Study of ordering and local atomic environments in iron-based magnetostrictive alloys

The α-phases of Fe-Ga and Fe-W alloys show large low field magnetostriction and the observed magnetostrictive behavior is sensitive to the solute content and thermal history. In order to improve understanding of magnetostriction, the first part of this work examines the influence of W substitution for Ga in Fe-Ga alloys and a more detailed examination of Fe-W alloys. The variation of magnetostriction coefficient λ100 with W substitution for Ga in various Fe-x at.% Ga alloys was examined. Fe-12.5 at.% Ga-2.5 at.% W, Fe-15 at.% Ga-2.5 at.% W and Fe-17.5 at.% Ga-2.5 at.% W single crystals were prepared by vertical Bridgman technique and annealed in the α-phase region followed by rapid water quenching. Increase of the Ga content in Fe-x at.% Ga-2.5 at.% W increased λ100 magnetostriction values up to 307 x 10-6 for 15 at.% Ga and further increase in Ga content decreased the magnetostriction value. Comparing the magnetostriction of Fe-Ga-W alloys to those of Fe-Ga alloys, the substitution of W for Ga caused more than a 7% drop in magnetostriction. The magnetostriction coefficients λ111 were also measured using of [220]- or [211]-oriented single crystals of Fe-12.5 at.% Ga-2.5 at.% W, Fe-3 at.% W, Fe-6 at.% W and Fe-7.5 at.% W alloys. The magnetostriction increases as the W content increased from 3 at.% to 7.5 at.% W. In the second part, the B2 and DO3 types of ordering were examined in annealed and quenched magnetostrictive Fe-27.5 at.% Ga single crystal and the presence of both B2 and DO3 ordered domains is demonstrated. The relative amounts of B2 phase domains increased with increasing depth up to about 100 um depth. After that, the volume ratio of B2 and DO3 domains showed little variation beyond this depth. In the third part, the extended x-ray fluorescence spectroscopy (EXAFS) studies of local atomic environment of Fe-27.5 at.% Ga, Fe-20 at.% Ga and Fe-15 at.% Ga single crystals were carried out. The study focused on the first and second nearest neighbors. The results obtained in the examination of Ga-Ga first nearest neighbor bond distance show a large positive increase after long-term annealing and the value of this positive strain increases as the Ga content increases. As the change of magnetostriction value with the changes in Ga content or thermal history follows the same tendency, it suggests a correlation between local lattice distortions to the material's magnetostrictive behavior.