Irradiated Nickel Research Articles

Positron lifetime measurement and latent vacancy clusters in 14 MeV neutron irradiated nickel

The existence of electron microscopically invisible uncollapsed small vacancy clusters (latent vacancy clusters) in MeV neutron irradiated Ni has been investigated by comparison between electron microscopy observation and positron lifetime measurement. In the specimen irradiated at 300 K, a long lifetime component ( > 300 ps) was observed. This component was not observed in specimens irradiated at 363 K but appeared again in specimens irradiated at 423 K and higher temperatures. Considering the observed defect structures (stacking fault tetrahedra and interstitial loops up to the irradiation temperature of 423 K, and voids in addition to these above 473 K), the long lifetime component at 300 K was reasonably assigned to be latent vacancy clusters and that above 423 K to be voids.

Depth distribution of damage in helium-ion and proton irradiated nickel

Bulk samples of pure nickel are irradiated with 1.5 MeV H + ions and 1.8 MeV He + ions at 300 and 823 K. In order to study the effect of helium and hydrogen atoms on the nucleation of damage clusters, the depth distribution of the damage structure is observed by electron microscopy using the cross-section method. The measured depth distributions of the defect clusters which are dislocation loops, cavities and probably hydrides are compared with the theoretical depth distribution of the production rate of point defects and gas atoms, and are analyzed by diffusivities of point defects and gas atoms. It is suggested that the nuclei of the dislocation loops formed by He +-ion irradiation are vacancy-helium atom complexes and the nuclei of dislocation loops for H +-ion irradiation are interstitials and/or interstitial-hydrogen atom complexes.

Determination of sink concentrations in ion irradiated nickel and copper by diffusion measurements

Diffusion coefficients of nickel in nickel and in copper were measured after 300 keV self-ion irradiation at temperatures between 293 and 1000 K with dose rates between 10−5and 3 × 10−2 dpa/s and doses between 0.2 and 102 dpa. The results were analyzed by means of a rate equation model characterizing the sink strength of the substructure by an effective sink concentration. At lower temperatures, sink concentrations (in terms of atomic units) of 2 × 10−6 in nickel and 2 × 10−5 in copper were deduced. Above 850 K. in nickel and 500 K in copper these sink concentrations decreased considerably. In nickel the effective sink concentration became stationary for doses greater than 0.2 dpa. The present results indicate a higher effective sink concentration than has been deduced so far from electron microscopy investigations.

Effects of preinjected helium in heavy-ion irradiated nickel and nickel-copper alloys

Pure nickel and two nickel-copper alloys (Ni-10 at. pct Cu and Ni-50 at. pct Cu) containing 50 appm preinjected helium have been irradiated with 14 MeV nickel ions at a constant homologous temperature of 0.45T m to 3 to 5 dpa at a depth of 1 μm from the surface. The radiationinduced lattice defects have been analyzed by transmission electron microscopy (TEM) with samples prepared in cross section. In the helium preinjected region of the pure nickel specimen, a substantial density of voids with an average diameter of 35 nm was observed. The nickelcopper alloys were found to contain only a high density of small helium bubbles (under 5 nm in diameter) and dislocation loops. The density of both dislocation loops and helium bubbles increases with increasing copper content, while the size decreases. The observed resistance of the nickel-copper alloys to void formation regardless of the presence of helium bubbles is considered to be the result of local clustering of like atoms which generates a high density of traps, and, thereby, disperses vacancies and gas atoms.

