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
Thursday, AM Room: Grand H
February 8, 1996 Location: Anaheim Marriott Hotel
Session Chairperson: T. A. Gabriel, Computational Physics and Engineering Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 378316364
8:30 am Invited
POINT DEFECT CLUSTER MOBILITY IN RADIATION DAMAGE MODELS: R. E. Stoller, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge, TN 37831-6376
Results of recent molecular dynamics simulations of displacement cascades indicate that the mobility of small point defect clusters may be relatively high. The impact of their mobility on the predictions of radiation damage models using the chemical rate theory will be discussed. Although this theory has been broadly applied in models simulating such radiation-induced phenomena as void swelling and embrittlement, the mobility of defects other than single vacancies and interstitials has generally been ignored. As a result, both the point defect concentrations and point defect cluster sink strengths can reach levels that are not physically reasonable at low temperatures. The results of the new rate theory simulations indicate that including the role of mobile clusters may extend the applicability of the kinetic models to lower temperatures. Research sponsored by 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 and by the Division of Materials Sciences, U.S. Department of Energy under contract DEAC05-840R21400 with Lockheed Martin Energy Systems.
MIGRATION OF INTERSTITIAL CLUSTERS IN [[alpha]]-IRON: B. D. Wirth, G. R. Odette, D. Maroudas, G. E. Lucas, Departments of Mechanical Engineering and Chemical Engineering, University of California, Santa Barbara, CA 93106
Energetic primary recoil atoms from fast neutron irradiations generate both isolated point defects and clusters of vacancies and interstitials. Cluster stability and mobility play key roles in the subsequent fate of the defects and, hence, in the overall microstructural evolution under irradiation. Single interstitials, di-interstitials, and probably larger interstitial clusters are highly mobile at low temperatures. Indeed, recent molecular dynamics simulations have demonstrated the mobility of larger interstitial clusters. Interstitial cluster mobility has profound significance to the kinetics of microstructural evolution: immobile clusters can serve as stable nuclei for interstitial-type loops while mobile clusters diffuse to and are annihilated at sinks. For example, a population of clusters that anneal during irradiation would mediate flux and temperature dependent recombination rates. In this study, the mobility of small interstitial clusters in -iron is evaluated by molecular-statics and molecular-dynamics simulations using interatomic potentials based on the embedded-atom method. Clusters of <lll>-type split interstitials migrate along  planes in amoebae-like fashion by sequential local dissociation and re-association processes coupled with collective cluster vibration modes. The cluster mobility can be described in terms of an effective diffusion coefficient. The interaction of mobile clusters with dislocations, other point-defect clusters, and free surfaces is also evaluated. These calculations provide the necessary position dependent reaction probabilities for incorporation into lattice Monte Carlo simulations of defect production following intermediate term cascade evolution and sink strengths. Supported by the U. S. Nuclear Regulatory Commission Contract: NRC-04-94-049.
INTERSTITIAL MIGRATION AND INTERACTIONS IN IRON: B. D. Wirth, G. R. Odette, D. Maroudas, G. E. Lucas, Departments of Mechanical Engineering and Chemical Engineering, University of California, Santa Barbara, CA 93106
Energetic primary recoil atoms from fast neutron irradiations generate both isolated point defects and clusters of vacancies and interstitials. The mobility of and interactions between point defects play a key role in the subsequent fate of the defects and, hence, in the overall microstructural evolution under irradiation. In this study, the mobility of single interstitials and the interactions between interstitials and point-defect clusters in iron is evaluated by molecular-statics and dynamics and Monte Carlo simulations using embedded-atom-type interatomic potentials. Limiting cases of defect interactions with free surfaces and dislocations are also examined. Dynamically, single split interstitials re-orient from the static <110> direction towards the <111> direction which has only a slightly higher energy. The interstitials can migrate by rotation and translation to equivalent sites, off-axis <110> sites, or fully rearrange into a crowdion configuration; the crowdions migrate along their axis in what is often observed to be a correlated sequence of jumps before returning to the <110> configuration. The mixed mode process can be described by an effective diffusion coefficient. The interactions between interstitials and single point defects as well as point-defect clusters are characterized by calculating binding energies and position-dependent reaction probabilities. These results are incorporated into lattice Monte Carlo simulations of residual defect production following intermediate-term cascade evolution and of defect-sink interactions. Supported by the U.S. Nuclear Regulatory Commission Contract: NRC-04-94-049.
