Solute diffusion in the γ′ phase of Ni based alloys

We systematically investigate the diffusion mechanisms of 3d (Ti–Cu), 4d (Zr–Ag) and 5d (Hf–Au) transition metal solutes in γ′-Ni₃Al phase using first-principles calculations. The results reveal that the diffusion of Ni-substituting solute is mainly controlled by the sublattice diffusion mechanism via Ni vacancies and the diffusion of Al-substituting solute is mostly governed by the formation of the anti-structure defects on the Ni sublattice with negligible contribution of the anti-structure bridge mechanism. The elements which occupy both the Al and Ni sites show a diffusion behavior similar to that of the Ni-substituting solutes. Our calculations show that larger atoms can move much faster than smaller atoms, which disprove the traditional view that larger atoms move slower than smaller atoms.     

Clustering and solute-vacancy binding energies in Al-4.4% Zn alloy with 0.01% ternary additions

Isochronal and isothermal ageing experiments have been carried out to determine the influence of 0.01 at. % addition of a second solute on the clustering rate in the quenched Al-4,4 a/o Zn alloy. The influence of quenching and ageing temperatures has been interpreted to obtain the apparent vacancy formation and vacancy migration energies in the various ternary alloys. Using a vacancy-aided clustering model the following values of binding free energy have been evaluated: Ce-0.18; Dy-0.24; Fe-0.18; Li-0.25; Mn-0.27; Nb-0.18; Pt-0.23; Sb-0.21; Si-0.30; Y-0.25; and Yb-0.23 (± 0.02 eV). These binding energy values refer to that between a solute atom and a single vacancy. The values of vacancy migration energy (c. 0.4 eV) and the experimental activation energy for solute diffusion (c. 1.1 eV) are unaffected by the presence of the ternary atoms in the Al-Zn alloy.     

Chemically-biased diffusion and segregation impede void growth in irradiated Ni-Fe alloys

Recent irradiations of Ni-Fe concentrated solid solution alloys have demonstrated significant improvement of radiation performance. This improvement is attributed to redistribution of the alloying elements near sinks of point defects (voids, dislocations) due to chemically-biased atomic diffusion, where vacancies have preference to migrate via Fe atoms and interstitials via Ni atoms. In Ni-Fe, all sinks are enriched by Ni atoms, which strongly affects further interactions of radiation-produced mobile defects with voids and dislocations, hence void growth and dislocation climb. Ni-decorated sinks interact stronger with interstitial atoms than vacancies, which enhances dislocation loops growth. At the same time, Ni segregation creates Fe-enriched “channels” for vacancy migration out of the damage region to agglomerate in the outer regions, inaccessible to interstitial atoms. Strong effect of chemically-biased diffusion is supported by transmission electron microscope characterization and calls for special attention in designing alloys with desired properties through tuning defect mobilities.     

Interactions between transition metals and defective carbon nanotubes

Interactions between 3d transition-metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) and (5,5) carbon nanotube (CNT) with a vacancy defect are quantitatively characterized using first-principles calculations. The binding energies between CNT and transition metals are found to be significantly enhanced when vacancy defects are introduced into the CNT. For the defective CNTs doped with Sc, Cr and Zn atoms, the structures of defective CNTs are found to be intact. The doping of Ti, Mn, Cu, Fe, Ni and Co alternates the structures of defective CNTs. Among all 3d transition metals, only the ferromagnetic metal atoms Fe, Co and Ni form bonds with carbon atoms of CNT, suggesting the important role of magnetic exchange interaction in the p–d hybridisation between carbons and transition-metal atoms. The results also indicate that the 3d transition-metal atoms acting as substitutional defects can substantially modify the electronic structure of CNT. It is suggested that these stable CNT-metal systems could become promising engineering materials in many fields such as CNT devices for various spintronics applications and CNT metal–matrix composites.     

