Computer modelling of isothermal crystallisation kinetics of electroless and melt quenched amorphous solids using Johnson-Mehl-Avrami theory

The isothermal crystallisation kinetics of electroless and melt quenched amorphous solids are modelled based on the framework of the Johnson-Mehl-Avrami (JMA) theory. The alloy systems studied are electroless and melt quenched Ni-P with varying phosphorus concentrations around the eutectic composition, and melt quenched Fe40Ni40P14B6. The JMA kinetic parameters at various temperatures are derived, based on published differential scanning calorimetry (DSC) experimental data. The isothermal crystallisation produces nanocrystals, before the grain growth that normally occurs upon linear heating to elevated temperatures. Therefore, it has the advantage of separating crystallisation from grain growth, the two stages that are not separated during linear heating experiments and modelling based on such experiments. It is found that the Avrami index n varies within a small range from 3.1 to 3.8 in the case of electroless Ni-P, indicating that the mechanism of transformation remains the same for the range of isothermal annealing temperatures (560-590 K).