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.     

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.     

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.     

Influence of electric field on the quenched-in vacancy and solute clustering during early stage ageing of Al-Cu alloy

The effects of electric field on the evolution of excess quenched-in vacancy as well as solute clustering in Al-4wt%Cu alloy, and on the vacancy migration and formation enthalpy of pure aluminum were investigated, using positron annihilation lifetime spectroscopy, high-angle annular dark-field scanning transmission electron microscopy, transmission electron microscopy, hardness measurement and four-probe electrical resistivity measurement. The results showed that the electric field improved age hardening response obviously and postponed the decay of excess vacancies for 30 min during the early stage ageing of Al-4wt%Cu alloy. A large number of 2–4 nm GP zones with dense distribution were observed after 1 min ageing with an electric field applied. The electric field-assisted-aged sample owned a lower coarsening rate of GP zone, which was about three fifths of that in the aged sample without an electric field, from 1 min to 120 min ageing. The electric field contributed 8% increase of the vacancy migration enthalpy(0.663 ± 0.021 e V) of pure Al, comparing with that(0.611 ± 0.023 e V) of pure Al without an electric field. The increase of vacancy migration enthalpy, induced by the electric field, was responsible for the difference on evolution of quenched-in vacancy, rapid solute clustering and age hardening improvement during the early stage ageing of Al-4wt%Cu alloy.     

Oxidation stacking faults in epitaxial silicon crystals

Oxidation stacking faults (SFs) in epitaxial silicon crystals have been studied by means of preferential etch and X-ray topography. SFs lie on {111} planes and are surrounded by a partial Frank dislocation. Nucleation takes place at crystallographic defects located near the surface of epitaxial layers. These defects appear as etch hillocks after preferential etch. SF length as a function of temperature and time for different thermal treatments is reported. The SF length is controlled by an activation energy of 2.6 eV. This energy, which is nearly half the self diffusion activation energy of silicon, supports a SF growth mechanism controlled by vacancy emission.     

Clustering phenomena in Al-4.4 at. % Zn alloy with 0.03 and 0.08 at. % Ag additions

The influence of 0.03 and 0.08 at. % Ag additions on the clustering of Zn atoms in an Al-4.4 at. % Zn alloy has been studied by resistometry. The effect of quenching and ageing temperatures shows that the ageing-ratio method of calculating the vacancy-solute atom binding energy is not applicable to these alloys. Zone-formation in Al-Zn is unaffected by Ag additions, but the zone-reversion process seems to be influenced. Apparent vacancy-formation energies in the binary and ternary alloys have been used to evaluate the v-Ag atom binding energy as 0.21 eV. It is proposed that, Ag and Zn being similar in size, the relative vacancy binding results from valency effects, and that in Al-Zn-Ag alloys clusters of Zn and Ag may form simultaneously, unaffected by the presence of each other.     

Energetics and interdiffusion at the Cu/Ru(0001) interface: density functional calculations

Density functional theory calculations were performed on the energetics of an adatom and adlayer of Cu on a Ru(0001) slab and on the interdiffusion of Cu or Ru at the interface of Cu/Ru(0001) slab. The total energy calculations showed the equal possibilities of both pseudomorphic hcp- and fcc-adlayers of Cu on the Ru(0001) slab. The formation energies of mono-vacancy at the Cu/Ru(0001) interface and the barrier energies of the vacancy-mediated interdiffusion were calculated. The formation energies of mono-vacancy at the Cu/Ru(0001) interface were determined to be 1.31 eV for Cu atom and 1.83 eV for Ru atom, respectively. The diffusion of Ru atom into the vacancy of Cu across the Cu/Ru(0001) interface required energy increase of 0.98 eV, and the barrier energy was substantially high, 1.80 eV. On the other hand, the diffusion of Cu atom into the vacancy of Ru across the Ru/Cu(0001) interface was favored with the energy reduction of 0.35 eV. However, under this most favorable situation, the barrier energy is 0.52 eV. The calculation results imply that, as far as energetics is concerned, the diffusion of both Cu and Ru atoms via the vacancy-mediated diffusion mechanism at the Cu/Ru(0001) interface seems rather restricted unless considerable thermal activation is provided.     

