United States Patent [19J
`Pelton et al.
`
`111111111111111111111111111111111111111111111111111111111111111111111111111
`US005843244A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,843,244
`Dec. 1, 1998
`
`[54] SHAPE MEMORY ALLOY TREATMENT
`
`FOREIGN PATENT DOCUMENTS
`
`[75]
`
`Inventors: Alan Pelton; Thomas Duerig, both of
`Fremont, Calif.
`
`9/1985
`201028/85
`3/1987
`62-60851
`62-284047 12/1987
`
`Japan .
`Japan ..................................... 148/563
`Japan ..................................... 148/563
`
`[73] Assignee: Nitinol Devices and Components,
`Freemont, Calif.
`
`Primary Examiner-George Wyszomierski
`Attorney, Agent, or Firm-Dean Gamer
`
`[21] Appl. No.: 661,3115
`
`[22] Filed:
`
`Jun. 13, 1996
`
`•••••••••••••••••.•••.•..•..••.•.•.•.•.•.••.•.•.•.•••.•. C22F 1/10
`Int. Cl.6
`[51]
`[52] U.S. Cl. ............................................. 148/563; 148/676
`[58] Field of Search ..................................... 148/402, 563,
`148/676, 670
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`[57]
`
`ABSTRACT
`
`A method of treating a component formed from a Ni-Ti
`based shape memory alloy, so that the component exhibits
`superelasticity, comprises cold working the component,
`annealing the alloy while the component is restrained in the
`configuration resulting from the cold working step, and
`e),:posing the component to a temperature that is less than the
`solvus temperature of the alloy and greater than the tem(cid:173)
`perature to which it is exposed in the annealing step to cause
`the Pv temperature to be reduced.
`
`4,502,896
`
`3/1985 Duerig el a!. ........................... 148/402
`
`8 Claims, 1 Drawing Sheet
`
`(a)
`
`COLD
`WORKING
`
`OPTIONAL
`STRESS RELIEF
`
`{b)
`
`FORMING ~
`/
`
`CONSTRAINED
`ANNEALING
`
`OPTIONAL
`REPETITION
`
`(c)
`
`PARTIAL
`SOLUTION/ZING
`
`GOLD STANDARD EXHIBIT 2033
`US ENDODONTICS v. GOLD STANDARD
`CASE IPR2015-00632
`
`

`
`U.S. Patent
`
`Dec. 1, 1998
`
`5,843,244
`
`FIG. 1
`
`(a)
`
`COLD
`WORKING
`
`OPTIONAL
`STRESS RELIEF
`
`FORMING ~..------------.
`~ OPTIONAL
`REPETITION
`
`v._____.
`
`(b)
`
`CONSTRAINED
`ANNEALING
`
`(c)
`
`PARTIAL
`SOLUTION/ZING
`
`

`
`5,843,244
`
`1
`SHAPE MEMORY ALLOY TREATMENT
`
`BACKGROUND TO THE INVENTION
`
`This invention relates to a method of treating a component
`formed from a shape memory alloy, in particular so that the 5
`component exhibits enhanced elastic properties.
`Articles formed from shape memory alloys can exhibit
`shape memory properties associated with transformations
`between martensite and austenite phases of the alloys. These
`properties include thermally induced changes in configura(cid:173)
`tion in which an article is first deformed from a heat-stable
`configuration to a heat-unstable configuration while the
`alloy is in its martensite phase. Subsequent exposure to
`increased temperature results in a change in configuration
`from the heat-unstable configuration towards the original
`heat-stable configuration as the alloy reverts from its mar(cid:173)
`tensite phase to its austenite phase.
`Shape memory alloys can exhibit enhanced elastic prop(cid:173)
`erties compared with materials which do not exhibit
`martensite-austenite transformations and it is these proper(cid:173)
`ties that the present invention is concerned with in particular.
`The nature of the superelastic transformations of shape
`memory alloys is discussed in "Engineering Aspects of
`Shape Memory Alloys", T. W. Due rig et al, on page 370,
`Butterworth-Heinemann (1990). Subject matter disclosed in
`that document is incorporated in this specification by this
`reference to the document. A principal transformation of
`shape memory alloys involves an initial increase in strain,
`approximately linearly with stress. This behaviour is
`reversible, and corresponds to conventional elastic defor(cid:173)
`mation. Subsequent increases in strain are accompanied by
`little or no increase in stress, over a limited range of strain
`to the end of the "loading plateau". The loading plateau
`stress is defined by the inflection point on the stress/strain
`graph. Subsequent increases in strain are accompanied by
`increases in stress. On unloading, there is a decline in stress
`with reducing strain to the start of the "unloading plateau"
`evidenced by the existence of an inflection point along
`which stress changes little with reducing strain. At the end
`of the unloading plateau, stress reduces with reducing strain.
