Faulted Dislocation Loops Research Articles

Phase stability of precipitate-hardened CoCrFeNiTi high entropy alloys fabricated via laser-directed energy deposition under high-temperature irradiation

This study investigated the irradiation response of precipitate-hardened CoCrFeNiTi high entropy alloys (HEAs) fabricated by laser-directed energy deposition (LDED). These HEAs were in-situ irradiated with 1 MeV krypton ions (Kr2+) at 400 °C and 600 °C up to 10 displacements per atom (dpa) at the IVEM-Tandem Facility at Argonne National Laboratory. Transmission electron microscopy (TEM) characterized precipitate stability and defect evolution under irradiation. At 400 °C and 10 dpa, precipitates showed high phase stability, while irradiation at 600 °C caused intense dissolution mainly due to its annealing effect. A high density of faulted dislocation loops formed at 400 °C. The annihilation rate of dislocation loops seemed higher at 600 °C. Increasing the dose increased the number and size of voids in the face-centered cubic (FCC) matrix.

Microstructural evolution of compositionally complex solid-solution alloys under in-situ dual-beam irradiation

This work attempts to link the microstructural evolution of single-phase compositionally complex alloy (CCA) compositions under dual-beam irradiation to their Mn-content via the stacking-fault energy (SFE) and vacancy migration energies. Two alloys, Cr18Fe27Mn27Ni28 and Cr15Fe35Mn15Ni35, along with less compositionally complex pure Ni and Fe56Ni44 binary were irradiated at 500 and 600 °C under dual-beam 1 MeV Kr2+ and 16 keV He+ heavy-ions up to 7 displacements per atom (dpa) with a He/dpa ratio of 0.75 %/dpa using in situ transmission electron microscopy (TEM). Due to the bubble-stabilizing effect of implanted He, bubbles were observed in all irradiations, and populations of faulted interstitial loops were characterized in Cr18Fe27Mn27Ni28 and Cr15Fe35Mn15Ni35. A reduction in swelling was observed in the two CCAs compared to pure Ni and Fe56Ni44. Although swelling increased from 500 to 600 °C in Fe56Ni44, Cr15Fe35Mn15Ni35 and Cr18Fe27Mn27Ni28 both swelled slightly more at 500 °C. This was attributed to the difference in vacancy mobility, stronger pinning effect of vacancies on He, and the sink strength of faulted dislocation loops. Faulted interstitial loops nucleated with a higher number density and dislocation line density in Cr15Fe35Mn15Ni35 at both temperatures, and at 600 °C in both materials. The differences in faulted loop population and temperature effect on swelling are correlated to the Mn-content and the measured SFE (20.2 ± 6.7 mJ/m2 for Cr18Fe27Mn27Ni28 and 9.2 ± 3.4 mJ/m2 for Cr15Fe35Mn15Ni35).

Compact A15 Frank-Kasper nano-phases at the origin of dislocation loops in face-centred cubic metals

It is generally considered that the elementary building blocks of defects in face-centred cubic (fcc) metals, e.g., interstitial dumbbells, coalesce directly into ever larger 2D dislocation loops, implying a continuous coarsening process. Here, we reveal that, prior to the formation of dislocation loops, interstitial atoms in fcc metals cluster into compact 3D inclusions of A15 Frank-Kasper phase. After reaching the critical size, A15 nano-phase inclusions act as a source of prismatic or faulted dislocation loops, dependent on the energy landscape of the host material. Using cutting-edge atomistic simulations we demonstrate this scenario in Al, Cu, and Ni. Our results explain the enigmatic 3D cluster structures observed in experiments combining diffuse X-ray scattering and resistivity recovery. Formation of compact nano-phase inclusions in fcc structure, along with previous observations in bcc structure, suggests that the fundamental mechanisms of interstitial defect formation are more complex than historically assumed and require a general revision. Interstitial-mediated formation of compact 3D precipitates can be a generic phenomenon, which should be further explored in systems with different crystallographic lattices.

