On the correlation between electron structure and short range atomic order in iron-based alloys

Abstract

The experimental data on the concentration of free electrons in fcc iron-based alloys, results of theoretical calculations on the electronic structure and experimental data of atomic distribution are analysed. The electron structure of iron-based substitutional solid solutions and CrNi austenitic steels alloyed by Mn, Mo, Cu, Si, Al and C, N was studied by means of the measurement of conduction electron spin resonance. The electron exchange in binary fcc Fe–N and Fe–C alloys was also calculated using an ab initio norm-conserving pseudopotential method. It is shown that Ni, Cu, Si and Al increase the concentration of free electrons, whereas Cr, Mn and Mo decrease it. Theoretical calculations as well as experimental data show that nitrogen in fcc iron and iron-based solid solutions increases the state density at the Fermi surface, whereas carbon contributes its electrons to the states below the Fermi surface. Mössbauer spectroscopy was used to study the distribution of carbon and nitrogen in binary fcc Fe–C and Fe–N alloys, while the data on the distribution of d-solutes in multicomponent solid solutions were obtained from the analysis of the contributions of different electronic subsystems, namely free electrons, isolated localized d-electrons (single solute d-atoms) and superparamagnetic clusters (clusters of d-atoms), to the temperature dependence of the magnetic susceptibility. The results of studies concerning the atomic distribution are consistent with the available data on the short range order in iron-based alloys. The following correlation is found: an increase in the concentration of free electrons assists the short range atomic ordering in iron-based alloys, whereas the localization of electrons promotes clustering of solute atoms. The state of atomic order influences properties like austenite stability, corrosion resistance and strength.