Nucleation and growth mechanisms of phase with single-unit-cell height in Mg-RE-Zn(Ag) series alloys:a first-principles study

The highly thermal stability precipitates strengthen creep resistance for alloys,which require precipitates to resist coarsen at elevated temperature.In the Mg-RE-Zn (Ag) series alloys,the key strengthening phase-γ'phase,remains its single-unit-cell height throughout aging process.The origin of such extremely thermal stability of γ'phase is still unclear.By using the first-principles calculations,it is found that the formation ofγ'phase introduces compressive strain to its surrounding α-Mg lattice and simultaneously alter charge distribution of α-Mg lattice near the α-Mg/γ'hetero-interface.These two variations over α-Mg lattice lead to lower vacancy formation energies and migration energy barriers for solute atoms near the α-Mg/γ'hetero-interface in comparison with that of bulk condition.Consequently,the variations facilitate rapid diffusion of solute atoms near the γ'phase and promote nucleation rate of other γ'plates around it.On the basis of ledge-thickening model,the origin of nucleation/growth of γ'phase with single-unit-cell height was explained.Thermodynamically and kinetically,the solute atoms are not apt to migrate into the nearest basal plane adjacent to the γ'phase.Moreover,the lower aging temperature(～200 ℃) and almost completely coherent α-Mg/γ'hetero-interface lead to the ledges are extremely hard to nucleate on the hetero-interface.Therefore,γ'plates maintain single-unit-cell height throughout aging process.     

Formation and reversion of G-P zones in Al-1.3 at. % Ag alloy

The kinetics of clustering and reversion of G-P zones in an Al-1.3 at. % Ag alloy have been investigated by resistivity measurements, with special attention to the initial parts of reversion. From an analysis of clustering kinetics, vacancy formation, solute migration and solute-vacancy binding energies have been deduced to be 0.70, 0.53 and 0.10 eV respectively.The activation energy for reversion, which is identified as the activation energy for solute atom diffusion, is found to vary from 0.98 to 1.46 eV with increasing reversion time. This is attributed to variations in the vacancy concentration in equilibrium with small dislocation loops.     

Impact of Electronic Structure Calculations on the Study of Diffusion in Metals

Developments in the methods of electronic structure calculations combined with the impressive advances in computer performances have made it possible to perform ab‐initio calculations of a variety of structural properties of materials efficiently and at a predictable level, as illustrated here in the case of defect parameters in transition metals. The values of the vacancy formation and migration enthalpies calculated in the framework of the density‐functional theory in the local density approximation (DFT‐LDA) agree with experimental data within typically less than 10 %. Reliable self‐diffusion coefficients for the vacancy mechanism can be deduced. This approach sheds new light on the so‐called self‐diffusion anomaly in body centered cubic (bcc) metals. It outlines in particular the role of structural relaxations around the vacancy and it reveals that electronic excitations may have a contribution to the formation and migration entropies comparable to the vibrational contribution.     

Solute–point defect interactions in bcc systems: Focus on first principles modelling in W and RPV steels

The microstructure of materials and its evolution are influenced by the interaction of point defects with the solute atoms, already present or introduced by irradiation. Electronic structure calculations are nowadays intensively used to characterise these interactions. Prediction of the consequences of these interactions on the microstructure evolution, for instance on the diffusion properties, can be obtained from raw first principles data or more often, by introducing these data in higher scale models. This paper reviews the current knowledge, gained from recent intensive sets of first principles calculations, of the interactions between point defects (vacancies and self interstitial atoms) in Fe and W matrix, and solute elements, both substitutional (mostly 3d, 4d and 5d transition metals) and interstitial (C, N, H and He).     

硅铝分子筛骨架脱铝模型

通过把硅铝分子筛的骨架脱铝过程简化为分子筛骨架中杂原子的扩散和空位的形成及迁移过程,使用费克第二定律建立扩散方程,得到任意时刻铝原子的浓度关于位置的函数,并讨论了影响脱铝的几个重要因素.用XPS、红外光谱和27Al-NMR测定在一定条件下处理后的氢型丝光沸石和ZSM-5的骨架硅铝比的变化,并与由所建立的数学模型计算出的骨架硅铝比比较,验证了公式的准确性.     

