Radiation damage in non-fissile metallic alloys

A general survey is presented of radiation-induced displacement damage in non-fissile metallic alloys. The importance of the spatial arrangement of the vacancies and interstitials so produced is highlighted, especially as a guide to formulating an appropriate gauge for the various radiation-induced phenomena considered—i.e. hardening, embrittlement, growth, creep, swelling and fracture. The present level of theoretical understanding and the technological import of these phenomena are also assessed.     

A correlation between irradiation creep strength and yield stress of FCC metals and alloys

Transient and stationary creep under 6.2 MeV proton irradiation was investigated at temperatures between 100 and 400°C in pure metals (Ag, Cu, Pt, Ni) and in solution-hardened alloys (Ni-1.2 at% W, Ni-8.5 at% Al, Fe-18Cr-14Ni, Fe-19Cr-20 Ni-3Mo and AISI type 316 stainless steel). The creep rates of all materials depend linearly on dose rate and show below a stress σ_lin also a linear dependence on stress. An empirical correlation of the stationary irradiation creep rate and of σ_lin with the yield stress of the unirradiated material is established. Inferences from this correlation on current irradiation creep theories are discussed.     

Irradiation creep in transient irradiation environments

Creep in a transient irradiation environment is examined theoretically. Pulsing the irradiation flux can enhance the predicted climb-glide creep rate. The enhancement is due to cyclic transients in the point defect fluxes which are shown to be a function of pulse duty cycle, pulse frequency and temperature. The pulsed mechanism is only effective if the climb barrier height is of the order of a nanometer or less. Furthermore, the enhanced creep rate is expected to have a weak stress dependence. The SIPA creep rate, on the other hand, is reduced with pulsing because pulsing reduces the time-averaged interstitial flux as compared with a continuous irradiation having the same time-averaged damage rate. A final consideration in the present analysis is that of discontinuously stepping the irradiation temperature to induce a transient-enhanced climb-glide creep rate.     

Irradiation creep in Zr single crystals

A single crystal of crystal bar Zr was irradiated, unstressed, at 570 K in a fast (> 1 MeV) neutron flux of $\text{5.5 × 10}{}^{16}\text{n/m}{}^2\text{-s}$. After a dose of $\text{6 × 10}{}^{23}\text{n/m}{}^2$ a tensile stress of 25 MPa was applied during a period of steady reactor power. The loading strain was an order of magnitude smaller than that observed when an identical, unirradiated, crystal was loaded to the same stress. There followed a period of primary creep during which the creep rate decreased to a value of $\text{5 × 10}{}^{\mathrm{?6}}\text{h}{}^{\mathrm{?1}}$ in the first 24 hours of the test. For the final 2000 hours of the test the specimen was observed to creep at a rate of $\text{1 × 10}{}^{\mathrm{?6}}\text{h}{}^{\mathrm{?1}}$ when the reactor was at full power. During shutdowns, the creep rate decreased with time. The results will be discussed and compared with predictions from current theories for the mechanism of irradiation enhanced creep in light of the micro-structures observed.     

Irradiation creep due to elastodiffusion

Irradiation creep due to elastodiffusion, i.e. anisotropic diffusion induced by the application of a uniform external stress, has been studied. This is found to be a first order effect as compared to the usual SIPA which is a second order effect. The sink strength of dislocations is derived, as a function of the orientation of the dislocation line relative to the stress direction. The creep rate due to the resulting bias differential among dislocations is calculated and compared with that due to the usual SIPA. Using point-defect data generated by computer simulation, the creep rates due to elastodiffusion in iron and copper are found to be up to thirty times larger than those due to the usual SIPA.The effects of an anisotropic dislocation structure on the irradiation creep rate is also investigated. It is found that the anisotropy of the dislocation structure causes an anisotropy of the creep rate, which is largest in the direction where there is an excess of dislocation line directions.     

High-temperature creep and superplasticity in zirconium alloys

High-temperature (≈ 900?1400 K) steady-state creep test data on as-received zirconium alloys, Zr-1wt%Nb and Zircaloy-4 used as fuel cladding materials in light water reactors are evaluated by employing two sets of models. In particular, the focus of the paper is on the former alloy and in the two-phase coexistence region, i.e. the (α+β)-domain of the alloy. In one modeling approach, the constitutive relations for the two single phase regions (α and β) are combined through a phase transition kinetic model and a phase mixing rule; in another, a superplasticity model is used directly to calculate the creep deformation rate as a function of stress and temperature in the (α+β)-domain. The results show that the former approach is inadequate in retrodicting the experimental data, while the latter one gives a fair overall agreement. The paper describes the details of the models, the data, and derivations of the constitutive laws.     

Irradiation creep in deuteron bombarded stainless steel

A commercial and a high purity version of cold worked type 316 stainless steel was irradiated with 9 MeV deuterons at 300°C under tensile stresses between 100 and 350 MPa and the irradiation creep rate was measured. The results are qualitatively discussed in the light of present theoretical models.     

