Frank Loops Research Articles

Dynamic Interaction between Dislocation and Irradiation-Induced Defects in Stainless Steels during Tensile Deformation

A series of in-situ transmission electron microscopy (TEM) observations during tensile deformation were conducted on the ion-irradiated stainless steel. The jerky motion of dislocations appeared, and dislocations were pinned and depinned at the defects through the in-situ TEM observation. The jump distance traveled by dislocation was measured and discussed as the mean interval of defects interacting with the dislocation motion. Microstructural information of irradiation defects such as obstacle interval was obtained by TEM and atom probe tomography (APT), and the type of pinning site was identified. It was found that Frank loops and black dots were irradiation defects that strongly interacted with dislocations. It was suggested that solute atom clusters act as weak obstacles for dislocations in the dynamic interaction behavior with dislocation motion.

Dislocation loop evolution in Kr‐irradiated ThO2

AbstractThe early stage of microstructural evolution of ThO2, under krypton irradiation at 600, 800, and 1000°C, was investigated using in situ transmission electron microscopy (TEM). Dislocation loops grew faster, whereas their number density decreased with increasing irradiation temperature. Loop density was found to decrease with ion dose. Interstitial dislocation loops, including Frank loops with Burgers vector of a/3〈111〉 and perfect loops with Burgers vector of a/2〈110〉, were determined by traditional TEM and atomic resolution–scanning TEM techniques. Atomistic and mesoscale level modeling are performed to interpret experimental observations. The migration energy barriers of defects in ThO2 were calculated using density‐functional theory. The energetics of different dislocation loop types were studied using molecular dynamics simulations. Loop density and diameter were analyzed using a kinetic rate theory model that considers stoichiometric loop evolution. This analysis reveals that loop growth is governed by the mobility of cation interstitials, whereas loop nucleation is determined by the mobility of anion defects. Lastly, a rate theory model was used to extract the diffusion coefficients of thorium interstitials, oxygen interstitials, and vacancies.

Investigation of Exfoliation Efficiency of 6H-SiC Implanted Sequentially with He+ and H2+ Ions.

Silicon carbide (SiC) is a promising material used in the advanced semiconductor industry. Fabricating SiC-on-insulator via H implantation is a good method. He and H co-implantation into Si can efficiently enhance exfoliation efficiency compared to only H implantation. In this study, 6H-SiC single crystals were implanted with He+ and H2+ dual beams at room temperature, followed by annealing at 1100 °C for 15 min, and irradiations with 60 keV He ions with a fluence of 1.5 × 1016 ions/cm−2 or 5.0 × 1016 ions/cm−2 and 100 keV H2+ ions with a fluence of 5 × 1016 ions/cm−2 were carried out. The lattice disorder was characterized by both Raman spectroscopy and transmission electron microscopy. The intensity of Raman peaks decreased with increasing fluence. No Raman shift or new phases were found. A very high numerical density of bubbles was observed as compared to single H or He implantation. Moreover, stacking faults, Frank loops and tangled dislocations were formed in the damaged layer. Surface exfoliation was inhibited by co-implantation. A possible reason for this is an increase in fracture toughness and a decrease in elastic out-of-plane strain due to dense bubbles and stacking faults.

Dislocation reaction-based formation mechanism of stacking fault tetrahedra in FCC high-entropy alloy

In conventional FCC metals and alloys, the formation mechanisms of stacking fault tetrahedron (SFT) based on the vacancy cluster and Frank loop have been widely accepted; however, the dislocation reaction-based formation mechanism of SFT without Frank loop and vacancy cluster remains controversial. In this work, we studied the formation of the SFTs in CoCrFeNiCu high-entropy alloy (HEA) bicrystals and their single-crystal counterparts subjected to shear deformation using molecular dynamics (MD) simulations. The migration and thickening of grain boundaries, and the nucleation and glide of dislocations from the grain boundaries are the predominant carriers of deformation, which are the prerequisite for the formation of SFT. We observed two types of dislocation reaction-based formation mechanisms, where there is no vacancy aggregation and Frank loop participation. The formation of SFT originates from the growth of the bottom face of the pyramid in the bicrystal HEA, while in the single-crystal HEA, SFT nucleates at its vertex, and extends towards its bottom.

