Evolution Of Defect Structure Research Articles

High resolution transmission electron microscopy study on the development of nanostructured precipitates in Al–Cu obtained by mechanical alloying

Aluminum alloy 2014 is used to obtain nanostructured powders via mechanical alloying. The evolution of the diffusion processes is observed by the development of defect structures and nanoprecipitates after 10 h of milling. The characterization includes analytical and high resolution transmission electron microscopy. Dislocations associated with different Al/Cu ratio affect the material. These defects act as nucleation sites where precipitates of the ɛAl 2Cu 3 hexagonal phase have been identified. Moiré fringes show the interference of {1 1 1} Al with { 1 0 − 1 0 } ε Al 2 Cu 3 glide planes and locally small shifts of 1/3{1 1 1} Al and 1 / 3 { 1 0 − 1 0 } ε Al 2 Cu 3 . Changes in the Al/Cu ratio lead to the formation of other solid solutions identified in the Cu rich area and could correspond to transition phases.

CuO-doped NaNbO3antiferroelectrics: Impact of aliovalent doping and nonstoichiometry on the defect structure and formation of secondary phases

The interplay between aliovalent CuO doping and nonstoichiometry on the development of defect structures and the formation of secondary phases of antiferroelectric NaNbO${}_{3}$ ceramics has been investigated by means of x-ray diffraction (XRD), first-principles calculations using density functional theory (DFT), and electron paramagnetic resonance spectroscopy. The results indicate that, for stoichiometric 0.25 mol% CuO-doped NaNbO${}_{3}$, as well as for 2.0 mol% Nb-excess sodium niobate, the Cu${}^{2+}$ functional centers are incorporated at the Nb site (${\mathrm{Cu}}_{\mathrm{Nb}}^{\ensuremath{'}\ensuremath{'}\ensuremath{'}}$). For reasons of charge compensation, two kinds of mutually compensating defect complexes ${({\mathrm{Cu}}_{\mathrm{Nb}}^{\ensuremath{'}\ensuremath{'}\ensuremath{'}}\ensuremath{-}{V}_{\mathrm{O}}^{\ifmmode\bullet\else\textbullet\fi{}\ifmmode\bullet\else\textbullet\fi{}})}^{\ensuremath{'}}$ and ${({V}_{\mathrm{O}}^{\ifmmode\bullet\else\textbullet\fi{}\ifmmode\bullet\else\textbullet\fi{}}\ensuremath{-}{\mathrm{Cu}}_{\mathrm{Nb}}^{\ensuremath{'}\ensuremath{'}\ensuremath{'}}\ensuremath{-}{V}_{\mathrm{O}}^{\ifmmode\bullet\else\textbullet\fi{}\ifmmode\bullet\else\textbullet\fi{}})}^{\ifmmode\bullet\else\textbullet\fi{}}$ are formed where, for the niobium-excess compound, additionally, ${V}_{\mathrm{Na}}^{\ensuremath{'}}$ contribute to the mechanism of charge compensation. In contrast, for 2.0 mol% Na-excess sodium niobate, a Na${}_{3}$NbO${}_{4}$ secondary phase has been detected by XRD, and only part of the Cu${}^{2+}$ forms these types of defect complexes. The major part of the Cu${}^{2+}$ is incorporated in a fundamentally different way by forming Cu${}^{2+}$-Cu${}^{2+}$ dimeric defect complexes.

The evolution of vacancy-type defects in silicon-on-insulator structures studied by positron annihilation spectroscopy

Variable-energy positron annihilation spectroscopy (VEPAS) has been applied to the study of the formation and evolution of vacancy-type defect structures in silicon (Si) and the 1.5 μm thick Si top layer of silicon-on-insulator (SOI) samples. The samples were implanted with 2 MeV Si ions at fluences between 1013 and 1015 cm−2, and probed in the as-implanted state and after annealing for 30 min at temperatures between 350 and 800 °C. In the case of SOI the ions were implanted such that their profile was predominantly in the insulating buried oxide layer, and thus their ability to combine with vacancies in the top Si layer, and that of other interstitials beyond the buried oxide, was effectively negated. No measurable differences in the positron response to the evolution of small clusters of n vacancies (Vn, n ∼ 3) in the top Si layer of the Si and SOI samples were observed after annealing up to 500 °C; at higher temperatures, however, this response persisted in the SOI samples as that in Si decreased toward zero. At 700 and 800 °C the damage in Si was below detectable levels, but the VEPAS response in the top Si layer in the SOI was consistent with the development of nanovoids.