Irradiation creep of stainless steel in bending

The development is described of a test to measure irradiation enhanced creep in bending of 20% cold-worked Type-316 stainless steel. The test will be irradiated in the experimental fast reactor EBR-II. The rationale used in design selection is described. The selected beam designs, the supportive tests in other stress states and the measurement techniques are described in detail.     

Irradiation-induced swelling and creep of metals

Conclusions The assumption of the existence in the microstructure of cold-worked material of dislocations having a preference for interstitial atoms (edge), and dislocations not having any preference (screw or mixed) can explain the observed decrease in dislocation density during the early stages of irradiation. This assumption also enables us to understand why the formation of a high dislocation density in a metal has a relatively small effect on the concentration and dimensions of interstitial dislocation loops formed in irradiation. Within the framework of the existing theory it is difficult to justify the assumption that screw dislocations are neutral sinks for point defects, since these dislocations have an edge component, and it would seem they should have a preference for interstitial atoms as pure edge dislocations do. However, the efficiency of dislocations as sinks may possibly not be determined by the drift of vacancies and interstitial atoms in elastic dislocation fields, but by a structural core of dislocations where trapping of point defects occurs. Unfortunately, the problem of the effect of a core of dislocations on the trapping of vacancies and interstitial atoms has not been adequately studied. Within the framework of the Heald and Speight mechanism of irradiation creep, relations were derived and analyzed which characterize the rates of irradiation swelling and irradiation creep in the presence of different types of dislocations, including those having a preference for interstitial atoms. It was established that taking account of neutral dislocation sinks does not change the theoretical conclusion that there is no simple linear relation between swelling and irradiation creep of a metal. The direct proportionality between swelling and creep deformation observed in certain cases can be explained by assuming that the strength of the sinks for dislocations predominates over the sink for point defects.Translated from Atomnaya Énergiya, Vol. 50, No. 1, pp. 17–21, January, 1981.     

Irradiation effects in magnesium and aluminum alloys

Effects of neutron irradiation on microstructure, mechanical properties and swelling of several magnesium and aluminum alloys were studied. The neutron fluences of $\text{2?3 × 10}{}^{22}\text{n/cm}{}^2$, $\text{> 0.2}\text{MeV}$ produced displacement doses of 20 to 45 displacements per atom (dpa). Ductility of the magnesium alloys was severely reduced by irradiation induced recrystallization and precipitation of various forms. Precipitation of transmuted silicon occurred in the aluminum alloys. However, the effect on ductility was much less than for the magnesium alloys. The magnesium and aluminum alloys had excellent resistance to swelling: The best magnesium alloy was Mg/3.0 wt% Al/0.19 wt% Ca; its density decreased by only 0.13%. The best aluminum alloy was 6063, with a density decrease of 0.22%.     

Irradiation enhanced creep in LMFBR fuel element cladding

Rate changes observed in irradiation enhanced creep and swelling in stainless steel cladding are ascribed to the precipitation of carbide. Empirical equations modified according to precipitation kinetics are consistent with results from fuel element irradiation and in particular describe the “second peak” phenomenon.     

Irradiation creep and interrelation with swelling in austenitic stainless steels

The available experimental data on irradiation-induced creep in austenitic stainless steels are summarized and the existing theories reviewed. Attention is paid to the influence of material composition and pretreatments on irradiation creep. In particular the stress, flux, fluence and temperature dependencies are reported and possible correlations of irradiation creep with the microstructural evolution, the swelling behaviour and the precipitation kinetics of the materials are outlined. The consequences of stress effects connected with swelling for the irradiation-creep behaviour, especially the stress-dependence, are discussed.     

Point defects and the creep of metals

Basic concepts felt to be important in diffusion-controlled creep of metals are reviewed and it is suggested that such creep is controlled by edge-dislocation climb under a rather wide range of conditions. The effect of a damage-producing flux on such creep processes is explored. It is shown that processes such as Herring-Nabarro creep are unaffected by irradiation. Evidence is presented for a climb-plus-glide mechanism of radiation creep for stresses above unirradiated yield or flow stresses. At lower stresses a preferential dislocation loop nucleation model is suggested.     

Irradiation creep under 60 MEV alpha irradiation

Accelerator-produced charged-particle beams have advantages over neutron irradiation for studying radiation effects in materials, the primary advantage being the ability to control precisely the experimental conditions and improve the accuracy in measuring effects of the irradiation. An apparatus has recently been built at ORNL to exploit this advantage in studying irradiation creep. These experiments employ a beam of 60 MeV alpha particles from the Oak Ridge Isochronous Cyclotron (ORIC). The experimental approach and capabilities of the apparatus are described. The damage cross section, including events associated with inelastic scattering and nuclear reactions, is estimated. The amount of helium that is introduced during the experiments through inelastic processes and through backscattering is reported. Based on the damage rate, the damage processes and the helium-to-dpa ratio, the degree to which fast reactor and fusion reactor conditions may be simulated is discussed. Recent experimental results on the irradiation creep of type 316 stainless steel are presented, and are compared to light ion results obtained elsewhere. These results include the stress and temperature dependence of the formation rate under irradiation. The results are discussed in relation to various irradiation creep mechanisms and to damage microstructure as it evolves during these experiments.     

