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Yield Functions and Plastic Potentials for BCC Metals and Possibly Other Materials

Description: Yield functions and plastic potentials are expressed in terms of the invariants of the stress tensor for polycrystalline metals and other isotropic materials. The plastic volume change data of Richmond is used to evaluate the embedded materials properties for some bcc metals and one polymer. A general form for the plastic potential is found that is intended to represent and cover a wide range of materials types.
Date: September 29, 2005
Creator: Christensen, R M
Partner: UNT Libraries Government Documents Department

The c/a Ratio in Quenched Fe-C and Fe-N steels - a Heuristic Story

Description: The body-centered tetragonal (BCT) structure in quenched Fe-C steels is usually illustrated to show a linear change in the c and a axes with an increase in carbon content from 0 to 1.4%C. The work of Campbell and Fink, however, shows that this continuous linear relationship is not correct. Rather, it was shown that the body-centered-cubic (BCC) structure is the stable structure from 0 to 0.6 wt%C with the c/a ratio equal to unity. An abrupt change in the c/a ratio to 1.02 occurs at 0.6 wt%C. The BCT structure forms, and the c/a ratio increases with further increase in carbon content. An identical observation is noted in quenched Fe-N steels. This discontinuity is explained by a change in the transformation process. It is proposed that a two-step transformation process occurs in the low carbon region, with the FCC first transforming to HCP and then from HCP to BCC. In the high carbon region, the FCC structure transforms to the BCT structure. The results are explained with the Engel-Brewer theory of valence and crystal structure of the elements. An understanding of the strength of quenched iron-carbon steels plays a key role in the proposed explanation of the c/a anomaly based on interstitial solutes and precipitates.
Date: January 31, 2006
Creator: Sherby, O; Wadsworth, J; Lesuer, D & Syn, C
Partner: UNT Libraries Government Documents Department

The Deformation-DIA: A Novel Apparatus for Measuring the Strength of Materials at High Strain to Pressures at Elevated Temperature

Description: The primary focus of this 3-year project was to develop and put to use an instrument to test experimentally the effect of pressure on body centered cubic (BCC) metals and other materials of interest to the Stockpile Stewardship program. Well-resolved materials testing requires measurements of load and deformation rate be measured at separable conditions of temperature, pressure, and plastic strain. The new apparatus at the heart of this work, the Deformation-DIA (D-DIA), began the project as a design concept. Its principal feature would be the capability to extend the conditions for such controlled materials testing from the current pressure limit of about 3 to almost 15 GPa, a factor of 5 increase. Once constructed and successfully tested, the plan of the project was to deform samples of BCC metals at arbitrary temperature and high pressures in order to provide preliminary measurements of strength and to prove its worth to the Stockpile Stewardship program. The project has been a stunning success. Progress toward demonstrating the worth of the D-DIA as a workhorse instrument for materials strength measurement at high pressure was given a huge boost by the fact that the machine itself functioned flawlessly from the very start, allowing the investigators to focus on measurement quality rather than technical operational issues. By the end of the project, we had deformed several samples of polycrystalline molybdenum (Mo) and tantalum (Ta) under very precisely controlled conditions, and for the Ta, had produced the first rudimentary measurements of strength to pressures of 8 GPa.
Date: March 10, 2004
Creator: Durham, W
Partner: UNT Libraries Government Documents Department

Theoretical confirmation of a high-pressure rhombohedral phase in vanadium metal

Description: Recent diamond-anvil-cell (DAC) experiments revealed a new phase in vanadium metal at high pressure. Here we present results from first-principles electronic-structure calculations confirming the existence of such phase. The new phase is due to a rhombohedral distortion of the body-centered-cubic (bcc) ambient-pressure phase. The calculated transition pressure of 0.84 Mbar and density compare favorably with the measured data. Interestingly, a re-entrant bcc phase is discovered at an ultra high pressure, close to the limit of DAC experimental capabilities, of about 2.8 Mbar. We show, extending prior work, that the phase transitions in vanadium are driven by subtle electronic-structure effects.
Date: February 27, 2007
Creator: Lee, B; Rudd, R E; Klepeis, J; Soderlind, P & Landa, A
Partner: UNT Libraries Government Documents Department


