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Boron is the most important p-type dopant in Si and it is essential that, especially for low energy implantation, both as-implanted B distributions and those produced by annealing should be characterized in very great detail to obtain the required process control for advanced device applications. While secondary ion mass spectrometry (SIMS) is ordinarily employed for this purpose, in the present studies implant concentration profiles have been determined by direct B imaging with approximately nanometer depth and lateral resolution using energy-filtered imaging in the transmission electron microscopy. The as-implanted B impurity profile is correlated with theoretical expectations: differences with respect to the results of SIMS measurements are discussed. Changes in the B distribution and clustering that occur after annealing of the implanted layers are also described.
Neutron hardening and embrittlement of pressure vessel steels is due to a high density of nanometer scale features, including Cu-rich precipitates which form as a result of radiation enhanced diffusion. High-energy displacement cascades generate large numbers of both isolated point defects and clusters of vacancies and interstitials. The subsequent clustering, diffusion and ultimate annihilation of primary damage is inherently coupled with solute transport and hence, the overall chemical and microstructural evolutions under irradiation. In this work, we present atomistic simulation results, based on many-body interatomic potentials, of the migration of vacancies, solute and self-interstitial atoms (SIA) in pure Fe and binary Fe-0.9 and 1.0 at.% Cu alloys. Cu diffusion occurs by a vacancy mechanism and the calculated Cu diffusivity is in good agreement with experimental data. Strain field interactions between the oversized substitutional Cu solute atoms and SIA and SIA clusters are predominantly repulsive and result in both a decreased activation energy and diffusion pre-factor for SIA and small (N <5) SIA cluster migration, which occurs by three-dimensional motion. The Cu appears to enhance the re- orientation of the SIA clusters to different <111> directions, as well as the transition from <110> to mobile <111> configurations. The migration behavior of larger SIA clusters, which undergo only one-dimensional diffusion during molecular dynamics timescales, is largely unaffected by the Fe-Cu alloy, although SIA clusters are effectively repelled by coherent Cu precipitates.
Precipitation of copper-rich clusters is a major cause of in-service hardening of reactor pressure vessel steels and has attracted much attention. Experimental studies of microstructural changes in alloys under various conditions have revealed similarities and differences. It has been established that under ageing the precipitate ensemble experiences normal nucleation, growth and Ostwald ripening, a distinguishing feature of which is the bcc-9R-3R-fcc transformations the precipitates undergo during growth. The main effect of electron irradiation is believed to be enhancement of the diffusion of copper and hence acceleration of the kinetics. In the case of neutron irradiation, however, there are many aspects that are not clear. One is that at temperatures less than about 300°C the precipitate size is observed to be very small (∼1-3 nm), i.e. the coarsening rate is very low. In this paper we study this phenomenon by computer simulations based on the “mean-field” approach for describing microstructural evolution.
A TEM observation of fission neutron-irradiated copper at 300°C shows that the maximum size of stacking fault tetrahedra (sft) observed is 6 nm of edge length which corresponds to a cluster of 280 vacancies and the minimum size of voids is 2.2 nm in diameter which corresponds to a cluster of 470 vacancies. The result suggests that a vacancy cluster whose size is smaller than 300 vacancies grows to sft while a cluster whose size is larger than 500 vacancies relaxes to a void in 300°C-irradiated copper. A computer simulation of molecular dynamics (MD) with an isotropic EAM potential examined this model. It is found that a vacancy cluster that is smaller than 300 vacancy segregates to a (111) platelet and relaxes to an sft. Small vacancy clusters which are generated at damage cascade cores aggregate to spherically distributed vacancies for the size of more than 500 vacancies, and relax to several (111) platelets, which finally form a vacancy (111) polyhedron. Inside a polyhedral vacancy platelet, vacancies are confined and grow to a void at high temperature.
The objective of this work is to investigate the addition of misfit elements in both size and mass on the evolution of irradiated microstructure in 316 SS. Alloys were modified by the addition of Pt and Hf to suppress the radiation damage. Pt and Hf were added as a lattice perturbation to catalyze defect recombination within the early stage of cascade formation and defect migration. Irradiations were conducted with 5 MeV Ni-ions at 500 °C to doses up to 50 dpa or with 3.2 MeV protons at 400 °C. Microstructures were characterized using transmission electron microscopy. While no beneficial effect was seen for Pt addition, Hf appears to effectively alter the microstructural response to irradiation.
