ASM Handbook, Volume 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials Copyright © 1990 ASM International® ASM Handbook Committee, p 897-902 All rights reserved. www.asminternational.org

Shape Memory Alloys

Darel E. Hodgson, Shape Memory Applications, Inc., Ming H. Wu, Memry Corporation, c

THE TERM SHAPE MEMORY AL- versed when the twinned structure reverts system. Of all these systems, the Ni-Ti LOYS (SMA) is applied to that group of upon heating to the parent phase. alloys and a few of the copper-base alloys metallic materials that demonstrate the abil- have received the most development effort ity to return to some previously defined History and commercial exploitation. These will be shape or size when subjected to the appro- the focus of the balance of this article. priate thermal procedure. Generally, these The first recorded observation of the materials can be plastically deformed at shape memory transformation was by General Characteristics some relatively low temperature, and upon Chang and Read in 1932 (Ref 1). They exposure to some higher temperature will noted the reversibility of the transforma- The martensitic transformation that oc- return to their shape prior to the deforma- tion in AuCd by metallographic observa- curs in the shape memory alloys yields a tion. Materials that exhibit shape memory tions and resistivity changes, and in 1951 thermoelastic martensite and develops from only upon heating are referred to as having the shape memory effect (SME) was ob- a high-temperature phase with a one-way shape memory. Some materials served in a bent bar of AuCd. In 1938, the long-range order. The martensite typically also undergo a change in shape upon recool- transformation was seen in (copper- occurs as alternately sheared platelets, ing. These materials have a two-way shape zinc). However, it was not until 1962, which are seen as a herringbone structure memory. when Buehler and co-workers (Ref 2) dis- when viewed metallographically. The trans- Although a relatively wide variety of covered the effect in equiatomic - formation, although a first-order phase alloys are known to exhibit the shape (Ni-Ti), that research into both change, does not occur at a single temper- memory effect, only those that can recover the metallurgy and potential practical uses ature but over a range of temperatures that substantial amounts of strain or that gen- began in earnest. Within 10 years, a num- varies with each system. The usual erate significant force upon changing ber of commercial products were on the way of characterizing the transformation shape are of commercial interest. To date, market, and understanding of the effect and naming each point in the cycle is shown this has been the nickel-titanium alloys was much advanced. Study of shape mem- in Fig. 1. Most of the transformation occurs and copper-base alloys such as Cu-Zn-Al ory alloys has continued at an increasing over a relatively narrow temperature range, and Cu-AI-Ni. pace since then, and more products using although the beginning and end of the trans- A shape memory alloy may be further these materials are coming to the market formation during heating or cooling actually defined as one that yields a thermoelastic each year (Ref 3, 4). extends over a much larger temperature martensite. In this case, the alloy undergoes As the shape memory effect became bet- range. The transformation also exhibits hys- a martensitic transformation of a type that ter understood, a number of other alloy teresis in that the transformation on heating allows the alloy to be deformed by a twin- systems that exhibited shape memory were and on cooling does not overlap (Fig. 1). This ning mechanism below the transformation investigated. Table 1 lists a number of these transformation (shown as T in Fig. temperature. The deformation is then re- systems (Ref 5) with some details of each 1) varies with the alloy system (Table 1).

Table 1 Alloys having a shape memory effect

Transformation Transformation-temperature range hysteresis Alloy Composition *C *F AoC AOF '~~ i 100 o~

Ag-Cd ...... 44/49 at.% Cd -190 to -50 -310 to -60 ~15 ~25 AL - "~ Au-Cd ...... 46.5/50 at.% Cd 30 to 100 85 to 212 ~15 ~25 Cu-AI-Ni ...... 14/14.5 wt% AI -140 to 100 -220 to 212 ~35 ~65 3/4.5 wt% Ni Cu-Sn ...... ~15 at.% Sn -120 to 30 -185 to 85 Cu-Zn ...... 38.5/41.5 wt% Zn -180 to -10 -290 to 15 ~10 ~20 i I Cu-Zn-X (X = Si, Sn, AI)... a few wt% of X -180 to 200 -290 to 390 ~10 ~20 I In-Ti ...... 18/23 at.% Ti 60 to 100 140 to 212 ~4 ~7 Mf Ms A6 Af Ni-AI ...... 36/38 at.% AI -180 to 100 -290 to 212 ~10 ~20 Temperature ~, Ni-Ti ...... 49/51 at.% Ni -50 to 110 -60 to 230 ~30 ~55 Fe-Pt ...... ~25 at.% Pt ~- 130 ~-200 ~4 ~7 Typical transformation versus temperature Mn-Cu ...... 5/35 at.% Cu -250 to 180 -420 to 355 ~25 ~45 Fig. 1 curve for a specimen under constant load Fe-Mn-Si ...... 32 wt% Mn, 6 wt% Si -200 to 150 -330 to 300 =100 ~180 (stress) as it is cooled and heated. T, transformation Source: Ref 5 hysteresis.Ms, martensite start; Mf, martensite finish; As, austenite start; Af, austenite finish 898 / Special-Purpose Materials

