Microstructure and magnetostriction of Fe_1-xTb_x alloys prepared by solid-state synthesis

Fe_1−xTb_x was mechanically alloyed over a range of compositions 0.1<x<0.9. Based on X-ray diffraction measurements, a complete diagram of the resulting phases was drawn as a function of alloying time and composition. Depending on the composition, the microstructure of the as-milled powder is nanocrystalline or amorphous, with solubility ranges differing from those observed in the thermodynamically equilibrium structures. Terbium is soluble in the iron lattice with 0<x<0.33. A single-phase amorphous structure was isolated with x=0.33, and a single-phase Laves Fe₂Tb structure with 0.4<x<0.5. At higher terbium concentrations, a mixture of Fe₂Tb and terbium was obtained. The magnetic properties of the mechanically alloyed binary Fe₂Tb were studied in more detail. The mechanically alloyed binary Fe₂Tb powders were cold compacted and annealed. The highest magnetostriction was measured after annealing the samples at 500°C, resulting in a crystal size of 7–8 nm. Without an external load, the saturation magnetostriction λs for the nanocrystalline Fe₂Tb was measured to be 1225×10⁻⁶. This is 30% higher than that of the zone-melted coarse-grained Terfenol-D with the composition of Fe₂Tb_0.3Dy_0.7. When a constant magnetic field was applied perpendicular to the primary driving magnetic field, the saturation magnetostriction λs of the nanocrystalline Fe₂Tb sample was measured to be 1582×10⁻⁶. This value is 80% higher than the magnetostriction in unstressed zone-melted Terfenol-D.