Materials Science Nitinol Memory Wire

Materials Science Nitinol Memory Wire

Shape memory alloys (SMAs) are metals that "remember" their original shapes. Nickel-titanium alloys, which are known by the generic name Nitinol, are the most useful of the SMAs, although other alloys are also known. Nitinol has two different solid phases. At higher temperatures, its atoms are arranged in a crystal lattice known as the austenite phase. At lower temperatures, the atoms shift to a different crystal structure called the martensite phase. This transformation occurs around 50 °C, although it can vary based on the ratio of Ni to Ti in the wire. The crystal lattices for the two solid phases are shown below. When Nitinol changes from austenite to martensite, its atoms simply shift position, much like a square shifting into a parallelogram.

When Nitinol wire is in the austenite phase, it is stiff and springy. In the martensite phase, it is soft and bendable. When most metals are bent or deformed, their atoms move along defects or slip planes, and a complete rearrangement of atoms occurs. This is not the case when Nitinol is deformed. It simply changes the orientation of its crystal structure. The atoms shift, but the relative location of atoms remains unchanged. In this way, the original arrangement of atoms is “remembered” by the Nitinol. When it is heated, the atoms snap back to their original positions in the austenite phase, and the Nitinol wire returns to its original shape.

Materials

Nitinol wire, beaker and hotplate, candle or Bunsen burner, 9-volt battery and battery snap

Procedure

1. Bend or coil a small piece of Nitinol wire. While holding one end of the wire with tongs, slowly lower it into a beaker of hot water. What happens? Is this reversible? How hot does the water have to be?

2. Get a feel for the force released when Nitinol returns to its austenite phase. Bend the wire in half, and try holding each end of the wire in place as you lower the bent end into a beaker of hot water (this is easier if your beaker is filled up at least ¾ of the way). Would you describe this as a strong or a weak force?

3. Now try heating up the Nitinol wire with electricity instead of hot water. Bend the Nitinol wire, then connect each end of it to a 9-volt battery for a few seconds (don’t let the battery get hot, or you will drain the battery!). What happens?

4. The shape of a piece of Nitinol can be reset by heating it above 500 °C. The high temperature causes the atoms to reorganize into the most compact and regular pattern possible, resulting in a new austenite phase.

To do this with your piece of Nitinol, bend it and hold it in position. Carefully bring the bent end of the wire close to a candle or Bunsen burner flame (NOT in the flame – if it gets too hot it will lose its shape memory ability). Heat it SLOWLY until you feel a release of tension. At that point, remove the wire from the vicinity of the flame. Do NOT heat the wire more than is necessary to release the tension.

Let the wire cool. It has now been set into a new shape. Repeat step 1 to see if it remembers its new shape. What do you observe?

Postlab Questions (answer in your lab notebook)

1. Use the unit cells given to determine the chemical formula for Nitinol. Show that the austenite and martensite phases give the same formula.

2. Calculate the density of Nitinol in the austenite phase. The average atomic radii of nickel and titanium are 124 pm and 147 pm, respectively.

3. Search online for an interesting application or use of Nitinol wire. Try to find an application that is different from one we talked about in class (and different from one your friends found!). Describe the application, and give a reference.