Faulted Loops Research Articles

Coarsening of End-of-Range defects in ion-implanted silicon annealed in neutral and oxidizing ambients

The evolution of End-of-Range defects during post-implantation anneals has been investigated in order to elucidate the role of the surface on the self-interstitial supersaturation near the damage region. In this work we have investigated both high dose Si +-implanted and Ge +-implanted silicon samples thermally treated under N 2, N 2O and O 2 ambients. It has been found that independently of the implanted species and annealing ambient conditions the EOR damage evolves into two distinguishable categories of extended defects, namely perfect and faulted dislocation loops. The main feature observed is that a non-conservative coarsening of the two populations of defects occurs as function of time and temperature and strongly depends on the chemical state of the surface and on the defect distance from the surface.

Temperature dependence of the radiation damage microstructure in V–4Cr–4Ti neutron irradiated to low dose

Transmission electron microscopy (TEM) was performed on a V–4Cr–4Ti alloy irradiated to damage levels of 0.1–0.5 displacements per atom (dpa) at 110–505°C. A high density of small faulted dislocation loops were observed at irradiation temperatures below 275°C. These dislocation loops became unfaulted at temperatures above ∼275°C, and a high density of small Ti-rich defect clusters lying on {0 0 1} planes appeared along with the unfaulted loops at temperatures above 300°C. Based on the TEM and tensile measurements, the dislocation barrier strengths of the faulted dislocation loops and {0 0 1} defect clusters are ∼0.4–0.5 and 0.25, respectively. This indicates that both types of defects can be easily sheared by dislocations during deformation. Cleared dislocation channels were observed following tensile deformation in a specimen irradiated at 268°C.

A molecular dynamics study of high-energy displacement cascades in α-zirconium

The damage produced in α-zirconium at 100 K by displacement cascades with energy, E p, up to 20 keV has been investigated by molecular dynamics using a many-body interatomic potential. The results are compared with similar data for cascades of energy up to 10 keV in α-titanium. The production efficiency of Frenkel pairs falls to about 25% of the NRT value as E p rises above 10 keV in zirconium, and to about 30% at 10 keV in titanium. The power-law dependence of the number of Frenkel pairs, N F, on E p found previously is obeyed, i.e., N F= A( E p) m . Interstitial and vacancy clusters with sizes of the same order are created in the cascade process, and clusters containing up to 25 interstitials and 30 vacancies were formed in zirconium by 20 keV cascades. Two thirds of the SIAs are produced in clusters in zirconium at high cascade energy. Most interstitial clusters have dislocation character with perfect Burgers vectors of the form 1/3〈112̄0〉, but a few metastable clusters are formed and are persistent over the timescale of MD simulations. Collapse of the 30-vacancy cluster to a faulted loop on the prism plane was found to occur over a period of more than 100 ps. Annealing over this timescale has a stronger effect on the number and clustering of defects in cascades that are dispersed over a large region of crystal than in cascades that form a compact region of damage.

Comparative study of microdefects in dislocation-free, heavily Si doped VB GaAs by DSL etching, NIR phase contrast microscopy, TEM and X-ray diffuse scattering

Microdefects in as-grown and annealed VB GaAs (Si > 5 x 10 18 cm −3) have been studied by four methods. The nature and density of microdefects has been established by TEM: perfect or faulted interstitial type dislocation loops lying in the 110 and 111 planes have been found. Direct comparison between the results of TEM study and other methods has been performed. Photo-etching reveals microdefects in the form of microroughness. Large loops (micrometer size) in as-grown and annealed material could be clearly discerned by this method. NIR phase contrast microscopy is effective in analysing the loops which are enlarged during high temperature annealing. Diffuse X-ray scattering is suitable for qualitative assessment of the density and type of microdefects. From etching experiments a speculative argument has been obtained on the involvement of Si in the formation of interstitial type faulted loops.

A computational study of dislocation evolution and radiation-induced segregation at a grain boundary

Radiation-induced segregation (RIS) influences the chemical and mechanical properties of the structural materials of fusion and fission reactors. In this study, such RIS and dislocation sink strength evolution were investigated on the basis of computer simulation for solute segregation at and near a grain boundary. A new continuity equation model for RIS, based on the rate theory, which includes the simultaneous evolution of faulted dislocation loops and network dislocations, has been developed. A di-interstitial was assumed to be the nucleus of a dislocation loop in the present work, and we calculated dislocation evolution due to absorption of interstitials and vacancies. The dependencies of irradiation temperature, irradiation rate and dose on the RIS at a grain boundary under electron irradiation in high-voltage electron microscope (HVEM) were clarified. We also considered the effect of probe size used in TEM-EDS analysis in order to satisfy the relationship with measured compositional profiles. The present work sheds a light on the accomplishment of performing ‘combined calculation’ which deals with both RIS and various kinds of internal sink evolution.

