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First principles study of helium, carbon and nitrogen in austenite, dilute austenitic iron alloys and nickel  [PDF]
D. J. Hepburn,D. Ferguson,S. Gardner,G. J. Ackland
Physics , 2013,
Abstract: An extensive set of first-principles density functional theory calculations have been performed to study the behaviour of He, C and N solutes in austenite, dilute Fe-Cr-Ni austenitic alloys and Ni in order to investigate their influence on the microstructural evolution of austenitic steel alloys under irradiation.
Numerical simulation of solute trapping phenomena using phase-field solidification model for dilute binary alloys
Furtado, Henrique Silva;Bernardes, Américo Trist?o;Machado, Romuel Figueiredo;Silva, Carlos Ant?nio;
Materials Research , 2009, DOI: 10.1590/S1516-14392009000300016
Abstract: numerical simulation of solute trapping during solidification, using two phase-field model for dilute binary alloys developed by kim et al. [phys. rev. e, 60, 7186 (1999)] and ramirez et al. [phys. rev. e, 69, 05167 (2004)] is presented here. the simulations on dilute cu-ni alloy are in good agreement with one dimensional analytic solution of sharp interface model. simulation conducted under small solidification velocity using solid-liquid interface thickness (2λ) of 8 nanometers reproduced the solute (cu) equilibrium partition coefficient. the spurious numerical solute trapping in solid phase, due to the interface thickness was negligible. a parameter used in analytical solute trapping model was determined by isothermal phase-field simulation of ni-cu alloy. its application to si-as and si-bi alloys reproduced results that agree reasonably well with experimental data. a comparison between the three models of solute trapping (aziz, sobolev and galenko [phys. rev. e, 76, 031606 (2007)]) was performed. it resulted in large differences in predicting the solidification velocity for partition-less solidification, indicating the necessity for new and more acute experimental data.
Transition metal solute interactions with point defects in austenitic iron from first principles  [PDF]
D. J. Hepburn,E. MacLeod,G. J. Ackland
Physics , 2014,
Abstract: We present a comprehensive set of first principles electronic structure calculations to study transition metal solutes and their interactions with point defects in austenite. Clear trends were observed across the series. Solute-defect interactions were strongly correlated to the solute size factors, consistent with local strain field effects. Strong correlations with results in ferrite show insensitivity to the underlying crystal structure in Fe. Oversized solutes act as strong traps for vacancy and self-interstitial defects and as nucleation sites for the development of proto-voids and small self-interstitial loops. The reduction in defect mobility and net defect concentrations explains the observed radiation-damage resistance in austenitic steels doped with oversized solutes. Oversized solutes remaining dissolved in oxide dispersion-strengthened (ODS) steels could contribute to their radiation-damage resistance. Ni and Co diffuse more slowly than Fe, along with any vacancy flux produced under irradiation below a critical temperature, which is 400 K for Co and their concentrations should be enhanced at defect sinks. Cr and Cu diffuse more quickly than Fe, against a vacancy flux and will be depleted at defect sinks. Oversized solutes early in the transition metal series form highly-stable solute-centred divacancy (SCD) defects with a nearest-neighbour vacancy. The vacancy-mediated diffusion of these solutes is dominated by the dissociation and reassociation of the SCDs, with a lower activation energy than for self-diffusion, which has important implications for the nucleation and growth of complex oxide nanoparticles containing these solutes in ODS steels. Interstitial-mediated solute diffusion is energetically disfavoured for all except Cr, Mn, Co and Ni. The central role that solute size plays in the results presented here means they should apply to other solvent metals and alloys.
Thermodynamic model of solute site preferences in ordered alloys  [PDF]
Gary S. Collins,Matthew O. Zacate
Physics , 2015,
Abstract: A thermodynamic model based on the law of mass action is used to calculate concentrations of elementary point defects and to determine site preferences of solute atoms in ordered alloys. Combinations of lattice vacancies, antisite atoms and host interstitials that form equilibrium defects are enumerated for the CsCl (B2) and Ni2Al3 structures. For CsCl, in addition to the two substitutional sites, a distorted tetrahedral interstitial site is considered. For Ni2Al3, the Ni site, two distinct Al sites and a vacant, insterstitial-type Ni-site are considered. An equation of constraint among concentrations of elementary defects is derived that is valid for any crystal structure. The concentration of a selected defect can be solved using the equation of constraint in conjunction with mass-action equations for defect combinations. The method leads directly to defect concentrations without the need to evaluate composition-dependent chemical potentials, resulting in a more transparent formalism. The model is used to explore the phenomenology of site-preferences of dilute ternary solute atoms. Findings are in agreement with previous treatments restricted to substitutional sites. General rules for how site-preferences depend on temperature and on composition in non-stoichiometric compounds are obtained through algebraic analysis and numerical simulations: (1) Solute S tends to occupy substitutional sites of the element in which there is a deficiency. (2) If the difference of energies of S on sites A and B is very positive or negative, then S will occupy site B or A exclusively, independent of composition. If the difference of site energies is intermediate, the solute will switch from one site to the other as the composition changes. (3) Solutes have a tendency to occupy interstitial or empty-lattice sites with a maximum site-fraction near the stoichiometric composition.
