Nickel-Titanium Memory Metal A "Smart" Material Exhibiting a Solid-state Phase Change and Superelasticity Kathleen R. C. Gisser, Margret J. Geselbracht, Ann Cappellari, Lynn Hunsberger, and Arthur 6. ~llis' Department of Chemistry, University of Wisconsin-Madison, Madison. WI 53706 John Perepezko Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706 George C. Lisensky Department of Chemistry, Beloit College, Beloit, WI 53511 Phase changes commonly are presented in general chemistry courses a s interconversions of solids, liquids, and gases, as in, for example, ice melting to liquid water and solid carbon dioxide (dry ice) subliming into the gaseous state. There are many technologically important classes of phase changes, however, that occur exclusively in the solid state, In one class, the so.called martensitic transformations, changes in temperature can cause a revemible shift of atomic positions. As a result, the solid exhibits two phases with different structures and strikingly different physical properties. In any martensitie phase transformation, the high-temperature phase is called austenite, and the low-temperature phase is called martensite, regardless of the crystal structures of the phases. This nomenclature originally was derived from the martensitic phase transformation in steel. Amartensitic phase transformation is also responsible for the remarkable shapememorv effect found in a few select allovs. One the most interesting solids the memory effect is memory metal, a n alloy of nickel and titanium. Memory metal is sometimes called Nitinol, which is short for Nickel Titanium Naval Ordnance Laboratory and acknowledges the site of its discovery in 1965 ( I ) . The relatively low cost of the alloy and its ready availability make it ideal for lecture demonstrations and laboratory experiments in general chemistry courses. In addition to illustrating the martensitic phase change, the ability of NiTi to remember its shape under certain experimental conditions makes it an entry point for discuss~ng"smart" materials. "Smart" materials have the capability to sense changes in their environment and respond to the changes in a pre-programmed way These new high-tech solids are being used in a variety of artistic, medical, and engineering applications. In this paper, we describe several simple experiments that illustrate the shape-memory, mechanical, and acoustic properties of NiTi. These properties are discussed in terms of the crystallography of NiTi and general thermodynamic principles of phase tranformations.
if
'Author to whom correspondence should be addressed. 'Other suppliers include Mondotronics, San Leandro, CA; Innovative Technology International,Beltsville. MD; Flinn Scientific,Batavia. IL; and Edmund Scientific Co., Barrington, NJ (packages of two wires). %ending a metal can result in work hardening. Adescription of this process may be found in "Mechanical Propenies of Metals";J. Chern. Educ. 1994, 71,254.
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Demonstrations and Laboratory Experiments Samples of NiTi are available commercially as wires and small rods from several sources. For our experiments we obtained samples from Shape Memory Applications, Sunnyvale, CA. Samples ofNiTi wire (3 in. inlengthby 0.01 in. in diameter) with transition temperatures between 30 to 50 "C, were $0.20 each in lots of 3000; rods of differingstoichiometry (2.5 in. in length by 0.10 in. in diameter) were $3.00 each and available in both the low-temperature and high-temperature phase at room temperature.2 The rods a, reusable indefinitely, and the wires canbe used several times if they are not bent too far out of shape.s Shape Recovery As an initial experiment, bend a sample of low-temperature phase martensite in wire. par a particularly dramatic demonstration, coil the sample around a finger to form a helical shape. Then, holding one end of the coiled sample, either immerse the remainder of the sample in hot water or place it in the hot air stream of a heat g i n or hair dryer. As the sample is heated and transformed into the austenite phase, it straightens back into the linear shape it had been -trained=to To demonstrate this effedin a large ledure room, a petri dish hot water may be placed on an overhead projector so that the straightening of the wire is readily visible. Alternativelv. the coiled wire may he taped onto-the overhead project& (to prevent the wire from blowing away), and a heat gun may be used to straighten the wire. In a laboratory setting, ~ t f i e n t sCan do this experiment with their own strip of Ni'll.
