A brief review of works is given in which, using the method of transmission electron microscopy, the phase transition ‘ordering-phase separation’ has been found, which indicates that the sign of the energy of the chemical bond between the atoms of the components changes at a change in the temperature or composition of the alloy. It has been shown that in the case of the tendency to ordering, this sign is negative and chemical compounds are formed in the alloys, in the case of the tendency to phase separation, the sign is positive and formation of either clusters enriched with solute atoms or grains of solute atoms takes place in the alloys. Proofs are presented that the phase transition ‘ordering-phase separation’ begins with changes in the electronic structure of an A(B) alloy. It leads to delocalization of the valence electrons on the atoms of A and B and hybridization of the valence electrons on the atoms of B. Along with the metallic component, the covalent component of the chemical bond is formed. The transition ‘phase separation-ordering’ begins with dehybridization of the valence electrons on the atoms of B and localization of the valence electrons on the atoms of A and B. Along with the metallic component, the ionic component of the chemical bond is formed. Such changes in the electronic structure are the cause of subsequent changes in the microstructure of alloys.
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