Formation of Nanocrystalline Phases During Thermal Decomposition of Amorphous Ni-P Alloys by Isothermal Annealing

The microstructure and the average grain size were investigated by x-ray diffraction and transmission electron microscopy for nanocrystalline (n) Ni-P alloys with 18, 19, and 22 at.% P. A detailed study of the nanocrystalline states obtained along different heat treatment routes has been performed: (1) a → n_i by isothermal annealing of the melt-quenched amorphous (a) Ni-P alloys; (2) n_i → n_ii by isothermal annealing of the nanocrystalline n_i state; (3) n_i → n_il by linear heating of the n_i state. The heats evolved during the structural transformations were determined by differential scanning calorimetry. From these studies, a scheme of the structural transformations and their energetics was constructed, which also includes previous results on phases obtained by linear heating of the as-quenched amorphous state of the same alloys. Grain boundary energies also have been estimated. In some cases it was necessary to assume a variation of the specific grain boundary energy during the phase transformation to understand the enthalpy and microstructure changes during the different heat treatments.