Overview
`
`Physical Metallurgy
`
`The Physical Metallurgy of Nitinol for
`Medical Applications
`
` Alan R. Pelton, Scott M. Russell, and John DiCello
`
` The purpose of this paper is to review
`the current processing and resultant
`properties of Nitinol for medical device
`applications. The melting and fabrica-
`tion of Nitinol present a number of
`unique challenges because of the
`strong sensitivity of the alloy system to
`chemistry and processing. The first part
`of this paper will summarize the effect
`of alloy fabrication on key material
`properties, vacuum-melting techniques,
`hot working, and cold working. The
`effects of the final shape-setting heat
`treatments on transformation tempera-
`ture and mechanical properties for
`medical devices will also be reviewed.
`INTRODUCTION
`
` The growth of the use of Nitinol in
`the medical industries has exploded
`in the past ten years due to increasing
`demand for minimally invasive surgical
`and diagnostic procedures. Nitinol is the
`enabling component in an increasing
`number of devices such as endoscopic
`instruments, stents (see Figure 1), filters,
`and orthopedic devices; examples of
`these medical applications are richly
`illustrated in other publications.1–5
`The preference for Nitinol over more
`standard engineering alloys is based on
`its unique combination of mechanical
`properties, including shape memory
`and superelasticity, coupled with superb
`biocompatibility.5
` The physics of the phase transforma-
`tions that give rise to the shape-memory
`effect in Nitinol have been known since
`the 1960s and have been the source of
`many technical articles and books.6–11
`However, Nitinol producers have only
`recently focused on optimizing processes
`for a few standard alloys rather than
`pursuing a myriad of “boutique alloys”
`with niche applications. The industry
`workhorse alloy is 50.8 at% nickel–49.2
`
`2003 May • JOM
`
`at.% titanium, hereafter referred to as
`Ni50.8Ti49.2. This alloy is used in many
`thousands of kilometers of wire and
`microtubing for such diverse products
`as cellular telephone antennas, eyeglass
`frame components, guidewires, under-
`garment supports, and orthodontic
`archwires.
` To ensure the quality and consistency
`of the Nitinol materials used in medical
`devices, it is important to understand
`some of the fabrication processes and
`their effects on the final properties and
`performance. Therefore, the purpose
`of this paper is to summarize Nitinol
`processing methods from the selection
`of raw components and melting practices
`to hot working and cold working.
`Additionally, the metallurgical influences
`of time and temperature for shape setting
`will be illustrated with examples.
`NITINOL PHASE
`TRANSFORMATIONS
`
` The unique shape-memory and
`superelastic properties of Nitinol are due
`to a diffusionless phase transformation
`between a higher-temperature austenite
`
`phase (B2 structure) and a lower-
`temperature martensite phase (B19´
`structure).6–11 The hysteresis curves that
`characterize the thermal and mechanical
`behavior are explained in great detail in
`these publications. For many medical
`devices, the temperature at which
`martensite fully transforms to austenite
`(Af) is the most important transforma-
`tion temperature since it dictates the
`transition between shape memory
`and superelastic properties. As will
`be discussed in this article, the Af
`temperature can be adjusted through
`thermomechanical treatments in order
`to optimize device performance.
` Nitinol can be considered an ordered
`intermetallic that has an extremely
`narrow composition range below the
`630°C eutectoid as shown in the binary
`phase diagram (refer, for example, to
`Reference 11). Therefore, any deviation
`from the stoichiometric 50:50 composi-
`tion requires that the alloy is in a
`two-phase field. Slight deviations in
`composition also affect the transforma-
`tion temperatures, which are determined
`by the nickel and titanium ratio. As
`
`Figure1. A Cordis Corporation SMART Nitinol endovascular
`stent that was manufactured through processes outlined
`in this paper.
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`
`100
`
`50
`
`0
`
`51
`(56)
`
`49
`(54)
`
`50
`(55)
`Ni Content, at. % (wt.%)
`Figure 2. A schematic of the effect of the
`nickel content in Nitinol on the alloy
`phase transformation temperature, Af.