The Sluggish Diffusion of Cations in CeO2 Probed through Molecular Dynamics and Metadynamics Simulations

Cation diffusion in fluorite-structured CeO₂, though far slower than anion diffusion, is an important, high-temperature process because it governs diverse fabrication and degradation phenomena. Herein, cation diffusion is studied by means of classical molecular dynamics and metadynamics simulations. Three different mechanisms are examined: migration involving an isolated cerium vacancy, migration involving a cerium vacancy in a defect associate with an oxygen vacancy, and migration involving a cation divacancy. For each mechanism, defect diffusion coefficients are calculated as a function of temperature, from which the respective activation enthalpy of defect migration is obtained. Through comparisons with experimental cation diffusion data (specifically, of the absolute magnitude of the cation diffusivity as well as its activation enthalpy), it is concluded that cation diffusion takes place predominantly neither by isolated vacancies nor by cation vacancy–oxygen vacancy associates but by cation divacancies.     

Computer modeling of oxygen migration in HgBa2CuO4+δ

A molecular statics method is used to calculate the energy barriers for the migration of oxygen ions. Vacancy and interstitial diffusion mechanisms are considered. The lowest migration barrier is obtained for the vacancy mechanism. Pis’ma Zh. Tekh. Fiz. 25, 1–4 (August 12, 1999)     

Impact of electronic structure calculations on study of diffusion in metals

Abstract

Developments in the methods of electronic structure calculations combined with the impressive advances in computer performance have made it possible to perform ab initio calculations of a variety of structural properties of materials efficiently and at a predictable level, as illustrated here in the case of defect parameters in transition metals. The values of the vacancy formation and migration enthalpies calculated in the framework of the density functional theory in the local density approximation agree with experimental data to typically within 10%. Reliable self-diffusion coefficients for the vacancy mechanism can be deduced. This approach sheds new light on the so called self-diffusion anomaly in bcc metals. It outlines in particular the role of structural relaxations around the vacancy and it reveals that electronic excitations may make a contribution to the formation and migration entropies comparable with the vibrational contribution. The link between the electronic structure and vacancy properties in bcc transition metals is analysed using a simple tight binding d band model: the position of the Fermi level with respect to the characteristic dip in the density of states is shown to govern the sharp variations along a transition metal series of the vacancy parameters.     

Study of the diffusion of Al–Li alloys subjected to an electric field

An experimental investigation of the homogenization treatment of 2091 Al–Li alloy in the presence of an electric field, has revealed the phenomena of reduced volume fraction, small size, spherical shape and random distribution of second-phase particles, which bring about an increase in ductility. The results show that the dissolution of second-phase particles is promoted by means of the vacancy mechanism, because of the larger diffusion coefficient of solute atoms than the vacancy–solute complexes at the beginning of the homogenization treatment. By increasing the homogenization time and applying an electric field, the diffusion coefficient of vacancy-solute complexes is raised, whereas that of solute atoms is reduced, because of the decreased potential energy of the second phase at grain boundaries. Therefore, the non-equilibrium segregation of magnesium and copper elements is generated near the surface of the ingot by a complex mechanism. An experimental study of the solution treatment under an electric field, revealed that the lithium non-equilibrium segregation is induced at grain boundaries responsible for the precipitates. © 1998 Chapman & Hall     

Magnetism and solid solution effects in NiAl (40% Al) alloys

The solid solution effects of ternary additions of transition elements in intermetallic Ni–40% Al were investigated by both experimental studies and theoretical calculations. Co solute atoms when sitting at Ni sublattice sites do not affect the lattice parameter and hardening behavior of Ni–40Al. On the other hand, Fe, Mn, and Cr solutes, which are mainly on Al sublattice sites, substantially expand the lattice parameter and produce an unusual solid solution softening effect. First-principles calculations predict that these solute atoms with large unfilled d-band electrons develop large magnetic moments and effectively expand the lattice parameter when occupying Al sublattice sites. The theoretical predictions were verified by both electron loss-energy spectroscopy (EELS) analyses and magnetic susceptibility measurements. The observed softening behavior can be explained quantitatively by the replacement of Ni anti-site defects (potent hardeners) by Fe, Mn, and Cr anti-site defects with smaller atom size mismatch between solute and Al atoms. This study has led to the identification of magnetic interaction as an important physical parameter affecting the solid solution hardening in intermetallic alloys containing transition elements.     

