Rassolov S. Influence of the thermal prehistory on the relaxation and crystallization processes in liquid and amorphous iron-based alloys under non-isothermal conditions

The thesis is devoted to studies of the thermal regime of cooling during melt-spinning processing, analysis of mechanisms and kinetics of relaxation and crystallization processes in liquid and amorphous iron-based alloys Fe100-x-Bx (х = 15, 16, 20) and Fe40Ni40P14B6 under non-isothermal conditions, as well as to establishment of interrelations between the processing conditions and the thermal stability of glassy state. The dependencies of the cooling rates on the ribbon thickness d were experimentally established. It was found that behavior of the cooling rates vs d curves is essentially determined by which of the technological parameters (the quenching wheel velocity, melt temperature) the value of d was changed. By using of combinations both of the structural and structure-sensitive techniques and the annealing regimes features of structural relaxation in Fe40Ni40P14B6 glassy alloy were observed and four elemental superimposed processes were established. The exact and approximated analytical equations which describe of the diffusion-limited crystal growth in metallic glasses as a function of temperature and heating rate were firstly obtained. Modeling of this process showed that the grain size is continuously decreased with heating rate and, in part, for nanocrystalline structure formation in the Fe85B15 glass the values of heating rate > 10000 К/с are required. It was shown that the experimentally observed strong dependence of the crystallization temperature in the Fe80B20 amorphous alloy might be quantitatively interpreted within the model proposed, which is based on the concept of the fictive temperature relaxation. It was found that the melt quenching rate is determined both the quenched-in nuclei density and the degree of deviation of glassy structure from its equilibrium and the experimental conditions were established under which each of the factors is influenced on the thermal stability of amorphous phases.