Atomistic mechanism of cyclic phase transitions in Nd–Fe–B based intermetallics

Mechanical milling induced atomic disorder and the mechanism of the cyclic crystal → amorphous → crystal phase transitions in a Nd 2Fe 14B intermetallic – based Nd 10Fe 85B 5 alloy were investigated. Least square first shell fitting of Extended X-ray Absorption Fine-Structure (EXAFS) data of mechanically milled samples revealed that phase evolution during mechanical milling of Nd 10Fe 85B 5 alloy occurred in two stages as a result of two dynamical effects: milling induced forced atomic mixing and enhanced diffusion due to continuous production of point defects. In the first stage of milling, atomic mixing of Nd and Fe atoms dominated, resulting in extensive chemical and structural disorder in the initially ordered intermetallic, followed by amorphization. The second stage of milling was characterized by a remarkable increase in point defect density. Enhanced diffusion resulted in redistribution of alloy atoms and annihilation of these point defects leading to the precipitation of α-Fe nanocrystals in the amorphous matrix. Thus the time evolution of defect structures formed during mechanical milling and their atomic scale interactions led to the cyclic phase transitions in the Nd 10Fe 85B 5 alloy. These milling induced changes in atomic order were found to strongly influence magnetic properties. Crystallization of mechanically milled alloy and effect of annealing parameters on magnetic properties was also analyzed.

TEM investigation of irradiation damage in single crystal CeO 2

In order to understand the evolution of radiation damage in oxide nuclear fuel, 150–1000 keV Kr ions were implanted into single crystal CeO 2, as a simulation of fluorite ceramic UO 2, while in situ transmission electron microscopy (TEM) observations were carried out. Two characteristic defect structures were investigated: dislocation/dislocation loops and nano-size gas bubbles. The growth behavior of defect clusters induced by 1 MeV Kr ions up to doses of 5 × 10 15 ions/cm 2 were followed at 600 °C and 800 °C. TEM micrographs clearly show the development of defect structures: nucleation of dislocation loops, transformation to extended dislocation lines, and the formation of tangled dislocation networks. The difference in dislocation growth rates at 600 °C and 800 °C revealed the important role which Ce–vacancies play in the loop formation process. Bubble formation, studied through 150 keV Kr implantations at room temperature and 600 °C, might be influenced by either the mobility of metal–vacancies correlated with at threshold temperature or the limitation of gas solubility as a function of temperature.

Characterising texture formation in fibre lattices embedded in a nematic liquid crystal matrix

A two-dimensional computational study is performed on the texturing of fibre-filled nematic liquid crystals using the Landau-de Gennes model describing the spatio-temporal evolution of the second moment of the orientation distribution function or quadrupolar tensor order parameter. The investigation is performed on a consistent computational domain comprising a square array of four circular fibres embedded within a unit square containing a uniaxial low molar mass calamitic liquid crystal. Interest is focused on the role of temperature, boundary conditions and their effect on the nucleation and evolution of defect structures. Thermal effects are characterised below and above the temperature at which the nematic state is stable. Simulations in the stable nematic state serves as a scenario for investigating the effect of imposing different external boundary conditions, namely periodic and Dirichlet; the former describes a square lattice array of fibres embedded in a nematic liquid crystal, and the latterdescribes a four-fibre arrangement in an aligned nematic material. In each case, the influence of temperature is characterised, with defect structures forming and either remaining or splitting into lower strength defects. For fibre lattices, splitting transitions of defects at the centre of the domain occur at a critical temperature, but for the four-fibre arrangement, defect transitions occur continuously over a temperature range. The discontinuous defect splitting transition in fibre arrays occurs at lower temperatures than the continuous defect transformation in the four-fibre arrangement. At sufficiently low temperatures, the four-fibre arrangement and the fibre lattice give the same texture consisting of two disclination lines close to each fibre. The evolution of the texture with respect to temperature can be characterised as a change from single-fibre mode at low temperature to a collective mode with a centre-located heterogeneity at higher temperature. At higher temperatures, in the stable isotropic state, it is shown that surface-induced ordering arising from the fibre/liquid crystal interaction propagates into the bulk forming thin disclination lattices around the four-fibre configuration.