Cu diffusion in α-Fe: determination of solute diffusivities using atomic-scale simulations

The nucleation and growth of radiation-induced Cu-rich clusters have proved an important topic in the study of microstructural and chemical evolution responsible for property changes in pressure vessel steels subjected to neutron irradiation. Small Cu clusters can act as precipitate precursors and contribute to the hardening of the material by impeding dislocation glide. Reliable Cu diffusion coefficients provide critical input to Monte Carlo and rate theory approaches to study the kinetics of diffusion and clustering that lead to the formation of such Cu-rich features. In this paper we report the results of a molecular dynamics study of Cu solute-atom diffusion in a Fe–0.9at%Cu alloy, using high-temperatures and high-vacancy concentrations to hasten the convergence of the calculation. We find a value of 0.61 eV for the migration energy of Cu in the b.c.c Fe matrix, in good agreement with available experimental results. The main diffusion mechanisms under the conditions simulated are identified and discussed, and the effect of vacancy and solute clustering on Cu migration is assessed. In addition, a rigorous computation of the solute enhancement factor, correlation factor and solute diffusivity is performed within the five-frequency theoretical framework and the harmonic approximation. The results obtained are compared with those yielded by the direct MD simulations and the observed differences discussed.     

Divalent cation diffusion in calcium fluorapatite

The mechanisms by which calcium ions are transported through the fluorapatite lattice are investigated using computer simulations. Cation vacancy assisted pathways are considered and migration energy barriers are calculated using a nudged elastic band transition state search. Migration activation energies for a range of divalent cations, in addition to calcium are compared. Generally there is very little change in activation energy as cation radius increases. Furthermore, in all cases the lowest energy migration pathways occur parallel to the c-axis either associated with the central anion channel via Ca(2) sites or away from the channel and via Ca(1) sites.     

Formation of solute atmospheres around dislocations by excess vacancies

Long range solute atmospheres tend to form around dislocations by diffusion. They scatter phonons and can be studied through their effect on the lattice thermal conductivity. Previous studies indicate that these atmospheres are formed after plastic deformation in Cu-Al but not in Cu-Ge alloys. Ordinary diffusion at room temperature is too slow to permit atmosphere formation, but excess vacancies originally produced during plastic deformation can enhance diffusion. The vacancies diffuse towards the dislocation, where they are annihilated, and their presence causes solute diffusion and hence atmosphere formation. The atmospheres attain equilibrium over a range R, and R² can be expressed in terms of a time-integrated diffusion coefficient due to excess vacancies until the excess is exhausted. This range R is independent of the vacancy jump rate, and depends only on the initial vacancy concentration and the dislocation density. The values of R thus calculated are much smaller than those observed. However, R is increased if there is enhancement of the solute diffusion relative to the solvent diffusion and also if dislocations do not act as perfect vacancy sinks. Estimates of R are given for Cu-Al, Cu-Ge and Al-Mg.     

Calculation of solute-vacancy binding energy in dilute fcc and bcc alloys by diffusion

Due to excess charge of the solute with respect to solvent, the free energy of vacancy formation and migration in the neighbourhood of the solute will change. This results in a change in the solvent diffusivity. A relation for the solute vacancy binding energy for fcc and bcc lattices using enhancement factor has been derived considering the solute vacancy interactions to be limited to first neighbour and neglecting the changes in the solvent correlation factor.     

空位缺陷对ZnNb2O6光电特性影响的第一性原理研究

铌酸盐类物质,如LiNbO3,KNbO3,LnNbO4(Ln=Pr,La,Ga,Y)等,表现出优良的光敏特性,受到广泛关注,但过渡金属类铌酸盐研究较少,其光电特性与空位缺陷的关系尚无深入探讨.基于密度泛函理论第一性原理方法,本研究探讨了空位缺陷对ZnNb2O6体系光电特性的影响.通过对各体系几何结构、电子结构和光电谱的...     

Interaction of propagating discrete breathers with a vacancy in a two-dimensional crystal

The interaction of discrete breathers (DBs) propagating along a close-packed atomic row with a vacancy situated in the same row has been studied by molecular dynamics in a two-dimensional monoatomic crystal with the Morse interatomic potential. It is established that DBs moving at a velocity below a certain threshold exhibit elastic repulsion from the vacancy, while DBs with velocities above this threshold are scattered on the vacancy. The interaction of DBs with a vacancy decreases the energy barrier for vacancy migration in the crystal.