`Note that a very small change in alloy
`composition can have a very large
`effect on the transformation temperature.
`The shaded region represents the area
`covered by typical binary superelastic
`Nitinol alloys.
`
`shown in Figure 2, a 1% shift in the
`amount of either nickel or titanium in
`the alloy will result in a 100°C change
`in alloy-transformation temperature.
`Most applications require the alloy-
`transformation temperatures to be
`controlled within ± 5°C, which means that
`the alloy composition must be controlled
`within ± 0.05%. These compositional
`requirements are significantly tighter than
`300 series stainless steel. Furthermore,
`most standard chemical-analysis methods
`are not precise enough to measure such
`subtle differences in alloy composition,
`so a differential scanning calorimetric
`measurement of the alloy-transformation
`temperatures is used to confirm proper
`ingot formulation.12
` The presence of impurities in Nitinol
`can affect the transformation tempera-
`tures of the material as well as the
`mechanical properties (see Table I). Most
`impurities depress the transformation
`(“down” arrows); they react with the
`titanium in the melt to form precipitates,
`resulting in nickel enrichment in the
`base alloy. As illustrated in Figure 2,
`higher nickel contents tend to lower
`the transformation temperature. For
`example, titanium strongly reacts with
`oxygen, nitrogen, and carbon to form
`oxides, oxi-nitrides, and carbides;
`therefore, the melting methods are
`selected to minimize their presence.
`Copper and niobium do not directly
`
`34
`
`Table I. A Schematic Table Illustrating the Effects of Various Melting Impurities on the
`Resulting Ingot Properties*
`
`
`
`Temperature
`Strength
`Ductility
`
`O
`
`↓
`↑
`↓
`
`N
`
`↓
`↑
`↓
`
`H
`
`↓
`↑
`↓
`
`C
`
`Cu
`
`Cr
`
`Co
`
`Fe
`
`V
`
`Nb
`
`↓ →
`↑
`
`↓
`↓ →
`
`↓
`↑
`↓
`
`↓
`↑
`↓
`
`↓
`↑
`↑
`
`↓ →
`↑
`↑
`↓ →
`
`* See text for explanation of the symbols.
`
`decrease the transformation temperature,
`although copper additions modify the
`phase transformation, and niobium
`has virtually no solubility in the NiTi
`phase.13 Chromium, cobalt, and iron
`all substitute for nickel in the B2
`lattice and, therefore, have an additive
`effect in determining the transformation
`temperature. In turn, most of these
`impurities simultaneously increase the
`strength of the ingot and decrease the
`ductility. Recent ASTM specifications
`set strict limits on these impurities for
`medical-grade binary Nitinol.14
`MELTING METHODS
`
` The sensitivity of the transformation
`temperatures to alloy composition is
`an important consideration in selecting
`an alloy melting method. As discussed
`previously, any contaminant will affect
`the amount and chemistry of phases
`present, possibly resulting in an unus-
`able material. Therefore, both an alloy
`melting method and the elemental raw
`materials must be chosen to ensure high
`purity. Further, the melting method must
`be one in which the molten material is
`very well mixed throughout to achieve
`
`Vacuum Chamber
`
`Induction Coils
`
`ingot homogeneity and uniformity
`of properties. As such, the two most
`common commercial melting methods
`are vacuum-induction melting (VIM)
`and vacuum-arc remelting (VAR).15,16
`Both melting methods start with high-
`purity components of nickel (>99.94%
`purity) and titanium (>99.99% purity).
` For VIM ingots, the nickel and
`titanium are weighed out in the proper
`ratio and placed in an electrically
`conductive crucible (usually graphite)
`inside a vacuum chamber, as shown
`schematically in Figure 3. The crucible
`is heated from the outside by electrical
`induction coils. Once the constituents
`are molten, the induction fields stir the
`alloy completely, which results in a
`homogeneous melt. Homogeneity is
`confirmed in the solid ingots, where
`transformation temperature uniformity
`within a degree or two is achievable.
