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Atomistic simulations of the plasticity behavior of polycrystalline metals.

Description: Recent advances in computers and atomistic modeling have made the realistic simulation of materials behavior possible. Two decades ago, modeling of materials at the atomic level used simple pair potentials. These potentials did not provide an accurate description of the elastic properties of materials or of the formation of free surfaces, a phenomenon critical in the fracture process. This paper will review the evolution of the Embedded Atom Method (EAM), a modern theory of metallic cohesion that was developed to overcome the limitations of pair potentials. The EAM includes many body effects that are necessary to describe such processes as bond weakening (or strengthening) by impurities. We examine the effects of deformation on polycrystalline FCC metals. We perform simple shear molecular dynamics simulations using the EAM on nickel samples of -10000 atoms to study yield and work hardening. It is found that the deformation is always inhomogeneous when a grain boundary or free surface is present. The atomistic simulations reveal that dislocations nucleating at grain boundaries or free surfaces are critical to causing yielding in pristine material as observed in experiment. Detailed investigation shows that the grain boundaries are significantly weaker than the bulk material and yield at a lower stress. Even so, the yield stress of the polycrystalline samples with either low angle and high angle grain boundaries are found to be similar and only slightly lower than the yield stress of single crystals with the same characteristic dimensions. The local yield stress at the boundaries if found to be significantly less than the average yield stress.
Date: January 1, 2002
Creator: Baskes, M. I. (Michael I.)
Partner: UNT Libraries Government Documents Department

Increasing the fracture toughness of silicon by ion implantation

Description: This study was motivated by some earlier indications that the fracture toughness of silicon could be increased by ion implantation. The location of fracture in hydrogen implanted silicon was found to change depending on the dose of implanted ions. For relatively low doses, fracture occurred at the center of the damage region created by implantation. However, for larger doses, the fracture location switched to the deeper edge of the implanted zone. This implied that the center of the implanted region experienced toughening due to ion implantation at the higher dose level. In addition, an initial increase in fracture toughness with radiation dose has been observed experimentally in some ceramics. After the initial increase, the fracture toughness reaches a peak and then decreases with further irradiation. The toughness increases found thus far are modest (25-100%). In attempts to explain the experimental results, several toughening mechanisms (such as deflection of the crack by the irradiated damage) have been proposed. However, the proposed mechanisms predict only a 40-80% increase in fracture toughness, which does not account for the highest levels of toughness observed. Our recent molecular dynamics (MD) calculations have found a previously unknown toughening mechanism acting in silicon, which can also explain the earlier experimental observations of toughening induced by irradiation. In our MD simulations, ion implantation produced clusters of disordered atoms. The presence of these clusters allowed silicon to deform plastically as a crack approached, blunting the crack tip and arresting crack growth. The MD calculations show a factor of 3 increase in fracture toughness. We have conducted experiments with silicon implanted with a small dose (ions/cm2) of alpha particles uniformly distributed to a depth of 25 pm. A 20% increase in fracture toughness is observed. Additional experiments with higher implantation doses are planned for the immediate future.
Date: January 1, 2002
Creator: Swadener, J. G. (John G.); Baskes, M. I. (Michael I.) & Nastasi, Michael Anthony,
Partner: UNT Libraries Government Documents Department

Atomic interactions between plutonium and helium.

Description: An essential issue in gallium (Ga)-stabilized fcc-phase plutonium ({delta}-Pu) is the formation of helium (He) voids and bubbles emanating from the radiolytic decay of the Pu. The rate of formation of He voids and bubbles is related to the He-defect formation energies and their associated migration barriers. The size and shape distributions of the bubbles are coupled to these critical migration processes. The values of the defect formation energies, internal pressure, and migration barriers can be estimated from atomistic calculations. Complicating this picture is the destruction of He-filled voids and bubbles by subsequent radiolytic decay events. The present study concerns the construction of the necessary potential energy surfaces for the Pu-He and He-He interactions within the modified embedded atom method (MEAM). Once fully tested, the potentials will be used to estimate the He-defect formation energies and barriers to the migration of these defects for both interstitial and substitutional He on an fcc Pu lattice. The He-He interactions are modeled from ab initio electronic structure calculations for the He{sub 2} dimer and the equilateral He, trimer. The experimental data and the electronic structure calculations on He{sub 2} agree very well. These data were fit to a Rose function fn{sub R}(x) = A P({alpha}x) exp(-{alpha}x), where P is a polynomial, x = R/R{sub 0}-1, R is the bond length, and R{sub 0} is its equilibrium value. The fits are very satisfactory. Both linear (P = 1+{alpha}x, zeroth-order Rose) and rational (P = 1+{alpha}x+a{sub 3} ({alpha}x){sup 3}/(1+x) first-order Rose) polynomials in the Rose function were tried. The more flexible rational form does improve the fit, but only marginally. Only the linear form was used thereafter. The resulting MEAM potential was used to predict the behavior of the linear trimer and the fcc cold compression curve. The results are shown in Fig. 2 and ...
Date: January 1, 2002
Creator: Valone, S. M. (Steven M.); Baskes, M. I. (Michael I.) & Martin, R. L. (Richard L.)
Partner: UNT Libraries Government Documents Department

