The world population is growing, globalization has resulted in a higher standard of living in many countries, and people are living longer. With increased living standards and choices people make, lifestyle-related illnesses, such as cardiovascular diseases, are on the increase. Companies race to make medical devices to cure challenging physical conditions and diseases. Novel materials are an integral part of supporting such design and development. One such material is Nitinol (NiTi), a serendipitous discovery in 1959 by William J. Buehler during research at the U.S. Naval Ordnance Laboratory, White Oak, MD. Nitinol, which saw use in medical devices beginning in the late 1980s, stands for Nickel Titanium Naval Ordnance Laboratory.
Nitinol is one among many shape memory alloys (SMAs) that have the ability to restore their original shape after deformation. Used in a variety of applications in industries ranging from consumer appliances to automotive to aerospace and medical, SMAs have gained a strong foothold because they offer designers incredible flexibility replacing conventional materials. In medical devices, Nitinol is popular due to its biocompatibility and superelasticity. Nitinol is used to manufacture stents, guide wires, stone retrieval baskets, filters, needles, dental files, and other surgical instruments. (See Figure 1)
The Shape Memory Effect
The most common demonstration of the shape memory effect is that a piece of this metal can be deformed—for example, by winding a piece of straight wire into a tight coil–and then the deformation can be completely removed by heating the metal a small amount, such as dipping it into hot water. As it is heated, the metal instantly “remembers” its old shape and springs back to the form of a straight wire. The shape memory effect is caused when the material undergoes a change in the crystal form as it is cooled or heated through its characteristic transformation temperature. This change in crystal structure in the NiTi alloys is from an ordered cubic crystal form above its transformation temperature (austenite) to a monoclinic crystal phase below the transformation temperature (martensite).
The majority of commercial applications utilize another useful property, which is its exceptional elasticity, commonly referred to as “superelasticity,” when one deforms the alloys at a temperature above the transformation temperature. Above the transformation temperature, the material is in the high temperature or austenitic phase. When stress is applied, the deformation causes a stress-induced phase transformation from austenite to deformed martensite. When the applied stress is removed, the material immediately springs back, and the crystal form returns to the austenite phase.
Most NiTi materials are a simple alloy of nickel and titanium with the ratio of the two constituents at about 50 atomic percent each (about 55 percent by weight of nickel). However, subtle adjustments in the ratio of the two elements can make a large difference in the properties of the NiTi alloy, particularly its transformation temperatures, i.e., the temperatures at which the crystal structure of the alloy changes from austenite to martensite or vice versa. If there is any excess nickel over the 50/50 ratio, one sees a dramatic decrease in the transformation temperature and an increase in the austenite yield strength. Increasing the nickel-to-titanium ratio to 51/49 causes the transformation temperature to drop by more than 100°C. This sensitivity of the properties to very small increases in the percent of nickel makes it difficult to manufacture Nitinol of uniform and repeatable properties. But, at the same time, this gives manufacturers a powerful method to control properties and to make ingots of the desired transformation temperature.
In fact, the sensitivity of the transformation temperature to alloy composition is so great that chemistry is not recommended as a way to specify the alloy of interest. Instead, the transformation temperature is a much more accurate means to specify the alloy. One of the most widely used methods of transformation temperature measurement of the ingot is use of a differential scanning calorimeter (DSC). ASTM F2004 is the standard for the DSC test method. The type of transformation information recorded by the DSC is shown Figure 2. The various transformation temperatures are marked.
While the DSC is used for characterizing raw materials, the temperature most frequently specified for the finished product like wires or tubes is the Active Austenite Finish (Active Af) Temperature which is generally determined by a “bend free recovery” (BFR) test. In this test, one deforms a sample of the material after cooling it into the martensitic region and then records the amount of shape recovery that occurs as it is warmed. A graph of temperature versus sample displacement is plotted and used to determine the temperature (Active Af) where the shape recovery is complete. The BFR is a very good functional test that shows distinct shape recovery. ASTM F2082 is the standard for the BFR test method. In most applications, specifying the transformation temperature of the final product (Active Af) is sufficient. However, the transformation temperature of the original ingot may be specified as well, if required.