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Influence of shockwave obliquity on deformation twin formation in Ta

Description: Energetic loading subjects a material to a 'Taylor wave' (triangular wave) loading profile that experiences an evolving balance of hydrostatic (spherical) and deviatoric stresses. While much has been learned over the past five decades concerning the propensity of deformation twinning in samples shockloaded using 'square-topped' profiles as a function of peak stress, achieved most commonly via flyer plate loading, less is known concerning twinning propensity during non-I-dimensional sweeping detonation wave loading. Systematic small-scale energetically-driven shock loading experiments were conducted on Ta samples shock loaded with PEFN that was edge detonated. Deformation twinning was quantified in post-mortem samples as a function of detonation geometry and radial position. In the edge detonated loading geometry examined in this paper, the average volume fraction of deformation twins was observed to drastically increase with increasing shock obliquity. The results of this study are discussed in light of the formation mechanisms of deformation twins, previous literature studies of twinning in shocked materials, and modeling of the effects of shock obliquity on the evolution of the stress tensor during shock loading.
Date: January 1, 2009
Creator: Gray Iii, George T; Livescu, V; Cerreta, E K; Mason, T A; Maudlin, P J & Bingert, J F
Partner: UNT Libraries Government Documents Department

Effect of Microstructure on the Deformation Mechanism of Friction Stir-Processed A₁₀.₁CoCrFeNi High Entropy Alloy

Description: This article reports grain refinement using friction stir processing and a detailed study on the microstructural evolution at various locations in the processed region using scanning electron microscopy, in addition to the effect of grain refinement on the tensile properties and the underlying deformation mechanisms in both coarse-grained and fine-grained materials.
Date: August 7, 2014
Creator: Komarasamy, Mageshwari; Kumar, N.; Tang, Z.; Mishra, Rajiv & Liaw, P.K.
Partner: UNT College of Engineering

Role of twinning in the optimization of the grain boundary character distribution

Description: The grain boundary character distribution (GBCD) is a microstructural property that describes the proportions of �special� and �random� boundaries as defined by the coincident site lattice model. Recently, there has been increased attention on determination of the GBCD and manipulation of the relative fractions in the microstructure through thermomechanical processing in order to improve material� s properties like corrosion and creep resistance. Most of the �optimization� treatments reported in the literature have been performed on fee materials with relatively low stacking fault energies and have resulted in microstructures with high fractions of {Sigma}3, {Sigma}9, and {Sigma}27 boundaries. It could be interpreted that annealing twins are solely required to improve the GBCD. However, in order to optimize the properties, it appears imperative that the formation of annealing twins disrupt the connectivity of the random boundary network, thus implying that {Sigma}3{sup n} reactions and resultant triple lines are critical. Experiments to modify the GBCD of oxygen-free electronic Cu and Inconel 600 through thermomechanical processing are presented and discussed in light of observations of the deformed and recrystallized microstructures.
Date: January 8, 1999
Creator: King, W E; Kumar, M & Schwartz, A J
Partner: UNT Libraries Government Documents Department

Twinning in Laves Phases by Synchroshear: Atomic Mechanisms and Compositional Contro

Description: Compression tests between 4.2 and 1273 K, compositional variation and conventional transmission electron microscopy were used to investigate the deformation and twinning behavior of C15 HfV{sub 2}+Nb - based Laves phase alloys. We chose two phase C15/bcc alloys to improve the ambient temperature ductility which has not been found in the single phase Laves intermetallic compound. Transmission electron microscopy revealed that substantial mechanical twinning takes place in the C15 Laves phase matrix at room temperature and 77 K. A deep minimum in the flow stress, with a drop of nearly 500 MPa, appears at around 77 K in the C15/bcc two-phase alloy with C15 matrix. Since no such anomaly is seen in the bcc phase, we believe that the cause of this can be attributed to mechanical twinning in the C15 Laves phase. Twin bands observed in the C15 matrix of deformed samples at both 298 K and 77 K can be classified into three categories by their thicknesses; coarse twin bands about 10 nm to several hundreds nm thick, fine twin bands around 3 to 10 nm thick, and ultra fine twin bands with average thickness of 1.5 nm. A high density of ultra-fine twin bands is the characteristic feature of twinning in the C15 matrix. They belong to the <112>(111) twinning system and commonly intersect with each other.
Date: April 7, 2003
Creator: Luzzi, David E. & Pope, David P.
Partner: UNT Libraries Government Documents Department

