S. E. Danilov

Effects of phosphorus on defects accumulation and annealing in electron-irradiated Fe–Ni austenitic alloys

Abstract Pure austenitic alloy Fe–36% Ni and the same alloy doped with phosphorus (0.1% P) were irradiated with 5 MeV electrons at different temperatures (270–573 K). Using the methods of positron annihilation and residual resistivity it was shown that at the irradiation temperature of 270 K vacancies are mobile in both alloys and form vacancy clusters. In Fe–36Ni–0.1P alloy vacancy clusters are decorated with phosphorus atoms. After the irradiation at 423 K the decoration effect is not observed. At the irradiation temperature of 573 K the addition of phosphorus results in the suppression of vacancy supersaturation. A conclusion is made that, at the elevated irradiation temperatures, suppression of vacancy supersaturation occurs due to the interstitial–phosphorus interaction.

A Study of the Vacancy–Impurity Interaction in Dilute Nickel Alloys by Core Electron Annihilation

It is shown that the angular correlation of annihilation radiation can be used to identify vacancy-impurity complexes in dilute alloys. Annihilation of trapped positrons with core electrons bears information about the chemical environment of a vacancy defect. The method is especially effective for d-matrices doped with sp-impurities since annihilation parameters of positrons with d- and sp-shell electrons differ considerably. The potentialities of the method of core-electron annihilation of positrons are demonstrated taking electron-irradiated dilute Ni-P and Ni-Si alloys as an example. It is shown that the interaction between the vacancies, which migrate at the III stage of annealing, and P atoms in Ni-P causes a considerable change in the annihilation parameters of positrons with core electrons compared to pure Ni. In Ni-Si alloys the annihilation parameters of trapped positrons with core electrons do not differ from those in Ni. This fact is an evidence that Si atoms do not interact with vacancies in Ni.

Structural and phase states and irradiation-induced defects in Ni-Cr alloys

The structural and phase states in alloys of the Ni-Cr-Mo system which were induced by both heat aging and electron irradiation at elevated temperatures have been studied by the methods of measurement of residual resistivity and positron annihilation. Migration of irradiation-induced defects during irradiation at 300°C is shown to initiate processes of ordering or phase separation depending on the initial alloy microstructure and chromium content. It has been established that in the alloy with 32 wt % Cr the concentration of accumulated vacancy defects in the state of short-range ordering after irradiation with 5-MeV electrons to a dose of ∼1.5 × 10−4 dpa at 200°C is half as high as that in the state of long-range ordering with a homogeneous distribution of domains (to 10 nm in size) of the ordered Ni2Cr phase in the matrix.

Effect of neutron and electron irradiation on radiation-induced separation of solid solutions in the Fe-Ni and Fe-Ni-P alloys

Processes of radiation-induced separation of solid solutions in Fe-34.7 at % Ni and Fe-34.7 at % Ni-0.1 at % P alloys have been investigated at Tirr ∼ 320K. A comparative analysis of these processes upon electron (cascade-free) and neutron (cascade-inducing) irradiations has been performed. It has been shown that the efficiency of electron irradiation is higher than that of neutron irradiation. The minimum fraction of freely migrating vacancies upon neutron and electron irradiations is 0.5–0.6 and 6–9%, respectively. Upon annealing of the irradiated samples, several substages of the processes of dissociation of vacancy clusters have been observed. Their energies for activation have been determined. For both types of irradiation the presence of phosphorus increases the fraction of vacancies retained after irradiation. The efficiency of the irradiation in the realization of processes of separation is determined by the concentration of freely migrating point defects and the diffusion length of their migration to sinks.

Radiation-induced strengthening of Al- and Ti-modified Fe-Ni alloys during electron irradiation

A complex study of Fe-Ni, Fe-Ni-Ti, and Fe-Ni-Al alloys irradiated by electrons with an energy of 5 MeV at a temperature of 423 K has been carried out. The relationship between radiation-induced strengthening, radiation-induced defects, and radiation-induced structural and phase transformations has been considered. In the Fe-Ni alloy, vacancy clusters play a leading role in radiation-induced strengthening; in the Fe-Ni-Ti and Fe-Ni-Al alloys, in addition to the vacancy clusters, the evolution of secondary-phase precipitates and the relief of solid-solution hardening are of great significance.

