Radiation-enhanced Diffusion Research Articles

Ion irradiation effects on commercial PH 13-8 Mo maraging steel Corrax

The effects of irradiation on the precipitation behavior of commercial PH 13-8 Mo maraging steel a.k.a. Corrax are investigated through in-situ ion irradiation. Samples of the alloy in its solution annealed state are irradiated up to 10 dpa at 573 and 773 K using 1 MeV Kr ions, in-situ in a transmission electron microscope in order to probe irradiation effects on the precipitation usually observed in this alloy under thermal aging. Indeed, the alloy is known to develop a relatively fine distribution of precipitates during thermal aging which gives the martensitic alloy its strength. The effects of irradiation are substantiated by comparing with the same material thermally aged at 773 and 873 K for similar amounts of experimental time. Both radiation and thermal aging induced segregation and precipitation are characterized using analytical transmission electron microscopy (TEM) techniques.The diffusion coefficients under irradiation are estimated using the point defect balance equations based on Rate Theory and then compared with the thermal diffusion coefficients, demonstrating the accelerated precipitation of β-phase and Laves-phase in the irradiation case at relatively lower temperature is attributed to the radiation-enhanced diffusion. In addition, a numerical model based on classical precipitate nucleation and growth theories is introduced and shows a relatively good agreement with the experimental results in terms of precipitate density. This study serves to generate baseline data on ion irradiation effects on Corrax to learn how this steel responds to irradiation.

Phase stability of single phase Al0.12CrNiFeCo high entropy alloy upon irradiation

Phase stability of NiCoFeCr and Al0.12NiCoFeCr single phase high entropy alloys (HEAs) was studied under 3 MeV Ni2+ irradiation at 500 °C to a fluence of 1 × 1017/cm2 reaching a peak dose of ~100 displacements per atom (dpa). Transmission electron microscopy (TEM) diffraction pattern and scanning transmission electron microscopy-energy dispersive X-Ray spectroscopy (STEM-EDS) were utilized to detect any second phases that formed in the irradiated regions of the specimens. While the single phase of NiCoFeCr alloy has remained stable under the irradiation, the phase in Al0.12NiCoFeCr alloy decomposed into FCC matrix phase, and Ni3Al type nanoprecipitates with L12 ordered structure. On the other hand, in Al0.12NiCoFeCr alloy, the non-irradiated region of the TEM specimen (annealed for 10 h at 500 °C) and the as-cast alloy annealed for 490 h at 500 °C exhibited no such precipitates similar to those observed in the irradiated region. Based upon the thermodynamic stability of these alloys, we propose that radiation-enhanced diffusion under the irradiation accelerated the formation of the Ni3Al type equilibrium phase. Our study clearly demonstrates that the phase stability of HEAs under irradiation should be considered as one of the deciding factors in the materials design and selection for nuclear applications.

Flux effects in precipitation under irradiation – Simulation of Fe-Cr alloys

Radiation-enhanced precipitation of Cr-rich α′ in irradiated Fe-Cr alloys, which results in hardening and embrittlement, depends on the irradiating particle and the displacement per atom (dpa) rate. Here, we utilize a Cahn-Hilliard phase-field based approach, that includes simple models for nucleation, irradiating particle and rate dependent radiation-enhanced diffusion and cascade mixing to simulate α′ evolution under neutrons, heavy ions, and electron irradiations. Different irradiating particles manifest very different cascade mixing efficiencies. The model was calibrated using neutron data. For cascade inducing neutron/heavy-ion dpa rates at 300 °C between 10−8 and 10−6 dpa/s the model predicts approximately constant number density, decreasing radius, decreasing α′ Cr composition, and lower α′ volume fraction. The model then predicts a dramatic transition to no α’ formation above approximately 10−5 dpa/s, while electron irradiation, with weak mixing, has little effect at dpa rates up to 10−3 dpa/s. These model predictions are consistent with experiments. We explain the results in terms of the flux dependence of the radiation-enhanced diffusion, cascade mixing, and their ratio, which all vary significantly in relevant flux ranges for neutron and cascade inducing ion irradiations. These results show that both cascade mixing and radiation-enhanced diffusion must be accounted for when attempting to emulate neutron-irradiation effects using accelerated ion irradiations.

