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A sound gradual type system ensures that untyped components of a program can never break the guarantees of statically typed components. This assurance relies on runtime checks, which in turn impose performance overhead in proportion to the frequency and nature of interaction between typed and untyped components. The literature on gradual typing lacks rigorous descriptions of methods for measuring the performance of gradual type systems. This gap has consequences for the implementors of gradual type systems and developers who use such systems. Without systematic evaluation of mixed-typed programs, implementors cannot precisely determine how improvements to a gradual type system affect performance. Developers cannot predict whether adding types to part of a program will significantly degrade (or improve) its performance. This paper presents the first method for evaluating the performance of sound gradual type systems. The method quantifies both the absolute performance of a gradual type system and the relative performance of two implementations of the same gradual type system. To validate the method, the paper reports on its application to 20 programs and 3 implementations of Typed Racket.
The Burst Observer and Optical Transient Exploring System (BOOTES), is a global robotic
observatory network, which started in 1998 with Spanish leadership devoted to study
optical emissions from gamma ray bursts (GRBs) that occur in the Universe. We present shot
history and current status of BOOTES network. The Network philosophy, science and some
details of 117 GRBs followed-up are discussed.
The brittle-to-ductile transition (BDT) and the strain-rate dependence of the brittle-to-ductile transition temperature (BDTT) have been recently investigated in single crystals of TiAl . It was found that the activation energy associated with the BDTT is 1.4 eV when the slip is dominated by ordinary dislocations and 4.9 eV when it is dominated by superdislocations. Despite this difference in the activation energies, the BDTT, while varying with the strain-rate, remains in the same temperature range, viz., between 516–750C and 635–685C for ordinary and superdislocations, respectively. In this paper, we examine how the activation energy of the BDTT can vary with the type of dislocation activity and explain why it can attain values which are clearly much larger than the activation energy for dislocation motion. We describe a strain-rate dependent mechanism of cooperative dislocation generation in loaded solids above a critical temperature and use it to explain the characteristics of the BDT in TiAl. We show that the activation energy associated with the BDTT is a composite value determined by two or more inter-dependent thermally activated processes and its magnitude can be much larger than the activation energy for dislocation motion in certain materials. The predictions of the model are in good agreement with observations in TiAl.
The tensile test in transition metal disilicides with C113 structure is simulated by ab initio electronic structure calculations using full potential linearized augmented plane wave method (FLAPW). Full relaxation of both external and internal parameters is performed. The theoretical tensile strength of MOS12 and WSi2 for  loading is determined and compared with those of other materials.
We have used embedded atom potentials to simulate the surfaces, thin films and grain boundaries in metals (Ni and Al) and alloys (NiAl and Ni3Al). The calculated surface relaxations and ripplings of free surfaces are in good agreement with experiments. A new interference phenomena of interlayer relaxation in thin films are observed in the simulation. The segregation behavior of B and S and their effects on the mechanical properties of Ni3Al are correctly predicted with potentials fitted to data obtained by electronic band structure calculations.
The Embedded Atom Method (EAM), a modem theory of metallic bonding, has been developed to provide a simple but accurate method of evaluating the energy and forces for an arbitrary arrangement of atoms. The relationships between the EAM and the underlying electron density theory will be discussed. Specific examples of EAM calculations of surface reconstruction for (110) fcc materials will be predicted and compared to experiment. The examples will include temperature effects in gold. The results of molecular dynamics calculations of the mechanical properties of nickel also will be presented. Topics to be discussed include dislocation mobility and dislocation emission from a stressed crack in nickel. The dislocation calculations will be related to continuum modelling.
Stacking faults in close-packed metals are known to play a crucial role in determining mechanical behaviour. Extending recent layer Korringa-Kohn-Rostoker calculations on twin faults in a variety of FCC crystals, we study in detail the aluminium defect and develop an atomistic understanding of the modifying behaviour of small concentrations of impurity atoms.
An empirical three-body potential, suitable for molecular dynamics (MD) simulations, has been developed to model the natural covalency of the Si-O bond in vitreous silica and silicate glass systems. Through the addition of a small directional-dependent three-body term to a previously used modified ionic pair interaction, a narrow distribution of tetrahedral angles and a low concentration of defects were obtained, in good agreement with experiment. The structure of bulk silica resulting from the MD technique also contained a larger average ring size, no edge-sharing tetrahedra, and a calculated static structure factor in good agreement with neutron diffraction results. The simulated sodium silicate glass was also largely improved over previous simulations using pair interactions alone. All silicon atoms were found to be exactly four coordinated while the number of non-bridging oxygen nearly equaled the number of sodium ions present with a reasonable distribution of Qi species.
