Shape memory metal gets new job on Mars

Two metals, nickel and titanium, come together in a unique alloy that is gaining popularity in new applications, including in outer space. Known as "nitinol," this high-demand metal is prized for its ability to snap back to a heat-trained shape after being pulled, twisted or deformed.

Accidentally discovered by metallurgist William Buehler at the Navy Ordinance Laboratories in 1959, nitinol has been used in increasingly sophisticated applications over the years, finding utility in everything from flexible eyeglass frames to tiny mesh tubes that can dilate a heart patient's blood vessels without mechanical inputs or wires to help.

Now the shape-shifting capabilities of this critical metal alloy is again turning heads with engineers at NASA's Glenn Research Center using nitinol in a recent multiyear venture. The metal is expected to face its most challenging application yet, as within the next decade a rover equipped with wheels made of nitinol embarks on a sample-return mission on Mars.

With wheels designed to flex around rocky obstacles and reshape back to their original form, the rover will retrieve soil samples dropped by Perseverance during its exploration of the red planet's surface.

Nitinol works its magic through heat.

Othmane Benafan, Ph.D., a materials research engineer at NASA Glenn Research Center, told the science and technology website, The Verge, that share memory alloys are governed by the principle of transformation.

While most transformations involve a material changing from a solid to a liquid or a gas, nitinol transforms, or rearranges, its atoms by going from one solid state to another solid state.

"It's a solid-to-solid transformation," said Benafan. "The key to shape memory alloys is that this transformation is reversible. So, you can go back and forth. There's always a path to go from one state to another."

To 'train' a paper clip made of nitinol, for example, you heat it at 500 degrees Celsius in its desired shape, then splash it in cold water. Bend it out of shape, then return the same heat source, and the metal will eerily slink back into its original form, according to The Verge.

Temperatures that can trigger nitinol's transformation vary, depending on the ratio of nickel to titanium. Engineers can train the metal to adapt to a wide range of conditions, making it a key tool in places where complex mechanics do not fit, like a hinge that positions a solar panel by responding to the sun's heat.

Research leads to discovery

Interestingly, Nitinol was not the first shape memory alloy discovered.

Researchers had been experimenting with gold-cadmium since 1939, but the shape memory effect was minimal, and the material was extremely expensive ($100/gram).

When Buehler made his discovery in 1959, he had been working on a project to develop a nose cone for the Polaris missile that was capable of withstanding the heat of re-entry into the Earth's atmosphere.

He described the project as "boring" and said he hoped something "interesting" would pop up.

Buehler had selected roughly 60 alloys for further examination from a book titled, "Constitution of Binary Alloys" – nitinol being one of them. When he made ingots of the alloys for testing, he intentionally dropped one of the cold ones on the floor. Hoping to hear a clear bell-like ring, indicating that the metal had the properties he was hoping for. One ingot returned a dull thud, similar to the sound of dropping a sack of flour on the ground. Worried that the ingot was filled with internal flaws, he dropped one of the ingots that had not cooled yet. This returned a wonderful bell-like ring. However, after the ingot had been cooled in water, it returned the same dull, leaden thud. This was the first indication that nitinol had a substantially different double state.

Buehler named the alloy "nitinol" for "Nickel-Titanium Naval Ordinance Laboratories." However, he did not discover the shape memory aspect of the alloy until a lab meeting in 1961.

The scientist had been performing tests to determine the fatigue life of nitinol by bending a strip into an accordion like shape over and over again. His project was brought under review and his technician was demonstrating the fatigue properties to senior officials. During this visit, one of the officials present heated the nitinol with a lighter, at which point it rapidly straightened out. This, of course, sent ripples throughout the scientific community.

The material could take low grade heat and generate mechanical energy!

