Seminar Report-2011 Shape Memory Alloys

Seminar Report-2011 Shape Memory Alloys

1. Introduction

Shape memory alloys (SMAs) are metals that "remember" their original shapes. SMAs are useful for such things as actuators which are materials that "change shape, stiffness, position, natural frequency, and other mechanical characteristics in response to temperature or electromagnetic fields". The potential uses for SMAs especially as actuators have broadened the spectrum of many scientific fields. The study of the history and development of SMAs can provide an insight into a material involved in cutting-edge technology. The diverse applications for these metals have made them increasingly important and visible to the world.

Nickel-titanium alloys have been found to be the most useful of all SMAs. Other shape memory alloys include copper-aluminum-nickel, copper-zinc-aluminum, and iron- manganese-silicon alloys.(Borden, 67) The generic name for the family of nickel-titanium alloys is Nitinol. In 1961, Nitinol, which stands for Nickel Titanium Naval Ordnance Laboratory, was discovered to possess the unique property of having shape memory. William J. Buehler, a researcher at the Naval Ordnance Laboratory in White Oak, Maryland, was the one to discover this shape memory alloy. The actual discovery of the shape memory property of Nitinol came about by accident. At a laboratory management meeting, a strip of Nitinol was presented that was bent out of shape many times. One of the people present, Dr. David S. Muzzey, heated it with his pipe lighter, and surprisingly, the strip stretched back to its original form.

1. History

Between 1952 and 1958, at the Naval Ordnance Laboratory, Buehler a metallurgist, to cure boredom experienced in between projects, would experiment on iron-aluminum alloy. William J. Buehler had completed research on a series of iron-aluminum alloys, for the Naval Ordnance Laboratory (NOL) in 1958. At NOL, Buehler was working on the in-house project which was to find an appreciate metal that could handle the heat and turbulenceexperienced by a spacecraft on reentry into the atmosphere from low space orbit. Buehler’s job on the in-house project was to provide physical and mechanical property data on existing metals and alloys for computer-assisted boundary layer calculations. These calculations were to simulate the heating, etc. of a reentry body through the earth’s atmosphere. The job of working out calculation started to become boring and Buehler started to think of different alloy conditions that may solve the reentry problem. (Kauffman, 1996)

Buehler consulted Max Hansen’s recently published Constitution of Binary Alloys which was the latest text available about binary constitution diagrams, showing the solid-state phase relationships of two–component metallic alloys as a function of composition and temperature. Starting with sixty intermetallic compound alloys and then narrowing down to twelve, Buehler, was able to select an alloy that exhibited considerably more impact resistance and ductility than the other eleven alloys. That metal combination was an equiatomicnickel–titanium alloy. (Kauffman, 1996)

In 1959, Buehler, decided to concentrate his research efforts on nickel-titanium alloy which he gave new name Nitinol. Nitinol exhibited favorable attributes that were needed for the nose cone of spacecraft during orbital reentry. (Kauffman, 1996)

1. Accidental Discovery

In 1961, preparing for meeting to demonstrate the fatigue-resistant properties of Nitinol, Buehler, prepared a (.010 inch thick) strip. At room temperature he bent the strip into an accordion shape, so it could be pulled out of shape and bounce back. Buehler gave the Nitinol strip to his assistant to bring to the laboratory management meeting, because he was able to attend. At the laboratory management meeting, the strip was passed around the members of the meeting, as a prop. The members of the meeting pulled and twisted the nickel–titanium alloy. One of the Associate Technical Directors, Dr. David S. Muzzey, who was a pipe smoker, applied heat from his pipe lighter to the compressed strip. To everyone’s amazement, the Nitinol stretched out longitudinally. The mechanical memory discovery, while not made in Buehler’s metallurgical laboratory, was the missing piece of the puzzle of the earlier mentioned acoustic damping and other unique changes during temperature variation. The unattended actions during a management meeting made accidental discovery of an amazing alloy, that will be used many new and innovative inventions. (Kauffman, 1996)

1. General principles

Shape memory metal alloy can exist in two different temperature dependent crystal structures (phases) called martensite (lower temperature ) and austenite ( higher temperature or parent phase ). Several properties of austenite and martensite are notably different

Martensite, is the relatively soft and easily deformed phase of shape memory alloys, which exists at lower temperatures. The molecular structure in this phase is twinned which is the configuration shown in the middle of Figure 2. Upon deformation this phase takes on the second form shown in Figure 2, on the right. Austenite, the stronger phase of shape memory alloys, occurs at higher temperatures. The shape of the Austenite structure is cubic, the structure shown on the left side of Figure 2. The un-deformed Martensite phase is the same size and shape as the cubic Austenite phase on a macroscopic scale, so that no change in size or shape is visible in shape memory alloys until the Martensite is deformed.

5.Shape Memory Effect

The shape memory effect is observed when the temperature of a piece of shape memory alloy is cooled to below the temperature Mf. At this stage the alloy is completely composed of Martensite which can be easily deformed. After distorting the SMA the original shape can be recovered simply by heating the wire above the temperature Af. The heat transferred to the wire is the power driving the molecular rearrangement of the alloy, similar to heat melting ice into water, but the alloy remains solid. The deformed Martensite is now transformed to the cubic Austenite phase, which is configured in the original shapeof the wire.

