The term Shape Memory Alloys (SMA) is applied to that group of metallic materials that demonstrate the ability to return to some previously defined shape or size when subjected to the appropriate thermal procedure. Generally, these materials can be plastically deformed at some relatively low temperature, and upon exposure to some higher temperature will return to their shape prior to the deformation. Materials that exhibit shape memory only upon heating are referred to as having a one-way shape memory. Some materials also undergo a change in shape upon recooling. These materials have a two-way shape memory.
Although a relatively wide variety of alloys are know to exhibit the shape memory effect, only those that can recover substantial amounts of strain or that generate significant force upon changing shape are of commercial interest. To date, this has been the nickel-titanium alloys and copper-base alloys such as CuZnAl and CuAlNi.
A shape memory alloy may be further defined as one that yields a thermoelastic martensite. In this case, the alloy undergoes a martensitic transformation of a type that allows the alloy to be deformed by a twinning mechanism below the transformation temperature. The deformation is then reversed when the twinned structure reverts upon heating to the parent phase.
As the shape memory effect became better understood, a number of other alloy systems that exhibited shape memory were investigated. Table 1 lists a number of these systems (Ref. 5) with some details of each system. Of all these systems, the NiTi alloys and a few of the copper-base alloys have received the most development effort and commercial exploitation. These will be the focus of the balance of this article.
Transformation Transformation Temperature Range Hysteresis (approx.) Alloy Composition deg.C deg.F deg.C deg.F Ag-Cd 44/49 at.% Cd -190 to -50 -310 to -60 15 25 Au-Cd 46.5/50 at.% Cd 30 to 100 85 to 212 15 25 Cu-Al-Ni 14/14.5 wt.% Al -140 to 100 -220 to 212 35 65 3/4.5 wt.% Ni Cu-Sn approx. 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 Cu-Zn-X (X = Si,Sn,Al) a few wt.% of X -180 to 200 -290 to 390 10 20 In-Ti 18/23 at.% Ti 60 to 100 140 to 212 4 7 Ni-Al 36/38 at.% Al -180 to 100 -290 to 212 10 20 Ni-Ti 49/51 at.% Ni -50 to 110 -60 to 230 30 55 Fe-Pt approx. 25 at.% Pt approx.-130 approx.-200 4 7 Mn-Cu 5/35 at.% Cu -250 to 180 -420 to 355 25 45 Fe-Mn-Si 32 wt.% Mn, 6 wt.% Si -200 to 150 -330 to 300 100 180 Source: Ref. 5
The herringbone structure of athermal martensites essentially consists of twin-related, self-accommodating variants (Fig. 2b). The shape change among the variants tends to cause them to eliminate each other. As a result, little macroscopic strain is generated. In the case of stress-induced martensites, or when stressing a self-accommodating structure, the variant that can transform and yield the greatest shape change in the direction of the applied stress is stabilized and becomes dominant in the configuration (Fig. 2c). This process creates a macroscopic strain, which is recoverable as the crystal structure reverts to austenite during reverse transformation.
The mechanical properties of shape memory alloys vary greatly over the temperature range spanning their transformation. This is seen in Fig. 3, where simple stress-strain curves are shown for a nickel titanium alloy that was tested in tension below, in the middle of, and above its transformation temperature range. The martensite is easily deformed to several percent strain at quite a low stress, whereas the austenite (high temperature phase) has much higher yield and flow stresses. The dashed line on the martensite curve indicates that upon heating after removing the stress, the sample remembered its unstrained shape and reverted to it as the material transformed to austenite. No such shape recovery is found in the austenite phase upon straining and heating, because no phase change occurs.
An interesting feature of the stress-strain behavior is seen in Fig. 3c, where the material is tested slightly above its transformation temperature. At this temperature, martensite can be stress-induced. It then immediately strains and exhibits the increasing strain at constant stress behavior, seen in AB. Upon unloading, though, the material reverts to austenite at a lower stress, as seen in line CD, and shape recovery occurs, not upon the application of heat but upon a reduction of stress. This effect, which causes the material to be extremely elastic, is known as pseudoelasticity. Pseudoelasticity is nonlinear. The Young's modulus is therefore difficult to define in this temperature range as it exhibits both temperature and strain dependence.