Theoretical prediction and direct observation of dislocation-free zone formation near a grain boundary in austenitic stainless steel under electron irradiation

We have investigated the radiation-induced segregation (RIS) near a grain boundary in an Fe-Cr-Ni alloy under electron irradiation taking account of the evolution of faulted dislocation loops and network dislocations. First, we predicted, on the basis of the rate theory, the formation dislocation-free zone (DLFZ) in the vicinity (about 30 nm) of a grain boundary and then carried out direct observation of the DLFZ by in-situ irradiation with high-voltage electron microscopy. The inter-relationship between RIS and heterogeneous defect clustering near a grain boundary has been substantially clarified.

On Correlations between Extended Defects Formation and Electron Concentration Changes Caused by Annealing of GaAs:Te

Extended type defects in heavily doped GaAs:Te (n 1 x 10 19 cm-3 ) after annealing at 700°C and 1150°C were studied by transmission electron microscopy, by high resolution technique and energy-dispersive X-ray analysis. Assuming, according to the literature that these defects are enriched in impurity atoms it is suggested that this solely is not sufficient to explain changes of electrical properties during annealing. Estimated amount of atoms involved in faulted dislocation loops seems to be too small, energy-dispersive X-ray microanalysis of precipitates did not show large differences in composition with the matrix. PACS numbers: 61.72.Nn, 61.72.Vv, 72.80.Ey

Computer Simulation of Displacement Cascades in α-Zirconium

AbstractThe damage produced in α-zirconium at 100 K by displacement cascades with energy up to 20 keV has been investigated by MD simulations. In agreement with modelling of fcc and bcc metals, the defect production efficiency in zirconium is well below the NRT estimate. The number and size of clusters, both vacancy and interstitial, are increased by increasing PKA energy, and clusters containing up to 25 interstitials and 30 vacancies were formed by 20 keV cascades. Most interstitial clusters have dislocation character with perfect Burgers vectors of the form 1/3<1120>, but a few metastable clusters are formed and are persistent over the timescale of MD simulations. Collapse of the 30-vacancy cluster to a faulted loop on the prism plane was found to occur over a period of more than 100 ps

Temperature Dependence of Yield Strength and Elongation of the Nickel-base Superalloy IN 738 LC and the Corresponding Microstructural Evolution

Tensile test data were obtained in the temperature range 20 C through 950 C with strain rates between 10{sup -2} and 10{sup -6} s{sup -1}. Extensive TEM investigations were performed after 0.2 % plastic strain and after fracture. Furthermore the fracture surfaces were studied in the SEM. Like in other superalloys, the yield strength decreased slightly between room temperature and 450 C. At higher temperatures it increased until it reached a maximum at about 750 C. Beyond 750 C a sharp decrease of strength was observed. These three extrema and the degradation of strength at high temperatures could be attributed to characteristic deformation mechanisms. With increasing temperature the following sequence of features was observed: faulted loops, anti-phase boundary-(APB)-coupled dislocation pairs, stacking faults bound by partial dislocations, and Orowan loops between {gamma}` precipitates. In a similar way tensile elongation exhibited a temperature dependence. In respect to fracture behaviour especially two aspects are important: (i) primary carbides crack after some few percent of plastic deformation and (ii) in principle every change in deformation mechanism was accompanied by inhomogeneous slip distribution. Hence poor ductility was caused by localized deformation in single slip bands leading to glide plane decohesion fracture. (orig.)

Impurity Atoms Responsible for the Void Formation in Quenched Pure Aluminum

To remove dissolved residual gas atoms, pure aluminum of 99.9995% nominal purity was melted on an alumina boat (specimen A) and a graphite boat (specimen B) in a high vacuum of 5×10−6 Pa. Both of the specimens were rolled down to 0.1 mm thickness and quenched from a high temperature in helium gas. Only faulted loops were observed in the quenched specimen A while both faulted loops and voids were observed in the quenched specimen B. When the specimen A was quenched in hydrogen gas, a large number of voids were formed. The results suggest that both hydrogen and carbon atoms are responsible for the formation of voids in quenched aluminum. To confirm the role of carbon atom, specimen C was prepared by melting pure aluminum in a vacuum at first on an alumina boat and subsequently melted on a graphite boat. In the quenched specimen C, both faulted loops and voids were observed as much as those in the quenched specimen B. Atomistics of the formation of faulted loops and voids in aluminum studied by computer simulation is described.