The Shape memory effect is currently being implemented in:

• The space shuttle
• Thermostats
• Vascular Stents
• Hydraulic Fittings (for Airplanes)

1. Pseudo-elasticity

Pseudo-elasticity occurs in shape memory alloys when the alloy is completely composed of Austenite (temperature is greater than Af). Unlike the shape memory effect, pseudo-elasticity occurs without a change in temperature. The load on the shape memory alloy is increased until the Austenite becomes transformed into Martensite simply due to the loading; this process is shown in Figure 5. The loading is absorbed by the softer Martensite, but as soon as the loading is decreased the Martensite begins to transform back to Austenite since the temperature of the wire is still above Af, and the wire springs back to its original shape.
Some examples of applications in which pseudo-elasticity is used are:

• Eyeglass Frames
• Medical Tools
• Cellular Phone Antennae

1. Alloy Types

Since the discovery of Ni-Ti, at least fifteen different binary, ternary and quaternary alloy types have been discovered that exhibit shape changes and unusual elastic properties consequent to deformation. Some of these alloy types and variants are shown in table 1.

Table 1. Shape memory alloy types.

· Titanium-palladium-nickel
· Nickel-titanium-copper
· Gold-cadmium
·Iron-zinc-copper-aluminium
·Titanium-niobium-luminium
· Uranium-niobium
· Hafnium-titanium-nickel / · Iron-manganese-silicon
· Nickel-titanium
· Nickel-iron-zinc-aluminium
· Copper-aluminium-iron
· Titanium-niobium
· Zirconium-copper-zinc
· Nickel-zirconium-titanium

The original nickel-titanium alloy has some of the most useful characteristics in terms of its active temperature range, cyclic performance, recoverable strain energy and relatively simple thermal processing. Ni-Ti and other alloys have two generic properties thermally induced shape recovery and super- or pseudo-elasticity. The latter means that an SMA in its elastic form can undergo a deformation approximately ten times greater than that of a spring-steel equivalent, and full elastic recovery to the original geometry may be expected. This may be possible through several million cycles. The energy density of the alloy can be used to good effect to make high-force actuators - a modern DC brushless electric motor has a mass of 5-10 times that of a thermally activated Ni-Ti alloy, to do the same work.

The super elastic Ni-Ti alloys are “stressed” by simply working the alloy. These stresses can be removed, just as with many other alloys, by an annealing process. The stressed condition is termed stress-induced martensite, which is the equivalent of being cold/hot worked.

SMAs, particularly nickel-titanium, are commercially available from several sources. However, world production is small compared to other metal commodities (about 200 tonnes were produced 1998) owing to difficulties in the melt/forging production process, and so the cost of the material high US$0.30-US$1.50 (UK£0.20-£1.00) per gram for wire forms 1999 prices). Fortunately, most current applications require only small amount of the material. As world production increases (as it has done quite dramatically in the 1990s) so prices should decrease. Wires, strip, rod, bar and sheet are all readily available and alloy foams, sintering powders and sputtering targets of high purity are also produced.

1. Nitinol Phases and Properties

Nitinol has phase change while still solid; these phase changes are known as martensite and austenite. Martensite and austenite phase changes "involve the rearrangement of the position of particles within the crystal structure of the solid" the discovery of the shape-memory effect. Dr. Frederick E. Wang. (Kauffman, 1993) Nitinol is in the martensite phase under the shift of temperature. The alteration temperature varies from different compositions from -50 °C to 166 °C. (Jackson, 1997) Nitinol can be bend into varies shapes in the martensite phase, to reshape the Nitinol back into its original character the Nitinol must held into position and heated to approximately 500 °C. By heating the Nitinol the atoms are realigned into a compact and regular pattern resulting into a rigid cubic arrangement known as the austenite phase. (Kauffman, 1993) The parent shape is achieved in the austenite phase. The Nitinol can phase shifted back and forth from martensite to austenite for millions of cycles with no breakdown on the composite alloy. (Jackson, 1997)

The production method of Nitinol varies, current existing techniques of producing nickel-titanium alloys include vacuum melting techniques such as electron-beam melting, vacuum arc melting or vacuum induction melting. The Nitinol is made into cast ingot in a press forge or rotary forge into in to rods or wire. The working temperature for Nitinol is between 700 °C and 900 °C. The cold working method for Nitinol is similar to the fabrication of titanium wire. To produce wires ranging in size from .075mm to 1.25mm in diameter carbide and diamond dies must be used to produce the wire. A change to the mechanical and physical properties of Nitinol will occur when the alloy is cold worked. (Jackson, 1997)

General the properties of Nitinol is comparable to other alloys, its melting point is around 1240 °C to 1310 °C, and its density is around 6.5 g/cm³. Other physical properties due differ from other alloys such as temperatures with various compositions of elements include electrical resistivity, thermoelectric power, Hall coefficient, velocity of sound, damping, heat capacity, magnetic susceptibility, and thermal conductivity. (Jackson, 1997) The large force generated upon returning to its original shape is a very useful property. Other useful properties of Nitinol are its "excellent damping characteristics at temperatures below the transition temperature range, its corrosion resistance, its nonmagnetic nature, its low density and its high fatigue strength" these properties translate into many uses for Nitinol. Reference Table 1. (Jackson, 1997)

