DISTINCTIVE ANNEALING BEHAVIOR OF SI-POOR FENBCUBSI ALLOYS

Unlike more Si-rich FeNb3Cu1By-xSix (where y=18-22.5) alloys, for x le ss than or equal to 10 at.% the building of hard-ribbon-axis magnetic anisotropy is observed after isothermal annealing of toroids at about 540 degrees C. The hysteresis loops increase their tilting with anneal ing time while the sense of the variation crosses over after a period depending on the Si contents. Primary increase of a.c. initial permeab ility is reverted after a short annealing period. Tiny (d less than or equal to 20 nm) alpha-Fe(Si) crystalline grains are formed in the ini tial stages of crystallization; their volume share is about 50% after 30 min annealing. Using transmission electron microscopy (TEM), it has been observed that after continued annealing a new crystalline phase is formed on the boundaries of the alpha-Fe(Si) grains. X-ray diffract ion then shows extra reflections, indicating the presence of Fe boride s. Several effects that could give rise to the observed behaviour are discussed. (C) 1997 Elsevier Science S.A.