The non-Arrhenius migration of interstitial defects in bcc transition metals (original) (raw)
Related papers
First-principles theory of the energetics of He defects in bcc transition metals
Physical Review B, 2008
Helium defect properties in V, Nb, Ta, Mo, and W were studied using first-principles electronic structure calculations. The most stable position for the He in all bcc metals is a substitutional site; the tetrahedral interstitial position is more favorable than the octahedral position. The formation energy of He substitutional defect is nearly the same for all the metals, while the formation energy of He interstitial defect strongly depends on the electronic structure of the host and insignificantly on its atomic size. The obtained He formation energies were used to calculate He binding energy to the vacancy. For V, Nb, and Ta He-vacancy binding energy is about one-half of the vacancy formation energy; for Mo and W it is about 40% higher than the vacancy formation energy. Both pair potentials and effective-medium theory fall to reproduce the preference order or the relationship between the formation energies. Calculated He formation energies and He-vacancy binding energies improve understanding of He behavior and diffusion mechanisms in metals.
Atomic Defects in Intermetallic Compounds and Diffusion Processes
MRS Proceedings, 1998
For the understanding of the properties of the technically important intermetallic compounds as transition metal aluminides and silicides the understanding of atomic defect properties is pivotal. From recent positron lifetime studies high or low values for the effective vacancy formation enthalpy H' were found for close-packed or for more open-structured bcc type compounds, respectively, which can be well understood theoretically. In B2-FeAl the vacancy migration enthalpy H' could be additionally derived at high temperatures from vacancy equilibration processes. It is shown here by a study on B2-FeAl in comparison to the positron annihilation experiments that the thermal formation and migration of defects can be specifically investigated by time-dependent length-change experiments at high temperatures by the defect equilibration behavior after temperature changes. With the present data on vacancy formation and migration the wide variation of the transition-metal self-diffusivities in intermetallic compounds can be understood.
Effect of impurities on vacancy migration energy in Fe-based alloys
Journal of Nuclear Materials, 2014
Effects of impurities, such as carbon, nitrogen, helium and hydrogen, on microstructural evolution in pure iron were investigated by means of a multi-beam electron microscope. Growth rate of dislocation loops were measured to calculate vacancy migration energies. In all irradiation temperature conditions, both the size and the number density of dislocation loops were increased as a function of dose. Irradiation with more impurities showed an increase in the temperature dependence of the dislocation loop growth rate compared to irradiation with little impurities. The in situ experiment indicated that the net migration energy of vacancies could be increased due to trapping by impurities, and the effect of C and N on the migration energy of vacancy would be larger than that of W, V, Ta. Furthermore, H and He would increase vacancy migration energy greater than C and N, as well as W, V, Ta. The density functional theory (DFT), applied to the atomic models of BCC iron, indicated an increase in vacancy migration energy by the trapping of impurity atoms, that is a good agreement with this in situ experiment.
Self-interstitial atom defects in bcc transition metals: Group-specific trends
Physical Review B, 2006
We present an investigation of systematic trends for the self-interstitial atom ͑SIA͒ defect behavior in body-centered cubic ͑bcc͒ transition metals using density-functional calculations. In all the nonmagnetic bcc metals the most stable SIA defect configuration has the ͗111͘ symmetry. Metals in group 5B of the periodic table ͑V, Nb, Ta͒ have significantly different energies of formation of the ͗111͘ and ͗110͘ SIA configurations, while for the group 6B metals ͑Cr, Mo, W͒ the two configurations are linked by a soft bending mode. The relative energies of SIA defects in the nonmagnetic bcc metals are fundamentally different from those in ferromagnetic bcc ␣-Fe. The systematic trend exhibited by the SIA defect structures in groups 5B and 6B transition metals correlates with the observed thermally activated mobility of SIA defects.
Multiscale modeling of crowdion and vacancy defects in body-centered-cubic transition metals
Physical Review B, 2007
We investigate the structure and mobility of single self-interstitial atom and vacancy defects in bodycentered-cubic transition metals forming groups 5B ͑vanadium, niobium, and tantalum͒ and 6B ͑chromium, molybdenum, and tungsten͒ of the Periodic Density-functional calculations show that in all these metals the axially symmetric ͗111͘ self-interstitial atom configuration has the lowest formation energy. In chromium, the difference between the energies of the ͗111͘ and the ͗110͘ self-interstitial configurations is very small, making the two structures almost degenerate. Local densities of states for the atoms forming the core of crowdion configurations exhibit systematic widening of the "local" d band and an upward shift of the antibonding peak. Using the information provided by electronic structure calculations, we derive a family of Finnis-Sinclair-type interatomic potentials for vanadium, niobium, tantalum, molybdenum, and tungsten. Using these potentials, we investigate the thermally activated migration of self-interstitial atom defects in tungsten. We rationalize the results of simulations using analytical solutions of the multistring Frenkel-Kontorova model describing nonlinear elastic interactions between a defect and phonon excitations. We find that the discreteness of the crystal lattice plays a dominant part in the picture of mobility of defects. We are also able to explain the origin of the non-Arrhenius diffusion of crowdions and to show that at elevated temperatures the diffusion coefficient varies linearly as a function of absolute temperature.