Microstructure and defect evolution in oxygen ion-irradiated pure nickel – Insights from experimental probes and molecular dynamics simulations

Pure nickel samples were irradiated to various doses by 160 MeV oxygen ions and the change in microstructure (domain size, microstrain, dislocation density etc.) were investigated with the help of X-ray diffraction (XRD) data from synchrotron source and Transmission electron microscopy (TEM). The coherent domain size decreased, whereas the microstrain and dislocation density increased in all irradiated samples as compared to the unirradiated one. However, these parameters showed saturation with increasing dose. The analysis reveals that there is an increase in the stacking fault probability with irradiation. TEM study confirmed the presence of stacking fault tetrahedra and dislocation loops in the irradiated samples. Microhardness measurements show an increase in the hardness with irradiation due to formation of defect clusters and dislocations. The effect of irradiation on the microstructure of the pure Ni has been simulated by molecular dynamics using overlapping simulation cascades of PKA generated by oxygen ion.

Enhanced helium ion irradiation tolerance in a Fe-Co-Ni-Cr-Al-Ti high-entropy alloy with L12 nanoparticles

• Helium bubbles and large stacking faulted loops are observed as the dominant structural damage in the He ion irradiated HEA reinforced by L1 2 nanoparticles. • The L1 2 nanoparticles provide numerous interfaces for He entrapment and damage elimination, which suppresses He bubble growth. • A correlative TEM/APT characterization reveals that the RIS around He bubbles is dominated by the inverse Kirkendall mechanism. • Irradiation-induced dissolution and re-precipitation of the L1 2 nanoparticles can retain the main microstructure of the L1 2 -strengthened HEA and provide a sustainable irradiation resistance. L1 2 -strengthened high entropy alloys (HEAs) with excellent room and high-temperature mechanical properties have been proposed as promising candidates as structural materials for advanced nuclear systems. However, knowledge about their radiation response is fairly limited. In the present work, a novel HEA with a high density of L1 2 nanoparticles was irradiated with He ion at 500°C. Transmission electron microscope (TEM) and atom probe tomography (APT) were employed to study the evolution of microstructural stability and radiation-induced segregation. Similar to the single-phase FeCoNiCr HEA, the main microstructural features were numerous large faulted dislocation loops and helium bubbles. While the irradiation resistance of the present L1 2 -strengthened HEA is much improved in terms of reduced bubble size, which could be attributed to the considerable He trapping efficiency of the coherent precipitate/matrix interface and the enhanced capability of the interface for damage elimination when the matrix channel width is narrow. APT analysis revealed that an inverse Kirkendall mechanism-dominated radiation-induced segregation (RIS) occurs around bubbles, where a significant Co enrichment and Ni depletion can be clearly observed. In addition, the competing dynamics of ballistic mixing and elemental clustering that raised from the irradiation-enhanced diffusion in a highly supersaturated matrix, along with the low precipitation nucleation barrier due to the small lattice misfit, lead to a dynamical precipitation dissolution and re-precipitation appears under irradiation. Such a promising phenomenon is expected to promote a potential self-healing effect and could in turn provide a sustainable irradiation tolerance over the operational lifetime of a reactor.

Emulation of neutron-irradiated microstructure of austenitic 21Cr32Ni model alloy using dual-ion irradiation

In this study, the capability of heavy-ion irradiation to emulate neutron irradiation was demonstrated on an austenitic 21Cr32Ni type ternary model alloy. The model alloy used in this study is chemically analogous but compositionally simpler than of alloy 800H, which is a candidate austenitic Ni alloy which has been proposed for use in Generation IV reactors.The microstructure of the 21Cr32Ni model alloy irradiated in the BOR-60 fast reactor to 17.1 dpa and 35 dpa at ∼380 °C was characterized using transmission electron microscopy (TEM). The 17.1 dpa BOR-60 irradiated microstructure was then compared with the microstructure of the same material developed under dual-ion (DI) irradiation using various He/dpa ratios between 0.1 and 16.6 appm He/dpa in the temperature range of 430 °C-500 °C. The results showed that both neutron and DI irradiation of 21Cr32Ni model alloy produced dislocations in the form of a dislocation network as well as {111}-type faulted dislocation loops, cavities, and radiation-induced Ni enrichment at radiation-induced sinks. When the dose and the He/dpa ratio were kept similar to those in neutron irradiation, DI irradiation of the 21Cr32Ni model alloy at 460 °C resulted in over-nucleation of small cavities and in a high density of faulted dislocation loops compared to those observed in the fast-neutron irradiated alloy of the same heat irradiated at ∼380 °C. The optimal condition for reproducing the neutron-irradiated microstructure was DI irradiation at 460 °C and 0.1 appm He/dpa. In that case, the faulted loop and cavity size distributions in the BOR-60 irradiated 21Cr32Ni model alloy samples closely matched with those measured in the DI irradiated 21Cr32Ni model alloy sample. The fact that the He/dpa is an order of magnitude smaller than the helium generation rate for fast neutron irradiation, stops over nucleation and allows for the development of a similar microstructure as for neutron irradiation.

