What Causes the Shape Memory Effect?

This chapter makes use of a very large amount of terminology.  The first time each term is used, it will be defined and described as best as possible.  For a complete list of terms, please consult the Glossary.   What causes the shape memory effect?  From the time the shape memory effect was first observed, this question was on the tip of everyone’s tongue.  Before the shape memory effect could be utilized to do anything useful, the mechanism behind it must be understood. In traditional materials, there are two kinds of deformation: elastic (or linear elastic) and plastic deformation.  Linear elastic deformation refers to deformation that is fully (or very close) recovered when the force is removed.  The equation governing elastic deformation is known as Hooke’s Law: where F is force, k is the spring constant, and x is the distance that the spring is deformed.  Linear elastic materials are called this because, if you plot force vs. displacement (also known as stress vs. strain) on a graph, a straight line is drawn. Plastic deformation refers to deformation that is permanent.  When the force is removed, the deformation is not recovered. Shape memory materials appear to undergo plastic deformation, but after heating, elastically return to their original shape.  This has been termed thermoelastic deformation. Clearly, at the atomic level, the deformation in the shape memory materials is substantially different from the deformation in linear elastic materials, so let’s take a look at them. Metals are known as crystalline objects because all of the atoms are held together in a very regular spacing, known as the crystal lattice.  The most common kind of deformation at the atomic level is called diffusion.  This is when the atoms trade places with each other and move around the crystal lattice. Each time this happens, the electron bonds between the atoms are broken and then re-formed.  This type of movement within the crystal lattice is largely temperature dependent and generally doesn’t have any noticeable effects at low temperatures. Slip is the primary mechanism, or main way, plastic deformation occurs.  In slip, an entire sheet of atoms breaks their electronic bonds and moves one or more atomic spacings.  Note, this only happens for a whole number of atomic spacings (this will be important very soon).  If enough crystal planes move enough spacings, a micro-crack forms–usually much smaller than 1mm in length.  Once enough micro-cracks form, they coalesce, or come together, to form a crack which is soon followed by the catastrophic failure of the object. It was Dr. Frederick Wang who determined how the shape memory effect works, while working at the Naval Ordinance Laboratories.  The shape memory effect is not governed by either of these mechanisms.  Instead, it is caused by a phenomenon called the martensitic transformation.  The martensitic transformation happens when a material transfers between two different, solid, crystal states, namely austenite and martensite.  Austenite, the high temperature state has a very strong, cubic structure where the atoms form cubes.  Martensite, on the other hand, has an atomic spacing more like a parallelogram.  These two crystal states were first observed in steel, but the transformation occurs at very high temperatures and the martensitic transformation in steel is less powerful than plastic deformation by crystal slip. What makes nitinol exhibit such a powerful shape memory effect is the presence of the phenomenon twinning.  Twinning is when atomic bonds rotate through an angle to make a sort of mirror image. Figure 2.1 is an artist’s rendition of twinning at the atomic level.  These bonds can be described as being very ‘floppy’ meaning that they can rotate with relative ease and stay put after they have rotated.  This gives the appearance of a soft, easily deformable material.  However, when the material is heated so that it goes back into the austenite phase, since none of the electronic bonds have been broken, the original shape returns. For more information on twinning, please read Twinning and Diffusionless Transformations in Metalsby E. O. Hall, Butterworth Publishing, 1954. According to Hall, there are two types of twinning: impact induced twinning and heat induced twinning.  Impact induced twinning occurs when the metal is struck with a very large point force–for example being hit by a bullet.  This causes the crystal structure to be distorted into the twinned structure. Heat induced twinning occurs when a metal is heated at the appropriate temperature for an appropriate period of time. This is why nitinol must be heat treated before it exhibits the shape memory effect. Going back to our discussion on the martensitic transformation, there are really two types of martensitic transformations.  The first is the thermally induced martensitic transformation which results in the shape memory effect.  The second martensitic transformation is the stress induced martensitic (SIM) transformation.  This happens when a sample of nitinol is in the austenitic phase and a force, or stress, is exerted on the sample.  This stress causes a portion of the austenite to transform into martensite. Since martensite can undergo substantial deformation without experiencing any crystal slip, SIM allows austenitic nitinol to experience deformations up to 50%.  Since this is one hundred times the amount of deformation that can be sustained by conventional materials like stainless steel, the SIM transformation in nitinol is referred to as superelastic. The transformation temperature of nitinol is determined by the ratio of nickel to titanium.  Increasing the amount of titanium increases the transition temperature while increasing the amount of nickel decreases the transition temperature.  After the initial ingot is prepared, the transition temperature can be affected by heat treating.  Generally speaking, heat treating at temperatures below 600℃ (1,100℉) causes the transition temperature to rise while heat treating at temperatures above 600℃ (1,100℉) causes the transition temperature to drop.  This is because, at low temperatures, Ti₂Ni₃crystals begin to form in the matrix, thereby reducing the effective ratio of nickel to titanium and, at high temperatures, the nickel and titanium return to the matrix, balancing it back out. Without a doubt, the most common misconception that customers have when they call the lab phone is that the shape memory transformation occurs AT the transition temperature and the reverse transformation happens immediately below the transformation temperature.  Unfortunately, this is not the case.  The temperature that is commonly referred to as “transition temperature” is actually the austenite finish (A_f) temperature.  The austenite start (A_s) temperature is anywhere from a few degrees to fifty degrees lower, depending on how the nitinol is prepared, the elemental ratio, and numerous other factors. Likewise, the martensite transformation also has a start and finish temperature (M_sand M_f, respectively).  To make things interesting, M_sfrequently is a higher temperature than A_s.  This means that between these two temperatures the transformation is continually occurring. The critical temperatures of nitinol are covered in greater detail in Chapter 4. This is just a brief overview of the martensitic transformation.  For a more in depth look at the martensitic transformation and the shape memory effect, please read the book Microstructure of Martensite: Why It Forms and How It Gives Rise to the Shape Memory Effectby Kaushik Bhattacharya, published by Oxford University Press.