Shape Memory Alloys

An alloy is defined as a substance with metallic properties that is composed of two or more chemical elements of which at least one is an elemental metal (1). The internal structure of most alloys starts to change only when it is no longer stable. When external influences, such as pressure and temperature, are varied, it will tend to transform spontaneously into a mixture of phases, the structures, compositions, and morphologies of which differ from the initial one. Such microstructural changes are known as phase transformation and may involve considerable atomic rearrangement and compositional change (2,3).
Shape memory alloys (SMAs) exhibit a unique mechanical ''memory'', or restoration force characteristic, when heated above a certain phase-transformation temperature range (TTR), after having been deformed below the TTR. This thermally activated shape recovering behavior is called the shape memory effect (SME) (3?5). This particular effect is closely related to a martensitic phase transformation accompanied by subatomic shear deformation resulting from the diffusion less, cooperative movement of atoms (6,7). The name marten site was originally used to describe the very fine, hard microstructure found in quenched steels (8). The meaning of this word has been extended gradually to describe the microstructure of nonferrous alloys that have similar characteristics.
SMAs have two stable phases: a high temperature stable phase, called the parent or austenite phase and a low temperature stable marten site phase. Marten site phases can be induced by cooling or stressing and are called thermally induced marten site (TIM) or stress induced marten site (SIM), respectively (8). The TIM forms and grows continuously as the temperature is lowered, and it shrinks and vanishes as the temperature is raised. The SIM is generated continuously with increasing applied stress on the alloy. On removing the applied stress, SIM disappears gradually at a constant temperature. If the temperature is sufficiently low when stressing, however, the SIM cannot return to its initial structure when the stress is removed. When the temperature is increased above the TTR, the residual SIM restores the original structure, resulting in shape recovery (9). Surprisingly, this process can be reliably repeated millions of times, provided that the strain limits are not breached. If dislocations or slips intervene in this process, the shape memory becomes imperfect. When the applied stress on a SMA is too great, irreversible slip occurs, and the SMA cannot recover its original shape even after heating above TTR (10). However, it can remember this hot parent pattern. In the next cooling cycle, the SMA changes slightly and remembers the cool-marten site pattern. A SMA trained with this repeated cyclic treatment is called a two-way SMA (9). A schematic explanation of the SME related to the two dimensional (2D) crystal structure (11) is shown in Fig. 1.
When a SMA is cooled below its TTR, the parent phase begins to form TIM without an external shape change. This TIM can be changed into SIM easily by mechanical deformation below the TTR. When the deformed SMA is heated above its TTR, however, it cannot hold the deformed shape anymore, and the SMA returns to its original shape, resulting in a reverse martensitic phase transformation.

IMAGE( Memory Alloy 1.gif)
Figure 1. Schematic illustration of the shape memory effect. The parent phase is cooled below TTR to form a twinned (selfaccommodated) martensite without an external shape change. Deformed martensite is produced with twin boundary movement and a change of shape by deformation below the TTR. Heating above the TTR results in reverse transformation and leads to shape recovery.

A SMA also shows rubber-like behavior at temperatures above its TTR. When a SMA is deformed isothermally above its TTR, only SIM is produced, until plastic deformation occurs. Then, the SIM disappears immediately after removing the applied load, resulting in a much greater amount of recovering strain, in excess of the elastic limit, compared to the conventional elastic strain of a metal. This rubber-like behavior at a constant temperature above TTR is called superelasticity (12). A schematic explanation of superelasticity is shown in Fig. 2.

IMAGE( Memory Alloy 2.gif)
Figure 2. Schematic illustration of the superelasticity of a SMA above TTR. During the loading process, the applied load changes the parent phase into stress-induced martensite, which disappears instantly on unloading.

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