We present results of a weak-beam transmission electron microscopy study of “matrix damage” in two nearly-pure irons (designated alloys 1A and 2A) produced by neutron irradiation to a fluence of 0.06 dpa at 280°C. The matrix damage in both materials was found to consist of small (2-6 nm) dislocation loops. About 80 % have Burgers vectors b = a<100>, and the remainder have b = a/2<111>. The loops in alloy 1A have a mean image size dmean = 2.8± 0.1 nm and a mean maximum image size dmax = 4.2± 0.3 nm, while those in 2A have d mean = 3.4± 0.1 nm and d max = 4.5± 0.3 nm. The number densities are about 8.5 × 1021 m−3 in alloy 1A, and 6.6 × 1021 m−3 in 2A. It can be shown that the loops can account for the observed irradiation hardening. At least some loops are stable under thermal annealing to temperatures of at least 430°C. This and other indirect evidence suggests that their nature is interstitial.
Densification and crystallization kinetics of bulk SiC amorphized by neutron irradiation is studied. The temperature of crystallization onset of this highly pure, fully amorphous bulk SiC was found to be between 875-885°C and crystallization is nearly complete by 950°C. In-situ TEM imaging confirms the onset of crystallization, though thin-film effects apparently alter the kinetics of crystallization above this temperature. It requires >1125°C for complete crystallization of the TEM foil. Annealing at temperatures between the irradiation and crystallization onset temperature is seen to cause significant densification attributed to a relaxation, or reordering, of the as-amorphized structure.
Tensile properties of Ti-35Al-15V and Ti-30Al-10V alloys were examined. These alloys contain β phase with ordered bcc structure. The total elongation of Ti-35Al-15V alloy abruptly changes from 10% to 60% around 620 ºC. Many transformation bands were observed in β grains that were highly deformed above this temperature. The hexagonal α2-phase grains in bands and the β–phase grains in bands and matrix were in Burgers type of orientation relationship. The ductility of Ti-30Al-10V alloy was highly affected by neutron irradiation. The specimens irradiated and tested at 400 °C showed almost no ductility. The total elongation of specimens irradiated and tested at 600 °C was about 10 %, while that of unirradiated ones was larger than 60 %. Specimens irradiated at 400 °C were also tested at 600 °C and the elongation was only 6 %, showing little recovery in ductility. This embrittlement suggests phase decomposition during irradiation.
Under ion irradiation collisional mixing competes with phase separation if the irradiated solid consists of immiscible components. If a component is a chemical compound, there is another competition between the collisional forced chemical dissociation of the compound and its thermally activated re-formation. Especially at interfaces between immiscible components, irradiation processes far from thermodynamical equilibrium may lead to new phenomena. If the formation of nanoclusters (NCs) occurs during ion implantation, the phase separation caused by ion implantation induced supersaturation can be superimposed by phenomena caused by collisional mixing. In this contribution it will be studied how collisional mixing during high-fluence ion implantation affects NC synthesis and how ion irradiation through a layer of NCs modifies their size and size distribution. Inverse Ostwald ripening of NCs will be predicted theoretically and by kinetic lattice Monte-Carlo simulations. The mathematical treatment of the competition between irradiation-induced detachment of atoms from clusters and their thermally activated diffusion leads to a Gibbs-Thomson relation with modified parameters. The predictions have been confirmed by experimental studies of the evolution of Au NCs in SiO2 irradiated by MeV ions. The unusual behavior results from an effective negative capillary length, which will be shown to be the reason for inverse Ostwald ripening. Another new phenomenon to be addressed is self-organization of NCs in a δ-layer parallel to the Si/SiO2 interface. Such δ-layers were found when the damage level at the interface was of the order of 1-3 dpa. It will be discussed that the origin of the δ-layer of NCs can be assigned to two different mechanisms: (i) The negative interface energy due to collisional mixing gives rise to the formation of tiny clusters of substrate material in front of the interface, which promotes heteronucleation of the implanted impurities. (ii) Collisional mixing in the SiO2produces diffusing oxygen, which may be consumed by the Si/SiO2 interface. A thin layer parallel to the interface becomes denuded of diffusing oxygen, which results in a strong pile up of Si excess. This Si excess promotes heteronucleation too. Independent of the dominating mechanism of self-organization of a d-layer of NCs, its location in SiO2 close to the SiO2/Si interface makes it interesting for non-volatile memory application.
Two high copper irradiated welds, one containing very low Ni and the other containing very high Ni, have been examined using 3-D atom probe (3DAP) microscopy, small angle neutron scattering (SANS) and field emission gun-scanning transmission electron microscopy (FEG- STEM).
Irradiation induced clusters were observed in both welds. They were found to be significantly smaller and exist at a higher number density in the high Ni weld. A new algorithm was developed to precisely identify the shape, composition and size of clusters observed in the atom probe data. Representative irradiation induced clusters from each weld were then examined in greater detail. They were shown to be ramified and have a significant Fe content (∼60at.%). Cu was found to be more strongly associated with the core of the clusters than Mn or Ni. In the low Ni weld, there was evidence for P at the interfaces between the clusters and matrix. Cluster composition estimates from FEG-STEM analyses were consistent with those observed by 3DAP microanalysis. For each weld, the mean radius of gyration of the clusters was found to be almost identical to the radius of gyration determined directly from SANS analyses of these materials. Finally, the number density of features was estimated from the SANS data by using the compositional information from the 3DAP observations. Consistency with the number density calculated directly from the 3DAP data was obtained provided that it is assumed that the clusters exhibit some magnetic properties.