(a) T>Af T1 Austenite ~J T 1 ~> T3 ~> Af > Ms > 72

A \ (c) T3 Pseudoelastic (b) T< Mf 'f~---~ C

Martensite

1¢1 T < Mf 4 (a) A ~ phase crystal. (b) Self-accommodat- Fig. 2 ing, twin-related variants A, B, C, and D, after (b) cooling and transformation to martensite. (c) Variant A becomes dominant when stress is applied.

Crystallography of Shape Memory Alloys Strain Typical stress-strain curves at different temperatures relative to the transformation, showing (a) Thermoelastic martensites are character- Fig. 3 Austenite. (b) Martensite. (c) Pseudoelastic behavior ized by their low energy and glissile inter- faces, which can be driven by small temper- ature or stress changes. As a consequence "remembered" its unstrained shape and heating and cooling. The amount of this of this, and of the constraint due to the loss reverted to it as the material transformed to shape change is always significantly less of symmetry during transformation, ther- austenite. No such shape recovery is found than obtained with one-way memory, and moelastic martensites are crystallographi- in the austenite phase upon straining and very little stress can be exerted by the alloy cally reversible. heating, because no phase change occurs. as it tries to assume its low-temperature The herringbone structure of athermal An interesting feature of the stress-strain shape. The heating shape change can still martensites essentially consists of twin-re- behavior is seen in Fig. 3(c), where the exert very high forces, as with the one-way lated, self-accommodating variants (Fig. material is tested slightly above its transfor- memory. 2b). The shape change among the variants mation temperature. At this temperature, A number of heat-treatment and mechan- tends to cause them to eliminate each other. martensite can be stress induced. It then ical training methods have been proposed to As a result, little macroscopic strain is immediately strains and exhibits the in- create the two-way shape memory effect generated. In the case of stress-induced creasing strain at constant stress behavior, (Ref 6, 7). All rely on the introduction of martensites, or when stressing a self-ac- seen in AB. Upon unloading, though, the microstructural stress concentrations, commodating structure, the variant that can material reverts to austenite at a lower which cause the martensite plates to initiate transform and the greatest shape stress, as seen in line CD, and shape recov- in particular directions when they form change in the direction of the applied stress ery occurs, not upon the application of heat upon cooling, resulting in an overall net- is stabilized and becomes dominant in the but upon a reduction of stress. This effect, shape change in the desired direction. configuration (Fig. 2c). This process creates which causes the material to be extremely a macroscopic strain, which is recoverable elastic, is known as . Pseu- Characterization Methods as the crystal structure reverts to austenite doelasticity is nonlinear. The Young's mod- during reverse transformation. ulus is therefore difficult to define in this There are four major methods of charac- temperature range as it exhibits both tem- terizing the transformation in SMAs and a Thermomechanical Behavior perature and strain dependence. large number of minor methods that are In most cases, the memory effect is one only rarely used and will not be discussed. The mechanical properties of shape mem- way. That is, upon cooling, a shape memory The most direct method is by differential ory alloys vary greatly over the temperature alloy does not undergo any shape change, scanning calorimeter (DSC). This technique range spanning their transformation. This is even though the structure changes to mar- measures the heat absorbed or given off by seen in Fig. 3, where simple stress-strain tensite. When the martensite is strained up a small sample of the material as it is heated curves are shown for a nickel-titanium alloy to several percent, however, that strain is and cooled through the transformation-tem- that was tested in tension below, in the retained until the material is heated, at perature range. The sample can be very middle of, and above its transformation- which time shape recovery occurs. Upon small, such as a few milligrams, and be- temperature range. The martensite is easily recooling, the material does not spontane- cause the sample is unstressed this is not a deformed to several percent strain at quite a ously change shape, but must be deliberate- factor in the measurement. The endotherm low stress, whereas the austenite (high- ly strained if shape recovery is again de- and exotherm peaks, as the sample absorbs temperature phase) has much higher yield sired. or gives off energy due to the transforma- and flow stresses. The dashed line on the It is possible in some of the shape mem- tion, are easily measured for the beginning, martensite curve indicates that upon heat- ory alloys to cause two-way shape memory. peak, and end of the phase change in each ing after removing the stress, the sample That is, shape change occurs upon both direction. Shape Memory Alloys / 899

