On the growth of dislocation loops in electron irradiated copper foils

We have irradiated foils of single-crystal pure copper in the Harwell AEI EM-7 microscope at several temperatures from 140 to 320°C, and have measured interstitial dislocation loop densities and growth rates. We have interpreted the experimental results using the rate theory of microstructural evolution during radiation damage.

We have achieved a good agreement between loop growth measurements and rate theory calculations only when using a very recently developed dislocation sink strength. In this case we predict a temperature-dependent dislocation bias for interstitials and a vacancy migration energy of about 0.75 eV, which is consistent with recent independent experimental data.     

The nature of dislocation loops in neutron irradiated zirconium

The nature of the damage structure in neutron-irradiated zirconium has been studied using transmission electron microscopy. The damage structure consists of dislocation loops with $\text{a/3?11}\text{2}\text{?}\text{0〉}$ Burgers vectors. No evidence was found for $\text{c}$ component dislocation loops. The dislocation loops are non-edge in character and are elliptical with the loop minor axis in the (0001) plane and the major axis in the vicinity of the [0001] direction. This ellipticity was particularly marked for large vacancy loops whereas interstitial loops tended to be circular. Under all irradiation conditions studied, the loop populations were mixed interstitial/vacancy.     

Confinement of motion of interstitial clusters and dislocation loops in BCC Fe-Cr alloys

This work is devoted to the study of the effect of Cr solutes on the mobility of self interstitial atom (SIA) clusters and small interstitial dislocation loops (of size up to a few nanometers) in concentrated Fe-Cr alloys. Atomistic simulations have been performed to characterize the variation of the free energy of interstitial loops in the Fe-15Cr alloy using the experimentally determined profile of Cr distribution along the path of a loop. It is shown that the presence of randomly distributed Cr in Fe leads to the creation of local trapping configurations for small SIA clusters. The strength (trapping energy) and density of these configurations depend on the Cr content. On the contrary, large SIA clusters (which can be described as 1/2〈1 1 1〉 dislocation loops) are strongly affected by the presence Cr-Cr pairs and larger Cr clusters, which act as barriers to their motion.     

Void nucleation, growth, and coalescence in irradiated metals

A novel computational treatment of dense, stiff, coupled reaction rate equations is introduced to study the nucleation, growth, and possible coalescence of cavities during neutron irradiation of metals. Radiation damage is modeled by the creation of Frenkel pair defects and helium impurity atoms. A multi-dimensional cluster size distribution function allows independent evolution of the vacancy and helium content of cavities, distinguishing voids and bubbles. A model with sessile cavities and no cluster–cluster coalescence can result in a bimodal final cavity size distribution with coexistence of small, high-pressure bubbles and large, low-pressure voids. A model that includes unhindered cavity diffusion and coalescence ultimately removes the small helium bubbles from the system, leaving only large voids. The terminal void density is also reduced and the incubation period and terminal swelling rate can be greatly altered by cavity coalescence. Temperature-dependent trapping of voids/bubbles by precipitates and alterations in void surface diffusion from adsorbed impurities and internal gas pressure may give rise to intermediate swelling behavior through their effects on cavity mobility and coalescence.     

The contribution of dislocation loops to radiation growth and creep of Zircaloy - 2

Annealed specimens of Zircaloy-2 have been examined using transmission electron microscopy after irradiation to various fluences up to $\text{1 × 10}{}^{26}\text{n · m}{}^{\mathrm{?2}}$ ($\text{E > 1}\text{MeV}$) at 573 K. Measurements of the size and concentration of the radiation-induced defects show that they make a negligible contribution to radiation growth. An alternative mechanism is sug-gested, based on the bias interaction between interstitials and dislocations. A specimen of cold-worked and stress-relieved Zircaloy-2, creep tested at 138 MPa in a reactor at 573 K, was also examined. It was shown that the total number of point defects in the form of clusters was the same as in an unstressed, annealed specimen. It can be concluded that the stress-induced orientation of dislocation loops makes a negligible contribution to radiation creep of Zircaloy-2 at 573 K, regardless of the magnitude of the applied stress.     

