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Shape Memory Alloys: Part One
Abstract: Shape memory alloys (SMAs) are metals which exhibit
two very unique properties, pseudo-elasticity and the shape memory
effect. Because these materials are relatively new, some of the
engineering aspects of the materials are still not well understood.
Many of the typical engineering descriptors, such as Young's
modulus and yield strength, do not apply to shape memory alloys
since they are very strongly temperature dependent. On the other
hand, a new set of descriptors must be introduced, such as stress
rate and amnesia.
Shape memory alloys (SMAs) are metals which exhibit two very
unique properties, pseudo-elasticity and the shape memory effect.
Arne Olander first observed these unusual properties in 1938, but
not until the 1960s were any serious research advances made in the
field of shape memory alloys.
The basic phenomenon of the shape memory effect was widely
reported a decade later by Russian metallurgist G. Kurdjumov and
also by Chang and Read. However, presentation of this property to
the wider public came only after the development of the
nickel-titanium alloy (Nitinol) by Buehler and Wang. Since then,
research activity in this field has been intense, and a number of
alloys have been investigated, including Ag-Cd, Au-Cd, Cu-Zn,
Cu-Zn-AI, Cu-AI-Ni, Cu-Sn, Cu-Au-Zn, Ni-AI, Ti-Ni, Ti-Ni-Cu,
Ni-Ti-Nb, Ti-Pd-Ni, In-Ti, In-Cd and others. The most effective and
widely used alloys include Ni-Ti, Cu-Zn-Al, and Cu-Al-Ni.
Crystallography of shape memory alloys have been studied for the
last four decades. Because these materials are relatively new, some
of the engineering aspects of the materials are still not well
understood. Many of the typical engineering descriptors, such as
Young's modulus and yield strength, do not apply to shape memory
alloys since they are very strongly temperature dependent. On the
other hand, a new set of descriptors must be introduced, such as
stress rate and amnesia.
SMAs are useful for such things as actuators which are materials
that change shape, stiffness, position, 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 diverse
applications for these metals have made them increasingly important
and visible to the world.
The alloys that exhibit so-called shape memory can undergo
surprisingly large amounts of strain and then, upon temperature
increase or unloading, revert to their original shape. Ni-Ti based
shape memory alloys have to date provided the best combination of
materials properties for most commercial applications. However they
are very expensive compared with Cu-based shape memory alloys. In
many applications, Cu-based alloys provide a more economical
alternative to Ni-Ti. Cu-Zn based shape memory alloys have actually
been used and Cu-AI based shape memory alloys have shown
promise.
The two unique properties described above are made possible
through a solid state phase change that is a molecular
rearrangement, which occurs in the shape memory alloy. The two
phases, which occur in shape memory alloys, are martensite and
austenite. 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 the
configuration is shown in the middle of Figure 1. Upon deformation
this phase takes on the second form shown in Figure 1, 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 1. The
undeformed 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.
Figure 1: Microscopic and macroscopic views of the two phases of
shape memory alloys
The temperatures at which each of these phases begin and finish
forming are represented by the following variables: Ms, Mf, As, Af.
The amount of loading placed on a piece of shape memory alloy
increases the values of these four variables as shown in Figure 2.
The initial values of these four variables are also dramatically
affected by the composition of the wire (i.e. what amounts of each
element are present).
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
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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 shape of the wire.
Figure 2: The dependency of phase change temperature on
loading
The shape memory effect is currently being implemented in:
The space shuttle Hydraulic fittings for airplanes Thermostats
Vascular stents Coffeepots
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. 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.
It was discovered that Nitinol had phase changes while still a
solid. These phase changes, known as martensite and austenite,
involve the rearrangement of the position of particles within the
crystal structure of the solid. Under the transition temperature,
Nitinol is in the martensite phase.
The transition temperature varies for different compositions
from about -50C to 166C. In the martensite phase, Nitinol can be
bent into various shapes. To fix the parent shape (as it is
called), the metal must be held in position and heated to about
500C. The high temperature causes the atoms to arrange themselves
into the most compact and regular pattern possible resulting in a
rigid cubic arrangement known as the austenite phase. Above the
transition temperature, Nitinol reverts from the martensite to the
austenite phase which changes it back into its parent shape. This
cycle can be repeated millions of times.
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.
Figure 3: Microscopic diagram of the shape memory effect
Among the shape memory alloys that have been developed, Au-Cd,
Cu-Zn-Al, Cu-Al-Ni and the like have been noted as being comparable
with the Ti-Ni type alloys. Particularly, the latter two Cu-type
alloys are generally considered prominent in the view of their low
cost. When comparing the two Cu-type alloys with each other,
Cu-Al-Ni alloy is superior both in the shape memory performance and
heat stability (heat resistivity). However, the defect in the
Cu-Al-Ni alloy of low cold workability somewhat hinders its
application. It has already been found that the workability of the
Cu-Al-Ni alloy can be improved by blending Ti therewith to render
the crystal grain finder.
Some of the main advantages of shape memory alloys include:
1. Bio-compatibility 2. Diverse fields of application 3. Good
mechanical properties (strong, corrosion resistant).
There are still some difficulties with shape memory alloys that
must be overcome before they can live up to their full potential.
These alloys are still relatively expensive to manufacture and
machine compared to other materials such as steel and aluminum.
Most SMAs have poor fatigue properties; this means that under same
loading conditions (i.e. twisting, bending, compressing) a steel
component may survive for more than one hundred times more cycles
than an SMA element.
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