Metal with a memory

Imagine fixing a dent in your car by pouring hot water on it and watching the panel pop back. Shape-memory alloys are otherworldly metals that “remember” their original shape and return to it when warmed. Scientists have recently discovered one such alloy that can recover its shape 10 million times without fatigue, a thousand-fold improvement on previous materials. The results, published in Science, could lead to the development of artificial muscles, or a refrigerator that works without liquid coolant.

It’s a “breathtaking development,” says Richard James, a materials scientist at the University of Minnesota. “Before seeing this paper, if somebody told me there's an alloy that could do this, I’d have said ‘You're crazy!’”

The inventors of the best-known shape-memory material, Nitinol, were themselves astonished by its properties back in 1959. Nitinol is now used to make “indestructible” spectacle frames and self-folding staples that help close bone fractures – but that’s not what researchers at Maryland’s Naval Ordnance Laboratory were seeking. In the hunt to make a better missile nose cone in the late 1950s, William Buehler was excited to find that an alloy of nickel and titanium might do the job. To show off its elasticity, he crumpled a sample into an accordion shape and passed it around at a team meeting. One of the other scientists held his lighter underneath the sample to test its heat resistance, and to everyone’s surprise, it sprang back to its original shape.

“Imagine you have a spoon made from Nitinol,” says James. You could bend it over double and twist it. “Then you just heat it up by putting it in a cup of hot water. It will return to its original shape, and it will do so quite fast … It'll surprise you!”

The force of the transformation can make the spoon jump right out of the cup. This gave researchers the idea of putting this force to work – to lift, pull or squeeze. But metal fatigue has been a problem. The alloys gradually stop snapping perfectly back into shape after a few thousand times, and can even break.

By understanding how shape memory works, Eckhardt Quandt at Germany’s University of Kiel  and Manfred Wuttig at the University of Maryland, plus colleagues, came up with a radically better material.

Shape-memory materials take advantage of quirky phase transformations. Some phase transformations, such as water freezing to ice, are obvious. But there are also phase transformations within solids – when the atoms or molecules that make up a material adopt a different pattern, like a regiment of soldiers shifting to a new formation.

In metals, these phase changes can have dramatic consequences.

Below -30 °C, tin disintegrates as the reorganisation of its atoms tears the structure apart – a transformation, legend has it, that contributed to Napoleon’s defeat in Russia. As the bitter wind rolled in, his army’s tin buttons disintegrated, causing their coats to pop open and his soldiers to freeze.

In Nitinol, the phase change is just above room temperature. A cup of hot water causes the atoms in our mangled spoon to leap back into the shape they were in when the spoon was made. But each time the spoon springs back, some of the atoms step out of line. This gradual breaking of ranks has so far limited Nitinol’s applications.

Quandt and Wuttig’s new alloy of nickel, titanium and copper, has a much longer lasting memory. That’s because the material’s two phases (the pattern its atoms adopt at high and at low temperatures) fit seamlessly together. It’s like when you tile a room with a parquet floor and try to combine two patterns of tiles, Wuttig explains. “You can do that only if the two patterns are compatible, if they share some common features.”

A similar strategy could reveal materials that not only change shape, but also their magnetic or electrical properties

The material also contains tiny particles of titanium-copper that act like marshals overseeing a military parade. When the material is warmed, these particles are trigger sites for the phase transformation, ensuring it goes smoothly. If an atom does step out of line, these marshals stop the defect from propagating through the rest of the troops.

The resulting material can be deformed – and regain its shape – at least 10 million times without fatigue. As Wuttig explains, this opens new possibilities for tiny motors with very few moving parts, artificial heart valves that can expand and contract to help regulate blood flow, or  refrigerators incorporating the new material that changes shape as it absorbs heat, as a way of keeping their contents cool.

To test the material the team stretched and relaxed it 20 times per second for about six days, repeatedly placing the alloy under almost double the stress needed to buckle structural steel. Perhaps even more significantly, a swathe of new transforming materials could just be around the corner, James says. Researchers have learned from Quandt and Wuttig's technique and are zeroing in on materials that not only change shape, but also have magnetic or electrical properties. “The field is moving rapidly forward due to these results,” says James.