Radiation-induced segregation on dislocation loops in austenitic Fe-Cr-Ni alloys

Radiation-induced segregation of alloying elements to crystallographic defects is commonly observed in irradiated austenitic stainless steels. The interaction between solutes and radiation-induced defects changes the physical distribution of solutes and thus affects the formation and growth of defects. The change of the microstructure consequently affects the mechanical properties of the material. A qualitative and quantitative understanding of the interaction between solutes and defects is desirable to better predict the service lifetime of nuclear materials. We used atom probe tomography to measure the distribution of solutes at dislocation loops in 304L stainless steel, irradiated with 2 MeV protons up to 1.5 displacements per atom at 373 and 633 K. No segregation at dislocation loops was found in samples irradiated at 373 K, whereas Ni and Si enrichment and Cr depletion were detected at dislocation loops irradiated at 633 K. The experimentally observed perfect and faulted dislocation loops in vacancy and interstitial types were reproduced by molecular dynamics (MD). A hybrid MD/Monte Carlo method was used to predict the redistribution of alloying atoms at all possible types of dislocation loops in face-centered cubic Fe-Cr-Ni alloys at the same irradiated temperatures (373 and 633 K). The simulations show that, at both temperatures, Cr clusters were formed and distributed randomly, and Ni atoms enriched or depleted at interstitial or vacancy dislocation loops, respectively. The change of solute concentration reaches the highest at the edge of the loop. Ni profiles exhibit characteristic behavior in terms of the stress field of the loops: tension inside of vacancy loops showing depletion of Ni atoms compared with compression inside of interstitial loops showing enrichment of Ni atoms. In addition, the stress field is reduced after solute redistribution. The absence of alloying segregation observed in experiments at a lower temperature (373 K) is explained by a rate theory model: Low-temperature irradiation requires significantly longer irradiation time to see the same amount of segregation as at high temperatures because of the extremely low diffusion of vacancies at low temperatures.

Dislocation Loops in Proton Irradiated Uranium-Nitrogen-Oxygen System

In this study, we investigated the type of dislocation loops formed in the proton-irradiated uranium-nitrogen-oxygen (U-N-O) system, which involves uranium mononitride (UN), uranium sesquinitride (α-U 2 N 3 ), and uranium dioxide (UO 2 ) phases. The dislocation loop formation is examined using specimens irradiated at 400°C and 710°C. Based on the detailed transmission-based electron microscopy characterization with i) the morphology-based on-zone and ii) the invisibility-criterion based two-beam condition imaging techniques, only a single type of dislocation loop in each phase is found: a/2⟨110⟩, a/2⟨111⟩, or a/3⟨111⟩ dislocation loops in UN, α-U 2 N 3 , and UO 2 phases, respectively. Molecular statics calculations for the formation energy of perfect and faulted dislocation loops in the UN phase indicate a critical loop size of ∼ 6 nm, above which perfect loops are thermodynamically favorable. This could explain the absence of faulted loops in the experimental observation of the irradiated UN phase at two temperatures. This work will enhance the understanding of irradiation induced microstructural evolution for uranium mononitride as an advanced nuclear fuel for the next-generation nuclear reactors.

In-situ TEM observation of the evolution of helium bubbles & dislocation loops and their interaction in Pd during He+ irradiation

The microstructural evolution of purity Pd under 30 keV He+ irradiation at 573 K was investigated by in-situ transmission electron microscopy. The nucleation, growth, merging, annihilation, size change, number density variation, and types of dislocation loops were analyzed under the influence of irradiation fluence and sample thickness. Both perfect dislocation loops with b = 1/2 < 110> and faulted dislocation loops with b = 1/3 < 111> were formed. However, at low irradiation fluence, most of the loops were 1/3 < 111> loops. The thickness of TEM foil obviously affected the ratio of 1/3 < 111> loop variants, the size and number density of dislocation loops, and the characteristics of bubble-loop complexes. With the increase of irradiation fluence, the size of dislocation loops increased, but loop volume number density remained almost constant until dislocation loops merged and evolved into dislocation network. There was an obvious interaction between dislocation loops and bubbles, indicating that 1/3 < 111> loop was first formed at the initial stage of irradiation, and when the loop grew to a certain size, obvious helium bubbles appeared inside its region.

