Small Defect Clusters Research Articles

Tensile properties and microstructure of martensitic steel DIN 1.4926 after 800 MeV proton irradiation

A double-wall window of martensitic steel DIN 1.4926 (11% Cr) was irradiated with 800 MeV protons in the LANSCE facility of the Los Alamos National Laboratory (LANL) to a total number of about 6.3×10 22 protons (2.8 Ah) in a temperature range from 50°C to 230°C. Tensile tests show that irradiation hardening increases with fluence up to the maximum attained dose of about 6.6 dpa. All irradiated specimens show significant embrittlement, ⩽1.5% uniform elongation and 7.5–9% total elongation as compared to about 11% uniform elongation and 21% total elongation for the unirradiated specimens. SEM observations illustrate that the fracture of specimens changes gradually from ductile mode in unirradiated and low dose specimens to cleavage mode in specimens of high dose (⩾5.6 dpa) . Intergranular brittle fracture mode has not been observed. Irradiation induced small defect clusters exist in all samples of irradiated material. Both of the size and the density of clusters increase with fluence. At the highest dose of 6.6 dpa large dislocation loops of a size ⩾10 nm have been observed in addition to the clusters.

Defect production, annealing kinetics and damage evolution in α-Fe: An atomic-scale computer simulation

Abstract Radiation-induced microstructural and compositional changes in solids are governed by the interaction between the fraction of defects that escape their nascent cascade and the material. We use a combination of molecular dynamics (MD) and kinetic Monte Carlo (KMC) simulations to calculate the damage production efficiency and the fraction of freely migrating defects in α-Fe at 600 K. MD simulations provide information on the nature of the primary damage state as a function of recoil energy, and on the kinetics and energetics of point defects and small defect clusters. The KMC simulations use as input the MD results and provide a description of defect diffusion and interaction over long time and length scales. For the MD simulations, we employ the analytical embedded-atom potential developed by Johnson and Oh for α-Fe, including a modification of the short-range repulsive interaction. We use MD to calculate the diffusivities of point defects and small defect clusters and the binding energy of small ...

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.

Microstructural examination of irradiated V–(4–5%)Cr–(4–5%)Ti

Microstructural examination results are reported for two heats of V–(4–5%)Cr–(4–5%)Ti irradiated in the EBR-II X530 experiment to 4.5 dpa at ∼400°C to provide an understanding of the microstructural evolution that may be associated with degradation of mechanical properties. Fine precipitates were observed in high density intermixed with small defect clusters for all conditions examined following the irradiation. The irradiation-induced precipitation does not appear to be affected by preirradiation heat treatment at 950–1125°C. There was no evidence for a significant density of large (diameter > 10 nm) dislocation loops or network dislocations. Analytical investigations successfully demonstrated that the precipitates were enriched in titanium, depleted in vanadium and contained no nitrogen. These results are discussed in terms of future alloy development options.

Strain and defect microstructure in ion-irradiated GeSi/Si strained layers as a function of annealing temperature

High-energy (1 MeV), ion irradiation of GeSi/Si strained layers at elevated temperatures can cause strain relaxation. This study examines the defects responsible for relaxation and for the evolution of the strain during subsequent annealing. Three distinct annealing stages are identified and correlated with the defect microstructure. In the temperature range from 350 to 600 °C, a gradual recovery of strain is observed. This is shown to correlate with annealing of small defect clusters and the growth of voids. The voids are visible at annealing temperatures in excess of 600 °C, consistent with an excess vacancy concentration in the irradiated alloy layer. In the temperature range 600–750 °C, the strain recovers to a maximum value which is correlated with the ripening of voids, dissolution of alloy layer {113} rodlike defects, and {113} planar interstitial defects in the substrate. At temperatures in the range 750–1000 °C, strain relaxation is observed and is correlated with the growth of intrinsic dislocations within the alloy layer. These intrinsic, looplike dislocations nucleate at the alloy–substrate interface and grow within the alloy layer, toward the surface.