In most cases, the memory effect is one-way. That is, upon cooling, a shape memory alloy does not undergo any shape change, even though the structure changes to martensite. When the martensite is strained up to several percent, however, that strain is retained until the material is heated, at which time shape recovery occurs. Upon recooling, the material does not spontaneously change shape, but must be deliberately strained if shape recovery is again desired.
It is possible in some of the shape memory alloys to cause two-way shape memory. That is, shape change occurs upon both heating and cooling. The amount of this shape change is always significantly less than obtained with one-way memory, and very little stress can be exerted by the alloy as it tries to assume its low-temperature shape. The heating shape change can still exert very high forces, as with the one-way memory.
A number of heat-treatment and mechanical training methods have been proposed to create the two-way shape memory effect (Ref. 6, 7). All rely on the introduction of microstructural stress concentrations which cause the martensite plates to initiate particular directions when they form upon cooling, resulting in an overall net shape change in the desired direction.
There are four major methods of characterizing the transformation in SMAs and a large number of minor methods that are only rarely used and will not be discussed.
The most direct method is by differential scanning calorimetry (DSC). This technique measures the heat absorbed or given off by a small sample of the material as it is heated and cooled through the transformation temperature range. The sample can be very small, such as a few milligrams, and because the sample is unstressed, this is not a factor in the measurement. The endotherm and exotherm peaks, as the sample absorbs or gives off energy due to the transformation, are easily measured for the beginning peak, and end of the phase change in each direction.
The second method often used is to measure the resistivity of the sample as it is heated and cooled. The alloys exhibit interesting changes and peaks in the resistivity (by up to 20%) over the transformation temperature range; however, correlating these changes with measured phase changes or mechanical properties has not always been very successful. Also, there are often large changes in the resistivity curves after cycling samples through the transformation a number of times. Thus, resistivity is often measured as a phenomenon in its own right, but is rarely used to definitely characterize one alloy versus another.
The most direct method of characterizing an alloy mechanically is to prepare an appropriate sample, then apply a constant stress to the sample and cycle it through the transformation while measuring the strain that occurs during the transformation in both directions. The curve shown in Fig. 1 is the direct information one obtains from this test. The values obtained for the transformation points, such as Ms and Af, from this method are offset to slightly higher temperatures from the values obtained from DSC testing. This happens because the DSC test occurs at no applied stress, and the transformation is not stress-induced; therefore, increasing test stress will lead to increasing transformation-temperature results. This test is directly indicative of the property one can expect in a mechanical device used to perform some function using shape memory. Its disadvantages are that specimens are often difficult to make, and results are quite susceptible to the way the test is conducted.
Finally, the stress-strain properties can be measured in a standard tensile test at a number of temperatures across the transformation-temperature range, and from the change in properties the approximate transformation-temperature values can be interpolated. This is very imprecise, though, and is much better applied as a measure of the change in properties of each phase, due to such things as work hardening or different heat treatments.
The only two alloy systems that have achieved any level of commercial exploitation are the NiTi alloys and the copper-base alloys. Properties of the two systems are quite different. The NiTi alloys have greater shape memory strain (up to 8% versus 4 to 5% for the copper-base alloys), tend to be much more thermally stable, have excellent corrosion resistance compared to the copper-base alloys' medium corrosion resistance and susceptibility to stress-corrosion cracking, and have much higher ductility. On the other hand, the copper-base alloys are much less expensive, can be melted and extruded in air with ease, and have a wider range of potential transformation temperatures. The two alloy systems thus have advantages and disadvantages that must be considered in a particular application.