Irradiation-induced segregation at dislocation loops in CoCrFeMnNi high entropy alloy

To understand the redistribution of alloying elements in high entropy alloys under irradiation, a CoCrFeMnNi alloy was irradiated with 1 MeV Kr ions at room temperature and at 500°C, and characterized with atom probe tomography and transmission electron microscopy. At 500°C, Co and Ni were enriched around the interstitial, faulted and perfect, dislocation loops resulted from the ion irradiation. In contrast, no segregation was observed at room temperature. The inverse Kirkendall effect through vacancy flux, as opposed to the interstitial binding mechanism, was the primary underlying process attributing to the observed segregation. In addition, a ring-shaped segregation pattern was observed at the faulted dislocation loops, indicating a non-equilibrium nature of the defect clustering and solute segregation process in CoCrFeMnNi under irradiation at high temperature.

In situ microstructural evolution in face-centered and body-centered cubic complex concentrated solid-solution alloys under heavy ion irradiation

This study characterizes the microstructural evolution of single-phase complex concentrated solid-solution alloy (CSA) compositions under heavy ion irradiation with the goal of evaluating mechanisms for CSA radiation tolerance in advanced fission systems. Three such alloys, Cr18Fe27Mn27Ni28, Cr15Fe35Mn15Ni35, and equimolar NbTaTiV, along with reference materials (pure Ni and E90 for the CrFeMnNi family and pure V for NbTaTiV) were irradiated at 50 K and 773 K with 1 MeV Kr++ ions to various levels of displacements per atom (dpa) using in-situ transmission electron microscopy. Cryogenic irradiation resulted in small defect clusters and faulted dislocation loops as large as 12 nm in face-centered cubic (FCC) CSAs. With thermal diffusion suppressed at cryogenic temperatures, defect densities were lower in all CSAs than in their less compositionally complex reference materials indicating that point defect production is reduced during the displacement cascade stage. High temperature irradiation of the two FCC CSA resulted in the formation of interstitial dislocation loops which by 2 dpa grew to an average size of 27 nm in Cr18Fe27Mn27Ni28 and 10 nm in Cr15Fe35Mn15Ni35. This difference in loop growth kinetics was attributed to the difference in Mn-content due to its effect on the nucleation rate by increasing vacancy mobility or reducing the stacking-fault energy.

In Situ Microstructural Evolution in Fcc and Bcc Complex Concentrated Solid-Solution Alloys Under Heavy Ion Irradiation

This study characterizes the microstructural evolution of single-phase complex concentrated solid-solution alloy (CSA) compositions under heavy ion irradiation with the goal of evaluating mechanisms for CSA radiation tolerance in advanced fission systems. Three such alloys, Cr18Fe27Mn27Ni28, Cr15Fe35Mn15Ni35, and equimolar NbTaTiV, along with reference materials (pure Ni and E90 for the CrFeMnNi family and pure V for NbTaTiV) were irradiated at 50 K and 773 K with 1 MeV Kr++ ions to various levels of dpa using in-situ TEM. Cryogenic irradiation resulted in small defect clusters and faulted dislocation loops as large as 12 nm in FCC CSAs. With thermal diffusion suppressed at cryogenic temperatures, defect densities were lower in all CSAs than in their less compositionally complex reference materials indicating that point defect accumulation is reduced during the displacement cascade stage. High temperature irradiation of the two FCC CSA resulted in the formation of interstitial dislocation loops which by 2 dpa grew to an average size of 27 nm in Cr18Fe27Mn27Ni28 and 10 nm in Cr15Fe35Mn15Ni35. This difference in loop growth kinetics was attributed to the difference in Mn-content due to its effect on the nucleation rate by increasing vacancy mobility or reducing the stacking-fault energy.

