Interaction of dislocations with nanoprecipitates of the metastable phase and dispersion strengthening of the Fe-Cu alloy

The interaction of dislocations with copper-enriched precipitates in the matrix of body-centered cubic iron has been investigated by the molecular dynamics method. It has been shown that dislocations stimulate the development of a phase instability of body-centered cubic copper precipitates in a specific range of their sizes. This process is accompanied by the pinning of dislocations by precipitates and makes a significant contribution to strengthening. The results obtained provide an adequate explanation for the observed dependence of the strengthening in the Fe-Cu system on the precipitate size.     

Surface phase transformation induced by the dezincification of a beta Cu-Zn alloy on highly deformed systems

When Cu-Zn alloys are annealed under dynamical vacuum the Zn component evaporates. The process is called dezincification. This paper presents the results of the dezincification of highly mechanically deformed surfaces of samples initially in the beta (bcc) phase by a combination of in situ optical microscopy observations together with TEM measurements. It is shown that grinding lines remaining from the sample preparation process act as nucleation centers for the alpha (fcc) phase. Under this surface preparation conditions the new fcc phase nucleates with a different geometry than the one reported in previous papers in which surfaces were finished by electropolishing. In the present case, we observe individual fcc precipitates with a well defined geometry. The typical size of precipitates is in the micron range, and depends on the dezincification parameters: final temperature, dezincification time and prior surface preparation. TEM observations show that the fcc precipitates contain a large density of defects, mainly dislocations and twin boundaries.     

TEM study of NbC heterogeneous precipitation in ferrite

The heterogeneous precipitation of NbC in ferrite has been quantitatively characterized by transmission electron microscopy in a Fe–C–Nb model alloy for different isothermal heat treatments. The elongation and size distribution of precipitates were derived using dark field imaging. For each precipitation state, the precipitation of NbC occurs on dislocations due to the as-quenched state. This precipitation mechanism leads to characteristic arrays of precipitates in which precipitates grow in a self-similar manner. A detailed study of these arrays has shown that most dislocations decorated by these arrays are edge dislocations with ?112? type line vectors. There is only one variant on a given dislocation. This selection can be interpreted by the interaction between dislocation and precipitate strain fields.     

Early precipitation stages of aluminum alloys—The role of quenched-in vacancies

We present preliminary results on the structure of nano-sized particles (clusters of solute atoms or Guinier-Preston-Bagaryatsky (GPB) zones) in aluminum alloys. Hindering the motion of dislocations, these GPB zones (precipitates) are responsible for the strength of AlCuMg-alloys - used e.g. as AA2024 - for the outer shell plates in aircraft industry. We will discuss the role of quenched-in vacancies for the formation and growth of the precipitates. Using positron annihilation (Doppler broadening together with the recently developed tool of high-momentum analysis (HMA)) and X-ray absorption spectroscopy measurements, we are able to characterize the local atomic environment in the vicinity of vacancies and selected elements (Cu) forming the precipitates. The interpretation will be based on a comparison to numerical calculations of positron lifetimes and momentum distributions as well as simulated X-ray absorption spectra. Often reliable numerical calculations of experimental quantities are possible only if relaxed atomic positions - calculated employing an ab-initio code like SIESTA - are used as input.     

Electrical properties of plastically deformed silicon due to its interaction with an iron impurity

The change in the recombination properties of plastically deformed silicon as a result of its interaction with an iron impurity introduced by diffusion at 1000°C has been studied using scanning electron microscopy and deep level transient spectroscopy. The recombination activity of dislocations and slip planes behind dislocations has been found to increase due to the accelerated formation of iron precipitates at these defects. Based on the measurements of the electron-beam-induced current, the concentration of the formed precipitates has been estimated, and their sizes have been calculated to be several hundred nanometers.     