Resistive heating may be used in lieu of hot air or water to demonstrate the shape-memory effect. For this experiment, conned a coiled NiTi wire h a 9V battery with two alligator clips soldered to a 9V battery snap (Radio Shack, 270-3251, The current provided by the battery heats the NiTi, thereby transforming it back into the austenite phase. Caution: Due to the large current drain, do not maintain the connection for more than 10s. If the battery becomes hot, disconnect the wire immediately. Annealing a New Shape The ability to re-train the NiTi wire to remember a new shape can be shown using a candle flame. Grasp the two ends of the wire, and place the middle of the wire in the center of the candle flame. Bend the wire into a V-shave as soon as the hot wirc readily bends, at whch p i n t it should be removed immcdiatelv from the flame. The wlre will cool off in a few seconds by waving it in the air or blowing on it. After the wire has returned to room temperature, the ex-
periment described above can be repeated. The wire can be coiled, but upon heatinn with hot water or the heat m,it will now re&m to the V-shape, not the linear s h a p e r ~ o r e complicated shapes usually require a mold, since the heat treatment will cause the wire to try to recover its original shape before it relaxes into the new shape. Mechanical Properties of the Two Phases
The thicker NiTi rods can be used to illustrate the critical role that atomic structure plays in defining mechanical properties. Arod that is in the high temperature austenite phase at room temperature is extremely difficult to bend into a V-shape. In contrast, a rod that is in the low temperature martensite phase at room temperature is quite flexible. Of course, the bent rod can be placed in hot water or in front of a heat gun to restore its linear shape. Interconversion of the austenite and martensite phases can be monitored by the effect of temperature on the mechanical properties of the rods. Thus, if the inflexible rod that is in the austenite phase a t room temperature is cooled by immersing it into liquid nitrogen and is then removed, it can be bent into a V-shape as long as it is cold. Caution: use appmpriate gloves or tongs to protect fingers
As the rod warms back to room temperature, it will return to a linear shaoe. Similarlv.. if a rod that is in the martensite phase at room temperature is immersed in very hot water. it will become inflexible when hot and flexible as it cools back into the martensite phase. Another difference in the mechanical properties is the comparative hardness of the two phases. The end of an austenitic rod will scratch the martensite, but a martensitic rod will not scratch the surface of the austenite, demonstrating the hardness of the austenite compared to the martensite.
.
Acoustic Properties of the Two Phases
Another fascinating characteristic difference in the two phases of NiTi is revealed upon dropping a rod of each
phase at room temperature. If they are dropped from a height of a meter or so onto a hard, nonvibrating surface like concrete or a slate laboratory bench, the austenite samples will ring like a bell and the martensite samples will yield a dull thud! A sample that contains both phases will yield a sound that is intermediate between a ring and a thud. Mapping the Phase Dansfonation The marked acoustical difference in the two phases permits students to determine the approximate temperature of the phase transformation for the NiTi rods. Workine in teams,-students can take a rod that is in the martensite phase a t room temperature and heat it to different temperatures using a water bath. Suspending the rod on a thread (like a tea bag) allows it to be quickly retrieved from the water. After thermal equilibrium has been attained, typically in a minute or two, the rod is removed from the bath, droooed. and the audio sienal (rine. thud. or intermediate souyd) is noted. For belt result:; drop the rod from a roughly reproducible initial position that is approximately parallel to the surface it will strike, and from a height that ensures an easily rewenizable sound. In addition. the rod should be returned to 'the water bath quickly after taking each data point (vide infra). students can first determine the approximate transition temperature by taking data points at 10 'C intervals while progressively heating the sample fmm mom temperature (-25 'CI up to about 85 'C. The audio signal converts from a thud to a ring as temperature increases, and this audio signal may be graphed as a function of temperature. The audio characteristics can then be measured on the reverse cycle as the temperature of the water bath is reduced. During the cooling cycle, readings can be taken at larger intervals where the audio signal is constant, and smaller intervals (-5'C) i n the ohase-chanee reeion to refine the transition temperature range on cooling. In some samples, the heating curve may be displaced toward higher temperature from the woling curve, as shown in Figure 1, a phenomenon known as hysteresis. However, if the heating and cooling curves are not separated by at least 10'C. it may not be possible to control the temperature well &oughto reveaithe bysteresis4,5.
