A New Paradigm for Metallic Alloys in Materials Science

Abstract

In the article, taking into account the phase transition "ordering-phase separation" discovered in alloys, new concepts about the diffusion phase transformations in alloys are formulated: chemical interaction between dissimilar atoms exists always in all alloys and at any temperature of heating; alloys offer a surprising and not previously known property of changing the sign of the chemical interaction between dissimilar atoms at a change of the temperature or composition of alloys; diffusion processes occurring in alloys at different temperatures depend on the sign and the absolute magnitude of the energy of the chemical interaction between dissimilar atoms. All these three concepts are analyzed in detail, by the example of Ni-based and Co-based binary alloys using experimental results obtained by transmission electron microscopy. It is shown, on these ideas, how to carry out heat treatment of alloys more rationally, what principles should underpinned in the base of the construction of phase diagrams, how the microstructures of ordering and phase separation affect some properties of alloys.

Chemical Bond in Metallic Alloys: Electron Microscopic Study

“…In the first case, this distance is approximately 300°C, in the second -about 50°C. Based on this scheme, the following conclusions have been made [20]:a. The sign of the energy of the chemical interaction between dissimilar atoms is not a constant for the majority of metallic systems but varies with the change of the temperature of the alloy (sometimes more than once).…”

“…Everything depends on the nature of the alloy. The latter may mean that near the phase transition line on the diagram there exists not a line but a range of temperatures within which a disordered solid solution structure is formed [20]. The latter may mean that near the phase transition does not exist in the graph line, but the temperature range in which the formed structure is a disordered solid solution [20].…”

“…Everything depends on the nature of the alloy. The latter means that in a certain temperature range, in the vicinity of the phase transition line in the diagram, there may exist a stripe, within which a disordered solid solution structure is formed [19,20]. This scheme shows experimentally determined points (1 and 2) of the phase transition 'orderingphase separation'.…”

“…The latter may mean that near the phase transition line on the diagram there exists not a line but a range of temperatures within which a disordered solid solution structure is formed [20]. The latter may mean that near the phase transition does not exist in the graph line, but the temperature range in which the formed structure is a disordered solid solution [20].The phase transition 'ordering-phase separation' occurs at a temperature, which is quite definite for each binary system. With the help of the method of X-ray photoelectron spectroscopy, it has been shown that this transition occurs at the level of changes in the electronic structure of alloys (the shape of the valence band at such a transition changes [23,35]).…”

“…It is obvious that the chemical bond inside these particles is different from the metallic bond existing in the rest of the matrix. In alloys, in which the microstructure is a eutectic or structures similar to it, clusters of atoms of the solute component, for example, inside these precipitations a covalent bond should take place; in alloys, in which chemical compounds precipitate-in such compounds there should exist an ionic bond.A detailed study of the type of chemical bonds existing in alloys at various temperatures of heat treatment was conducted in a series of electron microscopic studies, the reviews of them published in References [10,20]. It has been shown experimentally by the example of alloys of 17 binary systems that dissimilar atoms can participate in the formation of a chemical compound only if the sign of the chemical interaction energy is negative, i.e.…”