Structure,impurity composition,and photoluminescence of mechanically polished layers of single-crystal silicon

The introduction of optically active defects (such as atomic clusters, dislocations, precipitates) into a silicon single crystal using irradiation, plastic deformation, or heat treatment has been considered a possible approach to the design of silicon-based light-emitting structures in the near infrared region. Defects were introduced into silicon plates by traditional mechanical polishing. The changes in the defect structure and the impurity composition of damaged silicon layers during thermal annealing (TA) of a crystal were examined using transmission electronic microscopy and x-ray fluorescence. Optical properties of the defects were studied at 77 K using photoluminescence (PL) in the near infrared region. It has been shown that the defects generated by mechanical polishing transform into dislocations and dislocation loops and that SiO₂ precipitates also form as a result of annealing at temperatures of 850 to 1000°C. Depending on the annealing temperature, either oxide precipitates or dislocations decorated by copper atoms, which are gettered from the crystal bulk, make the predominant contribution to PL spectra.     

Bulk interfaces in a Ni-rich Ni-Au alloy investigated by high-resolution Z-contrast imaging

Interfaces between Au-rich precipitates and the Ni-rich matrix in a decomposed Ni-10 at.% Au alloy were investigated by low-magnification and high-resolution Z-contrast imaging. During aging at 923 K, the originally single crystalline sample decomposed and recrystallized resulting in a microstructure consisting of subgrains separated by small-angle grain boundaries. These small-angle grain boundaries are decorated by Au-rich precipitates. The interfaces between the Au-rich precipitates and the Ni-rich matrix were characterized with respect to the orientation relationship between precipitates and matrix, misfit dislocations and concentration gradients. Two transformation modes were identified that are involved in the decomposition of bulk Ni-rich Ni–Au alloys. While in the first mode the interface is semi-coherent, in the second mode the interface corresponds to an incoherent twin boundary. It is further shown that strain fields around misfit dislocations can result in systematic errors in the determination of the concentration gradients across interfaces between precipitates and matrix.     

Dislocation nucleation and precipitation of impurity in calcium-doped NaCl-KCl crystals

In calcium-doped mixed NaCl-KCl crystals, Ca²⁺ ion sites were selectively sensed by a suitable etchant. Different impurity substructures, viz. arrays, close-networks, unevenly distributed clusters were formed by the dopant ions in the lattice. Non-uniform distributions of the impurity ions in the lattice does not nucleate dislocations. Non-structural impurities like precipitates were found to take different forms, viz. elongated rods, rectangular forms, with different orientations. Dislocations showed strong preference for the nucleation of the precipitates. The nucleation of the precipitates at sites other than those of dislocations is discussed.     

3D DDD modelling of dislocation–precipitate interaction in a nickel-based single crystal superalloy under cyclic deformation

Strain-controlled cyclic deformation of a nickel-based single crystal superalloy has been modelled using three-dimensional (3D) discrete dislocation dynamics (DDD) for both [0?0?1] and [1?1?1] orientations. The work focused on the interaction between dislocations and precipitates during cyclic plastic deformation at elevated temperature, which has not been well studied yet. A representative volume element with cubic γ′-precipitates was chosen to represent the material, with enforced periodical boundary conditions. In particular, cutting of superdislocations into precipitates was simulated by a back-force method. The global cyclic stress–strain responses were captured well by the DDD model when compared to experimental data, particularly the effects of crystallographic orientation. Dislocation evolution showed that considerably high density of dislocations was produced for [1?1?1] orientation when compared to [0?0?1] orientation. Cutting of dislocations into the precipitates had a significant effect on the plastic deformation, leading to material softening. Contour plots of in-plane shear strain proved the development of heterogeneous strain field, resulting in the formation of shear-band embryos.     

Optical properties and origin of infrared light scattering centers in undoped semi-insulating GaAs crystals

The wavelength dependence and polarization characteristics of the infrared light scattered from an undoped GaAs crystal were investigated in the 90° angle infrared light scattering configuration. The scattering is Rayleigh scattering from scatterers which are always associated with the dislocations, and they are classified into three types,S, L _A , andL _G scatterers, according to their polarization characteristics. TheS, L _A , andL _G -scatterers are thought to be small As clusters, large As precipitates and large Ga precipitates, respectively.     