An atomistic study of dynamic brittle fracture in silicon

Description: Dynamic fracture has been modeled using a modified embedded atom method (MEAM) potential for silicon. For Mode I dynamic fracture along (1 1 1) crystallographic planes, the molecular dynamics model predicts crack speeds and fracture energies in agreement with previous experimental results [l]. In this orientation, hcture occurs almost exclusively along (1 1 1) planes for energy release rates up to 30 J/m2. For Mode I fracture oriented initially on (1 10) planes, fracture occurs by cleavage on (1 10) planes for a static energy release rate (J,) less than 8 J/m2. For greater values of J,, the fracture surfaces switch to alternating (111) planes, which is in agreement with previous experimental results [2]. Crack speed predictions for the (1 10) orientation are somewhat In the atomistic simulations, the dynamically propagating cracks generate dislocations, which are primarily produced on the (1 1 1) and (1 10) planes. Differences in the type and quantity of dislocations produced have been observed for different orientations. Molecular dynamics has the ability to calculate the energy consumed by dislocations and other lattice defects produced during fracture and the total surface energy of the main crack, side branches and secondary cracks. The sum of the surface energy and the energy consumed by lattice defects determines the dynamic fracture less than the high speeds observed experimentally. toughness, J(v). The dynamic fkacture toughness has been found to vary linearly with J,. For the (111) orientation with cracks propagating in the [211] direction, J(v) asymptotically approached a value of 1/3 of J,. The remainder of the strain energy that is released during fracture is converted into kinetic energy at the crack tip during the fracture process, which occurs atom by atom.
Date: January 1, 2002
Creator: Swadener, J. G. (John G.); Baskes, M. I. (Michael I.) & Nastasi, Michael Anthony,
Partner: UNT Libraries Government Documents Department

The effect of stoichiometry in C15 HfCo[sub 2]

Description: In binary Laves phases (AB{sub 2}) that exhibit a homogeneity range, the ability to accommodate nonstoichiometry is related to the atomic size requirements for the topologically close-packed structure. Based upon a hard sphere model, Laves phases with a metallic atom size ratio R{sub A}/R{sub B} close to the ideal value of {approx}1.225 exhibit homogeneity ranges. The C15 phase, HfCo{sub 2}, has a radius ratio of R{sub H}/R{sub Co} = 1.26 and a reported range of solubility of 9 at.%. The solubility limits and constitutional defects have been investigated, and with this information, the elastic and mechanical properties as a function composition were experimentally investigated. The results indicate that anti-site substitution is the governing defect mechanism on both sides of stoichiometry, The elastic and mechanical properties show a maximum at stoichiometry (unlike most intermetallics), with the brittle behavior decreasing with increasing Co content. The properties are described in terms of geometric models of space filling.
Date: January 1, 2001
Creator: Thoma, D. J. (Dan J.); Chen, K. C. (Katherine C.); Baskes, M. I. (Michael I.) & Peterson, E. J. (Eric J.)
Partner: UNT Libraries Government Documents Department

Multi-component gas transport in CANDU fuel rods during severe accidents.

Description: The multi-component transport of steam, hydrogen and stable fission gas in the fuel-to-clad gap of defective CANDU fuel rods, during severe accident conditions, is investigated. Based on a general Stefan-Maxwell treatment this work considers how incoming steam will diffuse into a breached rod against a counter-current flow of non-condensable fission gases and out-flowing hydrogen that is produced from the internal reaction of steam with the Zircaloy cladding or urania. The ability of the oxidized clad to act as a physical barrier to either hydrogen or oxygen diffusion was further investigated in the current work with a molecular-dynamics approach, with the interactions between atoms represented by a Modified Embedded Atom Method. During the initial Zircaloy oxidation phase in the CRL experiments, the model was able to predict the reduced fission product release kinetics as well as the timing for the completion of the clad-oxidation process. In this simulation, the model (with an effective gap size of 20 {micro}m) was able to successfully predict whether singlesided or double-sided oxidation had occurred in accordance with the metallographic examination. However, in order to account for the observed release kinetics after the completion of clad oxidation, it was necessary to assume a greater atmospheric exchange due to possible cracking of the brittle oxide layer. With the assumption of cracking (by assuming a reduced path length for gas transport), the model was successfully able to reproduce the fission product release kinetics and the final fuel stoichiometry as determined from end-of-test weight gain measurements. This analysis particularly shows that local hydrogen production (from the internal fuel oxidation process) will result in a reduced local oxygen potential in the fuel-to-clad gap compared to that which occurs in the bulk coolant.
Date: January 1, 2001
Creator: Szpunar, B; Lewis, B. J.; Arimescu, V. I.; Dickson, R. S.; Dickson, L. W. & Baskes, M. I. (Michael I.)
Partner: UNT Libraries Government Documents Department

