Crystallization of ternary Zr based glasses-kinetics and microstructure

Abstract

The effect of ternary addition on the thermal stability and the sequence and the kinetics of crystallization of metallic glasses Zr₇₆Fe_(24-x)Ni_x (x = 0, 4, 8, 12, 16, 20, 24) have been examined. It has been found that the surface crystallization occurs in the composition range 16 < x < 20, leading to the formation of an ordered Fe-rich (Fe, Ni)₃Zr cubic phase, followed by the transformation of the bulk to a mixture of α -Zr and Zr₂Ni. Crystallization of alloys containing 12 to 20% Fe occurs at lower temperatures by primary crystallization of Zr₃(Fe, Ni), followed by decomposition of the remaining amorphous matrix by eutectic crystallization giving rise to α-Zr + Zr₂Ni. At higher temperatures these alloys transform polymorphically to Zr₃(Fe, Ni) in which Ni partially substitutes Fe in the Zr₃Fe lattice. Copious nucleation of Zr₃(Fe, Ni) phase in these alloys, leading to the formation of a nanophase structure, has been observed. This is consistent with the prediction of increasing nucleation rate for Fe-rich compositions. The crystal nucleation and growth kinetics have been examined for primary, eutectic, and polymorphic crystallization processes. The observed nucleation and growth behaviors have been rationalized by considering the role of the quenched in nuclei and the activation energies of nucleation and growth.