In-situ TEM investigation of dislocation loop evolution in Al-forming austenitic stainless steels during Fe+ irradiation: Effects of irradiation dose and pre-existing dislocations

Alumina-forming austenitic (AFA) stainless steel with excellent properties has been regarded as a promising candidate of fuel claddings in Supercritical carbon dioxide (S-CO2) gas cooled nuclear reactor. In current work, in-situ irradiation with 400 keV Fe+ in transmission electron microscope were performed on AFA stainless steel to investigate the influences of irradiation dose and pre-existing dislocation lines on loop evolution at 773 K. The irradiation damage caused by irradiation-induced loops was also analyzed. In-situ TEM observation displayed the evolution and characteristics of dislocation loops, including initiation, migration, aggregation, growth, annihilation, and combination. Both Frank loops with b = 1/3<111> and perfect loops with b = 1/2<110> were formed, and at 0.54 dpa, 1/2<110> loops accounted for the majority and was up to 64.6%. Pre-existing dislocation lines obviously affected not only the evolution and distribution of dislocation loops, but also the degree of irradiation hardening. At the same irradiation dose, not only the size and number density of dislocation loops in the region with dislocation lines were smaller than those in the region without dislocation lines, but also the increment of yield strength and irradiation damage was lower before the formation of dislocation network. Increasing pre-existing dislocation density should be beneficial to improve the irradiation resistance of materials, but it needed to be comprehensively considered with the mechanical properties of the material. Al component played an important role in irradiation behavior of AFA steels and the corresponding mechanism was discussed subsequently.

Atomistic structure and thermal stability of dislocation loops, stacking fault tetrahedra, and voids in face-centered cubic Fe

Irradiation-induced hardening and changes to mechanical properties can often lead to material degradation and can limit the operation of reactor components. In addition, changes to mechanical properties can be a precursor to other material environmental degradation. Dislocation loops, stacking fault tetrahedra (SFT), and voids are major irradiation-induced lattice defects that occur in structural materials and are often observed post-irradiation. However, these defects are often considered only as obstacles to dislocation motion, with hardening assumed to be dependent on the habit plane and Burgers vector of the defect. The stability and nature of dislocation loops and SFTs with temperature are often overlooked, and this may play a crucial role in the mechanical properties of structural materials. We combine high-resolution transmission electron microscopy (HR-TEM) and atomistic simulations to determine the atomistic configurations and thermal stability of dislocation loops and SFTs. Dislocation dissociations are observed in both vacancy and interstitial-type perfect dislocation loops as well as in vacancy-type Frank loops, whereas interstitial-type Frank loops remain stable and are the least influenced by temperature. Triangle-shaped intrinsic stacking faults are formed at the edges of Frank vacancy loops, and a direct transformation from triangle-shaped Frank vacancy loops to SFTs is observed in atomistic simulations. SFTs are energetically more stable than Frank vacancy loops based on formation energy calculations in pure face-centered cubic Fe, which may explain why vacancy-type Frank loops are infrequently observed experimentally. The motivation for this study is to develop a framework for the stability and nature of irradiation defects as a function of temperature, which will be subsequently used for more complex (alloy) systems.

Hydrogen trapping by irradiation-induced defects in 316 Lstainless steel: A combined experimental and modeling study

The irradiation-induced defects in stainless steel internal components of pressurized water reactors combined with hydrogen uptake during the oxidation process could be a key parameter in the mechanism for Irradiation-Assisted Stress Corrosion Cracking (IASCC). A heat-treated 316 L SS containing a low amount of defects was Fe3+ ions-implanted; irradiation-induced defects types and depth distributions were characterized by Transmission electron microscopy (TEM). Deuterium was then inserted in the specimens either by cathodic charging or by sample exposure to deuterated primary water. Secondary Ions Mass Spectrometry – SIMS – permitted to access the deuterium distribution at the implanted stainless steel surface. A finite difference numerical solver accounting for hydrogen diffusion/trapping coupling was used to simulate the hydrogen transport in the implanted material, taken into consideration the specific heterogeneous character of the irradiation-induced defects distribution in matter. Taking as input data the experimental defects distribution associated with Frank loops or voids, the main trapping sites for hydrogen were assigned to voids, not Frank loops. Such numerical approach was able to deal accurately with the problem of hydrogen transport in a heterogeneous material as well as to differentiate two potential trap sites contributions to the experimental deuterium distribution. In addition, according to results obtained after primary water exposure, trapping at voids was still effective at 320 °C, signature of a high binding energy of hydrogen in voids.