Irradiation induced creep of ceramic nuclear fuels

This contribution gives a review of the experimental results and accompanying theoretical considerations. The mechanisms considered for irradiation creep are: relaxation of elastic stresses by fission spikes, promotion of dislocation slide by thermal spikes, preferential, stress-orientated nucleation of dislocation loops and preferential growth of dislocation loops. A survey over the irradiation creep rates attributed to steady-state creep shows $\text{ε}{}_{\mathrm{irr}}\text{~ σ · F}$ for oxide fuel in the stress and fission rate ranges of $\text{σ = 10–50}\text{MN/m}{}^2$ and $\text{F = 3 × 10}{}^{12}\text{–1 × 10}{}^{14}\text{f/cm}{}^3\text{· s}$ at burnups < 3%. There seems to be a continuous increase of the irradiation creep rate of oxide fuels with increasing temperature. However, that increase cannot be directly interpreted through a thermally activated process. It seems that the irradiation creep rate will also depend on fuel porosity, on plutonium distribution in mechanically blended UO₂-PuO₂, but not substantially on the plutonium content per se. Some results were already given for carbide and nitride fuels, which show the irradiation creep rate to be lower by about a factor of 10 than for oxide fuel under comparable conditions. Primary irradiation creep has been observed up to $\text{(3–5) × 10}{}^{19}\text{f/cm}{}^3$ and could prevail up to $\text{1 × 10}{}^{20}\text{f/cm}{}^3$.     

Irradiation creep relaxation of void swelling-driven stresses

Swelling-driven-creep test specimens are used to measure the compressive stresses that develop due to constraint of irradiation void swelling. These specimens use a previously non-irradiated 20% CW Type 316 stainless steel holder to axially restrain two Type 304 stainless steel tubular specimens that were previously irradiated in the US Experimental Breeder Reactor (EBR-II) at 490 °C. One specimen was previously irradiated to fluence levels in the void nucleation regime (9 dpa) and the other in the quasi-steady void growth regime (28 dpa). A lift-off compliance measurement technique was used post-irradiation to determine compressive stresses developed during reirradiation of the two specimen assemblies in Row 7 of EBR-II at temperatures of 547 °C and 504 °C, respectively, to additional damage levels each of about 5 dpa. Results obtained on the higher fluence swelling-driven-creep specimen show that compressive stress due to constraint of swelling retards void swelling to a degree that is consistent with active load uniaxial compression specimens that were irradiated as part of a previously reported multiaxial in-reactor creep experiment. Swelling results obtained on the lower fluence swelling-driven creep specimen show a much larger effect of compressive stress in reducing swelling, demonstrating that the larger effect of stress on swelling is on void nucleation as compared to void growth. Test results are analyzed using a recently proposed multiaxial creep-swelling model.     

Radiation induced creep and growth of zirconium alloys

The strain under irradiation of zirconium and its alloys is calculated within a simple rate theory approach. Network dislocations and interstitial dislocation loops with their Burgers vector oriented parallel to the crystal basal plane are assumed to climb by preferentially attracting interstitials with respect to vacancies, while the grain boundaries act as neutral sinks and absorb therefore more vacancies than interstitials. This same theory has been applied by Fainstein-Pedraza, Savino and Pedraza for modelling the irradiation growth of cold worked zirconium alloys. It is now extended by including the effect of vacancy traps and the stress induced preferential bias for interstitials of those dislocations favourably oriented with respect to an external or internal stress field. In addition, a model which allows to correlate the deformation of the individual grains with the strain of the polycrystalline specimen where they pertain is developed. The stresses induced within the same grain while it deforms inside the textured crystal are also numerically calculated. Those stresses modify the grain strain via the SIPA mechanism and the stresses-strains are then coupled. The calculated crystal deformation is strongly dependent on texture. For tubes with the c axis oriented preferentially on an axial plane, a rapid increase of the longitudinal strain rate is predicted at high doses.     

Irradiation creep transients in Ni-4 at.% Si

In the course of irradiation creep experiments on Ni-4 at.% Si alloy, two types of creep transients were observed on the termination of irradiation. The short term transient was completed within one minute while the long term transient persisted for nearly ten hours. A change in the temperature distribution was excluded from the possible causes, partly because the stress dependence of the observed transient strains was not linear, and partly because the strain increase expected from the temperature change was much smaller than the observed value. Transient behavior of point defects was examined in conjunction with the climb-glide mechanism and the steady-state irradiation creep data. Calculated creep transient due to excess vacancy flux to dislocations was in good agreement with the observed short term transient. The long term transient appears to be a result of dislocation microstructure change. The present results suggest an enhanced irradiation creep under cyclic irradiation conditions which will be encountered in the early generations of fusion reactors.