`The unloading plateau stress is also defined by the inflection
`point on the stress/strain graph. Any residual strain after
`unloading to zero stress is the permanent set of the sample.
`Characteristics of this deformation, the loading plateau, the
`unloading plateau, the elastic modulus, the plateau length
`and the permanent set (defined with respect to a specific total
`deformation) are established, and are defined in, for
`example, "Engineering Aspects of Shape Memory Alloys",
`on page 376.
`Shape memory alloys are also discussed in an article by
`L. McDonald Schetky in the Encyclopedia of Chemical
`Technology (edited by Kirk-Othmer), volume 20 pages 726
`to 736. Subject matter disclosed in that document is incor(cid:173)
`porated in this specification by this reference to the docu(cid:173)
`ment.
`The transformation from austenite to martensite on cool(cid:173)
`ing begins at a temperature known as the Ms temperature,
`and is completed at a temperature known as the M1 tem(cid:173)
`perature. The transformation of martensite to austenite upon
`heating begins at a temperature known as the As temperature
`and is complete at a temperature known as the~ tempera(cid:173)
`ture. The application of a load tends to favour, or stabilise
`the martensite phase. Non-linear superelastic properties are
`exhibited when the austenitic phase is stable in the absence
`of a load, yet the martensitic phase can temporarily become
`the stable phase when a load of sufficient magnitude is
`
`2
`introduced. Thus these properties require that one maintains
`the material temperature slightly above the~ temperature.
`The temperature above which all traces of superelasticity are
`lost is called the Ma temperature.
`A preferred way in which non-linear superelastic proper-
`ties can be introduced in a shape memory alloy involves cold
`working the alloy by one of several deformation methods,
`for example, swaging, drawing, pressing, stretching or bend(cid:173)
`ing. The cold working step is followed by an annealing step
`10 at a temperature less than the recrystallization temperature
`of the alloy, for a time sufficient to cause dislocations to
`rearrange, combine and align themselves (so-called "recov(cid:173)
`ery" processes). The resulting recovered dislocation struc(cid:173)
`ture should ideally be dense enough to make plastic defor-
`15 mation difficult, but not so dense as to prevent the martensite
`phase from transforming upon the application of a load, and
`growing in a relatively unimpeded manner.
`Since many preferred superelastic alloys are thermally
`unstable in the temperature range in which these recovery
`20 processes occur, a second unavoidable result of this recovery
`heat treatment step is to age the material, that is to cause
`Ni-rich particles to precipitate, having the effect of enriching
`the matrix phase in titanium, and thus increasing the trans(cid:173)
`formation temperatures (including the~ temperature). Opti-
`25 mum superelastic properties are only realized when using
`shape memory alloys above the ~ temperature, though it
`should be noted that some indications of superelasticity are
`observed above the A" temperature (typically 2° to 20° C.
`below~) . Thus a second requirement for this recovery heat
`30 treatment is that ~not be increased above the temperature
`at which the alloy is to be used. Practically speaking this
`places upper limits on the time and temperature which can
`be used in the recovery heat treatment.
`It is often the case that a device is to be used in a shape
`35 other than that which can be practically produced by cold
`working processes. For example, a straight wire can be
`conveniently produced by cold drawing, but a wire loop or
`other formed shape cannot be. In this case, it is customary
`to form the drawn, cold worked wire into the desired "heat
`40 stable" shape, to constrain the wire in that shape, and then
`to perform the above described recovery heat treatment to
`"shape set" the component. In this case the final annealing
`operation has two purposes: to adjust the superelastic prop(cid:173)
`erties of the alloy, and to properly set the shape of the article.
`45 The time and temperature of this heat treatment step are
`critical. If held too long at temperature, the material over(cid:173)
`ages, causing the~ temperature to rise above the applica(cid:173)
`tion temperature. If the annealing temperature is too short, or
`the temperature too low, the shape will be insufficiently
`50 formed, and too much of the original dislocation structure
`will remain to allow free martensite movement. This "form(cid:173)
`ing" treatment may introduce still further cold work into the
`part, but that cold work is usually small compared to that
`introduced into the wire by drawing. Moreover, forming
`55 operations are often not uniform, and thus forming itself is
`not generally a convenient way to introduce cold work.