A MOLECULAR DYNAMICS STUDY OF DEFECT-INDUCED AMORPHIZATION OF INTERMETALLIC COMPOUNDS: Ram Devanathan, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545; Nghi Q. Lam, Paul R. Okamoto, Materials Science Division, Argonne National Laboratory, Argonne, IL 60439
A molecular dynamics (MD) study of the thermodynamic and elastic properties of intermetallic compounds has led to a deeper understanding of the effects of simple lattice defects in destabilizing crystalline materials. In particular, MD simulations of defect-induced amorphization of intermetallic compounds show that the static component of the relative mean square atomic displacements (rmsd) provides a quantitative measure of the excess enthalpy stored in the lattice. This static component produces a softening of the average shear modulus nearly identical in magnitude to that produced by the dynamic component associated with heating. In fact, the MD results show that the average shear modulus is a unique function of the total (static+dynamic) rmsd. These results provide the first direct confirmation of a generalized version of the Lindemann melting criterion. In its simplest form, this criterion states that a metastable crystal will melt when the total rmsd exceeds a critical value. Thus, solid-state amorphization can be understood as defect-induced melting of a critically-disordered crystal.
10:00 am BREAK
10:20 am Invited
COLLISION CASCADES IN POLYATOMIC COMPOUNDS: K. E. Sickafus, Los Alamos National Laboratory, Los Alamos, NM 87545; W. J. Weber, R. E. Willford, Pacific Northwest Laboratory, PO Box 999, Richland, WA 99352; R. A. Sutton, Carnegie-Mellon University, Pittsburgh, PA 15213
Irradiation damage is dependent on: (1) the number of secondary knock-on atoms (ska) displaced from their atomic sites by a primary knock-on atom (pka); (2) the kinetic energy imparted to ska atoms in a cascade; and (3) the spatial extent of cascades. We have solved for item 1 in polyatomic materials by calculating solutions to simultaneous integrodifferential transport equations, by the method of Parkin and Coulter (J. Nucl. Mater. 101 (1981) 261). We have evaluated items 2 and 3 using the Monte Carlo code TRIM. Transport equation solutions allow us to evaluate the number of displaced atoms in a polyatomic compound, as a function of threshold displacement energies for individual atomic constituents, and mass disparity between constituent atoms. We use items 2 and 3 to evaluate the mean cascade energy density by the method of Walker and Thompson (Rad. Effects 37 (1978) 113). We have determined, however, that it is best to employ the geometric mean volume to evaluate the average cascade energy density from a compilation of numerous Monte Carlo ion simulations. We will present results for three similar ternary compounds, with and without mass disparities: spinel (MgAl204), chrysoberyl (BeAl204), and gahnite (ZnAl204).
NET DISPLACEMENT FUNCTIONS FOR SiC, Al2O3, and MgO: W. J. Weber, R. E. Williford, Pacific Northwest Laboratory, PO Box 999, Richland, WA 99352; K. E. Sickafus, Los Alamos National Laboratory, PO Box 1663, MS K-762, Los Alamos, NM 87545
Solutions to polyatomic integrodifferential equations for net displacements were determined numerically for SiC, Al2O3, and MgO using the Ziegler, Biersack,:and Littmark (ZBL) universal scattering potential. The effect of electronic stopping powers was investigated by comparing results calculated using electronic stopping powers derived from LSS and Bethe-Bloch models with results calculated using the electronic stopping powers provided by the TRIM-95 code. In both cases, the sum of the net displacement functions for each self-ion (pka) were compared to the results determined using the Monte Carlo code TRIM-95. The numerical results derived using the LSS/Bethe-Bloch electronic stopping powers were significantly higher than the net displacements calculated by TRTM-95. The numerical solutions calculated using the TRIM-95 electronic stopping powers were in closer agreement to the full cascade TRIM-95 calculations.
DEVELOPING Si POTENTIALS FOR MODELLING ION IMPLANTATION: David F. Richards, James B. Adams, Jing Zhu, Lin H. Yang, Christian Mailhiot, Department of Materials Science and Engineering, University of Illinois, 105 S. Goodwin Avenue, Urbana, IL 61801
We have used the Force Matching Method to generate a new interatomic potential
for silicon which is fit to both experimental properties and a quantum
mechanical database of force and energy data for many atomic configurations.
These configurations include surfaces, point defects, clusters, bulk solids
(crystalline and amorphous), and dimers at very close separations. We discuss
the reliability of this new potential and applications to modeling damage
cascades produced by ion implantation.
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