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Discussion All of the phenomena illustrated above can be interpreted in terms of the atomic structure, microstructure, and composition of NiTi. Structural Cycle
The origin ofthe shape memory effect lies in the relationship between the structures of the high- and low-temperat u 6 ~hases.These structures have been investieated in detaii using X-ray diffraction (2). In Figure 2a the structure of austenite is shown from two different views. The austenite phase adopts the CsCl structure shown at the
H Temperature Figure 1. The maltensitic phase transformation in Nix is evident in a number of physical propelties. For example, the general characteris-
tics of the magnetic susceptibility as a function of temperature show four transition temperatures:4,thestart ofthe maltensite-to-austenite transition on heating; 4, the end of the maltensite-to-austenite transition on heating; M,, the start ofthe austenite-to-martensitetransition on coolino: and M.. the end of the austenite-to-martensitetransition on molino.'~he hietino and molino culves are often seoarated ~-~ by as much as30 'c. T ~ I separat S on, or hysteres s, allows ootn the a~steniteand manensite to be aaessiole at tne same remperatdre (marked HI aependlng Jpon wnelher the sample was prev~oJsly heated or cooled. ~
~
~
7~
~
~~~
-~~
~~~~~~~
~
~~
4The shape-memory effect in samples that show little hysteresis may be produced by the transition between the austenite and the so-called "R-" (rhombohedral)phase. The R-phase is a stable intermediate phase of NiTi, that can grow during the austenite-to-martensite transition. 5Anv samoies that are found to have a sianificant disolacement in the hesting and waling curves lend themseives to the fdilowing striking experiment: There will be temperatures in the middle of the tranwherein sition region (one such temperature is labeled "ti"in Fig. I), the sample will thud if it is first cooled into the martensite phase, then heated to temperature "Hand dropped. In contrast, if the sample is first heated into the austenite phase, then cooled in the bath totemperature "H and dropped, it will ring. Volume 71 Number 4 April 1994
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b,
%-.-I Martensite
Austenite
F~gure2 (a)Two d fferentvlews ofthe CsCl stmure adopted by N T n tne austenlte phase The common depmon IS at the eft.showmg a cLbe of one type of atom w tn tne otner atom at the center of the cube. At the right, the structure is depicted as a series of stacked planes, with the cube also shown for comparison. The arrows indicate the sliding of the planes that lead to changes in the atomic positions durina the austenite to martensite Dhase transition. Ib) The structures &the austenite and martensite phases are represented by a two-dimensional projection from above the middle layer (larger spheres) and bottom layer (smaller spheres) of the stacked planes shown in part (a).An additional shearing mechanism (notshown in A) has changed the angles in the structure of the martensite phase from 90' to 96'. left, which can be described as having Ni atoms at each corner of the cubes and Ti atoms in the centers of the cubes, or vice versa. (In Figure 2, the white and black spheres represent the two kinds of atoms). Smaller spheres lie behind the plane of the paper in this view. If the CsCl cube is rotated 45', i.e., it is balanced on an edge, the structure of austenite also may be represented as a stack of planes, as shown at the right in Figure 2a. This view is particularly useful in describing the relationship of the structure of austenite to the structure of martensite. Alternately, the stack of planes can be reduced to a two-dimensional projection from above. Two of the layers in the stack, are depicted in this manner at the left of Figure 2b. In this view. laree spheres represent the atoms in the middle laye* of ~:gure 2a, and small spheres represent the atoms in the next layer above or below. The structural transformation from austenite to martensite involves slight shifts in atomic positions. During the transformation, the rectangular planes of the austenite structure slide relative to one another (indicated by arrows on the right of Fig. 2a), and deform by shearing, changing the 9W angles to about 9V.The two-dimensional projection of the martensitic structure, at the right in Figure 2b, shows the new atomic positions after the shifting and shearing motions. Although the motions involved in the transformation from austenite to martensite are relatively simple, there are 24 different ways to cany out the transformation. To understand the origin of these 24 different variants, the directions of the shifting and shearing during the transformation are shown in three dimensions in Figure 3a. The planes can shift relative to one another in each of two directions (marked by the dark arrows), and shear in each of two directions (marked by the double-headed open arrows). In addition, there are six equivalent face-diagonal planes in the CsCl structure. hiehliehted in erav in Fieure 5b. Each of these six sets of planes can sh& 'i two dTrections and shear in two directions. The result is 6 x 2 x 2 or
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Journal of Chemical Education
Figure 3. (a)A total of four martensite variants may grow from each plane passing through a face diagonal in the CsCl structure. The planes may shift in the direction of either of the dashed arrows to offsetthe stacking of the planes. The planes may shear in the direction of either of the dark or the light arrows to destroy the 90' angles in the planes. (b) There are six equivalent planes passing through face diagonals in the CsCl structure. Thus, a total of 6 x 2 x 2 = 24 different variants may grow from the planes.
