WO2000070105A1

WO2000070105A1 - Eyeglasses and parts thereof using shape memory alloys

Info

Publication number: WO2000070105A1
Application number: PCT/US2000/013151
Authority: WO
Original assignee: Memory Corp PLC
Current assignee: Memory Corp PLC
Priority date: 1999-05-15
Filing date: 2000-05-12
Publication date: 2000-11-23
Anticipated expiration: 2001-11-15

Abstract

An eyeglass frame or component thereof (1, 2, 3, 4, 5) fabricated from Ni-Ti based shape memory alloy, the alloy being at least 50.5 atomic % nickel. The alloy is solution treated at a temperature of 650 to 1100 °C for 10-60 minutes. After cooling, the alloy is aged by heating it at a temperature of approximately 350 °C for 10-60 minutes. The alloy is characterized by having pseudoelastie properties without cold working, greater than 2.5 % elasticity over a temperature range of -20 to +40 °C, and allowing large plastic deformations during the fabrication of the component before the desired pseudoelastic properties are established. The methods of making the alloy and the component are described which provide greater flexibility in design and avoid the need for substantial cold working of the alloy. The new process and product provides a ductile alloy for ease of forming and a unique heat treatment which renders the fabricated eyeglass components highly elastic, enhancing wearer comfort combined with improved resistance to accidental damage.

Description

EYEGLASSES AND PARTS THEREOF USING SHAPE MEMORY ALLOYS

FIELD OF THE INVENTION

The present invention relates generally to the fabrication of eyeglasses, and, more particularly, to the making of eyeglass frames and parts thereof from a specially processed Nickel-Titanium shape memory alloy.

BACKGROUND OF THE INVENTION

Alloys used in conventional eyeglass frames include stainless steel, copper based alloys and nickel-silver, which all can be given quite high yield strength through work hardening, but in use can be fairly easily permanently deformed. Normal metals, even with very high yield strength, cannot sustain strains much greater than 0.2% without suffering a permanent set. Once a bend or kink has been sustained in a metal eyeglass frame fabricated from one of the above conventional alloys it is virtually impossible to remove. The concept of using shape memory alloys for eyeglass components has been suggested in numerous articles and patents. Y. Suzuki, at that time head of shape memory alloy research at Furukawa Electric in Japan, published in Kinzoku Journal, vol. 31, No. 11, pll5, 1981, the advantages of pseudoelastic shape memory alloy wire for fixing a lens into a frame. These findings were incorporated in one of the earliest patents on shape memory alloy applications for eyeglasses, Kokai Patent 56- 89715, (Publication Date: July 21, 1981) whose applications date back to 1979. Since these earlier studies, many other patents have issued claiming the advantages of using shape memory alloys for eyeglass components. The driving force for making metal eyeglass frames from shape memory alloys lies in their great resistance to permanent deformation as compared to conventional alloys employed in this application.

Shape memory alloys belong to a class which exhibit what is termed thermoelastic martensite transformation. The term martensite refers to the crystalline phase which is produced in steels when quenched from a high temperature. The phase which exists at the elevated temperature is referred to as austenite; these terms have been carried over to describe the transformations which occur in shape memory alloys. When a steel has been quenched from the austenitic temperature to martensite, to again form austenite requires heating the structure to quite high temperatures, usually in excess of 1400° F. By contrast, the thermoelastic shape memory alloys can change from martensite to austenite and back again on heating and cooling over a very small temperature range, typically from 18 to 55° F. The transformation of a shape memory alloy is usually described by its hysteresis curve. See FIG. 1 for an example of one.