On the background damping in the vicinity of the grain-boundary damping peak in zinc

The background damping in the vicinity of the grain-boundary damping decreases with increasing grain size. An analysis of the strain amplitude dependence of this damping shows that as the grain size increases the distance between solute pinning atoms on dislocation decreases. This can be explained in terms of a movement of solute away from grain boundaries and to dislocations. Thus as the grain size increases the total number of solute atoms at grain boundaries decreases and is rejected both into the lattice and to dislocations. A mathematical model is used to explain this result. As a consequence an activation energy of 0.05 eV is obtained for the binding energy of the solute to the dislocation.     

Positron Annihilation Study of the Quenched-in Vacancies in CuZnAl Alloys

The excess vacancies in ternary CuZnAl and Ti-doped CuZnAl alloys quenched from temperatures between 673 K and 1223 K, and the behaviour of the quenched-in vacancies on postquench aging in martensite phase have been studied by Doppler broadening measurements. It has been found that adding Ti to the alloys resuIts in the formation of vacancy-Ti solute atom complexes which lowers the mobility of the vacancies. In the Ti-free alloy, the prominent changes in the lineshape S-parameter during the early stage of martensite aging have evidenced the occurrence of vacancy migration to sinks, clustering or vacancy related atomic distortion. The role of the vacancies in martensite stabilization has been discussed     

First-principles study of 4d solute diffusion in nickel

Diffusion of the 4d transition elements in Ni has been investigated within the five-frequency model framework using migration energy barriers calculated from the first principles. Agreement with counterintuitive experimental/calculated data is observed; atoms in the middle of 4d row have the smallest atomic radii while exhibiting the lowest diffusivity as compared to larger atoms at the beginning and the end of 4d row. We show that 4d solute diffusion is controlled mainly by the size misfit. The larger atoms have higher solute–vacancy binding energies and lower migration barriers. Both were shown to correlate with a displacement of the equilibrium solute position toward the adjacent vacancy. The difference in mechanisms controlling sp- and transition elements diffusion rates in Ni is discussed.     

Impurity band in in-doped CdCr2Se4 observed in the spectral dependence of the photoconductance

The spectral dependence of the photoconductance of In-doped CdCr₂Se₄ crystals was measured in the photon energy range from 0.2 eV to 2.5 eV. At a temperature of 80 K, i.e. below the Curie temperature of 132 K, two peaks of the impurity photoconductivity are observed. The peak energy of 0.38 eV agrees with the thermal activation energy determined from the slope of the ? vs T curves at higher temperatures. This agreement is an experimental evidence for the impurity band formed by the In donors and the Se vacancy donors which should be responsible for the large negative magnetoresistance due to “magnetic impurity states” in these crystals. At a temperature of 155 K, i.e. above the Curie temperature, the structure of the photoconductance vanishes due to the thermal activation of electrons from occupied donor levels to the conduction band.     

Kinetics of WSi2 formation at low and high temperatures

Tungsten is evaporated onto an Si〈100〉 substrate at 500°C. Then WSi₂ is formed by furnace (600–800°C) or laser (1200–1400°C) annealing. In both temperature ranges, the kinetics of formation are diffusion controlled. At high temperature, the thicknesses of the WSi₂ layers formed in the middle of the laser-scanned lines are measured using an Auger electron spectroscopy profiler and an Alpha-step profiler; an activation energy of 2.2 eV is found. Between 600 and 800°C, using Rutherford backscattering spectroscopy to measure the thickness of the layer formed, we find a surprisingly high activation energy of 3.4 eV. To our knowledge this is the first time that the formation kinetics of WSi₂ have been determined at low temperature.     

Simulation of vacancy migration energy in Cu under high strain

The activation energy for the migration of vacancies in Cu under high strain was calculated by computer simulation using static methods. The migration energy of vacancies was 0.98 eV in the absence of deformation. It varied with the migration direction and stress direction because the distance between a vacancy and its neighboring atoms changes by deformation. For example, the migration energy for the shortest migration distance was reduced to 9.6 and 39.4% of its initial value by 10% compression and 20% elongation, respectively, while that for the longest migration distance was raised to 171.7 by 20% elongation. If many vacancies are created during high-speed deformation, the lowering of migration energy enables vacancies to escape to sinks such as surfaces, even during the shorter deformation period. The critical strain rate above which the strain rate dependence of vacancy accumulation ceases to exist increases with the lowering of vacancy migration energy.     