Irradiation-Induced Segregation at Dislocation Loops in CoCrFeMnNi High Entropy Alloy

To understand the redistribution of alloying element in high entropy alloys (HEAs) under irradiation, a CoCrFeMnNi alloy was irradiated with 1 MeV Kr ions at room temperature and 500oC, and characterized with atom probe tomography (APT) and transmission electron microscopy. At 500oC, Co and Ni were enriched at interstitial dislocation loops resulted from the ion irradiation. In contrast, no segregation was observed at interstitial dislocation loops formed at room temperature. It is inferred that the inverse Kirkendall effect through vacancy flux, as opposed to interstitial binding, was the primary mechanism attributing to the observed segregation. In addition, APT concentration contour plots show a ring-shaped segregation pattern instead of a disk-shaped pattern at the faulted dislocation loops, indicating the non-equilibrium nature of the defect clustering and solute segregation process of the CoCrFeMnNi under irradiation at high temperature.

Atomistic simulation of the obstacle strengths of radiation-induced defects in an Fe–Ni–Cr austenitic stainless steel

A molecular dynamics method is employed to simulate the interaction of an edge dislocation with a combined solute cluster-dislocation loop defect, as well as independent faulted interstitial dislocation loops and solute clusters in an Fe–12Ni–20Cr at a temperature of 300 K. The examined defects are typical radiation-induced defects in austenitic stainless steels used as core structural components in nuclear reactors, while the selected composition matches with commercial grade 304L alloy. The dislocation-defect interaction is examined, and the peak shear stress required to overcome the obstacle array, and its dependence on obstacle size, solute concentration, and orientation is determined. The peak shear stress is then converted to an effective obstacle strength (α). Results show that dislocation loops are stronger obstacles than solute clusters, and their strength is heavily dependent on orientation, while the strength of solute clusters is largely dependent on the solute concentration. The total strength of the combined solute cluster-dislocation loop defect is largely contributed by the dislocation loop, though a fraction of the cluster strength is added as well.

Impact of irradiation induced dislocation loops on thermal conductivity in ceramics

AbstractExperimental work aimed at understanding the role of dislocation loops in limiting phonon mediated thermal transport in ceramics is presented. Faulted dislocation loops, having diameters of a few nanometers, were introduced by irradiating a polycrystalline cerium dioxide sample with 1.6 MeV protons at 700°C. XRD analysis indicated that irradiated samples retained their crystalline structure and exhibit very little lattice expansion suggesting a low concentration of point defects. Further microstructure characterization using transmission electron microscopy revealed that interstitial type faulted dislocation loops were primarily created as expected for these irradiation conditions. Thermal conductivity of the damaged layer was measured using a modulated thermoreflectance approach. Analysis of the experimental data using the classical Klemens‐Callaway approach reveals that the conductivity reduction is primarily due to dislocation loops, while point defects and voids play only a minor role. These results provide experimental confirmation that faulted loops offer a unique arrangement for displaced atoms that leads to an unusually large reduction of thermal conductivity.

Nano-scale microstructure damage by neutron irradiations in a novel Boron-11 enriched TiB2 ultra-high temperature ceramic

Ultra-high temperature transition-metal ceramics are potential candidates for fusion reactor structural/plasma-facing components. We reveal the irradiation damage microstructural phenomena in Boron-11 enriched titanium diboride (TiB2) using mixed-spectrum neutron irradiations, combined with state-of-art characterization using transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Irradiations were performed using High Flux Isotope Reactor at ∼220 and 620 °C up to 2.4 × 1025 n.m−2 (E > 0.1 MeV). Total dose including contribution from residual Boron-10 (10B) transmutation recoils, was ∼4.2 displacements per atom. TiB2 is susceptible to irradiation damage in terms of dislocation loop formation, cavities and anisotropic lattice parameter swelling induced micro-cracking. At both 220 and 620 °C, TEM revealed dislocation loops on basal and prism planes, with nearly two orders of magnitude higher number density of prism-plane loops. HRTEM, electron diffraction and relrod imaging revealed additional defects on {101¯0} prism planes, identified as faulted dislocation loops. High defect cluster density on prism planes explains anisotropic a-lattice parameter swelling of TiB2 reported in literature which caused grain boundary micro-cracking, the extent of which decreased with increasing irradiation temperature. Dominance of irradiation-induced defect clusters on prism planes in TiB2 is different than typical hexagonal ceramics where dislocation loops predominantly form on basal planes causing c-lattice parameter swelling, thereby revealing a potential role of c/a ratio on defect formation/aggregation. Helium generation and temperature rise from residual 10B transmutation caused matrix and grain boundary cavities for the irradiation at 620 °C. The study additionally signifies isotopic enrichment as a viable approach to produce transition-metal diborides for potential nuclear structural applications.