`The main disadvantage of VIM is that
`the molten Nitinol picks up a small
`amount of carbon contaminant from
`the graphite crucible. Carbon impurity
`levels of 300 wppm to 700 wppm are
`typical for VIM-melted materials. Major
`commercial suppliers of VIM Nitinol
`
`Molten Alloy
`
`Graphite Crucible
`
`Mold
`
`To Vacuum Pump
`
`Figure 3. A schematic of vac-
`uum-induction melting.
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`reduce the cross-sectional dimension
`to less than 15 cm. Depending on the
`final product shape, techniques such as
`press forging, rotary forging, extrusion,
`swaging, bar rolling, rod rolling, and
`sheet rolling may be used in the hot-
`working stage. Hot working is typically
`performed at temperatures of 600°C
`to 900°C. These elevated temperatures
`(approximately 0.55–0.75 Tm) are
`sufficient to lower the flow stresses to
`allow relatively large deformation steps.
`In addition to dimensional reduction,
`these initial hot-deformation processes
`are effective in breaking up the as-cast
`microstructure, which has very little
`ductility and does not exhibit much
`shape memory, superelasticity, or
`resistance to fracture.
`COLD WORKING
`
` Cold-working processes provide the
`final product shape, surface finish,
`refined microstructure, and mechanical
`properties. Figure 5 shows the change
`in the ultimate tensile strength of
`cold-drawn 1 mm diameter wire. As
`illustrated in this figure, Nitinol alloys
`work harden rapidly, so the material
`must be fully annealed (at 600°C to
`800°C) after cold-working operations.
`A series of such cold-working and
`annealing steps are usually required to
`bring the material down to its finished
`size. Figure 6a and 6b shows the
`microstructures of as-drawn and fully
`annealed 1 mm diameter wires, respec-
`tively. The as-drawn microstructure
`consists of deformed martensite and
`a high density of dislocations. The
`annealed grain size is ASTM 4 or finer
`(on the order of 100 µm).
` Typical cold-worked Nitinol (semi-
`finished) products include wire, tubing,
`and sheet. For most applications,
`however, Nitinol does not exhibit the
`final desired properties in its cold-
`
`Figure 5. The effect of cold work
`on the ultimate tensile strength of
`a Ni50.8Ti49.2 Nitinol alloy. Note the
`high work-hardening rate.
`
`60
`
`35
`
`Power Cable
`
`Electrode
`Withdrawal
`System
`
`To Vacuum Pump
`
`Water
`Cooled
`Crucible
`
`Electrode
`
`(Elemental
`Compact or
`Previous
`Ingot)
`
`Molten
`Zone
`
`New Ingot
`
`Figure 4. A schematic of vacuum-arc remelting.
`
`Wah Chang (United States).
` References 15 and 16 summarize
`the differences between VIM and VAR
`melting. In general, the cost of Nitinol
`produced by either technique is similar.
`Further, the major commercial suppliers
`of Nitinol around the world are currently
`producing acceptable high-quality
`materials for the exacting needs of
`the medical device industry by either
`melting method.
`HOT WORKING
`
` Commercial VAR or VIM/VAR ingots
`are generally 50 cm in diameter and
`weigh in excess of 2,500 kg. Hot-
`working processes are typically used to
`
`1mm Cold -Drawn Nitinol Wire
`
`2,000
`
`1,800
`
`1,600
`
`1,400
`
`UTS (MPa)
`
`30
`
`40
`
`50
`
`Cold Work (%)
`
`include Furukawa Electric (Japan) and
`Special Metals (United States).
