Using Commercial-Grade Tb/Dy in Magnetostrictive Actuators

Single crystals of Tb-Dy alloys exhibit magnetostrictive strains approaching 1 percent and can generate forces sufficiently large to make them useful as actuators in cryogenic mechanical devices. These actuators offer many advantages over piezoelectric actuators and actuators using motion feedthroughs from higher temperature. Unfortunately, the preparation of single crystals of Tb-Dy alloys is difficult and costly, and will likely remain so. The limited availability of single crystals could impede the development of this new actuator technology. Since polycrystals are much simpler to prepare and are less costly, textured polycrystalline materials are being developed as alternatives to single crystals, as discussed in "Polycrystalline Tb/Dy Alloy for Magnetostrictive Actuators" (NPO-20273), NASA Tech Briefs, Vol. 23, No. 8, (August 1999), page 44. Continued efforts to reduce the cost of magnetostrictive polycrystals have led to the use of lower-cost commercial grade (total purity 99.7 percent) Tb-Dy material [as opposed to expensive high-purity (99.94 percent) material required for the growth of single crystals].