Stochastic effects on dislocation structure development under cascade-damage irradiation

Dislocation climb, loop growth, and void swelling in the presence of the continuous generation of intracascade primary clusters are considered in this paper. Due to the ran- dom nature of cascade initiation, annihilation of point defects at sinks undergoes continuous random fluctuations that of- ten have sizes comparable to the sizes of the primary clus- ters. This makes them particularly important to the evolution of small defect clusters. In this paper, a reaction kinetics approach is generalized to include the effects of stochastic fluctuations of point-defect fluxes introduced by the random point-defect production in discrete packages during cascade irradiation. The reaction constant consistent with this ap- proach is derived for the annihilation of immobile clusters by the climbing dislocations. Taking the immobile primary inter- stitial clusters (PICs) as the interstitial loop nuclei, the evolu- tion of loops and network dislocations, and the resulting void swelling are calculated as a function of dose. Using param- eters appropriate to stainless steel, the loop number density, loop size distribution, network line density, and correspond- ing swelling are calculated and compared with experimental observations. It is found that this calculation explains the ob- servations very well. One of the central issues in the study of the accumulation of damage produced by cascade irradiation, in the form of both free defects and defect clusters, is the reaction kinet- ics between the primary clusters and extended sinks such as dislocations. Indeed, it is immediately clear, from the con- tinuous generation of clusters in cascades, that if they are only produced and not annihilated, they will become domin- ant sinks at very low dose. Acting as recombination centers for point defects, they may suppress any further development of the microstructure. Therefore, the evolution of the primary clusters (particularly the thermally stable primary interstitial clusters (PICs)), as governed by reactions among themselves, and with extended dislocations (both loops and network), is an essential ingredient in any theory of irradiation-induced deformation. It is evident that reaction among the primary clusters, and between the primary clusters and the dislocations, can only take place when the reaction partners come within the reac- tion volumes of each other. In general, such reactions can be envisaged to occur in two steps. During the first step, the relative motion of the reaction partners brings them together in a position in which they are bound by the elastic inter- action. During the second step, one of the reaction partners, the primary cluster, is annihilated, with the absorption of its point-defect content accommodated by dislocation climb or

Identification of the nature of neutron-irradiation-induced defects in copper by means of electron irradiation

Identification of the nature of small point defect clusters introduced in copper by neutron irradiation was made based on the behavior of the analyses of the defect clusters under electron irradiation in a high voltage electron microscope. The point defect clusters examined were those produced in thin foil and bulk specimens by three neutron irradiation facilities at temperatures between 300 and 473 K: a fusion neutron source RTNS-II (mono-chromatic 14MeV), a fission reactor JMTR (up to 2 MeV) and a spallation neutron source LASREF (up to 100 MeV). The electron irradiation using a high voltage electron microscope was performed with 1 MeV electrons at 300 and 473 K. About 95% of the clusters in the thin foils and 85% in the bulk samples were identified to be of the vacancy type. Fewer vacancy clusters and more interstitial type dislocation loops were observed in the bulk specimens than in the thin foil specimens. There were some differences in the defect structures between the different neutron irradiation facilities; the proportion of interstitial type clusters introduced by JMTR irradiation was less than those by RTNS-II and LASREF. The judgment validity of the nature of the defect clusters from electron microscope images was examined from the results of the present additional electron irradiation.

Identification of the nature of small point defect clusters in neutron irradiated Fe–16Ni–15Cr by means of electron irradiation

The nature of very small point defect clusters in Fe–16Ni–15Cr irradiated with fission neutrons was identified from their behavior under electron irradiation with a high voltage electron microscope. In this analysis, the defect clusters which grew during electron irradiation were judged to be of interstitial (I)-type and those which shrank or disappeared to be of vacancy (V)-type. The fraction of the number of I- and V-type defect clusters in a specimen irradiated as a bulk at 353 K were found to be 7% and 93%, respectively. In a specimen irradiated at 623 K as a bulk, most of defect clusters were I-type with a very small fraction of V-type. In a thin foil specimen irradiated at 573 K, the fraction of I-type defect clusters increased from 20% near the thin specimen edge to 70% at a thicker part of 120 nm. These results were consistent with those in the previous judgement from the shape and contrast of transmission electron microscope image.

