Program Organizers: L. K. Mansur, Metals and Ceramics Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6376; C. L. Snead, Jr., Applied Technologies Division, Brookhaven National Laboratory, PO Box 5000, Upton, NY 11973-5000
Wednesday, AM Room: Grand H
February 7, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: C. Lewis Snead, Jr., Applied Technologies Division, Brookhaven National Laboratory, PO Box 5000, Upton, NY 11973-5000
8:30 am Invited
STRENGTH SUPERPOSITION IN STRUCTURAL ALLOYS: IMPLICATIONS TO THE IRRADIATION HARDENING AND ANNEALING OF PRESSURE VESSEL STEELS: G. Tedesky, G. R. Odette, G. E. Lucas, Departments of Mechanical Engineering and Materials, University of California, Santa Barbara, CA 93106
A model describing how various individual contributions ([[sigma]]i, i = 1,2,...) combine to produce the net yield stress ([[sigma]]n) is described. Typically, n contains both linear sum LS: [[sigma]]n = [[sigma]]1 + [[sigma]]2 + ...) and root sum of the squares (RSS: [[sigma]]n = [[radical]][[alpha]]12 + [[sigma]]22 +.... ) as well as intermediate contributions. For discrete dislocation pinning points (PP), both [[sigma]]i and their combined strength contribution depend on the PP number densities (N), diameters (d) and strength factors ([[beta]]). The [[beta]] directly govern the maximum force per PP and indirectly influence the effective PP spacing as mediated by N, d and the shapes of the stressed dislocation lines. Computer simulations of dislocation motion through random point arrays of PP with varying ratios (NW/Ns) of a range of weak ([[beta]]w) and strong ([[beta]]s) obstacles extends earlier work of Foreman and Makin, yielding an general expression S = ([[sigma]]nsim-[[sigma]]nrss) / ([[sigma]]nls - [[sigma]]nrss)[[radical]][[beta]]s- -(A - B[[beta]]s)([[beta]]w)2/3- C - DNw/Ns where A to D are fit constants (note, the best fit values depend on the application range of the parameters and weighting of the simulation data). Model predictions are in good agreement with increases of [[sigma]]n in reactor pressure vessel steels caused by small copper rich precipitates (CRP) that form under irradiation. The as-irradiated CRP are weak PP and combine with a pre-irradiation population of strong carbide PP with a S > 0. The model also predicts the large annealing (A) recovery observed in irradiated steels; as the CRP coarsen, their [[beta]] increases, hence, S decreases, approaching 0 at large sizes. Thus, decreases in [[sigma]]n under PIA are much larger than the corresponding recovery of the isolated CRP strength increment alone. Supported by the Office of Fusion Energy, U.S. Department of Energy Grant: E-FG03-94ER54275.
THE EFFECT OF SOLUTES ON DEFECT DISTRIBUTIONS IN ION IRRADIATED MODEL LWR PRESSURE VESSEL STEELS: P. M. Rice, R. E. Stoller, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6376
A series of nine model LWR pressure vessel steels were ion irradiated at ~300deg.C using 2.5 MeV He ions, to a dose of 1.4 x 10l7 ion/cm2, which corresponds to ~0.1 dpa at a depth of 2 u and ~3.5 dpa at the peak damage region which occurs at about 4 u deep. The resultant changes in hardness as a function of depth were measured using a nanoindenter. TEM is being used to measure defect distributions down to ~1 nm. The effects of the various solutes, Cu and N in particular, but C and Mn as well, on the number and size distribution of the defect clusters caused by the ion irradiation will be discussed. Research sponsored by the Division of Materials Science, U.S. Department of Energy and the Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission under inter-agency agreement DOE 1886-8109-8L with the U.S. Department of Energy under contract DEAC05-840R21400 with Lockheed Martin Energy Systems and by an appointment to the Oak Ridge National Laboratory Postdoctoral Research Program administered by the Oak Ridge Institute for Science and Education.
EVOLUTION OF FINE SCALE PRECIPITATES IN IRRADIATED PRESSURE VESSEL STEELS: G. R. Odette, B. L. Chao, B. Wirth, Departments of Mechanical Engineering and Materials, University of California, Santa Barbara, CA 93106
Formation of a high density (>>1023/m3) of fine scale (~1 nm) precipitates leads to irradiation hardening and embrittlement of light-water reactor pressure vessel (RPV) steels. Copper rapidly precipitates from supersaturated solutions at around 300deg.C as a consequence of radiation enhanced diffusion. The precipitates are typically copper rich (CRP) but contain manganese and nickel; however, at high alloy contents of these elements the precipitates are manganese and nickel rich (MNP). Detailed thermodynamic and kinetic precipitation model predictions are compared to small angle neutron scattering data. The quaternary thermodynamic model includes the effect of the interface, which is significant at small sizes. Kinetics models, are based on either: 1) a full treatment of overlapping regimes of nucleation, growth and coarsening kinetics by integration of the full set of clustering equations; or 2) a simpler treatment of coupled time-dependent nucleation and multigroup growth and coarsening. In both cases, copper clustering is assumed to be rate controlling. While involving a minimum number of adjustable parameters, model predictions are shown to be in remarkable agreement with experiment, particularly at higher copper contents. Preliminary results of atomistic lattice Monte Carlo simulations that mitigate imitations of the model, including the assumption of a discrete interfaces and neglect of the potential role of cascades assisted nucleation are discussed. Supported by the U.S. Nuclear Regulatory Commission Contract: NRC-04-94-049.