Nickel-Titanium Alloys. The basis of the nickel-titanium system of alloy is the binary, equiatomic intermetallic compound of NiTi. The intermetallic compound is extraordinary because it has a moderate solubility range for excess nickel or titanium, as well as most other metallic elements, and it also exhibits a ductility comparable to most ordinary alloys. This solubility allows alloying with many of the elements to modify both the mechanical properties and the transformation properties of the system. Excess nickel, in amounts up to about 1%, is the most common alloying addition. Excess nickel strongly depresses the transformation temperature and increases the yield strength of the austenite. Other frequently used elements are iron and chromium (to lower the transformation temperature), and copper (to decrease the hysteresis and lower the deformation stress of the martensite). Because common contaminants such as oxygen and carbon can also shift the transformation temperature and degrade the mechanical properties, it is also desirable to minimize the amount of these elements.
The major physical properties of the basic binary NiTi system and some of the mechanical properties of the alloy in the annealed condition are shown in Table 2. Note that this is for the equiatomic alloy with an Af value of about 100 deg.C (230 deg.F).
Melting Temperature, deg.C (deg.F).........................1300 (2370) Density, g/cu.cm (lb/cu.in.).............6.45 (0.233) Resistivity, micro-ohms * cm Austenite.............................approx. 100 Martensite............................approx. 70 Thermal Conductivity, W/cm * deg.C (BTU/ft * hr * deg.F) Austenite.............................18 (10) Martensite............................8.5 (4.9) Corrosion Resistance.....................Similar to 300 series stainless steel or titanium alloys Young's Modulus, GPa, (1,000 ksi) Austenite.............................approx. 83 (12) Martensite............................approx. 28 to 41 (4 to 6) Yield Strength, MPa (ksi) Austenite.............................195 to 690 (28 to 100) Martensite............................70 to 140 (10 to 20) Ultimate Tensile Strength, MPa (ksi).............................895 (130) Transformation Temperatures, deg.C (deg.F).........................-200 to 110 (-325 to 230) Latent Heat of Transformation, kJ/kg * atom (cal/g * atom)...........167 (40) Shape Memory Strain......................8.5% maximum
Selective work hardening, which can exceed 50% reduction in some cases, and proper heat treatment can greatly improve the ease with which the martensite is deformed, give an austenite with much greater strength, and create material that spontaneously moves itself both on heating and on cooling (two-way shape memory). One of the biggest challenges in using this family of alloys is in developing the proper processing procedures to yield the properties desired.
Because of the reactivity of the titanium in these alloys, all melting of them must be done in a vacuum or an inert atmosphere. Methods such as plasma-arc melting, electron-beam melting, and vacuum-induction melting are all used commercially. After ingots are melted, standard hot-forming processes such as forging, bar rolling, and extrusion can be used for initial breakdown. The alloys react slowly with air, so hot working in air is quite successful. Most cold-working processes can also be applied to these alloys, but they work harden extremely rapidly, and frequent annealing is required. Wire drawing is probably the most widely used of the techniques, and excellent surface properties and sizes as small as 0.05 mm (0.002 in.) are made routinely.
Fabrication of articles from the NiTi alloys can usually be done with care, but some of the normal processes are difficult. Machining by turning or milling is very difficult except with special tools and practices. Welding, brazing, or soldering the alloys is generally difficult. The materials do respond well to abrasive removal, such as grinding, and shearing or punching can be done if thicknesses are kept small.
Heat treating to impart the desired memory shape is often done at 500 to 800 deg.C (950 to 1450 deg.F), but it can be done as low as 300 to 350 deg.C (600 to 650 deg.F) if sufficient time is allowed. The SMA component may need to be restrained in the desired memory shape during the heat treatment; otherwise, it may not remain there.
Commercial copper-base shape memory alloys are available in ternary CuZnAl and CuAlNi alloys, or in their quaternary modifications containing manganese. Elements such as boron, cerium, cobalt, iron, titanium, vanadium, and zirconium are also added for grain refinement. The major alloy properties are listed in Table 3.
