The Metal With A Memory
A combination of accident, luck, and hard work produced nitinol, an advanced “intelligent” metal
The patient lies on the operating table. The surgeon is implanting a filter to trap potentially fatal pulmonary embolisms (blood clots). Will this be a risky major surgical procedure involving general anesthesia, major incisions, and high costs? Not if the surgeon is using a filter made of nitinol. The surgeon can take the mushroom-shaped filter, cool it below body temperature, pull it into a straight bundle of wire, and then insert and position the bundle through a cooled catheter in one of the patient’s larger veins. When the bundle of wires warms to body temperature, it will spring back to its original mushroom shape, with six splayed feet to hold it snugly in place. The patient, who required only local anesthesia, can go home the same day.
The road that led to that operating table begins with William J. Buehler, who was born in Detroit on October 25, 1923. Buehler always had a leaning toward science and technology; his father, a schoolteacher, fostered his early interest in chemistry, physics, and mathematics. Buehler earned a B. S. in chemical engineering from Michigan State in December 1944. After graduating, he served as a main battery turret officer on a light cruiser in the Navy during the waning days of World War II.
After the war ended, Buehler earned an M.S. in metallurgical engineering at Michigan State, and in 1948 he got a job as an instructor in metallurgy at North Carolina State University. In June 1951 the Naval Ordnance Laboratory (or the NOL), now the Naval Surface Warfare Center, in White Oak, Maryland, hired Buehler as a mechanical engineer. By July 1956 he was the NOL’s supervisory physical metallurgist.
The work that resulted in the discovery of nitinol began in early 1958. Buehler was testing intermetallic compounds for the nose cone of the Navy’s below-the-surface launch missile SUBROC with a view to finding one with a suitably high melting point and other desirable properties. Having recently separated from his first wife, Buehler sought solace in the laboratory. In the three years between his separation and his divorce, he worked day and night; arising at 4:00 A.M. , he would go to the laboratory and often stay past 11:00 P.M. “Between working at the laboratory and playing golf, I really didn’t do anything other than eat or sleep,” he said later.
With the assistance of the metallurgical classic The Constitution of Binary Alloys , by Max Hansen (“my periodic table,” he called it), Buehler selected about sixty intermetalHc compounds for study. After narrowing the number of candidates to twelve, he began measuring impact resistance. Using a crude but effective test—hitting arcmelted buttons of each alloy with a hammer—Buehler noticed that a nickel-titanium alloy seemed to exhibit the greatest impact resistance. It also scored well on elasticity, malleability, and fatigue resistance. Buehler’s intuition told him that something interesting was going on with that alloy. He named it nitinol, after the chemical symbols of its two metallic components and the initials of the lab.
While varying the percentage composition of nickel and titanium, he made an observation that hinted at its extraordinary but still undiscovered property. One day in 1959 Buehler and his assistant cast six nitinol bars in their arc-melting furnace and laid them out on a table. The ones that came out of the furnace the earliest were naturally the first to cool, and Buehler took one of them to a shop grinder to smooth its surface. On the way he intentionally dropped it on the concrete floor out of curiosity and noticed that it made an unexpectedly dull thud, like a bar of lead. Puzzled, he decided to drop the other bars on the floor to see what sort of sound they made.
“To my amazement,” Buehler says, “the warmer bars rang with a bell-like quality. Following this, I literally ran with one of the warmer bars to the closest source of cold water—the drinking fountain—and chilled the warm bar. After thorough cooling the bar was again dropped on the floor. To my continued amazement it now exhibited the leaden-like acoustic response. To confirm this unique change, the cooled bars were heated through in boiling water, they now rang brilliantly when dropped upon the concrete floor.” Buehler knew that the acoustic damping signaled a change in atomic structure, one that could be turned off and on by simple heating and cooling near room temperature. He did not yet know that this atomic rearrangement would later show up as the phenomenon we now call shape memory.
It was clear that nitinol was worth a detailed Investigation. In 1960 Buehler was joined by Raymond C. Wiley, from an NOL group that had been working on failure analysis in various materials. Wiley would generate much of the data essential for understanding nitinol’s properties. The following year Wiley was scheduled to address a meeting of NOL management to review the laboratory’s ongoing projects. He brought along a prop: a long strip of nitinol a hundredth of an inch thick, bent into short accordion-like folds. The plan was to rapidly compress and stretch the strip to demonstrate nitinol’s fatigue resistance. During the presentation the directors passed the nitinol strip around the table, repeatedly flexing and unflexing it. Everyone was impressed with how well it held up.