The second method often used is to mea- ease, and have a wider range of potential Table 2 Properties of binary Ni-Ti shape sure the resistivity of the sample as it is transformation temperatures. The two alloy memory alloys heated and cooled. The alloys exhibit inter- systems thus have advantages and disad- Properties Property value esting changes and peaks in the resistivity vantages that must be considered in a par- Melting temperatures, (by up to 20%) over the transformation- ticular application. °C (°F) ...... 1300 (2370) temperature range; however, correlating Nickel-Titanium Alloys. The basis of the , g/cm 3 (lb/in. 3) .... 6.45 (0.233) these changes with measured phase changes nickel-titanium system of alloys is the bina- Resistivity, I~l~ - cm or mechanical properties has not always ry, equiatomic intermetallic compound of Austenite ...... ~ 100 Martensite ...... ~70 been very successful. Also, there are often Ni-Ti. This intermetallic compound is ex- , large changes in the resistivity curves after traordinary because it has a moderate solu- W/m • °C (Btu/ft • h • cycling samples through the transformation bility range for excess nickel or titanium, as °F) a number of times. Thus, resistivity is often well as most other metallic elements, and it Austenite ...... 18 (10) Martensite ...... 8.5 (4.9) measured as a phenomenon in its own right, also exhibits a ductility comparable to most Corrosion resistance ...... Similar to 300 series but is rarely used to definitely characterize ordinary alloys. This solubility allows alloy- or one alloy versus another. ing with many of the elements to modify titanium alloys The most direct method of characterizing both the mechanical properties and the Young's modulus, GPa (106 psi) an alloy mechanically is to prepare an ap- transformation properties of the system. Austenite ...... ~83 (~12) propriate sample, then apply a constant Excess nickel, in amounts up to about 1%, Martensite ...... ~28--41 (~4-6) stress to the sample and cycle it through the is the most common alloying addition. Ex- Yield strength, MPa (ksi) transformation while measuring the strain cess nickel strongly depresses the transfor- Austenite ...... 195-690 (28-100) Martensite ...... 70-140 (10-20) that occurs during the transformation in mation temperature and increases the yield Ultimate tensile strength, both directions. The curve shown in Fig. 1 strength of the austenite. Other frequently MPa (ksi) ...... 895 (130) is the direct information one obtains from used elements are iron and chromium (to Transformation this test. The values obtained for the trans- lower the transformation temperature), and temperatures, °C (°F) .... -200 to 110 (-325 to 230) Latent heat of trans- formation points, such as Ms and Af, from copper (to decrease the hysteresis and low- formation, kJ/kg • atom this method are offset to slightly higher er the deformation stress of the martensite). (cal/g • atom) ...... 167 (40) temperatures from the values obtained from Because common contaminants such as ox- Shape memory strain ...... 8.5% maximum DSC testing. This happens because the ygen and carbon can also shift the transfor- DSC test occurs at no applied stress, and mation temperature and degrade the me- the transformation is not stress induced; chanical properties, it is also desirable to some of the normal processes are difficult. therefore, increasing test stress will lead to minimize the amount of these elements. Machining by turning or milling is very increasing transformation-temperature re- The major physical properties of the basic difficult except with special tools and prac- sults. This test is directly indicative of the binary Ni-Ti system and some of the me- tices. Welding, brazing, or soldering the property one can expect in a mechanical chanical properties of the alloy in the an- alloys is generally difficult. The materials device used to perform some function using nealed condition are shown in Table 2. Note do respond well to abrasive removal, such shape memory. Its disadvantages are that that this is for the equiatomic alloy with an as grinding, and shearing or punching can specimens are often difficult to make, and Af value of about 110 °C (230 °F), Selective be done if thicknesses are kept small. results are quite susceptible to the way the work hardening, which can exceed 50% Heat treating to impart the desired mem- test is conducted. reduction in some cases, and proper heat ory shape is often done at 500 to 800 °C (950 Finally, the stress-strain properties can treatment can greatly improve the ease with to 1450 °F), but it can be done as low as 300 be measured in a standard tensile test at a which the martensite is deformed, give an to 350 °C (600 to 650 °F) if sufficient time is number of temperatures across the transfor- austenite with much greater strength, and allowed. The SMA component may need to mation-temperature range, and from the create material that spontaneously moves be restrained in the desired memory shape change in properties the approximate trans- itself both on heating and on cooling (two- during the heat treatment; otherwise, it may formation-temperature values can be inter- way shape memory). One of the biggest not remain there. polated. This is very imprecise, though, and challenges in using this family of alloys is in Commercial copper-base shape memory is much better applied as a measure of the developing the proper processing proce- alloys are available in ternary Cu-Zn-AI and change in properties of each phase, due to dures to yield the properties desired. Cu-AI-Ni alloys, or in their quaternary mod- such things as work hardening or different Because of the reactivity of the titanium ifications containing manganese. Elements heat treatments. in these alloys, all melting of them must be such as boron, cerium, cobalt, iron, titani- done in a vacuum or an inert atmosphere. um, vanadium, and zirconium are also add- Methods such as plasma-arc melting, elec- Commercial SME Alloys ed for grain refinement. tron-beam melting, and vacuum-induction The major alloy properties are listed in The only two alloy systems that have melting are all used commercially. After Table 3. The martensite-start (Ms) temper- achieved any level of commercial exploita- ingots are melted, standard hot-forming atures and the compositions of Cu-Zn-AI tion are the Ni-Ti alloys and the copper- processes such as forging, bar rolling, and alloys are plotted in Fig. 4. Compositions of base alloys. Properties of the two systems extrusion can be used for initial breakdown. Cu-AI-Ni alloys usually fall in the range of are quite different. The Ni-Ti alloys have The alloys react slowly with air, so hot 11 to 14.5 wt% AI and 3 to 5 wt% Ni. The greater shape memory strain (up to 8% working in air is quite successful. Most martensitic transformation temperatures versus 4 to 5% for the copper-base alloys), cold-working processes can also be applied can be adjusted by varying chemical com- tend to be much more thermally stable, to these alloys, but they work harden ex- position. Figure 4 and the following empir- have excellent corrosion resistance com- tremely rapidly, and frequent annealing is ical relationships are useful in obtaining a pared to the copper-base alloys' medium required. Wire drawing is probably the most first estimate: corrosion resistance and susceptibility to widely used of the techniques, and excellent stress-corrosion cracking, and have much surface properties and sizes as small as 0.05 • Cu-Zn-AI: Ms(°C) = 2212 - 66.9 (at.% higher ductility. On the other hand, the mm (0.002 in.) are made routinely. Zn) - 90.65 (at.% A1) (Ref 8) copper-base alloys are much less expen- Fabrication of articles from the Ni-Ti • Cu-A1-Ni: Ms(°C) = 2020 - 134 (wt% AI) sive, can be melted and extruded in air with alloys can usually be done with care, but - 45 (wt% Ni) (Ref 9) 900 / Special-Purpose Materials

