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Crystallization kinetics of Fe_55-xCr₁₈Mo₇B₁₆C₄Nb_x(x= 0, 3) bulk amorphous alloys were analyzed using X-ray diffraction and differential scanning  calorimetric (DSC) tests. In practice, crystallization and growth mechanism were evaluated using DSC tests at four different heating rates (10, 20, 30, and 40 K/min) and kinetic models. Two-step crystallization behavior was observed when Fe₅₅Cr₁₈Mo₇B₁₆C₄ and Fe₅₂Cr₁₈Mo₇B₁₆C₄Nb₃ bulk amorphous alloys were annealed, where Fe₃₆Cr₁₂Mo₁₀ phase was crystallized in the first step of crystallization. Results show that Fe₃₆Cr₁₂Mo₁₀ and Fe₃C phases were crystallized in the structures of the alloys after further annealing process. Activation energy for the crystallization of Fe₃₆Cr₁₂Mo₁₀ phase was measured to be 543 kJ/mol in Fe₅₂Cr₁₈Mo₇B₁₆C₄Nb₃ alloy and 375 kJ/mol for Fe₅₅Cr₁₈Mo₇B₁₆C₄ alloy according to Kissinger-Starink model. Moreover, a two-dimensional diffusion controlled growth mechanism with decreasing nucleation rate was found in Fe₅₂Cr₁₈Mo₇B₁₆C₄Nb₃ alloy whereas a three-dimensional diffusion controlled growth mechanism with decreasing nucleation rate was found in crystallization of Fe₃₆Cr₁₂Mo₁₀ phase during annealing of  Fe₅₅Cr₁₈Mo₇B₁₆C₄ alloy. TEM (transmission electron microscopy) observations reveal that crystalline Fe₃₆Cr₁₂Mo₁₀ phase nucleated in the structures of the alloys in an average size of 10 nm with completely mottled morphology.