Atomic-Migration-Controlled Processes in Intermetallics
Defect and Diffusion Forum, 2008
Chemical ordering kinetics in L1 0 -and B2-ordered AB binary intermetallics was simulated by means of Monte Carlo (MC) technique implemented with vacancy mechanism of atomic migration. While vacancy concentration is usually much lower than the antisite defect concentration in L1 0 -ordered systems, triple defects are generated in particular B2-ordered systems. The latter definitely affects the chemical ordering process and requires that full thermal vacancy thermodynamics is involved in B2-ordering simulations. The study on L1 0 -ordered binaries was dedicated to FePt thin layers considered as a material for ultra-high-density magnetic storage media. Metastability of the L1 0 c-variant with monoatomic planes parallel to the layer surface and off-plane easy magnetization was revealed. Thermal vacancy formation in B2-ordered binaries was modelled by implementing a mean-field Hamiltonian with a specific formalism of phase equilibria in a latticegas composed of atoms and vacancies. It was demonstrated that for particular pair-interaction energetics, equilibrium concentrations of vacancies and antisites result mutually proportional in well-defined temperature ranges. The MC simulations of B2-ordering kinetics involved the modelled equilibrium vacancy concentration and reproduced the experimentally observed low rate of the process.
Evolution of dislocation patterns in fcc metals
IOP Conference Series: Materials Science and Engineering, 2009
The interaction between dislocation climb and glide is a central issue in understanding creep. While there are atomistic methods to treat glide, the study of climb is still a challenge. The long time scale nature of dislocation climb mandates that modeling such phenomenon requires coarse graining the atomistic details of one or more vacancies binding to dislocation core. We use an empirical interatomic potential for BCC Fe 1 to obtain atomistic details of vacancy diffusion near a (110)<111> dislocation core, the intention being to extract mechanisms that can be fed into kinetic Monte Carlo simulations of climb. We will present results on binding and migration activation of the single vacancy as a function of distance from the core, along with evidence showing the adparticle nature of binding of additional vacancies. We then build upon this understanding to discuss results on the migration energetics of the additional vacancies relative to that of the single vacancy migration. From these results we obtain a general set of selfconsistent observations on vacancy-core binding, and discuss their applicability in a kinetic Monte Carlo simulation framework. We gratefully acknowledge financial support from SKF Global, Inc. and from the US National Defense Science and Engineering Graduate program (T. T. L.).
Journal of Nuclear Materials, 2012
The results of DFT calculations on radiation point defects in tungsten are presented. The lowest energy configuration of the self-interstitial has exactly the h1 1 1i orientation and no tilt from this direction is observed when using appropriate cell geometry and pseudopotential. The present DFT calculations confirm that in pure tungsten the interactions between two vacancies are unexpectedly repulsive until the fifth nearest-neighbor and that the second nearest-neighbor di-vacancy is the most repulsive. The electronic entropy contribution to the free energy makes the nearest-neighbor configuration attractive at high temperature. A comparison with other bcc metals shows that the binding energies between two vacancies are strongly metal dependent and that tungsten leads to the largest deviation from empirical potential predictions. In tungsten, the effect on vacancy properties of alloying by tantalum and rhenium has been investigated using the Virtual Crystal Approximation (VCA). The effect of these alloying elements is essentially to change the filling of the d-band and the vacancy formation energy is found to be maximal and the relaxation to be minimal when the Fermi level is at the minimum of the pseudogap, as predicted by previous tight-binding calculations. Di-vacancies are shown to become attractive at first and second nearest-neighbor upon tantalum alloying and even more repulsive upon rhenium alloying.
High-temperature atomic defect properties and diffusion processes in intermetallic compounds
Intermetallics, 1999
Data on thermal vacancy formation in intermetallic compounds obtained from positron lifetime spectroscopy yield high eective formation enthalpies H F V in close-packed structures and low values in bcc-type structures which can be well understood theoretically. The vacancy migration enthalpy H M V could be determined at high temperatures for B2-FeAl by studying the equilibration process after temperature changes. As demonstrated here in a comparative study on B2-FeAl the thermal formation and migration of defects can also be sensitively investigated by time-dierential length-change studies after temperature changes in the vicinity of the equilibration temperatures. The present vacancy data can explain the wide variation of the transition metal self-diusivities in intermetallic compounds. For B2-FeAl it is shown that the high-temperature mechanical properties are closely linked to the formation of thermal defects as evidenced by the temperature variation of the yield stress anomaly and its time dependence after fast heating.
Systematic group-specific trends for point defects in bcc transition metals: An ab initio study
Journal of Nuclear Materials, 2007
Density functional theory calculations have been performed to study the systematic trends of point defect behaviours in bcc transition metals. We found that in all non-magnetic bcc transition metals, the most stable self-interstitial atom (SIAs) defect configuration has the h1 1 1i symmetry. The calculated formation energy differences between the h1 1 0i dumbbell and the lowest-energy h1 1 1i configuration of metals in group 5B (V, Nb, Ta) are consistently larger than those of the corresponding element in group 6B (Cr, Mo, W). The predicted trends of SIA defects are fundamentally different from those in ferromagnetic a-Fe and correlate very well with the pronounced group-specific variation of thermally activated migration of SIAs under irradiation depending on the position of bcc metals in the periodic table.