Radiation-enhanced Precipitation Research Articles

Rate theory coupling multi-phase-field simulation for neutron irradiation nanophase and vacancy evolution

Radiation damage is a critical problem for the alloys serving under irradiation circumstances where the numerous defects is induced by the cascade collision of radiation particles, the microstructure and mechanical properties will be damaged by the irradiation. By combination of rate theory (RT) and multi-phase-field (MPF) model, the influences of neutron irradiation dose rate (DR) and irradiation temperature are studied on the radiation-enhanced precipitation, vacancies and interstitial atoms in Fe-35Cr-10Al (at.%) alloys. The RT coupled with phase-field simulations capture the radiation induced nanoscale precipitates, clustering of vacancies and interstitial atoms. With the increase of DR, the diffusion of solute atoms is accelerated for the generation of vacancies and interstitial atoms, and the growth and coarsening rate of α′ phase is quickened. The DR affects the distribution of elements between α/α′ phases, high DR promotes the Fe and Al to partition into α phase, Cr into α′ phase in the formation of α′ phase. Therefore, in the steady-state coarsening stage, the high DR leads to the α′ phase to reach the equilibrium composition earlier. As the increase of neutron irradiation temperature, the separation and coarsening rate of α′ phase are accelerated. The simulation morphology and quantitative characteristics are consistent with the experimental results. The coupling of RT and MPF simulation in alloys with point defects and nanophase under neutron irradiation put forward the study tactics and theory understanding for radiation-enhanced multiple microstructure evolutions.

Radiation-enhanced precipitation and the impact on He bubble formation in V-Ti-based refractory alloys containing interstitial impurities

V-Ti-based refractory high entropy alloys (RHEAs), e.g., VTiTa and VTiTaNb, have been found with outstanding mechanical properties and irradiation resistance, granting them application potential in nuclear engineering. Nonetheless, similar to conventional V-Ti-based alloys such as V-4Ti-4Cr, these alloys are sensitive to C/N/O interstitial impurities. Under irradiation, such impurities may promote the formation of precipitates which may affect the defect evolution, especially at the appearance of helium. Previous studies have observed different helium behavior in the above alloys, including the heterogeneous distribution of helium bubbles in VTiTa, but the origin was not clear. In this work, detailed microstructural characterization is performed to analyze the irradiation induced precipitates and their impact on helium bubble formation in these alloys. Under helium ion irradiation at 700°C, the TiN precipitates with the NaCl-type structure are formed in all the three alloys. Nonetheless, the interfaces between precipitates and matrix in VTiTa and VTiTaNb are semi-coherent while those in V-4Ti-4Cr are coherent, due to the various lattice parameters of the alloys. Moreover, severe lattice distortion is also found inside the precipitates in the two RHEAs. Consequently, significant heterogeneous nucleation of He bubbles is observed in these two RHEAs but not in V-4Ti-4Cr. Among the three alloys, the growth of precipitates is the fastest in VTiTa, causing the formation of bubble clusters. Furthermore, the growth direction as well as the shape of precipitates affects the shape of bubbles, leading to the {001} cuboids shape in V-4Ti-4Cr and the truncated {110} dodecahedra shape in V-Ti-Ta and V-Ti-Ta-Nb.

Phase-field simulation of Cu enriched nanoparticles with variation of defects migration energy under neutron irradiation

Neutron radiation induces point defects and affects the diffusivity of atoms and the kinetics of precipitation. The phase-field simulation reveals the influence of migration energy of vacancy on the radiation-enhanced precipitation in Fe–Cu alloy. The study shows that radiation-enhanced diffusion (RED) also depends on the diffusivity of vacancy-associated migration energy and not only on the dose rate; the low migration energy of vacancy results in accelerated precipitation and a higher volume fraction of Cu precipitates. Interestingly, decreasing migration energy from 1.0 eV to 0.9 eV results in a 30% increase in the precipitates’ volume fraction. Also, the combination of the lowest dose rate 5.0 × 10−3 dpa s−1 and highest migration energy 1.0 eV delays the precipitation. The study also examines the influence of migration energy of vacancy on the radius of Cu precipitates. The lowest migration energy, 0.9 eV, increases the radius up to one-third. Finally, the work presents the drawbacks of the analytical digital image processing technique in the quantitative comparison with the script.

Phase-field simulation of dose rate effect on the Cu precipitation with neutron irradiation.