Characterization of X-ray data sets

Description: With the emergence of structural genomics, more effort is being invested into developing methods that incorporate basic crystallographic knowledge to enhance decision making procedures (e.g. Panjikar, 2005). A key area where some crystallographic knowledge is often vital for the smooth progress of structure solution is that of judging the quality or characteristics of an X-ray dataset. For instance, detecting the presence of anisotropic diffraction or twinning while a crystal is on the beam line, may allow the user to change the data collection strategy in order to obtain a better or a more complete data set. In post-collection analyses, the presence of (for instance) non-crystallographic translational symmetry might help the user (or program!) to solve the structure more easily. Of course, the identification of problems is by no means a guarantee that the problems can be overcome, but knowledge of the idiosyncrasies of a given X-ray data set permits the user or software pipeline to tailor the structure solution and refinement procedures to increase the chances of success. In this report, a number of routines are presented that assist the user in detecting specific problems or features within a given dataset. The routines are made available via the open source CCTBX libraries (http://cctbx.sourceforge.net) and will also be included in the next available PHENIX (Adams, et al., 2004) release.
Date: July 21, 2005
Creator: Zwart, Peter H.; Grosse-Kunsteleve, Ralf W. & Adams, Paul D.
Partner: UNT Libraries Government Documents Department

DYNAMIC RESPONSE OF COPPER SUBJECTED TO QUASI-ISENTROPIC, GAS-GUN DRIVEN LOADING

Description: A transmission electron microscopy study of quasi-isentropic high-pressure loading (peak pressures between 18 GPa and 52 GPa) of polycrystalline and monocrystalline copper was carried out. Deformation mechanisms and defect substructures at different pressures were analyzed. Current evidence suggests a deformation substructure consisting of twinning at the higher pressures and heavily dislocated laths and dislocation cells at the intermediate and lower pressures, respectively. Evidence of stacking faults at the intermediate pressures was also found. Dislocation cell sizes decreased with increasing pressure and increased with distance away from the surface of impact.
Date: September 29, 2005
Creator: Jarmakani, H; McNaney, J M; Schneider, M S; Orlikowski, D; Nguyen, J H; Kad, B et al.
Partner: UNT Libraries Government Documents Department

Exploring Metric Symmetry

Description: Relatively minor perturbations to a crystal structure can in some cases result in apparently large changes in symmetry. Changes in space group or even lattice can be induced by heavy metal or halide soaking (Dauter et al, 2001), flash freezing (Skrzypczak-Jankun et al, 1996), and Se-Met substitution (Poulsen et al, 2001). Relations between various space groups and lattices can provide insight in the underlying structural causes for the symmetry or lattice transformations. Furthermore, these relations can be useful in understanding twinning and how to efficiently solve two different but related crystal structures. Although (pseudo) symmetric properties of a certain combination of unit cell parameters and a space group are immediately obvious (such as a pseudo four-fold axis if a is approximately equal to b in an orthorhombic space group), other relations (e.g. Lehtio, et al, 2005) that are less obvious might be crucial to the understanding and detection of certain idiosyncrasies of experimental data. We have developed a set of tools that allows straightforward exploration of possible metric symmetry relations given unit cell parameters and a space group. The new iotbx.explore{_}metric{_}symmetry command produces an overview of the various relations between several possible point groups for a given lattice. Methods for finding relations between a pair of unit cells are also available. The tools described in this newsletter are part of the CCTBX libraries, which are included in the latest (versions July 2006 and up) PHENIX and CCI Apps distributions.
Date: July 31, 2006
Creator: Zwart, P. H.; Grosse-Kunstleve, R. W. & Adams, P. D.
Partner: UNT Libraries Government Documents Department