In this figure it is shown that on cooling from the austenitic phase, often called the parent phase, martensite starts to form at a temperature designated as M_s and upon reaching the lower temperature, M_f the alloy is completely martensitic. Upon heating from below the M_f temperature the martensite starts to revert to the austenitic structure at A_s and when the temperature designated as A_f is reached the alloy is completely austenitic. These two phases or crystalline structures have very different mechanical properties; the Young's Modulus of austenite is about 12xl0⁶ psi while that for martensite is about 4x10⁶ psi and the yield strength, which depends on the amount of cold work the alloy is given, ranges from 28 to 100 ksi for austenite and from 10 to 20 ksi for martensite.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a nickel-titanium alloy which is particularly useful for eyeglass frames and parts thereof. Another object of the present invention is to provide an alloy having pseudo- elastic properties and which is useful for eyeglass frames and parts thereof.

A further object of the present invention is to provide a material for making eyeglass frames and parts thereof which is formable with the creation of cracks. These and other objects of the present invention are accomplished by providing a nickel-titanium shape memory alloy which is especially useful in making eyeglass frame parts and has desired pseudoelastic properties, characterized by: allowing large plastic deformations during fabrication of the part before the desired pseudoelastic properties are established, having pseudoelastic properties without using cold working, having greater than 2.5% elasticity over the temperature range where eyeglass frames are usually located, and being capable of undergoing large amounts of cold or hot forming without danger of cracking /fracturing during the forming operations required to make the part.

The unique feature of shape memory alloys is their ability to recover deformation. When a shape memory alloy specimen, hereinafter referred to as SMA, in its martensitic form is subjected to stress, the strain is accommodated by the growth and shrinkage of individual martensite variants rather than by the mechanisms which prevail in conventional alloys: slip, grain boundary sliding and dislocation motion. When deformed martensite is heated to the austenite finish temperature A_f the part reverts to its original undeformed state. This process is illustrated in FIG. 2. Although this process could be utilised in eyeglasses to recover accidental bending and kinking, the mechanical properties of martensite, its yield strength and its modulus of elasticity, are too low for this application, and, in addition, heating an eyeglass frame is not a convenient process.

Fortunately another mode of deformation of SMAs provides the properties and behaviour ideally suited to this service; this is pseudoelastic behaviour.

The unusual property of pseudoelasticily exhibited Dy shape memory alloys such as Au-Cd, Cu-Zn-Al, Ni-Ti and many others makes possible the complete "elastic" recovery of strains as great as 8%. Due to its high recoverable strain and its excellent resistance to corrosion, the shape memory alloy of preference for eyeglass frames and components has been within the Ni-Ti family of alloys.

The requirement of forming an eyeglass component from a piece of SMA wire or strip and controlling the amount of cold work it receives, both initially and in the final steps of component fabrication, followed by an annealing step which may require several hours, is considerably more complicated than the method of the present invention.

Prior practitioners of the art of applying SMAs to eyeglass components have recognised that pseudoelasticity is observed over a limited temperature range and have resorted to the use of an SMA which has been cold worked in the martensitic state followed by a low temperature anneal to give a combination of shape memory behaviour and superelastic characteristics. This processing gives a component with an elastic range of approximately 3% over a temperature range of -20 to +40° C Nickel-titanium alloys rendered pseudoelastic by a combination of cold work and heat treatment have a high yield strength which must be reduced by an annealing treatment requiring long periods of time to arrive at a satisfactory yield strength for eyeglass service. If the starting material for forming the eye glass component has already been cold work then subsequent forging or forming of the part may result in breakage. In pseudoelastic behaviour arising out of SIM, the upper plateau stress in this process can be changed by a combination of cold work following by an annealing treatment. Another form of superelastic behaviour is obtained when a shape memory alloy in the martensitic state is cold worked, yielding a material with the low modulus characteristic of martensite but with complete elastic behaviour up to a 4% strain. In addition, this behaviour is observed over a temperature range of from -200 to +150° C.

Past experiments on the precipitation hardening process, for instance by Nishida et al, Scripta Met, Vol 18, ppl299-1302, 1984, show that there is an optimum aging temperature to achieve the fine precipitates needed to increase austenite strengthening. Austenite yield strength must be high in order to have SIM proceed without having slip deformation of the matrix and permanent strain. A range of solution treatments and aging times and temperatures have been studied and reported in the literature for nickel titanium alloys.

