The effect of grain size on workhardening and superplasticity in Zn/0.4% Al Alloy

Superplasticity*requires, amongst other things, a metal with a grain-size in the range 0.5-5μ. Theories of SP invoking dynamic recovery require that the cell-Size of the substructure for the alloy in question is larger than the SP grain-size, so that gliding dislocations are always annihilated in the grain boundaries and workhardening cannot occur (1,2). .Thus the grain-size is critical, and for a given set of conditions, there must be a grain-size greater than which SP cannot be achieved

Extended plasticity in commercial-purity zinc

90% rolling-reduction of annealed commercial-purity zinc sheet (grain size 100 - 150 μ) results in the fragmentation of the large grains into, finally, stable micro-grains, 1 - 211 in diameter. The stability of the micro-grains is due to the presence of soluble and insoluble impurities which prevent recrystallization. This micro-grain material is strain-rate sensitive, and elongations of 200% have been obtained at room temperature. Although this as-rolled, 90% reduction zinc sheet is not super-plastic according to the current definition, its behaviour has led to the coining of the phase 'extended plasticity'. Evidence of grain-boundary sliding is found on examination of the surface by scanning electron microscopy, while the examination of thin foils and activation energy measurements support the dynamic softening (recovery) theory; thus, both these mechanisms must be operating, to a greater or less extent, to confer on this material the observe mechanical behaviour. It is finally concluded that it is dangerous to draw conclusions regarding the mechanism of plastic deformation from surface observations alone

Hydra: An Adaptive--Mesh Implementation of PPPM--SPH

We present an implementation of Smoothed Particle Hydrodynamics (SPH) in an adaptive-mesh PPPM algorithm. The code evolves a mixture of purely gravitational particles and gas particles. The code retains the desirable properties of previous PPPM--SPH implementations; speed under light clustering, naturally periodic boundary conditions and accurate pairwise forces. Under heavy clustering the cycle time of the new code is only 2--3 times slower than for a uniform particle distribution, overcoming the principal disadvantage of previous implementations\dash a dramatic loss of efficiency as clustering develops. A 1000 step simulation with 65,536 particles (half dark, half gas) runs in one day on a Sun Sparc10 workstation. The choice of time integration scheme is investigated in detail. A simple single-step Predictor--Corrector type integrator is most efficient. A method for generating an initial distribution of particles by allowing a a uniform temperature gas of SPH particles to relax within a periodic box is presented. The average SPH density that results varies by $\sim\pm1.3$\%. We present a modified form of the Layzer--Irvine equation which includes the thermal contribution of the gas together with radiative cooling. Tests of sound waves, shocks, spherical infall and collapse are presented. Appropriate timestep constraints sufficient to ensure both energy and entropy conservation are discussed. A cluster simulation, repeating Thomas andComment: 29 pp, uuencoded Postscrip

Anisotropic superplasticity

Anisotropy has already been mentioned in connexion with SP. Johnson et al (1) have shown that specimens of circular cross-section, machined from hot-rolled SP Zn/Al eutectic and eutectoid plate, become elliptical on straining in the rolling direction, while the fine-grain Zn/Al eutectoid produced by the quench-4 spinoidal decomposition method did not

Smoothed Particle Hydrodynamics in cosmology: a comparative study of implementations

We analyse the performance of twelve different implementations of Smoothed Particle Hydrodynamics (SPH) using seven tests designed to isolate key hydrodynamic elements of cosmological simulations which are known to cause the SPH algorithm problems. In order, we consider a shock tube, spherical adiabatic collapse, cooling flow model, drag, a cosmological simulation, rotating cloud-collapse and disc stability. In the implementations special attention is given to the way in which force symmetry is enforced in the equations of motion. We study in detail how the hydrodynamics are affected by different implementations of the artificial viscosity including those with a shear-correction modification. We present an improved first-order smoothing-length update algorithm that is designed to remove instabilities that are present in the Hernquist and Katz (1989) algorithm. For all tests we find that the artificial viscosity is the most important factor distinguishing the results from the various implementations. The second most important factor is the way force symmetry is achieved in the equation of motion. Most results favour a kernel symmetrization approach. The exact method by which SPH pressure forces are included has comparatively little effect on the results. Combining the equation of motion presented in Thomas and Couchman (1992) with a modification of the Monaghan and Gingold (1983) artificial viscosity leads to an SPH scheme that is both fast and reliable.Comment: 30 pages, 26 figures and 9 tables included. Submitted to MNRAS. Postscript version available at ftp://phobos.astro.uwo.ca/pub/etittley/papers/sphtest.ps.g

Simulation of associative learning with the replaced elements model

Associative learning theories can be categorised according to whether they treat the representation of stimulus compounds in an elemental or configural manner. Since it is clear that a simple elemental approach to stimulus representation is inadequate there have been several attempts to produce more elaborate elemental models. One recent approach, the Replaced Elements Model (Wagner, 2003), reproduces many results that have until recently been uniquely predicted by Pearce’s Configural Theory (Pearce, 1994). Although it is possible to simulate the Replaced Elements Model using “standard” simulation programs the generation of the correct stimulus representation is complex. The current paper describes a method for simulation of the Replaced Elements Model and presents the results of two example simulations that show differential predictions of Replaced Elements and Pearce’s Configural Theor

Integrals of Motion for Critical Dense Polymers and Symplectic Fermions

We consider critical dense polymers ${\cal L}(1,2)$. We obtain for this model the eigenvalues of the local integrals of motion of the underlying Conformal Field Theory by means of Thermodynamic Bethe Ansatz. We give a detailed description of the relation between this model and Symplectic Fermions including the indecomposable structure of the transfer matrix. Integrals of motion are defined directly on the lattice in terms of the Temperley Lieb Algebra and their eigenvalues are obtained and expressed as an infinite sum of the eigenvalues of the continuum integrals of motion. An elegant decomposition of the transfer matrix in terms of a finite number of lattice integrals of motion is obtained thus providing a reason for their introduction.Comment: 53 pages, version accepted for publishing on JSTA