Materials Science and Engineering

This paper reviews selected recent research on the atomistic simulation of dislocation and defect properties of materials relevant to the multiscale modeling of plasticity and strength, with special emphasis on bcc metals and including work at extreme conditions. Current topics discussed include elasticity and ideal strength, dislocation structure and mobility, grain boundaries, point defects, and rapid resolidification, as well as noteworthy examples of research that directly impacts the issue of linking of length and/or time scales, as required in multiscale materials modeling. The work reviewed has been inspired by the recent international Workshop on Multiscale Modeling of Materials Strength and Failure held in October 2001 at Bodega Bay, California.

A theory of intergranular creep crack growth in brittle materials has been developed. The mechanism of crack propagation is removal of atoms from the crack tip by stress induced grain boundary diffusion, and their deposition at the grain boundary. The width of the crack is assumed to be constant during crack propagation. Any possible plastic deformation at the crack tip has been neglected. The stress relaxation ahead of the crack tip which arises due to the non-uniform deposition of material onto the grain boundary is calculated and the diffusion process governed by this relaxed stress studied. Thus the theory is analogous to those developed previously for the growth of r-type voids in grain boundaries. Both the steady state and the transient were considered. It was found that the rate of crack growth at any moment was determined by the nominal stress intensity factor K. A minimum stress intensity, K,,~., exists below which no crack growth can take place. Conversely there is a maximum limiting rate of crack growth determined by the maximum surface diffusion rate. For K > Kin.. the steady state is always reached quickly and the length of the transient period t~r is proportional to K-6. For K ,> K,,~, the rate of crack growth is proportional to K ~. A comparison with experimental measurements on brittle ~-Cr-{Mo-~V steels show good agreement between the theory and experiment.

The intermetallic compound MoSi 2, which adopts the C11 b crystal structure, and related alloys exhibit an excellent corrosion resistance at high temperatures but tend to be brittle at room and even relatively high temperatures. The limited ductility of MoSi 2 in ...

We present a new Screened Bond-Order Potential (SBOP) for molybdenum in which the environmental dependence of two-center tight-binding bond integrals has been implemented via a recently developed analytic expression. These bond integrals reproduce very well the numerical ab-intio values of screened LMTO bond integrals. In particular, they display the large discontinuity in ddπ between the first and second nearest neighbor of the bcc lattice whereas they do not show any discontinuity in ddσ. This dependence can be traced directly to the angular character of the analytic screening function and is shown to be critical for the behavior of the second nearest neighbor force constants. The new BOP eliminates the problem of the very soft T2 phonon mode at the N point that is found in most two-center tight-binding models. Preliminary study of the core structure of 1/2<111> screw dislocations performed using SBOP indicates that the core is narrower and less asymmetric than structures found in previous studies, in agreement with recent ab-initio calculations.

ABSTRACTPST TiAl crystals oriented such that the deformation axis lies in the (111) interfacial planes have been deformed in compression. This deformation produces so-called “channeled flow” in which the strain perpendicular to the (111) interfaces is zero, while the other two strains are equal and opposite in sign. Thus the sample simply shortens axially and spreads laterally in the channels defined by the (111) interfacial planes. We have examined the fine structure of deformation bands on the free surface of these deformed samples using AFM to see how the deformation processes interact with the boundaries. By measuring the offset angle at the surface we have been able to show that not only is the macroscopic displacement vector parallel to the lamellar boundaries, but the total shear vector in each layer is also parallel to the lamellar boundaries. However these deformation bands have very different characters, requiring complex deformation processes at the boundaries in order to...

This paper describes a theoretical model and a Monte Carlo simulation of cooperative dislocation generation in loaded crystals at finite temperatures. The theoretical work, employing a meanfield approach, is presented for the three-dimensional case when dislocation loops are formed. It describes how extensive plasticity can be initiated in dislocation-free crystals above a critical temperature due to 'homogeneous' cooperative nucleation and expansion of many dislocation loops. The Monte Carlo simulation is carried out to demonstrate the feasibility of the theoretical model. It is performed for a two dimensional case when dislocation dipoles are formed on an underlying lattice under applied loads. The application of this model to the brittle-to-ductile transition and the yielding of whiskers is discussed.

The thermodynamic description of dislocation glide in BCC metals depends crucially on the shape of the Peierls barrier that 1/2⟨111⟩ screw dislocations have to overcome when moving in the lattice. While the height of this barrier can be obtained unequivocally using saddle-point search algorithms such as the Nudged Elastic Band (NEB) method, its exact shape depends on the chosen approximation of the transition pathway of the system. We formulate a procedure that allows to identify the position of the dislocation directly from the displacements of atoms in its core. We investigate the performance of this model by calculating curved paths of a 1/2⟨111⟩ screw dislocation in tungsten from a series of images obtained recently using the NEB method at zero applied stress and for positive/negative shear stresses perpendicular to the slip direction. The Peierls barriers plotted along these curved paths are shown to be quite different from those obtained previously by assuming a straight dislocation path. We demonstrate how these results can be utilized to develop a new thermodynamic model of plasticity of BCC metals that is systematically linked to the atomic-level properties of isolated 1/2⟨111⟩ screw dislocations.

