Imagine untwisting a finger-size spring, then holding the flame from a lighter underneath the unraveled section. Like magic, it twirls itself into a spring again because it’s made from a metal alloy that remembered its original shape.

Sandia researchers think such shape-memory alloys could be used to improve safety in weapons components in a fire or other accident. A thermal device made of a high-temperature shape-memory alloy might, for example, close or open a switch or lock a gear to prevent it from turning, says materials scientist Don Susan (1831).

"It’s almost unlimited what you can think of, what you can do with shape-changing alloys," he says.

Don is principal investigator for a Laboratory Directed Research and Development (LDRD) project, now in its third and final year, aimed at creating high-temperature shape-memory alloys for weapons components.

Accomplishments so far:

• Researchers made new alloy compositions, including nickel titanium platinum, nickel titanium palladium, and nickel titanium hafnium, and filed technical advances for those compositions, a first step in documenting an invention for a later patent application.
• The team has characterized such key properties of the materials as the way they change shape, strength, and ductility.
• Team members have produced prototype parts that show shape change at desired temperatures.
• Sandia is the first to demonstrate a property called super-elasticity in higher temperature shape-changing alloys, and filed a technical advance for that. Super-elasticity is a rubbery sort of behavior in metals, such as eyeglass frames that twist without snapping. Don says Sandia doesn’t currently intend to exploit this property but it could provide future design options.

Alloys remember original shape

Shape-memory alloys work somewhat like the thermal sensor in a building’s fire sprinkler system. That thermal sensor is made of a liquid that expands and breaks a glass enclosure, triggering a switch that turns on the sprinklers. Shape-memory alloys work in a similar way, but change shape instead of expanding.

"If you bend a wire, it’ll go back to straight if it was originally straight," Don explains. "If it was originally bent and you made it straight, it will go back to bent. It will remember a shape when you heat it up."

Such an alloy can trigger a process simply because it’s able to change shape, says project manager Jim McElhanon (1835), who started the LDRD with weapon safety engineer John Debassige. Don, part of the team from the start, became team lead when Jim became a Sandia manager after the project’s first year. Other members are Tom Buchheit (1814), Jordan Massad (1526), Don Bradley (1833), and Mark Reece and Tom Crenshaw (both 1831). Sandia also collaborates with Ron Noebe and his colleagues at NASA’s Glenn Research Center in Ohio.

"I truly believe this research on [high-temperature] shape-changing alloys will allow us to create new devices that significantly impact nuclear weapon safety. The shape-memory alloys we are developing can passively change shape via exposure to a particular temperature or actively change shape by passing current, which generates heat, through the material," Jim says.

Shape-memory alloys have been around for decades and various types are sold commercially. They’re commonly used in the human body in medical appliances such as stents that change shape at body temperature. A tiny stent, stored at below-body temperature, can be squeezed small enough to fit into an artery, then opens up the artery when warmed to body temperature, Don says.

Shape change needed at specific temperatures

The Sandia alloys can change shape at temperatures below room temperature to greater than 500 degrees Celsius, or about 930 degrees Fahrenheit.

Commercial alloys change shape at temperatures that don’t meet Sandia’s needs, Don and Jim say.

Sandia built upon recent research into higher temperature shape-memory alloys to create its own alloys.

"Folks at Sandia were studying these alloys decades ago, but the temperatures were always too low to be useful for our parts until these new alloys came along over the past 15 years or so," Don says.

Any shape change has to take place above the temperature at which components are manufactured, Don says.

"You don’t want this to happen when you’re making the parts," he says. "You don’t want it to happen when it’s sitting out in the sun either. It has to be higher than that."

In April, the Consortium for the Advancement of Shape Memory Alloy Research and Technology (CASMART) voted to add Sandia as a member. Government, academic, and industry experts in the field started CASMART in 2006 to share applied research on shape-memory alloys.

"Joining the consortium is a huge step forward for the Labs," Jim says. "We are collaborating with the world experts in the area."

Computer models to show behavior

In addition to its cooperation with NASA’s Glenn Research Center, Sandia also is working with Texas A&M University in College Station on shape memory alloys. NASA and Texas A&M are both consortium members. NASA is interested in the alloys for flight applications, while the university works on materials processing — turning the alloys into specific shapes, Don says. In addition, he and Jim say Texas A&M researcher Brian Lester is working with Sandia this summer on computer models of shape-memory alloy behavior.

"Our computer models can’t handle something that changes shape like that," Don says. "When you heat something up, it expands a little bit and when you cool it down it contracts. We can handle that in the computer codes, but not this more dramatic shape change."

Don envisions Sandia eventually studying shape-memory alloys for wind and solar energy and perhaps satellites.

"They are really interesting materials," he says. "Most of what we work on at Sandia is stainless steel, aluminum, the kinds of things we’ve worked on forever and that most of our parts are made of. So it’s interesting to work on something different and explore the possibilities."