Description: In-situ x-ray diffraction was used to study the response of single crystal iron under shock conditions. Measurements of the response of [001] iron showed a uniaxial compression of the initially bcc lattice along the shock direction by up to 6% at 13 GPa. Above this pressure, the lattice responded with a further collapse of the lattice by 15-18% and a transformation to a hcp structure. The in-situ measurements are discussed and results summarized.
Date: August 23, 2005
Creator: Kalantar, D H; Collins, G W; Colvin, J D; Davies, H M; Eggert, J H; Hawreliak, J et al.
Partner: UNT Libraries Government Documents Department

Atomic site-specific theory of H on {gamma}U surface

Description: This effort is a fundamental study of local chemical, electronic and physical behavior of metal and metal oxides as exposed to corrosive gases such as hydrogen and water vapor. The study benefits from state-of-art surface science experimental tools, such as scanning tunneling and atomic force microscopies (STM/AFM), photoelectron emission microscopy (PEEM), temperature programmed desorption (TPD) and modulated molecular beam mass spectrometry (MMBMS), to provide detailed knowledge of how, why and where a chemical reaction initiates and subsequently progresses with time, and theoretical/computational methods at the atomistic scale. The theoretical/computational component reported here consists of the first-principles calculation of the electronic structure of U surfaces exposed to H attack The calculations were designed to shed light on the energetic of various configurations of H interstitial impurities on the surface or in the substrate of U bcc metal. Predictions as to the preference of H to occupy surface or substrate interstitial positions can be made on the basis of the configuration with the lowest ground-state total energy.
Date: January 16, 1998
Creator: Gonis, A.
Partner: UNT Libraries Government Documents Department

Accurate atomistic simulations of the Peierls barrier and kink-pair formation energy for {lt}111{gt} screw dislocations in bcc-Mo

Description: Using multi-ion MGPT interatomic potentials derived from first- principles generalized pseudopotential theory, we have performed accurate atomistic simulations on the energetic of dislocation motion in the bcc transition metal Mo. Our calculated results include the (110) and (211) generalized stacking fault ({gamma}) energy surfaces, the Peierls stress required to move an ideal straight <111> screw dislocation, and the kink-pair formation energy for nonstraight screw dislocations. Many-body angular forces, which are accounted for in the present theory through explicit three- and four-ion potentials, are quantitatively important to such properties for the bcc transition metals. This is demonstrated explicitly through the calculated {gamma} surfaces, which are found to be 10-50% higher in energy than those obtained with pure radial-force models. The Peierls stress for an applied <111>/{l_brace}112{r_brace} shear is computed to be about 0.025{mu}, where {mu} is the bulk shear modulus. For zero applied stress, stable kink pairs are predicted to form for kink lengths greater than 4b, where b is the magnitude of the Burgers vector. For long kinks greater than 15b, the calculated asymptotic value of the kink-pair formation energy is 2.0 eV.
Date: May 23, 1997
Creator: Xu, W. & Moriarty, J.A.
Partner: UNT Libraries Government Documents Department

Multiple mechanisms in the thermally activated plastic flow of tantalum

Description: We argue that many of the features of the plastic flow behavior of tantalum can be described by a model that incorporates a two-component Peierls-type mechanism and an fcc-like obstacle mechanism in series. We compare the results of calculations based on such a model with flow data for unalloyed tantalum over a wide range of strain rates and a modest range of temperatures.
Date: June 27, 1995
Creator: Gourdin, W.H. & Lassila, D.H.
Partner: UNT Libraries Government Documents Department

Dislocation-drag contribution to high-rate plastic deformation in shock-loaded tantalum

Description: Time-resolved plastic waves in plate-impact experiments give information on relation between applied shear stress and plastic strain rate at low plastic strain. This information is different from that obtained at intermediate strain rates using Hopkinson bar techniques, because the material deformation state is driven briefly into the regime dominated by dislocation drag. Two VISAR records of particle velocity at the tantalum/sapphire (window) interface are obtained for symmetric impact producing peak in-situ longitudinal stresses of 75 and 111 kbar. Rise-times of plastic waves are about l00 and 50 ns, respectively, with peak strain rates of about 2 {times} l0{sup 5} and 8.5 {times} l0{sup 5}/s, respectively, as determined by weak-shock analysis. These data show a much stronger dependence of plastic strain rate on applied shear stress than predicted by linear viscous drag models in combination with thermal activation through a large Peierls barrier. The data also show complex evolution of the mobile dislocation density during early stages of high-rate plastic flow. This measurement and corresponding analysis aid significantly in establishing the fundamental picture of dynamic deformation of metals and the evolution of the internal material state at early times following shock compression.
Date: August 1, 1993
Creator: Tonks, D. L.; Hixson, R. S.; Johnson, J. N. & Gray, G. T. III
Partner: UNT Libraries Government Documents Department

