NONEQUILIBRIUM CRYSTALLIZATION OF ND13FE79B8 ALLOY

The nucleation frequency, the crystal growth rate and the transformation diagrams of temperature versus time and temperature versus heating rate for the primary crystallization of the Nd2Fe14B hard magnetic phase in Nd13Fe79B8 amorphous alloy are established. The treatment is based on nucleation and growth theories of the kinetics of non-equilibrium crystallization under both isothermal and non-isothermal regime. The modeling parameters used are assessed from calorimetric and microstructural experimental data. Values of the nucleation frequency as high as 10(17) cm-3s-1 are obtained at temperatures around 1015 K while the growth rate remains quite low with a maximum value of the order of 10(-4)cm s-1 between 1300 and 1400 K. The use of the time-temperature and temperature-heating rate diagrams allows onset crystallization temperatures to be predicted by the model when annealing at temperatures or heating at rates different from those experimentally used in the measurements. The modeling of both the nucleation frequency and the crystal growth rate makes it possible to design suitable heat treatments to produce the desired microstructure.