Then David S. Muzzey, one of the associate technical directors, decided to see how it would behave under heat. Muzzey was a pipe smoker, so he held the compressed nitinol strip in the flame of his lighter. To the great amazement of all, it stretched out longitudinally. When Buehler heard about the incident, he realized that it had to be related to the acoustic behavior he had noted earlier. But while the sonic phenomenon was essentially an interesting curiosity, it was clear from the start that shape memory could have very important applications.
No one understood the mechanical-memory property of nitinol at first. To figure out the atomic-level behavior behind it, Buehler recruited Frederick E. Wang, whose expertise in crystal physics gave the project new impetus. Wang was born on August 1, 1932, in Su-Tou, Formosa (now Taiwan). After emigrating to the United States, he did his undergraduate work in chemistry and physics at the University of Tennessee. He earned a master’s from the University of Illinois in 1957 and a doctorate from Syracuse University in 1960. He was working as a postdoctoral fellow at Harvard University with William N. Lipscomb, Jr. (who would win the Nobel Prize in chemistry in 1976), when he left to join Buehler.
Buehler and Wang began investigating the basic structural characteristics of nitinol. Buehler was unable to concentrate fully in the laboratory during this period because once nitinol’s properties became public, he had to perform much public-relations work for NOL, answering thousands of letters and hundreds of telephone calls requesting information. “I was constantly running from the laboratory to the phone,” he recalls. There were also “dogand-pony shows” for members of Congress and other high-ranking officials. Despite these distractions, Buehler and Wang’s persistence through the 1960s finally revealed how nitinol’s mechanical memory worked.
Phase changes between solid and liquid (i.e., melting or freezing) and between liquid and gas (i.e., evaporation or condensation) are familiar, everyday phenomena. Less familiar are changes that occur between two solid phases, which involve rearrangement of the particles (atoms, molecules, or ions) within a crystal lattice. For example, under great pressure one crystallic form of carbon—graphite—can be converted into another—diamond. Many materials undergo such transformations; nitinol’s unique property is that when it undergoes a phase transition due to a temperature change, it changes its shape as well. It “remembers” its shape because the phase change affects its structure on the atomic level only, without disturbing the arrangement of the crystals, which would be irreversible.
To fix the original shape, nitinol must be heated to about 500°C, or roughly 930°F, for an hour while held in a fixed position. During this process it enters what is called the austenite phase (after an English metallurgist, William Chandler RobertsAusten)—a rigid lattice of nickel atoms surrounded by titanium atoms. As nitinol cools below its transition temperature (which varies with the proportions of nickel and titanium), it adopts the martensite phase (named for the German metallographer Adolf Martens), in which the nickel and titanium atoms assume a different arrangement, one that is very easy to bend and deform. As the deformed nitinol is reheated, thermal motion causes the atoms to snap back into the austenite phase, restoring the original shape of the object.
With the mystery of nitinol’s properties revealed, Buehler and Wang worked on moving it from the experimental world of the laboratory to the commercial world of useful products. As early as 1969 they were working with John D. Harrison of the Raychem Corporation in Menlo Park, California, to produce Cryofit shrink-to-fit pipe couplers, which are still used to join hydraulic lines in F-14 fighter aircraft, as well as in many similar industrial applications. After being cooled in liquid nitrogen, the coupler is mechanically expanded until it is easy to slip over tubes to be joined. On warming to room temperature, it shrinks to form a totally sealed joint.
Today the use of nitinol in such hydraulic applications is well established, but in its early stages it caused Buehler considerable anxiety. While on a Christmas golfing vacation in Myrtle Beach, South Carolina, in December 1970, he saw a headline in the local paper: NAVY’S FIRST F14 CRASHES, EXPLODES ON TEST FLIGHT . Shocked, he read the opening paragraph: “The Navy’s only F14—it won’t get another one until February—developed a failure in its hydraulic system and crashed between 1,000 and 1,500 feet short of the runway, then exploded. A company spokesman said the plane was ‘wiped out.’” It was one of the biggest shocks Buehler has ever suffered. Navy investigators later learned, however, that the crash had been caused by a resonance-frequency failure in the hydraulic piping and had nothing to do with the fittings, which remained intact. In fact, the crash provided the ultimate test of nitinol’s ruggedness.
Another modern use of nitinol is in dentistry. In 1968 George F. Andreasen, a professor of orthodontics at the University of Iowa, read about Buehler and Wang’s strange alloy. At the time, orthodontists were using stainless-steel wire to constrain the teeth and push them in the right direction, but it was not springy enough and required constant readjustment. “It hit me like a bomb,” Andreasen recalled a decade later. “If we had a wire for orthodontic use that could return to its original set shape, we’d have the best of everything.” Andreasen got in touch with Buehler, who supplied technical details about the alloy. In the Cold War climate of the 1960s and under a huge backlog of requests for samples, it took a year for NOL to authorize the release of a small quantity of nitinol to Andreasen.
