Magnetic state effect upon the order-disorder phase transition in Fe-Co alloys: A first-principles study

Abstract

The aim of the thesis is to study ordering in binary alloys on the basis of first-principles or {\it ab-initio} techniques employing density functional theory (DFT). The ordering phenomena of materials are of crucial importance for technological applications. The results of the thesis are intended to demonstrate the applicability of the first-principles calculations to provide fundamental insight to the true, namely electronic structure, nature of ordering in binary alloys.The main part of the thesis focuses on atomic short- and long-range order phenomena in binary alloys as a function of both temperature and chemical composition in FeCo and NiCr alloys. In particular, the influence of magnetism on atomic ordering in FeCo alloys is investigated using the disordered local moment.A large number of concentration dependent effective cluster interactions, derived without the use of any adjustable parameters, are obtained by the SGPM as it is implemented in the EMTO within the CPA. The SGPM interactions can subsequently be used in thermodynamic Monte-Carlo simulations or mean field approximations to determine the ordering phenomena in binary alloys.First-principles calculations of intrinsic stacking-fault energies (SFE) andanti-phase boundary energies (APBE) in Al$_{3}$Sc and the effects of temperature on SFE and APBE are investigated by using the axial Ising model and supercellapproach. Temperature effects have been taken into consideration byincluding the one-electron thermal excitations in the electronicstructure calculations, and vibrational free energy in the harmonicapproximation as well as by using temperature dependent lattice constants.The latter has been determined within the Debye-Gr{\u}neisen model,which reproduces well the experimental data.Within the framework of the quasiharmonic approximation, the thermodynamics and elastic properties of B2- FeCo alloy are studied using first-principles calculations. The calculated thermal and elastic properties are found to be in good agreement withthe available measured values when the generalized gradientapproximations is used for the exchange correlation potential.The calculated finite temperature elastic constants show thatthe FeCo alloy is mechanically stable in the ordered phase.Meanwhile, a large elastic anisotropy exhibits a moderate dependence ontemperature.