Dislocation nucleation in bcc Ta single crystals studied by nanoindentation

Description: The study of dislocation nucleation in closed-packed metals by nanoindentation has recently attracted much interest. Here, we address the peculiarities of the incipient plasticity in body centered cubic (bcc) metals using low index Ta single-crystals as a model system. The combination of nanoindentation with high-resolution atomic force microscopy provides us with experimental atomic-scale information on the process of dislocation nucleation and multiplication. Our results reveal a unique deformation behavior of bcc Ta at the onset of plasticity which is distinctly different from that of closed-packed metals. Most noticeable, we observe only one rather than a sequence of discontinuities in the load-displacement curves. This and other differences are discussed in context of the characteristic plastic deformation behavior of bcc metals.
Date: August 8, 2007
Creator: Biener, M M; Biener, J; Hodge, A M & Hamza, A V
Partner: UNT Libraries Government Documents Department

Ferrimagnetic ordering of single crystal Fe1-xGax thin films

Description: Molecular beam epitaxy was used to deposit body centered cubic single crystal Fe{sub 1-x}Ga{sub x} thin films on MgO(001) and ZnSe/GaAs(001) substrates well beyond the bulk stability concentration of about 28%. The crystal quality of the substrate surface and each deposited layer was monitored in situ by reflection high energy electron diffraction. The magnetization of the samples as a function of Ga is found to decrease more rapidly than a simple dilution effect, and element-specific x-ray magnetic circular dichroism ascribes this trend to a decrease in the Fe moment and an induced moment in the Ga that is antialigned to the Fe moment.
Date: October 19, 2009
Creator: McClure, A.; Arenholz, E. & Idzerda, Y. U.
Partner: UNT Libraries Government Documents Department

Point defects in metals

Description: A review is presented of the knowledge of point defects in pure metals. Data are tabulated for vacancies and divacancies in fcc metals, and for defects in bcc metals. Production of Frenkel defects by irradiation is also discussed; data are tabulated for both fcc and bcc metals. Questions that need answering are posed. (16 references). (DLC)
Date: January 1, 1973
Creator: Koehler, J.S.
Partner: UNT Libraries Government Documents Department

Shock loading of Ta: yield and hardening behavior of polycrystalline and oriented single crystals

Description: We are undertaking a series of shock compression experiments on polycrystalline and oriented single crystal Ta to investigate the fundamental mechanisms controlling dislocation behavior in Ta and other bcc metals at high strain rate. We compare experimental results to those calculated using an explicit 1-D computer code using the Steinberg-Guinan-Lund rate dependent model (Steinberg and Lund [1989]) to describe the strength properties of Ta in these calculation
Date: October 23, 1998
Creator: Fiske, P. S.; Holmes, N. C. & Lassila, D. H.
Partner: UNT Libraries Government Documents Department

Atomistic simulations for multiscale modeling in bcc metal

Description: Quantum-based atomistic simulations are being used to study fundamental deformation and defect properties relevant to the multiscale modeling of plasticity in bcc metals at both ambient and extreme conditions. Ab initio electronic-structure calculations on the elastic and ideal-strength properties of Ta and Mo help constrain and validate many-body interatomic potentials used to study grain boundaries and dislocations. The predicted C(capital Sigma)5 (310)[100] grain boundary structure for Mo has recently been confirmed in HREM measurements. The core structure, (small gamma) surfaces, Peierls stress, and kink-pair formation energies associated with the motion of a/2(111) screw dislocations in Ta and Mo have also been calculated. Dislocation mobility and dislocation junction formation and breaking are currently under investigation.
Date: September 25, 1998
Creator: Belak, J.; Moriarty, J.A.; Soderlind, P.; Xu, W.; Yang, L.H. & Zhu
Partner: UNT Libraries Government Documents Department

Calculation of vacancy wind contributions in ternary diffusion

Description: In the absence of kinetic cross interactions between diffusing components, intrinsic diffusion can be described by a simple atomic mobility model. For systems where the diffusional interactions among components cannot be ignored, the interactions can be related to a vacancy wind effect in which the intrinsic flux of a component is influenced by the net vacancy flux. Atomic mobilities are calculated at selected composition points on the diffusion paths of y-phase U-Pu- Zr diffusion couples investigated at 750 {degrees}C to assess the contribution by the vacancy wind effect to the intrinsic diffusion of the individual components. The results point to the possibility that a large vacancy wind contribution may cause a component to diffuse intrinsically up its own chemical potential gradient.
Date: August 1, 1996
Creator: Petri, M.C. & Dayananda, M.A.
Partner: UNT Libraries Government Documents Department

