Popov V. Effect of replacement of nickel with cobalt on thermal stability, kinetics of crystallization and physical properties of Fe- and Al-based amorphous alloys.

The thesis deals with establishment of the mechanisms of alloying on thermal stability, crystallization kinetics and physical properties of new groups of Fe80-xCoxP14B6 (x = 20, 25, 30, 32, 35) and Al86(Ni,Co)8Gd6 amorphous metallic alloys (AMA). The studies have been performed by X-ray analysis, electron microscopy, differential scanning calorimetry, resistometry, thermomagnetic analysis as well as by measurements of the magnetic and strength properties. It has been found that full (in FeNiPB system) and partial (in Al(Ni,Co)Gd alloys) replacement of Ni with Co results in an essential increase of thermal stability of amorphous phases and in enhancement of the soft magnetic and strength characteristics of Fe- and Al-based AMA, respectively. The crystallization mechanisms of the AMA studied have been established and both the thermodynamic and kinetic parameters governing the nucleation and growth of crystals have been determined. It has been shown that enhancement of thermal stability due to replacement of Ni with Co is caused by the lowered rates of nucleation and growth of crystals in FeCo-based alloys (in comparison with those in FeNi-based) and by changing of crystallization mechanism on Al-based alloys. It has been established that non-monotonic behaviour with annealing temperature of the level of nucleation nonstationarity in Fe-based AMA is caused by the differences in the temperature dependencies of the characteristic transient and crystallization times. It has been shown that the full and partial replacement of Ni with Co leads to essential lowering (by 2-3 orders of magnitude) of the effective diffusion coefficients which govern the interfacial controlled eutectic growth (in Fe-based AMA) and the diffusion-limited growth of Al nanocrystals under conditions of impingement of the diffusion fields (in Al-based AMA). The threshold values of the diffusivities determining thermal stability of the glasses have been found.