THE EFFECT OF CRYSTALLIZATION FRACTION ON IN-SITU AC-INITIAL-PERMEABILITY OF AN FESI-B AMORPHOUS ALLOY DURING ISOTHERMAL ANNEALING

The crystallization fraction (X-f) was quantitatively determined by th e decrement in the crystallization enthalpy of an annealed Fe78Si13B9 alloy as compared with that of the as-cast one. The X-f was used to in terpret the variation in ac-initial permeability (mu(i)) of the Fe78Si 13B9 alloy in- situ measured during isothermal annealing at 646 and 66 6 K, respectively. There are two mu(i) maxima as soaking time elapses for both annealings. The first mu(i) maximum is attributed mainly to t he stress-relaxation effect and some crystallization effect with a mea sured X-f of 2-3.5%. The second mu i maximum, appearing at about 210 a nd 36 ks for the annealings at 646 and 666 K, respectively, is invaria ntly much higher than that of the first one. The two corresponding cry stallization fractions (X-f) are the same, about 9% for both annealing s. The second maximum is thus associated with crystallization effect. The Curie temperature of the amorphous matrix, being 668.5 K for the a s-cast sample, increases sharply with annealing time. This makes diffe rence in the shape of the first maximum, leveling off for the 646 K an nealing and sharpening for the other. Transmission electron microscopy (TEM) micrographs and X-ray diffractometry (XRD) studies of the cryst allized alpha-Fe and Fe(Si) phases in the alloy annealed at 646 K for more than 180 ks were depicted.