Small Frank Loops Research Articles

Nonconservative Ostwald ripening of a dislocation loop layer under inert nitrogen-rich SiO2∕Si interfaces

In this work we perform a systematic study of the dissolution of a dislocation loop layer under the influence of inert SiO2∕Si and nitrogen-rich SiO2∕Si interfaces. The composition of the dislocation loop layer was just after its formation 10%–20% Frank dislocation loops and 90%–80% perfect prismatic loops. During subsequent inert (N2) ambient annealing the differences of the kinetics between the two loop populations have been studied as a function of the interface type. It has been shown that during the nonconservative Ostwald ripening process the defect band loses interstitials mainly due to the dissolution of perfect prismatic loops, while Frank loops remain almost unaffected by the presence of both interfaces. In parallel a competition between the interface and the population of Frank loops in absorbing the interstitials released by the prismatic loops took place. The nitrogen-rich SiO2∕Si interface has been proved in general a less effective interstitial sink than the common one and under specific annealing conditions less effective even than the small Frank loops population.

Evolution of Microstructure and Microchemistry in Cold-worked 316 Stainless Steels under PWR Irradiation

The evolution of microstructures and microchemistry was examined by transmission electron microscopy in cold-worked SUS 316 stainless steel components irradiated in a pressurized water reactor to 1–73 dpa at 565–596 K. Homogenous nucleation of dislocation loops, helium bubbles and γ′ precipitates was detected. The dislocation loops consisted of a high density of Frank loops and black dots. The black dots are considered to be small Frank loops, some fraction of which could be vacancy-type. The size distribution showed a double peak, which remained unchanged up to 73 dpa, suggesting that the dislocation structure was saturated under a balance of nucleation and disappearance. The helium bubbles were extremely dense and fine, resulting in swelling of less than 0.1%. Such bubble structure was formed under irradiation with a high He/dpa ratio. The γ′ precipitation was detected at doses higher than 4 dpa with an increasing density for higher doses. The measured radiation hardening was almost explained by these visible microstructural features. Radiation-induced segregation at grain boundaries was confirmed to continuously develop up to 73 dpa whereas no significant segregation could be detected at Frank loops.

Evolution of fine-scale defects in stainless steels neutron-irradiated at 275 °C

Six austenitic stainless steel heats (three heats each of 304SS and 316SS) neutron-irradiated at 275 °C from 0.6 to 13.3 dpa have been carefully characterized by TEM and their hardness measured as a function of dose. The characterization revealed that the microstructure is dominated by a very high density of small Frank loops present in sizes as small as 1 nm and perhaps lower, which could be of both vacancy and interstitial-type. Frank loop density saturated at the lowest doses characterized, whereas the Frank loop size distributions changed with increasing dose from an initially narrow, symmetric shape to a broader, asymmetric shape. Although substantial hardening is caused by the small defects, a simple correlation between hardness changes and density and size of defects does not exist. These results indicate that radiation-induced segregation to the Frank loops could play a role in both defect evolution and hardening response.

The contribution of various defects to irradiation-induced hardening in an austenitic model alloy

The combination of charged particle irradiation using MeV energy range accelerators and an ultra-low load indentation is a potential technique to study the mechanical property changes of fusion structural materials due to high-energy neutron irradiation. This work was intended to examine the contribution of various defects to irradiation-induced hardening in an Fe–15Cr–20Ni model alloy by means of single/dual-beam ion irradiation and a micro-indentation. In single/dual ion-irradiated specimens, a significant reduction in Frank loop number density was observed in a region where indentation-induced dislocations evolved. It was observed that only Frank loops with diameters of about 20 nm partially survived in the plastically deformed region. In the dual ion-irradiated specimens, cavities shear-deformed along the mass-flow directions were observed in the region beneath the indent. It was concluded through the indentation hardness determination and post-irradiation indentation micro-structural examination that small Frank loops cause hardening to a larger extent than expected from their actual sizes and induce plastic instability by annihilating by interactions with moving dislocations, while clean cavities produced by ion irradiation are not strong deformation barriers as estimated from neutron data.