The treatment and the alloy selection provided by the present invention is a modification of those commonly proposed. Prior studies have not provided detailed information on the temperature range over which the pseudoelastic behaviour is observed in alloys subjected to solution treatment and aging. With the treatment described, the present invention provides a method of producing a pseudoelastic nickel-titanium alloy which exhibits properties ideal for easy fabrication of eyeglass frames combined with those properties desired for an eyeglass frame which features wearer comfort and high resistance to accidental damage.

A process to obtain pseudoelastic behaviour is by a solution heat treatment of a high nickel SMA at about 850° C followed by water quenching and then precipitation hardening at a lower temperature. High nickel alloy means alloys with a nickel content in excess of 50.5 atomic %.

The present invention provides using an alloy with higher than the equiatomic

Ni/Ti ratio, subjecting it to a high temperature solution treatment, followed by water quenching, and a subsequent aging treatment, that a pseudoelastic behaviour is obtained combined with excellent forming characteristics and a strain recovery of at least 3% over a temperature range from -20 to +40° C. The treated alloy yield strength ranges from 42 to 72 Ksi.

The present invention seeks to provide a shape memory alloy and process which reduces the complexity of producing components for eyeglass frames by using a precipitation hardening treatment of a high nickel allo"" rather than the presently used cold working and heat treating. The resulting components are characterised by pseudoelastic properties which dramatically reduce the chance for accidental deformation or kinking. The precipitation process combined with the particular nickel-titanium alloy composition employed feature.- a relative low upper plateau stress which renders the components flexible which, in turn, make eyeglasses fabricated in the described manner comfortable to wear.

By contrast with the prior art, forming the eyeglass components when the alloy of this invention has been solution treated is quite easy, since in this condition it has excellent ductility. After forming, the component is subjected to an aging treatment which gives the part the pseudoelastic properties desired in eyeglass components. Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a hysteresis curve showing the transformation of a shape memory alloy.

FIG. 2 is a schematic view of the process of the present invention.

FIG. 3 is a curve showing the behaviour of SIM.

FIG. 4 is a curve showing the stress required to produce SIM and the temperature dependence of stress-induced martensite. FIG. 5 is a stress-strain diagram for cold worked martensite yielding linear superelastic behaviour.

FIG. 6 is a stress-strain curve for the alloy with the treatment.

FIG. 7 is a stress-strain curve for the alloy of FIG. 6 tested at -10° C. FIG. 8 is the stress-strain curve of the alloy of FIG. 6 tested at 0° C.

FIG. 9 is the stress-strain curve of the alloy of FIG. 6 tested at +10° C.

FIG. 10 is the stress-strain curve of the alloy of FIG. 6 tested at 30° C.

FIG. 11 is the stress-strain curve of the alloy of FIG. 6 tested at 40° C.

FIG. 12 is the stress-strain curve of the alloy with a different treatment from that of FIG. 6.

FIG. 13 is the stress-strain curve of the alloy of FIG. 12 tested at 0° C.

FIG. 14 is the stress-strain curve of the alloy of FIG. 12 tested at 25° C.

FIG. 15 is the stress-strain curve of the alloy of FIG. 12 tested at 30° C.

FIG. 16 is the stress-strain curve of the alloy of FIG. 12 tested at 40° C. FIG. 17 is a stress-strain curve for the alloy with a modified treatment.

FIG. 18 is a stress-strain curve showing that performance is maintained over many cycles at 5% strain.

FIG. 19 is a stress-strain curve showing that performance is maintained over many cycles at 8% strain. FIG. 20 is an isometric view of a eyeglass frame showing the various parts thereof.