PHYSICAL PROPERTIES
Melting Point / 2390°F / 1310°C
Density / 0.234 lb/in3 / 6.5 g/cm3
Electrical Resistivity / 30 μohm-in / 76 μohm-cm
Modulus of Elasticity / 4-6 x 106psi / 28-41 x 103MPa
Coefficient of Thermal Expansion / 3.7 x 10-6/°F / 6.6 x 10-6/°C
MECHANICAL PROPERTIES
Ultimate Tensile Strength (min. UTS) / 160 x 103psi / 1100 MPa
Total Elongation (min) / 10% / 10%
SHAPE MEMORY PROPERTIES
Loading Plateau Stress @ 3%/ strain (min) / 15 x 103psi / 100 MPa
Shape Memory Strain (max) / 8.0% / 8.0%
Transformation Temperature (Af) / 140° F / 60° C

Table 1 - Nitinol SM495 Wire Properties (Nitinol, 2010)

1. PROGRAMMING

The use of the one way shape memory or super elastic property of NiTi for a specific application requires a piece of SMA to be molded into the desired shape . the characteristic heat treatment is then done to set the specimen to its final shape . The heat treatment methods used to set shapes in both the shape memory and the super elastic forms of NiTi are similar. Adequate heat treatment parameters are needed to set the shape and properties of the item.

The two way memory training procedure can be made by SMEtraining or SIM training . In SME training the specimen is cooled below Mf and bent the desired shape . It is then heated to a temperature above Af and allowed freely to take its austenite shape . The procedure is repeated 20 – 30 times which completes the training . The sample now assumes its programmed shape upon cooling under Mf and to another shape when heated above Af.

In SIM (stress induced martensite ) training the specimen is bent just above Ms to produce the preferred variants of SIM and then cooled below Mf temperature. Upon subsequent heating above the Af temperature the specimen takes its original austenitic shape . This procedure is repeated 20-30 times.

1. Future Prediction of Shape Memory Alloy (SMA)

Shape Memory Alloy (SMA) or Nitinol with it potential use as a muscle metal; it is like an actuator without all the extra parts. Present day actuators use different methods mechanics to achieve movement such as pneumatics, electricity, and hydraulics. A Nitinol wire has only a wire strain and a heat source that heat source can be direct or induced by electric current. Nitinol simplicity lends itself to diverse applications in different industries such as medicine, industrial, robotics, and etc. the potential is unlimited.

10.1.Medicine

The application of Shape Memory Alloy (SMA) or Nitinol in medicine is not new; its use in medicine has been around for few decades. The present day uses of Nitinol are for such devices as tension wires on dental orthodontics braces and in cardiovascular medicine Nitinol is being used for heart stints and blood vessel catheters. Nitinol wire is being used to make nearly indestructible frame for eye glasses, because SMA eyeglass frames will bounce back to the original shape after being bent. (Kauffman, 1993)

10.1.1 Stents

The property of thermally induced elastic recovery can be used to change a small volume to a larger one. An example of a device using this is a stent. A stent, either in conjunction with a dilation balloon or simply by self-expansion, can dilate or support a blocked conduit in the human body. Coronary artery disease, which is a major cause of death around the world, is caused by a plaque in-growth developing on and within an artery’s inner wall. This reduces the cross-section of the artery and consequently reduces blood flow to the heart muscle. A stent can be introduced in a “deformed” shape, in other words with a smaller diameter. This is achieved by travelling through the arteries with the stent contained in a catheter. When deployed, the stent expands to the appropriate diameter with sufficient force to open the vessel lumen and reinstate blood flow.

10.1.2. Vena-cava Filters

Vena-cava filters have a relatively long record of successful in-vivo application. The filters are constructed from Ni-Ti wires and are used in one of the outer heart chambers to trap blood clots, which might be the cause of a fatality if allowed to travel freely around the blood circulation system. The specially designed filters trap these small clots, preventing them from entering the pulmonary system and causing a pulmonary embolism. The vena-cava filter is introduced in a compact cylindrical form about 2.0-2.5mm in diameter. When released it forms an umbrella shape. The construction is designed with a wire mesh spacing sufficiently small to trap clots. This is an example of the use of superelastic properties, although there are also some thermally actuated vena cava filters on the market.

10.1.3. Dental and Orthodontic Applications

Another commercially important application is the use of superelastic and thermal shape recovery alloys for orthodontic applications. Archwires made of stainless steel have been employed as a corrective measure for misaligned teeth for many years. Owing to the limited “stretch” and tensile properties of these wires, considerable forces are applied to teeth, which can cause a great deal of discomfort. When the teeth succumb to the corrective forces applied, the stainless steel wire has to be re-tensioned. Visits may be needed to the orthodontist for re-tensioning every three to four weeks in the initial stages of treatment.