Radiation damage during HRTEM studies in pure Al and Al alloys

Abstract During transmission electron microscopy (TEM) investigations of Al alloys, defects caused by the electron irradiation can occur. Since the image contrast of these irradiation defects is similar to that of early stages of precipitates, care is needed to avoid confusion. In the present paper the formation and the coarsening of radiation damage defects were studied by in situ TEM in both, pure Al and Al alloys. High-resolution (HR)TEM images show on an atomic level that the radiation defects are extrinsic Frank loops (some converted into unfaulted prismatic loops); they are distributed homogeneously within the TEM foil but inhomogeneously on the four {111} planes. Applying HRTEM imaging conditions, the minimum electron energy causing defects is found to be as low as 110 keV using a [110] beam direction. The results for pure Al are very similar to those of the Al alloys. Therefore, during HRTEM studies (using accelerating voltages &gt; 100 kV) the formation of radiation defects seems inevitable; they can be distinguished from the early stages of precipitates if they lie on different planes.

A Combined Atomistic-Continuum Study on the Unfaulting of Single and Multi-layer Interstitial Dislocation Loops in Irradiated FCC and HCP Metals

Unfaulting of single and multi-layer interstitial loops in face centered cubic (FCC) and hexagonal close-packed (HCP) metals, with Ni, Al, Mg and Zr as model systems, have been comprehensively studied employing molecular dynamics simulations and continuum modeling. A versatile approach properly accounting for atom insertion/removal and displacement field was developed, able to correctly construct single and multi-layer interstitial loop structures of arbitrary geometries. The unfaulting mechanisms and corresponding dislocation reaction intermediates in different unfaulting scenarios were clarified. Critical conditions necessary for unfaulting have been identified, and an analytical model has been developed to accurately predict these conditions. Particularly for unfaulting in FCC materials, the role of Double Shockley (D-Shockley) partial in unfaulting has been explicitly elucidated, and the mechanisms of unfaulting initiating from periphery of the Frank loop and induced by a moving straight edge dislocation have been clarified. For unfaulting in HCP, despite certain similarity to its FCC counterpart, clear difference in unfaulting behaviors due to the HCP stacking sequence has been demonstrated. A comprehensive mapping of unfaulting routes for double-layer interstitial loops in HCP has been established to reveal the relationships between the various loop morphologies observed in experiments. The present work provides important mechanistic insights to address key knowledge deficits regarding interstitial Frank loop unfaulting, essential for better understanding of deformation and irradiation-induced failure in structural metals. The new modeling techniques and predictive tools offered are expected to be of great merits to not only the study of unfaulting of dislocation loops, but also future investigation of other complex dislocation or defect ensembles.

A molecular dynamics study of a cascade induced irradiation creep mechanism in pure copper

Recently, in-situ TEM straining experiments on pure copper have unraveled a high stress irradiation creep mechanism. Irradiation induced unpinning of dislocations from defects has been observed and the mean pinning lifetime has been determined. In the present study, molecular dynamics simulations are performed on pure copper to investigate the impact of collision cascades on screw dislocations pinned on Frank loops under high-applied stresses at 300 K in order to further quantitatively elucidate this mechanism. The simulations indicate two possible dislocation unpinning mechanisms. Unpinning can occur through loop destruction when the cascade is generated on the pinning points of the dislocation (type 1 unpinning). Unpinning can also be triggered by the shear stresses building up around a cascade generated in front of the dislocation in the glide plane (type 2 unpinning). Type 2 unpinning generally leads to dislocation repinning on cascade residues, so that it should only marginally contribute to irradiation creep. The mean pinning lifetime due to type 1 unpinning in the conditions of the in-situ TEM experiments is derived from a simple model previously developed for zirconium, and is found in the same orders of magnitude as in the experiments.

