Smart+Materials-Mohamed+essawy

1. Metallurgical Basis for the Shape-Memory Effect Shape memory or the shape-memory effect (SME) is a phenomenon associated with the martensite transformation, a first-order displacive transformation usually related to the hardening of steel (qv). In steel an alloy heated to the temperature where the elevated temperature face-centered cubic (fcc) austenite phase is stable, is rapidly cooled to produce the hard martensite phase. In certain alloys, however, the martensite transformation is thermoelastic, ie, the martensite forms and disappears on heating and cooling over a relatively small temperature range. The intermetallic phase in these alloys undergoes a displacive, shear-like transformation when cooled below a critical temperature designated as MS. Upon further cooling, to a temperature designated as MF, the transformation is complete and the alloy is said to be in its martensitic state. When this martensite is deformed, it undergoes a strain that is completely recovered when the alloy is heated. The recovery process starts at the temperature AS and is completed at a higher temperature, AF. Because this is a first-order phase transformation, there is hysteresis associated with the formation of martensite and its reverse transformation to the elevated temperature parent phase, which is usually also referred to as austenite. The temperatures MS, MF, AS, and AF depend on the particular alloy, alloy composition, and processing. The hysteresis loop associated with the transformation in a typical shape-memory alloy is illustrated in Figure 1. Figure Figure

Figure 1. Schematic of the hysteresis loop associated with a shape-memory alloy transformation, where MS and MF correspond to the martensite start and finish temperatures, respectively, and AS and AF correspond to the start and finish of the reverse transformation of martensite, respectively. The physical property can be volume, length, electrical resistance, etc. On cooling the body-centered cubic (bcc) austenite (parent) transforms to an ordered B2 or DO3 phase and then to one of the various martensite structures. The martensite in shape-memory alloys (SMAs) may also be isothermally generated by applying stress at a temperature above MS. Because the martensite is unstable at this temperature, when the stress is removed the martensite disappears. Above a temperature designated as MD, martensite cannot be generated, no matter how high the stress. The stress-induced martensite (SIM) gives rise to a mechanical type of shape memory called pseudoelasticity. These alloys exhibit, in addition to SME and SIM, another unique property, that of very high damping. Damping is the property which causes a vibration, once induced in a material, to decay. Bell bronzes have a low damping, usually expressed as the specific damping capacity. A gray cast iron formulated for use as a machine-tool bed for resistance to vibration might have a specific damping capacity of 10%. By contrast, SMAs can have a damping capacity of greater than 40%. This characteristic can be exploited in smart or adaptive materials (see Smart materials (Supplement)). Damping is the result of the high mobility of the interfaces between martensite variants or plates. When martensite is stressed, the deformation is not accommodated by the usual mechanisms that operate in conventional metals, such as a dislocation motion, slip, or grain boundary motion, but by the growth and shrinkage of martensite variants or the motion of twin boundaries. When subjected to an oscillating stress these mobile boundaries move back and forth, giving rise to a frictional loss that accounts for the damping. The SME was first reported in 1951 involving a Au–Cd alloy (1). Many other alloys exhibiting this behavior have since been discovered; some of these are listed in Table 1. It was, however, the discovery of the SME in the nominally equiatomic Ni–Ti alloy that led to commercial applications. This alloy is usually called nitinol, after Ni–Ti Naval Ordinance Laboratory (2). Nitinol and its variations are the most frequently employed shape-memory alloys, although two other SMAs, Cu–Zn–Al and Cu–Al–Ni, also find use in special applications. Nitinol is quite expensive, owing to the extremely tight composition control required to prepare an alloy having specific transformation temperatures. Moreover, nitinol is difficult to process into forms such as wire, rod, and ribbon that are required for commercial application. As a result, the copper-based SMAs were developed as less expensive alternatives. Although these latter alloys were employed for a period as actuators of various designs, shortcomings related to fatigue strength and thermodynamic stability restrict use. Certain ferrous alloys, listed in Table 2, have also been discovered. These are potentially less expensive SMA candidates, but as of 1996 have not achieved broad commercial use.