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Compositional changes during ion bombardment

Description: Ion irradiation initiates several processes that can alter the composition of the target. This presentation provides an overview of our current understanding of these kinetics processes, which include implantation, sputtering, displacement mixing, radiation-enhanced diffusion, and radiation-induced segregation. The latter two effects can alter the target composition to depths that are substantially greater than the projected ion range. 45 refs., 8 figs.
Date: September 1, 1988
Creator: Rehn, L.E.
Partner: UNT Libraries Government Documents Department

Ion-beam-induced changes in alloy composition. [Ni-Si; Cu-Ni]

Description: Non-equilibrium, radioinduced segregation (RIS) occurs in ion implantation, RIS studies in Ni-Si alloys show that heavy ions are much less efficient than light ions at producing long-range migrating defects. Defect diffusion can be very large at elevated temperature can produce changes to depths beyond the ion range, particularly in Cu-Ni alloys. (DLC)
Date: April 1, 1983
Creator: Rehn, L.E. & Wiedersich, H.
Partner: UNT Libraries Government Documents Department

Surface segregation during irradiation

Description: Gibbsian adsorption is known to alter the surface composition of many alloys. During irradiation, four additional processes that affect the near-surface alloy composition become operative: preferential sputtering, displacement mixing, radiation-enhanced diffusion and radiation-induced segregation. Because of the mutual competition of these five processes, near-surface compositional changes in an irradiation environment can be extremely complex. Although ion-beam induced surface compositional changes were noted as long as fifty years ago, it is only during the past several years that individual mechanisms have been clearly identified. In this paper, a simple physical description of each of the processes is given, and selected examples of recent important progress are discussed. With the notable exception of preferential sputtering, it is shown that a reasonable qualitative understanding of the relative contributions from the individual processes under various irradiation conditions has been attained. However, considerably more effort will be required before a quantitative, predictive capability can be achieved. 29 refs., 8 figs.
Date: October 1, 1985
Creator: Rehn, L.E. & Lam, N.Q.
Partner: UNT Libraries Government Documents Department

Ion-implantation effects on the thermal oxidation of metals. [He-implanted Ni-1 at. % Pt alloy]

Description: In the past decade, ion implantation has been shown to reduce oxide thicknesses up to tenfold in survey experiments on Ti, Zr, Ni, Cu, and Cr, and to enhance the long-term oxidation resistance of some high-temperature alloys. This review summarizes the major results of previous work. Important concepts are illustrated using recent experimental results obtained from a He-implanted Ni-1 at.% Pt alloy. Collectively, the results indicate that ion implantation has considerable potential for reducing oxidation, and as a research tool to investigate the mechanisms of thermal oxidation in metals.
Date: March 1, 1982
Creator: Grabowski, K.s. & Rehn, L.E.
Partner: UNT Libraries Government Documents Department

Anelastic relaxation in irradiated Cu-Be

Description: Ultrasonic velocity measurements were made after thermal neutron irradiation of single-crystal Cu specimens containing 700 or 3700 appM Be. Ultrasonic attenuation was measured in similar specimens after 3 MeV electron irradiation. Three anelastic relaxation processes due to self-interstitial-Be complexes were observed. Cu-Be 1 was found to occur near liquid helium temperature; the temperature dependence of the velocity change suggests that reorientation of the Cu-Be 1 complex may involve quantum mechanical tunneling. Cu-Be produced an attenuation peak at approx.2.5 K at a frequency of 10 MHz. Cu-Be 3 appeared simultaneously with 2 as a shoulder on the high temperature side of the 2.5 K attenuation peak.
Date: May 1, 1985
Creator: Rehn, L.E.; Wiedersich, H. & Granato, A.V.
Partner: UNT Libraries Government Documents Department

The role of gamma rays and freely-migrating defects in reactor pressure vessel embrittlement

Description: Gamma ray effects are often neglected when evaluating reactor pressure vessel (RPV) embrittlement. However, recent analyses indicate that in newer style light water reactors, gamma damage can be a substantial fraction of the total displacement damage experienced by the (RPV); ignoring this damage will lead to errors in embrittlement predictions. Furthermore, gamma rays may be more efficient than fast neutrons at producing freely-migrating defects and as such can impact certain embrittlement mechanisms more effectively than fast neutrons. Consideration of these gamma effects are therefore essential for a more complete understanding of radiation embrittlement.
Date: September 1, 1996
Creator: Alexander, D.E. & Rehn, L.E.
Partner: UNT Libraries Government Documents Department

