1. Introduction to Shape Memory Alloys

A typical MR fluid consists of carbonyl iron particles in oil, petroleum products [13].

Thermally responsive SMP’s have shown the ability to recover large deformations when subjected to external thermal stimuli. Above the glass transition temperature \(T_g\), there is an onset of long range molecular motion i.e. it changes from a glassy solid phase to an unordered rubbery phase and vice versa [2]. During this process, SMP’s can exhibit large changes in shape and moduli which can used for many applications [2].

The word diffusionless suggests that the atoms do not leave their lattice positions and there is no “long range diffusion” of atoms or species. Some literature on SMA commonly refer the twinning events in phase transformations as “short range diffusion” where the atomic displacements are less than the average interatomic distances.

Phase transformations in SMA (responsible for their functionality) under both SE and SME are between austenite and single variant martensite variants. The “self-accommodated martensite” is generally a combination of single variant martensite species [21]. For example, under tension and compression loading cases, two unique single variant martensite species (say \(M^t\) and \(M^c\) for tension and compression loading cases) can exist. In such a case, the “self-accommodated martensite” version would be 50 % \(M^t\) and 50 % \(M^c\).

Commercially, the NiTi SMA’s are available as actuator wires or superelastic wires. In case of actuator wires, the SMA is martensitic at room temperature, that is, the \(M_f\) is slightly above the room temperature and the \(A_f\) may be around 60 to 100 \(^\circ \)C depending on the alloy composition and other material processing conditions. Such actuator wires under external mechanical loads at room temperature causes the self accommodated twins to detwin into more stress preferred martensite variants and thus demonstrating macroscopic shape change. Such wires would show superelastic behaviour above \(A_f\). On the other hand, the superelastic wires are austenitic at room temperature, that is, its \(A_f\) is below room temperature and its \(M_f\) is far below sub zero temperatures (around -60 to 100 \(^\circ \)C again depending on the alloy composition and other material processing conditions). Such superelastic wires will show SE behaviour at room temperature and demonstrate martensite detwinning at temperatures below zero (i.e., below \(M_f\)). A more detailed discussion on the effect of composition and other material processing conditions will be taken up in future chapters of this book.

Biocompatibility is dependent on the allergic reactions between the foreign material and the host [41]. Material characteristics, patients health’s and several other factors play an important role for assessing biocompatibility. In case of SMAs, several clinical studies were performed to establish Ni–Ti alloys biocompatibility and FDA^® has cleared many SMA products for medical use [41]. FDA^® is a registered trademark of “The Food and Drug Administration (FDA or USFDA)” affiliated with the United States Department of Health and Human Services (an United States’ Federal executive department) [42].

These applications indicate the use of SMA like a “Metallic rubber band” without the component loosing its shape. Superelastic SMA devices in their austenitic state provide high resistance to deformation/kinks and keeping them in desired shape without any permanent deformation even under considerable loads.