Molecular Dynamics (MD) is a very powerful tool for studying displacement cascades initiated by the neutrons when they interact with matter and thus evaluate the primary damage. The mean number of point defects created can be obtained with a fair standard error with a reasonable number of cascade simulations (10 to 20 ), however other cascades characteristics (spatial distribution, size and amount of defect clusters …) display a huge variability. Therefore, they may need to be studied using faster methods such as the Binary Collision Approximation (BCA) which is several order of magnitude less time consuming. We have investigated the point defect distributions subsequent to atomic collision cascades by both MD (using EAM potentials for Fe) and its BCA. MD and its BCA lead to comparable point defect predictions. The significant similarities and differences are discussed.
Crystallization of spatially isolated amorphous zones in Si, Ge, GaP, InP and GaAs was stimulated thermally and by irradiation with electrons and photons. The amorphous zones were created by a 50 keV Xe+ implantation. Significant thermal crystallization occurred at temperatures greater than 425 K, 375 K and 200 K in Si, Ge and GaAs, respectively. Electrons with energies between 25 and 300 keV stimulated crystallization in all materials at temperatures between 90 K and room temperature. For electron energies above the displacement threshold, the crystallization rate decreased as the electron energy decreased. As the electron energy was decreased below approximately 100 keV, the crystallization rate unexpectedly increased. The crystallization rate was independent of temperature for all electron irradiations. Irradiation with a 532 nm green laser (hv= 2.33 eV) caused crystallization in Si (Eg = 1.11 eV) and Ge (Eg = 0.67 eV) at a rate comparable to a thermal anneal at 425 K and 375 K, respectively, and caused minimal crystallization in GaP (Eg = 2.26 eV). The electron and photon irradiation results are consistent with the model that crystallization is controlled by defects (dangling bonds and kinks) created by electronic excitation at the amorphous-crystalline interface.
Advances in computational capability and modeling techniques, as well as improvements in experimental characterization methods offer the possibility of directly comparing modeling and experiment investigations of irradiation effects in metals. As part of a collaboration among the Instituto de Fusión Nuclear (DENIM), Lawrence Livermore National Laboratory (LLNL) and CIEMAT, single and polycrystalline α-Fe samples have been irradiated with 150 keV Fe- ions to doses up to several dpa. The irradiated microstructure is to be examined with both transmission electron microscopy (TEM) and positron annihilation spectroscopy (PAS). Concurrently, we have modeled the damage accumulation in Fe under these irradiation conditions using a combination of molecular dynamics (MD) and kinetic Monte Carlo (KMC). We aim to make direct comparison between the simulation results and the experiments by simulating TEM images and estimating positron lifetimes for the predicted microstructures. While the identity of the matrix defect features cannot be determined from TEM observations alone, we propose that both large self-interstitial loops, trapped at impurities within the material, and small, spherical nanovoids form.
The effect of bulk P contents on hardening, non-equilibrium intergranular segregation and embrittlement has been studied in Mn-doped ferritic alloys subjected to neutron irradiation (E>0.1MeV: fluence of 1 × 1025 n/m2 at 711K for 2120 h) or irradiation-equivalent thermal aging. Neutron irradiation-induced intergranular P segregation became more prominent with decreasing bulk P content. Thermal aging slightly enhanced the amount of segregated P independent of the bulk P content. Intergranular C segregation in all the alloys was suppressed by the irradiation. An alloy with low bulk P content showed only moderate irradiation-induced hardening. The ductile-brittle transition temperature (DBTT) in alloys with low and intermediate amounts of P increased by the same shift during the irradiation but not at all during the thermal aging. Doping high bulk P led to a high DBTT in the as-heat-treated alloy while the irradiation decreased the DBTT. The irradiation effect on the DBTT in the model ferritic alloys containing the different levels of P is discussed in light of embrittling or toughening effects caused by the changes in the P or C segregation, and hardness.
This paper explores the effect of hydrogen on the luminescence properties of silicon nanocrystals formed in silica by high-dose ion-implantation and thermal annealing. For samples implanted to low fluence (small nanocrystals), passivation is shown to result in a uniform enhancement of the PL emission for all wavelengths. However, for samples implanted to high fluence, preferential enhancement of the emission from larger nanocrystals is evident, resulting in a red-shift of emission spectra. Both the intensity enhancement and the red-shift are shown to be reversible, with spectra returning to their pre-passivation form when H is removed from the samples by annealing. The luminescence lifetime is also shown to increase after passivation, confirming that defect-containing nanocrystals luminesce.
