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Eight complex austenitic stainless steel alloys based on the composition Fe-13Cr-15Ni-2Mo-2Mn-0.2Ti-0.8Si-0.06C were implanted simultaneously with 400-keV B+ and 550-keV N+ ions and were investigated for changes in fatigue properties and surface microhardness. The nearsurface hardness of all eight alloys improved, but the fatigue life of each decreased. These findings were contrary to those obtained in an earlier study using four simple Fe-13Cr-15Ni alloys, where the dual implantation improved fatigue life by up to 250 pct. While unimplanted specimens failed by slip-band crack initiation, it was hypothesized that the dual implantation suppressed slip to the extent that fewer slip-band cracks were initiated and these were subjected to accelerated crack propagation. In addition, grain-boundary cracking was promoted, yielding a lower fatigue life. Support for this hypothesis was obtained by a study of single crystals of Fe-15Cr-15Ni, which were also implanted with B+ and N+. The dual implantation caused a lower fatigue life due to concentration of slip along a few slip bands to relieve applied stress. Evidence of grain-boundary cracking was obtained using the four simple alloys, which were subjected to triple ion implantation with B+, N+, and C+. The triple implantation decreased the fatigue life of the alloys and caused accelerated growth of fewer slip bands and grain-boundary cracking due to suppression of surface slip bands. This study thus shows the existence of an optimum level of strengthening when multiple ion implantation is used to improve the fatigue properties of alloys.

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.

Polyimide Kapton and spin-cast polyamic acid (PAA) on sapphire have been implanted with 1 MeV Ar ions to a dose of 4.7 × 1015 cm−2 at ambient temperature. The properties of both pristine and implanted surfaces were characterized by a depth-sensing low-load indentation technique. Experiments were carried out to investigate the effects of substrate, indentation rate, relaxation, and indentation technique. The results showed that (1) hardness was depth-dependent and decreased with increasing indentation depth, (2) measurements of the ion beam hardened surface with the untreated material as a substrate underestimated the hardness while measurements over the sapphire substrate overestimated it, (3) the effects of loading/unloading rates were apparent in the load displacement results, and (4) hardness values measured using the force modulation technique showed very little depth dependence. The hardness value at 100 nm depth is used for comparison purposes since the hardness value at this depth was almost independent of substrate, indentation rate, and indentation method. The hardness of Kapton, which was measured using the techniques described herein, was increased by over 30 times after Ar implantation, from 0.43 to 13 GPa at 100 nm indentation depth. A similar increase in hardness was also observed for polyamic acid. This result suggests that spin-cast PAA film may have potential technological applications for protective coatings where hardness and wear resistance are required.

Polycarbonate (Lexan) was implanted with 100 and 200 keV B+ ions to doses of 0.26, 0.78, and 2.6 × 1015 ions/cm2 at room temperature (15 ions/cm2 for 200 keV ions, which is more than 10 times that of the unimplanted polymer. Wear properties were characterized using a reciprocating tribometer with nylon, brass, and SAE 52100 Cr-steel balls with 0.5 and 1 N normal forces for 10000 cycles. The wear mode varied widely as a function of ion energy, dose, wear ball type, and normal load. For given ion energy, load, and ball type conditions, there was an optimum dose that produced the greatest wear resistance and lowest friction coefficient. For polycarbonate implanted with 0.78 × 1015 ions/cm2, the nylon ball produced no wear after 10000 cycles. Moreover, the overall friction coefficient was reduced by over 40% by implantation. The results suggest that the potential of ion-beam technology for improving polycarbonate is significant, and that surface-sensitive mechanical properties can be tailored to meet the requirements for applications demanding hardness, wear, and abrasion resistance.

