Evolution Of Dislocation Loops Research Articles

Evolution of dislocation loops in austenitic stainless steels implanted with high concentration of hydrogen

It has been found that under certain conditions, hydrogen retention would be strongly enhanced in irradiated austenitic stainless steels. To investigate the effect of the retained hydrogen on the defect microstructure, AL-6XN stainless steel specimens were irradiated with low energy (100 keV) H2+ so that high concentration of hydrogen was injected into the specimens while considerable displacement damage dose (up to 7 dpa) was also achieved. Irradiation induced dislocation loops and voids were characterised by transmission electron microscopy. For specimens irradiated to 7 dpa at 290 °C, dislocation loops with high number density were found and the void swelling was observed. At 380 °C, most of dislocation loops were unfaulted and tangled at 7 dpa, and the void swellings were observed at 5 dpa and above. Combining the data from low dose in previous work to high dose, four stages of dislocation loops evolution with hydrogen retention were suggested. Finally, molecular dynamics simulation was made to elucidate the division of large dislocation loops under irradiation.

Accumulation of dislocation loops in the α phase of Zr Excel alloy under heavy ion irradiation

In-situ heavy ion irradiations were performed on the high Sn content Zr alloy ‘Excel’, measuring <a> type dislocation loop accumulation up to irradiation damage doses of 10 dpa at a range of temperatures. The high content of Sn, which diffuses slowly, and the thin foil geometry of the sample provide a unique opportunity to study an extreme case where displacement cascades dominate the loop formation and evolution. The dynamic observation of dislocation loop evolution under irradiation at 200 °C reveals that <a> type dislocation loops can form at very low dose (0.0025 dpa). The size of the dislocation loops increases slightly with irradiation damage dose. The mechanism controlling loop growth in this study is different from that in neutron irradiation; in this study, larger dislocation loops can condense directly from the interaction of displacement cascades and the high concentration of point defects in the matrix. The size of the dislocation loop is dependent on the point defect concentration in the matrix. A negative correlation between the irradiation temperature and the dislocation loop size was observed. A comparison between cascade dominated loop evolution (this study), diffusion dominated loop evolution (electron irradiation) and neutron irradiation suggests that heavy ion irradiation alone may not be enough to accurately reproduce neutron irradiation induced loop structures. An alternative method is proposed in this paper. The effects of Sn on the displacement cascades, defect yield, and the diffusion behavior of point defects are established.

Evolution of nanoscale interstitial dislocation loops under coupling effect of stress and temperature

The properties of nano-scale interstitial dislocation loops under the coupling effect of stress and temperature are studied using atomistic simulation methods and experiments. The decomposition of a loop by the emission of smaller loops is identified as one of the major mechanisms to release the localized stress induced by the coupling effect, which is validated by the TEM observations. The classical conservation law of Burgers vector cannot be applied during such decomposition process. The dislocation network is formed from the decomposed loops, which may initiate the irradiation creep much earlier than expected through the mechanism of climb-controlled glide of dislocations.

Idealized vs. Realistic Microstructures: An Atomistic Simulation Case Study on γ/γ' Microstructures.

Single-crystal Ni-base superalloys, consisting of a two-phase γ/ microstructure, retain high strengths at elevated temperatures and are key materials for high temperature applications, like, e.g., turbine blades of aircraft engines. The lattice misfit between the γ and phases results in internal stresses, which significantly influence the deformation and creep behavior of the material. Large-scale atomistic simulations that are often used to enhance our understanding of the deformation mechanisms in such materials must accurately account for such misfit stresses. In this work, we compare the internal stresses in both idealized and experimentally-informed, i.e., more realistic, γ/ microstructures. The idealized samples are generated by assuming, as is frequently done, a periodic arrangement of cube-shaped particles with planar γ/ interfaces. The experimentally-informed samples are generated from two different sources to produce three different samples—the scanning electron microscopy micrograph-informed quasi-2D atomistic sample and atom probe tomography-informed stoichiometric and non-stoichiometric atomistic samples. Additionally, we compare the stress state of an idealized embedded cube microstructure with finite element simulations incorporating 3D periodic boundary conditions. Subsequently, we study the influence of the resulting stress state on the evolution of dislocation loops in the different samples. The results show that the stresses in the atomistic and finite element simulations are almost identical. Furthermore, quasi-2D boundary conditions lead to a significantly different stress state and, consequently, different evolution of the dislocation loop, when compared to samples with fully 3D boundary conditions.

