88 research outputs found
From quantum mechanics to the physical metallurgy of steels
In the last decade there has been a breakthrough in the construction of
theories leading to models for the simulation of atomic scale processes in
steel. In this paper the theory is described and developed and used to
demonstrate calculations of the diffusivity and trapping of hydrogen in iron
and the structures of carbon vacancy complexes in steel.Comment: Paper presented at "Adventures in the Physical Metallurgy of Steels,"
Cambridge, August 2013. See http://www.msm.cam.ac.uk/apms where the slides
and video of the talk can also be seen. Paper is submitted to a special issue
of Materials Science and Technolog
A Stabilization Mechanism of Zirconia Based on Oxygen Vacancies Only
The microscopic mechanism leading to stabilization of cubic and tetragonal
forms of zirconia (ZrO) is analyzed by means of a self-consistent
tight-binding model. Using this model, energies and structures of zirconia
containing different vacancy concentrations are calculated, equivalent in
concentration to the charge compensating vacancies associated with dissolved
yttria (YO) in the tetragonal and cubic phase fields (3.2 and 14.4% mol
respectively). The model is shown to predict the large relaxations around an
oxygen vacancy, and the clustering of vacancies along the directions,
in good agreement with experiments and first principles calculations. The
vacancies alone are shown to explain the stabilization of cubic zirconia, and
the mechanism is analyzed.Comment: 19 pages, 6 figures. To be published in J. Am. Ceram. So
Local Volume Effects in the Generalized Pseudopotential Theory
The generalized pseudopotential theory (GPT) is a powerful method for
deriving real-space transferable interatomic potentials. Using a coarse-grained
electronic structure, one can explicitly calculate the pair ion-ion and
multi-ion interactions in simple and transition metals. Whilst successful in
determining bulk properties, in central force metals the GPT fails to describe
crystal defects for which there is a significant local volume change. A
previous paper [PhysRevLett.66.3036 (1991)] found that by allowing the GPT
total energy to depend upon some spatially-averaged local electron density, the
energetics of vacancies and surfaces could be calculated within experimental
ranges. In this paper, we develop the formalism further by explicitly
calculating the forces and stress tensor associated with this total energy. We
call this scheme the adaptive GPT (aGPT) and it is capable of both molecular
dynamics and molecular statics. We apply the aGPT to vacancy formation and
divacancy binding in hcp Mg and also calculate the local electron density
corrections to the bulk elastic constants and phonon dispersion for which there
is refinement over the baseline GPT treatment.Comment: 11 pages, 6 figure
Hydrogen Diffusion and Trapping in {\alpha}-Iron: The Role of Quantum and Anharmonic Fluctuations
We investigate the thermodynamics and kinetics of a hydrogen interstitial in
magnetic {\alpha}-iron, taking account of the quantum fluctuations of the
proton as well as the anharmonicities of lattice vibrations and hydrogen
hopping. We show that the diffusivity of hydrogen in the lattice of BCC iron
deviates strongly from an Arrhenius behavior at and below room temperature. We
compare a quantum transition state theory to explicit ring polymer molecular
dynamics in the calculation of diffusivity and we find that the role of phonons
is to inhibit, not to enhance, diffusivity at intermediate temperatures in
constrast to the usual polaron picture of hopping. We then address the trapping
of hydrogen by a vacancy as a prototype lattice defect. By a sequence of steps
in a thought experiment, each involving a thermodynamic integration, we are
able to separate out the binding free energy of a proton to a defect into
harmonic and anharmonic, and classical and quantum contributions. We find that
about 30% of a typical binding free energy of hydrogen to a lattice defect in
iron is accounted for by finite temperature effects and about half of these
arise from quantum proton fluctuations. This has huge implications for the
comparison between thermal desorption and permeation experiments and standard
electronic structure theory. The implications are even greater for the
interpretation of muon spin resonance experiments
The influence of hydrogen on plasticity in pure iron-theory and experiment
Tensile stress relaxation is combined with transmission electron microscopy
to reveal dramatic changes in dislocation structure and sub structure in pure
alpha iron as a result of the effects of dissolved hydrogen. We find that
hydrogen charged specimens after plastic deformation display a very
characteristic pattern of trailing dipoles and prismatic loops which are absent
in uncharged pure metal. We explain these observations by use of a new self
consistent kinetic Monte Carlo model, which in fact was initially used to
predict the now observed microstructure. The results of this combined theory
and experimental study is to shed light on the fundamental mechanism of
hydrogen enhanced localised plasticity
Magnetic tight-binding and the iron-chromium enthalpy anomaly
We describe a self consistent magnetic tight-binding theory based in an
expansion of the Hohenberg-Kohn density functional to second order, about a non
spin polarised reference density. We show how a first order expansion about a
density having a trial input magnetic moment leads to the Stoner--Slater rigid
band model. We employ a simple set of tight-binding parameters that accurately
describes electronic structure and energetics, and show these to be
transferable between first row transition metals and their alloys. We make a
number of calculations of the electronic structure of dilute Cr impurities in
Fe which we compare with results using the local spin density approximation.
The rigid band model provides a powerful means for interpreting complex
magnetic configurations in alloys; using this approach we are able to advance a
simple and readily understood explanation for the observed anomaly in the
enthalpy of mixing.Comment: Submitted to Phys Rev
Free energy and molecular dynamics calculations for the cubic-tetragonal phase transition in zirconia
The high-temperature cubic-tetragonal phase transition of pure stoichiometric
zirconia is studied by molecular dynamics (MD) simulations and within the
framework of the Landau theory of phase transformations. The interatomic forces
are calculated using an empirical, self-consistent, orthogonal tight-binding
(SC-TB) model, which includes atomic polarizabilities up to the quadrupolar
level. A first set of standard MD calculations shows that, on increasing
temperature, one particular vibrational frequency softens. The temperature
evolution of the free energy surfaces around the phase transition is then
studied with a second set of calculations. These combine the thermodynamic
integration technique with constrained MD simulations. The results seem to
support the thesis of a second-order phase transition but with unusual, very
anharmonic behaviour above the transition temperature
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