Nature of the Fine-Scale Defects and Radiation Hardening in Stainless Steels Neutron-Irradiated at 550K

ABSTRACTOur understanding of radiation effects in stainless steels irradiated at temperatures below annealing stage V (<573K) remains unclear. The microstructure under these conditions has been loosely described in terms of Frank loops and “black spot” damage, the latter of which is typically considered to be 4-5 nm or less in size. To further clarify the nature of these defects, 6 stainless steels heats neutron irradiated at 550K from 0.6 to 13.3 dpa have been characterized by TEM. The microstructure is comprised of a very high density of small Frank loops present in sizes as small as 1 nm and perhaps lower, which could be of both vacancy and interstitial-type. The density of the visible loops is already established at the lowest dose available, however, the microstructure continues to evolve as the dose increases. The size distributions of the Frank loops change from a narrow symmetric distribution to a much broader size distribution with a bimodal distribution beginning to form at the higher doses.

Sheet resistivity and transmission electron microscope investigations of BF+2 -implanted silicon

Sheet resistivities of post-implantation annealed BF+2 -implanted silicon were measured by four-point probe method as a function of implantation dose, annealing temperature, and time. The resistivities of the implanted layer were found to be lower for samples annealed at higher temperature as well as irradiated to higher dose. Detailed transmission electron microscope investigations were made for samples irradiated to 1×1014 cm−2. Disordered zones were found to transform to microscopic clusters and small Frank loops on {111} planes upon 600 °C annealing. The rodlike defects in samples annealed at 700–800 °C are identified to be elongated perfect dislocation loops lying on or near {111}, interstitial in nature, with Burgers vectors b = 1/2 〈110〉 perpendicular to their elongated directions. They disappeared upon 900 °C annealing. The dislocation loops that coexisted with the rodlike defects consisted of interstitial perfect prismatic loops and extrinsic Frank loops, mainly hexagonal or circular in shape. They persisted after 900–1000 °C annealing. Half circular faulted loops and larger faults were found in 1000–1200 °C annealed samples. The loops were identified to be extrinsic Frank loops, lying on {111}. The formation and growth of these loops is likely related to the effect of oxidation in annealing.

Electron microscope image contrast of small dislocation loops and stacking-fault tetrahedra

With the use of the method described in the preceding paper (to be referred to subsequently as I) for constructing the displacement fields, the electron microscope image contrast of small dislocation loops and of stacking-fault tetrahedra has been computed from numerical solutions of the Howie-Whelan (1961) equations. The computer-simulated images, displayed in the form of half-tone pictures, have been used to identify the nature and geometry of such defects in ion-irradiated foils. A systematic study of the contrast of small Frank loops in Cu + ion irradiated copper under a wide variety of diffraction conditions is reported. In particular the variations of the contrast of loops edge-on and inclined to the electron beam with the operating Bragg reflexion, the thickness and inclination of the foil, depth of the defect in the foil and deviation from the Bragg-reflecting condition have been studied. Methods of obtaining useful information, such as the diameters of the loops, are suggested. The contrast of stacking-fault tetrahedra, and of non-edge perfect dislocation loops in ion-irradiated molybdenum is also investigated.

The determination of the geometry and nature of small Frank loops using the weak‐beam method

SUMMARYTheoretical and experimental investigations have been carried out to determine under which conditions weak‐beam images can give useful information on the geometry, size and nature of small Frank loops. Suitable experimental conditions for obtaining such information were deduced from computed weak‐beam images of Frank loops under various selected loop and foil orientations and diffraction vectors. Experiments to check these deductions were performed in silicon damaged by irradiation with phosphorus ions. It was found that the geometry and size of loops of diameter 8 nm (80 Å) viewed normal to their habit plane was well defined by weak‐beam images taken in {220} reflections lying in the loop plane for which g.b = 0 and g.b × u ≠ 0. By imaging in reflections with g vectors not in the loop plane the enclosed stacking faults were imaged (thus confirming the loops to be Frank loops) and the sense of the Burgers vector (i.e. the loop nature) was determined.