DETAILED DESCRIPTION OF THE INVENTION

Martensite forms when a SMA is cooled from the austenitic region to below the M_s temperature; it can also form when the austenite is stressed to above some critical level. The martensite so formed is called stress-induced-martensite (SIM). Since the martensite formed under stress is at a temperature where it is not stable, when the stress is removed the alloy spontaneously reverts to its prior unstressed shape. This behaviour is illustrated in FIG. 3. It can be observed that the reversion stress is lower than the stress at which martensite from. These stresses are referred to as the upper and lower plateau stresses and their magnitude is dependent on the thermal and mechanical treatment which the SMA has received.

As the temperature of the specimen is raised, the stress magnitude required to produce SIM is increased, as shown in FIG. 4, however when the specimen reaches a critical temperature above A_f, designated as M_d, stress induced martensite canned be formed, no matter how high the stress. This behaviour gives rise to a limitation on using the pseudoelastic property in many situations since it places a limit on the temperature range over which pseudoelasticity is observed; typically in the NiTi alloys, this is a temperature range of about 60° C (108° F), although a 40° C (72° F) range is more typical. The desirable temperature range for eyeglass applications is from -20 to +40° C, a range of 60° C.

When the nickel content of a nickel-titanium alloy is higher than 50.5 At% [atomic percent] then such an alloy can be strengthened by an age hardening process. In this method of treatment the alloy is first solution treated at a temperature in excess of 750° C followed by water quenching. If the solution treated article is then heated to an intermediate temperature of from 300° C to 600° C a second phase of composition TiNi₃ is precipitated. By choosing the correct aging temperature and aging time, very fine precipitates are produced giving rise to optimum properties. Alloys with a composition in which nickel is present in excess of the stoichiometric ratio of 50/50 atomic percent, have low transformation temperatures, for example

A_s temperature lower than -20° C. Since in the present invention it is desired to have pseudoelastic properties to temperatures as low as -20° C, it is an additional advantage that the alloys capable of age hardening also have a low transformation temperature. The 50/50 At% alloy is by weight % 55 Ni-45 Ti. The alloy which has proven optimum for this application is one with a 56.1 wt% Ni and 43.9 wt% Ti. It should be understood that other high nickel NiTi binary compositions can also be used, and that these alloys may have additions of Mo, Ta, Nb, Zr, Cu, Co, Fe, Cr, Mn or V as partial substitution for the nickel with similar results. A NiTi alloy with a composition of 56.1 wt% Ni-43.5 wt% Ti is cold worked 29% and then solution treated at 850° C for 30 minutes followed by water quenching. The specimen is then aged at 350° C for 30 minutes and water quenched. The stress- strain curves for the alloy with this treatment are shown in FIGS. 6 through 11 for test temperatures from -20° C to +40° C. After the first cycle there is a residual strain which ranges from 1.7% at 40° C to 0.35% at 0° C. Subsequent cycles show a residual strain of approximately 0.15%. In all cases the recoverable strain is greater than 3%. A NiTi alloy with the same composition as shown in FIGS. 6-11 is cold worked

29% and then solution treated at 850° C for 30 minutes and water quenched. The specimen is then aged at a temperature of 350° C for 60 minutes and water quenched. The stress-strain curves for alloy specimens with this treatment are shown in FIGS. 12-16 for test temperatures from -20° C to +40° C. The longer aging times result in a greater residual strain after the first cycle but low residual strain in subsequent cycles; from 0.02 to 0.19. Although the recoverable strain in the 60 minute aging treatment is in excess of 4% in the temperature range from 0° C to +40° C, at -20° C the recoverable strain falls to 1.5%. To improve the low temperature pseudoelastic performance it has been found that if the material is cold worked 20% prior to the solution treatment and then aged that the temperature range for pseudoelastic behaviour is extended to -20° C. A curve illustrating this is shown in FIG. 17.

The use of the pseudoelastic NiTi in eyeglass frame components assumes that the performance with be maintained after many cycles of deformation in use. To check this specimens were cycled 10 times at strains to 5% and at strains to 8%. The curves in FIGS. 18 and 19 show that the recoverable strain remains essentially constant, and the upper plateau stress also is essentially unchanged.