First principles calculations of interlayer exchange coupling in bcc Fe/Cu/Fe structures

Description: The authors report on theoretical calculations of interlayer exchange coupling between two Fe layers separated by a modified Cu spacer. These calculations were motivated by experimental investigations of similar structures by the SFU group. The multilayer structures of interest have the general form: Fe/Cu(k)/Fe and Fe/Cu(m)/X(1)/Cu(n)/Fe where X indicates one AL (atomic layer) of foreign atoms X (Cr, Ag, or Fe) and k, m, n represent the number of atomic layers of Cu. The purpose of the experimental and theoretical work was to determine the effect of modifying the pure Cu spacer by replacing the central Cu atomic layer with the atomic layer of foreign atoms X. The first principles calculation were performed using the Layer Korringa-Kohn-Rostoker (LKKR) method. The theoretical thickness dependence of the exchange coupling between two semi-infinite Fe layers was calculated for pure Cu spacer thicknesses in the range of 0 < k < 16. The effect of the foreign atoms X on the exchange coupling was investigated using the structure with 9 AL Cu spacer as a reference sample. The calculated changes in the exchange coupling are in qualitative agreement with experiment.
Date: January 1, 1998
Creator: Kowalewski, M.; Heninrich, B.; Schulthess, T.C. & Butler, W.H.
Partner: UNT Libraries Government Documents Department

Design of high density gamma-phase uranium alloys for LEU dispersion fuel applications.

Description: Uranium alloys are candidates for the fuel phase in aluminum matrix dispersion fuels requiring high uranium loading. Certain uranium alloys have been shown to have good irradiation performance at intermediate burnup. Previous studies have shown that acceptable fission gas swelling behavior and fuel-aluminum interaction is possible only if the fuel alloy can be maintained in the high temperature body-centered-cubic {gamma}-phase during fabrication and irradiation, i.e., at temperatures at which {alpha}-U is the equilibrium phase. Transition metals in Groups V through VIII are known to allow metastable retention of the gamma phase below the equilibrium isotherm. These metals have varying degrees of effectiveness in stabilizing the gamma phase. Certain alloys are metastable for very long times at the relatively low fuel temperatures seen in research reactor operation. In this paper, the existing data on the gamma stability of binary and ternary uranium alloys is analyzed. The mechanism and kinetics of decomposition of the gamma phase are assessed with the help of metal alloy theory. Alloys with the highest possible uranium content, good gamma-phase stability, and good neutronic performance are identified for further metallurgical studies and irradiation tests. Results from theory will be compared with experimentally generated data.
Date: October 19, 1998
Creator: Hofman, G. L.
Partner: UNT Libraries Government Documents Department

3D dislocation dynamics: stress-strain behavior and hardening mechanisms in FCC and BCC metals

Description: A dislocation dynamics (DD) model for plastic deformation, connecting the macroscopic mechanical properties to basic physical laws governing dislocation mobility and related interaction mechanisms, has been under development. In this model there is a set of critical reactions that determine the overall results of the simulations, such as the stress-strain curve. These reactions are, annihilation, formation of jogs, junctions, and dipoles, and cross-slip. In this paper we discuss these reactions and the manner in which they influence the simulated stress- strain behavior in fcc and bcc metals. In particular, we examine the formation (zipping) and strength of dipoles and junctions, and effect of jogs, using the dislocation dynamics model. We show that the strengths (unzipping) of these reactions for various configurations can be determined by direct evaluation of the elastic interactions. Next, we investigate the phenomenon of hardening in metals subjected to cascade damage dislocations. The microstructure investigated consists of small dislocation loops decorating the mobile dislocations. Preliminary results reveal that these loops act as hardening agents, trapping the dislocations and resulting in increased hardening.
Date: February 19, 1999
Creator: Hirth, J P; Rhee, M; Zhib, H M & de la Rubia, T D
Partner: UNT Libraries Government Documents Department