Thermal Properties CuZnAl CuAlNi Melting Temperature, deg.C (deg.F).......950-1020 (1740-1870) 1000-1050 (1830-1920) Density, g/cu.cm (lb./cu.in.)............7.64 (0.276) 7.12 (0.257) Resistivity, micro-ohms * cm.............8.5 to 9.7 11 to 13 Thermal Conductivity, W/m * deg.C (BTU/ft. * h * deg.F).....120 (69) 30 to 43 (17 to 25) Heat Capacity, J/kg * deg.C (BTU/lb. * deg.F)........400 (0.096) 373 to 574 (0.089 to 0.138) Mechanical Properties Young's Modulus, GPa (1,000 ksi)(a) Beta Phase............................72 (10.4)(a) 85 (12.3)(a) Martensite............................70 (10.2)(a) 80 (11.6)(a) Yield Strength, MPa (ksi) Beta Phase............................350 (51) 400 (58) Martensite............................80 (11.5) 130 (19) Ultimate Tensile Strength, MPa (ksi).....600 (87) 500 to 800 (73 to 116) Shape Memory Properties Transformation Temperatures, deg.C (deg.F).........................<120 (250) <200 (390) Recoverable Strain, %....................4 4 Hysteresis, delta deg.C (delta deg.F)....15 to 25 (30 to 45) 15 to 20 (30 to 35) (a) The Young's Modulus of shape memory alloys becomes difficult to define between the Ms and the As transformation temperatures. At these temperatures, the alloys exhibit nonlinear elasticity, and the modulus is both temperature and strain-dependent.
The martensite-start (Ms) temperatures and the compositions of CuZnAl alloys are plotted in Fig. 4. Compositions of CuAlNi alloys usually fall in the range of 11 to 14.5 wt% Al and 3 to 5 wt% Ni. The martensitic transformation temperatures can be adjusted by varying chemical composition. Figure 4 and the following empirical relationships are useful in obtaining a first estimate:
The melting of Cu-base shape memory alloys is similar to that of aluminum bronzes. Most commercial alloys are induction melted. Protective flux on the melt and the use of nitrogen or inert-gas shielding during pouring are necessary to prevent zinc evaporation and aluminum oxidation. Powder metallurgy and rapid solidification processing are also used to produce fine-grain alloys without grain-refining additives.
Copper-base alloys can be readily hot worked in air. With low aluminum content (<6 wt%) CuZnAl alloys can be cold finished with interpass annealing. Alloys with higher aluminum content are not as easily cold workable. CuAlNi alloys, on the other hand, are quite brittle at low temperatures and can only be hot finished.
Manganese depresses transformation temperatures of both CuZnAl and CuAlNi alloys and shifts the eutectoid to higher aluminum content (Ref. 10). It often replaces aluminum for better ductility.
Because copper-base shape memory alloys are metastable in nature, solution heat treatment in the parent Beta-phase region and subsequent controlled cooling are necessary to retain Beta-phase for shape memory effects. Prolonged solution heat treatment causes zinc evaporation and grain growth and should be avoided. Water quench is widely used as a quenching process, but air cooling may be sufficient for some high aluminum content CuZnAl alloys and CuAlNi alloys. The as-quenched transformation temperature is usually unstable. Postquench aging at temperatures above the normal Af temperature is generally needed to establish stable transformation temperatures.
CuZnAl Alloys, when quenched rapidly and directly into the martensitic phase, are susceptible to the martensite stabilization effect (Ref. 11). This effect causes the reverse transformation to shift toward higher temperatures. It therefore delays and may completely inhibit the shape recovery. For alloys with Ms temperatures above the ambient, slow cooling or step quenching with intermediate aging in the parent Beta-phase state should be adopted.
The thermal stability of copper-base alloys is ultimately limited by the decomposition kinetics. For this reason, prolonged exposure of CuZnAl and CuAlNi alloys at temperatures above 150 deg.C (300 deg.F) and 200 deg.C (390 deg.F) respectively, should be avoided. Aging at lower temperatures may also shift the transformation temperatures. In case of aging the Beta phase, this results from the change in long-range order (Ref. 12). When aged in the martensitic state, the alloys exhibit an aging induced martensite stabilization effect (Ref. 11). For high-temperature stability, CuAlNi is generally a better alloy system than CuZnAl. However, even for moderate temperature applications, which demand tight control of transformation temperatures, these effects need to be evaluated.