Microstructural response of He+ irradiated FeCoNiCrTi0.2 high-entropy alloy

In the He+ irradiated FeCoNiCrTi0.2 high-entropy alloy (HEA), radiation-induced defects including helium bubbles and faulted dislocation loops were characterized by transmission electron microscopy. Compared with other face-centered cubic alloys irradiated in similar conditions, we found that the faulted loops in the FeCoNiCrTi0.2 HEA have a much large size, whereas the perfect loops were rarely detected. Our results reveal that the faulted-type defect structure is more energetically favorable in the FeCoNiCrTi0.2 HEA, whereas the transformation of the faulted to perfect dislocation loops was suppressed. For the underlying mechanism, we propose that the low stacking fault energy can stabilize the faulted planar defects during irradiation.

Characterization of faulted dislocation loops and cavities in ion irradiated alloy 800H

Alloy 800H is a high nickel austenitic stainless steel with good high temperature mechanical properties which is considered for use in current and advanced nuclear reactor designs. The irradiation response of 800H was examined by characterizing samples that had been bulk ion irradiated at the Michigan Ion Beam Laboratory with 5 MeV Fe2+ ions to 1, 10, and 20 dpa at 440 °C. Transmission electron microscopy was used to measure the size and density of both {111} faulted dislocation loops and cavities as functions of depth from the irradiated surface. The faulted loop density increased with dose from 1 dpa up to 10 dpa where it saturated and remained approximately the same until 20 dpa. The faulted loop average diameter decreased between 1 dpa and 10 dpa and again remained approximately constant from 10 dpa to 20 dpa. Cavities were observed after irradiation doses of 10 and 20 dpa, but not after 1 dpa. The average diameter of cavities increased with dose from 10 to 20 dpa, with a corresponding small decrease in density. Cavity denuded zones were observed near the irradiated surface and near the ion implantation peak. To further understand the microstructural evolution of this alloy, FIB lift-out samples from material irradiated in bulk to 1 and 10 dpa were re-irradiated in-situ in their thin-foil geometry with 1 MeV Kr2+ ions at 440 °C at the Intermediate Voltage Electron Microscope. It was observed that the cavities formed during bulk irradiation shrank under thin-foil irradiation in-situ while dislocation loops were observed to grow and incorporate into the dislocation network. The thin-foil geometry used for in-situ irradiation is believed to cause the cavities to shrink.

Radiation-induced segregation on defect clusters in single-phase concentrated solid-solution alloys

A group of single-phase concentrated solid-solution alloys (SP-CSAs), including NiFe, NiCoFe, NiCoFeCr, as well as a high entropy alloy NiCoFeCrMn, was irradiated with 3 MeV Ni2+ ions at 773 K to a fluence of 5 × 1016 ions/cm2 for the study of radiation response with increasing compositional complexity. Advanced transmission electron microscopy (TEM) with electron energy loss spectroscopy (EELS) was used to characterize the dislocation loop distribution and radiation-induced segregation (RIS) on defect clusters in the SP-CSAs. The results show that a higher fraction of faulted loops exists in the more compositionally complex alloys, which indicate that increasing compositional complexity can extend the incubation period and delay loop growth. The RIS behaviors of each element in the SP-CSAs were observed as follows: Ni and Co tend to enrich, but Cr, Fe and Mn prefer to deplete near the defect clusters. RIS level can be significantly suppressed by increasing compositional complexity due to the sluggish atom diffusion. According to molecular static (MS) simulations, “disk” like segregations may form near the faulted dislocation loops in the SP-CSAs. Segregated elements tend to distribute around the whole faulted loop as a disk rather than only around the edge of the loop.

Void formation in melt-grown silicon studied by molecular dynamics simulations: From grown-in faulted dislocation loops to vacancy clusters

Molecular dynamics simulations of a dislocation based mechanism for void formation in silicon are presented. By studying a moving solid-liquid interface in Si, we observe the formation of dislocation loops on (111) facets consisting of coherency and anticoherency dislocations, which disband within nanoseconds into vacancy clusters of 10 or more vacancies. These vacancy clusters can act as nucleation seeds for the experimentally observed octahedral single and double voids.