Directional solidification and heat treatment of terfenol-D magnetostrictive materials

Abstract

Directional solidification techniques have been developed to produce grain-aligned rods of the highly magnetostrictive material Terfenol-D, Tb_xDy_1-xFe_y The magnetostrictive properties have been compared to those of single-crystal, bicrystal, and polycrystal rod prepared by a float zone melting (ZM) technique. It was found that the properties of the grain-aligned material were comparable to three-and four-grain ZM material and the magnetostriction for both materials was less than the ZM single crystal and the better bicrystal rods by around 20 pct. It was demonstrated that a heat treatment consisting of a 1-hour hold at 950 °C was very effective at improving the magnetostrictive properties of all of the materials. Experiments have demonstrated that, to be effective, the heat-treatment temperature must be above somewhere between 850 °C and 900 °C. It is postulated that the mechanism for the improved properties involves a relief or redistribution of residual stresses produced by localized melting near second-phase particles at the predicted eutectic temperature of around 875 °C in the Terfenol-D material.