Electron-beam radiation induced degradation of silicon nitride and its impact to semiconductor failure analysis by TEM

By in-situ transmission electron microscopy (TEM), we performed a detailed study on the electron-beam radiation damage to nanostructured silicon nitride thin-film process layers in a typical semiconductor NVM device. It was found that high-dose electron-beam radiation at 200 kV led to rapid degradation of silicon nitride process layers, i.e. thin-downing of nanostructured silicon nitride, inter-diffusion of O and N, the formation of bubble-like defects and segregation of N at neighbouring interfaces. Further detailed analysis revealed that radiation-induced modification in the microstructure and chemical composition of silicon nitride layers could be ascribed to the electron radiation induced knock-on damage and ionization damage. The radiation enhanced diffusion (RED) accounted for the continuous thin-down of the nitride process layer and the formation of bubble-like defects in thick nitride spacer process layers. The work well demonstrated the electron-beam sensitivity of nanostructured silicon nitride materials in the semiconductor devices, and thus may give useful information about electron-dose control during TEM failure analysis of the semiconductor devices containing nanostructured silicon nitride process layers.

Temperature decrease and multiple acceleration of structural and phase transformations in metastable metals and alloys under cascade-forming irradiation. Part 1 – General questions and theory

Classical radiation physics describes well a number of known phenomena observed under irradiation of metals and alloys (radiation embrittlement, swelling, radiation creep), based on relatively slow processes of thermo- and radiation-enhanced diffusion. Mechanisms based on the description of the defect migration processes can not, however, explain the “low- dose effect” under neutron irradiation and the low-dose “long-range effect” under irradiation with accelerated ions E ∼ (104 – k×105) eV (1<k≲3). The paper is devoted to a brief review of the model that takes into account the nanoscale dynamic effects during cascade-forming irradiation. We are talking about explosive energy release in the regions of dense cascades of atomic displacements (thermal spikes) emitting powerful post-cascaded solitary waves, which can initiate structural-and-phase transformations in metastable media, theoretically, at unlimited distances. The distances at which the effect of accelerated (104 – k×105) eV ion beams is observed (in the continuous irradiation mode) are sometimes more than a few tens/hundreds of micrometers (at projected ion ranges of less than 1 μm) and, as recent studies have shown, can reach 1-10 millimeters. These effects are considered on the basis of experimental research data of more than ten different systems. The foundations of the theory of undamped propagation of plane and spherical waves in metastable media are presented. It is noted that the most probable energy of recoil atoms generated by reactor neutrons and fission fragments also belong to the above energy range, which indicates the need to take into account the nanoscale dynamic effects, regardless of the type of the cascade-forming irradiation.

Enhanced diffusion of Cr in 20Cr-25Ni type alloys under proton irradiation at 670 °C

Alloy 709 (Fe-20Cr-25Ni-1.5Mo-0.26Nb in weight percentages) and its model alloy (Fe-20Cr-25Ni) were subjected to 2 MeV proton irradiation up to 1.5 displacements per atom at ∼670 °C for 90 h. Significant compositional changes were observed in the irradiated zone of both alloys, which was correlated with the radiation-enhanced diffusion. The radiation-enhanced diffusion coefficient of Cr was experimentally determined by fitting the Cr depletion profiles and theoretically analyzed considering vacancy-assisted lattice diffusion, dislocation pipeline diffusion, etc. The estimation of the radiation-enhanced diffusion in the 20Cr-25Ni type alloy in this study is useful in predicting its performance in radiation damage environment. The methodology of investigating radiation-enhanced diffusion adds to the nuclear-material screening capability of ion-irradiation technologies.

Formation and dynamics of Au nanoparticles in a silica-glass: synergistic effects of temperature and fluences of ion irradiations

To investigate formation and dynamics of Au nanoparticles in a silica-glass, Au2+ ions are implanted in silica matrix at elevated temperatures up to 600 °C. Optical absorption, Raman spectroscopy and electron microscopy results confirm athermal growth of the Au nanoparticles. Growth dynamics has strong dependence on ion fluences and target temperatures. For example, with the highest ion fluences and at the maximum target temperature, Au nanoparticles have coalesced to form nanorods of Au. Rutherford backscattering spectrometry reveals surprising results about narrowing of implanted profiles of Au atoms with increase of the ion fluences and target temperatures. Observed phenomena are explained in the light of radiation-enhanced diffusion of Au atoms in the ion-implanted silica-glass samples.

