3064
`SCIENTIFIC OFFPRINTS
`AMERICAN
`
`Shape-Memory Alloys
`
`by L. McDonald Schetky
`
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`SCIENTIFIC
`AMERICAN
`NOVEMBER
`VOL. 241. NO. 5
`
`1979
`PP 74-82
`
`I -
`
`•
`
`PUBLISHED BY W. H. FREEMAN AND COMPANY 660 MARKET STREET, SAN FRANCISCO , CALIFORNIA 94104
`
`.
`
`Copyright © 1979 by Scientific American. Inc. All rights reserved . Printed in the U.S.A. No part of this offprint may be reproduced by any mechanical, photo(cid:173)
`graphIc or electronIc process. or In the form of a phonographIc recording. nor may It be stored In a retrieval system. transmitted or otherwise copied
`for public or private use without written permissIon of the publisher. The trademark and tradename "SCIENTIFIC AMERICAN" and the distinctive logo(cid:173)
`type pertaining thereto are the sole property of and are regIstered under the name of. and are used herein under license from. SCIentific American. Inc.
`
`Lombard Exhibit 1029, p. 1
`
`

`

`Shape-Memory Alloys
`
`If one of these new alloys has been formed into a shape at a certain
`temperature and is then deformed at another temperature, it can
`"remember" the original shape. The effect has many applications
`
`by 1. McDonald Schetky
`
`Metals are characterized by such
`
`tensile
`physical qualities as
`strength, ductility, malleability
`and conductivity. To these, in the case of
`a new family of alloys, one can now add
`the anthropomorphic qualities of mem(cid:173)
`ory and trainability. The new alloys ex(cid:173)
`hibit what is called the shape-memory
`effect. If such alloys are plastically de(cid:173)
`formed at one temperature, they will
`completely recover their original shape
`on being raised to a higher temperature.
`In recovering their shape the alloys can
`produce a displacement or a force, or a
`combination of the two, as a function of
`temperature. Because of these novel and
`remarkable properties shape-memory
`alloys are helping to solve a wide va(cid:173)
`riety of problems. In one well-devel(cid:173)
`oped application shape-memory alloys
`provide simple and virtually leakproof
`couplings for pneumatic and hydraulic
`lines. They are also serving as tight seals
`and couplings in electronic assemblies.
`Most recently the alloys have been ex(cid:173)
`ploited in mechanical and electrome(cid:173)
`chanical control systems to provide, for
`example, a precise mechanical response
`to small and repeated
`temperature
`changes. Several promising applications
`
`are being tested in medicine. More re(cid:173)
`mote, but nonetheless tantalizing, is the
`possibility of harnessing shape-memory
`alloys to convert low-grade heat (such
`as the heat of water at the surface of the
`ocean) into mechanical energy.
`In order for an alloy to exhibit the
`shape-memory effect it must have a
`crystal structure that can shift into
`the configuration known as martensite
`when it is subjected to a certain temper(cid:173)
`ature or stress and then shift out ofk In
`a simple example a wire of shape-mem(cid:173)
`ory alloy might be bent at room temper(cid:173)
`ature into the form of a four-leaf clover.
`Then the wire is heated until its crystal
`structure assumes a high-temperature
`configuration called the beta or parent
`phase. Next the wire is rapidly cooled so
`that the atoms in the metal rearrange
`themselves into the crystal form of mar(cid:173)
`tensite. One can now bend or twist the
`wire into any other form. If the wire is
`later heated to a temperature above that
`at which the martensite reverts to the
`parent phase, there is an orderly shift of
`large groups of atoms that restores the
`or-iginal cloverleaf form. Since the mar(cid:173)
`tensite transformation is essential for
`the shape-memory effect, alloys that ex-
`
`hibit memory are also known as mar(cid:173)
`mem alloys.
`The shape-memory effect had been
`noted as early as 1938, when Alden B.
`Greninger of Harvard University and V.
`G. Mooradian of the Massachusetts In(cid:173)
`stitute of Technology showed that the
`martensite phase in brass (an alloy of
`copper and zinc) could be made to form
`and disappear with a change in tempera(cid:173)
`ture. At about the same time G. V. Kur(cid:173)
`dyumov, a Russian metallurgist noted
`for his early work on the crystallogra(cid:173)
`phy of martensites, particularly in steel,
`studied the phase relations in brass be(cid:173)
`tween the high-temperature beta phase
`and the martensite formed by rapid
`cooling. Later Thomas A. Read and his
`associates at the University of Illinois
`investigated the shape-memory effect in
`gold-cadmium alloys and demonstrat(cid:173)
`ed . the forces that could be developed
`by the phase transitions. Alloys as di(cid:173)
`verse as iron-platinum, indium-cadmi(cid:173)
`um, iron-nickel, nickel-aluminum and
`stainless steel have been observed to ex(cid:173)
`hibit shape memory in varying degrees.
`It was not until 1962, however, that
`the phenomenon came to worldwide at(cid:173)
`tention with the announcement of shape
`
`SPONTANEOUS UNFOLDING of a wire hemisphere that could
`serve as a small antenna for a spacecraft is shown in this sequence of
`four photographs. The antenna is made of an alloy of nickel and tita.
`nium (Nitinol) that exhibits the shape-memory effect. In the photo-
`2
`
`graph at the left the fuUy formed antenna has been crushed at room
`temperature into a tight ball less than five centimeters across. As the
`temperature of the mass of wire is increased the antenna gradually
`unfolds. When the temperature reaches 77 degrees Celsius, the struc-
`
`Lombard Exhibit 1029, p. 2
`
`