Metastable bcc phase formation in the Nb-Cr-Ti system

Description: Metastable disordered bcc phases have been formed from the melt in the Nb-Cr-Ti system where primary Laves phases would develop under equilibrium solidification conditions. Three vertical temperature-composition sections in the ternary system incorporating NbCr, were evaluated: the Nb-Cr binary, the TiCr{sub 2}-NbCr{sub 2} isoplethal section, and the NbCr{sub 2}-Ti plethal section. In the rapid solidification of NbCr{sub 2}, metastable bcc phase formation was not observed, but deviations from NbCr{sub 2} stoichiometry or alloying with Ti was found to promote bcc phase formation by decreasing the required liquid undercooling to reach the metastable bcc liquidus and solidus. The metastable phases were characterized through x-ray diffraction (XRD), and systematic deviations from Vegard`s Rule have been defined in the three plethal sections. The metastable bcc phases decompose at temperatures >800{degrees}C to uniformly refined microstructures. As a result, novel microstructural tailoring schemes are possible through the metastable precursor microstructures.
Date: August 1, 1994
Creator: Thoma, D. J. & Perepezko, J. H.
Partner: UNT Libraries Government Documents Department

Interfacial structure of lattice mismatched bcc(110)/bcc(110) transition metal superlattices

Description: We present structural characterizations of a series of sputtered Fe/Nb and V/Nb superlattices by high-angle x-ray diffraction. Diffraction scans were performed with the scattering vector at various angles ({Chi}) with respect to the layers. {Chi}=0{degrees} diffraction spectra (normal to the layers) were fitted to a general structural model to determine the (110) lattice strains, interfacial disorder and interdiffusion. {Chi}>0{degrees} spectra probe the lattice strain of the individual layers and the in-plane interfacial coherence. Both systems form incoherent interfaces above a critical modulation wavelength ({Lambda}{sub C}). At {Lambda}{sub C}, the Fe/Nb system undergoes a crystalline-to-amorphous transition while the V/Nb forms in-plane coherent interfaces.
Date: March 1, 1993
Creator: Fullerton, E. E.; Sowers, C. H.; Bader, S. D.; Mini, S.; Bommannavar, A. S. & Ehrlich, S. N.
Partner: UNT Libraries Government Documents Department

Shock compression and quasielastic release in tantalum

Description: Previous studies of quasielastic release in shock-loaded FCC metals have shown a strong influence of the defect state on the leading edge, or first observable arrival, of release wave, due to large density of pinned dislocation segments behind the shock front, their relatively large pinning separation, and a very short response time as determined by drag coefficient in shock-compressed state. This effect is entirely equivalent to problems associated with elastic moduli determination using ultrasonic methods. This is particularly true for FCC metals, which have an especially low Peierls stress, or inherent lattice resistance, that has little influence in pinning dislocation segments and inhibiting anelastic deformation. BCC metals, on the other hand, have a large Peierls stress that essentially holds dislocation segments in place at low net applied shear stresses and thus allows fully elastic deformation to occur in the complete absence of anelastic behavior. Shock-compression and release experiments have been performed on tantalum (BCC), with the observation that the leading release disturbance is indeed elastic. This conclusion is established by examination of experimental VISAR records taken at the tantalum/sapphire (window) interface in a symmetric-impact experiment which subjects the sample to a peak longitudinal stress of approximately 7.3 GPa, in comparison with characteristic code calculations.
Date: June 1, 1993
Creator: Johnson, J. N.; Hixson, R. S.; Tonks, D. L. & Gray, G. T. III
Partner: UNT Libraries Government Documents Department

The HCP To BCC Phase Transformation in Ti Characterized by Nanosecond Electron Microscopy

Description: The general class of martensitic phase transformations occurs by a rapid lattice-distortive mechanism, where kinetics and morphology of the transformation are dominated by the strain energy. Since transformation is diffusionless, phase fronts propagate through a crystal with great speed that can approach the speed of sound. We have observed a particular example of this class of phase transformation, the hexagonal close packed (HCP) to body centered cubic (BCC) transformation in titanium that is driven by a rapid increase in temperature. We have used a novel nanosecond electron microscope (the dynamic transmission electron microscope, DTEM) to acquire diffraction and imaging information on the transformation, which is driven in-situ by nanosecond laser irradiation. Using nanosecond exposure times that are possible in the DTEM, data can be collected about the transient events in these fast transformations. We have identified the phase transformation with diffraction patterns and correlated the time of the phase transformation with calculated conditions in the sample.
Date: June 21, 2005
Creator: Campbell, G; LaGrange, T; King, W; Colvin, J; Ziegler, A; Browning, N et al.
Partner: UNT Libraries Government Documents Department