`Articles of complicated shape require extensive forming
`and are very difficult to produce according to the above
`process. If the forming process causes strains which are too
`60 severe, the article will fracture as it is heated to the shape
`setting and recovery temperature (one is able to restrain the
`formed article, but cannot maintain its shape during the
`heating process without causing fracture) . It is possible to
`overcome this problem by performing a series of smaller,
`65 intermediate shape setting operations which accumulate to
`provide the desired final shape, but unfortunately each of
`these shape setting operations requires sufficient annealing
`
`

`
`5,843,244
`
`3
`time to allow the material to soften, in preparation for the
`next. When accumulated, these heat treatments cause a
`cumulative ageing effect that can cause the~ temperature to
`rise beyond the expected service temperature (37° C. for
`most medical applications, for example).
`It is also known that one can introduce superelasticity by
`solution treating and ageing, abandoning all attempts to
`retain cold work. Although this approach resolves the above
`problems, it leads to inferior superelastic properties, pro(cid:173)
`ducing articles that are susceptible to fatigue and storage
`problems.
`
`SUMMARY OF THE INVENTION
`The present invention provides a technique for treating a
`component formed from a shape memory alloy so that the
`component exhibits enhanced elastic properties, which
`includes a step of briefly heating the component at an
`elevated temperature that is higher than the temperature of
`the shape setting treatment referred to above.
`Accordingly, in one aspect, the invention provides a
`method of treating a component formed from aNi-Ti based
`shape memory alloy so that the component exhibits
`superelasticity, which comprises:
`(a) cold working the component,
`(b) heat treating the alloy while the component is
`restrained in a desired shape at a temperature that is less
`than the recrystallisation temperature of the alloy, to
`cause the dislocations in the alloy resulting from the
`cold working step to combine, and
`(c) exposing the component to a temperature that is
`greater than the temperature to which it is exposed in
`the shape setting step and less than the solvus tempera(cid:173)
`ture of the alloy, to cause the ~ temperature to be
`reduced.
`It has been found surprisingly that brief exposure of the
`component to elevated temperature after the cold working
`and shape setting steps can result in increases in the ~
`temperature of the alloy caused by the cold working and
`annealing steps to be reduced and often in a reduction in the 40
`~ temperature. This has the significant advantage of
`enabling the method of the invention to be used to make a
`component whose configuration is so complicated that it
`requires several cold working steps, while still making it
`possible to achieve shape memory properties (including 45
`enhanced elastic properties) with appropriate transformation
`temperatures. Surprisingly, this adjustment in the shape
`memory properties of the alloy can occur rapidly, and can be
`obtained without significant loss of the cold work previously
`imparted to the article. The treatment provided by the 50
`present invention makes it possible to shape set complex
`formed articles with less concern for averaging and degra(cid:173)
`dation of superelastic properties.
`Preferably, the amount of cold work imparted to the
`component after any full annealing treatment and prior to 55
`step (a) of the method is greater than about 10%, more
`preferably greater than about 20%. The technique for
`imparting the cold work will be selected according to the
`configuration of the article that is to be made and to the
`nature and configuration of the starting materials. Examples 60
`of suitable techniques include swaging, drawing, pressing
`and so on. Examples of configurations for the starting
`materials include sheet, bar, wire, rod, tube and so on.
`The annealing temperature used in step (b) of the method
`will be selected so that configuration resulting from the cold 65
`working step is substantially retained after the means by
`which the component is restrained is released. Complex
`
`4
`shapes may require one or more intermediate constrained
`heat treatments in order to achieve the final desired shape.
`Preferably the temperature to which the component is heated
`in these shape setting steps is kept as low as possible to
`5 minimise aging effects on the alloy, which can result in loss
`of strength or ductility or both. Preferably, the temperature
`to which the component is heated in the annealing step of the
`method is no higher than about 40° C. below the solvus
`temperature, more preferably no higher than about 80°
`10 below the solvus temperature, for example no higher than
`about 100° below the solvus temperature. The temperature
`to which the component is heated will preferably be no
`lower than about 300° below the solvus temperature, more
`preferably no lower than about 250° below the solvus
`15 temperature, for example no lower than about 100° below
`the solvus temperature. Preferably, the temperature to which
`the component is heated in these shape setting steps of the
`method are no higher than 540° C. and preferably no higher
`than about 510° C. The temperature to which the component
`20 is heated will preferably be no lower than about 270° C.,
`more preferably no lower than about 300° C. The duration
`of each constrained heat treatment should be no less than 1
`minute.
