This paper describes a technique developed to determine values for the dislocation barrier strength of the defects believed to be responsible for the embrittlement of light water reactor (LWR) pressure vessel steels. Microstructures consisting of a single defect type were introduced by ion irradiation or thermal annealing, and the defect distributions were determined by TEM. Hardness changes were measured using a nano indenter and the dislocation barrier strengths for the defects involved were computed based on a dispersed barrier hardening model.

High-energy gamma rays emitted from the core of a nuclear reactor produce displacement damage in the reactor pressure vessel (RPV). The contribution of gamma damage to RPV embrittlement has in the past been discounted. However, in certain reactor designs the gamma flux at the RPV is sufficiently large that its contribution to displacement damage can be substantial. For example, gamma rays have been implicated in the RPV embrittlement observed in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory.

In the present study, mechanical property changes induced by 10-MeV electron irradiation of a model Fe alloy and an RPV alloy of interest to the HFIR were examined. Tensile specimens were irradiated with high-energy electrons to reproduce damage characteristic of the Compton recoil-electrons induced by gamma bombardment. Substantial increases in yield and ultimate stress were observed in the alloys after irradiation to doses up to 5.3x10−3 dpa at temperatures (~50°C) characteristic of the HFIR pressure vessel. These measured increases were similar to those previously obtained following neutron irradiation, despite the highly disparate nature of the damage generated during electron and neutron irradiations.

It is proposed that the spatial distribution of transmutation-produced silicon precipitates formed in aluminum by capture of thermal neutrons is systematically modified by the fast neutron portion of the spectrum, and the tensile strength is altered accordingly. The concept is used to rationalize some previously erratic tensile data.

Samples of austenitic 304L stainless steel have been irradiated with 3.4 MeV protons to atotal dose of 1 dpa. The microstructure of the irradiated stainless steel has been quantified by transmission electron microscopy and shown to be similar to that found in neutron-irradiated core components. Constant extension rate tensile tests have been performed at strain rates of 3x10−7 s−1 and 3x10−8 s−1 to strains of up to 27% at 23°C and 288°C. The resulting microstructures were characterized using electron and optical microscopy. Deformation of the unirradiated material is similar to that reported by others in previous work on austenitic steels, consisting of dislocation source activation and formation of dense dislocation networks with increasing strain. In the irradiated samples tested at 288°C, deformation consists of dislocation source activation at grain boundaries punching dislocations through the grain interior to the opposing grain boundaries. The dislocations create channels that are free of radiation-produced defects and on which dislocation motion is concentrated. Dislocations in the channels pile-up at the boundaries, creating regions of highly localized stress concentration at the grain boundary. The mechanism by which this stress is relieved is still unknown. Deformation at 23°C consists of the nucleation and propagation of microtwins across the width of the grains.

Analysis of the deformation microstructures in ion-irradiated stainless steel shows twinning to be the predominant deformation mode at room temperature. Dislocation channelling also occurs under slow strain rate conditions. Stresses required for twinning were calculated by the model of Venables and are compatible with observed yield stresses in neutron-irradiated material if loops are the principal twin source. Computation of the expected radiation hardening from the defect structure, based on a simple model, is consistent with yield strengths measured on neutron-irradiated steels. Lower yield stresses and greater thermal energy at 288°C lessen the probability of twinning and dislocation channeling becomes the primary deformation mode at the higher temperature. However, preliminary results show that some twinning does occur in the irradiated stainless steel even at the higher temperature when higher strain rates are used.

The binary and ternary intermetallic compounds Zr3Fe, Zr2 Fe, (Zr0.5,Nb0.5)3Fe, Zr3(Fe0.9,Ni0.1) and Zr3(Fe0.5,Ni0.5) were subjected to 900 keV electron irradiation until amorphous to study the change in the dose-to-amorphization with temperature. The critical temperatures were observed to vary with dose rate, and with the type of compound. Hexagonal (Zr0.5,Nb0.5)3Fe had an appreciably lower critical temperature and higher dose to amorphization at low temperature than orthorombic Zr3Fe, whereas other orthorombic Zr3(Fex,NiI-x) compounds were essentially identical in behavior to Zr3Fe. The electron energy dependence of the dose-to-amorphization was studied in Zr3Fe between 250 and 900 keV. The analysis of the results gives displacement energies of EZrd = 26 eV, EFed = 18 eV in the Zr3Fe compound.

The 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.

