The effect of addition of Si and variation of the Fe/Co ratio on the evolution of the nanostructure was studied in a modification of the Fe-Nb-B system. The entire system (Fe, Co)(73)Nb(7)(Si, B)(20) was prepared in an amorphous state by rapid quenching using the planar flow casting method over a wide range of Fe/Co atomic ratios, ranging from 0 to 1. Nanocrystallization was investigated by evolution of the electrical resistivity with time and temperature. The microstructural analysis was performed using transmission electron microscopy as well as electron and X-ray diffraction. The results from microscopy observations were used to determine the distribution of grain size, which in these alloys attain very small dimensions of approximately 5-8 nm. New algorithms of microscope image analysis were used for grain size determination, crucial for quantifying the microprocesses controlling nucleation and growth from the amorphous rapidly quenched phase.

Classical transformations from rapidly quenched amorphous into (nano)crystalline state monitored by time and temperature evolution of electrical resistivity are discussed as a typical case. Common and different features observed in these transformations, related to local structural and chemical arrangements, are compared with special cases, e.g. sharp increase of resistivity during energy-intensive crystallization observed in Fe–Ni based amorphous systems. A drastic increase of electrical resistivity during transformation in Al–V system is attributed to the formation of nanoquasicrystalline structures with a predicted pseudogap in the electron density at the Fermi level. The results are correlated with microstructure observations and with data from routinely used methods of kinetic analysis with respect to the expected local atomic ordering in amorphous precursor systems.

SummaryThe effect of the substitution of Fe by Co on the enhancement of glass‐forming ability limits and subsequent nanocrystallization was studied in a rapidly quenched amorphous system (FexCoy)79Mo8Cu1B12 for y/x ranging from 0 to 1. The effect of Cu on nanocrystallization was investigated by comparison with Cu‐free amorphous Fe80Mo8B12. Systems partially crystallized at the surface layer were prepared for y/x = 0 using different quenching conditions. The effect of heat treatment of master alloys used for ribbon casting was also assessed. The microstructure and surface/bulk crystallization effects were analysed using transmission electron microscopy and electron and X‐ray diffraction in relation to the expected enhancement of high‐temperature soft magnetic properties, drastically reduced grain sizes (∼5 nm) and Co content. Unusual surface phenomena were observed, indicating the origin of possible nucleation sites for preferential crystallization in samples with low Co content.

The formation of amorphous metallic systems by rapid quenching is influenced in a significant manner by the detailed path of the corresponding alloy from the melt through undercooled liquid down to the amorphous state. The thermodynamic history of the formation of an amorphous system (quenching the liquid state with specific structure, interatomic bonding and short range ordering) forces a specific template, reflecting the presence of ordered polyatomic clusters, into its potential energy structure. Subsequent transitions from amorphous to the (nano)crystalline state which then take place respect the existing potential energy landscape with such quenched-in local-scale heterogeneities. It will be shown that using a model-free continuous approach for the estimation of the distributions of initial energetic states (reflecting the cluster structure) it is possible to obtain information about the types of micromechanisms controlling the transformation processes as well as to use this information to obtain control of the phase selection during the transformation into the nanocrystalline state. A generalized view of the process of nanocrystallization from a clustered amorphous state will be presented. A special focus will be put on the nucleation processes and on the methods of their quantification. Two methods will be discussed.