Nanocrystallization and thermal stability of the Fe45Ni19.4Co8.5Cr5.7Mo1.9B14Si5.5 amorphous alloy

Abstract Thermal stability, structural changes and nanocrystallization kinetics of the Fe 80 B 14 Si 6 and Fe 45 Ni 19.4 Co 8.5 Cr 5.7 Мо 1.9 B 14 Si 5.5 amorphous alloys under continuous heating and isothermal annealings as well as the changes of microhardness caused by the phase transitions have been experimentally studied by a combination of X-ray diffraction analysis, differential scanning calorimetry, by measurements of electrical resistance and microhardness. The partial replacement of Fe in the Fe 80 B 14 Si 6 alloy by a number of transition metals results in lowering of the onset crystallization temperature, facilitating the nanocrystal forming tendency, and enhancement of the microhardness values of the samples in the non-equilibrium structural states. The temperature dependencies of effective diffusion coefficients governing nanocrystallization are determined by approximation of the experimentally measured changes of the α-Fe nanocrystal sizes and the kinetic curves of nanocrystallization within the appropriate analytical models. The essentially lower thermal stability of the Fe 45 Ni 19.4 Co 8.5 Cr 5.7 Мо 1.9 B 14 Si 5.5 amorphous alloy compared with that of Fe 80 B 14 Si 6 is associated with about two orders of magnitude lower effective diffusivity at the onset crystallization temperatures. From comparison of the rates of growth of α-Fe nanocrystals and their numbers of volume density in nanocomposite structures, it is suggested that the thermal stability of the former amorphous alloy is limited by slow diffusion-limited growth, while that of the latter one is controlled by nucleation.