Hypotheses on how first order phase transitions develop which deny a fundamental element of the classical thermal fluctuation theory, namely, that the nucleation of a new phase must take place when the crystal lattice changes, are considered. It is shown that the thermal fluctuation theory does not work for poly morphic transformations in pure metals, which are accompanied by lattice rearrangement in a macrovolume (over the entire volume of a sample under study) without substance transfer. However, the classical thermal fluctuation theory of new phase nucleation is valid in describing the supersaturated solid solution decompo sition in the case where the main pattern of the crystal lattice of the matrix is retained, and a new phase, which differs from the parent one in both crystal structure and chemical composition, precipitates in separate microvolumes.
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