Analysis of gamma ray displacement damage in Light Water Reactor pressure vessels

Description: In addition to fast neutrons, the copious energetic gamma rays, present in a reactor environment, induce displacement damage in the reactor pressure vessel. The contribution of gamma ray damage to embrittlement is most pronounced in reactors with large water gaps separating the core from the reactor pressure vessel. Water moderates the energies of fast neutrons much more effectively than it attenuates the high energy gamma flux, and thus enhances the high energy gamma flux, incident on the vessel relative to the fast neutron flux. In this paper, an analysis of computer transport calculations is presented which quantifies the relative contribution of gamma ray damage in various pressure vessels. The results indicate that gamma ray damage must be included for accurate predictions of radiation-induced embrittlement.
Date: May 1, 1995
Creator: Alexander, D.E. & Rehn, L.E.
Partner: UNT Libraries Government Documents Department

Radiation-induced segregation during sputtering

Description: Experimental measurements are reported of the time development of compositional changes which occur deep in a Cu-40 at. % Ni alloy during sputtering at elevated temperatures with 5-keV Ar ions. Large effects are observed due to radiation-induced segregation (RIS), i.e. the preferential transport of nickel atoms in the direction of the defect fluxes. Results are compared with calculated composition profiles obtained using the phenomenological model of Lam and Wiedersich. Qualitative agreement between the model calculations and experimental results is found. Quantitative differences between the model and experiment preclude a reliable estimate of RIS effects in the very near-surface layers. However, the fact that the calculations underestimate the RIS contribution very deep in the specimen while still predicting a substantial RIS contribution in the near-surface region suggests that RIS can play an important role in determining near- surface compositional changes during sputtering at temperatures where defects are mobile.
Date: June 1, 1984
Creator: Rehn, L.E.; Lam, N.Q. & Wiedersich, H.
Partner: UNT Libraries Government Documents Department

Characterization of near surface regions in irradiated Ni(Si) alloys

Description: Transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS), Auger electron spectroscopy (AES), and infra-red pyrometry (IRP) have been used to characterize the growth of Ni/sub 3/Si films on the surfaces of irradiated Ni(Si) alloys. Results from each of the four techniques are presented and discussed, and comparisons are made between the different techniques. AES measurements are reported which suggest that Si concentrations significantly in excess of that found for stoichiometric Ni/sub 3/Si are induced in regions very near to the surface during irradiation.
Date: January 1, 1980
Creator: Rehn, L.E.; Averback, R.S. & Okamoto, P.R.
Partner: UNT Libraries Government Documents Department

Freely-migrating defects: Their production and interaction with cascade remnants

Description: Many microstructural changes that occur during irradiation are driven primarily by freely-migrating defects, i.e. those defects which escape from nascent cascades to migrate over distances that are large relative to typical cascade dimensions. Several measurements during irradiation at elevated temperatures have shown that the survival rate of freely-migrating defects decreases much more strongly with increasing primary recoil energy than does the survival rate for defects generated at liquid helium temperatures. For typical fission or fusion recoil spectra, and for heavy-ion bombardment, the fraction of defects that migrate long-distances is apparently only {approximately}1% of the calculated dpa. This small surviving fraction of freely-migrating defects results at least partially from additional intracascade recombination at elevated temperatures. However, cascade remnants, e.g., vacancy and interstitial clusters, also contribute by enhancing intercascade defect annihilation. A recently developed rate-theory approach is used to discuss the relative importance of intra- and intercascade recombination to the survival rate of freely-migrating defects. Within the validity of certain simplifying assumptions, the additional sink density provided by defect clusters produced directly within individual cascades can explain the difference between a defect survival rate of about 30% for low dose, low temperature irradiations with heavy ions, and a survival rate of only {approximately}1% for freely-migrating defects at elevated temperatures. The status of our current understanding of freely-migrating defects, including remaining unanswered questions, is also discussed. 33 refs., 5 figs.
Date: May 1, 1991
Creator: Rehn, L.E. & Wiedersich, H.
Partner: UNT Libraries Government Documents Department