The testing of less Ni-rich alloys, for example, an alloy with 55.9 wt% Ni-44.1 wt% Ti did not result in pseudoelastic behaviour as good as the alloy cited above. In addition, aging the first cited alloy at other temperatures, 300° C/ 30 min. or 60 min, 400° C/30 min or 60 min, and 450° C/30 min or 60 min did not give acceptable properties; as such, the 350° C treatment is considered unique, and the alloy composition is considered optimum for this processing schedule. A typical processing of a component such as a temple piece requires different levels of cold work since the ear piece is round and the temple portion is usually rectangular. By using a solution treated high nickel alloy the effect of different levels of cold work generated during the forming operation is minimised. The final aging treatment renders the piece pseudoelastic and ready for any final processing steps such as plating or joining. It has been observed that a small amount of cold work before the aging treatment gives slightly better performance but this is not considered a necessary step in the normal processing of eyeglass components. FIG. 20 shows an eyeglass frame, part(s) or all of which may be made of the material provided by the present invention. The eyeglass frame includes the lens frames 2, the bridge 1, the hinges 3, the ear pieces 4 and the nose pieces 5.

One processing schedule which may be used in accordance with the present invention for eyeglass components, such as, the bridge or ear piece, is provided below.

After drawing wire to the required diameter wi*h interpass anneals of 800°C for three minutes, final forming of the part is carried out. After forming 800°C there is an aging treatment at 350°C for 30 minutes. The transformation temperature T is checked with DSC. The austenite peak temperature should be -30 to -50°C. This yields a pseudoelastic behavior over the temperature range of -20°C to 40°C.

If a lower loading and unloading plateau is desired to give a softer feel, a treatment at 720°C for two minutes is used instead of the treatment at 800°C for three minutes. The low temperature pseudoelastic behavior will not, however, be as good with this treatment.

It will now be apparent to those skilled in the art that other embodiments, improvements, details and uses can be made consisten with the letter and spirit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents.

Claims

1. In an eyeglass frame having at least a portion thereof fabricated from Ni-Ti based shape memory alloy, the improvement comprising: (a) said alloy being more than 50 atomic % nickel;

(b) said alloy having been solution treated at a temperature of 650 to 1100° C for 10-60 minutes;

(c) after cooling, said alloy having been aged by heating it at a temperature of approximately 350° C for 10-60 minutes; (d) said portion being characterized by:

(i) having pseudoelastic properties without cold working, (ii) having greater than 2.5% elasticity over a temperature range of -20° C to +40° C, and

(iii) allowing large plastic deformations during the fabrication of said portion.

2. An eyeglass frame as defined in claim 1 wherein said portion includes a pair of temples.

3. An eyeglass frame as defined in claim 1 wherein said portion includes a bridge.

4. An eyeglass frame as defined in claim 1 wherein said portion includes a bridge and a pair of temples.

5. An eyeglass frame as defined in claim 1 wherein said portion includes a pair of lens rims.

6. An eyeglass frame as defined in claim 1 wherein said frame includes a pair of lens rims, and a pair of nose pads wires, each nose pad being connected to a respective rim by a nose pad wire.

7. An eyeglass frame as defined in claim 1 wherein said portion includes at least one of a pair of temples, a bridge, a pair of lens rims and nose pads wires.

8. An eyeglass frame as defined in claim 1 wherein said alloy composition is about 56.1wt% Ni-43.9wt% Ti.

9. An eyeglass frame as defined in claim 1, wherein said portion is given relatively small amounts of cold work, either before, or after, or before and after the ageing treatment to further improve the pseudoelastic behaviour.

10. An eyeglass frame as defined in claim 9, wherein the relatively small amounts of cold work before the ageing treatment do not exceed 30%.

11. An eyeglass frame as defined in claim 1 wherein said portion is further characterized by a pseudoelastic or superelastic behaviour over the temperature range from -20°C to +40°C.