Dislocation loop and cavity formation under He-ion irradiation in a Ti-rich TiAl intermetallic compound

Damage structure in a Ti-rich TiAl intermetallic compound has been investigated by transmission electron microscopy after He-ion irradiations up to 3 × 10 21 ions/m 2 at 623 and 773 K. The irradiations resulted in loop-shaped or dot clusters in the γ-TiAl and α 2- Ti 3 Al grains. The cluster density in TiAl was over one order of magnitude lower than that in Ti 3Al. The clusters in TiAl irradiated to 3 × 10 21 ions/m 2 at 773 K were identified as interstitial-type faulted loops lying on {111} planes with the Burgers vector direction of 〈111〉. Cavities were created along the grain boundaries as well as in the matrix in TiAl at 773 K. The EELS analysis indicated that the cavity formation was associated with injected He atoms.

Radiation-induced segregation in FFTF-irradiated austenitic stainless steels

Analytical electron microscopy has been applied to radiation-induced segragation (RIS) to defefct sinks in two austenitic stainless steels irradiated in the Fast Flux Test Facility to ∼15 dpa at 520°C. RIS to faulted dislocation loops has been shown to be strongly dependent on loop size. Several mechanisms for this effect are discussed, including increased vacancy-driven RIS for large dislocation loop sizes. The spatial distribution of solutes near large dislocation loops is consistent with dominance of the inverse Kirkendall effect and not with an interstitial mechanism of RIS. In a manganese-stabilized austenite (EP838), manganese is the species strongly depleted at boundaries, resulting in little chromium depletion relative to that in USPCA. Therefore, EP838 should exhibit greater resistance to radiation-induced sensitization. Manganese depletion at grain boundaries in the EP838 may induce a loss of austenite stability, but that effect will be offset in part by an enrichment of nickel.

Characterization of extended defects in highly Te-doped 〈111〉 GaSb single crystals grown by the Czochralski technique

For the first time a detailed description of grown-in extended defects in highly Te-doped 〈111〉 CZ-grown GaSb single crystals is given. The nature and density of the defects is correlated with the local tellurium concentration. We observed prismatic dislocation loops with {110} and {111} habit planes (perfect or faulted loops) as the predominant defects, extended dislocation clusters consisting of climbed sessile perfect dislocations, and larger, often multi-layered planar defects with displacement fringe contrast (probably Ga 2Te 3 precipitates) lying in {111} lattice planes. While the prismatic dislocation loops are already generated at a tellurium concentration of 3×10 18 cm -3, dislocation clusters and large planar defects first occur at a tellurium concentration of about 1×10 19 cm -3. The prismatic loops and the planar defects are exclusively of extrinsic (interstitial) type. Altogether one can establish a far-reaching similarity with the defect population found in highly Te-doped GaAs. The mechanisms which lead to the formation of defects are discussed on the basis of concepts of the known behaviour of point defects in GaAs.

High-temperature ion implantation in silicon

The properties of silicon implanted with various doping and inert gas ions at target temperatures Ti from 600 to 1100°C are reviewed. Ion mass and T1 are the main factors governing the behaviour of lattice defects and impurity atoms. The observed residual damage is the result of decomposition of the supersaturated solid solution of point defects. Transition from the homogeneous to heterogeneous nucleation of the stable damage (extrinsic dislocation loops) occurs with the increase of the ion mass due to increasing probability to form loop nuclei within more dense displacement spikes. Heterogeneous nucleation leads to the formation of a large number of stable faulted loops in the region of maximal elastic losses. Homogeneous nucleation results in the growth of coarse perfect dislocation loops penetrating deeper than 1 μm. Their formation may be prevented by heating the target over 900°C. Dislocation loops are suggested to be the reason for incomplete electrical activity of the implanted dopants both for the “hot” and the “cold” implantation followed by annealing, provided the temperatures in both cases are higher than 800°C. The excess of mobile point defects causes an impurity diffusion enhancement which becomes observable at Ti > 700°C. Retrograde temperature dependence of the dopant diffusivity and the occurence of up-hill diffusion are noted at very high T1.

