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makes possible actuator devices that can produce large forces and motions in response to a temperature change in the ambient or a temperature change effected by I2R ohmic heating where I is current and R is resistance. The shape-recovery temperature of the most useful SMA, NiTi, can be varied from cryogenic to 90°C, though the maximum practical actuation temperature is about 80°C. The transformation temperature is a function of composition, with further smaller variation effected through thermal-mechanical processing. In addition to the shape-memory effect, these alloys can also exhibit pseudoelas-

Table 1: Nonferrous Shape-Memory Alloys.

Shape-Memory Alloys These unusual alloys are ordered B2 or D03 IMCs that on cooling transform to thermoelastic martensites, which are also ordered and twinned. The habit plane for the martensite transformation is the (110) plane of the ordered parent. Since there are four twin-related variants formed for each of the six (110) planes in the body-centered-cubic B2 or DO3, there are a total of 24 variants. When these martensitic variants are deformed, the strain is accommodated by variant-to-variant coalescence and by twinning. When the deformed structure is heated to the martensite-to-austenite transformation temperature, recovery of the original shape is initiated. The recovery is complete when the entire structure has transformed back to the parent phase. 1 The shape-recovery process

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tic behavior, which is particularly useful in medical devices. When an SMA is subjected to sufficient stress at a temperature above the A4S—the temperature at which martensite starts to form on cooling, martensite will form. When the stress is released, since the martensite is not stable at this temperature, the martensite reverts back to austenite. This stress-induced martensite (SIM) is a form of mechanical memory as opposed to thermal memory in the shapememory process first described. Shapememory alloys in their martensitic form have many very mobile variant boundaries that move under stress. When the stress is oscillatory, there is a frictional dissipation at these moving boundaries that gives rise to a very high vibration damping. Applications for these alloys have been developed using each of these three characteristics. In addition to the NiTi alloys, copperbased SMAs have also found applications in various devices. The family of alloys based on CuAINi and CuZnAl with modifications are the two that have achieved commercial status. The more important SMAs are presented in Table 1. The applications for SMAs fall into two categories: (1) one-way devices that provide a permanent coupling or fastening action and (2) two-way devices that are usually in the form of an actuator that provides force and motion in response to a temperature change. The former class contains the first largescale application for these alloys, hydraulic tube couplings for military aircraft. Similar couplings

Alloy

Composition Crystallographic Hysteresis Structure Change Ordering (at.%) °C B2-2H Ordered 44-49 Cd 15 B2