Numerous scientists began experimenting with ways to build engines with nitinol that would take low-grade energy and transform it into very high-grade energy that could do work. The efforts culminated in the Nitinol Heat Engine Conference, hosted by the Naval Surface Weapons Center (previously Naval Ordinance Labs) in 1974. At the Nitinol Heat Engine Conference, the NSWC gathered together the top scientists who had been working on nitinol to discuss what had been done and what still needed to be done to make nitinol heat engines a reality.

Modern applications abound

Twenty years later, Marchon Eyewear introduced an application of nitinol into the market under the trade name Flexon and advertised it as a product you could bend into incredible distortions and see it snap back to its original shape once released.

Nike began to use the material in its Vision line of glasses, and athletes everywhere snapped them up. Gone were the days of breaking your glasses on a regular basis just because you lived an active lifestyle. People who broke their glasses every six months could now go several years on a single "super-elastic" frame.

It was not long before surgeons began using nitinol as well. Vascular stents were an early medical application because the stents could be folded so flat, they could be inserted through the tiniest of holes into the patient's bloodstream – minimizing recovery time. Once in place, the nitinol wire could withstand severe deformation and outlast stainless steel by an order of magnitude. For example, nitinol stents were capable of undergoing a 30% deformation with a cycles-to-failure life expectancy greater than 10 million. Stainless steel, on the other hand, can withstand a deformation of just 0.5% with fatigue life of around 1 million cycles. This made intravascular stents relatively permanent, never needing replacement.

The one question the medical community posed revolved around nitinol being more than 50% nickel, and some people being allergic to nickel. This could potentially cause huge problems for patients if enough nickel were to dissolve out of the nitinol into the bloodstream.

Fortunately, researchers soon discovered that nitinol's biocompatibility is unsurpassed and that doctors could implant it into patients in any way they saw fit. Since then, nitinol has replaced other alloys in just about every kind of implant in the human body. Researchers have found that it makes a great hip replacement material because the super-elastic phenomenon damps out the vibrations caused by walking – greatly extending the useful life of a joint replacement.

Nitinol wire is also highly biocompatible, resisting corrosion in the human body and proving safe in vascular, soft tissue and orthopedic applications.

Because of the broad spectrum use of nitinol in surgical implants, the medical field is the largest consumer of nitinol worldwide, with the second-largest consumer of the alloy being the eyeglass industry.

The National Science Foundation has sponsored materials science research since the 1990s at universities across the United States. These small, independent research groups turned up all sorts of interesting, useful knowledge about nitinol. To organize results of this research and to share knowledge gained, the American Society of Materials created the Shape Memory and Superelastic Technologies group, which meets every 18 months to discuss the latest improvements in the material science of nitinol.

From its first meeting in 1997 with a gathering of experts from all over the world, SMST began corroborating findings about all sorts of things. New alloys of nitinol were discovered. Termed 55-Nitinol, 60-Nitinol, and 65-Nitinol for the approximate percent nickel content, these alloys had slightly different properties than regular nitinol. It was not long before scientists were discussing adding third and fourth elements into nitinol alloys to alter the properties to give something more desirable.

The research also sparked renewed interest from mechanical engineering scientists. In 2012, General Motors announced that they were working on a nitinol heat engine that would capture the waste heat from the exhaust from the engine and generate electricity. The hope here was to replace the alternator and use less fuel. Then in 2013, Kellogg's Research Labs reported building a generator capable of harnessing the energy from the daily change in air temperature. In its report, KRL said their thermal efficiency was between 1 and 4 percent. Hardly impressive compared to automobiles, which run at roughly 25% efficiency, or to power plants which run at 43% efficiency. Unlike those systems, however, the amount of heat energy discharged by the atmosphere on a daily basis is several orders of magnitude greater than all of the power generated by all power plants on earth, and it is free. So, 1% of a very large number is still a very large number, the scientists noted.

Today, nitinol wire is also widely used in the aerospace, energy and industrial sectors.

Nitinol alloy wire is ideal for laser cutting or electro discharge machining. Photo-etching and stamping are also common methods of producing components made from nitinol alloy wire.