Table 3 Properties of copper-base shape memory alloys inserted into the vein, then body heat is Property value sufficient to turn the part to its functional Property I Cu-Zn-AI Cu-AI-Ni I shape. Constrained Recovery. The most success- Thermal properties ful example of this type of product is un- Melting temperature, °C (°F) ...... 950-1020 (1740-1870) 1000-1050 (1830-1920) doubtedly the Cryofit hydraulic couplings Density, g/cm 3 (lb/in)) ...... 7.64 (0.276) 7.12 (0.257) made by Raychem Corporation (Ref 14). Resistivity, ~ • cm ...... 8.5-9.7 11-13 These fittings are manufactured as cylindri- Thermal conductivity, W/m • °C (Btu/ft • h - °F) ...... 120 (69) 30--43 (17-25) Heat capacity, J/kg • °C (Btu/lb • °F) ...... 400 (0.096) 373--574 (0.089-0.138) cal sleeves slightly smaller than the tubing they are to join. Their diameters are Mechanical properties then expanded while martensitic, and, upon Young's modulus, GPa (106 psi)(a) warming to austenite, they shrink in diam- 13 phase ...... 72 (10.4)(a) 85 (12.3)(a) Martensite ...... 70 (10.2)(a) 80 (11.6)(a) eter and strongly hold the tube ends. The Yield strength, MPa (ksi) tubes prevent the coupling from fully recov- 13 phase ...... 350 (51) 400(58) ering its manufactured shape, and the Martensite ...... 80 (11.5) 130 (19) stresses created as the coupling attempts to Ultimate tensile strength, MPa (ksi) ...... 600 (87) 500-800(73-116) do so are great enough to create a joint that, Shape memory properties in many ways, is superior to a weld. Transformation temperatures, °C (°F) ...... < 120 (250) <200 (390) Similar to the Cryofit coupling, the Betal- Recoverable strain, % ...... 4 4 loy coupling (Ref 15) is a Cu-Zn-AI coupling Hysteresis, A°C (A°F) ...... 15-25 (30-45) 15-20 (30-35) also designed and marketed by Raychem (a) The Young's modulus of shape memory alloys becomes difficult to define between the Ms and the A~ transformation temperatures. At these temperatures, the alloys exhibit nonlinear elasticity, and the modulus is both temperature- and strain-dependent. Corporation for copper and aluminum tub- ing. In this application, the Cu-Zn-A1 shape memory cylinder shrinks on heating and acts as a driver to squeeze a tubular liner The melting of Cu-base shape memory effect (Ref 11). This effect causes the re- onto the tubes being joined. The joint alloys is similar to that of aluminum bronz- verse transformation to shift toward higher strength is enhanced by a sealant coating on es. Most commercial alloys are induction temperatures. It therefore delays and may the liner. melted. Protective flux on the melt and the completely inhibit the shape recovery. For Force Actuators. In some applications, use of nitrogen or inert-gas shielding during alloys with Ms temperatures above the am- the shape memory component is designed pouring are necessary to prevent zinc evap- bient, slow cooling or step quenching with to exert force over a considerable range of oration and aluminum oxidation. Powder intermediate aging in the parent 13-phase motion, often for many cycles. Such an metallurgy and rapid solidification process- state should be adopted. application is the circuit-board edge con- ing are also used to produce fine-grain al- The thermal stability of copper-base alloys nector made by Beta Phase Inc. (Ref 16). In loys without grain-refining additives. is ultimately limited by the decomposition this electrical connector system, the SMA Copper-base alloys can be readily hot kinetics. For this reason, prolonged exposure component is used to force open a spring worked in air. With low aluminum content of Cu-Zn-AI and Cu-A1-Ni alloys at tempera- when the connector is heated. This allows (<6 wt%), Cu-Zn-AI alloys can be cold tures above 150 °C (300 °F) and 200 °C (390 force-free insertion or withdrawal of a cir- finished with interpass annealing. Alloys °F) respectively, should be avoided. Aging at cuit board in the connector. Upon cooling, with higher aluminum content are not as lower temperatures may also shift the trans- the Ni-Ti actuator becomes weaker and the easily cold workable. Cu-A1-Ni alloys, on formation temperatures. In case of aging in spring easily deforms the actuator while it the other hand, are quite brittle at low the 13 phase, this results from the change in closes tightly on the circuit board and forms temperatures and can only be hot finished. long-range order (Ref 12). When aged in the the connections. Manganese depresses transformation martensitic state, the alloys exhibit an aging- Based on the same principle, Cu-Zn-AI temperatures of both Cu-Zn-AI and Cu- induced martensite stabilization effect (Ref shape memory alloys have found several ap- AI-Ni alloys and shifts the eutectoid to 11). For high-temperature stability, Cu-AI-Ni plications in this area. One such example is a higher aluminum content (Ref 10). It often is generally a better alloy system than Cu- fire safety valve, which incorporates a Cu- replaces aluminum for better ductility. Zn-AI. However, even for moderate temper- Zn-AI actuator designed to shut off toxic or Because copper-base shape memory al- ature applications, which demand tight con- flammable gas flow when fire occurs (Ref 17). loys are metastable in nature, solution heat trol of transformation temperatures, these Proportional Control. It is possible to use treatment in the parent 13-phase region and effects need to be evaluated. only a part of the shape recovery to accurate- subsequent controlled cooling are neces- ly position a mechanism by using only a sary to retain 13 phase for shape memory Applications selected portion of the recovery because the effects. Prolonged solution heat treatment transformation occurs over a range of temper- causes zinc evaporation and grain growth There is a wide variety of uses for the atures rather than at a single temperature. A and should be avoided. Water quench is shape memory alloys. The following will device has been developed by Beta Phase widely used as a quenching process, but air illustrate one or two products in several Inc. (Ref 18) in which a valve controls the rate cooling may be sufficient for some high- categories of application. of fluid flow by carefully heating a shape- aluminum content Cu-Zn-AI alloys and Cu- Free recovery is illustrated when an SMA memory-alloy component just enough to AI-Ni alloys. The as-quenched transforma- component is deformed while martensitic, close the valve the desired amount. Repeat- tion temperature is usually unstable. and the only function required of the shape able positioning within 0.25 Ixm (10 -5 in.) is Postquench aging at temperatures above the memory is that the component return to its possible with this technique. nominal Af temperature is generally needed previous shape (while doing minimal work) Superelastic Applications. A number of to establish stable transformation tempera- upon heating. A prime application of this is products have been brought to market that tures. the blood-clot f'flter developed by M. Simon use the pseudoelastic (or superelastic) prop- Cu-Zn-AI alloys, when quenched rapidly (Ref 13). The Ni-Ti wire is shaped to anchor erty of these alloys. Eyeglass frames that and directly into the martensitic phase, are itself in a vein and catch passing clots. The use superelastic Ni-Ti to absorb large defor- susceptible to the martensite stabilization part is chilled so it can be collapsed and mations without damaging the frames are Shape Memory Alloys / 901