Distinct He-induced damage evolution in nickel-based alloys irradiated at elevated temperatures

The nickel-based alloy Inconel 617 and Alloy 800H have been envisaged as the leading candidate for the higher-temperature Molten Salt Reactors (MSRs) due to their superior high-temperature properties. To evaluate their potential applications, both alloys were irradiated by He ion at 750 and 850 °C with corresponding He ion fluence. Same irradiation tests were performed on Hastelloy N alloy to study its microstructure evolution at elevated irradiation temperature, as well as being the reference alloy to reveal candidate alloys irradiation tolerance. Transmission electron microscopy (TEM) reveals the depth distribution of irradiation induced He bubbles and Frank loops. The TEM graphs and quantitative analysis shows that when irradiated at 850 °C, the formation of precipitated clusters lead to denser and boarder distribution of He bubbles in Inconel 617, while the bubble-loop complexes form and contribute to larger volume fraction of He bubbles in Hastelloy N alloy. The nanoindentation technique is used to measure their mechanical changes, and the results show that the Alloy 800H has the lowest degree of irradiation induced hardening when irradiated at 850 °C, whereas that of all alloys irradiated at 750 °C are basically similar. Further, the yield strength increment is calculated and deviations between experimental and calculated values are caused by the formation of precipitated clusters in Inconel 617 and the selection of smaller obstacle strength in Hastelloy N alloy. By comparison, the Alloy 800H demonstrates a greater potential to function as the structural material for higher-temperature MSRs due to smaller volume fraction and better resistance to irradiation induced hardening.

Quantitative characterization of frank loops and their effects on the plastic degradation in heavy ion irradiated alloy 800H

Alloy 800H has recently received significant attention owing to its potential applications in molten salt reactors. However, its irradiation tolerance and plastic degradation after irradiation are not well understood. In this study, Alloy 800H was irradiated by 3 MeV Cu+ ions with irradiation damage up to 10 dpa. The microstructure changes were characterized using two Rel-rods cases and weak beam dark field technology by performing transmission electron microscopy. The plastic mechanical properties before and after the irradiation were extracted from P-h curves derived by performing nanoindentation at loading rates of 0.05, 0.1, and 0.5 nm/s. The microscopy studies demonstrate that the characterization and evolution of the two variations of Frank loops are distinct and the 13⟨1‾11⟩ Frank loops firstly nucleate and coarsening. The nanoindentation results demonstrate that the measured yield stress increases with the increasing irradiation damage and decreasing loading rate. The good agreement on yield stress between the calculated and measured values suggests that using the characteristics of two variations of Frank loops to calculate the contribution of yield stress is appropriate. In addition, the decreasing measured values with increasing loading rate are attributed to the increased geometrically necessary dislocations, which are correlated with the indent size effect.

Irradiation hardening of stainless steel model alloy after Fe-ion irradiation and post-irradiation annealing treatment

Microstructural observation with atom probe tomography, transmission electron microscopy, and mechanical examination with a nano-indenter were performed on stainless steel model alloy that had been irradiated with Fe ions and subjected to post-irradiation annealing treatment (PIA). Significant irradiation hardening occurred in the as-irradiated specimen and irradiation hardening recovery occurred at 400 °C of cumulative PIA, Tiny black dots, Frank loops and Ni-Si clusters formed in the as-irradiated specimen and their recovery was observed with the cumulative PIA. A classic dispersed-barrier hardening model was used to evaluate irradiation hardening connected to microstructural information by applying the density and size of defect variants. Ni-Si clusters and Frank loops contributed substantially to the increase of irradiation hardening, whereas the black dots contributed little for the stainless steel model alloy. The barrier strength factor of Ni-Si clusters, αSC, was obtained as 0.029 and 0.015 for two different obstacle strength model, which suggests that an individual Ni-Si cluster is a weak barrier obstacle to mobile dislocation on the dispersed-barrier hardening model, but that the dense formation of Ni-Si clusters controls irradiation hardening behavior. A quantitative analysis was performed of the increase in strength from the Ni-Si clusters that was caused by coherency strains, and an irradiation hardening mechanism for the stainless steel, including the solute atom clusters, is discussed.