Austenitic Fe-Cr-Ni steels are potential candidate alloys for structural materials in both fast breeder and magnetic fusion reactors. However, void swelling and phase instability during irradiation have been major problems limiting the properties and useful lifetimes of these materials and thus have been the subject of intensive investigations. Cavity nucleation in steels subjected to displacement damage is strongly influenced by the interactions between helium atoms and precipitates which are formed during irradiation. Several important mechanisms regarding gas atom-precipitate interactions and principles for the design of radiation-resistant alloys are critically examined. Central concepts derived from theory, including the critical radius and critical number of gas atoms, cavity-precipitate interactions, and relative sink strengths for point defects, are applied to the interpretation of experimental data.

The surface reaction rate controlled void growth model is extended to include the effects of point defect recombination and thermal emission of vacancies from the void surface. Experimental data on solution annealed Type 316 stainless steel from both neutron irradiation and ion bombardment are examined using this model. It is concluded that void growth in this material is controlled by reaction rate at the void surface; also, the temperature dependence of void growth rate mainly reflects the temperature dependence of dislocation density. Approaches for swelling data extrapolation and translation are discussed.

The theory of point defect trapping was applied to predict the effect of solutes on the steady-state dislocation climb creep rate. Vacancy loops generated in the cascades of point defect production by heavy particles were included. It is shown that the effect of vacancy loops on the creep rate is mathematically equivalent to a simultaneous reduction in the vacancy trapping rate at solutes as well as in the free point defect generation rate and recombination coefficient. These parameters are reduced by the ratio of the sink strength without vacancy loops to the total sink strength. Solute trapping and vacancy loops do not reduce the unperturbed creep rates by more than an order of magnitude for the parametric range considered.

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 unimplanted 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.

Irradiation of solid materials with energetic neutrons or charged particles can lead to profound changes in defect structure, microcomposition, and macroscopic properties. Such changes occur by atomic and microstructural mechanisms, some of which are familiar in "classical" physical metallurgy and materials science. However, other cases appear to be unique to irradiation. Irradiation has considerably broadened and indeed provided an entirely new dimension in materials science, since the energetic displacement of atoms potentially reaches to every property or process. The initial damaging events leading to the creation of point defects are generally complete in times of order 10−11 s. Subsequent changes in structure, composition, and properties take place in a span of much longer time scales corresponding to interstitial and vacancy diffusion, clustering, solute segregation, and precipitation. An extensive theoretical framework has been developed to understand the kinetics of these processes. Emphasis has been placed on both steady cumulative processes and on fluctuations, and on the appropriate application of stochastic and deterministic descriptions. Parallel and interactive experimental activities for both applied and basic programs over the past two decades have increased the level of phenomenological knowledge enormously. Much of the work has emphasized either high-dose phenomena such as irradiation-induced swelling, creep, embrittlement, phase instability, and solute segregation relevant to materials applications, or the properties, structures, and interactions of defects, which underlie more fundamental issues.

Mechanisms by which oxygen affects swelling during Fe-ion irradiation are investigated in a series of Fe-15Ni-13Cr-base alloys containing minor additions of titanium, silicon and carbon. The irradiations are carried out at 948 and 1000 K to doses up to 100 dpa. Effects of both residual oxygen content and accelerator-injected oxygen are studied. It is found that pre-injected oxygen promotes cavity swelling and, in some cases, leads to a fine distribution of stable bubbles for oxygen injection levels as high as 1000 at.p.p.m. The results are interpreted in terms of the chemical activity of oxygen in forming precipitates with alloying elements, and in terms of radiation dissolution of oxygen leading to oxygen-pressurized bubbles. Accounting for the interaction with alloying elements, we conclude that oxygen exhibits a direct cavity-pressurization effect on swelling similar to that previously reported for helium. The results are analyzed in terms of the theory of the critical radius and critical numb...