`The elevated temperature to which the component is
`25 heated in step (c) of the method is preferably at least about
`10° C. higher than the shape setting temperature to which the
`component is heated in step (b), more preferably at least
`about 20° higher. Preferably, the temperature to which the
`component is heated in step (c) of the method is no lower
`30 than about 70° C. below the solvus temperature, more
`preferably no lower than about 50° below the solvus
`temperature, for example no lower than about 30° below the
`solvus temperature. The treatment will generally be per(cid:173)
`formed at a temperature of at least 480° C. The temperature
`35 will be below the solvus temperature of the alloy. The solvus
`temperature is the temperature below which a Ni-rich pre(cid:173)
`cipitate forms when the alloy is subjected to an aging heat
`treatment.
`Preferably, the article is exposed to the elevated tempera(cid:173)
`ture in step (c) of the method for not more than about 10
`minutes, more preferably not more than about 3 minutes.
`Preferably, it is exposed to the temperature for at least about
`10 seconds, more preferably at least about 1 minute.
`Although cooling rates after this treatment are not consid(cid:173)
`ered critical, it is preferable to avoid cooling rates so slow
`as to allow re-precipitation of nickel-rich particles.
`Preferably, step (c) of the method is carried out while the
`component is constrained in a desired shape, which will
`frequently be the same as the shape in which it is constrained
`during step (b). If step (c) is carried out while the component
`is not so constrained, the exposure of the component to
`elevated temperature can result in a change in the shape of
`the component.
`It will often be appropriate for the method to include a
`forming step after the heat treatment of step (b) but before
`the heating step (c) of the method, especially by cold
`working the component. One or more such forming steps
`can enable high levels of deformations to be imparted to an
`article without breaking the component. The heat treatment
`step (b) can be repeated after the forming step. The method
`of the invention can include one, two, three or more such
`forming operations. There will generally be heat treatment
`steps (b) between the forming operations, and often also
`after the last of the forming operations and before the
`subsequent heating step (c) of the method.
`The method of the invention can be performed on Ni-Ti
`based shape memory alloys. Such alloys include binary
`
`

`
`5,843,244
`
`5
`alloys, such as those in which the nickel content is at least
`about 50 at. %, preferably at least about 50.5 at. %. The
`nickel content will usefully be less than about 52 at. %,
`preferably less than about 51 at. %. The method can also be
`performed on other Ni-Ti based alloys, including alloys 5
`with ternary and quaternary additions. Examples of elements
`that can be incorporated in the alloy include Cr, Fe, Co, Al
`and Cu. Added elements can be present in amounts up to
`about 10 at. %, preferably up to about 5 at. %. Alloys which
`are suitable for this treatment are preferably thermally
`unstable, meaning that their properties can be altered
`through thermal ageing after quenching from above the
`solvus temperature. It is understood that certain alloying
`additions may cause the specific temperatures cited to
`change somewhat, and that appropriate corrections be made. 15
`Preferably, the alloy for the component and the treatment
`of the component are selected so that the ~temperature of
`the alloy in the treated component is approximately 0° to 40°
`C. below the operating temperature of the component when
`muse.
`The method of the invention can be used to produce
`components for use in a wide variety of applications. The
`components can be used in medical devices, for example
`tools and devices which are to be implanted in a human or
`animal body. An example of an implant device is a stent 25
`which can be positioned in a lumen in a body while in a
`transversely compressed configuration and then made to
`expand transversely into contact with the wall of the lumen,
`to support the lumen or to force the wall outwardly. This
`transverse expansion can result from discharge of the stent 30
`from a tubular constraint, for example in a catheter. The
`expansion can involve the release of energy stored in the
`component by virtue of the enhanced elastic properties of a
`shape memory alloy.
`
`6
`the solvus temperature of the alloy, for a short period to
`cause the ~ temperature of the component to be reduced.
`This final treatment can be performed with or without
`constraining the component in a desired shape.
`The method of the invention can be used to make a stent
`for delivery to a blood vessel in a human or animal body in
`a catheter. Such a stent might be formed from a tube of a
`Ni-Ti binary alloy containing from 50.5 to 52 at. % nickel.
`The solvus temperature of such an alloy is about 580° C. The
`10 tube will have an external diameter of about 1.5 mm and a
`wall thickness of about 0.25 mm. The characteristic trans(cid:173)
`formation temperature ~of the tube (before shape setting)
`will be approximately 10° C. A pattern can be cut into the
`tube, for example by means of a laser (as is known). The
`pattern will be selected so that the resulting part has a
`desired flexibility an ability to exert force against the wall of
`the blood vessel or other lumen.
