Crystallization and magnetic field processing of cobalt-rich cobalt,iron-based nanocrystalline and amorphous soft magnetic alloys

Crystallization and thermal-magnetic processing of Co-rich Co,Fe based nanocrystalline and amorphous soft magnetic alloys of composition (Co1--xFex)89--yZr7B 4Cuy, (y = 0,1 and 0 ≤ x ≤ l) were investigated. Primary emphasis is placed on (1) the phases observed during crystallization of Co-rich compositions and (2) the magnitude and origin of field induced anisotropy as a function of composition and annealing temperature. For the former, standard x-ray diffraction and electron microscopy techniques are employed. For the latter, experimental observation of structural changes associated with magnetic field-annealing is difficult or impossible using standard techniques. Therefore, ac permeametry of magnetic properties is employed to probe this issue. In addition to the two primary issues addressed in this work, a number of other relevant physical phenomenae are discussed including: (1) Theoretical estimates of magnetic field effects on phase stability in the Fe-Co binary system. (2) Directional pair ordering theory of field induced anisotropy of ternary ferromagnetic alloys and alloys with tendencies toward CsCI-type chemical ordering. (3) Stability, structure, and magnetic properties of cubic phases of 23:6 stoichiometry observed as crystallization products after high temperature annealing treatments in this genre of materials. The major accomplishments of this thesis include the following: (1) Confirmation of preferential nucleation of bcc phase during crystallization in Co-rich nanocomposite alloys as observed in previous work. (2) Elucidation of potential mechanisms responsible for preferential nucleation of bcc phase through a combination of 3-D atomic probe experiments and classical nucleation theory arguments. (3) Application of a Monte Carlo model to directional pair ordering theory to investigate the effect in ternary alloys and in the presence of tendencies for chemical ordering or chemical segregation. (4) Improved understanding of the potential mechanisms responsible for field induced anisotropies in Fe,Co-based "nanocomposite" alloys. (5) Identification of enhanced temperature stability of field crystallized "nanocomposite" alloys as compared to field annealed amorphous alloys making the former more applicable to high temperature inductive applications. (6) Development of a more sophisticated understanding of the phase stability, structure, and magnetic properties of complex cubic structures of 23:6 stoichiometry often observed as secondary crystallization products in the Fe,Co-based "nanocomposite" forming alloys.