24 different ways to transform the structure into martensite. The phase transformation and shape memory cycle is presented in Figure 4. At the top of the figure is the two-dimensional representation of the structure of austenite. The matrix of rectangles shown below this representation depicts how the structural units pack to fill space on a larger dimension. As a sample of austenite is cooled through the phase transition temperature, the structure transforms to martensite. To reflect the 24 different orientations of martensite (called "variants"), the long-range structure of martensite is represented in two dimensions in Figure 4 as a set of tilted parallelograms. These parallelomams can be packed toeether so that the total volume of tce solid does not change-significantly during the phase change. Note that the shape of the matrix of rectangles and the matrix of tilted parallelograms is roughly the same. The density differencein the two phases is less than 0.5% (the martensite is slightly denser) (3), and thus cooling NiTi from austenite to martensite does not change the shape of the metal. The NiTi wire, as purchased, is tmicallv in the low temperature martensite-phase at rook&mp&-ature. The wire has been trained previously to remember a linear shape in the austenite phase. When the NiTi is bent or coiled in the low-tem~eraturemartensite phase. the effect on the microstructurk is a reorientation if the variants corresponding to a macroscopic change in:shape. This is shown in the
-
L
Bend
Martensite
Martensite
Figure 5. The effect of nickel mncentration on the transition temperature, adapted from ref. 7.This is a compilation of data by several researchers, who measured the composition and transition temperatures by differentmethods. displacement of the heating and cooling curves will occur, although it may be small. Annealing to Change the Shape
Figure 4. The struitural features of NiT that give rise to the shapememory effect. The cycle starts with the N i l in the austenite phase shown at the too of the fioure. As the NiTi is cooled. aoina clockwise around Into the marlenslte ,~ oroceeds ~ ~the cvcie - . tr&sformat~on ,- - -the ~~ ~ ~ The dlagonal panes slde past eacn other (as shown in rne wper r ghr of F!g. 2a) ana deforms ghly to a parallelogram in two aimens.ons. In tnis proection, two dinerent y or entea martenslte var ants. one angled to me left,an0 one lo me right, may oe arawn. When the martensite is bent, the variants can reorient from leftto right to relieve the stress. When the N i l is heated, the lowest energy pathway that returns the atoms to their original positions and maintain the ordering of the atoms with nickels surrounded exclusively by titaniums and vice versa.
- -
~
~
~~
~
~
~~~~
~~~~~~
lower leR of Fieure 4 as the inclination of most of the Darallelograms to common direction. This reversible mehanism for accommodating stress distinguishes NiTi from most metals, in which similar stress would introduce defects into the crvstal structure. or cause lanes of atoms to slip past each ;her, permaner;t?y defoAing the metal3. In the last step of the shape-memory cycle, heat is used to transform the martensite back into the austenite ~ h a s e . The atoms recover their initial positions, and the initial macroscopic shape of the sample is restored. Although several pathways back to the austenite structure exist, only the lowest energy pathway restores the initial ordered CsCl structure in which Ni atoms are surrounded exclusively by Ti atoms and vice-versa. Other paths would produce higher-energy structures in which Ni and Ti atoms have a mixture of Ni and Ti atoms as nearest neighbors in the crystal lattice. The enthalpy associated with the phase change is relatively small, but typical for a solid state transformation: the conversion of austenite to martensite is modestly exothermic with an enthalpy change on the order of only 2 kJlmol(4).
a
Hysteresis
The hysteresis reflects the fact that one solid phase needs to grow within a lattice of the other. Growth of the new phase creates elastic strain in the lattice around it, adding to the chemical free energy necessary for enlarging the grain of the new phase (5).Thus, no matter how slowly the conversion takes place, this thermodynamically-based
As purchased, samples of NiTi are polycrystalline, meaning that regions of crystallinity on the order of microns in size (occasionally as large as millimeters) are separated from one another by grain boundaries. Within the grains, the nickel and titanium atoms are arranged with almost perfect order. However, occasional mistakes in the packing, such as dislocations and other defects, may occur (6). Since the nickel and titanium atoms must return to exactly the same position each time the NiTi is heated into the austenite phase, the configuration of defects in this phase effectively pins the austenite into a given shape. To give the NiTi a new shape to remember requires substantial energy, provided, for example, by heating the NiTi in a candle flame while it is constrained in the new position. During the annealing (heating) process, the atoms surrounding the defects gain enough enerm to relax into lower energy contiwraijons, and ;his new.configuration of defectz effectively pins the austenite into its new wsition. The gentle h e a t 6 used to return to the austenite phase from the martensite phase does not provide enough energy to allow the defects to readjust.