On the Micromechanisms of Shock-Induced Martensitic Transformation in Tantalum

Description: Shock-induced twinning and martensitic transformation in tantalum, which exhibits no solid-state phase transformation under hydrostatic pressures up to 100 GPa, have been further investigated. Since the volume fraction and size of twin and phase domains are small in scale, they are considered foming by heterogeneous nucleation that is catalyzed by high density lattice dislocations. A dynamic dislocation mechanism is accordingly proposed based upon the observation of dense dislocation clustering within shock-recovered tantalum. The dense dislocation clustering can cause a significant increase of strain energy in local regions of {beta} (bcc) matrix, which renders mechanical instability and initiates the nucleation of twin and phase domains through the spontaneous reactions of dislocation dissociation within the dislocation clusters. That is, twin domains can be nucleated within the clusters through the homogeneous dissociation of 1/2<111> dislocations into 1/6<111> partial dislocations, and {omega} phase domains can be nucleated within the closters through the inhomogeneous dissociation of 1/2<111> dislocations into 1/12<111>, 1/3<111> and 1/12<111> partial dislocations.
Date: December 7, 2005
Creator: Hsiung, L L
Partner: UNT Libraries Government Documents Department

Determining the DUF55-domain structure of human thymocyte nuclear protein 1 from crystals partially twinned by tetartohedry

Description: Human thymocyte nuclear protein 1 (hTHYN1) contains a unique DUF55 domain of 167 residues (55-221), but its cellular function is unclear. Crystals of DUF55 belong to the trigonal space group P3{sub 1}, but twinning causes the data to approach an apparent 622 symmetry. Two datasets to 2.3 {angstrom} resolution were collected. Statistical analysis confirmed that both datasets were partially twinned by tetartohedry. Tetartohedral twin fractions were estimated. After the structure was determined, only one twofold axis of rotational pseudosymmetry was found in the crystal structure. Using the DALI program, a YTH domain, which is a potential RNA binding domain from human YTH domain-containing protein 2, was identified to have the most similar three-dimensional fold to DUF55. It is implied that DUF55 might be a potential RNA-related domain.
Date: June 6, 2009
Creator: Yu, Feng; Song, Aixin; Xu, Chunyan; Sun, Lihua; Li, Jian; Tang, Lin et al.
Partner: UNT Libraries Government Documents Department

ULTRA-HIGH STRENGTH IN NANOCRYSTALLINE MATERIALS UNDER SHOCK LOADING

Description: Molecular dynamics simulations of nanocrystalline (nc) copper under shock loading show an unexpected ultra-high strength behind the shock front. The strength at high pressure can be up to twice the value at low pressure, for all grain sizes studied here (5-50 nm grains, with up to {approx}4 10{sup 8} atoms). Partial and perfect dislocations, twinning, and debris from dislocation interactions are found behind the shock front. Results are interpreted in terms of the pressure dependence of both deformation mechanisms active at these grain sizes, namely dislocation plasticity and grain boundary sliding. These simulations, together with new shock experiments on nc nickel, raise the possibility of achieving ultra-hard materials during and after shock loading.
Date: April 11, 2005
Creator: Bringa, E M; Caro, A; Wang, Y M; Victoria, M; McNaney, J; Remington, B A et al.
Partner: UNT Libraries Government Documents Department

A Quasicontinuum Study of Nanovoid Collapse under Uniaxial Loading in Ta

Description: The mechanisms underlying the deformation of nanovoids in Ta single crystals are analyzed when they are subjected to cyclic uniaxial deformation using numerical simulations. Boundary and cell-size effects have been mitigated by means of the Quasicontinuum (QC) method. We have considered {approx} 1 billion-atom systems containing 10.9 nm voids. Two kinds of simulations have been performed, each characterized by a different boundary condition. First, we compress the material along the nominal [0 0 1] direction, resulting in a highly symmetric configuration that results in high stresses. Second, we load the material along the high-index [{bar 4}819] direction to confine plasticity to a single slip system and break the symmetry. We find that the plastic response under these two conditions is strikingly different, the former governed by dislocation loop emission and dipole formation, while the latter is dominated by twinning. We calculate the irreversible plastic work budget derived from a loading-unloading cycle and identify the most relevant yield points. These calculations represent the first fully three-dimensional, fully non-local simulations of any body-centered cubic metal using QC.
Date: December 2, 2007
Creator: Marian, J; Knap, J & Campbell, G
Partner: UNT Libraries Government Documents Department

