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Shape memory alloy types

Author: Date:6/7/2012 4:15:22 AM

Since the discovery of Ni-Ti, at least fifteen different binary, ternary and quaternary alloy types have been discovered that exhibit shape changes and unusual elastic properties consequent to deformation. Some of these alloy types and variants are shown in table 1.

Table 1. Shape memory alloy types.















The original nickel-titanium alloy has some of the most useful characteristics in terms of its active temperature range, cyclic performance, recoverable strain energy and relatively simple thermal processing. Ni-Ti and other alloys have two generic properties thermally induced shape recovery and super- or pseudo-elasticity. The latter means that an SMA in its elastic form can undergo a deformation approximately ten times greater than that of a spring-steel equivalent, and full elastic recovery to the original geometry may be expected. This may be possible through several million cycles. The energy density of the alloy can be used to good effect to make high-force actuators - a modern DC brushless electric motor has a mass of 5-10 times that of a thermally activated Ni-Ti alloy, to do the same work.

The superelastic Ni-Ti alloys are “stressed” by simply working the alloy. These stresses can be removed, just as with many other alloys, by an annealing process. The stressed condition is termed stress-induced martensite, which is the equivalent of being cold/hot worked.

SMAs, particularly nickel-titanium, are commercially available from several sources. However, world production is small compared to other metal commodities (about 200 tonnes were produced 1998) owing to difficulties in the melt/forging production process, and so the cost of the material high US$0.30-US$1.50 (UK£0.20-£1.00) per gram for wire forms 1999 prices). Fortunately, most current applications require only small amount of the material. As world production increases (as it has done quite dramatically in the 1990s) so prices should decrease. Wires, strip, rod, bar and sheet are all readily available and alloy foams, sintering powders and sputtering targets of high purity are also produced.