In this introduction to the second chapter of Nitinol: The Book, Tom introduces the shape memory effect as it relates to phase transformations in metals.
2.0 The Thermal Transformation from Austenite to Martensite and the Origin of Shape Memory
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.
A martensitic transformation is a specific type of crystal structure change that occurs when cooling certain specific metals, including Nitinol. The crystal structure found at high temperatures is the parent phase, often referred to austenite, and the phase that results from a martensitic transformation is called martensite. The shape memory effect is a direct consequence of a reversible transformation between austenite and martensite.
A detailed description of martensitic transformations can be very complex  but fortunately is not necessary to understand the engineering aspects of shape memory materials. In this chapter, an accurate but simplified description of the transformation is presented, confined to aspects important to design and engineering. More specifically, the crystallography and properties of the two key phases of Nitinol are described, followed by discussions of how the transformation between the two takes place. With the crystal structures adequately defined, the text examines how the crystallography of martensite provides for a unique type of deformation behavior, and then finally, it examines how these factors combine to produce the thermal Shape Memory Effect (SME).
In the Chapter 3, it will be demonstrated that martensitic transformation can also be driven by the application of a stress, and that doing so gives rise to superelasticity, but here we confine the discussion to the effects of temperature alone. We also note that there is yet a third phase called the “R-Phase,” found in many conditions of Nitinol. The R-Phase will be discussed in detail in Chapter 4, but in this introductory chapter we will treat only the direct martensite-austenite transformation without any R-phase intervention.
Readers completely unfamiliar with the mechanism of the shape memory effect are encouraged to familiarize themselves with Figure 1-1 before attacking the material in this chapter; it serves as a valuable road map to the topics covered here.
- GB Olson and WS Owen, Martensite, ASM International (1992).