist or arm, without exerting increasing or uncomfortable forces.

[0067] The invention thus conceived is susceptible to modifications and variants, all falling within the same inventive concept. For example, the at least two zones 14a, 14b, 14c with different mechanical properties between them, in particular different mechanical bending response properties between them, may comprise any combination of a first zone with mechanical bending response properties with quasi-linear behaviour with hysteresis proximal to linear behaviour, a second zone with mechanical bending response properties with quasi-linear behaviour with hysteresis proximal to pseudoelastic behaviour and a third zone with mechanical response properties with substantially pure pseudoelastic behaviour.

Claims

1. A process for the thermoforming of articles made of shape memory metal alloys with pseudoelastic properties, where said process is characterised by comprising at least the following steps:

- a first step, comprising, or consisting of, an initial heat treatment to homogenise the microstructure of the starting article, said heat treatment being carried out by heating the article due to the Joule effect generated by a continuous electric current passed through the article itself; said first step being followed by

- a second step, comprising a further second heat treatment to produce the final shape of the article obtained from said first step, said second heat treatment being carried out by treatment in a furnace under appropriate temperature conditions.

2. The process according to claim 1, wherein said shape memory metal alloys are selected from NiTi alloys.

3. The process according to claim 2, wherein the percentage weight composition (%) of said NiTi alloy is comprised between Ni(46%)Ti(54%) (w:w) and Ni(55%)Ti(45%) (w:w).

4. The process according to claim 3, wherein said NiTi alloy is present in the form of a round wire with a thickness (diameter) comprised between 0.10 mm and 30 mm; or a square wire with sides comprised between 0.10 mm x 0.10 mm and 5 mm x 5 mm; or a rectangular wire with sides comprised between 0.10 mm x 0.15 mm and 0.15 mm x 30 mm.

5. The process according to any of the preceding claims, wherein said direct electric current has a voltage comprised between 3 volts and 180 volts and wherein the intensity of said electric current is comprised between 0.2 amperes and 15 amperes.

6. The process according to any one of the preceding claims, wherein said further second heat treatment in a furnace is carried out in an electric furnace at a temperature comprised from 200 °C to 650 °C for a period comprised from 5 to 120 minutes.

7. The process according to any one of the preceding claims, wherein, prior to heat treatment in a furnace according to claim 6, the obtained wire/article is wound/applied to a suitable mould reproducing the desired final shape.

8. The process according to any one of the preceding claims, further comprising, after the heat treatment in a furnace according to claim 6, a further step of heat treatment by Joule effect of the article manufactured therein, said further heat treatment step comprising:

- passing a direct current, with a voltage of 40 V to 80 V and a current intensity of 0.5 to 3 amperes into said article, for times comprised from 5 to 10 seconds.

9. The process according to any one of the preceding claims, wherein the first and/or further heat treatment comprises:

- passing a direct current, with a voltage of 40 V to 80 V and current intensity of 0.5 to 3 amperes, into selected portions of the wire, or the article, applying the electrodes to these areas for times comprised from 5 to 10 seconds.

10. An article (14), preferably a jewel, made of a shape memory NiTi alloy with improved pseudoelastic properties, obtained by a process according to any one of the preceding claims 1 to 9, characterised by at least one bending response curve comprising at least one portion of the curve having a plateau slope comprised between 10%-85% of the characteristic slope of an elastic deformation.

11. Article (14) according to claim 10, comprising at least one portion made of a NiTi alloy comprising at least two zones (14a,14b) with different mechanical properties from one another, in particular with different mechanical bending response properties from one another.

12. Article (14) according to claim 11, wherein the portion of the article comprising at least two zones (14a,14b) with different mechanical bending response properties from one another comprises any combination of zones (14a, 14b, 14c) with different mechanical properties from one another, preferably different mechanical bending response properties from one another:

- a first zone (14a) with mechanical bending response properties with quasi-linear behaviour with hysteresis proximal to linear behaviour;

- a second zone (14b) with mechanical bending response properties with quasi-linear behaviour with hysteresis proximal to pseudoelastic behaviour; and

- a third zone (14c) with mechanical bending response properties with essentially pure pseudoelastic behaviour.

13. Article (14) according to claim 11 or 12, wherein the portion of the article comprising at least two zones (14a,14b) having different mechanical bending response properties from one other is characterised by a bending response represented by a curve (20) with at least two curve sections (21,22) with different trends, preferably two curve sections (21,22) having different plateau slopes.

14. Article (14) according to claim 13, wherein the bending response curve (20) of the at least one portion of article comprises any combination of curve sections (21,22) between:

- a first curve section (21) having a plateau slope less than a characteristic slope of elastic deformation and greater than 50% of that characteristic slope, preferably comprised between 60%-85% of the characteristic slope, more preferably between 65%-80% of the characteristic slope;

- a second curve section (22) having a plateau slope comprised between 20%-50% of the characteristic slope, preferably comprised between 25%-45% of the characteristic slope, more preferably comprised between 30%-40% of the characteristic slope; and

- a third curve section having a plateau slope less than 20% of the characteristic slope, preferably less than 15% of the characteristic slope, more preferably less than 10% of the characteristic slope.

15. Article (14) according to any one of claims 11 to 14, wherein

- the at least two zones (14a, 14b) of the article portion with different mechanical properties from one another both exhibit quasi-linear behaviour with hysteresis, the quasi-linear behaviour with hysteresis being different between a first (14a) and a second (14b) zone of the at least two zones; and/or

- wherein the at least one portion of the article made of a NiTi alloy comprises three zones (14a, 14b, 14c) with different mechanical bending response properties, preferably a first zone (14a) with mechanical bending response properties with quasi-linear behaviour with hysteresis closer to linear behaviour, at least a second zone (14b) with mechanical bending response properties with quasi-linear behaviour with hysteresis more proximal to pseudoelastic behaviour, and at least a third zone (14c) with mechanical bending response properties with substantially pure pseudoelastic behaviour.

Drawing