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A Short Orientation

The “shape memory triangle” introduces the concepts of shape memory and superelasticity effects in Nitinol. This quick installment includes the first of many figures that we’ll refer to in this series. Read on for more.

1.1 A Short Orientation

This post is an excerpt from Nitinol: The Book, a working draft of an upcoming publication by Tom Duerig, Alan Pelton, and others. Visit the Table of Contents or Introduction for more information.

We begin by defining the three terms embodied in title of the book:

  • Shape memory refers to the ability of certain materials to “remember” their shape, even after relatively severe deformations.  Once deformed at low temperatures (in their martensitic phase), these materials remain in that deformed shape until heated to their “transformation temperature,” whereupon they spontaneously return to their original, undeformed shape.
  • Superelasticity is simply a manifestation of the same shape memory effect, observed when the material is deformed slightly above its transformation temperature.  In this case, shape recovery occurs spontaneously when the deforming stress is removed (without the need to apply heat).  Because shape memory and superelasticity are really one and the same, this text will often use the phrase “Shape Memory Effect”, or even SME as a collective term, referring to both.  In fact, early literature, prior to the solidifying the term “superelasticity” often referred to the effect as the elastic shape memory effect.
  • Nitinol refers to a class of alloys composed of nearly equiatomic quantities of nickel and titanium.  The specific term is derived from the words:  “Nickel-Titanium-Naval Ordnance Laboratory,” where the alloys were first researched.  Here we intend the term to refer to Ni-Ti-based alloys that exhibit either of the above effects, even if third or fourth elements have been added.

Probably the most important and best-known figure in shape memory is the “shape memory triangle” shown in Figure 1-1.  Chapter 2 will delve into the detail of each corner of the triangle as well as the processes that occur as one goes from corner to corner, but before becoming immersed in the detail, a quick glance at the triangle may serve as a useful orientation.  At high temperatures, Nitinol is stable in the austenitic phase with properties that are similar to many titanium alloys (top station in triangle).  When Nitinol is sufficiently cooled, it adopts a new crystal structure called “twinned martensite” (lower right of triangle) with dramatically different properties, more like those of lead or tin.  When twinned martensite is deformed, it does not deform through conventional mechanisms, but by moving and eliminating twin boundaries—a kind of “unfolding” in a sense.  When the deformed martensite (lower left) is sufficiently heated, it returns to the original austenite structure. All martensite (whether deformed or not) reverts to the same austenitic structure upon heating, and thus the original structure and shape is recovered.

Figure 1-1: The basic “Shape Memory Triangle,” schematically illustrated crystallographically (left), and by macroscopically through the bending of a simple rod (right). See text for further explanation

The above scenario describes the essence of the shape memory effect. As will be shown in Chapter 3, superelasticity is little more than a short cut directly from austenite to detwinned martensite, more or less skipping the station on the lower right.

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