`
`In VAR melting, the nickel and
`titanium are weighed out in the proper
`ratio and are pressed into a large
`compact, which is used as a consumable
`electrode. An electrical arc is initially
`struck between the electrode and the
`bottom of the crucible and sufficient
`current is passed to melt the electrode
`continuously (see Figure 4). As molten
`metal is formed, the electrode is slowly
`withdrawn and the metal solidifies at
`the bottom of the melt pool. The molten
`pool is contained by a water-cooled
`copper crucible that forms a frozen skull
`on the outside of the pool, preventing
`any contamination of the melt by the
`crucible. Because of this, VAR melting
`achieves extremely high purity of the
`resultant alloy. The disadvantage of this
`method is that the entire ingot is not
`molten at the same time; consequently,
`the ingot is remelted several times to
`achieve high homogeneity. Vacuum-arc
`remelting is also used for further refining
`of VIM ingots; the final products are
`known as VIM/VAR ingots. The major
`commercial supplier of VAR Nitinol is
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`10
`
`Time (minutes)
`
`100
`
`1,000
`
`700
`
`600
`
`500
`
`400
`
`300
`
`200
`
`1
`
`Temperature (°C)
`
`Figure 7. The effects of aging temperature and time on the transformation temperature of
`Ni50.8Ti49.2 wire with a starting Af temperature of 11°C. The data are plotted to illustrate a
`novel Af-TTT diagram. Note that the maximum precipitation rate is at about 450°C. Between
`500°C and 600°C the precipitates re-solutionize and tend to lower the Af. A new precipitate
`forms at longer times at these higher temperatures.
`
`uniform properties. Continuous strand
`straightening usually occurs in a
`temperature range of 450°C to 550°C
`under a stress of 35–100 MPa, depend-
`ing on the requirements of the applica-
`tion. As the wire moves through the
`heat zone, it will initially try to shrink in
`length and grow in diameter due to the
`shape-memory effect not suppressed by
`the cold work (i.e., springback). As the
`wire reaches the furnace temperature,
`the internal stresses will begin to relax
`with a concomitant decrease in wire
`strength. Under these conditions, the
`applied stress induces strain, which
`shape sets the wire straight. Strains
`have been measured on the order of
`2–3% during the wire-straightening
`process.17,18
` The final heat treatment controls
`the final properties of the product,
`including shape, mechanical properties,
`and the active Af. Longer times or higher
`temperatures tend to anneal the product,
`but result in more exact shapes with
`less springback. At the other extreme,
`shorter times or lower temperatures
`leave the material closer to the high-
`strength, cold-worked state with more
`shape springback. A balance must be
`developed between these two condi-
`tions to optimize the shape, Af, and
`mechanical properties.
`EFFECTS OF AGING
`TREATMENTS ON Af
` Several investigators have shown that
`optimal superelastic performance can
`be achieved in Nitinol alloys that have
`a combination of cold work and aging
`heat treatments.17–20 Precise control of
`
`these thermomechanical treatments can
`lead to reproducible mechanical proper-
`ties and transformation temperatures.
`Nickel-rich Nitinol alloys respond well
`to aging heat treatments to tune in the
`desired properties.
` Nishida et al.21 established the effects
`of aging time and temperature on
`the precipitation reactions in 52 at.%
`nickel–48 at.% titanium alloys by
`various metallographic techniques. They
`observed a precipitation sequence of
`Ni14Ti11 → Ni3Ti2 → Ni3Ti at tempera-
`tures between 500°C and 800°C and for
`times up to 10,000 hours and presented
`their data in a time-temperature-
`transformation (TTT) diagram. A similar
`approach was demonstrated for the Af
`changes of Ni50.8Ti49.2 wire that was
`cold drawn 42% and continuously
`straightened, as described previously.
`The wire had an initial Af of 11°C, which
`is appropriate for many endovascular
`guidewire applications.17,18 Figure 7
`shows the Af-TTT diagram, where each
`c-curve represents the loci of time and
`temperatures that produce a constant Af
`after thermal treatments between 300°C
`and 600°C and for times up to 180 min.
`This figure demonstrates that there is a
`maximum in the precipitation reaction
`at about 450°C (i.e., the Af increases
`most rapidly after heat treatments at
`450°C). From an industrial standpoint,
`this indicates that slight variations in
`heat-treatment times at these tempera-
`tures may greatly affect the Af. On
`the other hand, heat treatments around
`300°C or 500°C tend to be less affected
`by time fluctuations.
