First-principles Study Of Dislocation Core Structure And Impurity-dislocation Interactions

Based on density functional theory, the dislocation core structure and theimpurity-dislocation interactions are theoretically investigated in thisdissertation.The effects of spin polarization and generalized gradient corrections onthe generalized stacking-fault (GSF) energies and the edge dislocationproperties in bcc Fe are firstly studied by using the first-principles totalenergy calculations within the framework of Peierls-Nabarro model. Ourcalculation shows that the GSF energy is sensitive to the spin state of thesystem and only spin-polarized calculations can give a reasonable GSF energyfor the [1-11](110) slip system. Hence, it is crucial to include thespin-polarization for calculating the GSF energies in bcc Fe. We demonstratethat the spin-polarized calculations give a narrower dislocation core width,higher unstable stacking fault energy, and larger maximum restoring stress, ascompared with the non-spin-polarized calculations.Using the first-principles real-space DMol method, we have investigatedthe site preferences, energetics, and doping effects of interstitial carbon in thebcc Fe [100](010) edge dislocation core. It is found that: (i) aninhomogeneous electron density distribution occurs in the dislocation coreregion;(ii) an evident splitting of the local densities of states near the Fermienergy can be identified for the Fe atoms in the dislocation core;(iii) thehybridization between C and Fe mainly comes from C-2p and Fe-3d orbitals.Energetic calculations show that C prefers to segregate to the expansionregion and the center in the dislocation core. The strongest trapping effectappears at the dislocation core center, indicating the formation ofC-dislocation complex.The interactions of light impurities B, C, N, O, P, and S with [100](010)edge dislocation in B2-ordered FeAl as well as their doping effects are studiedin detail. The impurity-dislocation interaction is attributed to two parts: themechanical effect which is correlated with atomic size misfit, and thechemical effect which is related to the electronic bonding. The underlyingfactors governing the impurity-dislocation interactions are analyzed.The segregation of boron to [100](010) edge dislocation core inB2-ordered FeAl is studied through the first-principles total energycalculations. Several possible microscopic mechanisms are initially proposedaccording to the experimental observations. We demonstrate that the observedboron Cottrell atmosphere accompanied with large Al depletion in thecompressed region of dislocation core can not be ascribed to the singlesubstitution of boron for Al or simple interstitial states. Instead, ourcalculation results suggest that: (i) boron can segregate to the dilated region asinterstitials, which is consistent with the classical Cottrell theory;(ii) theboron interstitials possibly cluster together via the bridges of B substitution(for Al) both along the dislocation line and in the compressed regions, and (iii)boron and vacancy tend to form boron-vacancy complex and segregate to thedislocation line.