Kravets O. Peculiarities of heat transfer and crystallization processes under quenching of metals from liquid state.

The results of both experimental studies and analysis of the heat transfer and crystallization processes in thin layers metallic melts solidifying on a substrate with high thermal conductivity are presented in the thesis. By using of numerical solutions of a set of the heat conduction equations and the developed kinetic equation of mass crystallization the studies of the thermal regimes of quenching from liquid state, of the undercooling and glass-forming abilities of metals and alloys, of crystal growth kinetics during mass crystallization, as well as the peculiarities of formation of structure under conditions of several polymorphic phases competition. The interrelations between the cooling rates and thickness of the melt layer as well as between the abilities of metals to undercooling and to glass formation are established. The most probable crystal sizes in rapidly solidified metals and alloys and their dependencies on the cooling rate are calculated. It is shown that the results of the model calculation are in good accordance with the appropriate experimental data. The formation of the metastable modifications with seven-layered hexagonal lattice having ABCABAC… stacking sequence of the layers is experimentally established in structure of rapidly quenched foils of La, Ce and Nd. The mechanisms of polytype transformations which lead to formation of these phases are proposed which validity has been confirmed by the results of simulations of crystallization kinetics of two equilibrium and metastable modifications of Nd.