Shape memory alloys (SMA’s) 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 1960’s were any serious research advances made in the field of shape memory alloys. The most effective and widely used alloys include NiTi (Nickel – Titanium), CuZnAl, and CuAlNi.
The unusual properties mentioned above are being applied to a wide variety of applications in a number of different fields. Among the most promising are biomedical applications, aeronautical applications, micro-electromechanical systems (MEMS), and surgical tools.
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. Typically when one thinks of a phase change a solid to liquid or liquid to gas change is the first idea which comes to mind. A solid state phase change is similar in that a molecular rearrangement is occurring, but the molecules remain closely packed so that that the substance remains a solid. In most shape memory alloys, a temperature change of only about 10°C is necessary to initiate this phase change. The two solid phases, whichoccur in shape memory alloys, are Martensite, and Austenite.
Figure 1: The Martensite and Austenite phases
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 Figure 2, upon deformation this phase takes on the second form shown in the same Figure. Austenite is the stronger phase of shape memory alloys, which occurs at higher temperatures. The shape of the Austenite structure is cubic as shown in 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.
Figure 2: 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 3. The initial values of these four variables are also dramatically affected by the composition of the SMA (i.e. what amounts of each element are present).
Figure
3: The Dependency of Phase Change Temperature on
Loading
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 deforming the material the original shape can be recovered simply by heating the wire above the temperature of 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 shape of the wire. Some situations in which the shape memory effect is currently employed are:
-Pipe Couplings
-Electrical Connectors
-Coffeepot Thermostats
-Robotics
-Aerospace Actuator
-Satellite Release Bolts
Figure 4: Microscopic Diagram of the Shape Memory Effect
Pseudo-elasticity occurs in shape memory alloys when the alloy is at temperature higher than Af. The alloy is composed completely of Austenite at this point. Unlike the shape memory effect pseudo-elasticity occurs at a constant 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, the wire springs back to its original shape. Some examples of situations in which pseudo-elasticity is used are:
-Eyeglass Frames
-Orthodontic Arches
-Cell phone Antennas
-Bone Suture Anchors
-Brassiere Under wires
-Medical Guide wires
Figure 5: Load Diagram of the pseudo-elastic effect Occurring
As shown here some of the main advantages of shape memory alloys include: Bio-compatibility, Diverse Fields of Application, Good Mechanical Properties (strong, corrosion resistant)
There are still some difficulties with shape memory alloys, which must be overcome before they can live up to there full potential. These alloys are still relatively expensive to manufacture and machine compared to other materials such as Steel and Aluminum. Fatigue is also a problem for shape memory alloys. Components of machinery made of a SMA cannot be cycled through the same loading (twisting, bending, stretching etc.) many times. Most SMA’s survive 2000 cycles, while a Steel piece for example may withstand the same conditions for hundreds of thousands of cycles.
If you would like to learn more about shape memory alloys or study them more closely, please contact Dr. Abhijit Bhattacharyya at the University of Alberta, in Edmonton.
Dr. Abhijit Bhattacharyya
University of Alberta
5-8C Mechanical Engineering Building
Edmonton, Alberta, Canada
T6G 2G8
Phone: (780) 492-3705
Email: “a.bhatta@ualberta.ca”