Effect of Low Temperature ion Irradiation on the Microstructure of Nitride Ceramics

AbstractCross-section transmission electron microscopy was used to investigate the microstructure of polycrystalline silicon nitride (Si3N4) and aluminum nitride (AIN) following 2 MeV Si ion irradiation at 80 and 400 K up to a fluence of 4×1020ions/m2(maximum damage of ∼ 10 displacements per atom, dpa). A buried amorphous band was observed at both temperatures in Si3N4in the region corresponding to the peaks in the implanted ion and displacement damage. From a comparison of Si3N4specimens irradiated at different fluences, it is concluded that the amorphization is primarily controlled by the implanted Si concentration rather than the displacement damage level. Si3N4 amorphization did not occur in regions well-separated from the implanted ions for doses up to at least 3 dpa at 80 K, whereas amorphization occurred in the ion implanted region (calculated Si concentration >0.01 at.%) for damage levels as low as ∼0.6 dpa. The volumetric swelling associated with the amorphization of Si3N4, is <10%. Amorphization was not observed in any of the irradiated AIN specimens. A moderate density of small (∼3 nm) defect clusters were observed in the crystalline damaged regions of both the Si3N4and AIN specimens at both irradiation temperatures. Aligned network dislocations were also observed in the AIN specimen irradiated to high dose at 80 K.

Defect interaction processes controlling the accumulation of defects produced by high energy recoils

Defect interaction processes involved in the microstructural evolution during irradiation with high energy recoils are discussed on the basis of experimental observations. First of all, an experimental procedure is outlined for identifying the nature of small defect clusters produced in cascades and subcascades. The procedure was the growth and shrinkage behavior of clusters produced in the cascades during subsequent irradiations with 1 MeV electrons in a high voltage electron microscope. It is shown that the analysis of the clusters produced during irradiation with high energy recoils can be used to identify the formation of sub-cascades. Various aspects of the one-dimensional glide of small self-interstitial atom (SIA) clusters and their role in defect accumulation are considered. The experimentally observed dose dependence of the cluster density is used to discuss the problem of fission-fusion correlation. Finally, some comments are made on the role of stochastic fluctuation of point defect reaction in the stability and lifetime of SIA clusters under cascade damage conditions.

Defect structures in deformed f.c.c. metals

A high density of small defect clusters, similar to those observed in irradiated or quenched metals, has been observed in the deformed f.c.c. metals Cu, Au and Ni. The preliminary results show that the defect clusters are predominantly stacking fault tetrahedral (SFT). The SFT number density, rather than the size distribution, is deformation dependent. The defect cluster density is greater in the vicinities of dislocation tangles and grain boundaries. Their size distribution is wider than that produced by irradiation with an important number of larger clusters being formed. It is argued that these deformation-produced clusters may play a role in determining the flow stress and work hardening at low deformations.

Structure and property changes in spinel irradiated with heavy ions

We have studied the changes in microstructure and elastic modulus of single crystal MgAl 2O 4 irradiated with 12 MeV Au 3+ at about 100 K to a fluence of 1.5 × 10 16 ions/cm 2. High-resolution electron microscopy and electron diffraction from cross-sectional samples indicate that the irradiated layer remains crystalline even after a peak damage dose of about 33 displacements per atom (dpa). In addition, dark-field electron microscopy reveals that the irradiated layer contains small defect clusters which indicate a high point defect mobility. Young's modulus determined by nano-indentation does not show any significant changes. These results, which are in contrast to the amorphization and 30% decrease in Young's modulus previously observed following 400 keV Xe 2+ irradiation at 100 K, are interpreted in terms of the large electronic to nuclear stopping power ratio of the 12 MeV Au 3+ irradiation and the possibility of beam heating during irradiation.