NiTi has been prepared traditionally by heating the elet ments together above the NiTi meltiw .. ~ . o i n (120&1300 'C, depeiding on compositionl. Exclusion of oxygen in critical during - the synthesis because ofthe ease with which Ti oxidizes. NiTi can tolerate small deviations in chemical commsition around the 1:lstoichiometry before the shape memory effectis lost. A more accurate representation of NiTi within this range is Ni,Til .,wherein the shapememory effect has beenobsewed when x lies between 0.47 and 0.51. Ilowever, the temperature at which the martensitic phase change occurs in Nin is a sensitive function of stoichiometry. As indicated in F i e 5, variations of a few percent around the equiatomic point in NiiTi,, cause substantial changes in the temperature of the phase transformation (7). One of the reasons that the transition temperature begins to fall is that small domains of other begin to precipitate as the composition deviates from 1:l (x = 0.50). In addition. small aiantities of i m ~ u r i"t vatoms mav be present that also L a y influence the transition temperature. Tuning of the transition temperature to as high as several hundred degrees has been accomplished by partial
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tension
Figure 6. The simultaneous occurrence of tension and compression when a metal rod is bent. or complete substitution of Ni with Pd or Pt (8). Conversely, substitution of Ni with a few percent of Co or Fe substantially reduces the transition temperature (9).The result of this strong dependence of transition temperature upon composition is that samples of memory metal (such as the rods used above) can be made that are in either the martensite or the austenite phase at room temperature. Mechanical Properties and Superelasticity The CsCl austenite structure is relatively rigid. In contrast, the ability to re-orient variants of the martensite phase imparts mechanical flexibility, as noted above. However, factors other than a change in temperature cause the phase transformation to occur. Under certain conditions, the austenite phase may be transformed mechanically into the martens& phase, and bewme elastic; i.e., when the stress is removed, the martensite phase will transform back to the austenite phase and the NiTi will return to its 1'1 is pseudoelasticity and not elasticity because the strain, or fractional change in length of the wiie under tension or compression, is not quite linear with the amount of tension or compression that is applied. '111 the case of the martensitelaustenite transformation, a modified form of the Clausius-Clapeyron equation may be used to determine the change in the maltensite stalt temperature, M,, based on the applied stress, a, and the strain. E (the fractionalchange in length): daIdM, =-AH,%. The analogous equation in terms of pressure and volume changes would be dPld T= AHRAV.
undeformed shape. This mechanical property is sometimes known as "pseudoela~ticity"~, or more specifically "superelasticity", and many NiTi applications are based on it. Superelasticity may be understood as a solid-state application of Le Ch&telier'sprinciple, which says that applying pressure will favor formation of the denser phase: Applying mechanical pressure to NiTi in the austenite form will cause the transformation to the denser martensite phase to occur without a change in temperature. This mechanically-induced pressure, called stress by engineers, is a force exerted over an area of the material, and thus has units of pressure. Both tensile stress, in which the atoms are pulled apart (by applying tension), and compressive stress, in which the atoms are pushed together (by compression), can o m . When a material is bent, both kinds of stress occur, as shown in Figure 6. The conversion under pressure of the lower density, less stable austenite structure to the higher density, more stable martensite structure is roughly analogous to the transformation under pressure of low density, less stable gaseous water to the denser, more stable liquid form of water. In accord with Le CMtelier's principle, application of pressure will favor formation of the denser martensite phase (Fig. 7), even though the difference in densities of the two phases is less than 0.5%. Another way to visualize this relationship is with the traditional phase diagram for water that is shown in most general chemistry texts. On the pressure-temperature phase diagram, lines with positive slopes (like that representing the gas-liquid water equilibrium) indicate that an increase in pressure at a given temperature will raise the temperature at which interconversion of the two phases occurs (the boiling point, for example),permitting wnversion of some of the gas to the liquid state. The ClausiusClapeyron equation can be used to quantify the effect. Similarly, bending a sample of NiTi in the austenite form will cause an increase in the martensitic start temperature (M., Fig. 1) in the region of the solid that is being compressed by the bending motion. If the increase in M, is large enough, calculable with the Clausius-Clapeyron equation7, some of the austenite will be converted to martensite. Releasing the stress causes the martensite to transform back into austenite with restoration of the original shape.