The collapse of defect cascades to dislocation loops

We describe a number of experiments that we have recently performed to investigate the collapse of defect cascades to dislocation loops. This important ion and neutron irradiation phenomenon has been studied with in situ ion bombardment using the High Voltage Electron Microscope-Ion Accelerator Facility at Argonne National Laboratory in Cu₃Au, Cu, and Fe at temperatures of 30 and 300 K and in Ni at 30, 300 and 600 K. These experiments have demonstrated that individual defect cascades collapse to dislocation loops athermally at 30 K in some materials (Ni, Cu and Cu₃Au), while in Fe overlapping of cascades is necessary to produce dislocation loops. A slight sensitivity to the irradiation temperature is demonstrated in Cu₃Au and Fe, and a strong dependence on the irradiation temperature is seen in Ni. This phenomenon of cascade collapse to dislocation loops in metals at 30 K provides an understanding for previous neutron irradiation data. The more detailed dependencies of the collapse probability on material, temperature, bombarding ion dose, ion energy and ion mass contribute much information to a thermal spike model of the collision cascade which we will describe.     

The influence of pre-injected helium on void nucleation and growth in FV548 steel irradiated with 1 MeV electrons

A study has been made of the effect of helium pre-injection on the void populations produced in solution-treated specimens of FV548 austenitic steel irradiated with 1 MeV electrons in the temperature range 450–650°C. The results suggest that helium atom clusters, created during pre-injection, act as void nuclei during subsequent irradiation, although the helium cluster-void transition is limited at high irradiation temperatures by a rapid temperature-dependent increase in the critical void nucleus size. In most cases the void swelling rates at 600 and 650°C decreased with increasing void number density. This was not the case in those specimens in which helium had been pre-injected at elevated temperatures, suggesting that void swelling is an extremely sensitive function of the balance between the void and dislocation sink strengths.     

Numerical simulation of void nucleation in irradiated metals

An atomistics-based theory for void nucleation has been used to calculate — for the first time — terminal void number densities for irradiated nickel, type 316 stainless steel, and the Ni-base alloy, PE-16. Both the absolute magnitudes and temperature dependences of the void number densities are in agreement with experiment. The void nucleation parameter, ψ, which governs spontaneous void nucleation was evaluated for the three materials; the results are in agreement with experiment. The critical gas content for rapid void growth was calculated for PE-16 and type 316 stainless steel, and was found to increase from about 10 helium atoms at the lower end of the void swelling range to some 10⁴ atoms at the upper end. The theory was also found to predict re-nucleation of a new distribution of voids after a drop in temperature during irradiation.     

Modeling of the kinetics of dislocation loops

The precipitation of excess silicon interstitials into dislocation loops is modeled. This situation occurs when an amorphous layer is created at the surface in order to avoid boron channeling and form shallow p junctions. The modeling of the nucleation of these extended defects is included into the process simulator IMPACT-4. Their density and mean radius are calculated for several annealing times and temperatures and they are compared with experimental characterizations. This is the first step towards a full modeling of the complex processes involved in the transient enhanced diffusion of boron.     

Atomistic simulations of nanometric dislocation loops in bcc tungsten

Small dislocation loops formed from self-interstitial atoms (SIAs) are commonly found in irradiated metals. These defects significantly influence the mechanical properties of the materials. Atomistic simulations are used to describe nanometric circular dislocation loops with Burger’s vectors , and in bcc tungsten. Particular attention is paid to the habit plane of the loop. Two different embedded atom model (EAM) potentials are used. The energetics and geometry of the loops are studied as a function of their size.     

