XRD INVESTIGATION OF THE CRYSTALLIZATION PROCESS IN FE40NI40B20 METALLIC-GLASS

X-ray diffraction analysis showed that two crystalline solid solutions simultaneously arise when Fe40Ni40B20 glassy metallic ribbons, aged at room temperature for about 1 year, are heated in an inert atmosphere at 653 and at 663 K (first steps of the crystallization process). These two compounds are orthorhombic (Fe, Ni)3B and fcc gamma-(Fe, Ni). The first compound had larger unit cell parameters than those of both Ni3B and Co3B, while the second phase had a unit cell edge shorter than that of equiatomic gamma-(Fe, Ni). These unit cell parameters indicate the formation of an orthorhombic solid solution richer in Fe than in Ni and, conversely, of a cubic solid solution richer in Ni than in Fe. The orthorhombic phase had the largest volume fraction and crystallites larger than those of the fcc phase (average sizes of 36 and 6 nm, respectively). The crystalline fraction of the metallic ribbons, partially crystallized after isothermal heating at the above temperatures, was determined as a function of the heating time. A modified Avrami kinetic analysis was carried out with computed values of the crystallized fraction. Dependence of the 'local' Avrami exponent on crystallized fraction is indicative of a crystal growth mechanism based on pre-existing nuclei (surface plus quenched-in nuclei). By applying the well known Arrhenius equation to the kinetic constants, as obtained by the 'locally' applied Avrami analysis, an initial activation energy of 250 +/- 50 kJ mol-1 was determined for the growth process.