In situ deformation of proton-irradiated metals

For the understanding of the deformation mechanism in irradiated metals, “in situ” testing in a high-voltage electron microscope (HVEM) is a very useful method. In the present study, a special technique for the irradiation and thin-foil preparation has been developed in order to deform a micro-specimen dynamically in HVEM. This technique has enabled us to study the formation mechanism of defect-free zones and the interaction between glide dislocations and dislocation loops. New findings obtained by the in situ deformation are reported for Mo and Fe-Cr-Ni single crystals. Discussions are mainly concerned with the elimination of dislocation loops, 1 2 a〈111〉 perfect loops in Mo and 1 3 a〈111〉 faulted loops of extrinsic type in Fe-Cr-Ni.

In situ observation of defect cluster formation in metal and alloy under electron/ion dual-beam irradiation

To investigate the effect of gaseous elements on defect cluster formation, such as dislocation loops and voids, an in situ observation of dual-beam irradiation has been carried out for Ni and a commercial stainless steel by using a 1300 kV HVEM and 300 kV ion-accelerator facility. During the dual-beam irradiation of Ni, small cavities were nucleated only at the compression side of interstitial-type faulted loops. The growth of the loops was enhanced at the dual-beam condition compared to simple electron irradiation, which means that He has a strong binding energy to vacancies and moves to the compression side with them. In commercial 316 steel the void nucleation was strongly enhanced by the dual-beam irradiation up to high doses, and the bimodal distribution in void size was developed at higher He concentrations, more than 20 at ppm He/dpa. This means that the enhancement of the nucleation is complicated, and He stabilizes the void nucleus, but the growth could be decided by the sink nature. From these results, the usefulness of the HVEM/ion-accelerator facility is confirmed.

Production and migration of interstitials in deformed metals

An investigation was made of defect production during plastic deformation. Prior to cold-rolling, samples of Au were damaged and preannealed at temperatures in order to maximize site fractions of five different In-vacancy defect complexes, such as In-1V and In-2V. Evolutions of site fractions were monitored by perturbed γ-γ angular correlation spectroscopy after additional strains at 77 K. Observed transformations between sites are consistent with defect-antidefect annihilation reactions between the small complexes and mobile interstitials, and indicate a large production of interstitials during deformation. An extended defect complex attributed to a planar faulted loop became glissile under small strain.

Innovations in Testing Methodology for Fusion Reactor Materials Development

The successful development of fusion power critically depends on structural materials which can maintain their integrity and dimensional stability in an extremely hostile service environment involving high temperatures and heat fluxes, corrosive media, high stresses, and intense fluxes of neutrons. Neutron irradiation results in a variety of mechanical, physical, and chemical property changes that are collectively referred to as radiation effects. The primary sources of damage are neutron-induced atomic displacements and transmutation products.High-energy neutrons produce primary recoil atoms (PKAs) with energies ranging from less than 1 keV to more than 100 keV. The energetic PKAs create a cascade of additional displacements. About 10% of the displaced atoms survive cascade recombination as isolated vacancies and self-interstitials or small clusters of these defects. The mobile defects migrate to pre-existing sinks such as grain boundaries and dislocations, or cluster to form a variety of extended defects such as voids (vacancies) or faulted dislocation loops (interstitials).

Confirmation of vacancy-type stacking fault tetrahedra in quenched, deformed and irradiated face-centred cubic metals

Abstract An efficient and reliable method for characterizing the nature of stacking fault tetrahedra in f.c.c. metals using electron microscope diffraction image contrasts is presented. By using the 220 reflection, and thereby eliminating the contrast from overlapping stacking faults, one can differentiate between intrinsic-type stacking fault tetrahedra due to lattice vacancies and extrinsic type due to interstitial atoms. The validity of the method is examined by the observation of vacancy-type stacking fault tetrahedra in quenched metals, and by the observation of interstitial-type faulted dislocation loops. Stacking fault tetrahedra introduced by plastic deformation, electron irradiation, neutron irradiation and ion irradiation are all confirmed to be vacancy type. It was found that interstitial-type stacking fault tetrahedra do not exist.

Observations of structural defects in web-grown silicon ribbon

The dislocation configuration in web-grown silicon ribbon was investigated using chemical etching techniques. The presence of dislocation loops on twin planes is observed and accounted for by self-interstitial condensation. The interstitials were either quenched in, due to the rapid cooling of the ribbon from the solidification temperature, or produced by oxide precipitation on the twin plane. Very large faulted loops of mm size were also observed.