Central concepts in the theory of swelling based on defect reactions are reviewed. The critical radius and critical number of gas atoms, which determine the initiation of swelling, and the ratio of the dislocation and cavity sink strengths, which dictates the swelling rate, are demonstrated to be of great utility in the understanding and control of swelling. Over the past two decades, a large data base has been accumulated covering austenitic and ferritic/martensitic alloys, the leading candidate materials for both fusion and fast fission reactor applications. This collection of data naturally serves as the largest source of information on which to develop and test mechanistic understanding of swelling. Over wide ranges in materials and irradiation parameters, including composition, temperature, damage rate, and helium generation rate, we find that apparently divergent swelling behaviors can be explained in a unified manner within the present theoretical framework. Principles for microstructures that insure swelling resistance, together with results from the necessary confirmatory experiments, are described.

Three cold-worked phosphorous-modified Fe-15Ni-13Cr-base alloys were irradiated with simultaneous Fe- and He-ion beams to doses of about lOOdpa at a helium implantation rate of 20at.p.p.m./dpa. The three alloys correspond to compositions that led to increasing densities of a very fine phosphide precipitate. In the case of the alloy with the highest precipitate density, the swelling was suppressed to the greatest extent. The results are interpreted in terms of the theory of cavity swelling, which provides the central concept of a critical radius and critical number of gas atoms required for cavity swelling. The large interfacial area of the needle-shaped phosphide precipitates led to profuse bubble nucleation at the interface. The high density of bubbles slowed the accumulation of gas atoms in individual bubbles and thus delayed the onset of point-defect-driven cavity swelling. After interpreting the present data in terms of these concepts, more general features of cavity swelling reduction by an ...

Radiation doses from galactic cosmic rays (GCR) are a significant issue for spacecraft crew exposures in deep space. We report initial work to evaluate a range of materials for GCR shielding. Earlier work has shown that conventional spacecraft materials, aluminum and higher atomic number structural alloys, provide relatively little shielding and, under certain conditions, may increase radiation risk. Materials containing high proportions of hydrogen and other low atomic mass nuclei provide improved GCR shielding. Polyethylene (PE) is generally considered a good performance benchmark shield material. However, PE shielding occupies volume and adds mass to the spacecraft. In this work we investigate several materials that are shown to provide shielding similar to PE, but which could furnish additional spacecraft functions, possibly eliminating the need for materials currently used for structural support or thermal management. Carbon forms that can incorporate a large mass of hydrogen, as well as polymers and polymer composites are being explored. Calculations of shielding effectiveness in GCR spectra have been carried out. Experiments to measure shielding properties recently have been completed at the NASA Space Radiation Laboratory (NSRL) located at Brookhaven National Laboratory (BNL) using high energy beans of O16. In this paper we report preliminary shielding results.

Raman scattering studies were carried out to investigate the effects of ion implantation on the structure of diamond, graphite, and polymers. Carbon phases produced by chemical vapor deposition (CVD diamond) and rf discharge (diamondlike carbon or DLC) were also analyzed. Two types of amorphous carbon phases were distinguished with relevance to hardness. In general, amorphous carbon phases produced by electron beam evaporation and sputtering are soft (hardness \ensuremath{\ll}1 GPa), while DLC and some ion-beam-modified polymers are much harder. In all cases, the characteristic Raman bands of the starting material were lost upon ion implantation, and for the lowest fluences the one-phonon bands near 1360 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (D line) and 1580 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (G line) of disordered polycrystalline graphite appeared. With increasing fluence these bands coalesced into a broad, asymmetric peak with the D line shifting to higher wave number and the G line shifting to lower wave number. This trend was clearly distinguishable from the finite crystallite size effect seen in graphite, where, in addition to the appearance of the D line, the G line shifts to higher wave number with decreasing crystallite sizes. Raman scattering could not distinguish between soft and hard amorphous carbon. There was also no indication that the hardness of DLC films and ion-beam-modified polymers was due to diamondlike ${\mathit{sp}}^{3}$ bonds. Instead, hardness in these materials is related to the three-dimensional interconnectivity of chemical bonds. Experimental results suggest that the amorphous carbons examined in this study are composed of random networks of distorted sp, ${\mathit{sp}}^{2}$, and ${\mathit{sp}}^{3}$ bonded atoms, sometimes in a hydrogenated state. The hard carbons such as DLC films and ion beam modified polymers have long-range chemical connectivity while the soft carbons such as damaged graphite, and carbon films prepared by sputter deposition lack such connectivity.