`

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`ture assumes its original configuration (top right). The diameter of
`the unfolded antenna is about 25 centimeters. The restoration of the
`antenna's shape coincides with the disappearance of crystals of mar(cid:173)
`tensite in the material and their replacement by crystals of austenite,
`
`which has a simpler crystal structure. The crystallographic history of
`the sequence is illustrated on the next page. The antenna was designed
`by the Goodyear Aerospace Corporation. Although it has never seen
`actual service, it vividly demonstrates the properties of such alloys.
`3
`
`Lombard Exhibit 1029, p. 3
`
`

`

`

`

`

`

`a
`
`b
`
`I (110) PLANE
`
`ELEVATED-TEMPERATURE PHASE
`(BETA PHASE)
`
`8 2 SUPERLATTICE
`
`ORTHORHOMBIC
`MARTENSITE
`
`c
`
`8 2 SUPERLATTICE
`STACKING OF (110) PLANES
`
`d
`
`e
`
`of Nitinol triggered the release of three
`instrument booms. The release had to be
`rapid and reliable in order to avoid any
`dynamic imbalance that might affect the
`stability of the spinning satellite itself.
`In another early application Nitinol
`alloys solved the problem of coupling
`hydraulic-fluid lines in the F-14 jet fight(cid:173)
`er built by the Grumman Aerospace
`Corporation. Grumman engineers were
`seeking an alternative to the difficult
`task of brazing lines that lie close to the
`aircraft's aluminum skin. The Raychem
`Corporation, which had had wide expe(cid:173)
`rience in heat-shrinkable plastics, pro(cid:173)
`posed a coupling in which a Nitinol al(cid:173)
`loy was shrunk to a tight fit by being
`raised above its martensite-transforma(cid:173)
`tion temperature. The coupling would
`have to maintain its gripping integrity
`down to -120 degrees Celsius, so that
`the alloy chosen would have to have a
`transformation temperature in the cryo(cid:173)
`genic region.
`The transformation temperature of
`the Nitinol family of alloys can be
`manipulated over a remarkably wide
`range, from -273 to 100 degrees C., by
`altering the nickel-titanium ratio and
`by adding small amounts of other ele(cid:173)
`ments. For the hydraulic coupling an al(cid:173)
`loy with a transition temperature below
`-120 degrees C. was found. The cou(cid:173)
`pling, in the form of a sleeve, is ma(cid:173)
`chined at ordinary shop temperature to
`have an inner diameter about 4 percent
`smaller than the outer diameter of the
`tubes it is designed to join. The coupling
`is then cooled to below the temperature
`at which martensite forms. While it is at
`this low temperature it is forcibly ex(cid:173)
`panded to a diameter 4 percent greater
`than the tube diameter, for an overall
`internal strain of about 8 percent. Still at
`a cryogenic temperature to maintain the
`martensite phase, the coupling is fitted
`over the ends of the tubes that are to be
`joined. When it is warmed to room tem(cid:173)
`perature, the martensite is replaced by
`
`ORTHORHOMBIC CRYSTALS of mar(cid:173)
`teosite form spontaneously in a great variety
`of alloy systems when they are cooled rapidly
`from an elevated temperature. In the beta, or
`high-temperature, phase shape-memory al(cid:173)
`loys usually take the form of disordered body(cid:173)
`centered-cubic crystals (a). As the tempera(cid:173)
`ture is lowered one of the atomic species (co/(cid:173)
`or) in the alloy takes up a preferred site in the
`crystal cell. One such structure is a B2 super(cid:173)
`lattice (b). The marteosite traosition is best
`seen by coosidering an array of B2 superlat(cid:173)
`tice crystals in which the planes designated
`(110) are stacked horizontally. Such a stack,
`made up of 10 (110) planes that consist al(cid:173)
`ternately of eight atoms and seven atoms, is
`shown in c. On cooling, the cubic arrange(cid:173)
`ment of the B2 superlattice is replaced by the
`orthorhombic structure of marteosite (d). The
`numbered atoms help to clarify the nature of
`the rearrangement. In a ternary alloy coosist(cid:173)
`iog of copper, zinc and aluminum the parent
`beta phase gives rise to variants of martensite
`offset from one another by 60 degrees (e).
`
`Lombard Exhibit 1029, p. 6
`
`