Accommodation of the Lattice Mismatch at the VCx-WC Interface

A VC doped WC-Co alloy is investigated using high resolution transmission electron microscopy. The VC grain growth inhibitor induces the presence of a thin layer on the surfaces of the WC grains in contact with Co and precipitates in the corners of Co pockets. These (VW)C_x compounds adopt an epitaxial orientation relationship with regards to the (0001) base facets of the WC crystals. Due to the small difference in lattice parameters, misfit dislocations are expected in the interfaces. Unlike the thin layers where no defects are observed, two kinds of dislocations are pointed out for larger precipitates. 1/6〈112〉_VC interfacial dislocations are sometimes present while more often 1/2〈1¯10〉_VC dislocations lying above the interface in the (VW)C_x phase are visible.     

Electronic states at dislocations and metal silicide precipitates in?crystalline silicon and their role in?solar cell materials

Predominant dislocation types in solar silicon are dissociated into 30°- and 90°-partials with reconstructed cores. Besides shallow 1D-band localized in their strain field and a quasi-2D band at the stacking fault connecting the two partials, the existence of several intrinsic core defects with deep lying levels has been demonstrated by electron spin resonance. The majority of core defects occur in nonequilibrium situations and, with the exception of a small EPR-signal assigned to a reconstruction defect, vanish after careful annealing above 800°C. There is good evidence now that part of deep levels observed in dislocated silicon is associated with impurities, especially with transition metal impurities. Electron-hole-pair recombination at a dislocation mainly runs via its shallow bands and is strongly increased by impurities bound to its core or in the strain field. The concentration of these impurities can be reduced by gettering processes to such a low level that radiative recombination at dislocations yields a luminescence efficiency of 0.1% at room temperature. A quite coherent picture has emerged for metal impurity precipitation in silicon. Early stages of precipitation in defect-free silicon are characterised by kinetically selected metastable defects forming as a result of large chemical driving forces for precipitation. Such defects are associated with deep level spectra which show the properties of extended multielectron defects. The evolution of the system to energetically more favourable configurations proceeds via ordinary particle coarsening but also via internal ripening, a process reminiscent of the above-mentioned metastable defects. Electronically, the defects evolve into metal-like inclusions which in general seem to act as strong recombination centers for minority carriers. In the presence of dislocations metastable defects quickly transform into equilibrium structures in the course of precipitation or do not form at all. In the presence of several metal impurities silicide precipitates which can be described as solid solutions of the respective metal atoms are observed, which is at least qualitatively in accord with ternary phase diagrams. Like single-metal silicide precipitates, strong minority carrier recombination is also typical for those multi-metal silicide particles.     

Atomic-scale study of precipitates (NbC and Cu-rich phase) at the twin boundary in the long time ageing austenitic stainless steel

ABSTRACT

Microstructures of Cu-rich phases and NbC precipitated phases have been studied in a long time ageing austenitic stainless steel by high resolution scanning transmission electron microscopy. The interaction difference between the twins and the second phases found to be dependent on the nature of the precipitates. The Cu-rich phases were identified to be twinned at the twin boundary. Nevertheless, the NbC precipitates not only twinned at the twin boundary but also induced the twin boundary bypass them. A particle size dependence of the generation of misfit dislocations also was detected at interface between precipitates and the austenitic matrix.     

Collective effects in an ensemble of dislocations and vacancies with the formation of the localized deformation zone

Abstract

Paperi have reported about the formation of the superplastic zone as the result of defect production in high strength alloys with the fine precipitates of a non-metalic phase under the intensive stress of about (10^-2 ÷ 10^-1)μ, where μ is a shear modulus. It turned out, that the stopping of slipping dislocations near the precipitates leaded to vacancy influx, which promoted dislocations climbing, on the one hand, and increased its concentration on the other. The higher vacancy concentration, the higher dislocation density increases     

Study of the electrical and optical properties of silicon containing oxygen precipitates

The properties of n-type silicon with oxygen precipitates introduced by three-stage annealing were studied by the electron beam induced current (EBIC) method, deep-level transient spectroscopy (DLTS), and photoluminescence (PL). The presence of extended defects with concentration of ≤10⁹ cm^?3 is revealed by the EBIC method. The concentration of electrically active defects formed in silicon due to oxygen precipitation is estimated from the EBIC contrast and is compared to that obtained from the DLTS data. Comparing the spectra of samples with oxygen precipitates with those of plastically deformed crystals, we can assume that the DLTS and PL spectra of silicon with oxygen precipitates are mainly determined by dislocations.     