Modeling the impact of radiation-enhanced diffusion on implanted ion profiles

Ion irradiations are a valuable research tool for exploring radiation effects in materials. However, it is well recognized that the implanted ions can artificially modify the radiation damage evolution, e.g., enhancing amorphization processes at low irradiation temperatures and suppressing void swelling at elevated temperatures. Therefore, the implanted ion region should be avoided for most studies of ion irradiated materials. Due to increased interest in high damage, high temperature ion irradiations studying radiation effects in materials for proposed high dose Generation IV fission and fusion energy applications, it is crucial to quantify the extent of diffusional broadening of the implanted ion profile for a variety of temperatures, irradiation fluxes, and sink strengths. The present study summarizes computational analyses of thermal and depth-dependent radiation enhanced diffusion (RED) on diffusion broadening of the implanted ion profiles in Fe and Ni for a variety of irradiation conditions. For a low assumed RED coefficient (10−20-10−19 m2/s and 10−4 and 10−3 peak dpa/s, respectively) or high flux, broadening of the as-implanted ion profile is very small and a suitable artifact-free midrange region for analysis exists for ion energies above 5–6 MeV at 100 peak dpa. At high RED coefficients (10−19-10−18 m2/s and 10−4 and 10−3 peak dpa/s, respectively) broadening is much more significant, and no valid region for investigation exists below 6–8 MeV ion energies at any damage level >50 dpa. While increasing flux decreases irradiation time, it also increases the RED coefficient; these effects mostly offset except under recombination-dominant conditions. For typical high dose irradiation studies of void swelling that exceed ∼100 displacement per atom (dpa), ion energies >6–8 MeV must be employed in order to achieve suitable artifact-free midrange irradiation analysis regions, depending on the material system. Analysis of amorphization/precipitation require even higher energies (>10 MeV).

Electron radiation-induced material diffusion and nanocrystallization in nanostructured amorphous CoFeB thin film

Transmission electron microscopy (TEM) is widely used for physical characterization of CoFeB based magnetic tunneling junctions (MTJ) with its atomic-scale resolution. However, highly energetic electron radiation during TEM analysis may cause phase and microstructure modification of CoFeB and its associated MTJ layers. It is the intention of this work to address the issues of the electron-beam sensitivity of CoFeB material. With in-situ TEM, we investigated the electron beam radiation-induced material diffusion and the nanocrystallization behaviors in nanostructured amorphous CowFexByOz/Co60Fe20B20/SiO2 thin films. It was found that electron radiation with different electron dose led to massive diffusion of Co, Fe, B and O atoms across the whole thin film layers, which directly resulted in the modification of the phase and composition of the thin film layers, i.e. the oxidation of Co, Fe, B with O diffusion and the formation of pure Si phase from SiO2. Two stages of material diffusion were observed. While Stage-I material diffusion proceeded with a high diffusion speed, Stage-II had a relatively low diffusion rate accompanying with the nanocrystallization at the bottom of the CoFeB layer. A detailed kinetic study by in-situ TEM revealed the electron-beam radiation induced massive diffusion was a non-thermal process, and the underlying driving force arose from radiation-enhanced diffusion (RED) effects. Nanocrystallization during Stage-II electron-radiation experiment showed unique phase transformation phenomena, repeated nanocrystallization, amorphization, and nanocrystallization processes in the sequence before a stable grain growth could be achieved. A detailed TEM analysis revealed that RED-enhanced B diffusion was responsible for such unique repeated phase transformation processes. B diffusion and the associated structure distortion and the local short-range re-ordering may also account for the phase transformation from fcc-CoxFe23-xB6 to B-rich orthorhombic- CoxFe3-xB phase.