Non-Linear Arrhenius Behaviour of Diffusion Coefficients in Irradiated Nickel and Copper

Non-Linear Arrhenius Behaviour of Diffusion Coefficients in Irradiated Nickel and Copper

Observation of micro-cavities in nickel during He + and D + irradiation and post-irradiation annealing

Changes in sizes and morphology of small cavities in nickel irradiated by 25 keV helium ions and 20 keV deuterons were investigated during irradiation and on annealing after irradiation by means of transmission electron microscopy. In the early stage of He + irradiations at 600 and 700° C, roundish cubes appeared, gradually changed to octahedra. and, then, by the truncation of apexes, finally reached cubo-octahedra. Nucleation and growth behavior of cubic cavities in D + irradiated nickel was different from the case of He + irradiation. On annealing of the He + irradiated specimen, only octahedral cavities showed marked growth, finally changing to roundish cubes at 750° C. Cavities of roundish cubes and cubo-octahedra did not grow nor change their shapes remarkably by the annealing. The cubic cavities formed by D + irradiation at 360° C showed gradual shrinkage on annealing at 600° C and disappeared at 625° C. The changes of cavities during irradiation and on annealing were interpreted by the effect of the internal gas pressure.

Point defect sinks in self-ion-irradiated nickel: A self-diffusion investigation

The diffusion coefficient of 63Ni in pure nickel after irradiation with 300 keV Ni ions has been measured directly using a secondary ion mass spectrometer. The calculated displacement rate for this irradiation ranged from 1.2×10−5 to 3.1×10−2 dpa/s, the dose from 0.2 to 102 dpa, and the temperatures from 293 to 950 K. Between 293 and 650 K the irradiation-induced diffusion coefficient is temperature independent for a displacement rate of 1.2×10−2 dpa/s. In this temperature range mass transport by atomic mixing prevails over diffusion via point defects (radiation-enhanced diffusion). Normalized to the displacement rate K′ the diffusion coefficient describing atomic mixing attains a value of Dmix /K′=1.3×10−18 m2 /dpa. The temperature and displacement rate dependence of the radiation-enhanced diffusion coefficient is discussed in the framework of a rate equation model. The analysis yields a production rate of freely migrating defects of about 1.5% of the calculated displacement rate. The effective concentration of point defect sinks was derived for nickel under heavy ion irradiation. This effective sink concentration Cs showed no dependence on displacement rate but a considerable temperature dependence, e.g., Cs =2×10−6 at 850 K and Cs =2.5×10−7 at 950 K. The conditions for a stationary effective sink concentration in self-ion irradiated nickel are discussed.

Effect of solute elements on void formation in irradiated binary nickel alloys

Abstract Experimental results of a study of the effects of solute elements (carbon and silicon) on the formation of voids in nickel alloys irradiated with 100 keV nickel ions to a dose of 1 × 1016 ions/cm2 at 625°C using transmission electron microscopy are presented. Solute elements affect void formation by reducing void nucleation rate and silicon is found to be more effective than carbon. Effects of temperature in the range 475–675°C and dose (5 × 1015, 1 × 1016and 2.5 × 1016 ions/cm2) on swelling in nickel were also investigated. No voids were observed in Ni-3.6% Si at any temperature and dose. Void number density increases with increasing temperature, reaches a maximum and then decreases. With increasing dose void number density decreases due to void coalescence. Dislocation loops coexist with voids at 475°C and 525°C at a dose of 5 × 1015 ions/cm2.

Defect structure evolution from cascade damage in 14 Mev neutron irradiated nickel and nickel alloys

Abstract D-T neutron irradiation of nickel and 4 nickel based 2 at.% alloys were performed using RTNS-II at 300 to 700 K and fluences up to 1023 n/m2. The role of free point defects was detected by the comparison of defect structures produced by irradiation of material as thin foil and those irradiated as bulk. The analysis of vacancy clusters in thin foil nickel gives the average distance between sub-cascades to be 10 nm and 14 keV as sub-cascade energy. In bulk samples interstitial loops were formed; the major clustered vacancy defects in nickel at the lower temperatures were stacking fault tetrahedra, and voids at the higher temperatures. This difference reflects the variation of the stability of tetrahedra with temperature. Under-sized solutes (Si) and extremely over-sized solutes (Sn) enhanced the formation of loops in matrix, whereas moderately over-sized solutes (Cu,Ge) enhanced the formation of loops near dislocations. In the latter case, voids were observed since free point defects can diffuse long distances.

