A new temperature versus heating rate transformation (T-HR-T) diagram: Application to study the crystallization behaviour of Fe 67.5Co 15Nb 1.5B 16 metallic glass

Abstract

A new approach based on a temperature-heating rate-transformation (T-HR-T) curves, useful when heating an amorphous sample under continuous heating conditions, together with the traditional time-temperature-transformation (T-T-T) diagram is presented in the study of the crystallization behaviour of Fe 67.5Co 15Nb 1.5B 16 metallic glass, prepared planar flow casting. The study has been carried out by differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. The calorimetric results show that there are two separate stages of crystallization. The second stage of crystallization is described correctly by the Johnson-Mehl-Avrami-Erofe'ev equation, with an Arrhenius form of the rate constant, under both isothermal and continuous heating. A more complicated kinetic behaviour is obtained for the first stage of crystallization. Transmission electron microscopy of the specimens after controlled heat treatment shows that the primary crystallization of α-(Fe,Co) dendrites in the amorphous matrix occurs in the first stage of crystallization with, in addition, sporadically secondary precipitation of (Fe,Co) 3B crystals. X-ray diffraction experiments confirm these results and show that a progressive enrichment in Fe in the α-(Fe,Co) phase occurs on the first stage of crystallization. From the crystallization kinetic results the low temperature part of the T-T-T and T-HR-T curves of the two exothermic crystalization processes are deduced.