` The Af changes shown in this figure
`
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`
`a
`
`b
`
`c
`
`Ni3Ti
`
`0.1 µm
`
`0.1 µm
`
`Ni14Ti11
`
`0.5 µm
`
`Figure 6. The microstructures of (a) 40%
`cold drawn, (b) fully annealed, and (c) over-
`aged Ni50.8Ti49.2 alloys. The cold-worked
`structure consists of deformed martensite
`and a high density of dislocations. Annealed
`austenite forms well-developed equiaxed
`grain structures. The aged microstructure
`consists of Ni14Ti11 and Ni3Ti precipitates.
`
`worked or fully annealed conditions.
`To optimize superelastic and shape-
`memory performance, the material must
`be partially heat treated after the final
`cold-working step.
`SHAPE SETTING
`
`In general, shape setting involves a
`
`combination of strain, temperature, and
`time to optimize the “remembered”
`shape. For example, the stent shown
`in Figure 1 was laser cut from Nitinol
`microtubing, shape set by expansion
`on a mandrel of a specific diameter,
`and then heat treated. Nitinol wires and
`tubes are continuously strain annealed
`to ensure that the entire length has
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`
`50
`
`1600
`
`1400
`
`1200
`
`1000
`
`Ultimate Tensile Stress (MPa)
`
`800
`
`0
`
`100
`Time (minutes)
`Figure 8. The effect of aging time and temperature on the ultimate tensile strength. Aging temperatures
`between 350°C and 450°C tend to increase the tensile strength due to precipitation hardening. Between
`500°C and 600°C, annealing effects dominate with a dramatic decrease in the strength.
`
`150
`
`200
`
`are due to precipitation reactions as
`discussed by Nishida et al.21 As the
`volume fraction of these precipitates
`increases, the matrix becomes enriched
`in titanium and the Af increases accord-
`ing to the relationship shown in Figure
`2. These shifts of composition strongly
`affect the transformation temperatures
`even though the overall composition
`of the material remains unchanged.
`Figure 6c shows the microstructure of
`an overaged sample with a high volume
`fraction of nickel-rich precipitates.
` These Af-TTT diagrams can be used
`as tools to tune the transformation
`temperatures of the Nitinol products.
`For example, imagine that the stent
`discussed previously was heat treated
`at 400°C for 30 min. with a resultant
`transformation temperature of 37°C. If
`the specification calls for a maximum Af
`of 30°C (just below body temperature),
`then the stent can be tuned to the proper
`Af at 515°C for 2–3 min. The Ni14Ti11
`precipitates that formed during the initial
`400°C treatments will re-solutionize at
`515°C, with a corresponding decrease
`in the transformation temperature as
`the nickel atoms diffuse back into the
`matrix. At longer aging times above
`500°C, the Ni3Ti2 phase forms, with a
`corresponding increase in Af.
`EFFECTS OF AGING
`TREATMENTS ON ULTIMATE
`TENSILE STRENGTH
` The wire used for the Af-TTT diagram
`had an as-drawn ultimate tensile strength
`
`(UTS) of 1,550 MPa, which was reduced
`to 1,400 MPa during the straightening
`process. Figure 8 shows that the aging
`treatments also have an effect on the
`UTS. For example, aging between
`300°C and 450°C increases the UTS,
`which demonstrates that the Ni14Ti11
`precipitates are effective barriers
`to dislocation motion and act to
`strengthen the matrix. At higher tem-
`peratures (500–600°C), however, the
`UTS decreases even though precipitates
`form during aging. The decrease in UTS
`at these higher temperatures reflects
`that the onset of recrystallization
`begins around 475°C and that the
`Ni3Ti2 precipitates are less effective in
`strengthening.
`
`References
`
`1. D. Stöckel, Min. Invas. Ther. & Allied Technol., 9
`(2000), pp. 81–88.
`2. T.G. Frank, W. Xu, and A. Cuschieri, Proceedings of
`the International Conference on Shape Memory and
`Superelastic Technologies, ed. S.M. Russell and A.R.