Non-steady-state conditions and incascade clustering in radiation damage modeling

The results of molecular dynamics simulations of displacement cascades indicate that a large fraction of the point defects that survive after intracascade annealing may be found in clusters, rather than as individual point defects. In addition, the mobility of these small point defect clusters may be relatively high. The impact of these clusters is discussed in the framework of a radiation damage model for austenitic stainless steel using the chemical reaction rate theory. Although this theory has been broadly applied in models simulating such radiation-induced phenomena as void swelling, irradiation creep, and embrittlement, such models have generally not fully accounted for incascade clustering. Many of these models have also focused on the assumed steady state behavior and tended to neglect the explicit dose and temperature dependence of the sink structure. A previously-developed model has been modified to permit a more extensive investigation of the time (dose) dependence of the point defect and extended defect concentrations and of the impact of incascade clustering. A preliminary evaluation of the so-called production bias was also carried out with this model. The results indicate that defect behavior is complex and that the limiting behavior predicted by simple analytical solutions is frequently not achieved. The comparison of the production bias and the conventional dislocation/interstitial bias found that two were comparable if examined separately at 500°C, but that the production bias effect was minimized in the presence of a reasonable dislocation/interstitial bias.

Dose Rate Effects During Damage Accumulation in Silicon

AbstractWe combine molecular dynamics and Monte Carlo simulations to study damage accumulation and dose rate effects during irradiation of Silicon. We obtain the initial stage of the damage produced by heavy and light ions using classical molecular dynamics simulations. While heavy ions like As or Pt induce amorphization by single ion impact, light ions like B only produce point defects or small clusters of defects. The amorphous pockets generated by heavy ions are stable below room temperature and recrystallize at temperatures below the threshold for recrystallization of a planar amorphous-crystalline interface. The damage accumulation during light ion irradiation is simulated using a Monte Carlo model for defect diffusion. In this approach, we study the damage in the lattice as a function of dose and dose rate. A strong reduction in the total number of defects left in the lattice is observed for lower dose rates.

Dose Rate Effects During Damage Accumulation in Silicon

AbstractWe combine molecular dynamics and Monte Carlo simulations to study damage accumulation and dose rate effects during irradiation of Silicon. We obtain the initial stage of the damage produced by heavy and light ions using classical molecular dynamics simulations. While heavy ions like As or Pt induce amorphization by single ion impact, light ions like B only produce point defects or small clusters of defects. The amorphous pockets generated by heavy ions are stable below room temperature and recrystallize at temperatures below the threshold for recrystallization of a planar amorphous-crystalline interface. The damage accumulation during light ion irradiation is simulated using a Monte Carlo model for defect diffusion. In this approach, we study the damage in the lattice as a function of dose and dose rate. A strong reduction in the total number of defects left in the lattice is observed for lower dose rates.

Identification of the nature of neutron-irradiation-induced small point defect clusters in nickel by means of electron irradiation and defect structure development from collision cascade

In order to identify the nature of small point defect clusters produced from cascade collisions, either made of interstitials or vacancies, an effective method is introduced in which the identification is made from the behavior of defects under electron irradiation. Material investigated is nickel neutron-irradiated with several facilities. Under electron irradiation, neutron-irradiation-induced defects having stacking fault tetrahedron (SFT) contrast shrank and disappeared, others having dislocation loop contrast grew. Majority of extremely small defects (≤ 1 nm), over 3/4 of all the defect clusters (bulk, 300 K) and about 95% of all the defect clusters (thin foil, 573 K), disappeared and are clarified to be of vacancy type. The size and spatial distributions of defect clusters in foil specimens and comparison of defect structures in thin foils with those in bulk disclosed the role of freely migrating interstitials.

