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Register a new userInvestigating the electronic origins of the repulsion between substitutional and interstitial solutes in hcp Ti
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The high solubility of oxygen in Ti, Zr and Hf makes it difficult to stabilize the protective oxide scales on their surfaces as the subsurface regions can serve as boundless sinks that continuously dissolve oxygen. Alloying elements are crucial to reduce the oxygen solubility and diffusivity within early transition metals. Past studies have shown that substitutional alloying additions to titanium repel interstitial oxygen. Here we use first-principles calculations to show that this repulsion is short ranged and identify a variety of factors that are likely responsible for the repulsive interaction. We identify a unique hybridization phenomenon between dissolved substitutional elements and interstitial oxygen within hcp Ti that leads to a repulsive interaction at short distances, similar to that between closed-shell atoms. Calculations of Bader charges also suggest the existence of short-range Coulomb interactions due to the accumulation of charge on the substitutional solute and interstitial oxygen that is drawn from the Ti host.
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Nucleation and growth of Ti$_3$Al textalpha{}$_2$ ordered domains in textalpha{}-Ti--Al--X alloys were characterised using a combination of transmission electron microscopy, atom probe tomography and small angle X-ray scattering. Model alloys based on Ti--7Al~(wt.%) and containing O, V and Mo were aged at SI{550}{celsius} for times up to SI{120}{day} and the resulting precipitate dispersions were observed at intermediate points. Precipitates grew to around SI{30}{ anometre} in size, with a volume fraction of 6--10% depending on tertiary solutes. Interstitial O was found to increase the equilibrium volume fraction of textalpha{}$_2$, while V and Mo showed relatively little influence. Addition of any of the solutes in this study, but most prominently Mo, was found to increase nucleation density and decrease precipitate size and possibly coarsening rate. Coarsening can be described by the Lifshitz-Slyozov-Wagner model, suggesting a matrix diffusion-controlled coarsening mechanism (rather than control by interfacial coherency). Solutionising temperature was found to affect nucleation number density with an activation energy of $E_{mathrm{f}} = 1.5pm{}0.4$~eV, supporting the hypothesis that vacancy concentration affects textalpha{}$_2$ nucleation. The observation that all solutes increase nucleation number density is also consistent with a vacancy-controlled nucleation mechanism.
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Dallas R. Trinkle (Department of Materials Science
, Engineering
, n University of Illinois
2016
Computational atomic-scale methods continue to provide new information about geometry, energetics, and transition states for interstitial elements in crystalline lattices. This data can be used to determine the diffusivity of interstitials by finding steady-state solutions to the master equation. In addition, atomic-scale computations can provide not just the site energy, but also the stress in the cell due to the introduction of the defect to compute the elastic dipole. We derive a general expression for the fully anistropic diffusivity tensor from site and transition state energies, and three derivatives of the diffusivity: the elastodiffusion tensor (derivative of diffusivity with respect to strain), the activation barrier tensor (logarithmic derivative of diffusivity with respect to inverse temperature) and activation volume tensor (logarithmic derivative of diffusivity with respect to pressure). Computation of these quantities takes advantage of crystalline symmetry, and we provide an open-source implementation of the algorithm. We provide analytic results for octahedral-tetrahedral networks in face-centered cubic, body-centered cubic, and hexagonal closed-packed lattices, and conclude with numerical results for C in Fe.
The electronic structure of interstitial hydrogen in a compound semiconductor FeS$_2$ (naturally $n$-type) is inferred from a muon study. An implanted muon (Mu, a pseudo-hydrogen) forms electronically different defect centers discerned by the hyperfine parameter ($omega_{rm hf}$). A body of evidence indicates that one muon is situated at the center of an iron-cornered tetrahedron with nearly isotropic $omega_{rm hf}$ (Mu$_{rm p}$), and that the other exists as a diamagnetic state (Mu$_{rm d}$, $omega_{rm hf}simeq 0$). Their response to thermal agitation indicates that the Mu$_{rm d}$ center accompanies a shallow level (donor or acceptor) understood by effective mass model while the electronic structure of Mu$_{rm p}$ center is more isolated from host than Mu$_{rm d}$ to form a deeper donor level. These observations suggest that interstitial hydrogen also serves as an electronically active impurity in FeS$_2$. Based on earlier reports on the hydrogen diffusion in FeS$_2$, possibility of fast diffusion for Mu$_{rm p}$ leading to formation of a complex defect state (Mu$^*_{rm d}$, $Tle 100$ K) or to motional narrowing state (Mu$^*_{rm p}$, $Tge 150$ K) is also discussed.
Recent experiments showed that Co undergoes a phase transition from ferromagnetic hcp phase to non-magnetic fcc one around 100 GPa. Since the transition is of first order, a certain region of co-existence of the two phases is present. By means of textit{ab initio} calculations, we found that the hcp phase itself undergoes a series of electronic topological transitions (ETTs), which affects both elastic and magnetic properties of the material. Most importantly, we propose that the sequence of ETTs lead to the stabilisation of a non-collinear spin arrangement in highly compressed hcp Co. Details of this non-collinear magnetic state and the interatomic exchange parameters that are connected to it, are presented here.
We report on a quantitative investigation of the spin-dependent quasiparticle lifetimes and electron correlation effects in ferromagnetic hcp Co(0001) by means of spin and angle-resolved photoemission spectroscopy. The experimental spectra are compared in detail to state-of-the-art many-body calculations within the dynamical mean field theory and the three-body scattering approximation, including a full calculation of the one-step photoemission process. From this comparison we conclude that although strong local many-body Coulomb interactions are of major importance for the qualitative description of correlation effects in Co, more sophisticated many-body calculations are needed in order to improve the quantitative agreement between theory and experiment, in particular concerning the linewidths. The quality of the overall agreement obtained for Co indicates that the effect of non-local correlations becomes weaker with increasing atomic number.
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