Dislocation Dynamics Simulation of Interaction between Prismatic Dislocation Loops and Voids in Irradiated Thin Films

At the microscopic scale fast neutron irradiation brings about a high density of small point defect clusters in the form of dislocation loops and voids. And such radiation damage is of primary importance for materials used in nuclear energy production. In the present investigation emphasis is placed on the understanding of the mechanisms involved in the evolution of prismatic dislocation loops by glide in the presence of external free surfaces and those of the voids and in the interaction between dislocation loops and voids within irradiated thin films, so as to simulate in situ Transmission Electron Microscopy (TEM) images of dislocations, which is an indispensable tool for extracting information on radiation damage. By employing 3D dislocation dynamics based on isotropic elacticity and principle of superposition, the calculation results show that the image force is determined by the distance of the dislocation loop from the external and void surfaces and scales with the film thickness; the dislocation glide force is determined by the image stress as well as the loop–loop interaction stress which is in turn governed by the loop spacing. It is also shown that the presence of voids in the thin films has a strong influence on the behaviours of prismatic dislocations.

Evolution of Dislocation Loops in AL-6XN Stainless Steels Irradiated by Hydrogen Ions

AL-6XN stainless steels, one of the candidate structure materials for supercritical water-cooled reactor, were irradiated from 0.5 to 5 dpa using 100 keV H2 + ions at 290 and 380 °C. Microstructures were characterized by transmission electron microscopy (TEM). Dislocation loops were the dominant radiation-induced defects. All the dislocation loops had 1/3 <111> type Burgers vector. Number density and size of the loops have been measured. Nucleation and evolution of dislocation loops were also investigated. Voids were observed only in the condition of 5 dpa at 380 °C. Different evolution mechanisms of the radiation-induced dislocation loops were discussed. Effects of hydrogen and elevated temperature on the microstructural evolution were also investigated. Besides, the formed voids have a further effect on the evolution of dislocation loops.

Evolution of dislocation loops in He ion irradiated nickel under different temperature

The effect of temperature on the evolution of loops in nickel was investigated under 30 keV He+ irradiation. The size, Burgers vector, and nature of loops were analyzed by a transmission electron microscope (TEM). In addition, the Weibull statistical analysis was introduced to analyze the size distributions of the loops. The TEM results indicate that the sizes of loops increase with increasing irradiation temperature, where they are mainly in the range of 10–30, 20–50, and 30–80 nm at 300, 400, and 500 °C, respectively. In the irradiation temperature range of 300–500 °C, the size distributions of loops fit Weibull distribution very well, which suggests that the loops tend to grow larger and/or be merged. In addition, almost all the loops formed under He+ irradiation are the interstitial loops with Burgers vectors b = a2⟨110⟩, which is ascribed to the assistance of the He atom on the formation process of loops.

Evolution of dislocation loops in annealed iron pre-irradiated with hydrogen ion in high-voltage electron microscope

The nature of dislocation loops in the annealed pure iron pre-irradiated with hydrogen ion at room temperature was studied by the evolution of loops under electron irradiation in high-voltage electron microscope (HVEM). Only interstitial-type loops were observed when annealed and electron irradiated at 350 °C but only vacancy-type loops formed at temperature higher than 500 °C. When annealed at temperatures from 450 °C to 490 °C, both interstitial-type and vacancy-type loops formed simultaneously in the specimen and vacancy-type loops accounted for an increasing fraction with increasing annealing temperature, from 28.5% at 450 °C to 55% at 490 °C. The bias factor of interstitial-type and vacancy-type loops was compared based on the growth rate or shrinkage rate of the dislocation loops. The bias factor of interstitial-type loops was demonstrated to be higher than that of vacancy-type loops at all three annealing temperatures.

Minimal Variation of Defect Structure Due to the Order of Room Temperature Hydrogen Isotope Implantation and Self-Ion Irradiation in Nickel

ABSTRACTThe formation and stability of radiation-induced defects in structural materials in reactor environments significantly effects their integrity and performance. Hydrogen, which may be present in significant quantities in future reactors, may play an important role in defect evolution. To characterize the effect of hydrogen on cascade damage evolution, in-situ TEM self-ion irradiation and deuterium implantation was performed, both sequentially and concurrently, on nickel. This paper presents preliminary results characterizing dislocation loop formation and evolution during room temperature deuterium implantation and self-ion irradiation and the consequence of the sequence of irradiation. Hydrogen isotope implantation at room temperature appears to have little or no effect on the final dislocation loop structures that result from self-ion irradiation, regardless of the sequence of irradiation. Tilting experiments emphasize the importance of precise two-beam conditions for characterizing defect size and structure.