Free recovery is illustrated when an SMA component is deformed while martensitic, and the only function required of the shape memory is that the component return to its previous shape (while doing minimal work) upon heating. A prime application of this is the blood-clot filter developed by M. Simon (Ref. 13). The NiTi wire is shaped to anchor itself in a vein and catch passing clots. The part is chilled so it can be collapsed and inserted into the vein, then body heat is sufficient to turn the part to its functional shape.
Constrained Recovery. The most successful example of this type of product is undoubtedly the Cryofit hydraulic couplings made by Raychem Corporation (Ref. 14). These fittings are manufactured as cylindrical sleeves slightly smaller than the metal tubing they are to join. Their diameters are then expanded while martensitic, and, upon warming to austenite, they shrink in diameter and strongly hold the tube ends. The tubes prevent the coupling from fully recovering its manufactured shape, and the stresses created as the coupling attempts to do so are great enough to create a joint that, in may ways, is superior to a weld.
Similar to the Cryofit coupling, the Betalloy coupling (Ref. 15) is a CuZnAl coupling also designed and marketed by Raychem Corporation for copper and aluminum tubing. In this application, the CuZnAl shape memory cylinder shrinks on heating and acts as a driver to squeeze a tubular liner onto the tubes being joined. The joint strength is enhanced by a sealant coating on the liner.
Force Actuators. In some applications, the shape memory component is designed to exert force over a considerable range of motion, often for many cycles. Such an application is the circuit-board edge connector made by Beta Phase Inc. (Ref. 16). In this electrical connector system, the SMA component is used to force open a spring when the connector is heated. This allows force-free insertion or withdrawal of a circuit board in the connector. Upon cooling, the NiTi actuator becomes weaker and the spring easily deforms the actuator while it closes tightly on the circuit board and forms the connections.
Based on the same principle, CuZnAl shape memory alloys have found several applications in this area. One such example is a fire safety valve, which incorporates a CuZnAl actuator designed to shut off toxic or flammable gas flow when fire occurs (Ref. 17).
Proportional Control. It is possible to use only a part of the shape recovery to accurately position a mechanism by using only a selected portion of the recovery because the transformation occurs over a range of temperatures rather than at a single temperature. A device has been developed by Beta Phase Inc. (Ref. 18) in which a valve controls the rate of fluid flow by carefully heating a shape-memory alloy component just enough to close the valve the desired amount. Repeatable positioning within 0.25 mm (0.00001 in.) is possible with this technique.
Superelastic Applications. A number of products have been brought to market that use the pseudoelastic (or superelastic) property of these alloys. Eyeglass frames that use superelastic NiTi to absorb large deformations without damaging the frames are now marketed, and guide wires for steering catheters into vessels in the body have been developed using NiTi wire, which resists permanent deformation if bent severely. Arch wires for orthodontic correction using NiTi have been used for many years to give large rapid movement of teeth.
The properties of the NiTi alloys, particularly, indicate their probable greater use in biomedical applications. The material is extremely corrosion resistant, demonstrates excellent biocompatibility, can be fabricated into the very small sizes often required, and has properties of elasticity and force delivery that allow uses not possible any other way.
Recent interest in the development of iron-base shape memory alloys has challenged the concept that long-range order and thermoelastic martensitic transformation are necessary conditions for shape memory effect. Among the alloys, FePt (Ref. 20). FePd (Ref. 21), and FeNiCoTi (Ref. 22) can be heat treated to exhibit thermoelastic martensitic transformation, and, therefore, shape memory effect. However, alloys such as FeNiC (Ref. 23), FeMnSi (Ref. 24), and FeMnSiCrNi (Ref. 25) are not ordered and undergo nonthermoelastic transformation, and yet exhibit good shape memory effect. These alloys are characteristically different from conventional shape memory alloys in that they rely on stress-induced martensite for shape memory effect, exhibit fairly large transformation hysteresis, and, in general, have less than 4% recoverable strain. The commercial potential of these alloys has yet to be determined, but the effort has opened up new classes of alloys for exploration as shape memory alloys. These new classes include Beta-Ti alloys and iron-base alloys.
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