`

`

`

`

`

`give rise to large changes in the martens(cid:173)
`ite transformation, from values as low
`as -105 degrees C. to as high as 299
`degrees. For alloys that can be readily
`fabricated, however, the range is nar(cid:173)
`rower: roughly from - 100 degrees to
`100 degrees. For a given alloy the shape(cid:173)
`memory effect extends over a range of
`about 80 degrees. At higher tempera(cid:173)
`tures there is a tendency for the mar(cid:173)
`tensite to become unstable, so that the
`usual upper temperature for operation
`is 150 degrees. Some of the applications
`for Nitinol are also attractive for the
`copper-based ternary alloys: couplings,
`springs and actuators.
`In exploring new applications the Del(cid:173)
`ta company has developed a series of
`devices in which the shape-memory al(cid:173)
`loy serves as both a temperature sensor
`and an actuator. One such device is a
`thermostatic valve for individual radia(cid:173)
`tors in a home or office heating system.
`The temperature at which the valve
`opens can be set by turning a knob that
`alters the compression on a spring acting
`against a spring made of the memory
`alloy. The characteristic of the memory
`spring is such that the force it exerts in(cid:173)
`creases as its temperature rises. In crys(cid:173)
`tallographic terms, as the temperature
`increases an increasing fraction of the
`martensite is transformed into the par(cid:173)
`ent phase, which is associated with a
`memory of the spring in its expanded
`position.
`. .
`A similar memory spring can regulate .
`the clutch system that couples and un(cid:173)
`couples the radiator fan in an automo(cid:173)
`bile cooling system. Present devices op(cid:173)
`erate through a viscous-fluid clutch con(cid:173)
`trolled by a bimetallic spring. The mem(cid:173)
`ory-alloy clutch eliminates the need for
`fluid seals and provides a more flexible
`speed control. Automotive engineers
`are exploring the possibility of resorting
`to shape-memory systems to replace
`small electric motors, solenoid-actuated
`valves and various engine-control devic(cid:173)
`es. One promising application is a varia(cid:173)
`ble-throat or variable-jet carburetor to
`provide optimum fuel economy over a
`range of air and fuel temperatures.
`Each application involving shape(cid:173)
`memory actuators must take into ac(cid:173)
`count operating temperature along with
`the force and deflection wanted and
`whether the motion is to be linear or
`torsional. By working with a series of
`charts and graphs developed by the Del(cid:173)
`ta company a designer can determine
`for each particular alloy the amount of
`bias, or compensating, force necessary
`to have the shape-memory device oper(cid:173)
`ate at a specific temperature and deliver
`a given force. The number of devices
`proposed in patents is now approaching
`100 and is accelerating.
`
`Q ne final application of the shape(cid:173)
`
`memory effect deserves mention:
`the recovery of energy from waste heat