heat
Suggested Uses of NiTi a higher density lower volume
heat
+
martensite
lower density higher volume
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Fishing hook that straightens when heated for easy removal For spies: send ordinary looking wire, pop in hot water for message
austenite
Siding and roofing on houses so that sun repairs baseball damage Cookware for which heating repairs dents Meat thermometer that sticks out when done
higher density lower volume
t
as P , equilbriurn shifts
lower density higher volume
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2
Fioure 7.The awlication of of - - - - - will cause the - austenite --- - form - - ~- oressure N?-G transform into martenslte at IemperatLres aoove where tne transformation wo~looccLr at amb en1 pressxe. T n ~ 1ss anaogous to the ooservatlon that tne boi ing point of water r ses Lnoer con0 lions of applied uniform pressure 77
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Journal of Chemical Education
Jewelry Clamps and locks Fire detectors Safety device for household irons Windchimes made from austenitic rods Malleable "ChemistryAction Figures" made from maltensite Moving sculpture
~
Levers Taken from a list of student-suggestedapplications at the University of WisWnsircMadison. Some of these ar8,already available.
Acoustic Properties The regular structure of the austenite also is responsible for the ringing heard upon dropping the sample. A sound wave launched in the material travels relatively unimpeded through it.' Alarge rod (25 cm long, 1cm in diameter) of austenitic NiTi will ring (vibrate) for more than a minute. A microphone wnnected to a n oscilloscope reveals a simple sine wave? In the martensite phase, the boundaries between differently oriented variants appear to act as baffles for vibration of the rod, resulting in the more muffled-soundine thud when martensitic samples are dropped. When both phases co-exist in the samples, a combination of ring the - and thud is evident upon dropping .. sample.
NiTi spring
steel Spring
Applications Shape-memory alloys have been incorporated into a wide varietv of applications (10). Some creative sueeestions from undergraduates a t theuniversity of ~ i s c o i i i n Madison are presented in the table. The shape memorv eff e d creates temperature-sensitive "on-off" switch that has been used in coffee makers and scald-proof shower heads. Diesel-fueled Mercedes Benz cars hive a shapememor, alloybased valve that r e d a t e s the flow of transmission fluid in the engine as a ?unction of temperature (11).The NiTi memory-metal valve controls the shifting pressure in the automatic transmission (as the temperature in the engine goes up, a higher pressure is desirable), and this in turn smooths shifting between gears. As shown in Figure 8, one NiTi spring, one steel spring, and a piston provide the switching mechanism. The springs and the piston are housed in a case with an inlet and outlet at the top and bottom. When the engine is cold, the NiTi spring is in the martensite phase, and is flexible. The steel sorine ~rovidesenoueh force to push the oiston to the left &d coliapse the ~ i T i s ~ r i n his g . closes tce flow path through the valve. As the engine warms up, the NiTi transforms into the austenite phase and remembers its expanded shape. I t pushes against the piston and the steel spring, opening Lp the valve and allowing oil to flow through. If,the NiTi is not strongly deformed, it can be switched through millions of cycles. NiTi has been used to create scul~tureswith movine parts. Olivier Deschamps designed tGe sculpture entitlel "Les Trois Mains" (The Three Hands) (12). When the weather is cool the NiTi hands are in the martensite phase, and the hands are pointed down toward the center of the sculpture. If the day is warm, the NiTi transforms into the austenite phase, and the hands reach upward. Similarly, a sculpture of a skier designed by the same artist crouches close to the ground when it is cold and straightens up when it is warm. The pseudoelastic properties of NiTi have been used to make eyeglass frames that may be bent. Once the stress is released, the frames snap back into their original shape. The stress causes the NiTi to change h m the austenite to the martensite phase, instead of creating defects in the metallic structure. Once the stress is released, the reverse transformation occurs. Since the NiTi must be in the austenite phase for this application, the transition temperature must be set slightly below room temperature. To ver-
a
8Aboveabout 20 K, defectsin metals are the primary absorbers of sound waves traveling through metals, and boundaries between martensitic variants behave similarly. See propagation of sound through a medium,for example in Bhatia, A. B. UltrasonicAbsorption; Oxford University Press. Oxford. 1967. Cold-worked metalsthat have a high density of induced defects are good at absorbing sound. The sound wave recorded in our laboratory was extremely sinusoidal, with a frequency of 690 Hz.
