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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.
Highlights
The existing ideas about the microstructure of metallic alloys were formed as far back as the first half of the twentieth century
At a decrease of the temperature, when the atomic interaction potential became substantially larger than the thermal energy, the mutual arrangement of component atoms in the solid solution was determined from the condition of the minimum free energy of the alloy [1] and an order-disorder phase transition began in it
5) The microstructure of a disordered solid solution is formed over all the bulk of an alloy only in the temperature region adjacent to the temperature of the phase transition “ordering-phase separation”, because it is in this region that the chemical interaction energy between dissimilar atoms is close to zero
Summary
The existing ideas about the microstructure of metallic alloys were formed as far back as the first half of the twentieth century. Experimental studies of the crystal structure of alloys conducted later with the help of X-ray diffraction (XRD), seemed to have fully confirmed these views When this method was applied to alloys quenched from high temperatures, no other phases besides the solid solution were found. If the sign of the chemical interaction energy between the dissimilar atoms is positive, there takes place a process of phase separation of an alloy into clusters (grains) consisting of atoms of any one sort, which in conventional phase diagrams is commonly presented as formation of eutectics (eutectoids) Such a transition from one type of the chemical interaction (attraction between dissimilar atoms) to another type (repulsion between them) naturally passes through the point, where the energy of the chemical interaction is zero, i.e. where the microstructure of a disordered solid solution is to be formed [13].
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