David Pettifor was a theoretical physicist who changed the nature of materials science by raising the status of materials modelling to that of materials characterization and processing. He believed that the subject advanced through the development of simple models that withstood rigorous testing against experiments and the most accurate numerical computations. Having been a pioneer of total energy density functional theory calculations, he went on to derive analytic interatomic potentials for transition metals and nearly-free-electron metals and alloys from quantum mechanical principles. He is probably best known for the development of highly successful structure maps for binary and pseudo-binary alloys that were used by alloy developers in industry to create intermetallic alloys with improved properties. At Oxford he established the first materials modelling laboratory, bringing together physicists, chemists, materials scientists and engineers to model materials across length and t...

1. In biopsy samples of the lateral part of m. quadriceps femoris of 49 obese and 14 lean persons the activities of the following enzymes were investigated: triosephosphate dehydrogenase (TPDH), glyeerolphosphate: NAD dehydrogenase (GPDH), lactate dehydrogenase (LDH), hexokinase (HK), malate: NAB dehydrogenase (MDH), citrate synthase (CS) and hydroxyacyl-CoA dehydrogenase (HOAX)H).

Materials with high melting temperatures (over 2000°C) tend to be brittle at ambient and even relatively high temperatures. High melting temperatures originate in strong interatomic bonding arising from formation of dd or dp bonds that also affect and/or control crystal structures and properties of extended defects, such as dislocations, grain boundaries. These, in turn, govern plastic deformation and fracture. General goal: Establish relationship between electronic structure and mechanical behavior Comments Poster presented at The Search for a Sustainable Energy Future: Challenges for Basic Research, A MiniSymposium sponsored by the Energy Working Group at Penn, March 9, 2007. This other is available at ScholarlyCommons: http://repository.upenn.edu/pennergy_posters/1 Atomistic Studies of Deformation and Fracture in Materials with Mixed Metallic and Covalent Bonding V. Vitek, M. Cawkwell and R. Groger Department of Materials Science and Engineering, University of Pennsylvania, Phila...

The fourth and higher harmonic components available in large-amplitude Fourier transformed ac voltammetry have been employed in a study involving surface-confined redox proteins having multiple electron-transfer centers. Ferredoxin I from Azotobacter vinelandii (AvFdI) and two variants exhibit significant differences in the reversible reduction potentials (Eo′) of their two non-interacting FeS clusters which are located at a fixed distance 1.2nm apart.

The dominant slip directions in different intermetallic alloys with B2 structure are different, either h0 0 1i or h1 1 1i. The elastic energy of h1 1 1i dislocations is usually significantly higher than that of h0 0 1i dislocations and it is commonly assumed that h1 1 1i slip occurs if h1 1 1i dislocations can dissociate on {1 1 0} planes into ½h1 1 1i superpartials. However, we show in this paper that ½h1 1 1i anti-phase boundaries may not be metastable faults on {1 1 0} planes and the displacement vectors of metastable stacking-fault-like defects on these planes vary from material to material. This analysis involves calculations of {1 1 0} c-surfaces for eight B2 alloys (CuZn, FeAl, NiAl, FeTi, CoTi, NiTi, FeGa, PdAl) using a density functional theory-based method. Since both h1 1 1i and h0 0 1i screw dislocations may possess non-planar cores if undissociated and will then control plastic properties analogously as ½h1 1 1i screw dislocations in bodycentered cubic metals, the dissociations have been analyzed for screw dislocations. Subsequently, we assume that if the width of splitting in {1 1 0} planes exceeds the Burgers vector of the corresponding dislocation, the dislocation spreads in this plane and is glissile while undissociated dislocation is sessile. The ability to glide is then regarded as the determining factor for the choice of the slip direction. If no planar spreading occurs the dominant dislocations are determined by their energy. This analysis predicts the slip directions for all alloys studied and demonstrates that an interplay of elastic anisotropy, displacement vectors of metastable stacking-fault-like defects and their energies govern the choice of the slip direction in any specific B2 alloy.

ÐThe formation of networks of mis®t dislocations is investigated at the 0001 el2y3 k111 x interface using a recently proposed approach (Vitek et al., Phil. Mag. A, 1995, 71, 1219) which employs a very simple pair-potential to describe interaction between the metal and the substrate that contains the strength of interfacial adhesion as a parameter. The calculations demonstrate how the strength of bonding between the two materials decides both the form of the network and the atomic structure of the cores of these dislocations. At the same time it reveals that diusion is essential for the formation of the observed triangular network of 1/2h111i dislocations. The calculated structures are then used to investigate related high resolution electron microscope (HREM) images using a multislice technique. In these simulations translational symmetry along the electron beam was not assumed but for each slice of material along the beam dierent sub-structures were used. This allowed us to investigate fully the eect of the dislocation intersections upon the images of the dislocation cores. Their eect is, indeed, considerable if an intersection is in the region producing the image but if not, the images of the cores of mis®t dislocations are aected only marginally and HREM can capture ®ne details of the core structure. A direct comparison of an experimental observation in Mayer and co-workers (Acta metall. mater., 1992, 40, S217; Scripta metall. mater., 1994, 31, 1097) with the present simulations demonstrates this ability.