The effect of copper and manganese on magnetic minor hysteresis loops in neutron irradiated Fe model alloys

Changes of magnetic minor hysteresis loops in pure Fe, Fe-1 wt% Mn, Fe-0.9 wt% Cu, and Fe-0.9 wt% Cu-1 wt% Mn model alloys after neutron irradiation have been studied. Minor-loop coefficients which are obtained from scaling relations between minor-loop parameters and in proportion to internal stress, were found to decrease in all model alloys after the irradiation to a fluence of 3.32 × 10¹⁹ n cm⁻². The decrease of the coefficients is larger for alloys including Cu and is enhanced by 1 wt% Mn addition. Such decrease implying the reduction of internal stress during irradiation is in contrast with changes of yield strength after the irradiation that increase with Cu and Mn contents. A qualitative explanation was given on the basis of the preferential formation of Cu precipitates along pre-existing dislocations which reduces internal stress of the dislocations.     

Modeling dislocation structure development and creep-swelling coupling in neutron irradiated stainless steel

Anisotropic nucleation and growth of multi-classes of dislocation loops under the combined actions of fast-neutrons and an external applied stress are considered in modeling dislocation structure development in metals and alloys. The stochastic nature of the nucleation kinetics is formulated via the Fokker-Planck equation. The strain derived from the climb of the anisotropic dislocation structure is separable into volumetric and deviatoric components, corresponding respectively to swelling and creep. The creep contribution resulting from the development of the stress-induced dislocation anisotropy is found to be very significant and exhibits a strong correlation with swelling. For stainless steel, our model explains very well the complex deformation behavior observed in a wide variety of in-reactor experiments.     

Formation of dislocation loops in silicon by ion irradiation for silicon light emitting diodes

We have studied the influence of the ion species, ion energy, fluence, irradiation temperature and post-implantation annealing on the formation of shallow dislocation loops in silicon, for fabrication of silicon light emitting diodes. The substrates used were (1 0 0) Si, implanted with 20-80 keV boron at room temperature and 75-175 keV silicon at 100 and 200 °C. The implanted fluences were from 5 × 10¹⁴ to 1 × 10¹⁵ ions/cm². After irradiation the samples were processed for 15 s to 20 min at 950 °C by rapid thermal annealing. Structural analysis of the samples was done by transmission electron microscopy and Rutherford backscattering spectrometry. In all irradiations the silicon substrates were not amorphized, and that resulted in the formation of extrinsic perfect and faulted dislocation loops with Burgers vectors a/2〈1 1 0〉 and a/3〈1 1 1〉, respectively, sitting in {1 1 1} habit planes. It was demonstrated that by varying the ion implantation parameters and post-irradiation annealing, it is possible to form various shapes, concentration and distribution of dislocation loops in silicon.     

Effect of trapping on interstitial cluster nucleation at the onset of irradiation

Kinetic calculations have been carried out to study heterogeneous nucleation of interstitial clusters at solute trapping sites during the transient period from the onset of irradiation until the vacancy and interstitial concentrations approach their “quasi” steady-state values. The irradiation rate, temperature, and interstitial-solute binding energy were the primary parameters varied. It has been reported earlier that, for reasonable cluster binding energies, interstitial clusters nucleate homogeneously in high concentrations at the lower temperature range of void swelling and their concentration decreases with increasing temperature. In the present calculations it is found that the decrease in the free interstitial concentration due to trapping reduces the homogeneous nucleation but, except at very high interstitial-solute binding energies, this decrease is more than offset by heterogeneous nucleation, yielding a net increase in nucleated interstitial clusters. These effects are greater at lower temperatures and lower irradiation rates.     

Continuum modeling of localized deformation in irradiated bcc materials

A continuum constitutive crystal plasticity framework is implemented to model the post-irradiation tensile behavior of bcc structural materials, accounting for localized deformation due to the formation of dislocation channels. Both the mechanical response and deformed microstructure of the material are modeled for quasi-static tensile loading. The latter is studied to identify the stages of dislocation channel formation during localization, specifically with respect to the evolution of dislocations and irradiation-induced defects. Parametric studies of the cross-slip and flow softening (due to annihilation of irradiation-induced defects) models are performed to study their effects on the localization behavior. Results are compared to available experimental data.