M s temperature, °C doelasticity, and many more of these appli- cations are likely. Finally, the availability of small wire that is stable, is easily heated by °YK a small electrical current, and gives a large Zn , , , f , , , repeatable stroke should lead to a new family of actuator devices (Ref 19). These oA~No ::::::: I \ devices can be inexpensive, are reliable for o 80~20 ~ , : : , : : ; thousands of cycles, and are expected to ox~• 70~3 0 ~ . :: :: :: ": :: :: :~_105/~ k k ~--~. move Ni-Ti into the high-volume consumer marketplace. : : : : : :j Recent interest in the development of iron-base shape memory alloys has chal- lenged the concept that long-range order 10 ~VV~/~/~/x,//'x/~ van and thermoelastic martensitic transforma- CU ~AAAAA/kA/Vk°~I 10 90 80 70 60 50 40 30 20 10 65 tion are necessary conditions for shape memory effect. Among the alloys, Fe-Pt Copper,% ; , ; ; / (Ref 20), Fe-Pd (Ref 21), and Fe-Ni-Co-Ti ,:,: ,: :/: ":' (Ref 22) can be heat treated to exhibit thermoelastic martensitic transformation, i i l i f iii and, therefore, shape memory effect. How- ::,::.::_6o ever, alloys such as Fe-Ni-C (Ref 23), Fe- i i l ~ i i l Mn-Si (Ref 24), and Fe-Mn-Si-Cr-Ni (Ref ~:"!: o-90 ° 25) are not ordered and undergo nonther- / moelastic transformation, and yet exhibit ! 10~~: i: J:59°53 o o ~32 ° good shape memory effect. These alloys are 20 , , , , , , -- characteristically different from conven- :I::'' tional shape memory alloys in that they rely on stress-induced martensite for shape memory effect, exhibit fairly large transfor- ~o mation hysteresis, and, in general, have less than 4% recoverable strain. The commer- / cial potential of these alloys has yet to be : iiiii:: / determined, but the effort has opened up 170°'e; ' I ' ' ;\ 1 new classes of alloys for exploration as .... e, ; ;15 °/ 66,o, ,f ,j k ,i ; ,48u 0\/ shape memory alloys. These new classes 730; ; ; e; ; , , 15 include 13-Ti alloys and iron-base alloys. i i u m i i i ; ; ; ,e; ; ;67/ ° i i i i i i i / i i i i i J m REFERENCES 148°; ~ ; ' : : : / 146°: :e: : ,' : '/ 1. L.C. Chang and T.A. Read, Trans. AIME, Vol 191, 1951, p 47 2. W.J. Buehler, J.V. Gilfrich, and R.C. ll:::li Wiley, J. Appl. Phys., Vol 34, 1963, p