ÐDislocation core structures of h100i, h110i and h111i dislocations in NiAl have been studied by molecular static calculations using an embedded atom potential. The response of dislocation cores to applied homogeneous shear stresses is investigated and the Peierls stresses of straight dislocations having edge, screw and mixed character is determined. The results are in many details in excellent agreement with experimental observations. The motion of dislocations in single crystals oriented for hard slip is discussed in a simple line-tension model.

ÐThe anisotropy of plasticity in b.c.c. metals is decomposed into two parts. The ®rst, a variation of the Peierls stress with slip plane and sense of slip, is intrinsic to the b.c.c. structure and is re¯ected in the geometry of the screw dislocation core and of the generalized stacking fault energy (g) surface. The second part, a sensitivity of the Peierls stress to non-glide elements of the applied stress, is shown to be due to small edge fractional dislocation components in the screw dislocation core. These edge components and their change in response to non-glide applied stress can be explained by the characteristics of the g-surface. These concepts are illustrated by means of atomistic calculations, using Finnis±Sinclair interatomic force laws, for the b.c.c. transition elements of groups VB and VIB of the periodic table. Metals in the two groups are shown to fall into two classes of core structure and behavior; the contrasting properties can be explained in terms of the g-surfaces, and can be traced further to dierences in the elastic constants. # 1998 Acta Metallurgica Inc.

Bridging the gap between electronic and atomistic levels plays a crucial role in multi-scale modelling of mechanical behaviour of materials. In this review, we summarise a methodology for linking systematically these two levels starting from the first-principles density functional theory and proceeding via the screened tight-binding approximation to development of reliable and transferable many-body interatomic bond-order potentials. We focus our investigations on material properties related to the electron-to-atom ratio. An immediate area of application is studies of the structure and properties of crystal defects in transition metals and intermetallic compounds based on transition metals, where the mixed character of covalent and metallic bonds represents a very challenging issue for understanding mechanical properties at the engineering scale. The need for environmental dependence of bond-order potentials as well as the implication of screening effects on bonding properties of alloys are discussed in connection with modelling of the core structure of dislocations in materials with negative Cauchy pressures and in body-centered cubic (bcc) transition metals. The latter are prime candidates as fusion power-plant materials. We discuss our current work on multiscale modelling, the behaviour of bcc materials under high-energy neutron irradiation, and emphasize the importance of quantum-mechanics in constructing reliable interatomic potentials for large scale molecular dynamic simulations.

A theoretical explanation of warm prestressing (WPS) is proposed based on the J-integral. This is defined so that only elastic strains and distortions are included in the integrand compared with the definition of J common in finite element computation where total strains and distortions are used. The physical meaning of J as used here is that it represents the force on singularities (e.g., dislocations) enclosed by the contour of integration. Therefore, it is path dependent if the contour cuts through the plastic zone. After WPS there are regions of residual plastic strain, where dislocations are immobile, due to the change in yield stress resulting from the lowering in temperature of the cracked structure. A new plastic zone, where dislocations are mobile, forms on reloading at low temperature. It is postulated that failure occurs when the force on these mobile dislocations attains a critical value, J~,,. Hence the contour used to evaluate J should be chosen to include only this plastic region. The concepts and theory are experimentally validated by successfully analysing fracture data from centre cracked panels which were subjected to a range of WPS loads before being fractured at liquid nitrogen temperatures. Similar successes were also obtained on analysing other published experimental WPS data. It is shown that the finite element definition of J cannot explain these results. It is concluded that the definition of J used here extends the applicability of J to cases involving complicated loading histories which may include unloading prior to failure as well as changes in yield stress due to temperature or irradiation effects.

We present a bond-order potential (BOP) for the bcc transition metal tungsten. The bond-order potentials are a real-space semiempirical scheme for the description of interatomic interactions based on the tight-binding approximation. In the hierarchy of atomic-scale-modeling ...

The positions of the α-and β-bands in the electronic absorption spectra of twenty aromatic benzenoid hydrocarbons were calculated by the semiempirical method of limited configuration interaction in the πelectron approximation using the Huckel molecular orbitals. The agreement of the experimental and calculated values is good for the β-band whereas a systematic deviation is observed for the α-band. This deviation cannot be removed by extending the configuration interaction of the monoexcited states constructed from the molecular orbitals considered. However, the consideration of electronic repulsion enables us to explain the character of the dependences of the experimental excitation energies on the excitation energies obtained by the simple Huckel method of molecular orbitals. Using a suitable choice of semiempirical parameters different for various electronic transitions (showing no large mutual differences) yields semiempirical interpolation formulas for the; p-, α-, and β-bands which give very good agreement with the corresponding experimental excitation energies for the compounds studied. Disciplines