A new method for the calculation of radiation damage parameters in large geometries encompassing multiple materials using the MCNPX mesh tally has been developed. The method has been tested against previously published calculations of the displacement rate for protons and neutrons at the center of the SNS 316LN stainless steel target vessel nose. Displacement rates for neutrons, protons, and the total using the mesh tally method are shown to agree with previous work. The mesh tally method is also applied to the SNS aluminum moderator vessels and to the SNS inner reflector plug composed of aluminum, beryllium, and stainless steel. Results are given for displacement, helium, and silicon production rates.

A mechanism of irradiation creep arising from the climb-enabled glide of dislocations due to stress-induced preferred absorption of radiation-produced point defects is proposed. This creep component is here termed preferred absorption glide, PAG. PAG-creep operates in addition to the previously studied components of creep from climb by stress-induced preferred absorption, (SI)PA-creep, and the climb-enabled glide due to excess absorption of interstitials on dislocations during swelling, I-creep. A formulation of the various climb and climb-enabled glide processes which includes earlier results is presented. PAG-creep is comparable in magnitude to PA-creep in the parameter range of applications. While the PA-creep rate and the I-creep rate are linear in stress, the PAG-creep rate is quadratic in stress and thus dominates at high stresses.

Microhardness and cantilever beam fatigue measurements were conducted on Fe–13Cr–15Ni base austenitic alloys that were implanted with boron and nitrogen ions either singly or simultaneously. The microstructure of the modified surface layer and dislocation slip modes after fatigue tests were investigated by optical and transmission electron microscopy. Both hardness and fatigue life were improved by ion implantation, but the greatest improvement was achieved when boron and nitrogen were implanted simultaneously. The degree of fatigue life improvement also varied with minor changes in the base alloying compositions: nitrogen was detrimental or ineffective in the presence of titanium, and boron was much more effective in the presence of molybdenum. Comparison of slip band morphology between the compression and tension cycles indicated that implantation improved the reversibility of surface slip and delayed crack initiation.

Helium and its state of agglomeration in pure aluminium prior to neutron irradiation are investigated with respect to its influence on the microstructural features developed during irradiation to doses of 0·005 to 2·6d.p.a. at about 328 K (0·35 Tm). A level of 4 a.p.p.m. helium, in either submicroscopic form or as small bubbles at the resolvable limit of 2 nm diameter, induces large increases in both the cavity and dislocation components of the damage structure. It also causes swelling to begin much earlier in the neutron dose and to exhibit two different stages of evolution, one early in the dose where the dose exponent of swelling is high, the other at later stages with lower exponent. When the helium is coarsened into 7-nm bubbles by annealing at 0·96 Tm before irradiation it has relatively small effects on damage structure and swelling. The increases in concentrations of cavities and dislocation loops in the presence of helium are attributed to trapping of radiation-produced vacancits by the ...

Irradiations with 4 MeV Ni ions and 200–400 keV He ions were carried out on two alloys, Fe-15Cr-15Ni and Fe-15Cr-35Ni, at 675°C and doses up to 84dpa. Both dual-ion irradiation experiments and sequenced He injection-anneal-Ni irradiations were used. The dual-ion experiment showed that the two alloys exhibited large differences in microstructural development, with the low nickel alloy having significantly greater swelling. The injection-anneal-irradiation experiment was designed to test the hypothesis, suggested by our earlier work, that the lower swelling of the high nickel alloy may result from a larger critical radius/critical number of gas atoms required to achieve bias driven swelling. This experiment provided a direct measurement of these critical quantities by the induction of a bimodal cavity size distribution. The measurement gave minimum critical radii of about 5nm for the high nickel alloy and < 0·5nm for the low nickel alloy, values consistent with the hypothesized mechanism. The basis...