i J i i u i i 1475 3. Proceedings of Engineering Aspects of Shape Memory Alloys (Lansing, MI), 10 1988 299oe :1:80:II i J i i i i i 4. D.E. Hodgson, Using Shape Memory i i i i ~ i i Alloys, Shape Memory Applications, 1988 Fig. 4 M s temperatures and compositions of Cu-Zn-AI shape memoryalloys 5. K. Shimizu and T. Tadaki, Shape Mem- ory Alloys, H. Funakubo, Ed., Gordon now marketed, and guide wires for steering Future Prospects and Breach Science Publishers, 1987 catheters into vessels in the body have been 6. J.R. Willson, et al., U.S. Patent developed using Ni-Ti wire, which resists Although specific products that might use 3,625,969, 1972 permanent deformation if bent severely. the Ni-Ti alloys in the future cannot be 7. A.D. Johnson, U.S. Patent 4,435,229, Arch wires for orthodontic correction using foretold, some directions are obvious. The 1972 Ni-Ti have been used for many years to give cost of these alloys has slowly decreased as 8. L. Delaey, M. Chandrasekaran, W. De- large rapid movement of teeth. use has increased, so uses that require Jonghe, W. Rapacioli, and A. Deruyt- The properties of the Ni-Ti alloys, partic- lower-cost alloys to be viable are being tere, INCRA Research Report 238, Inter- ularly, indicate their probable greater use in explored. Alloy development has yielded national Copper Research Association biomedical applications. The material is ex- several ternary compositions with proper- 9. K. Sugimoto, Bull. Jpn. Inst. Met., Vol tremely corrosion resistant, demonstrates ties improved over those obtained with bi- 24, 1985, p 45 excellent biocompatibility, can be fabricat- nary material, and alloys tailored to specific 10. P.L. Brook, U.S. Patent 4,166,739, ed into the very small sizes often required, product needs are likely to multiply. The Sept 1979 and has properties of elasticity and force medical industry has developed a number of 11. M. Ahlers, Proceedings of Internation- delivery that allow uses not possible any products using Ni-Ti alloys because of their al Conference on Martensitic Transfor- other way. excellent biocompatibility and large pseu- mations (Nara, Japan), 1986, p 786 902 / Special-Purpose Materials