Helium trapping at dislocations,precipitates and grain boundaries

Helium bubble formation was observed in austenitic stainless steels by transmission electron microscopy following implantation of 30 to 1000 appm helium at room temperature and annealing at 700 to 800°C. Helium bubble distributions at dislocations and at various grain boundaries and precipitates were studied. It was found that interfacial dislocations play a dominant role in bubble nucleation at grain and interphase boundaries but not at Tic-matrix interfaces. Particularly high trapping of helium was observed at Tic precipitate-matrix interfaces which is attributed to an inhomogeneous ripening mechanism.     

A TEM study of precipitation in Al–Mg–Si alloys

A set of extrusion samples of Al–Si–Mg alloys (0.5 wt%Mg and 0.3–0.8 wt%Si) were, respectively, T1- and T4-heat-treated. Differential scanning calorimetry (DSC) was used to heat the samples to particular temperatures to promote the formation of precipitates for study by transmission electron microscopy (TEM). It was found that, apart from β″, β′, and B′, there were many precipitates showing rectangular lattices when viewed along the long axes of the precipitates. It is considered that the residual stresses (or dislocations) in the extrusion after the T1-treatment facilitated the nucleation and growth of the precipitates during the heating in the DSC.     

Solid solution decomposition and growth of precipitates in Be-Fe alloys from Mössbauer investigations

Mössbauer spectra of fine-grained hot-pressed beryllium and coarse-grained beryllium samples containing different amounts of impurities were obtained after homogenization and after annealing for different durations. Mössbauer spectra of solid solution of iron in beryllium and decomposed during isothermal annealing two different iron containing phases were fitted by a convolution equation of three Lorentz lines. The models of solid solution decomposition and growth of secondary particle precipitates were investigated. The average distance between dislocations and the average grain size were obtained from the application of the models. The dependencies between the decomposition mechanism, the average grain size, the impurity concentrations and the type of the secondary particles precipitates were revealed. The possibility of a coherent analysis of the decomposition process by means of a kinetic law classification and secondary particle precipitates growth processes based on diffusion models has been shown.     

Microstructure evolution of zircaloy-4 during Ne ion irradiation and annealing: An in situ TEM investigation

The microstructural evolution of zircaloy-4 was studied, including the amorphization and recrystallization of Zr（Fe, Cr）2 precipitates, and the density of dislocations under in situ Ne ion irradiation and post annealing. The results show that irradiation at a relatively high temperature and dose induces the formation of nanocrystals in pre-amorphized Zr（Fe, Cr）2 precipitates. The recrystallized nanocrystals also have the structure of hcp-Zr（Fe, Cr）2. The formation of the nanocrystals is thought to be the consequence of competition between atomistic disordering and the recrystallization of precipitates under ion irradiation. The free energy of the nanocrystal is lower than that of the amorphous state, which is another reason for the recrystallization of the precipitates. With increased annealing temperature, the density of the nanocrystals is increased. The dislocation density sharply decreases with the increase in the annealing temperature, and its size increases.     

Magnetosensitive defects generated during plastic straining of NaCl:Eu crystals

The excitation of an equilibrium subsystem of point defects in plastically strained NaCl:Eu crystals has been studied. It is established that the macroplastic straining of crystals containing three-dimensional EuCl₂ precipitates leads to a decrease in the amount of these precipitates, accompanied by the appearance of new emission centers of two types. In the easy glide stage, the plastic straining is additionally accompanied by a decrease in the fraction of flat EuCl₂ precipitates and an increase in the content of nonequilibrium small-size complexes of impurity-vacancy dipoles sensitive to an external magnetic field. In thoroughly annealed crystals with the point defect subsystem in a nearly equilibrium state, individual dislocations exhibit displacement caused by the action of a pulsed magnetic field and by the joint action of constant and microwave magnetic fields under the electron paramagnetic resonance conditions.