Temperature dependent evolution of dislocation loops in YSZ under high energy electron irradiation

Temperature dependent evolution of dislocation loops in YSZ under high energy electron irradiation

High-energy synchrotron study of in-pile-irradiated U–Mo fuels

Here synchrotron scattering analysis results on U–7wt.%Mo fuel specimens irradiated in the Advanced Test Reactor to three burnup levels (3.0, 5.2, and 6.3×1021fission/cm3) are reported. Mature fission gas bubble superlattice was observed to form at intermediate burnup. The superlattice constant was determined to be 11.7 and 12.0nm by wide-angle and small-angle scattering respectively. Grain sub-division takes place throughout the irradiation and causes the collapse of the superlattice at high burnup. The bubble superlattice expands the U–Mo lattice and acts as strong sink for radiation-induced defects. The evolution of dislocation loops was, therefore, suppressed until the bubble superlattice collapsed.

Microchemical and microstructural evolution of AISI 304 stainless steel irradiated in EBR-II at PWR-relevant dpa rates

AISI 304 stainless steel was irradiated at 416 °C and 450 °C at a 4.4 × 10−9 and 3.05 × 10−7 dpa/s to ∼0.4 and ∼28 dpa, respectively, in the reflector of the EBR-II fast reactor. Both unirradiated and irradiated conditions were examined using standard and scanning transmission electron microscopy, energy dispersive spectroscopy, and atom probe tomography on very small specimens produced by focused ion beam milling. These results are compared with previous electron microscopy examination of 3 mm disks from essentially the same material. By comparing a very low dose specimen with a much higher dose specimen, both derived from a single reactor assembly, it has been demonstrated that the coupled microstructural and microchemical evolution of dislocation loops and other sinks begins very early, with elemental segregation producing at these sinks what appears to be measurable precursors to fully formed precipitates found at higher doses. The nature of these sinks and their possible precursors are examined in detail.

Rate theory modeling of dislocation loops in RAFM steel under helium ion irradiation and comparison with experiments

A modified model based on kinetic rate theory is presented to simulate the dose dependence of defects nucleation and growth in reduced-activation ferritic/martensitic (RAFM) steel under helium ion irradiation by considering the motion of dislocation loops. The modeling results are compared with the transmission electron microscopy (TEM) experiments. The TEM analysis shows that the mean size of dislocation loops increases with the increasing atomic displacement damage dose, while the density of the observed loops exhibits minor change. To investigate the evolution of dislocation loops, we compare two cases incorporating mobile and immobile interstitial clusters in our modified model, respectively. The calculated results reveal a good agreement with experiments in the case of considering mobile interstitial clusters. It is included the motion of interstitial clusters plays an important role in the evolution of dislocation loops in RAFM steel.

Influences of strain rate on the low cycle fatigue behavior of gravity casting Al alloys

The strain-controlled low cycle fatigue properties were evaluated on specimens of 333 aluminum alloy at different strain rates. The material exhibited initial cyclic hardening followed by cyclic stabilization at the strain rates of 0.15min−1, 0.20min−1, 0.30min−1 and 0.32min−1, while the material showed continuous cyclic hardening at the strain rate of 0.25min−1. The observed phenomena in hysteresis loops were in accord with the cyclic hardening and stability characteristics. At the strain rates of 0.15min−1 and 0.32min−1, the cyclic hardening characteristics observed were mainly due to the existence of dislocation network structures, as well as the accumulation of dislocations around some obstacles. The cyclic stabilization observed in both samples was correlated closely with the cross-slip during cycling at the strain rates of 0.15min−1 and 0.32min−1. At the strain rate of 0.25min−1, the continued cyclic hardening observed in the specimen was bound up with the evolution of dislocation loops. The fracture surface for the alloy exhibited some dimples and tear ridges at the strain rates of 0.15min−1 and 0.25min−1, whereas some fatigue striations were observed at the strain rate of 0.32min−1. The damage process of the 333 aluminum alloy composed of several mixed events: α-Fe/silicon particle cracking, short crack initiation and propagation, and local linkage of short cracks.

