R.C. Birtcher

A thermal spike model of grain growth under irradiation

The experimental study of grain growth in nanocrystalline metallic foils under ion irradiation showed the existence of a low-temperature regime (below about 0.15–0.22Tm), where grain growth is independent of the irradiation temperature, and a thermally assisted regime where grain growth is enhanced with increasing irradiation temperature. A model is proposed to describe grain growth under irradiation in the temperature-independent regime, based on the direct impact of the thermal spikes on grain boundaries. In the model, grain-boundary migration occurs by atomic jumps, within the thermal spikes, biased by the local grain-boundary curvature driving. The jumps in the spike are calculated based on Vineyard’s analysis of thermal spikes and activated processes using a spherical geometry for the spike. The model incorporates cascade structure features such as subcascade formation, and the probability of subcascades occurring at grain boundaries. This results in a power law expression relating the average grain ...

The amorphization of complex silicates by ion-beam irradiation

Twenty-five silicates were irradiated at ambient temperature conditions with 1.5 MeV Kr[sup +]. Critical doses of amorphization were monitored [ital in] [ital situ] with transmission electron microscopy. The doses required for amorphization are compared with the structures, bond-types, compositions, and physical properties of the silicates using simple correlation methods and more complex multivariate statistical analysis. These analyses were made in order to determine which properties most affect the critical amorphization dose. Simple two-variable correlations indicate that melting point, efficiency of atomic packing, the dimensionality of SiO[sub 4] polymerization (DOSP), and bond ionicity have a relationship with critical amorphization dose. However, these relationships are evident only in selected portions of the data set; that is, for silicate phases with a common structure type.

Amorphization, morphological instability and crystallization of Krypton ion irradiated germanium

Abstract Krypton ion irradiation of crystalline Ge and subsequent thermal annealing were both carried out with in situ transmission electron microscopy observations. The temperature dependence of the amorphization dose, effect of foil thickness, morphological changes during continuous irradiation of the amorphous state as well as the effect of implanted gas have been determined. The dose of 1.5 MeV Kr required for amorphization increases with increasing temperature. At a fixed temperature, the amorphization dose is higher for thicker regions of the specimen. Continuous irradiation of amorphous Ge at room temperature results in a high density of small cavities which grow with increasing dose. Cavities do not coalesce during growth but develop into irregular-shaped holes that eventually transform the amorphous Ge into a sponge-like material. Formation of the spongy structure is independent of Kr implantation. The crystallization temperature and the morphology of recrystallized Ge depend on the Kr+ dose. Voi...

Annealing of isolated amorphous zones in silicon

In situ transmission electron microscopy has been used to observe the production and annealing of individual amorphous zones in silicon resulting from impacts of 200-keV Xe ions at room temperature. As has been observed previously, the total amorphous volume fraction decreases over a temperature range from room temperature to approximately 500 °C. When individual amorphous zones were monitored, however, there appeared to be no correlation of the annealing temperature with initial size: zones with similar starting sizes disappeared (crystallized) at temperatures anywhere from 70 °C to more than 400 °C. Frame-by-frame analysis of video recordings revealed that the recovery of individual zones is a two-step process that occurred in a stepwise manner with changes taking place over seconds, separated by longer periods of stability.

Irradiation-Induced Nanostructures

This paper summarizes the results of the studies of the irradiation-induced formation of nanostructures, where the injected interstitials from the source of irradiation are not major components of the nanophase. This phenomena has been observed by in situ transmission electron microscopy (TEM) in a number of intermetallic compounds and ceramics during high-energy electron or ion irradiations when the ions completely penetrate through the specimen. Beginning with single crystals, electron or ion irradiation in a certain temperature range may result in nanostructures composed of amorphous domains and nanocrystals with either the original composition and crystal structure or new nanophases formed by decomposition of the target material. The phenomenon has also been observed in natural materials which have suffered irradiation from the decay of constituent radioactive elements and in nuclear reactor fuels which have been irradiated by fission neutrons and other fission products. The mechanisms involved in the process of this nanophase formation are discussed in terms of the evolution of displacement cascades, radiation-induced defect accumulation, radiation-induced segregation and phase decomposition, as well as the competition between irradiation-induced amorphization and recrystallization.