Accumulation and annealing of radiation defects under low-temperature electron and neutron irradiation of ODS steel and Fe-Cr alloys

The processes of accumulation and annealing of radiation defects at low-temperature (77 K) electron and neutron irradiation and their effect on the physicomechanical properties of Fe-Cr alloys and oxide dispersion strengthened (ODS) steel have been studied. It has been shown that the behavior of radiation defects in ODS steel and Fe-Cr alloys is qualitatively similar. Above 250 K, radiation-induced processes of the solid solution decomposition become conspicuous. These processes are much less pronounced in ODS steel because of specific features of its microstructure. Processes related to the overlapping of displacement cascades under neutron irradiation have been considered. It has been shown that, in this case, it is the increase in the size of vacancy clusters, rather than the growth of their concentration, that is prevailing. Possible mechanisms of the radiation hardening of the ODS steel and the Fe-13Cr alloy upon irradiation and subsequent annealing have been discussed.

Radiation defects and hydrogen in austenitic and austenitic-martensitic steels under low-temperature neutron irradiation

The experimental data concerning the effect of hydrogen (300 appm), radiogenic helium, and low-temperature neutron irradiation (77 K) on the properties of the promising austenitic 16Cr15Ni3Mo1Ti and austenitic-martensitic 16Cr9Ni3Mo steels have been reported. It has been found that hydrogen saturation causes an increase in the yield stress, with this increase being larger in the martensitic than in the austenitic phase. The yield stress of both steels increases substantially after exposure to fast neutrons. The variation of the yield stress of the two-phase steel and its phase components under low-temperature neutron irradiation has been estimated. The displacement cascades begin overlapping under irradiation at a fluence larger than 1.5 × 1018 cm−2.

Separation of radiation defects in deformed nickel

Measurements of electrical resistivity and the electron-microscopy method have been used to study processes of the separation of radiation defects that arise upon the electron irradiation in pure nickel deformed by rolling. It has been shown that the effect of the separation of radiation defects is determined by the microstructure. In deformed Ni annealed at 450 K, there are two systems of sinks for point defects, i.e., the boundaries of subgrains and the dislocation structure in the bulk of subgrains. With an increase in dose, the increments of the electrical resistivity in deformed nickel (concentration of vacancies) approach a quasi-stationary level and depend on the degree of deformation nonmonotonically. The maximum of the increment is observed at a degree of deformation of about 40%. The kinetics of the postradiation annealing of vacancies accumulated upon irradiation is determined by the strength of sinks, just as for interstitial atoms in the process of irradiation.

The separation effect of radiation-induced defects in nickel

Pure nickel, the model material for austenitic steels used in reactors, with an electrical resistivity ratio of ρ300 K/ρ4.2 K ∼ 300 has been investigated under electron and neutron irradiation at TIRR ∼ 320–340 K. Three of its states have been subjected to irradiation: a recrystallized state at 873 K, a deformed one to 90%, and an annealed one at 450 K after deformation to remove deformation-induced vacancies. It is has been experimentally shown that neutron and electron irradiation of the deformed nickel results in the separation of radiation-induced defects. This separation occurs because a significant portion of the radiation-induced interstitial atoms is captured by dislocation sinks and does not participate in recombination with vacancies. As a result, the concentration of accumulated vacancies in the deformed nickel can exceed their concentration in the annealed nickel and be almost twice as high. At higher doses of neutron irradiation, above 1018 cm−2, separation does not occur, since vacancy sinks in the form of clusters are more powerful than dislocation sinks.

Accumulation and annealing of radiation defects and the hydrogen effect thereon in an austenitic steel 16Cr15Ni3Mo1Ti upon low-temperature neutron and electron irradiation

The effect of hydrogen, accumulation and annealing of radiation defects on the physicomechanical properties of an austenitic Kh16N15M3T1 steel (16Cr15Ni3Mo1Ti) has been investigated upon low-temperature (77 K) neutron and electron irradiations. It has been shown that, when its concentration is about 300 at ppm, hydrogen reduces plasticity by 25%. The presence of helium (2.0–2.5 at ppm) introduced by the tritium-trick method exerts an effect on the yield strength and hardly affects embrittlement. Upon both electron and neutron irradiation, there is a linear relation between the increment of the yield strength and the square root of the increment of the residual electrical resistivity (the concentration of radiation defects). The annealing of vacancies occurs in the neighborhood of 300 K (energy for vacancy migration is 1.0–1.0 eV). Vacancy clusters dissociate near 480 K (energy for dissociation is 1.4–1.5 eV).