In his basement at home Andreasen had a fully equipped metalworking shop where he made jewelry as a hobby. There he experimented with the new material for two years. He eventually came up with a fine wire, cold-worked to 19/1,000 of an inch in diameter, that was very springy and had a very high degree of elastic recovery. After molding it to fit the patient’s mouth, an orthodontist can easily bend the wire and attach it to malpositioned teeth. As body heat penetrates the wire, it attempts to return to the shape in which it was molded. In so doing, it exerts gentle but persistent pressure on the teeth, forcing them into alignment. When Andreasen began using nitinol braces in patients in 1975, he cut treatment times in half.
Aeronautics and orthodontics are just two of the scores of areas where nitinol is useful. Buehler estimates that if he and Wang had concentrated on developing different ways to utilize his discovery, they could have come up with many patent applications for nitinol-based products. But they decided instead to concentrate on the composition of nitinol and left manufacturing to the manufacturers.
In 1974 Buehler left NOL to join the faculty at Virginia Polytechnic Institute. He is now enjoying his retirement in Florida and expresses amazement at the diversity of products that nitinol has spawned. Wang, on the other hand, has continued to work in the field. After leaving NOL in 1980, he founded Innovative Technology International, Inc. (ITI), which produces and supplies nitinol to manufacturers. He is still president of the company.
With improvements in nitinol manufacturing techniques during the 1970s, commercial products have begun to enter the market, from the serious to the frivolous. On the serious side, perhaps the most important applications have been in cardiovascular and orthopedic surgery. Mitek Surgical Products’ nitinol-based Mitek Anchor is expected to replace screws, staples, and other bulky hardware as the primary method of reattaching broken bones and tendons. Buehler himself participated in an early phase of this product’s development. Shortly before his retirement he hand-ground threads on some bone screws that were going to be used in tissue-compatibility studies. The NOL machine shop couldn’t do it, so Buehler used his motor skills to complete the work. That piece of work may have expedited the everexpanding use of nitinol in medical products.
Wang’s company, ITI, has built prototypes of nitinolbased engines that convert thermal energy to mechanical energy. First the nitinol passes through a hot-water bath and contracts; then it passes through a cold-water bath and expands. The resulting force drives an engine. The nitinol engine is still years from production, but two nitinol products already in existence and saving lives are sprinkler systems and automatic tap turnoffs. Nitinol-based fire Sprinklers respond far more quickly than conventional ones, while a nitinol antiscalding device shuts off a tap when the temperature of the water rises too high.
Hinges made from a nitinol spring are now used to open greenhouse windows whenever the temperature inside becomes too high. A coffee maker is being marketed with a nitinol valve that will not release the water onto the coffee grounds until it reaches a predetermined temperature. Yet another innovative use of nitinol is in underwire brassieres. Conventional underwire bras have to be hand-laundered because a washing machine would deform the wire. Such deformation causes no problem with nitinol bras because the wire automatically goes back to its original shape when it returns to room temperature.
On the more frivolous side, a toy called Space Wings consists of a pair of wings, a piece of nitinol wire, and a printed circuit board/battery module that heats the wire electronically. When heated briefly, the wire contracts and flaps the wings. As it cools, it expands, moving the wings in the opposite direction. The cycle repeats again and again, and the toy flaps away. Other nitinol novelties include blinking movie posters and tail-swishing dinosaurs.
As with much science and technology, the discovery and development of nitinol resulted from a combination of Buehler and Wang’s hard work and good fortune. If David S. Muzzey had not held his pipe lighter to the nitinol sample at the 1961 management meeting, the alloy’s remarkable properties might have remained unknown. Nitinol was the first of what are now called “intelligent” materials—ones that can sense changes in their environments and respond appropriately. For now, most intelligent materials remain on scientists’ and engineers’ wish lists. But because of Buehler and Wang’s investigation of nitinol, other shape-memory alloys are currently in research and development.
Less than a year before his death in 1989, Dr. Andreasen wrote to Buehler after Buehler’s wife of thirty years had undergone some orthodontic work: “If it hadn’t been for you, your wife would be treated with all stainless steel wire. If you had not sent me the 3 foot piece of Nitinol wire I could not have applied it to orthodontics. In fact, I’ll be in your debt forever.” Because of nitinol’s unique properties and its myriad applications, not only Dr. Andreasen and Mrs. Buehler but all of us are in Buehler and Wang’s debt.