Both stress-relieved (SR) and recrystallized (RX) samples irradiated near 300°C tofluences between 1 and 9 x 1021 n/;cm2 (E>IMeV) were examined using analytical scanning transmission electron microscopy (STEM). The aim was to extend our knowledge of irradiation effects on microstructure by examining the effect of prior-cold-work on the dissolution of intermetallic precipitates in Zircaloy-2. Resulting from prior mechanical deformation and fast neutron irradiation, SR samples contain a high density of <a>-component, mixed <a+> line dislocations and “black dot” damage. On the other hand, RX samples contain mostly “black dot” damage. Pure <c>-dislocations are detected in the high fluence samples in RX materials. For identical irradiation conditions, different degrees of amorphization and dissolution are observed in RX and SR samples. Also, preferential diffusion of solute is observed to occur along <c>-dislocations. These results are discussed in terms of possible interactions between irradiation produced defects, precipitates and solutes.

Irradiation of large-grained Zircaloy-2 (Zy) and Zr-2.5Nb (ZN) with 1.5 MeV Ar+ ions to fluences of ≈ 1020/m2 (= 10-30 dpa) at 50, 300 and 420 °C, leads to enhanced ∝-phase Fe dissolution. Electron microprobe analyses showed ∝-phase Fe levels of 250-1500 ppma, compared to equivalent, non-irradiated state values of ≈ 70 ppma. In ZN the β-phase Fe levels fell from about 6000 to 3500 ppma: this result accords, qualitatively, with the loss of Fe from the β-phase following in-service neutron irradiation. Concentration vs depth measurements for Zy showed higher Fe levels towards the surface. Limited data for Ni in Zy show similar behaviour.

Transmission electron microscopy (TEM) specimens of a First Core heat of 316 stainless steel, in both the solution annealed and 20% cold worked condition, were irradiated to 46 dpa at 420°C, to 49 dpa at 520°C, and to 34 dpa at 600°C in FFTF/;MOTA. Prior to irradiation, about half of the specimens were pre-implanted with approximately 100 appm of helium, and of these, several of the solution annealed and pre-implanted specimens were aged at 800°C for 2h. Post-irradiation density measurements showed small differences in density between the unimplanted alloys and their helium implanted counterparts. Microstructural observations on specimens irradiated at 420°C and 520°C showed relatively minor differences in defect distributions between the unimplanted and the helium implanted materials; in all cases the defect sizes and number densities were consistent with data in the literature. Where possible, irradiation hardening of the alloys was experimentally evaluated by microhardness and shear punch; experimentally obtained values were compared to values calculated using a computer model based on barrier hardening and the microstructural data. All methods indicated relatively small effects of helium implantation, and both measured and calculated values were in agreement with the range of values reported in the literature.

57Fe-Mössbauer spectroscopy was used to examine a specimen of Zircaloy-4 from a pressure tube extracted from a high-flux fuel channel of a CANDU* reactor. During 25 years of service the alloy was exposed to a fast-neutron fluence of 5.6×1021 n/cm2. The obtained spectra of unirradiated and neutron-irradiated specimens differ considerably. This is attributed to the radiation-induced dissolution and crystalline-to-amorphous transformation of Zr(Cr,Fe)2 precipitates, accompanied by reprecipitation and partial recrystallization of metastable Zr2Fe particles in the zirconium matrix. The correspondence between Mtssbauer and electron microscopy observations, as well as the similarity between neutron-irradiated and ion-implanted specimens, is discussed.

A rate theory model is formulated wherein amorphous clusters are formed by a damage event. These clusters are considered centers of expansion (CEs), or excess-free-volume zones. Simultaneously, centers of compression (CCs) are created in the material. The CCs are local regions of increased density that travel through the material as an elastic (e.g., acoustic) shock wave. The CEs can be annihilated upon contact with a sufficient number of CCs, to form either a crystallized region indistinguishable from the host material, or a region with a slight disorientation (recrystallized grain). Recrystallized grains grow by the accumulation of additional CCs.

Preirradiation of U3Si above the critical temperature for amorphization results in the formation of nanometer-size grains. In addition, subsequent reirradiation of these samples in the same ion flux at temperatures below the critical temperature shows that the material has developed a resistance to radiation-induced amorphization (i.e., a higher dose is needed to amorphize preirradiated samples than those that have not been preirradiated). In the model, it is assumed that grain boundaries act as effective sinks for defects, and that enhanced defect annihilation is responsible for retarding amorphization below the critical temperature by, for example, preventing a buildup of vacancies adjacent to the grain boundaries. The calculations have been validated against data from ion-irradiation experiments with U3Si. For appropriate values of the activation energy of thermal crystallization, the model predicts the evolution of a two phase microstructure consisting of nanocrystalline grains and amorphous clusters.

We compare damage evolution in U3Si produced by neutron irradiation at 30°C and 350°C and measured by neutron diffraction. Initial studies found that increasing neutron irradiation dose at 30°C results in monotonic expansion of the a-axis and contraction of the c-axis that transforms the crystal structure from tetragonal to cubic [1]. Additional irradiation results in amorphization. Neutron irradiation at 350°C results in little change to the a-axis and expansion of the caxis.The complete alteration in lattice dilatation during irradiation is interpreted as due to modification of surviving defect configurations at the higher temperature. The high temperature lattice dilations can be explained by defect loop formation in the a-b plane. Confinement of lattice strain to the c-axis during irradiation at 350°C may be the mechanism that prevents the total lattice dilatation from exceeding the critical level required for amorphization.