Molecular dynamics studies of lattice defect effects on tritium diffusion in zirconium

Tritium diffusion in α-Zr containing point defects such as vacancies or self-interstitial atoms (SIAs) is simulated using molecular dynamics. Point defects rapidly aggregate to form extended defects, such as 3D nanoclusters and Frank loops. The geometry of extended defects is affected by the presence of tritium. At low temperature and in the absence of tritium, vacancies aggregate to form stacking fault pyramids. Addition of tritium at these temperatures promotes aggregation of vacancies to form 3D nanoclusters, within which the tritium concentration can be sufficiently high to suggest that these defects may serve as nucleation sites for hydride precipitation. Trapping of tritium in vacancy nanocluster reduces the calculated bulk diffusivity by an amount proportional to the vacancy concentration. At high temperature, vacancy clusters change shape to form planar basal dislocation loops, which bind tritium less strongly, leading to a sharp reduction in the fraction of trapped tritium and a corresponding increase in tritium diffusivity at high temperature. In contrast, SIAs increase tritium diffusion through α-Zr. Analysis of atomic trajectories shows that tritium does not interact directly with SIAs. Diffusion enhancement is instead related to expansion of the lattice.

In-situ TEM investigation of unfaulting behavior of Frank loops in FCC Pd during H2+ & He+dual-beam irradiation

A faulted Frank dislocation loop (FDL) can transform into an unfaulted perfect dislocation loop (PDL) under plastic deformation, quenching or particle irradiation. Here, we report our direct observation of the unfaulting processes of FDLs in palladium (Pd) by in-situ ion irradiation in transmission electron microscopy. Results indicate that the FDL unfaulting is a combination of three processes involving the interaction between adjacent FDLs, the interaction between FDL and PDL, and the nucleation and growth of Shockley partial dislocations within FDL. Meanwhile, bubbles tend to nucleate within dislocation loops, and the PDLs capture more He/H2 over FDLs, which confirms for the first time that the irradiation-induced loops play an important role in the aging of Pd and the unfaulting strengthens this process.

Collapse of stacking fault tetrahedron and dislocation evolution in copper under shock compression

• The SFT undergoes different metastable structures, such as triangular Frank loop, semi-faulted SFT, and truncated SFT with two FPDs, under shock compression along different orientations. • The dependence of dislocation loops and stacking faults formed from SFT on shock intensity is revealed. • The stress relaxation resulting from the structural transformation of SFT increases with the enhance of shock intensity. • The critical stress for plastic deformation approximately shows a linear decrease with the increasing temperatures for all the three shock directions we simulated. The stacking fault tetrahedron (SFT) is frequently observed in irradiated materials, which distinctly affects the mechanical properties of matrix materials. This work focuses on the collapse of SFT and the following dislocation evolution in single crystal copper under shock compression with molecular dynamics simulations. Our results reveal the collapse pattern of SFT and its dependence on the shock orientations. The SFT undergoes different metastable structures, e.g., a triangular Frank loop, a semi-faulted SFT, and a truncated SFT with two Frank partial dislocations for [111], [11̄0], and [112̄] shock orientation respectively, which provide nucleation sites for dislocation emission. The dependence of dislocation loops and stacking faults formed from SFT on the shock intensity is revealed. As the shock intensity increases, the stress relaxation resulting from the structural transformation of SFT becomes more obvious, and the dislocation density increases more rapidly to a higher peak value. The equilibrium values of von Mises stress and dislocation density after long-time evolution also increase. With the increase of initial temperatures, the critical stress for plastic deformation is found to decrease linearly and the reduction in [112̄] orientation is much smaller than that in [111] and [11̄0] orientations. The dislocation density increases and the stacking faults will decompose into smaller stacking fault pieces to some extent because of thermal fluctuation.

Effects of order and disorder on the defect evolution of NiFe binary alloys from atomistic simulations

The effects of the ordered and disordered arrangements of elements on radiation-induced defects production and evolution in NiFe alloys were investigated through atomistic simulations. Results present a sluggish evolution of the overall microstructure in ordered L10 NiFe. Although the disordered phase has fewer Frankel pair accumulation in cascade simulation attributed to the low thermal conductivity reduced by the intrinsic chemical disorder, the difference is negligible when PKA energy increases because of the direct formation of clusters. Interstitial diffusion is restricted in the ordered phase, where Ni and Fe layers are alternately arranged. This condition delays interstitials accumulation and leads to the formation of more Shockley partial loops rather than Frank loops which favor in the disordered phase. The higher stacking fault energy in the ordered phase renders it difficult to form stacking fault tetrahedra

TEM characterization of dislocation loops in proton irradiated single crystal ThO2