Comparative study of defect evolution in carbon implanted strained SiGe and SiSn layers

AbstractBy combining secondary ion‐mass spectrometry, transmission‐electron microscopy (TEM) and Rutherford‐backscattering spectrometry we show that the redistribution of implanted carbon atoms around epitaxially strained Si/SiGe layers results in their accumulation on the Si side and depletion on the SiGe side. On the contrary, uphill diffusion of carbon into SiSn layers takes place in the case of Si/SiSn structures. The TEM study demonstrates formation of dislocation loops, stacking faults and interstitial clusters in the Si/SiGe layers, but elimination of interstitial dislocation loops and suppression of tin precipitates in the Si/SiSn layers. We deduced different evolution of dislocation loops and a precipitate is due to dopant‐defect complexes. The complex formation is enhanced by separation of implanted point defects in strain‐fields of Si/SiSn and Si/SiGe layers. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Evolution of dislocation loops in iron under irradiation: The impact of carbon

The impact of carbon content on the evolution of dislocation loops in iron–carbon was studied by object kinetic Monte Carlo, explicitly introducing carbon atoms and their atomic features for the first time. We demonstrate that the saturated loop density strongly depends on carbon content and temperature, in good agreement with in situ irradiation microscopy studies. The physical processes responsible for the accumulation and long-range migration of the loops are rationalized with implications for nanostructural evolution in commercial steels upon low-dose-rate neutron irradiation.

Dislocation loop formation under various irradiations of laser and/or electron beams

A high-voltage electron microscope (HVEM) equipped with a laser head (laser-HVEM) was developed at Hokkaido University and is used to investigate the surface modification of semiconductors and the behaviour of lattice point defects in metals under various irradiations of laser (photon) and/or electron beams. In the present study, the annealing effect of pulsed laser irradiation on a face-centred-cubic metal was experimentally investigated and theoretically calculated. The systematic assessment of dislocation loop evolution under laser-electron sequential irradiation and laser-electron dual-beam irradiation was performed. Our results show that the rapid heating and quenching that occurred during pulsed laser irradiation caused vacancies to be introduced at the surface of the specimen and to diffuse to the interior, which led to the formation and growth of vacancy-type (V-type) dislocation loops. These loops gradually shrank and finally disappeared during the subsequent electron irradiation of the sample. During laser-electron simultaneous dual-beam irradiation, the type of loop formed, interstitial-type (I-type) or V-type loops, is determined by the relative intensities of the laser beam and electron beam, which indicates that the loop type can be controlled by changing the relative intensities of the beams. Accordingly, models of dislocation loop formation during various irradiations were proposed. The newly developed laser-HVEM instrument is expected to be employed in the exploration of mechanisms in material science, as well as in other scientific fields.

Multi-Scale Modeling of Interstitial Dislocation Loop Growth in Irradiated Materials

ABSTRACTIn order to reduce the inherent uncertainty in kinetic theory models and promote their transition to become predictive methodologies, a multi-scale modeling approach is proposed and demonstrated in this work. KiValues of key materials properties such as point defect (vacancy and interstitial) migration enthalpies, as well as kinetic factors, such as dimer formation and defect recombination coefficients and self-interstitial atom – interstitial loop reaction rates, were obtained by ab initio/molecular dynamics calculations. A rate theory model was used to interpret the evolution of dislocation loops in irradiated molybdenum. Calculations of the dose dependence of average loop diameter were performed and compared to experimental measurements obtained from irradiations with high-energy electrons. The comparison demonstrates reasonable agreement between model-predicted and experiment-measured data.

Spreading and retraction dynamics of a dye doped smectic liquid crystal domain at the air–water interface

We have studied the spreading and retraction dynamics of a dye doped smectic multilayer domain in the collapsed state of a Langmuir monolayer at the air–water interface. We find that the domain undergoes shearing when excited with an appropriate wavelength in the epifluorescence setting of the microscope. The shearing leads to displacement of the stack of layers and results in asymmetric spreading of the domain. Eventually, due to line tension, the domain transforms into a circular shape. Here, the domain area was about twice that of the initial area. Under reflection setting of the microscope, with white light, we observe that the domain retracts to a smaller area. During the course of retraction, we find successive generation and evolution of edge dislocation loops leading to thickening of the domain. Our analysis on the variation of the normalized area of the domain with time yields different characteristic time constants for spreading and retraction. The spreading and retraction of the domain can be understood by invoking changes in interfacial tension due to photobleached surface active products and the depletion of photobleached products, respectively.

Irradiation hardening of reactor pressure vessel steels due to the dislocation loop evolution

The irradiation hardening of reactor pressure vessel steels due to the formation of dislocation loops is analyzed. The analysis is based on the original model for the nucleation and subsequent evolution of dislocation loops in irradiated materials. The loop formation in displacement cascades is taken into account, along with the homogeneous clustering of point defects. The loop evolution is shown to contribute mainly to the athermal component of the yield stress, which is determined by interaction of gliding dislocations with strong barriers. Irradiation-induced hardening is evaluated as a function of irradiation dose and temperature, dose rate, material parameters and initial microstructure. The model results are compared with experimental data for neutron irradiated pressure vessel steels of various grades and with empirical low power expressions of the yield stress increase with increasing irradiation dose.