A MODEL FOR POST IRRADIATION ANNEALING PRESSURE VESSEL STEELS: E. Mader, G. R. Odette, Departments of Mechanical Engineering and Materials, University of California, Santa Barbara, CA 93106
Irradiation embrittlement of reactor vessels can be reduced by post irradiation annealing (PIA). Hardening and embrittlement results from a high density of fine nm scale irradiation induced features including: a) Cu (Mn- Ni) rich precipitates (CRP); b) small defect clusters that are unstable (UMD) and larger clusters that are stable (SMD) under irradiation; and c) a variety of other possible precipitates. A detailed thermodynamic-kinetic model for the evolution of these features is presented and compared to both small angle neutron scattering and microhardness recovery data. The CRP recovers by diffusion controlled precipitate dissolution (mostly Mn and Ni) and coarsening at rates are about 10 to 100 times faster than predicted using extrapolated diffusion coefficients. The UMD and SMD are modeled as a size distribution of vacancy clusters. They dissolve during PIA at rates controlled by self-diffusion and the Gibbs-Thompson effect. Recovery of hardness is based on a detailed treatment of the superposition of contributions from the irradiation features with pre-existing sources of the alloy strength. The model is in good agreement with the a variety of observations, including a recent semiempirical correlation developed for the data base PIA of transition temperature shifts. Detailed understating of the fate of the key alloy constituents and structure-property relations can be used to optimize PIA treatments. For example, treatments that result in significant recovery while leaving copper in solution would suffer greater re-embrittlement rates compared to those resulting in copper sequestered in the large precipitates. Supported by the U.S. Nuclear Regulatory Commission Contract: NRC-04-94-049.
10:00 am BREAK
10:20 am Invited
AMORPHIZATION OF INTERMETALLIC COMPOUNDS UNDER IRRADIATION-A REVIEW: Arthur T. Motta, Department of Nuclear Engineering, The Pennsylvania State University, University Park, PA 16802
A review is presented of the field of irradiation-induced amorphization of intermetallic compounds, with special attention to the kinetic aspects of the transformation. A full update will be given of recent experimental results using in-situ particle irradiation showing the effects of dose rate, temperature, crystal orientation, electron energy and the presence of stacking faults. Results from neutron, ion and electron irradiation will be reviewed, in the context of a kinetic description of the amorphization process, where the rate-limiting step is the accumulation of enough radiation damage in the lattice when opposed by thermal annealing. It will be shown how stability criteria, thermodynamic or otherwise, can be combined with kinetics of radiation damage and annealing to provide an overall description of the amorphization process, and how the experimentally measured quantities of the critical dose and the critical temperature fit in the model. Finally a comprehensive approach is proposed where a combination of computer simulation, in-situ irradiations and theoretical modeling can be used to glean a deeper understanding of the amorphization process under irradiation.
10:50 am Invited
AMORPHIZATION OF U3Si AND U3Si2 BY ION OR NEUTRON IRRADIATION: R. C. Birtcher, J. W. Richardson, Jr., M. H. Mueller, Materials Science Division, Argonne National Laboratory, Argonne, IL 60439
Electron diffraction (at ANL-HVEM National User Facility) during in situ 1.5 MeV Kr ion irradiation was used to determine the temperature dependence of amorphization doses for both U3Si and U3Si2. Neutron diffraction (at ANL-IPNS National User Facility) was used to follow crystallographic changes produced by neutron irradiation at 30deg.C. Both irradiations result in direct amorphization. The temperature limits for complete amorphization are 280deg.C for U3Si and 250deg.C for3 U2Si . Lattice strains from the localized amorphous zones develop in both materials at the same rate, however the total dilation upon amorphization is +2.3% in U3Si and -2.2% in U3Si2. At high doses, plastic flow in the amorphous volume fraction of U3Si relieves strain in the remaining crystalline volume fraction. The flow rate of U3Si2 is very small. Complete amorphization occurs between 0.88 and 1.13 x 1017 fissions/cm3 or between 0.29 to 0.38 dpa. This work supported by the U.S. Department of Energy, BES-Materials Sciences, under Contract W-31-109-Eng-38.
CRYSTAL AND MICROSTRUCTURE OF U3Si2 FUEL PLATES AFTER REACTOR IRRADIATION: G. L. Hofman, J. W. Richardson, Jr., R. C. Birtcher, Argonne National Laboratory, Argonne, IL 60439
Neutron diffraction (at ANL-IPNS National User Facility) and SEM have been
used to determine irradiation induced changes in reactor fuel plates containing
U3Si2. The fuel plates were fabricated by hot rolling low enriched U3Si2 (45
volume %) and Al powders. Plates were irradiated at 150deg.C in the ORR reactor
to 7% and 16% burnup of the uranium. Amorphization of U3Si2 by 4 x 10-6
burnup results in a 3.3% volume contraction. SEM examination shows the
development of a several um thick shell around each fuel particle due to Al
diffusion into the particles. The shell thickness varies as the square root of
the irradiation dose. Neutron diffraction shows that after the lower dose the
fuel is amorphous but that after the higher dose it also contains a crystalline
aluminide phase. The stability of U3Si2 fuel plates is believed to be
associated with the volume contraction upon amorphization and the subsequent
formation of the aluminide shell. This work supported by the U.S. Department of
Energy, BES-Materials Sciences, under Contract W-31-109-Eng-38.
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