Memory metal The invention: Known as nitinol, a metal alloy that returns to its original shape, after being deformed, when it is heated to the proper temperature. The person behind the invention: William Buehler (1923- ), an American metallurgist The Alloy with a Memory In 1960,William Buehler developed an alloy that consisted of 53 to 57 percent nickel (by weight) and the balance titanium. This alloy, which is called nitinol, turned out to have remarkable properties. Nitinol is a “memory metal,” which means that, given the proper conditions, objects made of nitinol can be restored to their original shapes even after they have been radically deformed. The return to the original shape
is triggered by heating the alloy to a moderate temperature. As the metal “snaps back” to its original shape, considerable force is exerted and mechanical work can be done. Alloys made of nickel and titanium have great potential in a wide variety of industrial and government applications. These include: for the computer market, a series of high-performance electronic connectors; for the medical market, intravenous fluid devices that feature precise fluid control; for the consumer market, eyeglass frame components; and, for the industrial market, power cable couplings that provide durability at welded joints. The Uncoiling Spring At one time, the “uncoiling spring experiment” was used to amuse audiences, and a number of scientists have had fun with nitinol in front of unsuspecting viewers. It is now generally recognized that the shape memory effect involves a thermoelastic transformation at the atomic level. This process is unique in that the transformation back to the original shape occurs as a result of stored elastic energy that assists the chemical driving force that is unleashed by heating the metal.The mechanism, simply stated, is that shape memory alloys are rather easily deformed below their “critical temperature.” Provided that the extent of the deformation is not too great, the original, undeformed state can be recovered by heating the alloy to a temperature just below the critical temperature. It is also significant that substantial stresses are generated when a deformed specimen “springs back” to its original shape. This phenomenon is very peculiar compared to the ordinary behavior of most materials. Researchers at the Naval Ordnance Laboratory discovered nitinol by accident in the process of trying to learn how to make titanium less brittle. They tried adding nickel, and when they were showing a wire of the alloy to some administrators, someone smoking a cigar held his match too close to the sample, causing the nitinol to spring back into shape. One of the first applications of the discovery was a new way to link hydraulic lines on the Navy’s F-14 fighter jets. The nitinol “sleeve” was cooled with liquid nitrogen, which enlarged the sample. Then it was slipped into place between two pipes. When the sleeve was warmed up, it contracted, clamping the pipes together and keeping them clamped with a force of nearly 50,000 pounds per square inch. Nitinol is not an easy alloy with which to work. When it is drilled or passed through a lathe, it becomes hardened and resists change. Welding nitinol and electroplating it have become manufacturing nightmares. It also resists taking on a desired shape. The frictional forces of many processes heat the nitinol, which activates its memory. Its fantastic elasticity also causes difficulties. If it is placed in a press with too little force, the spring comes out of the die unchanged.With too much force, the metal breaks into fragments. Using oil as a cooling lubricant and taking a step-wise approach to altering the alloy, however, allows it to be fashioned into particular shapes. One unique use of nitinol occurs in cardiac surgery. Surgical tools made of nitinol can be bent up to 90 degrees, allowing them to be passed into narrow vessels and then retrieved. The tools are then straightened out in an autoclave so that they can be reused.Many of the technical problems of working with nitinol have been solved, and manufacturers of the alloy are selling more than twenty different nitinol products to countless companies in the fields of medicine, transportation, consumer products, and toys. Nitinol toys include blinking movie posters, butterflies with flapping wings, and dinosaurs whose tails move; all these applications are driven by a contracting bit of wire that is connected to a watch battery. The “Thermobile” and the “Icemobile” are toys whose wheels are set in motion by hot water or by ice cubes. Orthodontists sometimes use nitinol wires and springs in braces because the alloy pulls with a force that is more gentle and even than that of stainless steel, thus causing less pain. Nitinol does not react with organic materials, and it is also useful as a new type of blood-clot filter. Best of all, however, is the use of nitinol for eyeglass frames. If the wearer deforms the frames by sitting on them (and people do so frequently), the optometrist simply dips the crumpled frames in hot water and the frames regain their original shape. From its beginnings as an “accidental” discovery, nitinol has gone on to affect various fields of science and technology, from the “Cryofit” couplings used in the hydraulic tubing of aircraft to the pin-and-socket contacts used in electrical circuits. Nitinol has also found its way into integrated circuit packages. In an age of energy conservation, the unique phase transformation of nickel-titanium alloys allows them to be used in lowtemperature heat engines. The world has abundant resources of low-grade thermal energy, and the recovery of this energy can be accomplished by the use of materials such as nitinol. Despite the limitations imposed on heat engines working at low temperatures across a small temperature change, sources of low-grade heat are so widespread that the economical conversion of a fractional percentage of that energy could have a significant impact on the world’s energy supply. Nitinol has also become useful as a material capable of absorbing internal vibrations in structural materials, and it has been used as “Harrington rods” to treat scoliosis (curvature of the spine).