Effects of irradiation damage on the structure in Cr2AlC thin film

The radiation damage effects of Cr2AlC film irradiated by 100 keV He ion with a fluence range of 5 × 1015 to 1 × 1017 ions cm−2 at room temperature have been investigated. A range of characterization techniques including grazing-incidence X-ray diffraction, Raman scattering spectroscopy and ion beam analysis were employed to analyze the irradiation damage on the structure in Cr2AlC thin film. The Cr2AlC film exhibits sensitivity to irradiation of He ions as a result of partial disorder at a relatively low fluence of 5 × 1015 ions cm−2, The atomic arrangement in the crystal structure is seriously damaged at the fluence of 1 × 1017 ions cm−2. The lattice expansion occurs along c direction and the lattice contracts along a/b direction compared to the un-irradiated Cr2AlC. No existence of secondary phase is observed after irradiation. The processes of decomposition and phase transition induced by ion irradiation are not influenced by the chemical environment. The carbon atoms in the lattice are easy to dislocate. The carbon precipitation is induced by radiation-enhanced diffusion in the near surface layer. The accumulation has a tendency to increase with the increasing fluence.

Nanoscale dynamic and long-range effects under cascade-forming irradiation

Classical radiation physics describes well a number of known phenomena in irradiating metals and alloys (radiation embrittlement, swelling, radiation creep) on the basis of relatively slow processes of thermo- and radiation-enhanced diffusion. Mechanisms based on the description of the defect migration processes cannot, however, explain the “small-dose effect” under neutron as well as low-dose “long-range effect” under ~(104–k · 105) eV (1 ≤ k ≲ 3) ion irradiation.11Relatively cheap and simple accelerators (sources) of ions of the indicated energy range, generating high-current ion beams of large cross section, are increasingly used to modify the properties of materials. They can easily be included in the technological processes, in contrast to high-energy accelerating equipment. This paper takes a brief look at the model taking into account the explosive energy release in the regions of the dense cascades of atomic displacements (formation of thermal spikes) and the emission of powerful post-cascade solitary waves that are able initiate structural-and-phase transformations at their front in metastable media, theoretically, at unlimited distances. In practice observed distances of accelerated (104–k · 105) eV ion beams action (in continuous mode) are sometimes more than a few tens/hundreds of micrometers (at Rp < 1 μm; Rp is the projected ion range) and up to 1–10 mm as it was shown in the last experiments. It should be noted that the energies of recoil atoms generated by reactor neutrons and fission fragments also partly belong to the above-mentioned energy range, but the consideration of post-cascade effects for these types of irradiation is beyond the scope of the survey.

Dose rate dependence of Cr precipitation in an ion-irradiated Fe[sbnd]18Cr alloy

Precipitation of α′ in FeCr alloys under neutron irradiation is thermodynamically driven while being accelerated by radiation-enhanced diffusion. However, similar alloys under ion irradiation at high dose rates (>10−4dpa/s) fail to exhibit α′ precipitation. Here, the microstructure of an Fe18Cr alloy under ion or neutron-irradiation at 300°C at dose rates from ~10−7 to 10−4dpa/s was analyzed by atom probe tomography. The steady-state composition content of the clusters depends on the ion irradiation dose and dose rate, confirming the contribution of ballistic mixing in diluting the Cr concentration in non-equilibrium α′ precipitates.

Influence of thermal annealing and radiation enhanced diffusion processes on surface plasmon resonance of gold implanted dielectric matrices

Gold nanoparticles (AuNPs) embedded in fused silica and sapphire dielectric matrices were synthesized by Au ion implantation. Systematic investigations were carried out to study the influence of implantation dose, post annealing temperature, swift heavy ion (SHI) irradiation and radiation enhanced diffusion (RED). Rutherford Backscattering Spectrometry (RBS) measurements were carried out to quantify concentration and depth profile of Au present in the host matrices. X-ray diffraction (XRD) was employed to characterize AuNPs formation. As-implanted and post-annealed films were irradiated using 100MeV Ag ions to investigate the effect of electronic energy deposition on size and shape of NPs, which is estimated indirectly by the peak shape analysis of surface plasmon resonance (SPR). The effect of volume fraction of Au and their redistribution is also reported. A strong absorption in near infra red region is also noticed and understood by the formation of percolated NPs in dielectric matrices. It is quite clear from these results that the effect of RED assisted Oswald ripening is much more pronounced than the conventional Oswald ripening for the growth of NPs in the case of silica host matrices. However for sapphire matrices, it seems that growth of NPs already completed during implantation and it may be attributed to the high diffusivity of Au in sapphire matrices during implantation process.

