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].
In general, a material containing many individual crystals with random orientation will not have a large bulk magnetostriction. Crystallographic texture describes the average orientation of the individual crystallites in the material. It is possible to achieve a preferred texture by various materials-processing methods, but one of the simplest is deformation processing. The Tb-Dy alloy was arc-melted and drop-cast into a chilled copper mold. Since the as-cast ingot shows strong crystallographic texture, the material was first cold rolled to a 35-percent reduction in thickness and heat treated for 1.5 h at 950 °C to induce recrystallization. This step was intended to provide a random initial orientation of spherical grains, although our bulk thermal expansion measurements indicate that significant texture remained after this step. The specimen was then plane-rolled by 55 percent and annealed at 350 °C to relieve strain. This rolling and annealing combination was performed once or repeated a number of times. It can be seen in the figure that the commercial specimen exhibits 2,400 ppm magnetostriction, while the high-purity specimen exhibits only 1,750 ppm magnetostriction. It may be that the additional impurities in the commercial-grade material limits grain boundary growth during heat treatments, and studies are currently underway to relate grain size and texture to magnetostriction. Although impurities that inhibit grain growth are detrimental to the fabrication of single-crystal materials, these impurities may not be a problem for polycrystalline materials. The use of Tb and Dy of lower purity may be an important practical advantage of polycrystalline materials, since the lower-purity materials can be obtained at significantly lower cost and with greater reliability.
This work was done by Jennifer Dooley and Brent Fultz of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
Technology Reporting Office., JPL, Mail Stop 122-116, 4800 Oak Grove Drive, Pasadena, CA 91109. (818) 354-2240
Refer to NPO-20697
This Brief includes a Technical Support Package (TSP).
Using Commercial-Grade Tb/Dy in Magnetostrictive Actuators
(reference NPO20697) is currently available for download from the TSP library.
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