Micromechanics of high temperature deformation and failure

The micromechanics of the constitutive behavior of elastoplastic materials at high temperatures was examined. The experimental work focused on the development of microscopic defects in superalloys (Waspaloy), especially the formation of voids at grain boundary carbides, and slip induced surface cracks within grains upon cyclic loading at high temperatures. The influence of these defects on the life expectancy of the material was examined. The theoretical work consists of two parts: (1) analytical description of the mechanisms that lead to defects observed experimentally; and (2) development of macroscopic elastoplastic nonlinear constitutive relations based on mechanical modeling

Strength and High-Temperature Stability of Dispersion Strengthened Nickel-MgO Alloys

Strength and high-temperature stability of dispersion strengthened nickel-magnesium oxide alloy

Predicting dislocation climb: Classical modeling versus atomistic simulations

The classical modeling of dislocation climb based on a continuous description of vacancy diffusion is compared to recent atomistic simulations of dislocation climb in body-centered cubic iron under vacancy supersaturation [Phys. Rev. Lett. 105 095501 (2010)]. A quantitative agreement is obtained, showing the ability of the classical approach to describe dislocation climb. The analytical model is then used to extrapolate dislocation climb velocities to lower dislocation densities, in the range corresponding to experiments. This allows testing of the validity of the pure climb creep model proposed by Kabir et al. [Phys. Rev. Lett. 105 095501 (2010)]

Earthquake Mechanism and Displacement Fields Close to Fault Zones

The Sixth Geodesy/Solid Earth and Ocean Physics (GEOP) Research Conference was held on February 4–5, 1974, at the Institute of Geophysics and Planetary Physics, University of California, San Diego, in La Jolla, California. It was attended by about 100 persons. James N. Brune, program chairman, opened the conference and delivered the introductory address, a somewhat extended version of which is printed elsewhere in this issue. Brune's paper and the following summaries of the sessions constitute a report of the conference

Four-field finite element solver and sensitivities for quasi-Newtonian flows

International audienceA computationally efficient finite element algorithm for power law fluid is elaborated in view of extensive direct and inverse simulations. We adopt a splitting technique to simplify the nonlinear structure of the fluids equations and derive a four-field saddle point formulation for which we prove the existence of a solution. The resolution of the corresponding variational inequalities is based on an augmented Lagrangian method and a mixed finite element discretization. The resulting iterative solver reveals to be fast and robust with low memory consumption. The time-saving provided by the algorithm compared to the standard algorithms of fixed point and Newton increases with the number of degrees of freedom and the nonlinearity of the problem. It is therefore well-suited for the solution of large problems with a great number of elements and for corresponding adjoint-based computations. Bidimensional numerical experiments are performed on two realistic situations of gravity flows: an experimental viscoplastic steady wave and a continental glacier. In the present study, results emphasize that for both cases, the modeling at bottom plays a strongly dominant role. Using surface velocitiy observations, the sensitivity analysis with respect to a spatially varying power-law exponent highlights the importance of an accurate knowledge of the rheology at high shear rate. The one on the basal sliding allows to detect the presence of a short wavelength (two times the thickness) free-slip area indetectable from surface velocities

Microhardness and elastic modulus of nanocrystalline Al-Zr

An investigation of the mechanical properties of nanocrystalline Al-Zr alloy composites has been conducted via nanoindentation and Vickers microhardness experiments. The microhardness of the samples exhibits a four-fold increase over the concentration range of 0-30 wt.% Zr, from {approximately}0.7 GPa to nearly 3 GPa. The aluminum grain size is found to be strongly correlated with the level of zirconium present in the samples, suggesting that the observed hardness increase can be attributed to the combined effects of alloying and grain size reduction. The elastic moduli of the nanocrystalline Al-Zr samples are determined to be similar to the modulus of coarse-grained aluminum and independent of zirconium content

Lattice Resistance and Peierls Stress in Finite-size Atomistic Dislocation Simulations

Atomistic computations of the Peierls stress in fcc metals are relatively scarce. By way of contrast, there are many more atomistic computations for bcc metals, as well as mixed discrete-continuum computations of the Peierls-Nabarro type for fcc metals. One of the reasons for this is the low Peierls stresses in fcc metals. Because atomistic computations of the Peierls stress take place in finite simulation cells, image forces caused by boundaries must either be relaxed or corrected for if system size independent results are to be obtained. One of the approaches that has been developed for treating such boundary forces is by computing them directly and subsequently subtracting their effects, as developed by V. B. Shenoy and R. Phillips [Phil. Mag. A, 76 (1997) 367]. That work was primarily analytic, and limited to screw dislocations and special symmetric geometries. We extend that work to edge and mixed dislocations, and to arbitrary two-dimensional geometries, through a numerical finite element computation. We also describe a method for estimating the boundary forces directly on the basis of atomistic calculations. We apply these methods to the numerical measurement of the Peierls stress and lattice resistance curves for a model aluminum (fcc) system using an embedded-atom potential.Comment: LaTeX 47 pages including 20 figure

High-Temperature Stress-Strain Behavior of MgO in Compression

Compressive stress-strain curves for several types of polycrystalline MgO specimens were correlated with those for single crystals and analyzed as a function of grain size and grain-boundary character at 1200 and 1400 C for several strain rates. The results for fully dense specimens were explained in terms of grain-boundary sliding and intergranular separation in addition to slip. The modification of grain-boundary nature concurrent with heat treatment for grain growth, caused by residual LUF, was associated with enhanced grain-boundary sliding and intergranular separation. For grain sizes <30 {micro}m, it was concluded that the von Miss criteria for ductility could be relaxed by the Occurrence of dislocation climb and, to a limited extent, by intergranular separation. Yield drop corresponding to dislocation multiplication occurred when grain-boundary sliding was initially promoted. Specimens with a liquid phase of adequate viscosity also indicated plasticity accompanied by high strength. Specimens with clean grain boundaries exhibited ductility and normal strain hardening with no intergranular separation

Atomic-scale modeling of the deformation of nanocrystalline metals

Nanocrystalline metals, i.e. metals with grain sizes from 5 to 50 nm, display technologically interesting properties, such as dramatically increased hardness, increasing with decreasing grain size. Due to the small grain size, direct atomic-scale simulations of plastic deformation of these materials are possible, as such a polycrystalline system can be modeled with the computational resources available today. We present molecular dynamics simulations of nanocrystalline copper with grain sizes up to 13 nm. Two different deformation mechanisms are active, one is deformation through the motion of dislocations, the other is sliding in the grain boundaries. At the grain sizes studied here the latter dominates, leading to a softening as the grain size is reduced. This implies that there is an ``optimal'' grain size, where the hardness is maximal. Since the grain boundaries participate actively in the deformation, it is interesting to study the effects of introducing impurity atoms in the grain boundaries. We study how silver atoms in the grain boundaries influence the mechanical properties of nanocrystalline copper.Comment: 10 pages, LaTeX2e, PS figures and sty files included. To appear in Mater. Res. Soc. Symp. Proc. vol 538 (invited paper). For related papers, see http://www.fysik.dtu.dk/~schiotz/publist.htm

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

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