Shape memory alloys (SMAs) are materials that, after being deformed, return to their original shape when heated. An alloy is a material which combines a metal and another element. Many alloys have multiple solid phases—several different ways the atoms of the two elements can be arranged. For instance, low carbon-steel (iron-carbon) may be found in the phase of pearlite, martensite, or austenite depending on temperature and cooling rates.
What makes SMAs unique is that there are two solid phases which can be transformed into each other by heating or cooling. By convention, the same names are used as for the phases of steel: the phase stable at high temperatures is called “austenite”, and the phase stable at low temperatures is called “martensite”. Such a reversible transition allows the SMA to return to an earlier shape after being deformed, by simply heating them to return them to the austenitic phase. Notably, titanium-nickel alloys have been found to have shape-memory properties.
SMAs have been proposed for many applications, including medical implants to keep blood vessels open, safety systems such as taps which automatically shut off when the water gets too hot, and aircraft components. However, one of the barriers to adoption of SMAs is that they are prone to fatigue; generally, after a few thousand cycles of deformation the constant transformations will either lead to crack formation or a change in the transformation temperature, causing the material to fail or become permanently deformed.
Recently, a team of scientists from the University of Kiel and the University of Maryland demonstrated that a titanium-nickel-copper (Ti-Ni-Cu) alloy could be used to create SMAs which could last for 10 million cycles. The breakthrough was discovering, in an experiment with thin films, that an alloy rich in titanium (containing Ti2Cu) exhibits more consistent stress-strain characteristics than an equiatomic (equal numbers of titanium and nickel atoms) alloy. After observing phase transformations using electron microscopes and X-rays, the scientists came to the conclusion that the causes are likely epitaxy and low cofactors. In other words, during the transformation from one phase to another, the Ti2Cu precipitates (which happen to be compatible with both phases) allow the nuclei of the new phase to form in an organized, repeatable manner, and it also happens that the two phases have a density and shape such that grains of the new phase grow in an equilibrium manner around the nuclei. Unlike other SMAs, which often have imperfect transformations with martensite “left over” after a transformation to austenite, essentially 100% of this Ti-Ni-Cu alloy transforms from phase to phase. Thus, phase transformations are reliable and repeatable, even for millions of cycles.
This discovery has the potential to make SMAs viable for applications where service life is an important consideration. Perhaps one day we’ll see doctors recommending the use of SMAs in mission-critical implants as a safe, low-risk treatment.
Leave a Reply