Hot corrosion behaviour of a laser beam irradiated nickel base superalloy

AbstractThe structure and the hot corrosion resistance of cast nickel‐base superalloy M 38 irradiated by CO2 laser beam of 1.3 KW output have been examined by means of optical microscopy, XRD, SEM and EPMA in comparison with un‐irradiated specimens. The hot corrosion tests were carried out in a crucible filled with a mixture of 75 Na2SO4 + 25 NaCl at 900°C for 15, 35, 55, 80 and 100 hrs, respectively. The results showed that the corrosion resistance of the irradiated alloy is by a factor 2–3 higher than that of the cast alloy. In contrast to significant internal oxidation/sulfidation along the grain boundaries in the cast alloy, under a dense protective oxide scale mainly consisting of Cr2O3, there was no sign of internal oxidation/sulfidation in the irradiated alloy. The irradiated surface layer may be regarded as an effective barrier forming part of the superalloy itself.

Effect of Si, Ge, Sb Solute Atoms on the Void Nucleation in Electron Irradiated Nickel

Effect of Si, Ge, Sb Solute Atoms on the Void Nucleation in Electron Irradiated Nickel

Grain boundary deformation in neutron irradiated nickel

The authors studied grain boundary deformation in nickel irradiated with neutrons in a VVR-K reactor and mechanically tested at different temperatures and various neutron fluences. The degree of radiation embrittlement was found to be significant. Results show that the general nature of intergranular displacement occurred in the form of relative sliding and mutual rotation of grains. Neutron irradiation was also found to suppress grain boundary migration. These conditions are assumed to cause the formation of grain boundary microcracks during high temperature tensile deformation. The microcracks increase with increasing deformation and merge together to form a potential main crack leading to ductility loss and intergranular fracture.

Ion irradiation induced mass transport of Ni in Ni and Fe-20Cr-20Ni

Diffusion coefficients of 63Ni in Ni and Fe-20Cr-20Ni were measured with SIMS after 300 keV Ni + irradiation at temperatures between 293 and 1000 K with an ion flux of 6.3 × 10 12 cm −2s −1. At lower temperatures up to about 600 K a temperature independent diffusion coefficient of about 1.4 × 10 −16 cm 2s −1 was observed, which indicates that transport is dominated at these temperatures by athermal recoil mixing. At higher temperatures the diffusion coefficients are temperature dependent. These results are discussed with respect to point defect reactions and diffusion. The activation energies of the thermally activated process of 0.58 eV for Ni and 0.67 eV for Fe-20Cr-20Ni are determined by half the respective vacancy migration enthalpies. The production rate of freely migrating defects is estimated to be about 2.5% of the calculated damage rate per atom in self-ion irradiated nickel.

A Study of Defect Structures in Fast Neutron Irradiated Nickel at Low Temperature Using Magnetic Anisotropy Measurements

Measurements of the magnetic anisotropy in pure nickel after fast neutron irradiation at 5 K and after successive isochronal annealings at various temperatures between 5 K and 300 K were carried out at liquid helium temperature. Observed torque curves after the irradiation consist of only a biaxial component. By analyzing the intensity and phase of the biaxial term after the irradiation and during isochronal annealings, it was found that the structure of interstitials in nickel is 100-dumbbell type and those interstitials are initially aggregated on (100) planes, then change their state gradually and finally onto (111) plane after isochronal annealings at 320 K. The intensity of induced torque curves in nickel was compared with the case of bcc iron.

Dislocation/obstacle interactions during low dose irradiation creep of nickel

Analyses of interactions between gliding dislocations and irradiation induced obstacles indicated that loop unfaulting, loop annihilation, and loop sweeping can control low dose irradiation creep in fcc metals. Dislocation reactions remove glide barriers from active slip planes and rationalize rapid creep rates observed in deuteron irradiated nickel. Irradiation creep experiments revealed that pre-irradiation reduced irradiation creep rates in annealed Ni whereas preirradiation had little effect on creep rates in cold-worked Ni. Dislocation/obstacle interactions and the role of the applied stress in barrier removal processes were related to observed irradiation creep responses.