Neutron diffraction data were collected from a highly radioactive, low-enrichment uraniumU3Si2/A1 fuel mini-plate that had been irradiated to 42% uranium burnup. The experiment was performed using the General Purpose Powder Diffractometer at IPNS. Most prominent in the diffraction pattern were the Bragg reflections from crystalline Al (fuel plate) and V (sample container). Rietveld refinement revealed a broad oscillatory signal from the U3Si2 typical of amorphous material. This “amorphous” scattering pattern has peaks at the same positions as the amorphous scattering components observed following room temperature neutron irradiation of U3Si2 powder. The irradiated U3Si2 is amorphous with a remote possibility that it consists of recrystallized nanocrystalline particles subject to the requirements that the crystallites: (i) are no larger than 100-200 Å in size, and (ii) possess a crystalline lattice which is distorted relative to the unirradiated material.

Investigations of the nature of defects in neutron-irradiated intermetallic U3Si2 as a function of dose have been carried out. Defect categories of vacancies and interstitials, their clusters and dislocation loops have been considered. The effects of the displacement field from each on the neutron scattering intensity profiles are presented. The theoretical predictions are compared with experimentally determined intensity profiles from time-of-flight data using the General Purpose Powder Diffractometer at the Intense Pulse Neutron Source of Argonne National Laboratory. The results are illustrated for a particular peak at various doses and the physical implications discussed.

The evolution of individual helium bubbles in thin foils of gold and aluminum irradiated with 400 keV Ar+ and 200 keV Xe+ has been followed with in-situ transmission electron microscopy for a comparison between the effects of dilute (Al) and dense (Au) collision cascades. Bubble shrinkage in Al has been attributed to direct displacement of the gas out of the bubbles. Effects in Au, include the disappearance and Brownian motion of bubbles under irradiation, and are consistent with thermal spike processes seen in molecular dynamics simulations.

Fe-Cr, Fe-Y and Fe-Cr-Y surface alloys were produced by direct ion implantation, ion beam mixing, and combinations of implantation and vapor deposition. The influence of these treatments on the oxidation behavior of iron was investigated in 1 atm. of oxygen at 520°C. The oxidation rates were less in all the ion beam alloyed iron samples than in untreated iron. The oxidation follows parabolic kinetics in most cases, with the rate constants, Kp, in the range (3-8)×10−6 mg2cm−4 sec−l versus 2.2×10−5 mg2 cm−4 sec−1 for untreated iron. Yttrium fluences between 5×1014 and 5×lO15 cm−2 did not alter the microstructures of iron significantly. However, fluences of 1×1016, 3×1016, 5x1016 and 1x1017 cm−2 caused the crystalline structure of iron to be replaced by an amorphous phase. The presence of this phase was demonstrated with selected area channeling patterns and transmission electron microscopy.

Fluences from 1×1017 to 9.6×1017 cm−2 of single and doubly charged iron ions accelerated through 170 kV were implanted into polycrystalline copper substrates at room temperature. An order of magnitude decrease in reflectivity was observed during implantation as a result of sputter-induced changes in surface topography and subsurface accumulation of iron. Both scanning electron microscopy and optical microscopy showed that the bombarded surfaces consisted of (110) and (100) grains having an intragranular, ridge-like structure. Significant height differences formed between grains as a result of their different sputtering yields. Transmission electron microscopy reveals that the implanted iron forms a face-centered-cubic metastable solid solution with the copper. Auger electron spectroscopy confirmed iron accumulation beneath the surface with maximum concentrations approaching in excess of 20 atomic percent.

Density changes in amorphous Pd80Si20 during ion irradiation below 100K were detected by in situ HVEM measurements of the changes in specimen length as a function of ion fluence. A decrease in mass density as a function of the ion fluence was observed. The saturation value of the change in mass density was determined to be approximately -1.2 %.

Diffusion mechanisms in fcc materials during irradiation with high energy particles due to vacancies, interstitials, di-interstitials, and dynamic crowdions are discussed. It is shown that in most alloys an increase in the degree of order is obtained by migrating vacancies and interstitials, and only in ∝-copper-zinc alloys mainly interstitials and in nickel-chromium alloys mainly vacancies are able to increase the degree of order during irradiation. The migration activation energies of interstitials and of vacancies for these two alloys are derived.

Mass transport also by channeling and by dynamic crowdions is shown for Ni63 in nickel irradiated with 1.85 MeV electrons.