Titanium-Platinum alloys remember their shape

Certain alloys of titanium and platinum have a unique tendency to bend back into their original shape when heated. Karem E. Tello Araya, a doctorate student in the Materials and Metallurgical Engineering department, has been studying how different alloys of titanium and platinum (Ti-Pt) behave under various temperatures.Certain alloys of titanium and platinum have a unique tendency to bend back into their original shape when heated. Karem E. Tello Araya, a doctorate student in the Materials and Metallurgical Engineering department, has been studying how different alloys of titanium and platinum (Ti-Pt) behave under various temperatures.

Araya, a native of the country of Chile, pointed out that these alloys have many important applications in various industries, from medical to industrial to transportation. Known as SMA’s (Shape Memory Alloy), these alloys are used in the medical field as stents (angioplasty), which is where a cylindrical mesh of the alloy is inserted into blood vessel of a patient. The metal expands in response to the heat of the body, forcing the blood vessel to remain open.

The Ti-Pt alloys are also used in aircraft jet engines, as adaptive chevrons. Chevrons are the outer ring around the jet exhaust port, usually triangle-shaped, that dampen the exhaust noise when the jet is idling on the ground. When the jet is not moving, the metal heats up and bends inward, dampening the sound from the engine. At high speed cruise, the chevrons cool because of the increased air flow over them and bend back out of the way of the jet exhaust. Araya said, “Regardless of the shape of the wire, under hot conditions the wire will recover it’s original shape.” One demonstration she showed was a video of what looked like a badly twisted wire being submerged in hot water, and the wire instantaneously recovered it’s original shape, which spelled the word “HOT.”

The present limitation in the utilization of this technology in industry is that the metal cannot withstand very high temperatures. Presently, the maximum temperature for most Ti-Pt alloys is roughly 1200 degrees Celsius, at which point it starts behaving very strangely. This strange behavior limits the use of these alloys to low-temperature applications. Araya’s research is primarily directed toward understanding the behavior of the alloys, and possibly discovering new ways to make the alloys that would allow for much higher temperature use.

In the mid-nineteenth century, new phases of the Ti-Pt alloy were being discovered, but very little was understood about their characteristics. Araya set up a testing apparatus that would allow her to closely examine the alloys as they were heated and cooled, with the purpose of better understanding the new phase that was discovered. “We designed the heat treatments to study the different phases at different temperatures,” Araya commented. As the Ti-Pt alloy was heated to above 1500 degrees Celsius, she noticed the development of interstitial oxygen within the metal itself. Interstitial oxygen is where the tiny spaces in between the metal atoms are filled with oxygen atoms, sometimes causing odd and unwanted effects.

Araya postulated that perhaps the interstitial oxygen may be causing the unique characteristics of the new phase, because an errant Ti-Pt alpha phase showed up when the alloys were heated to a specific temperature, the same temperature that produced the oxygen. At that point the properties of the alloy changed, leading Araya to take a closer look at the affects of the oxygen.

The purpose of Araya’s research is to find news ways to manufacture Shape Memory Alloys that allows them to be used in high-temperature applications. With this new understanding of how the Ti-Pt alloy acts at high temperatures, and what phases are being created, she will proceed to experiment with various methods to produce a stable alloy.