Mechanism of nucleation and incipient growth of Re clusters in irradiated W-Re alloys from kinetic Monte Carlo simulations

High-temperature, high-dose, neutron irradiation of W results in the formation of Re-rich clusters at concentrations one order of magnitude lower than the thermodynamic solubility limit. These clusters may eventually transform into brittle W-Re intermetallic phases, which can lead to high levels of hardening and thermal conductivity losses. Standard theories of radiation-enhanced diffusion and precipitation cannot explain the formation of these precipitates and so understanding the mechanism by which nonequilibrium clusters form under irradiation is crucial to predict material degradation and devise mitigation strategies. Here we carry out a thermodynamic study of W-Re alloys and conduct kinetic Monte Carlo simulations of Re cluster formation in irradiated W-2Re alloys. We use a generalized Hamiltonian for crystals containing point defects parametrized entirely with electronic structure calculations. Our model incorporates recently gained mechanistic information of mixed-interstitial solute transport, which is seen to control cluster nucleation and growth by forming quasispherical nuclei after an average incubation time of 13.5($\ifmmode\pm\else\textpm\fi{}8.5$) s at 1800 K. These nuclei are seen to grow by attracting more mixed interstitials bringing solute atoms, which in turn attracts vacancies leading to recombination and solute agglomeration. Owing to the arrival of both Re and W atoms from the mixed dumbbells, the clusters are not fully dense in Re, which amounts to no more than 50% of the atomic concentration of the cluster near the center. Our simulations are in qualitative agreement with recent atom probe examinations of ion-irradiated W-2Re systems at 773 K.

Oxide formation on chromium metal surfaces by low-energy oxygen implantation at room temperature

We have studied the formation of oxides on pure chromium metal surfaces by 1keV O2+ ion bombardment at room temperature (RT), using X-ray photoemission spectroscopy around Cr 2p or O 1s core-levels and the valence band photoemission. For comparison, we have also examined the oxidation mechanism of thermally oxidized Cr surfaces at RT. The results of thermal oxidation reveal the formation of a very thin, single Cr2O3 oxide with the saturation thickness of 0.7nm, obtained after exposure to 10L of O2. This layer reduces the reactivity of the surface and prevents any further adsorption or uptake of oxygen at higher oxygen doses. On the other hand, the oxygen-ion bombardment is more efficient in creating thicker Cr2O3 films with the logarithmic increase of film thickness with the implanted dose of oxygen, up to the saturation thickness of about 2.7nm. This thickness is in good agreement with the implantation kinetics of oxygen ions in Cr. We explain the observed direct logarithmic growth rate by the radiation-enhanced diffusion of Cr cations and O anions within the oxide film through the open volume defects, such as voids, created by ion bombardment.

Effects of impurities on the rate of radiation creep and vacancy swelling in Fe-based austenite alloys

In the framework of the model of radiation-enhanced diffusion in the systems with impurities, effects of the formation of “vacancy–impurity atom” complexes on accumulation and annealing of radiation defects, rate of the growth of vacancy voids, and radiation creep in Fe-based austenite alloys are investigated. The temperature dependences of the steady-state concentration of radiation point defects, the concentration of vacancy complexes, and the effective coefficient of mutual recombination of radiation defects in materials with impurity complexes with a different bonding energy are calculated. The possibility of the suppression of the vacancy swelling and radiation creep effects due to formation of “vacancy–impurity atom” complexes having a high bonding energy is shown.

In situ TEM study of electron-beam radiation induced boron diffusion and effects on phase and microstructure evolution in nanostructured CoFeB/SiO2 thin film