Thermal activation analysis of the structural and phase transformations in the Zr-Cu-Nb amorphous alloy

In the present work, the procedure of the estimation of the thermal activation parameters from the data of dilatometric measurements and the results of its application to the Zr-Cu-Nb amorphous alloy discussed. The determination of the thermal activation parameters of the processes occurring in materials under known temperature-force conditions can be useful for the identification of the structural mechanisms of phase transformations and the evolution of defect structure. We used the data of dilatometric measurements for evaluating the effective activation energy. This method exhibits some advantages over the conventional one due to the design features of dilatometers. First, it ensures the precise measurement of strains; second, the assigned temperature regime is very precisely maintained both at the stage of heating and upon isothermal holding; third, it ensures a high-speed continuous record of the experimental data. The developed method of evaluating the effective activation energy from the results of dilatometric experiments provides statistically reliable results. The data of the photometric analysis of structure images are in accordance with the results of dilatometric experiments

The effect of crystal geometry on the formation and crack development in molybdenum single crystals

The systematic study of the defect structure evolution in single crystal specimens deformed in situ in the high voltage transmission electron microscope (HVTEM) was started around 1977 in the Baikov Metallurgy Institute of the USSR Academy of Sciences. All the experience gathered for the past 30 years has set up the basis for the determination of the way that metals adapt to the different physical factors inside the HVTEM column. In the present work the detailed phenomenology of the generation and propagation process of cracks in single crystals of Mo with different crystallographic orientations as well as the kinetics of opening of primary and secondary cracks and the tip evolution for cracks with different dislocation generation sources is presented.

Unfaulting mechanism of trapped self-interstitial atom clusters in bcc Fe: A kinetic study based on the potential energy landscape

We report on the complete unfaulting mechanism of a trapped self-interstitial atom cluster in the form of a nonparallel configuration (NPC), investigated using the autonomous basin climbing (ABC) method. A detailed set of transition state atomic trajectories in the unfaulting process from the trapped to the mobile glide $⟨111⟩$ configuration and the corresponding potential energy landscape were identified. The breaking of the initial ring structure of the three trimers on (111) planes followed by the rotation of the $⟨111⟩$ crowdion in the NPC are the main rate limiting processes of the unfaulting mechanism. The effective activation barrier in the transition from the NPC to the glide $⟨111⟩$ configuration was calculated by combining the ABC and kinetic Monte Carlo methods and was further benchmarked against molecular dynamics (MD) simulations. The effective activation barrier was found as 0.82 eV; smaller than its previously reported value of 1.68 eV. The ABC method was confirmed to be more efficient than MD, especially for the defect structure evolution processes associated with high barriers and at low temperatures.

In situ transmission electron microscopy and ion irradiation of ferritic materials

The intermediate voltage electron microscope-tandem user facility in the Electron Microscopy Center at Argonne National Laboratory is described. The primary purpose of this facility is electron microscopy with in situ ion irradiation at controlled sample temperatures. To illustrate its capabilities and advantages a few results of two outside user projects are presented. The motion of dislocation loops formed during ion irradiation is illustrated in video data that reveals a striking reduction of motion in Fe-8%Cr over that in pure Fe. The development of extended defect structure is then shown to depend on this motion and the influence of nearby surfaces in the transmission electron microscopy thin samples. In a second project, the damage microstructure is followed to high dose (200 dpa) in an oxide dispersion strengthened ferritic alloy at 500 degrees C, and found to be qualitatively similar to that observed in the same alloy neutron irradiated at 420 degrees C.

Mean field rate theory and object kinetic Monte Carlo: A comparison of kinetic models

