Distinctive annealing behaviour of Si-poor FeNbCuBSi alloys

Unlike more Si-rich FeNb3Cu1By − xSix (where y = 18–22.5) alloys, for x ⩽ 10 at.% the building of hard-ribbon-axis magnetic anisotropy is observed after isothermal annealing of toroids at about 540 °C. The hysteresis loops increase their tilting with annealing time while the sense of the variation crosses over after a period depending on the Si contents. Primary increase of a.c. initial permeability is reverted after a short annealing period. Tiny (d ⩽ 20 nm) α-Fe(Si) crystalline grains are formed in the initial 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 α-Fe(Si) grains. X-ray diffraction then shows extra reflections, indicating the presence of Fe borides. Several effects that could give rise to the observed behaviour are discussed.