• This paper reports the full characterization of dislocation loops in single crystal ThO 2 after 0.47 dpa proton irradiation at 600 o C. • Dislocation loops were found to be mainly edge type with different variants of faulted 1 / 3 〈 111 〉 Frank loops along { 111 } habit plane. • Inside-outside contrast technique and atomic resolution STEM-HAADF imaging revealed the interstitial nature of dislocation loops. This work focuses on the full characterization of dislocation loops induced by proton irradiation in single-crystal ThO 2 . Irradiation was performed using 2 MeV H + ions with sample temperature at 600 o C and a dose of up to 0.47 displacements per atom (dpa). Transmission electron microscopy (TEM) characterization was performed on a large number of dislocation loops. Burgers vector ( b → ) analysis using standard g → . b → = 0 invisibility criterion revealed different variants of 1 / 3 〈 111 〉 type dislocation loops. TEM analysis of edge-on dislocation loops was used to determine habit planes as { 111 } type. The nature of dislocation loops was revealed using the inside-outside contrast method as interstitial type. Rel-rod dark field images were obtained by selecting streak at g → = 1 / 2 [ 31 1 ¯ ] in diffraction pattern to identify the Frank loops present in the microstructure. Subsequent analysis of a single dislocation loop using atomic resolution scanning transmission electron microscopy (STEM) confirmed the interstitial nature of the loop with { 111 } habit plane.

Molecular dynamic simulations evaluating the effect of the stacking fault energy on defect formations in face-centered cubic metals subjected to high-energy particle irradiation

Austenitic stainless steels, which are used as incore structural materials in light water reactors, are characterized by an extremely low stacking fault energy (SFE) among face-centered cubic (FCC) metals. To evaluate the effects of SFE on defect formation under high-energy particle irradiation, molecular dynamics simulations were performed using the interatomic potential sets for FCC metals with different SFEs and a primary knock-on atom energy (EPKA) of 100 keV at 600 K. The results show that the number of residual defects is independent of the SFE. However, the characteristics of self-interstitial atom (SIA) clusters do depend on the SFE. For clusters smaller than a certain size, the ratio of glissile SIA clusters decreases as the SFE increases, which is similar to the trend observed at the low EPKA. However, for larger clusters, which can be detected only at a high EPKA, the ratio of glissile clusters increases. These results correspond to static energy calculations, in which the difference in the formation energy between a Frank loop and perfect loop (ΔEF-P) for the small clusters decreases as the SFE increases. In contrast, for the larger clusters, the SFE dependence of ΔEF-P changes due to the shape restrictions of stable perfect loops. At a high temperature of 600 K, large vacancy clusters with stacking faults can be detected at EPKA = 100 keV, resulting in the enhanced formation of these clusters at lower SFEs. Furthermore, several of these clusters were similar to perfect loops, with the edges split into two partial dislocations with stacking faults, although the largest clusters detected at low EPKAs were similar to stacking fault tetrahedrons.

In situ transmission electron microscopy studies of 30 keV H2+ and He+ dual-beam irradiated Pd: defect generation and microstructural evolution under varied temperatures and foil thicknesses

To fully understand the tritium aging behavior of palladium (Pd), an important material applied in hydrogen isotope storage and tritium engineering, an in situ irradiation of 30 keV H2+ and He+ dual beam was performed for the investigation of the microstructural evolution in Pd. The irradiation dose, temperature, and sample thickness obviously affected the morphology, distribution characteristics, density, and size of dislocation loops, bubbles, and bubble-loop complexes. Highly dense loops were aggregating at lowered temperatures, whereas the loop density dropped dramatically, producing bubble-loop complexes at elevated temperatures. Most of loops were 1/3<111> Frank loops with quasi-circular under a low dose at 573 K and then transformed to 1/2<110> perfect loops with wavy and polygonal shape at a high dose. The difference in growth behavior between faulted and perfect loops suggested that there was a critical transform loop size ~32 nm in diameter. The heterogeneity in size and shape of bubbles could be attributed to the interaction between bubbles and the periphery loop. Moreover, the hydrogen resulted in the formation of larger bubbles when compared with the single-beam helium irradiation. The unfaulting transition of loops led to bubble redistribution. The corresponding mechanisms of in situ irradiation experimental results were discussed.