Dependence of ion beam mixing on projectile mass

Description: Ion beam mixing in Pt-Si bilayered samples was measured during irradiation with projectiles ranging in mass from 4 amu (He) to 131 amu (Xe) at 10/sup 0/K, 300/sup 0/K and 373/sup 0/K. Using deposited damage energy as a basis for comparing the different irradiations, it was found that the heavier ions were more efficient than the lighter ones for inducing mixing. Moreover, it was observed that the mixing was essentially independent of temperature below 373/sup 0/K. These results are interpreted on the basis that the mixing is caused by the stimulated motion of defects during the cooling phase of energetic cascades.
Date: November 1, 1983
Creator: Averback, R.S.; Thompson, L.J. & Rehn, L.E.
Partner: UNT Libraries Government Documents Department

Radiation-induced segregation in Cu-Au alloys

Description: Irradiation produced Au atom depletion in the near surface region. Since the analysis of measurements of Au diffusion in Cu in terms of the five-frequency model predicts that vacancies preferentially transport Au atoms toward the surface, interstitials are apparently responsible for the observed Au atom depletion. Segregation occurred in the temperature range between about 300 and 500/sup 0/C. It peaked near 400/sup 0/C for a calculated dose rate of 3.9 x 10/sup -5/ dpa/s. Theoretical analysis based on Johnson-Lam model predicted: (1) that the amount of segregation would be proportional to dose in the early stage of irradiation, would deviate from linearity with a continuously decreasing slope at intermediate doses, and finally approach a constant value after high doses; (2) that segregation rate would vary as -1/4th power of the dose rate at constant dose in the low temperature region. The ratio of segregation rate which was obtained by polynomial fitting agreed well with -4th root of dose rate ratio.
Date: December 1, 1985
Creator: Hashimoto, T.; Rehn, L.E. & Okamoto, P.R.
Partner: UNT Libraries Government Documents Department

Radiation--induced solute segregation in a V-15 Wt. % Cr alloy

Description: Measurements of the dose and temperature dependences of radiation-induced segregation in a V-ion irradiated alloy of V-15 Cr are reported. Very rapid migration of Cr atoms toward the external surface occurs during irradiation in the temperature range from 450 to 700/sup 0/C. A maximum in the degree of segregation is found near 650/sup 0/C for a peak dose rate of approx. 3 x 10/sup -3/ dpa s/sup -1/.
Date: January 1, 1979
Creator: Rehn, L.E.; Agarwal, S.C. & Nolfi, F.V. Jr.
Partner: UNT Libraries Government Documents Department

Radiation-induced solute segregation in a V-15 wt % Cr alloy

Description: Measurements of the dose and temperature dependences of radiation-induced segregation in a V-ion irradiated alloy of V-15 Cr are reported. Very rapid migration of Cr atoms toward the external surface occurs during irradiation in the temperature range from approx. 450/sup 0/C to 700/sup 0/C. A maximum in the degree of segregation is found near 650/sup 0/C for a peak dose rate of approx. 3 x 10/sup -3/ dpa s/sup -1/.
Date: January 1, 1979
Creator: Rehn, L.E.; Agarwal, S.C. & Nolfi, F.V. Jr.
Partner: UNT Libraries Government Documents Department

The effect of simultaneous electron and Kr/sup +/ irradiation on amorphization of CuTi

Description: CuTi was irradiated with 1-MeV electrons and Kr/sup +/ ions simultaneously at temperatures from 10 to 423 K. Retardation of Kr/sup +/-induced amorphization was observed with simultaneous electron irradiation at 295 and 423 K. The retardation effect increased with increasing irradiation temperature and relative electron-to-Kr dose rate. In contrast, simultaneous irradiation below 100 K showed an additive effect of electron- and Kr/sup +/-induced amorphization. The results can be explained by the mobility point defects introduced by electron irradiation interacting with Kr/sup +/-induced displacement cascades. 6 refs., 6 figs.
Date: January 1, 1989
Creator: Koike, J.; Okamoto, P.R.; Rehn, L.E. & Meshii, M.
Partner: UNT Libraries Government Documents Department

Modification of. mu. m thick surface layers using keV ion energies

Description: Root-mean-square diffusion distances for both vacancy and interstitial defects in metals can be very large at elevated temperatures, e.g. several ..mu..m's in one second at 500/sup 0/C. Consequently, defects that escape the implanted region at elevated temperature can produce conpositional and microstructural changes to depths which are much larger than the ion range. Because of the high defect mobilities, and of the fact that diffusion processes must compete with the rate of surface recession, the effects of defect production (ballistic mixing), radiation-enhanced diffusion and radiation-induced segregation become spatially separated during ion bombardment at elevated temperature. Results of such experimental studies in a Cu-Ni alloy are presented, discussed and compared with predictions of a phenomenological model. Contributions to the subsurface compositional changes from radiation-enhanced diffusion and radiation-induced segregation are clearly identified.
Date: November 1, 1983
Creator: Rehn, L.E.; Lam, N.Q. & Wiedersich, H.
Partner: UNT Libraries Government Documents Department