Reflection diffracted beam interferometry (RDBI) applied to the study of surfaces

Diffracted beam interferometry, DBI, (previously referred to as Convergent Beam Electron Diffraction + Electron Biprism Interferometry, CBED+EBI) which uses an electron biprism to deflect diffracted beams (convergent or parallel) can produce an interferogram between any two beams within the information envelope of the microscope such that the beam's amplitude and phase can be measured and studied. As well, the electron source need not be highly coherent. So far, DBI has been applied only to transmission electron diffraction, although there is no reason why it shouldn't be applicable to all electron diffraction methods including reflection high (low) energy electron diffraction, RH(L)EED and, possibly, back-scattered electron diffraction, BSED, in the SEM for the study of surfaces. DBI has already shown that substantial phase information, such as strain at interfaces and dislocations, compositional gradients and small defect clusters which are on the size scale of the unit cell, are retrievable from its holograms.

Numerical analysis of stress effects on Frank loop evolution during irradiation in austenitic Fe&z.sbnd;Cr&z.sbnd;Ni alloy

Effects of applied stress on early stages of interstitial type Frank loop evolution were investigated by both numerical calculation and irradiation experiments. The final objective of this research is to propose a comprehensive model of complex stress effects on microstructural evolution under various conditions. In the experimental part of this work, the microstructural analysis revealed that the differences in resolved normal stress caused those in the nucleation rates of Frank loops on {111} crystallographic family planes, and that with increasing external applied stress the total nucleation rate of Frank loops was increased. A numerical calculation was carried out primarily to evaluate the validity of models of stress effects on nucleation processes of Frank loop evolution. The calculation stands on rate equuations which describe evolution of point defects, small points defect clusters and Frank loops. The rate equations of Frank loop evolution were formulated for {111} planes, considering effects of resolved normal stress to clustering processes of small point defects and growth processes of Frank loops, separately. The experimental results and the predictions from the numerical calculation qualitatively coincided well with each other.

Defect microstructure in copper after low fluence irradiation with 440 MeV argon and 230 MeV neon ions

A saturation of the yield strength appears in the post irradiation hardening of copper irradiated with high energy heavy ions, in the range from 10 −3–10 −2 dpa. The saturation value of the yield strength is significantly lower than that measured after irradiation with 14 MeV neutrons to comparable doses. To investigate the origin of this difference in behaviour, a detailed transmission electron microscope study of the post-irradiation defect cluster microstructure was undertaken. In specimens irradiated with 440 MeV Ar and 230 MeV ions up to 0.01 dpa the microstructure is composed by large dislocation loops (up to 25 nm), stacking fault tetrahedra (3 nm) and small defect clusters (< 2 nm). In the case of the Ar irradiation, the total defect density is 2.8 × 10 23 m −3 at 10 −2 dpa. Of these, 40–50% are stacking fault tetrahedra, whereas only 1–5% are large loops. The total defect cluster concentration shows a (dose) 0.5 behaviour, with no saturation up to highest dose observed (10 −2 dpa).

Defect microstructure in copper alloys irradiated with 750 MeV protons

Transmission electron microscopy (TEM) disks of pure copper and solid solution copper alloys containing 5 at% of Al, Mn, or Ni were irradiated with 750 MeV protons to damage levels between 0.4 and 2 displacements per atom (dpa) at irradiation temperatures between 60 and 200°C. The defect cluster density in copper was observed to be constant for irradiation temperatures below about 130°C, and to decrease with increasing temperature above 150°C. About 60% of the defect clusters in copper were resolvable as stacking fault tetrahedra (SFT). Cavity formation was observed for irradiation temperatures above about 150°C. The dislocation loop and network densities were relatively low in all of the irradiated pure copper specimens. Contrary to expectations, the loop density and size both decreased with increasing irradiation temperature. Solute additions did not have any significant effect on the total density of small defect clusters, but they did cause a significant decrease in the fraction of defect clusters resolvable as SFT to ~ 20 to 25%. In addition, the dislocation loop density (> 5 nm diameter) was more than an order of magnitude higher in the alloys compared to pure copper.