Advanced characterization of twins using automated electron backscatter diffraction

Description: This paper describes results obtained using an automated, crystallographically-based technique for twin identification. The technique is based on the automated collection of spatially specific orientation measurements by electron backscatter diffraction (EBSD) in the scanning electron microscope (SEM). The key features of the analysis are identification of potential twin boundaries by their misorientation character, identification of the distinct boundary planes among the symmetrically equivalent candidates, and validation of these boundaries through comparison with the boundary and twin plane traces in the sample cross section. Results on the application of this technique to deformation twins in zirconium are analyzed for the effect of twin type and amount and sense of uniaxial deformation. The accumulation of strain tends to increase the misorientation deviation at least to the degree of the trace deviation compared with recrystallization twins in nickel. In addition to the results on characterizing the twin character, results on extending the twin analysis to automated identification of parent and daughter material for structures exhibiting twin deformation are reported as well.
Date: January 1, 2002
Creator: Wright, S. I. (Stuart I.); Bingert, J. F. (John F.); Mason, T. A. (Thomas A.) & Larson, R. J. (Ryan J.)
Partner: UNT Libraries Government Documents Department

Mechanistic Selection and Growth of Twinned Bicrystalline Primary Si in Near Eutectic Al-Si Alloys

Description: Morphological evolution and selection of angular primary silicon is investigated in near-eutectic Al-Si alloys. Angular silicon arrays are grown directionally in a Bridgman furnace at velocities in the regime of 10{sup -3} m/sec and with a temperature gradient of 7.5 x 10{sup 3} K/m. Under these conditions, the primary Si phase grows as an array of twinned bicrystalline dendrites, where the twinning gives rise to a characteristic 8-pointed star-shaped primary morphology. While this primary Si remains largely faceted at the growth front, a complex structure of coherent symmetric twin boundaries enables various adjustment mechanisms which operate to optimize the characteristic spacings within the primary array. In the work presented here, this primary silicon growth morphology is examined in detail. In particular, this thesis describes the investigation of: (1) morphological selection of the twinned bicrystalline primary starshape morphology; (2) primary array behavior, including the lateral propagation of the starshape grains and the associated evolution of a strong <100> texture; (3) the detailed structure of the 8-pointed star-shaped primary morphology, including the twin boundary configuration within the central core; (4) the mechanisms of lateral propagation and spacing adjustment during array evolution; and (5) the thermosolutal conditions (i.e. operating state) at the primary growth front, including composition and phase fraction in the vicinity of the primary tip.
Date: December 12, 2006
Creator: Jung, Choonho
Partner: UNT Libraries Government Documents Department

Dislocation accumulation at large plastic strains -- An approach to the theoretical strength of materials

Description: The usual method of introducing engineers to the concept of dislocations and their role in plastic flow is to compare an estimate of the theoretical strength of solid (of order {micro}/30 where {micro} is the shear modulus) and the observed strength of either single crystals ({mu}/10{sup 4}) or practical engineering material such as structural steels where the yield stress in shear is of order {mu}/10{sup 3}. However, if one considers the problem in reverse, one can consider the accumulation of dislocations as an important mechanism by which one can produce engineering materials in which the strength level approaches the theoretical strength. If one assumes that the flow stress can be expressed in terms of te mean free path between stored dislocations or as the square root of the global dislocation density, then one can see the influence of dislocation density in a diagrammatic form. It is clear that the strengthening by dislocation accumulation due to large imposed plastic strains represents an important approach both to the development of new, potentially valuable, engineering materials and an important area of basic understanding in terms of the mechanical response of materials close to their theoretical strength. Thus, this article will survey some of the factors which influence dislocation accumulation at large strains and the consequences of such accumulation processes.
Date: April 1, 1999
Creator: Embury, J.D. & Han, K.
Partner: UNT Libraries Government Documents Department