Using in situ transmission electron microscopy (TEM), we studied boron diffusion and segregation in CoFeB/SiO2 nanostructured thin film stacks. We also investigated how these phenomena affected the phase and microstructure of CoFeB thin films under electron beam irradiation at 300 kV. A unique phase transformation was observed in CoFeB thin films under high-dose electron irradiation, from a polycrystalline Co3Fe to a unilateral amorphous phase of Co3Fe and nanocrystalline FexCo23−xB6. The unilateral amorphization of the Co3Fe film showed an electron-dose-rate sensitivity with a threshold dose rate. Detailed in situ TEM studies revealed that the unilateral amorphization of the Co3Fe film arose from boron segregation at the bottom of the Co3Fe thin film induced by radiation-enhanced diffusion of boron atoms that were displaced by electron knock-on effects. The radiation-induced nanocrystallization of FexCo23−xB6 was also found to be dose-rate sensitive with a higher electron beam current leading to earlier nucleation and more rapid grain growth. The nanocrystallization of FexCo23−xB6 occurred preferentially at the CoFeB/SiO2 interface. Kinetic studies by in situ TEM revealed the surface crystallization and diffusion-controlled nucleation and grain growth mechanisms. The radiation-enhanced atomic diffusivity and high-concentration of radiation-induced point defects at the Co3Fe/SiO2 interface enhanced the local short-range ordering of Fe, Co, and B atoms, favoring nucleation and grain growth of FexCo23−xB6 at the interface.

Evolution of Kr Precipitates in Kr-Implanted Al as Observed by the Channelling Method

The evolution of Kr precipitates at room temperature from nucleation to the formation of solid Kr precipitates in Al implanted with 50 keV Kr+ ions has been studied through the site change of Kr atoms determined by the channelling method. A previous channelling study reported that nucleation centres are various types of Kr–vacancy (V) complexes formed at low implantation doses. In this study, the initial stage of growth of Kr precipitates to bubbles and a key process towards the formation of epitaxially aligned solid Kr precipitates are investigated. The growth of Kr precipitates to bubbles proceeds from the accumulation of Kr atoms migrating to Kr–V complexes by radiation-enhanced diffusion. The Kr bubbles are in the fluid state. As to the epitaxial alignment, the following mechanism is proposed. At implantation doses higher than 2 × 1015 Kr/cm2, small clusters of Kr atoms located at octahedral (O) or displaced O sites are formed on the planes parallel to {111} planes at the bubble-matrix interface at th...

Methods of local analysis for study of carbon in silicates: Nuclear microprobe analysis and secondary ion mass spectrometry

New approaches are proposed to analyze the content, distribution, and diffusion of carbon in silicates using nuclear microprobe analysis and secondary-ion mass spectrometry (SIMS). Techniques based on the nuclear reaction 12C(d,p)13C were developed to determine the coefficients of radiation-enhanced carbon diffusion in olivine at 300–370 K and deuteron doses that are comparable in terms of defect formation with those of α-particles generated by the decay of uranium and thorium isotopes for ~400 Ma (olivine age). The coefficients of thermal (Dth) and radiation-enhanced (Drad) carbon diffusion in synthetic forsterite were compared to those of natural olivines from alkaline basalt nodule (Shevaryn Tsaram volcano, Mongolia). It is demonstrated that the diffusion coefficients strongly depends on the migration mechanisms of carbon atoms in crystals. The developed techniques and software package for SIMS determination of carbon distribution in silicates allowed us to study simultaneously the carbon and hydrogen distribution in a glass vein of the Chelyabinsk meteorite. The possible presence of hydrocarbons in the studied silicate glass of meteorite is suggested.

Microscopic mechanism responsible for radiation-enhanced diffusion of impurity atoms

Modelling of radiation-enhanced diffusion (RED) of boron and phosphorus atoms during irradiation of silicon substrates respectively with high- and low-energy protons was carried out. The results obtained confirm the previously arrived conclusion that impurity diffusion occurs by means of the ‘impurity atom – intrinsic point defect’ pairs and that the condition of the local thermodynamic equilibrium between substitutional impurity atoms, nonequilibrium point defects created by irradiation, and the pairs is valid. It is shown that using RED, one can form a special impurity distribution in the semiconductor substrate including retrograde profiles with increasing impurity concentration in the bulk of the semiconductor. In addition, modelling of radiation-induced segregation of nitrogen implanted in stainless steel modified by titanium is carried out. It is shown that vacancy-impurity complexes are responsible for nitrogen diffusion in an implanted layer excluding the ‘tail’ region. The calculations performed give clear evidence in favour of further investigation of various doping processes based on RED, especially the processes of plasma doping, to develop a cheap method for forming specific impurity distributions in the near surface region.