`Pelton (Pacific Grove, CA: International Organization
`on SMST, 2001), pp. 549–560.
`3. T. Duerig and M. Wholey, Min. Invas. Ther. & Allied
`Technol., 11 (2002), pp. 173–178.
`4. D. Stöckel, C. Bonsignore, and S. Duda, Min. Invas.
`Ther. & Allied Technol., 11 (2002), pp. 137–147.
`5. D. Stöckel, A.R. Pelton, and T. Duerig, Euro Rad.
`(to be published 2003).
`6. W. Buehler and F.E. Wang, Ocean Eng., 1 (1968),
`pp. 105–120.
`7. T.W. Duerig et al., eds., Engineering Aspects of Shape
`Memory Alloys (London: Butterworth-Heinemann
`Ltd., 1990).
`8. H. Funakubo, ed., Shape Memory Alloys (New York:
`Gordon and Breach Science Publishers, 1987).
`9. L.Mc. Schetky, “Shape Memory Alloys,” Scientific
`American, 241 (5) (1979), pp. 74–82.
`10. J. Perkins, ed., Shape Memory Effects in Alloys
`
`(New York: Plenum Press, 1975).
`11. T.W. Duerig and A.R. Pelton, “Ti-Ni Shape Memory
`Alloys,” Materials Properties Handbook: Titanium
`Alloys, ed. R. Boyer, G. Welsch, and E.W. Collings
`(Materials Park, OH: ASM International, 1994), pp.
`1035–1048.
`12. ASTM F 2004-00 Test Method for Transformation
`Temperature of Nickel-Titanium Alloys by Thermal
`Analysis (West Conshohocken, PA: ASTM, 2002).
`13. C.M. Jackson, H.J. Wagner, and R.J. Wasilewski,
`NASA–SP 5110 (Washington, D.C.: DoE Technology
`Utilization Office, 1972).
`14. ASTM F 2063-00 Standard Specification for Wrought
`Nickel-Titanium Shape Memory Alloys for Medical
`Devices and Surgical Implants (West Conshohocken,
`PA: ASTM, 2002).
`15. S.M. Russell and D.E. Hodgson, Min. Invas. Ther. &
`Allied Technol., 9 (2000), pp. 61-65.
`16. S.M. Russell, Proceedings of the International
`Conference on Shape Memory and Superelastic
`Technologies, ed. S.M. Russell and A.R. Pelton (Pacific
`Grove, CA: International Organization on SMST,
`2001), pp. 1–10.
`17. A.R. Pelton, J. DiCello, and S. Miyazaki, Min. Invas.
`Ther. & Allied Technol., 9 (2000), pp. 107–118.
`18. A.R. Pelton, J. DiCello, and S. Miyazaki, Proceedings
`of the International Conference on Shape Memory and
`Superelastic Technologies, ed. S.M. Russell and A.R.
`Pelton (Pacific Grove, CA: International Organization
`on SMST, 2001), pp. 361–374.
`19. T.W. Duerig and R. Zadno, Engineering Aspects of
`Shape Memory Alloys, ed. T.W. Duerig et al. (London:
`Butterworth-Heinemann Ltd., 1990), pp. 369–393.
`20. S. Miyazaki, Engineering Aspects of Shape Memory
`Alloys, ed. T.W. Duerig et al. (London: Butterworth-
`Heinemann Ltd., 1990), pp. 394–413.
`21. M. Nishida, C.M. Wayman, and T. Honma, Met.
`Trans. A, 17A (1986), pp. 1505–1515.
`
`Alan R. Pelton, Scott M. Russell, and John
`DiCello are with Nitinol Devices & Components
`in Fremont, California.
`
`For more information, contact A.R. Pelton, Nitinol
`Devices & Components, a Johnson & Johnson
`Company, 47533 Westinghouse Drive, Fremont,
`California 94539 USA; (510) 623-6996; fax (510)
`623-6808; e-mail apelton@ndcus.jnj.com.
`
`
`2003 May • JOM
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