Microstructural Changes Accompanying Annealing of Cold-Worked Uranium

Description: Recovery of the capacity for plastic deformation by annealing previously cold-worked uranium plays a key role in the mechanism proposed for the cavitational swelling observed in irradiated uranium. Consequently, an investigation of recovery of yield strength was undertaken for unalloyed uranium and several selected alloys. During the course of this study, variations in the volume fraction of twins in the various specimens of unalloyed uranium suggested that twinning might be a mechanism of the recovery process. Results of four experiments in this study are described in this report.
Date: October 30, 2002
Creator: Caskey, G.R. Jr.
Partner: UNT Libraries Government Documents Department

Micromechanisms of Twin Nucleation in TiAl: Effects of Neutron Irradiation

Description: The so-called radiation-induced ductility (RID) reported in neutron-irradiated 47at%Al alloys is attributed to the formation of effective twin embryos in the presence of interstitial-type Frank loops in {gamma}-TiAl and the subsequent nucleation and growth of microtwins during post-irradiation tensile deformation. The stability of large faulted Frank loops is explained in terms of the repulsive interaction between Shockley and Frank partials. Interaction of only six ordinary slip dislocations with a Frank loop can facilitate a pole mechanism for twin formation to work. The relative ease of heterogeneous twin nucleation is the reason for the RID and the lack of changes in yield strength and work hardening.
Date: January 28, 1999
Creator: Hishinuma, A. & Yoo, M.H.
Partner: UNT Libraries Government Documents Department

Creep mechanisms and interface-enhanced deformation twinning in a two-phase lamellar TiAl alloy

Description: Deformation mechanisms and the role of interfaces in deformation twinning of a two-phase [TiAl({gamma})/Ti{sub 3}Al({alpha}{sub 2})] lamellar alloy creep deformed at elevated temperatures have been investigated. Since the multiplication of lattice dislocations within both {gamma} and {alpha}{sub 2} lamellae is very limited at a low stress level due to a refined lamellar microstructure, the glide of interfacial dislocations on both {gamma}/{alpha}{sub 2} and {gamma}/{gamma} interfaces (i.e interface sliding) becomes an important deformation mode. Obstacles such as impinged lattice dislocations can impede the movement of interfacial dislocations, which glide in a cooperative fashion along the lamellar interfaces. The impediment of dislocation motion subsequently causes a dislocation pile-up in front of obstacles as creep strain accumulates. When the crystals deform at high stress level, deformation twinning becomes a predominant deformation mode. Deformation twins are found to nucleate from the interfaces as a result of a local stress concentration generated from dislocation pile-ups. It is suggested that the deformation twinning in lamellar TiAl/Ti{sub 3}Al crystals can be vieived as a stress relaxation process for the concentration of stress at the head of each dislocation pile-up. An interface-assisted twinning mechanism is accordingly proposed and discussed.
Date: March 1, 1997
Creator: Hsiung, L.M., LLNL
Partner: UNT Libraries Government Documents Department

Residual stress, strain, and faults in nanocrystalline palladium and copper

Description: Nanocrystalline Pd and Cu, prepared by inert gas condensation and warm compaction, were studied using x-ray diffraction techniques. A sample of Cu with sub-micrometer grain size produced by severe plastic deformation was also examined. The Warren-Averbach technique was used to separate the line broadening due to grain size, root-mean-squared strain, and faults. Peak shifts and asymmetry were used to determine the long range surface stresses, stacking fault probability, and twin probability. The Young`s modulus of a Pd sample was determined by an ultrasonic technique, and compared with the coarse-grained, fully-dense value.
Date: February 1, 1995
Creator: Sanders, P.G.; Witney, A.B.; Weertman, J.R.; Valiev, R.Z. & Siegel, R.W.
Partner: UNT Libraries Government Documents Department

The nature of twin boundaries in the high-temperature superconductor YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}

Description: Twin boundaries are the most commonly observed lattice defect in the high-temperature superconductor YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}. Furthermore, the region around a twin boundary for which the structure and composition are seriously affected is of a scale comparable to the coherence length for the superconducting order parameter. Thus, twin boundaries can be important in the behavior of magnetic vortices and the critical current density in this material. In this paper the authors review the results of a wide range of investigations of twin boundary structure and composition by advanced transmission electron microscopy methods, both imaging and analytical in nature. A simple Landau model of twin boundary energy and width is proposed.
Date: June 1, 1999
Creator: Zhu, Y. & Welch, D.O.
Partner: UNT Libraries Government Documents Department