The effect of interstitial gas atoms on mustructural evolution in self-ion irradiated nickel

Annealed foils of pure nickel were injected with He in concentrations of 100 and 600 atomic parts per million (appm) at room temperature utilizing a 700 kV electrostatic accelerator. Injections were completed at various accelerating potentials over the range 700 kV to 200 kV. This variation produced a relatively uniform concentration of interstitial gas atoms extending from a depth of approximately 1.25 μm to the surface of the sample. The injected samples were irradiated at temperatures of 525°C and 625°C with 14 MeV Ni 3+ ions to a fluence of 1 × 10 16 ions/cm 2 and prepared for transmission electron muscopy evaluation utilizing a transverse sectioning technique. Results show the formation of a void-free zone which extended from the surface to a depth of 1.25 μm. The variation of void number density, average void diameter, and swelling and a function of depth from the surface is presented for each sample. The results are compared with previous studies completed without injected gas atoms.

Microstructural contributions to bubble growth in helium ion irradiated nickel

Pure nickel films were irradiated by 80 keV He + ions to fluences up to 2.8 × 10 22m −2 at 550°C or 750°C. The films were annealed in a TEM and were subsequently vaporized in a mass spectrometer to measure retained helium. For specimens containing no visible bubbles (lower temperature irradiation) a saturation level of retained helium near 8 × 10 20m −2 (0.8a%) was found; over twice as much retained helium was found in bubble containing specimens (higher temperature irradiation). For these latter specimens only 55% of the retained helium was necessary to fill each bubble to equilibrium at 650°C. Annealing at 600°C of a low temperature irradiated sample produced bubbles which increased in diameter with annealing time to the power of n. For bubbles on the grain boundary n~0.5 and for those in the grain interior n~l. Anneals at 600°C of a high temperature irradiated sample showed all bubbles growing; however, annealing at 650°C of the same sample showed small bubbles shrinking and large ones growing via an Ostwald ripening mechanism.

Investigation of electron irradiated iron alloys and nickel

μSR experiments were performed on electron-irradiated magnetic samples. In dilute iron alloys, the isolated vacancy (30 MHz line) was observed up toTA (annealing temperature) = 200 K. Above this temperature a new line (30 MHz +δv), assigned to vacancy-impurity pairs, was found in some alloys. The frequency shiftδv and the annealing behaviour of the new line are characteristic for each impurity. In electron-irradiated Nino newμSR line was observed.

Flow behavior of nickel irradiated with 15 MeV neutrons and 16 MeV protons

The yield strength of zone refined nickel that was irradiated at 300 K with T(d, n) and Be(d, n) neutrons and 16 MeV protons was found to increase by a similar amount when compared at equal fluence levels. A factor of 2.5 increase in the yield strength was determined for a 15 MeV neutron fluence of 1 × 10 22 m −2 and a 16 MeV proton fluence of 2 × 10 22 m −2 . The strength increase was associated with defect clusters or small prismatic loops and could be described by a strong barrier model for both neutron and proton irradiated nickel. Serrated plastic deformation was observed for samples irradiated with 15 MeV neutrons and 16 MeV protons to fluences higher than 3 × 10 21 m −2 . The similarity in the hardening process of 15 MeV neutron and 16 MeV proton irradiated nickel is consistent with their damage energy cross sections being equal at high primary knock-on energies (> 10 keV).

A Comparison of Electron Energy Loss Spectroscopy and Electron Diffraction for Polycrystalline and Xe+ Irradiated Nickel Silicides

ABSTRACTVariations in the valence and core level electron energy loss spectroscopy (EELS) peaks are studied as a function of nickel silicide composition.Analysis of the EELS spectra in comparison with the electron diffraction patterns show the plasmon energies fingerprint the silicide phases. The EELS core level spectra reflect the Si bonding and the electronic density of states in each phase.Irradiated Ni 2Si becomes amorphous causing a change in the plasmon peak energy and the Si L23peaks.