The structure of nanocrystalline Ni58.5Mo31.5B10 and structure evolution at heat treatment

The formation and thermal stability of a nanocrystalline structure in Ni58.5Mo31.5B10 alloy were studied by X-ray diffraction, transmission electron microscopy and high resolution electron microscopy. A nanocrystalline FCC structure was obtained by isothermal annealing of the amorphous alloy ribbons at 550–710 °C. The average grain size of the nanocrystalline phase varies from 10 to 28 nm depending on the heat treatment conditions. It has been observed that the nanocrystalline alloy has a two-phase (nanocrystal and residual amorphous phase) structure. The thickness of amorphous intergranular layers before the decomposition of nanocrystalline structure was about 2–3 nm. The dependence of the average grain size in the nanocrystalline structure on the heat treatment conditions was obtained. This structure was stable after isothermal annealing at 600 °C for several days. The decomposition of nanocrystalline structure occurred by crystallization of intergranular amorphous layers in some places. In these places the crystal grains begin to border each other directly and the coalescence of the grains and phase transformations occur rapidly. After a prolonged isothermal annealing at 600 °C the nanocrystalline structure is accompanied by Ni3Mo and Mo2B crystals with these crystal proportion increase with the annealing duration.