Multiscale modeling schemes encompass models from the atomistic to the continuum scale. Phenomena at the mesoscale have typically been simulated using models based on reaction rate theory, such as mean field rate theory (MFRT) or Monte Carlo. These mesoscale models are appropriate for application to problems that involve intermediate-length scales, and timescales from those characteristic of diffusion to long-term microstructural evolution (∼μs to years). Although the MFRT and Monte Carlo models can be used simulate the same phenomena, some of the details are handled quite differently in the two approaches. Models employing the rate theory have been extensively used to describe radiation-induced phenomena such as void swelling and irradiation creep. The primary approximations in such models are time and spatial averaging of the radiation damage source term, and spatial averaging of the microstructure into an effective medium. Kinetic Monte Carlo models can account for these spatial and temporal correlations; their primary limitation is the computational burden, which is related to the size of the simulation cell. Even with modern computers, the maximum simulation cell size and the maximum dose (typically much less than 1 dpa) that can be simulated are limited. In contrast, even very detailed MFRT models can simulate microstructural evolution for doses up 100 dpa or greater in clock times that are relatively short. Within the context of the effective medium, essentially any defect density can be simulated. A direct comparison of MFRT and object kinetic MC simulations has been made in the domain of point defect cluster dynamics modeling, which is relevant to the evolution (both nucleation and growth) of radiation-induced defect structures. Overall, the agreement between the two methods is best for irradiation conditions that produce a high density of defects (lower temperature and higher displacement rate) and for materials that have a relatively high density of fixed sinks such as dislocations.

Effects of transmutation elements on the defect structure development of W irradiated by protons and neutrons

A series of W–Re–Os alloys were fabricated by arc melting for investigating the effects of transmutation elements of tungsten on the defect structure development. Transmutation electron microscopy has been used to investigate the defect structure for proton-irradiated (E=1MeV) W, W–3Re, W–3Os and 0.15dpa neutron-irradiated (E>1MeV) W–5Re–3Os and W, W–3Re, W–5Re and W–26Re. The irradiation-induced voids and dislocation loops which directly cause the irradiation hardening were observed. The results show the combination of W with Re or Os effectively restrains irradiation damage since the number density and radius of both voids and dislocation loops remarkably decrease with increasing Re or Os content.

Displacement damage and transmutations in metals under neutron and proton irradiation

The knowledge of the defect and impurity generation rates, as well as the defect spatial distribution, is the corner stone for the understanding of the evolution of material properties under irradiation. This knowledge is also an essential element for comprehensive experimental simulations of the behavior of irradiated materials. In this article the interaction of neutron and proton irradiation with metals is discussed with respect to displacement damage production. Charged particle irradiation is also briefly illustrated. After discussion of the primary interaction of projectiles (neutrons, charged particles in general, and protons in particular) with target atoms/nuclei, we describe the interaction of a recoil atom with other target atoms resulting in the slowing down of the projectile, displacement damage, impurity atom production due to nuclear reactions, and the creation of atomic displacement cascades. Then the further evolution of defect structure is discussed. The next section, devoted to subcascade formation, is divided into two parts. The first experimental evidence of subcascade formation under neutron and charged particle irradiation is presented. Then the models of subcascade formation are described. Finally we review the models for the calculation of displacement damage and show how these models can be applied to displacement damage calculation under neutron irradiation with a demonstration of a real application of the methods discussed to several nuclear facilities. To cite this article: P. Vladimirov, S. Bouffard, C. R. Physique 9 (2008).

Evolution of Structural Defects in SiOx Films Fabricated by Electron Cyclotron Resonance Plasma Chemical Vapour Deposition upon Annealing Treatment

We study the structural defects in the SiOx film prepared by electron cyclotron resonance plasma chemical vapour deposition and annealing recovery evolution. The photoluminescence property is observed in the as-deposited and annealed samples.[-SiO3]2− defects are the luminescence centres of the ultraviolet photoluminescence (PL) from the Fourier transform infrared spectroscopy and PL measurements. [-SiO3]2− is observed by positron annihilation spectroscopy, and this defect can make the S parameters increase. After 1000°C annealing, [-SiO3]2− defects still exist in the films.

An optical spectroscopy study of defects in lithium tantalate single crystals

The congruent, stoichiometric, and Mg doped stoichiometric LiTaO3 single crystals have been successfully grown by the Czochralski technique. The evolution of defect structures caused by varying composition and post-growth processing has been evaluated from the optical absorption and photoluminescence measurements. Optical absorption studies showed that the UV absorption edge is very sensitive to the composition of LiTaO3 crystals. Photoluminescence of various LiTaO3 single crystals at room temperature was observed. The emission bands centered at 360, 430, and 530nm were assigned to different defects, which can well show the defect information in LiTaO3 crystals.