12. An eyeglass frame as defined in claim 1, wherein said alloy portion has additional alloying elements, which, without substantially altering the processing of the portion, extend the temperature range of the pseudoelastic behaviour of the portion.

13. An eyeglass frame as defined in claim 12 wherein said additional alloying elements are at least one selected from the group consisting of Ta, Mo, Nb, Co, Cr, Cu, V, Mn and Fe.

14. An eyeglass frame as defined in claim 11 wherein the shape memory portion exhibits pseudoelastic properties with an upper plateau stress which is between approximately 42Ksi and 72Ksi, whereby the stress level is comfortable to the wearer

15. A method of making at least a component of an eyeglass frame, comprising: (a) fabricating said component of an alloy which includes (i) an alloy of NiTi with a higher nickel content than an equiatomic Ni/Ti ratio in a Ni-Ti shape memory alloy,

(ii) and which has been solution treated at a temperature of 650 to 1100° C for 10-60 minutes; and (b) aging said component after it is fabricated, by heating it at a temperature of approximately 350° C for 10-60 minutes, said portion being characterized by: having pseudoelastic properties without could working, having greater than 2.5% elasticity over a temperature range of -20° C to +40°

C, and allowing large plastic deformations during the fabrication step before the desired pseudoelastic properties are established.

16. A method as defined in claim 15, wherein said step of fabricating includes large plastic deformations of said alloy before pseudoelasticity is imparted to the component.

17. A method as defined in claim 16, wherein said NiTi based shape memory component is characterised by having pseudoelastic properties without using cold working and greater than 2.5% elasticity over the temperature range of -20 to +40° C.

18. An alloy for use in fabricating at least a component of an eyeglass frame, comprising: an alloy of NiTi having a higher nickel content than an equiatomic Ni/Ti ratio in a Ni-Ti shape memory alloy; said alloy having been solution treated at a temperature of 650 to 1100° C for 10- 60 minutes; and after fabrication into a component, having been aged by heating it at a temperature of approximately 350° C for 10-60 minutes, said alloy having pseudoelastic properties without cold working, having greater than 2.5% elasticity over a temperature range of -20° C to +40° C, and allowing large plastic deformations during the fabrication step before the desired pseudoelastic properties are established.

19. The alloy defined in claim 18, wherein the alloy is characterised by good forming characteristics, a strain recovery of at least 3% over a temperature range from -20 to +40° C and having a yield strength ranging from about 42 Ksi to about

72 Ksi.

20. The alloy defined in claim 19, wherein the alloy is characterised by being capable of undergoing large amounts of cold or hot forming without danger of cracking/fracturing during the fabricating operations inquired to made said part.

21. An eyeglass frame as defined in claim 20 wherein an ageing treatment is at the same time employed to provide stress relief after secondary operations such as electroplating or joining.

22. An eyeglass frame as defined in claim 8 wherein the readily deformed solution-treated shape memory alloy, during fabrication, is subjected to a variety of cold working levels to create various cross sections of the portion, which nevertheless, after an ageing treatment exhibit substantially uniform pseudoelastic properties, this ageing treatment has to be proceeded by another solution treatment if the total cold work before the ageing treatment is not relatively small.

23. An eyeglass frame as defined in claim 20 wherein relatively small means not exceeding 30 %.

24. An eyeglass frame having at least a portion thereof fabricated from Ni-Ti based shape memory alloy, said portion being characterized by: having pseudoelastic properties without cold working, having greater than 2.5% elasticity over a temperature range of -20° C to +40° C, and allowing large plastic deformations during the fabrication of said portion before the desired pseudoelastic properties are established.

25. An eyeglass frame having at least a portion thereof fabricated from Ni-Ti based shape memory alloy, said portion being characterized by: having pseudoelastic properties without cold working, having been solution treated, having greater than 2.5% elasticity over a temperature range of -20° C to +40° C, having been heat aged, and allowing large plastic deformations during the fabrication of said portion.