Radiation-induced segregation in Cu-Au alloys. [1 at. % Au]

Description: Radiation-induced segregation in a Cu-1 at. % Au alloy was investigated using in situ Rutherford Backscattering Spectrometry. Irradiation with 1.8-MeV He produced nonequilibrium Au atom depletion in the near surface region. The amount of segregation was measured as a function of dose, dose rate and temperature. Segregation was observed between 300 and 500/sup 0/C. For a calculated dose rate of 3.9 x 10/sup -5/ dpa/s, the radiation-induced segregation rate peaked near 400/sup 0/C. Theoretical analysis based on the Johnson-Lam model predicted that the amount of segregation would be directly proportional to dose at the early stage of irradiation, would deviate from linearity with a continuously decreasing slope at intermediate doses, and finally approach a constant value after high doses. The analysis also predicted that the segregation rate would vary as the -1/4th power of the dose rate at constant dose in the low temperature region. These predictions were all verified experimentally. A procedure for extracting relative defect production efficiencies from similar measurements is discussed.
Date: April 1, 1986
Creator: Hashimoto, T.; Rehn, L.E. & Okamoto, P.R.
Partner: UNT Libraries Government Documents Department

Radiation-induced segregation and precipitation in molybdenum-rhenium alloys

Description: Specimens of Mo-7 at. % Re and Mo-30 at. % Re were irradiated with 1.8 MeV /sup 4/He/sup +/ ions at elevated temperatures. Radiation-induced segregation of Re was measured during irradiation by in situ Rutherford backscattering spectrometry. Segregation of the undersized Re atoms in the same direction as the defect fluxes, i.e., toward the external surface, was observed. The amount of Re enrichment in the near-surface region was measured as a function of temperature and of dose at a calculated near-surface displacement rate near 1 x 10/sup -4/dpa/s. Segregation was observed at temperatures from 800 to 1500/sup 0/C in Mo-7Re, and from 850 to 1225/sup 0/C in Mo-30Re. Irradiated disks were examined by transmission electron microscopy. Precipitates of Chi phase were observed on grain boundaries, or in a thin layer at the irradiated surface in Mo-30Re after irradiation at temperatures from 750 to 1075/sup 0/C. Frequently, Chi precipitates formed with a crystallographic twin orientation with respect to the host matrix. No voids were observed for doses up to 1.6 dpa.
Date: February 1, 1986
Creator: Erck, R.A.; Wayman, C.M. & Rehn, L.E.
Partner: UNT Libraries Government Documents Department

Experimental studies of free defect generation during irradiation: Implications for reactor environments

Description: Over the past several years, systematic experiments have revealed that irradiations which generate energetically dense cascades are much less effective than light-ion, MeV electron, or thermal neutron irradiations at producing freely-migrating defects. In this paper, the systematic results on freely-migrating defect production from ion irradiation studies are briefly summarized. Difficulties with applying a simple extrapolation of the ion-irradiation results to neutron environments are discussed. This discussion, coupled with our existing knowledge of neutron-induced property changes, indicates that Compton scattering, and the (n,[gamma]), (n,He) and (n,p) nuclear reactions, are considerably more important for producing freely-migrating defects than was previously realized.
Date: January 1, 1993
Creator: Rehn, L.E. & Birtcher, R.C.
Partner: UNT Libraries Government Documents Department

Effects of primary recoil energy on the production rate of mobile defects during elevated temperature irradiation

Description: Radiation-induced segregation rates in a Ni-12.7 at.% Si alloy have been measured as a function of temperature using ions of various masses and energies. An analysis of the segregation kinetics using a simple analytical model yielded the relative efficiency of each of the ions for producing mobile defects directly from ratios of their measured segregation rates. In this paper, we also show that the relative efficiencies can also be determined from measured shifts in the peak segregation temperature. Both methods yield a strong decrease in efficiency with increasing ion mass. The reduction in efficiency for the heavior ions was found to be significantly larger than that measured at very low temperatures by resistivity techniques. The latter are often used as a basis for correlating damage structures produced at elevated temperatures. Differences between the low and high temperature measurements indicate that relative efficiencies determined from segregation measurements are more reliable for correlating microstructural changes that are produced in different irradiation environments at high temperatures.
Date: November 1, 1984
Creator: Okamoto, P.R.; Rehn, L.E. & Averback, R.S.
Partner: UNT Libraries Government Documents Department