Radiation-enhanced precipitation (REP) of Cu in Fe-Cu alloys results in hardening and degradation of the mechanical properties. By combining the CALPHAD-based free energy for phase-field modeling, and radiation-enhanced diffusion (RED) with neutron irradiating energetic particle, the precipitation of Cu in binary Fe-Cu alloys is studied under different dose rates, concentrations, and temperatures. Rate theory (RT) provides the RED that serves as an input parameter for the phase-field simulation to capture the morphology of the precipitates. The REP results agree with the theoretical predictions: the increase in the dose rates increases the concentration of defects, and accelerates the kinetics of precipitation. The simulation predicts the stability of the precipitates even under high damage rates. The increase in radius is achieved for high damage rates. Precipitate dissolution is observed to be dependent on the combination of dose rate, concentration, and temperature. The work also outlines the limitations of the model and the potential future improvements.

The effect of flux on ion irradiation-enhanced precipitation in AISI-316L: An in-situ TEM study

Thin foils of AISI 316L stainless steel were irradiated in-situ in a transmission electron microscope (TEM) with 325 keV Xe ions at 550 °C at three different fluxes to study flux effects. The kinetics of the radiation-enhanced precipitation (REP) and the evolution of the precipitates were found to be correlated with the irradiation flux. At lower fluxes (1 and 2 × 1012 ions∙cm−2∙s−1), cascade mixing played an important role in the accumulation of point defects within the austenite matrix, facilitating the formation of clusters which act as sinks for heterogeneous nucleation of precipitates with high areal density. At the highest flux (4 × 1012 ions∙cm−2∙s−1) the cascade mixing favours the recombination of vacancies and interstitials which supresses the growth of existing precipitates beyond a certain total damage level. The results agree with a previous radiation-enhanced precipitation model proposed by Wiedersich, Okamoto and Lam and further studied by Bruemmer, but a small modification is proposed when the flux is close to the vacancy-interstitial recombination limit.

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.

Transmission electron microscopy study of second phase particles irradiated by 2 MeV protons at 350 °C in Zr alloys

In order to improve the understanding of the microscopic phenomena occurring during irradiation in zirconium alloys, ion beam irradiations are performed at 350 °C (dose-rate of 2 × 10−5 dpa/s) on recrystallized Zy-4 and M5® alloys, with 2 MeV protons. The aim of this study is to determine in which way proton irradiations can be representative of neutron irradiations, considering the second phase particle changes and the influence of these changes on the microstructural evolution of the material during irradiation.The 2 MeV proton irradiation at 350 °C, performed here, seems to reproduces well what happened in Zy-4 in PWR conditions with a progressive amorphisation of the Zr(Fe,Cr)2 Laves phases, but with a lower growth rate and a higher Fe/Cr ratio of the amorphous rim. The Zr(Fe,Nb)2 s phase particles in M5® undergo a uniform amorphisation, while the native βNb precipitates remain fully crystalline as evidenced in neutron irradiation at very low irradiation temperature. No radiation-enhanced precipitation of nanometric βNb particles was observed. Thus, for M5® alloy, the present irradiation seems to be representative of neutron irradiations at a very low irradiation temperature. Nevertheless it does not reproduce what happens in PWR conditions, where no amorphisation and a drastic loss of iron is reported for the Zr(Fe,Nb)2 Laves phase SPPs. Despite the lower iron rejection from the particles into the matrix during proton irradiation than during neutron irradiation, <c>-component loop distribution is found to be similar after both types of irradiations. These results underline the influence of both dose-rate and temperature on second phase particles behavior under irradiation and point out the complexity of iron rejection influence on the basal <c>-component loops. Indeed, although the <c>-component loop nucleation and growth seem locally correlated to iron dissolution into the matrix, they do not seem to be directly correlated to the global amount of iron rejected.

Irradiation-induced formation of nanorod precipitates in a dilute Cu–W alloy

Radiation-enhanced precipitation of W has been investigated in dilute Cu-W alloys. While spherical W precipitates are stabilized during irradiation below ~300°C and above ~600°C, irradiation at 500°C results in the formation of nanorods. Furthermore, use of two-step irradiations, first one at 600°C followed by one at 500°C, indicates that the morphological transformation is reversible. The stabilization of W nanorods is rationalized using a model introduced by Frost and Russell to explain size stabilization of precipitates formed during irradiation, but extended here to consider precipitate morphology.