The interaction between F atoms and crystalline Si, which is essential for etching processes in semiconductor device fabrication, is investigated with state-of-the-art theoretical techniques. The theory is based on the pseudopotential-density-functional method in a supercell geometry. A comprehensive picture of F reactions with the Si surface, the bulk, and the near-surface region is obtained, in terms of which the etching process is elucidated. Insertion of F into Si-Si bonds becomes possible because of relaxed steric constraints in the near-surface region. Dependence of the etch rate on doping follows naturally, in agreement with observations. Similarities and differences between F-Si and H-Si reactions are discussed.
The behavior of a metallic grain boundary at high temperatures is studied using an embedded atom potential. A recently developed molecular dynamics code is used which allows the simulation of an isolated grain boundary at temperatures as high as the bulk melting point. The stability of the boundary below the melting point is studied and compared with earlier investigations which have suggested the existence of a “premelting“ transition. It is found that the boundary migrates at high temperature but remains well defined up to the bulk melting point. In contrast to simulations of ideal crystals, it was not possible to superheat the grain boundary due to the nucleation of bulk melting at the boundary.
Amorphous silicon structures have been generated by quenching liquid silicon configurations using molecular-dynamics simulations. Localized vibrational modes have been identified in these models. The presence of under-coordinated atoms in these a-Si models leads to extra resonant modes at low frequencies. The vibrational densities of states, and dynamic structure factors for localized, resonant and extended modes, are discussed and compared with neutron scattering data. The amorphous networks have also been adapted to model amorphous silicon-germanium systems. Densities of states and localization characteristics have been calculated for a-SixGe1-x alloys and a-Si/a-Ge superlattices, and are compared to Raman measurements.
Temperature-composition phase diagrams of alloys are calculated by a new method combining (i) first principles total energy calculations (at T=0) for ordered structures, using the local density formalism, with (ii) finite-temperature statistical-mechanics approach (the Cluster Variation Method) to the solution of the multi-spin Ising model, using volume-dependent interaction energies obtained from (i). Novel features, including the appearance of metastable long-range ordered compounds at low temperatures are discovered.
The embedded-atom method was applied in computer simulations to study epitaxial Cu/Ag interfaces in cube-on-cube orientation relationship. Coherent and semicoherent interfaces were studied with inclinations parallel to (001), (011) and (111). The coherent boundary energy depends strongly on the predicted enthalpy of mixing. The interfacial energy for semicoherent boundaries was highly anisotropic, having its largest value (549 mJ/m2) for the (011) interface and its smallest value (231 mJ/m2) for the (111) interface. The periodic elastic relaxations correspond to networks of misfit dislocations lying in the plane of the interface; the maximum displacement in the (011) interface is about one-third the atomic diameter, but only one-eighth the atomic diameter in the (111) interface.
Using an Embedded Atom Method calculation of the interatomic potentials and volume forces in the Ni-Al alloy system, we have examined the plastic and elastic response of an ordered bcc Ni-Al crystal with a pre-existing crack under Mode I loading at various temperatures, stresses and crystal orientation. Depending upon those conditions we found evidence of slip and dislocation generation near the crack tip concomitant with crack propagation. we also saw evidence of a brittle to ductile transition above a certain temperature which is manifested by copious slip and dislocation production. Atomic arrays up to 4000 atoms have been studied.
The constrained density functional approach is used to calculate the energy surface as a function of local charge fluctuations in La2CuO4. This energy surface is then mapped onto a self consistent mean field solution of the Hubbard model which allows extraction of the Coulomb interaction parameters when combined with oneelectron parameters derived from band structure results. The present calculations indicate that La2CuO4 is intermediate between the extreme spin or charge fluctuation regimes. This severly restricts the range of parameter space for theories of quasiparticles, optical excitations and possible pairing mechanism based on the extended Hubbard model.
Atomic structures of 60° dislocations were calculated in bulk Si and Ge ,and at Ge/Si interfaces, using energy minimization techniques. An empirical three body potential developed by Stillinger-Weber was used for Si ,and the same potential with modified parameters was used for Ge. Two different configurations of a 60° dislocation (shuffle and glide) were compared. Energetics of a 60° dislocation in shuffle configuration in bulk Si and Ge , and at Ge/Si interfaces are discussed.
The growth kinetics of melting nucleated at a high-angle twist boundary in silicon are investigated using molecular dynamics. Melting is found to be a two-stage process. In the first stage order is lost within a single plane at the interface and the density of the solid increases to that of the liquid. In the second stage the atomic coordination changes and an isotropic liquid is formed.