Ionic Bond ⇄ Covalent Bond Electron Transitions in Alloys

Abstract

The ordering–separation phase transition, which has been earlier detected in many alloys and causes microstructural changes, is shown to begin with changes in the electronic structure of an alloy. In contrast to pure metals, where all valence electrons take part in the formation of a metallic bond, some pairs of valence electrons in an alloy turn out to be localized at two neighboring unlike atoms, which leads to the formation of common orbitals between them. This behavior is characteristic of the appearance of an ionic chemical bond. During the ordering–separation phase transition in an alloy, this localization of valence electrons is violated and some pairs of valence electrons become involved in the formation of hybridized orbitals between the pairs of neighboring like atoms. As a result, a covalent bond component appears instead of an ionic component in the alloy. It is concluded that the ionic bond covalent bond ⇄ electron transition precedes the microstructural ordering–separation phase transition.