Synchrotron radiation investigations of microstructural evolutions of ODS steels and Zr-based alloys irradiated in nuclear reactors

Abstract

Microstructural Stability of M5™ Alloy Irradiated up to High Neutron Fluences

Abstract The present paper is focused on the microstructural stability of the Framatome-ANP M5™ (Zr-1%NbO) alloy. Thin foils were cut from M5 cladding tubes irradiated from 1 to 6 annual cycles in EDF PWRs, and examined by analytical transmission electron microscopy (TEM). The results show that within the wide range of irradiation conditions investigated, the initial β-Nb particles undergo no amorphisation and only minor evolutions in size distribution, number density, and composition. Finally, very few c component basal loops were detected, so that no sign of accelerated growth regime has to be feared for the fuel burn-ups currently considered (∼70 GWd/t) for PWRs. In the second part of this paper, other industrial or experimental alloys (Zy-4, M4, Zr-2.5%Nb, ZrNbSnFe) are considered to improve our knowledge about the irradiation-induced evolution of β-Nb and other minor SPP. Thin foils were cut from specimens irradiated in PWRs or MTRs (OSIRIS, SILOE) and also examined by transmission electron microscopy. In addition, Small Angle Neutron Scattering (SANS) was used to provide quantitative and statistical information on the radiation-enhanced precipitation of β-Nb needles and possible other defects or solute clusters. All these results allow a general discussion of evolution of SPP with regard to their initial structure and composition, and of the noteworthy irradiation stability of the M5™ up to high neutron fluence with respect to other alloys.

Radiation enhanced copper clustering processes in Fe–Cu alloys during electron and ion irradiations as measured by electrical resistivity

To study the mechanism of radiation-enhanced clustering of copper atoms in Fe–Cu alloys, in situ electrical resistivity measurements are performed during irradiation with 100 MeV carbon ions and with 2 MeV electrons at 300 K. Two kinds of highly pure Fe–Cu alloys with Cu content of 0.02 and 0.6 wt% are used. The results are summarized as follows: (1) Although there is a steep initial resistivity increase below about 10 μdpa, the resistivity steadily decreases after this initial transient in Fe–0.6wt%Cu alloy, while in Fe–0.02wt%Cu alloy, the resistivity either decreases slowly or stays almost constant. The rate of change in resistivity depends on copper concentration. (2) The rate of change in resistivity per dpa is larger for electron irradiation than for ion irradiation. (3) Change in dose rate from 10 −8 to 10 −9 dpa/s slightly enhances the rate of resistivity change per dpa. The decrease in resistivity with dose is considered to be due to clustering or precipitation of copper atoms. The initial abrupt increase in resistivity is too large to be accounted for by initial introduction of point defects before copper clustering. Tentatively the phenomenon is explained as due to the formation of embryos of copper precipitates with a large strain field around them. Quantitative evaluation of the results using resistivity contribution of a unit concentration of Frenkel pairs and that of copper atoms gives an important conclusion that more than one copper atom are removed from solid solution by one Frenkel pair. The clustering efficiency is surprisingly high in the present case compared with the ordinary radiation-induced or radiation-enhanced precipitation processes.

On the Effect of Neutron Flux and Composition on Hardening of Reactor Pressure Vessel Steels and Model Alloys

ABSTRACTSingle variable experiments on a set of reactor pressure vessel and model alloys with a range of controlled compositions show a systematic effect of neutron flux (ϕ) on hardening in the intermediate fluence (ϕt) regime. The data suggest that high ϕ and enhanced recombination by vacancy trapping by solutes delays the onset of radiation-enhanced precipitation hardening.

Radiation-enhanced precipitation in FeCu(C) alloys studied by electron microscopy

Radiation-enhanced precipitation in FeCu(C) alloys studied by electron microscopy

Precipitation in Zr-2.5Nb During 10 MeV Electron Irradiation

AbstractThe corrosion resistance of Zr-2.5Nb is increased by neutron irradiation at 570K. This is due to the radiation-enhanced precipitation of β-Nb within the ∝-Zr grains, and the associated reduction in the Nb concentration in the ∝-Zr matrix during irradiation. This microstructure was observed in annealed Zr-2.5Nb irradiated with neutrons at 670K and 770K, and also with protons between 770 and 720K to 0.9 dpa. In this study, the same annealed Zr-2.5Nb, in the form of four 8 mm diameter, 0.5 mm thick discs, was irradiated with 10 MeV electrons for 370 hours. The damage accumulated on the discs ranged from 0.5 to 1.2 dpa, while the temperature of each disc varied from 720K at the centre to 600K at the edge. TEM examination of the 1.2 dpa, 710K sample revealed needle-like precipitates in the ∝-Zr matrix. These precipitates, having widths of I or 2 nm and lengths up to 70 nm, were shown by EDX analysis to contain Zr and Nb. Dark field observations using a precipitate reflection confirmed that the precipitates were β-Nb with about 88 % Nb. Specimens made from the same Zr-2.5Nb that were annealed at 720K for 370 hours but unirradiated had clear ∝-Zr grain interiors, with no precipitates. Thus electron irradiation produced a microstructure similar to that produced by neutrons and protons.

