Many people will have heard of 'memory metal', and perhaps seen a demonstration of its startling ability to take on a new shape when heated. It is certainly a surprising phenomenon, as a length of straight wire suddenly curls itself into, say, a paperclip.
The proper name for this unusual material is Nitinol, an alloy of approximately equal portions of nickel and titanium. Its name comes from the first two letters of each of those elements, plus the abbreviation of the US Naval Ordnance Laboratory where it was first created in 1961.
Nitinol is the best known and most successful example of a group of materials called Shape memory alloys, or SMAs. These metals can 'remember' a shape they are set into at high temperature and, once cooled, can be bent and deformed at will up to 8% strain. They will then recover the memorised or 'parent' shape when heated above a transition temperature.
The Chemistry of Shape Memory
The atoms of any solid material are arranged in a fairly rigid pattern, and in a metal that pattern is a regular crystal lattice. Shape memory alloys have the unusual property of being able to arrange their atoms in one of two different crystal patterns or phases, known as martensite and austenite.
The austenitic arrangement is the most compact, with the atoms as close to each other as they can get, but packing atoms this tightly requires energy. Under normal conditions, and below the transition temperature, Nitinol will automatically adopt the lower-energy martensite arrangement.
To set the shape of a sample of Nitinol, it must first be heated to about 500°C, which is far above the transition temperature. It can then be formed into the desired 'parent' shape, with its atoms tightly packed in the austenite phase. As it cools, its atoms re-adopt the martensite phase and the sample can be bent and deformed at will.
When heat is applied, the material absorbs the energy until it reaches its transition temperature. At this moment the crystal structure has taken on enough energy to re-arrange itself back into the austenite pattern. This instantaneous change causes the metal to revert to the previously-set parent shape.
Making use of Shape Memory
This remarkable property of Nitinol to instantly recover a set shape has many interesting applications. Because the material was first developed at the US Naval Ordnance Laboratory, it will be no surprise that it was first put to use in military hardware. One early application was to create leak-proof seals on fuel lines in fighter aircraft.
Many of the metal's more recent applications are in the medical world, and it is here that much of the high-grade Nitinol wire produced by Fort Wayne Metals is used. In dentistry, the appropriate grade of wire is fashioned into dental braces which can be deformed as needed to make fitting a comfortable experience for the patient.
The specific formulation of alloy used here will have a transition temperature a little below body temperature, so the cooled wires will spring into shape once warmed by the mouth. Futhermore, the parent shape will be slightly smaller than the shape of the teeth, so that the brace exerts a continuous gentle pressure to push the teeth into the desired arrangement. Orthodontics made this way do not require wire replacement as frequently as conventional alternatives.
The ability to exert a continuous pressure is key to another popular medical use of Nitinol, in stents for insertion into blood vessels. Folded stents are easier to insert, via a catheter, into the blood vessel at the site of a potential blockage. Once again a body-temperature trigger will cause the device to expand and exert a small but steady force to keep an artery or vein open.
It will be apparent that the material used for such surgical applications must be of the highest quality. An example is medical grade NiTi#1,
Fine-tuning the Recipe
Nitinol is made of approximately equal amounts of nickel and titanium, and small variations in these proportions have a radical effect on the properties of the alloy and in particular its transition temperature. This means that Fort Wayne Metals' modern advanced manufacturing processes can create memory metals which will trigger at carefully predetermined temperatures.
While body heat is a convenient transition for medical devices, some Nitinol applications require higher triggers. Safety devices like anti-scald mechanisms in showers, or fire sprinklers, make use of both fine-tuned transition temperatures and the large force that Nitinol can exert in order to control water flow.
Somewhere in between these temperatures is the trigger for self-repairing spectacles, which are made of Nitinol wire and will recover their set shape by simply being placed in warm water.
Although it is the shape memory phenomenon for which Nitinol is best known, the material also has other useful properties. In particular, it has excellent fatigue strength, which means it can be cycled many thousands of times without any significant change in its strength or shape.
Many more conventional metals undergo the process of work hardening, which causes them to become brittle after a few cycles and then snap. This is easily experienced by bending a paperclip back and forth a few times.
Another remarkable property is superelasticity. As the name suggests, this means that components such as springs made of Nitinol wire are capable of carrying much higher strain loads than conventional metals, returning to their original shape without deformation.
This property is exploited in high-precision mechanisms such as mechanical watch springs. Premium applications like these are candidates for the exceptional quality of surface finish.
The ability of Nitinol to exert a substantial force when reverting to its parent shape has been the object of some interesting experiments in energy conversion. Many machines are efficient at turning mechanical energy into heat, but turning heat into useful mechanical energy is an engineering challenge that has hardly been improved upon since the steam engine. Nitinol heat engines have been built on a small scale, as proof of concept, and it is possible that in future these will become a useful source of energy recovery.
Finally, perhaps the most exciting application of the metal is in conjuring tricks. Nitinol spoons can create a very convincing demonstration of magic to an unwary audience by bending with no effort other than the heat of the magician's hand.