Figure 8. NiT-actuated governor valve for automatic transmissions. The steel spring easily compresses the memory-metal spring when it is in the low-temperature martensite phase. As the temperature increases, the NiTi spring remembers its extended shape (it transforms into the austentite phase) and becomes rigid, forcing the steel spring to compress. This opens the pathway for transmission fluid through the valve. (Figure reprinted from ref. 11 by permission of the author). ify that memory-metal eyeglass frames are normally in the austenite phase, we have woled a frame in liquid nitrogen. The eyeglass frame is easily bent but returns to its original shape on warming to room temperature (see the cover of this month's Journal for an example of the phenomenon). The most common use of NiTi is in biomedical applieations where its combination of strength, flexibility, and biocompatibility is desirable. The pseudoelastic property of NiTi is used in orthodontic braces. To straighten teeth, an "archwire" is connected across each tooth by a brace. The archwire is pulled tight, and this aligns the teeth. If a NiTi archwire in the austenite phase is used, the stress from tightening causes it to transform into the martensite phase. After tightening, the force the wire exerts as it tries to return to the austenite eraduallv oulls the teeth into position. The advantage of NiTi is that the wire can be pulled tighter than ocher types of wire, and, therefore, fewer trips to the orthodontist are needed. Since the force comes &m the phase transformation, it is more even and continuous over time than the force from other types of wire, which tend to pull a lot at first, and then relax. Other biomedical applications for NiTi include staples that use body heat to cause the phase transformation and clamp broken bones together and guidewires for arthroscopic surgery that are strong but flexible.
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..
Conclusion The shape-memory effect is exciting to watch and easily engages the imagination and interest of students. These alloys may be used to demonstrate a number of basic chemical wncepts in introdudoq chemistry courses. The shape-memory effect may be understood in terms of the relationship between the crvstal structures of the two phases, and the mechanical a i d acoustical features of the two phases are directlv related to the crystal structures as well. Finally, the phenomenon of superelasticity may be Volume 71
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used to extend Le Ch2telier's Principle and the ClausiusClapeymn equation to solid-state transformations.
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.
..
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gamon:Oxford, 1974,p 104.
Acknowledgment
The authors would like to thank Brian Johnson and William Robinson for helpful discussions and the Dreyfus Foundation and the National Science Foundation (grant USE-9150484) for financial support.
6. GwelbraehZ M. J.: Penn, R. L.: Lisensky, G. C.; Stone, D. S.: Ellis, A. B. J. Chem. Educ. 1994,71.254. 7. Mumay, J . L.in Phase Dlagrom ofRinory ZlLhnium Alloys; Mumgy, J . L. Ed. ASM International:Metal Park. OH. 1987. D 203. 8 . (a) Iundqluat, P. G.;Wayman, C. M. inEnginowiqA8pects of Shape Memory Alloys; Duerig, T. W., Ed. Butterworth:London, 1990,p 58, (bl Yi, H,C.: M0ore.J. J. J. Metals 1990.1Aue.l. 31.
Literature Cited 1.Wang, F E.; Buehler, W. T.; Pickart, S. J. J. AppL Phys. 1965.36.3232. 2. (a) Miehal, G.M.; Sinelair, R.Aeto C w t . R 1881.37, 18. (b) Kudoh, Y.et al.Acfa
MddI 19%.
??. Z M 9 .
3. Shimizu, K;Tsdaki, T in Shap-Memory Alloys. FunaLubo. H . , Ed.; Gordon and Breach: New York, 1964,p 7. 4. Aimldi, G.;Rim G.; Riualta, B. Lo MRS lnfernnflad M@. on Ad". Mots VoI. 9, Doyama, M.: Somya, S.:Chang, R. P. H., Eds.; MRS:Pittsburgh, 1989,p 105.
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10. An overview ofrecent applications using shape memorya11oys may be found in h, X.: Stice. J J h ~. .C M o n u Svsf f 1990.~Jan.,.65.
11. Stoekel, D.;Tin%chert,F SAE l k h n i d Paper Series C910805. Society of Automo6ue Engineers: Warrendale, PA, 1991,p 145. 12. Desehamps. 0.JOM 19'31.43.64.
