Dale E. Alexander

Gamma-ray (electron) irradiation effects on tensile properties of ferritic alloys

Abstract We demonstrate that high-energy electron irradiation can be used to study gamma-ray embrittlement of reactor pressure vessels. Observed changes in tensile properties, in ferritic alloys electron irradiated at low temperature (⩽ 50° C ), correlate well with neutron irradiation data when compared simply on a displacement per atom basis. The similarity in embrittlement behavior between gamma-rays (electrons) and neutrons can be explained in terms of a reaction-rate theory model of defect production and clustering. The present results, obtained under well-controlled experimental conditions, provide strong support for a previous analysis implicating gamma-ray damage as the source of the “accelerated” embrittlement of the High Flux Isotope Reactor (HFIR) pressure vessel.

Post-yield strain hardening behavior as a clue to understanding irradiation hardening

We analyze the post-yield true-stress vs. true-strain flow behavior of neutron and electron irradiated reactor pressure vessel steels, A212B and A350, and binary alloys, Fe-0.28 Cu and Fe-0.74 Ni. The flow curves suggest that neutron-irradiation hardening has the same effect as strain hardening for all the materials examined. The post-yield flow curves, obtained after electron-irradiation hardening to yield strength levels similar to those achieved by neutron irradiation, behave differently.

Defect production considerations for gamma ray irradiation of reactor pressure vessel steels

Abstract Motivated by the recent interest in gamma ray embrittlement of nuclear reactor pressure vessels (RPVs), calculations were performed to evaluate aspects of defect production by gammas in iron and steel. In addition to determining displacement damage cross-sections, the atomic recoil energy dependence of gamma-induced defect production was described by integral recoil damage spectra, W(T), and their associated median recoil damage energies, T1/2. These latter characterizations, should be particularly useful in evaluating the contribution of gamma ray generated defects to microstructural changes causing radiation embrittlement. The results for monoenergetic gammas, as well as for gamma rays with a spectrum of energies characteristic of a RPV, reveal T1/2 values of

Enhancement of diffusion‐induced grain boundary migration by ion irradiation

Diffusion‐induced grain boundary migration (DIGM) was observed in Au/Cu bilayers irradiated with 1.5 MeV Kr at T≥400 K. Rutherford backscattering spectrometry (RBS) showed nearly uniform distributions of Cu present throughout polycrystalline Au films after irradiation and after annealing treatments. Irradiation increased the amount of Cu relative to annealed‐only areas. Cross‐section transmission electron microscopy combined with x‐ray energy dispersive spectroscopy (XEDS) identified alloyed zones (14–20 at. % Cu), confirming DIGM in the Au film of an ion bombarded bilayer. A description of DIGM is presented relating RBS measurements of the film‐averaged Cu composition with treatment time, average grain size, and film thickness. Application of this model to the experimental results in combination with XEDS work indicates that irradiation enhances DIGM by increasing the grain boundary velocity.

The effect of ion irradiation on inert gas bubble mobility

Abstract The effect of Al ion irradiation on the mobility of Xe gas bubbles in Al thin films was investigated. Transmission electron microscopy was used to determine bubble diffusivities in films irradiated and/or annealed at 673,723 and 773 K. Irradiation increased bubble diffusivity by a factor of 2–9 over that due to thermal annealing alone. The Arrhenius behavior and dose rate dependence of bubble diffusivity are consistent with a radiation enhanced diffusion phenomenon affecting a volume diffusion mechanism of bubble transport.

Thermal spike model of ion-induced grain growth

A thermal spike model has been developed to describe the phenomenon of ion irradiation-induced grain growth in metal alloy thin films. In single phase films where the driving force for grain growth is the reduction of grain boundary curvature, the model shows that ion-induced grain boundary mobility, M{sub ion}, is proportional to the quantity F{sub D}{sup 2}/{Delta}H{sub coh}{sup 3}, where F{sub D} is the deposited ion damage energy and {Delta}H{sub coh} is the cohesive energy of the element or alloy. Experimental strain growth results from ion irradiated coevaporated binary alloy films compare favorably with model predictions. 11 refs., 1 fig., 1 tab.

Neutron-induced collision cascade mixing in Nb/V superlattices☆

High-angle X-ray diffraction was used to determine cascade mixing efficiencies, Dt/dpa, in Nb/V superlattices irradiated to small doses (≤ 0.26 dpa). Samples were neutron irradiated in the core of the High Flux Beam Reactor at Brookhaven National Laboratory and ion irradiated with 1.5 MeV Ne. No significant differences were observed in cascade mixing efficiencies between the two types of irradiation. Values of Dt/dpa were observed to vary with the modulation wavelength of the superlattice, with thicker wavelength samples yielding values approaching that determined from ion mixing thick Nb/V bilayers (Dt/dpa ∼ 110 A2/dpa). The decrease in mixing efficiency observed at lower wavelengths may be related to an observed structural transition in which the interfaces of the superlattice become coherent.

In-Situ Irradiation Studies on the Effects of Helium on the Microstructural Evolution of V-3.8Cr-3.9Ti

Role of He in microstructural evolution of V-3.8Cr-3.9Ti was investigated by in-situ TEM of as-prepared and He implanted (<10 appM) samples subjected to 200 keV He irradiation at RT. Quantitative analysis showed an increase in defect density and size with irradiation in both. The unimplanted sample showed a defect density consistent with electron irradiation experiments. The He preimplanted sample had slightly larger defects and a substantially greater increase in number density of defects. This is consistent with a mechanism of He trapping by formation of He-vacancy-X (X=C,N,O) complexes.

Diffusion-induced grain boundary migration during ion beam mixing of Au/Cu bilayers

Experiments were performed to evaluate the effect of 1.5 MeV Kr irradiation on diffusion-induced grain boundary migration (DIGM) in Au/Cu bilayers in the temperature range of 300{le}T{le}050K. The experimental results were consistent with DIGM occurring in bilayers both during irradiation and during annealing treatments. Rutherford backscattering spectrometry showed a nearly uniform distribution of Cu present through the entire thickness of appropriately prepared polycrystalline Au films irradiated or annealed at temperatures {ge}400K. No parallel effect was seen in similarly treated single-crystal films. In each polycrystalline sample studied, irradiation resulted in greater amounts of Cu present uniformly in the Au compared to annealing-only. The magnitudes of measured Cu compositions were substantially greater than that expected solely from grain boundary diffusion. A simple analysis of the process indicated that ion irradiation affects DIGM by increasing the composition of Cu present in alloyed zones and/or by increasing the grain boundary velocity in the Au.

Grain growth and phase formation in ion irradiated/annealed thin Ni-Al alloys films

Ion irradiation and annealing studies were performed on Ni, Ni-20 at.%Al multilayers and Ni-20 at.%Al co-evaporated thin films. Xe{sup +} ions were used to irradiate the films and homogenize the multilayers at room temperature. Irradiation of alloy films formed a metastable, supersaturated solid solution of {gamma} phase and an HCP phase. Ion induced grain growth occurred in all films. A factor of 2 greater growth was observed in Ni--Al multilayers compared with coevaporated films irradiated to the same dose. The enhancement is attributed to a heat of mixing effect. Post irradiation annealing of the mixed multilayers formed {gamma}{prime}, the morphology of which was dependent upon the presence of Cu in the films due to substrate mixing from the support grid. 10 refs., 1 fig.