Multiscale simulations of alloy phase stability

Description: First principles, atomic scale and continuum level models are combined to predict thermodynamic properties of alloys and stability of phases. Many-body interactions, as well as vacancies, defects, and non-stoichiometry are included in the modeling process and the structural stability of hypothetical phases is evaluated. The resulted thermodynamic functions and phase diagrams are integrated in a casting simulation computer program. The process of relating microscopic modeling results to the macroscopic heat transfer and phase equilibrium calculations is detailed to emphasize the self-consistency of the approach and to identify the potential sources of errors. The sequence: data acquisition, modeling, prediction experimental validation, is illustrated for several recent results in actinide based alloys.
Date: January 1, 2002
Creator: Stan, M. (Marius); Baskes, M. I. (Michael I.); Valone, S. M. (Steven M.); Chen, S. P. (Shao-Ping) & Kothe, D. B. (Douglas B.)
Partner: UNT Libraries Government Documents Department

Atomistic models of point defects in plutonium metal.

Description: The aging properties of plutonium (Pu) metal and alloys are. driven by a combination of materials composit ion, p rocessing history, and self-irradiat ion effects . Understanding these driving forces requires a knowledge of both t h ermodynamic and defect properties of the material . The multiplicity of phases and the small changes in tempe rat u re, pressure, and/or stress that can induce phase changes lie at the heart of these properties . In terms of radiation damage, Pu metal represents a unique situation because of the large volume chan ges that accompany the phase changes . The most workable form of the meta l is the fcc (S-) phase, which in practice is stabi l ized by addit io n of a ll oying el eme n ts s u c h as Ga or Al. The thermodynamically stable phase at ambient conditions is the monoclinic (a-) phase, which, however, is 2 0 % lower i n volume th an the S phase . In stabilized Pu metal, there is an in t er play between th e n atu ral swe l li n g tendencies of fcc metals and the volume-contraction tendency of the u n d erlyin g thermodynamicall y stable phase. This study exp lores the point d efect pr operties that are necessary to model the long-term outcome of this interplay.
Date: January 1, 2003
Creator: Valone, S. M. (Steven M.); Baskes, M. I. (Michael I.); Uberuaga, B. P. (Blas Pedro) & Voter, A. F.
Partner: UNT Libraries Government Documents Department

Calculations of the structure and properties of rapidly quenched NI/ZR alloys.

Description: Using molecular dynamics and a modified embedded atom potential developed by our group we studied the diffusivity and viscosity of molten Nil-XZrX alloys as a function of composition, temperature, and cooling rate. Previous results indicate that these potentials represent the Ni-Zr system quite well . Liquid alloys were quenched at rates of 5 x 10{sup 11} and 10{sup 12} K/s. For x < 0.04 the solidified alloys were crystalline . For higher x values, the solidified alloys were amorphous . For the amorphous alloys, the composition dependence of the calculated glass transition temperature Tg follows the general trend of experimental Tg values . The calculated viscosity and diffusivity show systematic variation with composition . For the undercooled Ni-6 at .% Zr melt the calculated viscosity shows the Vogel-Fulcher-Tamman (VFT) behavior characteristic of a 'fragile' glass .
Date: January 1, 2003
Creator: Cherne, F. J. (Frank J.); Baskes, M. I. (Michael I.); Schwarz, R. B. (Ricardo B.) & Srivilliputhur, S. G. (Srinivasan G.)
Partner: UNT Libraries Government Documents Department

Point-detect production and migration in plutonium metal at ambient conditions

Description: Modeling thermodynamics and defect production in plutonium (Pu) metal and its alloys, has proven to be singularly difficult. The multiplicity of phases and the small changes in temperature, pressure, and/or stress that can induce phase changes lie at the heart of this difficulty, In terms of radiation damage, Pu metal represents a unique situation because of the large volume changes that accompany the phase changes. The most workable form of the metal is the fcc (6.) phase, which in practice the 6 phase is stabilized by addition of alloying elements such as Ga or AI. The thermodynamically stable phase at ambient conditions is the between monoclinic (a-) phase, which, however, is approximately 20 % lower in volume than the 6 phase. In stabilized Pu metal, there is an interplay between the natural swelling tendencies of fcc metals and the volume-contraction tendency of the underlying phase transformation to the thermodynamically stable phase. This study explores the point defect production and migration properties that are necessary to eventually model the long-term outcome of this interplay.
Date: January 1, 2001
Creator: Baskes, M. I. (Michael I.); Stan, M. (Marius); Sickafus, K. (Kurt E.) & Valone, S. M. (Steven M.)
Partner: UNT Libraries Government Documents Department