It is often observed experimentally in complex alloys, such as the austenitic stainless steels, that the largest cavities produced during irradiation are attached to second-phase precipitate particles. One hypothesis that such observations suggest is that the precipitate—matrix interface may assist in the collection of irradiation-produced point defects which are channelled to the attached cavities. A theoretical analysis is developed to evaluate this mechanism. It is found that the growth of cavities attached to precipitates is increased compared to the growth of cavities in the matrix. The relative growth rates of the two types of cavity are also affected by differences in bias and differences in sink strength. The relationships required to evaluate these effects are developed and the consequences of enhanced point-defect collection are explored in some detail. For the parameter range of interest, it is found by calculation that the cavity attached to the precipitate may grow several times larg...

The interaction energy of a center of dilatation with a spherical cavity or a sphere covered by a surface layer is derived. The surface layer and the substrate are assumed to consist of elastically isotropic materials with different elastic constants. In addition to the image interaction also the interaction energy is obtained when a homogeneous eigenstrain exists in the surface layer and when the spherical cavity is in a hydrostatic or uniaxial tension field. A comparison is made for the image interaction with the coated sphere, the cavity, and the flat surface, and approximate expressions are given for the point defect close to the coated cavity. Die Wechselwirkungsenergie zwischen einem Dilatationszentrum und einem spharischen Hohlraum oder einer Kugel, die beide mit einer Oberflachenschicht bedeckt sind, wird abgeleitet. Es wird angenommen, das die Oberflachenschicht und das Medium aus isotropen, elastischen Materialien bestehen, die unterschiedliche elastische Konstanten haben. Zusatzlich zu der Bild-Wechselwirkung wird auch die Wechselwirkungsenergie erhalten, wenn die Schicht eine homogene Eigenverzerrung enthalt, und wenn der Hohlraum sich in einem hydrostatischen oder einachsigen Spannungsfeld befindet. Die Bildkraft fur die bedeckte Kugel, den bedeckten Hohlraum oder die bedeckte ebene Flache werden miteinander verglichen, und Naherungsausdrucke werden erhalten, wenn der Punktfehler sich in der Nahe des Hohlraumes befindet.

Thermal aging and irradiation experiments were carried out in Fe–Ni–Cr base alloys containing Ti or Nb as well as low levels of oxygen, nitrogen, and carbon. After aging, fine dispersions of TiC and NbC were formed. However, after irradiation the Ti-containing alloy precipitated Ti(N,O) rather than TiC, while the Nb-containing alloy precipitated NbC. Analysis of the energetics of these reactions reveals that the Ti(N,O) phase is thermodynamically more stable than the TiC phase, while the NbC phase is more stable than the NbO phase. These results are therefore consistent with observed behavior. While it is well known that irradiation can drive a system away from equilibrium, the present results show that irradiation also can catalyze the formation of a more stable phase at the expense of a persistent metastable phase.

Ion implantation, ion beam mixing, and ion beam stimulated reactions can be discussed as somewhat distinct but related processes. The first two emphasize compositional changes in near-surface regions. The last relies mainly on microstructural and precipitation reactions caused by atomic displacements. In this last area much of the work has been carried out in metallic alloys. Here we summarize experiments in our laboratory that cover several ion-beam-induced reactions in Fe-15Ni-15Cr base alloys, to provide a perspective on related work in materials other than silicon. The techniques and mechanistic interpretations of results are applicable to a variety of materials.This is a powerful way to examine the interplay of characteristic relaxation times. Intervals of high point defect production and resultant processes, such as solute segregation and drift-directed clustering, are alternated with thermal annealing. Remarkable changes with respect to either steady bombardment or thermal aging at the same temperature can be produced. Figure 1(a) shows theoretically calculated fluctuating vacancy concentration in a steady irradiation.