Implantation of dopant ions in SiC has evolved according to the assumption that the best electrical results (i.e., carrier concentrations and mobility) are achieved by using the highest possible processing temperature. This includes implantation at > 600°C followed by furnace annealing at temperatures as high as 1750°C. Despite such aggressive and extreme processing, implantation suffers because of poor dopant activation, typically ranging between < 2%–50% with p-type dopants represented in the lower portion of this range and n-types in the upper. Additionally, high-temperature processing can led to several problems including changes in the stoichiometry and topography of the surface, as well as degradation of the electrical properties of devices. A novel approach for increasing activation of implanted dopants in SiC and lowering the activation temperature will be discussed. This approach utilizes the manipulation of the ion-induced damage to enhance activation of implanted dopants. It will be shown that nearly amorphous layers containing a small amount of residual crystallinity can be recrystallized at temperatures below 900°C with little residual damage. It will be shown that recrystallization traps a high fraction of the implanted dopant residing within the amorphous phase (prior to annealing) onto substitutional sites within the SiC lattice.
Polycrystalline specimens of hcp pure titanium have been irradiated at 300-320 K with 590 MeV protons to doses ranging between 10-3 and 3×10-2 dpa. Combination of tensile deformation experiments and transmission electron microscopy observations revealed that irradiation produces slight hardening of the material, related to the irradiation-induced formation of defect clusters, but not significant loss of ductility. Plastic deformation of irradiated titanium is homogeneous. It occurs via propagation of dislocations through a cloud of defect clusters, leading to their annihilation and the formation of a cellular dislocation structure together with twins. This mechanical behavior is similar to what was previously observed for pure fcc metals, the formation of twins being however intrinsic to deformation of hcp titanium.
A classic example of radiation-induced phase instability and degraded mechanical properties occurs in γ′-γ″-strengthened alloy 718. During irradiation with neutrons or protons at ∼ 30 to 288°C, the Ni 3Nb γ″ particles disappear after low doses. The γ′ (present only in the matrix) also disappears after <0.6 dpa at 30 to 55°C, but at 288°C it persists to higher doses and eventually reprecipitates as new γ′ with changed composition. Hardness of the alloy is unaffected by disappearance of the γ″, but decreases appreciably at 288°C as the original γ′ particles dissolve. Fine-probe compositional measurements in a TEM showed that the softening coincides with solute redistribution and reprecipitation rather than with the phase disappearance. Compositional changes at grain boundaries included leveling of the thermally segregated Mo as well as strong Ni enrichment and loss of Nb after high doses. The complex phase stability and solute redistribution behavior reflects mainly ballistic mixing at 30-70°C irradiation temperatures and the influence of significant thermal diffusivities at the higher temperatures.
The Pressurized Water Reactor vessel steels are embrittled by neutron irradiation. Among the solute atoms, copper play an important role in the embrittlement and different Cu-rich defects have been experimentally observed to form. We have investigated by Kinetic Monte Carlo (KMC) on rigid lattices the evolution of the primary damage. Since the point defects created by the displacement cascades have very different kinetics, their evolution is tracked in two steps. In a first step, we have studied their recombination in the cascade region and the formation of interstitial clusters using “object diffusion”. The parameters of this model are based on MD simulations, or on first principles calculations. In a second part, we have investigated the subsequent evolution of the primary damage with a model based on a vacancy jump mechanism. These simulations which rely on an adapted EAM potential show the formation of copper rich defects. Some of the potential's predictions that played a key role in the model were checked by ab initio calculations. The defects obtained from these simulations, subsequent to the primary damage created by displacement cascades, exhibit similarities with the ones observed by atom probe. The influence of temperature and Cu content on the final damage was investigated.
Irradiation-induced amorphization of Cd2Nb2O7 pyrochlore was investigated by means of in-situ temperature-dependent ion-irradiation experiments in a transmission electron microscope, combined with ex-situ ion-implantation (at ambient temperature) and RBS/channeling analysis. The in-situ experiments were performed using Ne or Xe ions with energies of 280 and 1200 keV, respectively. For the bulk implantation experiments, the incident ion energies were 70 keV (Ne+) and 320 keV (Xe2+). The critical amorphization temperature for Cd2Nb2O7 is ∼480 K (280 keV Ne+) or ∼620 K (1200 keV Xe2+). The dose for in-situ amorphization at room temperature is 0.22 dpa for Xe2+, but is 0.65 dpa for Ne+ irradiation. Both types of experiments suggest a cascade overlap mechanism of amorphization. The results were analyzed in light of available models for the crystalline-to-amorphous transformation and were compared to previous ionirradiation experiments on other pyrochlore compositions.