Influence of twinning on the constitutive response of Zr: Experiments and modeling

Description: The stress-strain response of Zr due to twinning is distinctly different from that due to slip as a function of temperature and strain rate. When the applied stress is lower than the transition stress, dislocation slip is the dominant deformation mechanism. The traditional MTS model is shown to adequately represent the constitutive behavior of Zr. Above the transition stress twinning becomes the dominant deformation mechanism where the flow stress increases linearly with strain. In this regime the rate-dependent strain hardening can be described by equations based on thermal activation theory that are very similar to the formula used in the MTS model.
Date: May 1, 1997
Creator: Chen, Shuh Rong & Gray, G.T. III
Partner: UNT Libraries Government Documents Department

Twinning mechanism in PrCo{sub 2}C{sub x} magnetic phase

Description: A magnetic rhombohedral PrCo{sub 2}C{sub x} (x=0.05{approximately}0.25) phase (space group R {bar 3} m), which is heavily twinned along the (110) and (211) planes, was identified. The twinning mechanism was explored by analyzing the reduction of crystal symmetry due to the cubic-rhombohedral phase transformation. The origin of the twinning and the formation of four twin variants were attributed to the insertion of carbon interstitials into Co{sub 4} tetrahedrons along the {bar 3} axis in the rhombohedral lattice, which corresponds to one of the four equivalent <111> axes of its parent PrCo{sub 2} cubic-lattice.
Date: July 1, 1997
Creator: Wu, Lijun; Bian, Weimin Bian & Zhu, Yimei
Partner: UNT Libraries Government Documents Department

Modeling complex phenomena: Multiple length and time scales in extended dynamical systems

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). Using nonlinear techniques and large-scale simulations, we have systematically studied meso-scale pattern formation and dynamics in nonlinear, nonequilibrium systems exhibiting topological excitations (dislocations, vortices, vortex lines, domain walls); dislocation generation from crack fronts in ductile materials; the smoothing of rough surfaces in solid-on-solid models; ordering and melting of moving flux lattices in three-dimensional Josephson junction arrays with external magnetic field, current, and disorder; filamentary and plastic vortex flow in disordered thin films superconductors; magnetic vortices in Heisenberg spin layers; and hierarchical twinning and tweed texture in elastic models.
Date: December 31, 1998
Creator: Lomdahl, P.; Bishop, A. & Jensen, N.G.
Partner: UNT Libraries Government Documents Department

Interfacial properties and mechanical behavior of titanium aluminides

Description: The role of various interfaces in deformation and fracture behavior of two phase TiAl-Ti{sub 3}Al alloys is analyzed on the basis of the specific interfacial and surface energies determined from ab initio calculations. The propensity of twinning observed in these alloys is consistent with the low true twin boundary energy. The strong plastic anisotropy reported in TiAl polysynthetically twinned (PST) crystals is attributed partly to the localized slip along lamellar interfaces, thus lowering the yield stress for soft orientations. Interfacial fracture energies are estimated to be the highest for the {alpha}{sub 2}/{gamma} lamellar boundary and the lowest for the 120 {degree} rotational {gamma}/{gamma} boundary. The fracture mode mixity plays an important role in the crack-tip plasticity by ordinary slip and true twinning, leading to translamellar and interfacial fracture.
Date: January 1, 1998
Creator: Yoo, M.H. & Fu, C.L.
Partner: UNT Libraries Government Documents Department

Dynamic Behavior of Beryllium as a Function of Texture

Description: The high-strain-rate stress-strain responses of commercial hot-pressed beryllium and rolled-sheet beryllium were studied as a function of orientation in compression and room temperature. Hot-pressed beryllium exhibits isotropic mechanical properties; whereas 16:1 rolled sheet was highly anisotropic. Rolled sheet displayed a factor of two difference in strength between the thickness and in-plane (lowest) directions. Twinning is a key deformation mechanism at high rates.
Date: January 5, 1999
Creator: Blumenthal, W.R.; Abeln, S.P.; Mataya, M.C.; Gray, G.T. III & Cannon, D.D.
Partner: UNT Libraries Government Documents Department