Phase Transformations in Metals

In Chapter 2 of Nitinol: The Book, Tom introduces some basic principles of phase transformations in metals. As unusual as Nitinol is, the superelastic and shape memory properties driving these are a byproduct of phase transformations that are ubiquitous in virtually all materials. Read on to learn more, including why Napoleon’s Russian invasion failed because of a phase transformation…

2.1 Phase Transformations in Metals

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.

Phase transformations in solids are common. Even pure metals such as titanium, carbon, tin and iron exist in different crystal structures, or phases, depending upon their temperature and other environmental factors such as pressure or even magnetic field strength. The phase of a metal, rather than just its composition, is a primary consideration in determining its mechanical, electrical and thermal properties. It is quite common to find one phase to be ductile and soft, and another of the identical composition to be brittle; or one phase to be magnetic and another not (such as in the case of iron). Napoleon Bonaparte, for example, learned of metallic phase transformations when his Russian invasion was halted by frigid winter temperatures that transformed the tin buttons on his army’s uniforms from ductile “beta” tin into a gray, powdery “alpha” tin [1]; it is often said that his lack of metallurgical knowledge led to his losing the war, even if some historians give a portion of the credit to the Russian army.

There are myriad different types of phase transformations that occur in metals, some very complex, but they can be divided into two general groups, both of which are encountered in Nitinol:

  • Diffusional transformations are those in which the new phase has a different chemical composition than the extant parent phase. Because it is compositionally different than its surroundings, the new phase can only be formed by transporting atoms over relatively long distances—for example, the precipitation of pure salt from a salt water solution changes the salt concentration in the “parent” liquid phase and thus requires salt to move, or diffuse, within the liquid phase. Since atomic diffusion is required, the progress of this type of transformation is dependent upon time, and can usually be suppressed by quenching to low temperatures at which atomic diffusion is very slow. That is why, for example, steel must be quenched to make it hard: Slowly cooling steel allows a diffusional transformation to soft phases. Diffusional transformations are often referred to as isothermal since they can progress with time at a constant temperature.
  • Displacive transformations do not change the composition of the parent phase, but rather only the crystal structure. Consequently, displacive transformations do not require long range atomic movement. Obviously, all phase changes in pure metals are of this variety. In such transformations, the new phase is formed through slight atomic shuffles of generally less than an atomic diameter, and atoms are cooperatively rearranged into a new, more stable crystal structure with the same chemical composition as the original parent phase. Because no atomic migration is necessary, displacive transformations usually progress in a time-independent fashion, with the speed of the interface between the two phases able to move at nearly the speed of sound. They are referred to as athermal transformations, since they cannot progress at a constant temperature*, but rather the amount of the new phase present depends only upon temperature, not time.

Martensitic transformations are of the displacive variety, with the term martensite referring specifically to the lower temperature phase, and the term austenite referring to the higher temperature phase from which martensite is formed.  The austenitic phase is also appropriately referred to as the parent phase, and martensite the daughter phase.  Some researchers use the terms austenite and parent interchangeably, while others argue that the terms “martensite” and “austenite” should be strictly reserved for the specific transformation in steel to which the names were originally assigned, insisting that the terms “parent” and “daughter” should be used in other alloy systems. Here we assume the broader definitions, using “martensite” to refer to the phase resulting from any athermal, diffusionless phase transformation, and “austenite” to refer to the phase from which martensite is formed.

Both diffusional and displacive transformations take place in most commercially available Nitinol alloys, with the two processes competing to find the lowest energy state. In such cases, quenching from the higher temperature phase usually suppresses diffusional transformations and decides the contest in favor of martensite. Controlling this competition is one of the key objectives in the processing of Nitinol that will be discussed extensively in Chapter 11. For the purposes of this chapter, however, it is assumed that the only transformation taking place is the displacive transformational between austenite and martensite. Indeed, this is the case in Nitinol of exactly 50 atomic percent each of nickel and titanium; in this case no isothermal decomposition is possible. As will be shown later, as excess nickel is added, diffusional transformations become increasingly important, and as we will see, understanding this competition is essential to controlling alloys that exhibit superelastic properties at room and body temperature.

  1. P LeCouteur and J Burreson, Napoleon’s Buttons, Penguin Books (2003) 2.

* Isothermal displacive transformations do exist, but do not exhibit shape memory properties, and can thus be ignored in the current context.

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