An important part of materials science is the study of the behavior of materials under difficult environmental conditions. Some operating conditions have been traditional objects of study—for example, high temperatures and corrosive atmospheres. The study of materials in radiation environments gained particular significance after the advent of nuclear reactors, and radiation has joined the list of insults to which materials are subjected. Technological applications that involve significant levels of radiation include but are not limited to fission reactors, proposed fusion reactors, nuclear waste, ion accelerators, and spacecraft, with types of radiation encompassing thermal and fast neutrons, light and heavy ions, and energetic photons. More detailed information on applications and accompanying radiation fields appears in the articles published in this issue of MRS Bulletin and in the references listed under Further Reading.

The four theme articles in this issue focus on radiation effects in four families of materials commonly used in radiation environments: metals, ceramics, graphite and carbon-based materials, and polymers.

Radiation-damage research started with the installation of the first nuclear reactors in the United States. In 1946 Wigner pointed out that energetic neutrons would displace atoms from their regular lattice sites and thus change the properties of irradiated materials—a prediction that was soon confirmed experimentally. Most of these changes are unfavorable for the performance of materials, justifying the influence of radiation being referred to as “radiation damage.”

Since radiation-induced materials degradation can have a drastic impact on the safe and economic operation of present fission reactors and, probably even more, on future fusion reactors, radiation-damage research comprised a large part of the research-and-development programs for nuclear materials. As a result of this effort, a broad database is now available, and materials have been developed that fulfill practically all requirements being encountered in present nuclear technology.

Besides this applied work, extensive fundamental research on radiation effects has been carried out because physicists soon recognized that bombardment with energetic particles offered a unique method to create controlled populations of defects in solids. Whereas the cross-linking between basic and applied research was rather weak in the early stages, a convergence of the two branches has been clearly noticeable during the last decade. This welcome development occurred for a variety of reasons. Examples are the need to employ simulation irradiations in cases where no prototypic devices exist (fusion reactors, high-power spallation neutron sources) and the increasing application of ion-beam techniques in microelectronics, thin-film technology, and metallurgy where the damage produced by the implanted ions needs clarification.

Ceramics are inorganic, nonmetallic materials, a class that includes solids with both crystalline and glassy (i.e., aperiodic) atomic structures. This article will emphasize radiation effects in conventional refractory oxide ceramics and glasses that display high-temperature strength, chemical inertness, and high electrical resistivity. The introduction to this issue cites various radiation environments to which materials may be exposed; a number of these environments receive mention in this article. However emphasis here is on giving the reader an overview of damage mechanisms and of the changes in key physical properties (optical, thermal, electrical, and mechanical) that can affect the behavior of these materials. The broad subject of radiation-damage phenomena in ceramics has recently been reviewed by Hobbs et al.

Even at low doses of energetic radiation, whether particulate or electromagnetic, isolated atomic-level defects may be generated and can offer the potential to induce changes in physical properties. However for some applications such as fission reactors, fusion reactors (now in the conceptual design stages), and high-energy accelerators, one must deal with high fluences capable of generating relatively large damage aggregates such as dislocation loops and voids. Thus one must understand the evolution of damage microstructures from point defects to aggregates in both model materials and engineering ceramics. Even when induced changes are only electronic in nature, the effects on some properties (e.g., electrical conductivity) can be dramatic. In the following sections, evolution of the damage state and consequent changes in key properties are described, with some attention to the often critical role of impurities.

Displacement damage in graphite and carbon-based materials can occur when energetic particles, such as neutrons, ions, or electrons impinge on the crystal lattice. The displacement of carbon atoms from their equilibrium positions results in lattice strain, bulk dimensional change, and profound changes in physical properties. This article will discuss the effects of displacement damage in graphites and carbon-based materials. The materials considered here are those whose bonding is sp2—that is, graphites, pyrolytic carbons and graphites, carbon fibers, and carbon-carbon (C/C) composites. Radiation damage in sp3 (diamond) carbon forms is not discussed.

Carbon-based materials and graphites are widely used in nuclear applications. For example, polygranular (manufactured) graphites have been employed as a moderator in nuclear reactors since the 1940s. More recently, pyrolytic graphites, artificial graphites, and C/C composites have been adopted as plasma-facing components in fusion devices. Engineering applications, such as those just cited, have necessitated a full understanding of the basic mechanisms of radiation damage, as well as the effects of radiation damage on the physical properties of carbon-based materials.

There have been many studies (see for instance Reference 1) directed at quantifying the effects of ionizing radiation on polymers. Mainly these studies have focused on assessing the degradation of mechanical properties. In addition to changes in mechanical properties, a mixture of gases results from the irradiation of polymeric compounds. The amount of gas produced is related to the total absorbed dose while the type of gas is independent of dose and is related only to polymer structure. For many polymers, hydrogen is the major component in the gaseous mixture but polyesters and anhydride-cured epoxide resins are a notable exception.

Some problems associated with radiation damage may be further complicated when the irradiation temperature is in the cryogenic range. When materials are irradiated at low temperatures (<150 K), the release of volatile degradation products is delayed and occurs—possibly very rapidly and exceeding normal diffusion rates—only when the polymer is warmed. The rapid release of gaseous degradation products could occur as a result of warming a superconducting magnet that has been resin bonded and irradiated at low temperatures during operation. Radiation damage-related problems may be summarized as follows: (1) changes in short-term mechanical properties, (2) production of gases and low-molecular-weight species, and (3) longer term and geometrical effects (unknown) of trapped gases on electrical and mechanical properties.

This article considers mechanisms of radiation-induced damage and relates the chemical structure of the polymer to the rate of damage accumulation. Changes in electrical properties measured on irradiated polymers are generally small, and mechanical damage is generally more significant than changes in electrical characteristics. However all such reports are a result of electrical measurements on thin films and do not consider the effects of trapped gases on long-term electrical properties in bulk specimens.