THE TEMPERATURE VS TIME TRANSFORMATION (T-T-T) DIAGRAM FOR A TRANSITION FROM THE AMORPHOUS TO THE NANOCRYSTALLINE STATE

Nanocrystalline (NC) phases can be formed during a crystallization of amorphous alloys annealed isothermally below the usual crystallization temperature. In this work, the transformation kinetics from the amorphous state to the NC phases in a Ni-P alloy was investigated by means of a differential scanning calorimeter (DSC) and a magnetothermal analysis (MTA) method. The temperature vs time transformation (T-T-T) diagrams for the amorphous-to-NC process determined by using the two experimental methods are comparatively analyzed. In the MTA measurements, the transformation was found to start much earlier than that in the DSC measurement, and the difference in the measured starting times for the transformation from the two measurements becomes larger for low temperatures where the average grain size in the NC sample is reduced. The transformation kinetics were analyzed by means of the Avrami exponent (based on the Johnson-Mehl-Avrami equation) and the activation energy (from the Arrhenius relation). It was found that the transformation process measured in the DSC experiments is only a bulk crystallization occurring in the amorphous samples, and the early surface crystallization and a segregation process of solute P atoms fail to be detected by the DSC.