ISOTHERMAL CRYSTALLIZATION KINETICS OF NI24ZR76 AND NI24(ZR-X)76 AMORPHOUS-ALLOYS

Isothermal devitrification kinetics of the melt-spun Ni24Zr76 and Ni24(Zr-X)76 amorphous alloys has been studied by means of DSC, optical microscopy, TEM and X-ray diffraction. These amorphous alloys exhibit eutectic crystallization. Isothermal devitrification kinetics, as obtained by DSC, has been analyzed in terms of Kolmogorov-Johnson-Mehl-Avrami (KJMA) model. The classical KJMA plots exhibit significant non-linearity from the beginning of the transformation. Such non-linearity has been attributed to a number of factors such as surface-induced crystallization, anisotropic growth of crystals, impingement effects (hard and soft) and variation of the nucleation rate during devitrification etc. In these alloys, the steady state nucleation frequency is established after a well-defined transient period. A quantitative analysis of the transient period has been performed in terms of non-steady state nucleation theory and the theoretically estimated transient times are found to be consistent with the experimentally observed incubation periods. By applying classical theory of homogeneous nucleation, it is found that the density of quenched-in nuclei will be very small in the absence of heterogeneous nucleation.