Research has shown that with suitable processing, one can produce a polycrystalline version of a terbium/dysprosium alloy suitable for use in a magnetostrictive actuator for generating small motions. Prior to this research, only the single-crystal version had been used, and that in only a limited number of applications. One advantage of polycrystalline Tb/Dy is that it costs much less than does the single-crystal version. A second advantage of the polycrystalline material is that the preload springs, which are necessary in most single-crystal applications, can be eliminated in many cases.
Magnetostrictive actuators are attractive for generating motion at cryogenic temperatures, where piezoelectric actuators lose much of their effectiveness. The Tb/Dy alloy exhibits a large magnetostriction at low temperatures, with the ratio of Tb to Dy chosen so that the anisotropy is minimized for the temperature range of operation.
The raw polycrystalline material is produced by casting. To obtain sufficient actuation performance (see figure) that results in saturation magnetostriction, which is 60 percent of the saturation magnetostriction of single-crystal Tb/Dy, it is necessary to orient a substantial fraction of the crystal grains by a suitable mechanical treatment. Experiments have shown that by rolling the cast material and subjecting the rolled material to multiple heat treatments, one can achieve a degree of crystalline orientation that results in 60 percent of the magnetostriction of single-crystal Tb/Dy; even at this reduced level, the low-temperature actuation performance of polycrystalline Tb/Dy is still about 50 times that of a piezoelectric material.
Magnetostriction Was Measured in specimens of Tb/Dy at a temperature of 77 K. Rolled Tb/Dy performed somewhat better than did as-cast Tb/Dy. Experiments have shown that optimized thermomechanical treatment can produce an even bigger increase in magnetostriction, up to a major fraction of the magnetostriction of single-crystal Tb/Dy.
This work was done by Robert Chave, Christian Lindensmith, Jennifer Dooley, Brent Fultz, and Marius Birsan of Caltech for NASA's Jet Propulsion Laboratory.
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