12. D. Schofield and A.P. Miodownik, 18. D.E. Hodgson, Proceedings of Engi- 1984, p 1105 Met. Technol., Vol 7, 1980, p 167 neering Aspects of Shape Memory Al- 23. S. Kajiwara, Trans. Jpn. Inst. Met., 13. M. Simon, et al., Radiology, Vol 172, loys (Lansing, MI), 1988 Vol 26, 1985, p 595 1989, p 99-103 19. Product brochure, Dynalloy Inc., Ir- 24. A. Sato, K. Soma, E. Chishima, and T. 14. J.D. Harrison and D.E. Hodgson, vine, CA Mori, in Proceedings, International Shape Memory Effects in Alloys, J. 20. M. Foos, C. Frantz, and M. Gantois, Conference on Martensitic Transforma- Perkins, Ed., Plenum Press, 1975, p 517 Shape Memory Effects in Alloys, J. tions (Louvain, Belgium), 1982, p C4- 15. Product Brochure, Raychem Corpora- Perkins, Ed., Plenum Press, 1975, p 797 tion, Menlo Park, CA 407 25. H. Otsuka, H. Yamada, H. Tanahashi, 16. J.F. Krumme, Connect. Technol., Vol 21. T. Sohmura, R. Oshima, and F.E. Fu- and T. Maruyama, in Proceedings, In- 3 (No. 4), April 1987, p 41 jita, Scr. Metall., Vol 14, 1980, p 855 ternational Conference on Martensitic 17. E. Waldbusser, Semicond. Saf. Assoc. 22. T. Maki, K. Kobayashi, M. Minato, Transformations (Sydney, Australia), J., Aug 1987, p 34 and I. Tamura, Scr. Metall., Vol 18, 1989