Radiation‐induced formation of cavities in amorphous germanium

Prethinned polycrystalline Ge TEM samples were irradiated with 1.5 MeV Kr+ ions at room temperature while structural and morphological changes were observed in situ in the Argonne High Voltage Electron Microscope‐Tandem Facility. After a Kr+ dose of 1.2×1014 ions/cm2, the irradiated Ge was completely amorphized. A high density of small void‐like cavities was observed after a Kr+ dose of 7×1014 ions/cm2. With increasing Kr+ ion dose, these cavities grew into large holes transforming the irradiated Ge into a sponge‐like porous material after 8.5×1015 ions/cm2. The radiation‐induced nucleation of void‐like cavities in amorphous material is astonishing, and the final structure of the irradiated Ge with enormous surface area may have potential applications.

Amorphization of U3Si2 by ion or neutron irradiation

Abstract Diffraction techniques have been used to monitor the crystal structure of U 3 Si 2 during irradiation. Neutron diffraction was used to follow crystallographic changes produced by neutron irradiation at 30°C. Neutron irradiation results in uranium fission into energetic fragments that produce tracks of damage in the form of amorphous zones whose volume change relative to the initial lattice produces lattice strains. The total lattice strain increases as the volume fraction of amorphous material increases, and the maximum unit cell volume change is − 2.2%. The amorphous volume fraction increases exponentially at an initial rate of (2.24 × 10 22 fissions/m 3 ) −1 or (0.076 dpa) −1 and complete amorphization occurs by a dose of 1.1 × 10 23 fissions/m 3 or 0.38 dpa. The unit cell volume decreases linearly with increasing volume fraction of amorphous material, indicating that there is little mechanical yielding or plastic flow during irradiation of amorphous U 3 Si 2 . Electron diffraction during in situ 1.5 MeV Kr ion irradiation was used to determine complete amorphization doses for U 3 Si 2 at temperatures above 30°C. As with fission fragments, individual Kr ions produce small amorphous volumes. The temperature limit for complete amorphization is approximately 250°C. Within the uncertainty of the neutron dose steps and damage calculations, the same amount of damage is required for amorphization of U 3 Si 2 by ion or neutron irradiation.

In situ transmission electron microscopy and ion irradiation of ferritic materials

The intermediate voltage electron microscope‐tandem user facility in the Electron Microscopy Center at Argonne National Laboratory is described. The primary purpose of this facility is electron microscopy with in situ ion irradiation at controlled sample temperatures. To illustrate its capabilities and advantages a few results of two outside user projects are presented. The motion of dislocation loops formed during ion irradiation is illustrated in video data that reveals a striking reduction of motion in Fe‐8%Cr over that in pure Fe. The development of extended defect structure is then shown to depend on this motion and the influence of nearby surfaces in the transmission electron microscopy thin samples. In a second project, the damage microstructure is followed to high dose (200 dpa) in an oxide dispersion strengthened ferritic alloy at 500°C, and found to be qualitatively similar to that observed in the same alloy neutron irradiated at 420°C. Microsc. Res. Tech., 2009.

In situ transmission electron microscopy investigation of radiation effects

In situ observation is of great value in the study of radiation damage utilizing electron or ion irradiation. We summarize the facilities and give examples of work found around the world. In situ observations of irradiation behavior have fallen into two broad classes. One class consists of long-term irradiation, with observations of microstructural evolution as a function of the radiation dose in which the advantage of in situ observation has been the maintenance of specimen position, orientation, and temperature. A second class has involved the recording of individual damage events in situations in which subsequent evolution would render the correct interpretation of ex situ observations impossible. In this review, examples of the first class of observation include ion-beam amorphization, damage accumulation, plastic flow, implant precipitation, precipitate evolution under irradiation, and damage recovery by thermal annealing. Examples of the second class of observation include single isolated ion impacts that produce defects in the form of dislocation loops, amorphous zones, or surface craters, and single ion impact-sputtering events. Experiments in both classes of observations attempt to reveal the kinetics underlying damage production, accumulation, and evolution.

Observation of second-phase particles in bulk zirconium alloys using synchrotron radiation

Abstract To further advance the mechanistic understanding of microstructural evolution in zirconium alloys for high burnup applications, it is important to obtain a quantitative measurement of the volume fractions of second-phase precipitates present in the bulk alloys as a function of the heat treatment and irradiation fluence. In this work, X-ray diffraction from a synchrotron radiation source was used to identify and follow the growth kinetics of second-phase particles in zirconium alloys. The high energy flux, energy resolution and signal-to-noise ratio of this light source allowed us to study the very small (