Chemical Bond in Metallic Alloys: Electron Microscopic Study

Abstract

Aplastic anemia in systemic lupus erythematosus: A better prognosis acquired aplastic anemia Copyright: © 2016 Pannu and VarmaThe first experimental study to verify the existence of the high-temperature phase separation in alloys of the ironchromium system was undertaken by the example of the Fe-45% Cr alloy [15]. Using the method of TEM, it was found that the microstructure of this alloy after quenching from 1150 -1200°С was similar to that which had been obtained in Reference [14]. However, they interpreted such precipitations as chromium nitrides, i.e., as a kind of an "added" phase formed by the chemical reaction of chromium atoms (from the alloy) with atoms of nitrogen (from the air) during a high temperature heat treatment for quenching. A second study was conducted using the Fe 51 Cr 49 alloy, with the help of Mossbauer spectroscopy [16]. It is obvious that the choice of the method of research in Reference was poor, since it was hardly possible to judge a local phase separation of the alloy by the change of the partial gamma-resonance peaks corresponding to pure iron. Therefore, they did not manage to find the structure of phase separation [16]. Figure 2 shows the iron-rich part of an iron-chromium phase diagram, built on the results of electron microscopic studies of the microstructure of alloys of iron with 20, 30, 40 and 50 wt. % of chromium [17]. From the diagram, it can be seen that in the Fe-Cr system, there occur two phase transitions, in the result of which, for example, the microstructure that has formed as a consequence of the tendency to ordering is dissolving and a microstructure of phase separation is forming in its place (and vice versa). This occurs within the temperature ranges of 1100 -850°C and 600 -550°C (Figure 2) On the basis of these data, it could be concluded that at the level of microstructures, such a phase transition is bound to pass through the stage of the existence of the solid solution in the alloy. The authors have given this phase transition the name "orderingphase separation" [17]. Since the discovery of this transition, 17 binary systems have so far been studied and in 16 of them the transition has been experimentally found. The transition occurs at a temperature, specific for each system, at which the sign of the chemical interaction between atoms of A and B is reversed. It is obvious that the transformation of the microstructure formed as a result of the tendency to ordering into the microstructure formed as a result of the tendency to phase separation, and vice versa, is a consequence of the phase transition ordering -phase separation. The transition itself, that is, the process of changing the sign of the chemical interaction between dissimilar atoms, occurs at the level of changes in the electronic structure of the alloy.Before considering the physical essence of this transition, we would like to decide on the issue of the use of such terms as "ordering energy", "mixing energy", "enthalpy of mixing" and so on. Each of the above terms refers to basically the same no...