Fundamental aspects on ion-beam surface modification: defect production and migration processes

Description: Ion-beam modification of metals is generating increasing scientific interest not only because it has exciting technological potential, but also because it has raised fundamental questions concerning radiation-induced diffusion processes. In addition to the implanted species, several defect production and migration mechanisms contribute to changes in the near-surface composition of an alloy during ion bombardment, e.g., atoms exchange positions via displacements and replacement sequences; preferential sputtering effects arise; radiation-enhanced diffusion and radiation-induced segregation occur. The latter two defect migration mechanisms are of particular significance since they can alter the composition to depths which are much greater than the implanted ion range. By altering various parameters such as irradiation temperature, ion mass, energy, and current density, and initial alloying distributions, a rich variety of near-surface composition profiles can be created. We have utilized changes in ion mass and energy, and irradiation temperature to distinguish defect production from defect migration effects. Experimental results are presented which provide a guide to the relative efficiencies of different mechanisms under various irradiation conditions. 46 references.
Date: September 1, 1984
Creator: Rehn, L.E.; Averback, R.S. & Okamoto, P.R.
Partner: UNT Libraries Government Documents Department

Ion beam mixing of marker layers in Al and Si. [300 keV Ar ions]

Description: Ion beam mixing experiments on thin Pt, Au, and Ni markers in Al and Si have performed at 17, 85, and 300 K. After irradiation with 300-keV Ar ions the broadening and relative shifts of the markers have been determined by RBS measurements. The marker broadenings are more pronounced in Si than in Al; in both matrices the broadenings decrease in the following order: Au, Pt, and Ni. No dependence of mixing on irradiation temperature was observed between 17 and 300 K. The shifts of the heavy Au and Pt markers relative to the Ni markers are approximately equal to the experimental accuracy. However, a shift of the Ni marker toward the surface relative to the heavier Au and Pt markers was consistently observed. 13 references, 2 figures.
Date: July 1, 1984
Creator: Mantl, S.; Rehn, L.E.; Averback, R.S. & Thompson, L.J. Jr.
Partner: UNT Libraries Government Documents Department

Transmission electron microscopy study of cascade collapse in copper during in-situ ion-irradiation at elevated temperatures.

Description: The basic mechanisms driving the collapse of point defects produced in collision cascades are investigated by transmission electron microscope (TEM) characterization of defect microstructure produced in fcc-Cu irradiated with low-fluences of heavy (100 keV Kr) ions at elevated temperature (23--600 C). Areal defect yields are determined from direct TEM observation of the total defect production integrated over the duration of the in-situ ion-irradiation. They are unequivocally demonstrated to decrease with increasing lattice temperature. This decrease in defect yield indicates a proportional decrease in the probability of collapse of cascade regions into defects of size where visible contrast is produced in a TEM.
Date: January 29, 1998
Creator: Daulton, T. L.; Kirk, M. A. & Rehn, L. E.
Partner: UNT Libraries Government Documents Department

Transmission electron microscopy study in-situ of radiation-induced defects in copper at elevated temperatures

Description: Neutrons and high-energy ions incident upon a solid can initiate a displacement collision cascade of lattice atoms resulting in localized regions within the solid containing a high concentration of interstitial and vacancy point defects. These point defects can collapse into various types of dislocation loops and stacking fault tetrahedra (SFT) large enough that their lattice strain fields are visible under diffraction-contrast imaging using a Transmission Electron Microscope (TEM). The basic mechanisms driving the collapse of point defects produced in collision cascades is investigated in situ with TEM for fcc-Cu irradiated with heavy (100 keV Kr) ions at elevated temperature. The isothermal stability of these clusters is also examined in situ. Areal defect yields were observed to decrease abruptly for temperatures greater than 300 C. This decrease in defect yield is attributed to a proportional decrease in the probability of collapse of point defects into clusters. The evolution of the defect density under isothermal conditions appears to be influenced by three different rate processes active in the decline of the total defect density. These rate constants can be attributed to differences in the stability of various types of defect clusters and to different loss mechanisms. Based upon observed stabilities, estimations for the average binding enthalpies of vacancies to SFT are calculated for copper.
Date: December 1, 1996
Creator: Daulton, T.L.; Kirk, M.A. & Rehn, L.E.
Partner: UNT Libraries Government Documents Department