Radiation-altered alloy decomposition for Cu-47 at% Ni-8 at% Fe

Abstract For the alloy Cu-47 at% Ni-8 at% Fe proton radiation-enhanced precipitation and Cu ion radiation-induced precipitate dissolution has been studied by means of atom probing in a field ion microscope. The radiation-enhanced decomposition differs from the thermally activated one. Besides the accelerated quasibinary decomposition into Cu-rich and Ni-rich phases, Fe-rich clusters precipitate. This finding confirms earlier conclusions drawn from small angle neutron scattering data. From atom probe studies of the precipitate dissolution process as caused by 300 keV Cu ion irradation at 573 K, the dissolution efficiency was determined to about 2 nm 2 /dpa.

Electron irradiation of aluminium-zinc alloys: 3. Radiation-enhanced precipitation in an aluminium-4.5 at% zinc alloy

An aluminium-4.5 at% zinc alloy was irradiated at temperatures between 25 and 170°C to doses of 7 dpa to 14 dpa at a dose rate of ~3.88 × 10 −3 dpa/s . Irradiation produced a very high density of coherent G.P. zones in the temperature range ~80–~130°C, coherent plate shaped precipitates in the temperature range ~130–~160°C and, under some irradiation conditions, large Zn precipitates on the foil surfaces. No precipitates occurred along the grain boundaries in the irradiated area. The matrix precipitation was interpreted in terms of the theory of Cauvin and Martin. The surface precipitation is ascribed to surface segregation of Zn and the non-appearance of precipitates along the grain boundaries in the irradiated area is thought to be due to the high metastability of the precipitates on the boundaries under irradiation.

A review of microstructure evolution in zirconium alloys during irradiation

The microstructure developed in Zr alloys during irradiation has proven to be increasingly complicated as specimens of varying composition and thermo-mechanical history have been irradiated to higher fluences. Cavities form in neutron and electron irradiated Zr at temperatures between 625 and 775 K. They are less common in the Zircaloys either because of vacancy trapping or because c- component dislocation loops compete for vacancies. There is a complex dependence on the purity of the material which is related to the presence of precipitates or interstitial solutes and on the presence of insoluble gases such as He. The latter is necessary to prevent the collapse of small vacancy clusters to form vacancy loops. Vacancy and interstitial dislocation loops having Burgers vectors of b = 1 3 〈11 2 ̄ 0〉 ( 〈a〉 type) coexist during neutron and electron irradiation. They are generally aligned in rows or layers parallel with (0001). There is a marked decrease in the stability of vacancy 〈a〉 type loops at temperatures ⩾ 723K. Dislocation loops having Burgers vectors of b = 1 2 [0001] (vacancy type), b = 1 3 〈11 2 ̄ 3〉 (interstitial type) and 1 6 〈20 2 ̄ 3〉 (undetermined character) have been identified in Zr following electron irradiation at temperatures between 573–773 K. Vacancy loops of b = 1 6 〈20 2 ̄ 3〉 have been observed in Zr and Zr alloys with a high interstitial solute content following neutron irradiation at temperatures between 560–773 K. Their formation and growth is determined by a combination of: (i) size effect interaction, dependent on their Burgers vector; (ii) stress, generated in collision cascades during neutron irradiation or in oxidised thin foils during electron irradiation; (iii) interstitial solute, which may lower the stacking-fault energy or segregate to the dislocations bounding the loops; and (iv) anisotropic interstitial diffusion (principally in the basal plane). The dislocation network in cold-worked materials is retained during neutron irradiation, at least for fluences up to 7 × 10 25 n m −2 at 573 K. Intermetallic precipitates of Zr with Fe, Cr and Ni undergo amorphous transformations (depending on the precipitate composition) during neutron irradiation at temperatures < 600 K. Radiation-induced dissolution and reprecipitation occurs at temperatures > 560 K. The secondary precipitates are often distributed in rows or layers parallel with (0001). Radiation-enhanced precipitation from solution has been observed in ZrSn and ZrNb alloys at temperatures between about 573–873 K.

Radiation-enhanced precipitation in Al-4%Cu by high voltage electron microscopy

Radiation-enhanced precipitation in Al-4%Cu by high voltage electron microscopy

On radiation-induced irreversible changes in metals and alloys and the “temperature-spike” concept of radiation damage

The temperature distribution in the immediate neighborhood of a primary knockon between fast neutrons and a heavy meta1, or the “temperature spike,” is calculated and correlated with the induced irreversible changes in the metal. The experimental observations of Kernohan on radiation-enhanced precipitation of Ni-Be compound in a supersaturated solid solution of Be in Ni and of Reynolds on radiation instability of austenitic-type 347 stainless steels can both be successfully interpreted this way. Further studies are in need to consolidate the “temperature spike” concept of radiation damage.