`The tube can be expanded from its configuration as cut so
`that its external dimension increases to 4 mm. This can be
`done be successively passing mandrels of diameters 2 mm,
`20 3 mm and 4 mm through the device, and heat treating the
`device on each of these mandrels at about 450° C. for about
`10 minutes. The characteristic transformation temperature
`~of the alloy after the all steps are performed is about 35°
`C. (producing a device that is no longer superelastic at room
`temperature).
`The component is then subjected to a final heat treatment
`after the end of the shape setting steps, at a temperature
`between 510° and 530° C. for about 1 minute. The charac(cid:173)
`teristic transformation temperature ~of the alloy after the
`heat treatment step is about 25° C., making the device
`substantially superelastic at room temperature, and ideally
`superelastic at body temperature.
`The component can be deformed inwardly from the final
`configuration resulting from the heat treatment step towards
`35 the configuration prior to cold work, and the deformation
`substantially recovered elastically relying on the superelas(cid:173)
`tic properties imparted to the alloy by the method of the
`invention.
`What is claimed is:
`1. A method of treating a component formed from a
`Ni-Ti based shape memory alloy so that the component
`exhibits superelasticity and to impart a desired shape to the
`component and thereafter lower the component's Af, which
`comprises:
`(a) cold working the component, the component compris(cid:173)
`ing greater than 50.5 at. % nickel;
`(b) restraining the alloy to a desired shape and heat
`treating the alloy at a temperature that is less than
`solvus temperature of the alloy, to cause dislocations in
`the alloy resulting from the cold working step to
`combine; and
`(c) after heat treating the component in the restrained
`desired shape in step (b), removing the restraint and
`thereafter heat treating the component at a temperature
`that is greater than the temperature to which it is
`exposed in step (b) and less than the solvus temperature
`of the alloy, causing the component to retain the desired
`shape and causing the Af temperature to be reduced so
`that said component is superelastic at body tempera(cid:173)
`ture.
`2. A method as claimed in claim 1, further including the
`step of annealing the component prior to step (a) and
`wherein the amount of cold work imparted to the component
`in step (a) is greater than about 10%.
`3. A method as claimed in claim 1, in which the compo(cid:173)
`nent is exposed to elevated temperature in step (c) for not
`more than about 10 minutes.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a block diagram providing schematic represen(cid:173)
`tation of the steps of the method of the invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`40
`
`A component formed from a Ni-Ti based shape memory
`alloy, which is to be treated by the method of the invention
`so that the component exhibits superelasticity, is produced 45
`by appropriate step(s) involving for example, machining,
`working, heat treatment steps and so on. After a prior
`annealing step, the component is subjected to one or more
`cold working steps, for example by drawing, swaging,
`pressing, mandrel expansion etc. A stress relief heat step can 50
`be included after the after cold working step in order to
`reduce the amount of retained cold work or to improve
`ductility. If such an optional heat treatment is performed, it
`is necessary to avoid removing too much cold work. This
`stress relief heating step is conducted without constraint.
`The component is then formed into a desired shape, and
`subjected to a heat treatment while it is restrained in the
`desired configuration. This operation can be repeated several
`times in order to achieve a final desired component con(cid:173)
`figuration. Alternatively, the forming and shape setting heat 60
`treatment might be combined into one hot forming opera(cid:173)
`tion. In this case, the temperature and time of the hot
`forming operation will be those discussed above for the
`shape setting heat treatment.
`When the desired configuration is achieved, it is subjected 65
`to an additional heating step at elevated temperature higher
`than that of the final shape setting treatment, but lower than
`
`55
`
`

`
`5,843,244
`
`7
`4. A method as claimed in claim 3, in which the compo(cid:173)
`nent is exposed to elevated temperature in step (c) for not
`more than about 5 minutes.
`5. A method as claimed in claim 1, in which the compo(cid:173)
`nent is exposed to elevated temperature in step (c) for at least
`about 30 seconds.
`6. A method as claimed in claim 1, in which step (b) is
`carried out more than once but before step (c) is carried out.
`
`8
`7. A method as claimed in claim 1, in which the alloy is
`a thermally unstable alloy.
`8. A method as claimed in claim 1, in which the heat
`treatment of step (c) is performed at a temperature above
`5 5100 c.
`
`* * * * *