Dynamics of Point Defects in Tellurium-Rich CdTe

The evolution of defect structure occurring during the cooling of Tellurium-rich CdTe is studied theoretically and experimentally. Classical nucleation theory is applied to comprehend the precipitation of excess Te and a formation of Te precipitates both in pure lattice and on extended defects is taken into account. Numerical simulations demonstrate significant effect of the cooling process on the room temperature precipitate magnitude and density. Theoretical results are compared with high temperature in-situ measurements of Te-rich CdTe:In. It is shown that the cooling rate significantly affects the temperature dependence of the conductivity. Characteristic temperatures, at which excess Te precipitates, are identified as apparent bows in measured dependencies. Optimum annealing-cooling treatment to obtain detector grade CdTe is suggested.

Evolution of defect structures during cold rolling of ultrafine-grained Cu and Cu–Zn alloys: Influence of stacking fault energy

Samples of pure Cu, bronze (Cu–10 wt.% Zn) and brass (Cu–30 wt.% Zn) with stacking fault energies (SFE) of 78, 35, and 14 mJ/m 2, respectively, were processed by high-pressure torsion (HPT) and by a combination of HPT followed by cold-rolling (CR). X-ray diffraction measurements indicate that a decrease in SFE leads both to a decrease in crystallite size and to increases in microstrain, dislocation and twin densities for the HPT and HPT + CR processed ultrafine-grained (UFG) samples. Compared with processing by HPT, subsequent processing by CR refines the crystallite size of all samples, increases the twin densities of UFG bronze and brass, and increases the dislocation density in UFG bronze. It also decreases the dislocation density in UFG brass and leads to an unchanged dislocation density in UFG copper. The results suggest there may be an optimum stacking fault energy for dislocation accumulation in UFG Cu–Zn alloys and this has important implications in the production of materials having reasonable strain hardening and good tensile ductility.

Positron annihilation study of hydrogen trapping at open-volume defects: Comparison of nanocrystalline and epitaxial Nb thin films

H interaction with defects in thin Nb films was investigated in this work. Thin Nb films were prepared by the cold cathode beam sputtering. First, microstructure of the as deposited films was characterized. The films sputtered at room temperature exhibit nanocrystalline grains, while those sputtered at high temperature ( T = 850 °C) are epitaxial. Subsequently, the films were step-by-step electrochemically charged with H. Development of microstructure and evolution of defect structure with increasing H concentration was investigated by slow positron implantation spectroscopy combined with X-ray diffraction. It was found that H is trapped at open-volume defects in the thin films of both kinds. The nanocrystalline films exhibit significantly extended H solubility in the α-phase. Formation of the hydride-phase (Nb-H) at higher H concentrations leads to introduction of new defects. These are most probably dislocation loops that are emitted by growing hydride-phase particles.

Effect of long anneals on the densities of threading dislocations in GaN films grown by metal-organic chemical vapor deposition

Effect of long anneals on densities of different types of threading dislocations (TDs) in GaN films grown onto sapphire substrate by metal-organic chemical vapor deposition was investigated by high-resolution X-ray diffraction. The results showed that the densities of both types of TDs changed obviously but oppositely, and residual stress in the GaN films was relaxed by generating edge-type TDs instead of screw-type TDs. The results obtained from chemical etching experiments and grazing-incidence X-ray diffraction (GIXRD) also supported the proposed defect structure evolution.

COOLING RATE EFFECTS ON DYNAMICS IN SUPERCOOLED Al2O3

The cooling rate effects in supercooled Al 2 O 3 have been investigated by Molecular Dynamics (MD) method. Simulations were done in the basic cube under periodic boundary conditions containing 3000 ions with Born–Mayer type pair potentials. The temperature of the system was decreased linearly in time as T(t)=T0–γt, where γ is the cooling rate. The cooling rate dependence of density, thermal expansion coefficient and enthalpy of the system was found. Structure of amorphous Al 2 O 3 model at the temperature of 0 K was in good agreement with Lamparter's experimental data. The cooling rate dependence of the dynamical heterogeneities in supercooled states has been studied through the comparison of the partial radial distribution functions (PRDFs) for the 10% most mobile or immobile particles with the corresponding mean PRDFs in the models. Also, cooling rate effects on the cluster size distributions of the most mobile or immobile particles have been obtained. Calculations show that the cooling rate effects on the dynamical heterogeneities are pronounced. Finally, the evolution of structural defects and cluster size distributions of the most mobile or immobile particles in the system upon cooling has been studied and presented.