Master equation approach to the theory of diffusion in alloys and calculations of enhancement factors for tracer solvent and tracer solute diffusion in FCC alloys  [PDF]
V. G. Vaks,A. Yu. Stroev,I. R. Pankratov,I. A. Zhuravlev
Physics , 2013,
Abstract: The earlier-suggested master equation approach is used to develop the consistent statistical theory of diffusion in alloys using the five-frequency model of FCC alloys as an example. Expressions for the Onsager coefficients in terms of microscopic interatomic interactions and some statistical averages are presented. We discuss methods of calculations of these averages using both the mean-field and the pair-cluster approximation to describe influence of vacancy-solute and solute-solute interactions, and both the nearest-neighbor and the second-shell approximation to describe vacancy correlation effects. The methods developed are used for calculations of enhancement factors which determine the concentration dependence of tracer self-diffusion and tracer solute diffusion in dilute FCC alloys. For the tracer self-diffusion, we show that some significant contribution to the enhancement factor related to the thermodynamic activity of vacancies was missed in the previous treatments of this problem. It implies that the most of existing estimates of parameters of the five-frequency model for real alloys should be revised. For the tracer solute diffusion, the expression for the enhancement factor seems to be presented for the first time. The results obtained are used to estimate the microscopic parameters important for diffusion, including the vacancy-solute interaction, in several FCC alloys for which necessary experimental data are available.
Effects of Composition on the Mechanical Properties and Microstructural Development of Dilute 6000 Series Alloys  [PDF]
Aiza Jaafar,Azmi Rahmat,Ismail Zainol,Zuhailawati Hussain
Journal of Applied Sciences , 2012,
Abstract: The aim of the present investigation is to study the effect of composition on the mechanical properties and microstructural development during thermal treatment of some dilute 6000 aluminium alloys. The alloys were based on 6000 series dilute aluminium alloys containing different levels of magnesium (Mg) and silicon (Si) contents. The effects of artificial ageing (T6) on the precipitation hardening behaviour were studied in the four dilute alloys. The solution treated alloys samples were quenched at 530°C for 5 min and then immediately subjected to ageing at 185 and 300°C. The ageing response and mechanical properties of the alloys were investigated by hardness and tensile test, respectively. The microstructures of the alloys were monitored using Transmission Electron Microscopy (TEM). The results from this study showed a correlation between the Mg and Si contents on the mechanical properties and microstructures of the alloys after ageing treatment. It is apparent that an increasing solute content of Mg2Si and ExSi has marked effect in increasing hardness and tensile properties of the alloys. It was found that Mg2Si is more effective than ExSi in hardening and strengthen the alloys. The precipitates formed during artificial ageing were needle-shaped with their major axes parallel to (100) directions of the matrix. The number density of precipitates increased as their solute contents of Mg2Si and ExSi increased. Overall results of TEM observation are in agreement with the results of hardness and tensile test.

金属学报(英文版) , 2003,
Abstract: Irradiation-induced grain boundary segregation of solute atoms frequently bring about grain boundary precipitation of a second phase because of its making the solubility limit of the solute surpassed at grain boundaries. Until now the kinetic models for irradiation-induced grain boundary precipitation have been sparse. For this reason, we have theoretically treated grain boundary precipitation under irradiation in dilute binary alloys. Predictions ofγ'-Ni3Si precipitation at grain boundaries ave made for a dilute Ni-Si alloy subjected to irradiation. It is demonstrated that grain boundary silicon segregation under irradiation may lead to grain boundaryγ'-Ni3 Si precipitation over a certain temperature range.
Impurity correlations in dilute Kondo alloys  [PDF]
Victor Barzykin,Ian Affleck
Physics , 1999, DOI: 10.1103/PhysRevB.61.6170
Abstract: The single impurity Kondo model is often used to describe metals with dilute concentrations (n_i) of magnetic impurities. Here we examine how dilute the impurities must be for this to be valid by developing a virial expansion in impurity density. The O(n_i^2) term is determined from results on the 2-impurity Kondo problem by averaging over the RKKY coupling. The non-trivial fixed point of the 2-impurity problem could produce novel singularities in the heat capacity of dilute alloys at O(n_i^2).
Skew scattering in dilute ferromagnetic alloys  [PDF]
Bernd Zimmermann,Kristina Chadova,Diemo K?dderitzsch,Stefan Blügel,Hubert Ebert,Dmitry V. Fedorov,Nguyen H. Long,Phivos Mavropoulos,Ingrid Mertig,Yuriy Mokrousov,Martin Gradhand
Physics , 2014, DOI: 10.1103/PhysRevB.90.220403
Abstract: The challenging problem of skew scattering for Hall effects in dilute ferromagnetic alloys, with intertwined effects of spin-orbit coupling, magnetism and impurity scattering, is studied here from first principles. Our main aim is to identify chemical trends and work out simple rules for large skew scattering in terms of the impurity and host states at the Fermi surface, with particular emphasis on the interplay of the spin and anomalous Hall effects in one and the same system. The predicted trends are benchmarked by referring to three different \emph{ab initio} methods based on different approximations with respect to the electronic structure and transport properties.
Theory of magnetoresistance in films of dilute magnetic alloys  [PDF]
L. Borda,A. Zawadowski
Physics , 1999, DOI: 10.1103/PhysRevB.61.3570
Abstract: Earlier a magnetic anisotropy for magnetic impurities nearby the surface of non-magnetic host was proposed in order to explain the size dependence of the Kondo effect in dilute magnetic alloys. Recently Giordano has measured the magnetoresistance of dilute Au(Fe) films for different thicknesses well above the Kondo temperature $T_K$. In this way he verified the existence of that anisotropy even for such a case where the Kondo effect is not dominating. For detailed comparison of that suggestion with experiments, the magnetic field dependence of the magnetoresistance is calculated in the lowest approximation, thus in the second order of the exchange coupling. The strength of the anisotropy is very close to earlier estimates deduced from the size dependence of the Kondo resistivity amplitude.
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