Interaction of C with vacancy in W: A first-principles study

We provide a vacancy trapping mechanism of C in W by investigating structure, stability, and diffusion properties of C in W using a first-principles method. C easily bonds onto the internal-surface of vacancy. A monovacancy is capable of trapping as many as 4 C atoms to form C_nV (n = 1, 2, 3, 4) complexes. Single C atom prefers to interact with neighboring W at vacancy with the trapping energy of ?1.93 eV. With the C atoms added, both of them are preferred to bind with each other to form covalent-like bond despite the metallic W environment. For the C_nV complexes, C₂V is the major one due to its largest average trapping energy (?1.97 eV). Kinetically, formation of the C_nV complexes can be ascribed to the interstitial mechanism due to the lower activation energy barrier of 1.46 eV for the interstitial C than 1.66 eV for the vacancy.     

Positron annihilation spectroscopy in materials science

The application of positron annihilation spectroscopy to the study of defects in materials is illustrated through several examples drawn from our work. These include the study of vacancy clustering in metals, clustering of He atoms to form bubbles, and solute clustering in alloys. Results of studies on novel materials such as quasicrystals, cuprate superconductors and fullerenes are also presented.     

Study of the lithium cluster breakage kinetics in germanium

The lithium cluster breakage in germanium obeys an exponential kinetics with an activation energy for the particle detachment from cluster equal to 0.31±0.05 eV.     

Analysis of the oxidation kinetics and barrier layer properties of ZrN and Pt/Ru thin films for DRAM applications

ZrN and Pt/Ru thin films have been grown by an automated ion beam sputter-deposition system. Both materials were evaluated for use as barrier layers (ZrN) and bottom electrodes (Pt/Ru) in dynamic random access memory (DRAM) applications. The ZrN films had resistivities on the order of 250–300 μω cm. The ZrN films were (002) oriented and were rather smooth with an average surface roughness of ±17 Å. Analysis of the oxidation kinetics of the ZrN thin films reveals a thermally activated, diffusion-limited oxidation process with an activation energy of 2.5 eV in the temperature range of 500–650 ° C. This implies that there is an advantage in using ZrN as a barrier layer instead of TiN since the activation energy for oxidation of TiN is 2.05 eV. In addition, preliminary data suggest that a Pt/Ru double layer may be a promising bottom electrode and oxygen diffusion barrier for use in DRAMs with high-permitivity dielectrics.     

Interstitialcy diffusion in electron-irradiated CuNi alloys

The well-known Ni cluster formation in solid solutions of CuNi around the 50:50 composition was formerly observed during thermal annealing down to temperatures of about 620 K, when the vacancy mobility practically vanishes. In the present work Ni cluster formation has been observed during electron irradiation at temperatures as low as 100 K. The required mass transport must be due to an interstitialcy mechanism. The activation energy of interstitial motion is of the order of 0.2 eV. From the observed cluster formation, kinetic details of the interaction of migrating interstitials with other lattice defects can be derived.     

Study of the kinetics of recrystallisation of dispersion-strengthened (Al/Al2O3) alloys

The kinetics of recrystallisation of an SAP alloy containing 5.25 wt % oxide has been studied, mainly by metallographic observations and transmission electron microscopy. At the beginning of the process the isothermal reaction follows the Avrami lawx=1– exp (–Bt ^k) withk – 1.7. A deviation from the above kinetics takes place with increasing time; it is shown by metallographic observations and from the effect of preliminary recovery that this behaviour results from competition between recovery and recrystallisation. The competing process has an activation energy close to the value of 1.57 eV, previously determined for the stage of dislocation recovery preceding recrystallisation.Results reported by other authors have been analysed, and it is noticed that competition is a general feature of recrystallisation in these alloys; this effect becomes more pronounced with increasing oxide concentration, and can ultimately stop recrystallisation. The analysis of the kinetics of recrystallisation indicates that the dispersed phase not only reduces the driving force for grain boundary migration, but also affects mobility by raising the activation energy of the process.