L.K. Mansur

Void swelling in metals and alloys under irradiation: an assessment of the theory

The theory of swelling is reviewed in terms of basic concepts and simulation and impurity effects. The basic theory employs the formalism of chemical reaction rates. Efficiencies of voids, dislocat...

Mechanisms of helium interaction with radiation effects in metals and alloys: A review

Abstract Helium has been shown to cause major changes in the radiation effects response of metals and alloys. Examples are described. Physical mechanisms underlying these effects are discussed in terms of the theory of radiation effects. An extended treatment of the critical cavity radius concept is developed. Several new analytical results are presented. Applications are reviewed covering the temperature extension of swelling by gas, the evolution of bimodal cavity size distributions and the necessity for gas as a prerequisite to swelling under some conditions. The effect of helium on the dose dependence of swelling is described in terms of its effects on the balance of point defect sink strengths. Mechanisms underlying the enhanced formation and growth of cavities on precipitates when helium is present are discussed. A proposal is described to explain recently observed large effects of helium on phase stability.

Theory and experimental background on dimensional changes in irradiated alloys

Abstract When a material is irradiated with energetic neutrons or charged particles, a complex sequence of reactions takes place. Structure, composition, and properties are altered over an extremely wide scale, spanning atomic defects, meso-scale microstructures and macroscopic properties. Particularly interesting are radiation-induced dimensional changes. Such changes, which can occur in engineered components of fission and fusion reactors on a scale of meters, are driven by lattice defects at the subnanometer level. Because of their technological importance and their high scientific challenge, the dimensional changes termed radiation-induced swelling and creep have elicited sustained intensive research by basic and applied materials scientists for many years. The present paper is intended as a brief tutorial on salient features of this work. The presentation is divided into three parts. Background is first sketched emphasizing experimentally observed features and applications. Next, the theoretical framework and specific models that have been developed to understand radiation-induced swelling and creep in isotropic materials are described. Lastly, selected experiments designed and/or interpreted in terms of theory are highlighted to illustrate the current state of understanding of the physical bases of these phenomena.

Improved surface properties of polymer materials by multiple ion beam treatment

Ion beam treatment studies have been carried out to investigate the potential for improvements in surface-sensitive properties of polymers. Kapton, Teflon, Tefzel, and Mylar have been implanted with boron, nitrogen, carbon, silicon, and iron ions, singly or simultaneously with dual or triple beams. The implanted materials were characterized by optical microscopy, transmission electron microscopy, nano-hardness indentation, wear testing, scanning tunneling microscopy, x-ray analysis, nuclear reaction analysis, Fourier transform infrared spectroscopy, and Raman spectroscopy. Although the polymers showed a color change and varying degrees of measurable surface depression in the bombarded area, the implanted surface revealed substantial improvements in surface smoothness, hardness, and wear resistance. In particular, B, N, C triple-beam implanted Kapton showed over 30 times larger hardness than unimpanted material, making it more than three times harder than stainless steel. Sliding wear properties were characterized using an oscillating nylon or high carbon steel wear ball. Severe wear tracks were observed in virgin Kapton, but no appreciable wear was observed in ion implanted Kapton. Mechanisms underlying the improved surface properties are addressed.

Control of helium effects in irradiated materials based on theory and experiment

Helium produced in materials by (n,..cap alpha..) transmutation reactions during neutron irradiations or subjected in ion bombardment experiments causes substantial changes in the response to displacement damage. In particular, swelling, phase transformations and embrittlement are strongly affected. Present understanding of the mechanisms underlying these effects is reviewed. Key theoretical relationships describing helium effects on swelling and helium diffusion are described. Experimental data in the areas of helium effects on swelling and precipitation is reviewed with emphasis on critical experiments that have been designed and evaluated in conjunction with theory. Confirmed principles for alloy design to control irradiation performance are described.

Effects of electronic and recoil processes in polymers during ion implantation

It has been shown that ion implantation produces remarkable improvements in surface-sensitive mechanical properties, as well as other physical and chemical properties in polymers. To understand mechanisms underlying such property changes, various polymeric materials were subjected to bombardment by energetic ions in the range of 200 keV to 2 meV. The magnitude of property changes is strongly dependent upon ion species, energy, and dose. Analysis indicated that hardness and electrical conductivity increased by employing ion species with larger electronic cross sections and with increasing ion energy and dose. The results showed that electronic stopping or linear energy transfer (LET, energy deposited per unit track length per ion) for ionization was the most important factor for the enhancement of hardness, while nuclear stopping or linear energy transfer for displacement generally appeared to reduce hardness.

LET effect on cross-linking and scission mechanisms of PMMA during irradiation

Abstract Mechanisms of scission and cross-linking are investigated for poly(methyl methacrylate) subjected to various irradiation sources, such as low LET radiation sources (MeV e -beams, Co 60 γ-rays) and high LET ions (MeV He + , Ar + ). PMMA properties were degraded upon irradiation by e -beam and γ-rays, but were improved upon bombardment with high energy ion beams (HEIB) as a result of cross-linking. The results indicate that high LET produces a high concentration of free radicals over many neighboring molecular chains, facilitating track overlap and enhancing cross-linking over scission, while low LET affects only a single molecular chain, leading to chain scission.

Ion beam application for improved polymer surface properties

Abstract Various polymeric materials were subjected to bombardment by different energetic ions with energies ranging from 200 to 1000 keV. Tests showed substantial improvements in hardness, wear resistance, oxidation resistance, resistance to chemicals, and electrical conductivity. The magnitude of property changes was strongly dependent upon ion species, energy, dose, and polymer structure. Both hardness and electrical conductivity increased with ion energy and dose. These properties were apparently related to the effectiveness of cross-linking. Ion species with a large electronic stopping cross-section are expected to produce more cross-linking. It is believed that the polymer property improvements are commensurate with the extent of cross-linking, which is responsible for the formation of three-dimensionally-connected, carbon-rich, rigid networks.

The effects of impurity trapping on irradiation-induced swelling and creep☆

Abstract A theory of the effects of point defect trapping on radiation-induced swelling and creep deformation rates is developed. Trapping increases point defect recombination and decreases the rates of deformation processes. For fixed trapping parameters, the reduction is largest for void nucleation, less for void growth and creep due to dislocation climb-glide, and least for creep due to dislocation climb. The effects of trapping at multiple traps and of spatial and temporal variation in trap concentrations are determined. Alternative pictures in terms of effective recombination and diffusion coefficients are derived. Previous derivations of these coefficients are incorrect. A rigorous explanation is given of the well-known numerical asymmetry in the effects of vacancy and interstitial trapping. Corrections which become necessary at solute concentrations above about 0.1% are described. Numerical results for a wide range of material and irradiation parameters are presented.

On the origin of deformation microstructures in austenitic stainless steel: part I—microstructures

Abstract A comprehensive characterization of room temperature deformation microstructures was carried out by transmission electron microscopy for ion irradiated and deformed AISI 316LN austenitic stainless steel. Deformation microstructures were produced by a recently developed disk-bend test method and also by a uniaxial tensile test. Cross-slip was dramatically suppressed by the radiation-induced defects and slip occurred predominantly by planar glide of Shockley partial dislocations. Deformed microstructures consisted of piled-up dislocations, nanotwin layers, stacking faults, and defect-reduced dislocation channel bands. Analyses revealed that all these features were different manifestations of the same type of deformation band, namely a composite of overlapping faulted layers produced by Shockley partial dislocations.