`or low-temperature water by the direct
`
`conversion of the heat into mechanical
`energy. Such a solid-state engine was
`considered soon after the discovery of
`the shape-memory effect in N itinol. The
`first patent issued to Buehler and David
`M. Goldstein for a shape-memory en(cid:173)
`gine in 1966 has been followed by what
`now seems an endless procession of en(cid:173)
`ergy devices. The first engine to really
`show the feasibility of generating useful
`amounts of energy was one invented by
`Ridgway Banks of the Lawrence Radia(cid:173)
`tion Laboratory of the University of
`California. Possibly because Banks has
`
`had professional training in music, his
`devices give something of the aesthetic
`pleasure of stringed' instruments.
`The theoretical efficiency of a solid(cid:173)
`state engine can be analyzed by taking
`into account the latent heat of transfor(cid:173)
`mation, the additional stress delivered
`for an additional change in temperature
`and the difference in free energy be(cid:173)
`tween the martensite phase and the ele(cid:173)
`vated-temperature phase. By means of
`such an analysis L. Delaey of the Uni(cid:173)
`versity of Louvain has arrived at a theo(cid:173)
`retical efficiency of 4 or 5 percent, which
`
`Ms TEMPERATURE (DEGREES C.)
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`PERCENT COPPER
`
`MARTENSITE.FORMATION TEMPERATURES, Jl." for ternary alloys of copper, zinc
`and aluminum can be made to vary over more tban 400 degrees C. as a result of only small
`cbanges in composition. Tbe sbape-memory alloys all lie in the copper.rich corner (c%r) of
`tbe triangle representing ternary mixtures. Tbe amount of aluminum varies from about 4 to 10
`percent, tbe amount of zinc from just under 10 to 28 percent. Balance in eacb case is copper.
`9
`
`Lombard Exhibit 1029, p. 9
`
`

`

`

`

`The Author
`L. McDONALD SCHETKY is tech(cid:173)
`nical director for metallurgy of the In(cid:173)
`ternational Copper Research Associa(cid:173)
`tion in New York. He was educated at
`the Rensselaer Polytechnic Institute,
`where he obtained his bachelor's degree
`in chemical engineering, his master's in
`metallurgical engineering and his Ph.D.
`in physical metallurgy. From 1953 to
`1956 he was on the research staff of the
`Massachusetts Institute of Technology '
`and served as director of materials re(cid:173)
`search at the M.l.T. Instrumentation La(cid:173)
`bortory. In 1956 he founded the Alloyd
`Corporation, a research and manufac(cid:173)
`turing company specializing in materi-
`
`als research. in his spare time Schetky
`operates a vineyard in Rhode Island that
`should produce its first vintage this year.
`
`Bibliography
`NITINOL HEAT ENGINES. Ridgway Banks
`in Shape Memory E;ffecls in Alloys, ed(cid:173)
`ited by Jeff Perkins. Plenum Press,
`1975.
`SHAPE MEMORY EFfECTS AND ApPLICA(cid:173)
`TIONS: AN OVERVIEW. Walter S. Owen
`in Shape Memory E;ffi!cls in Alloys, ed(cid:173)
`ited by Jeff Perkins. Plenum Press,
`1975.
`
`Lombard Exhibit 1029, p. 11
`
`