INFLUENCE OF PREANNEALING ON THE CRYSTALLIZATION OF FE75SI15B10 METALLIC-GLASS

The thermal stability of Fe75Si15B10 metallic ribbon was analysed. The kinetics of the two-step crystallization of as-quenched and isotherma lly preannealed samples was studied by differential scanning calorimet ry using various heating rates and the Kissinger, isoconversional and Surinach methods. Both crystallization stages are characterized by the Johnson-Mehl-Avrami kinetic law. The first step is dependent on both the heating rate and the preannealing. After sufficient heat treatment , the Avrami exponent decreases from 2.5 to 1.5, and the activation en ergy decreases to the value characteristic for the crystal growth mode . The second-step Avrami exponent n(2) = 3, and the activation energy E(2) = 395 kJ (g-atom)(-1), remain constant. The first step was relat ed to the crystallization of alpha-Fe(Si) and Fe3Si, during which some of these primary crystals nucleate heterogeneously on the Fe3B cores. The eutectic decomposition of Fe3B into stable Fe2B and alpha-Fe was observed during the second step.