Emission Of Dislocation Loops Research Articles

On the micromechanics of void growth by prismatic-dislocation loop emission

Experimental evidence and recent molecular dynamics simulations of void growth indicate that prismatic dislocation loop emission by externally applied stresses is a viable mechanism of void growth under shock loading conditions when diffusive processes are given no time to operate. In this paper, the process of growth by loop emission is studied in a model system comprised of a void in an infinite linearly elastic and isotropic solid loaded axisymmetrically by remote applied stresses. First, the interaction between applied stresses, the stress field of a single dislocation loop or a pile-up of loops next to the void, the surface energy expenditure on void surface change, and the lattice resistance to the motion of loops is reviewed. The necessary condition for interstitial loop emission is used to determine the equilibrium positions of the loops as well as the maximum number of loops in a pile-up under given applied stresses. For the parameters of the model-material with purely hydrostatic loading, the numerical results yield a volume change for the void, which when normalized by the initial undeformed volume, exhibits a strong dependence on the size of the void for radii less than ∼400 times the lattice Burgers vector. For larger voids, the normalized volume change was found to be independent of the void radius.

Laser shock compression of copper and copper–aluminum alloys

Copper and copper aluminum (2 and 6 wt% Al) with orientations [0 0 1] and [1 3 4] were subjected to high intensity laser shocks (energy levels of 40–300 J; energy densities of 15–70 MJ/m 2 and durations below 10 ns). In situ X-ray diffraction and VISAR wave profile measurements were used to study the response of the lattice to the shock. Recovered samples were characterized by transmission electron microscopy in order to study the defects within the lattice. The results are rationalized in terms of a criterion in which slip and twinning are considered as competing mechanisms. The slip-twinning transition is determined quantitatively and predicted as a function of orientation, temperature, and stacking fault energy (determined by the Al content of the alloy). A constitutive description is applied to the two orientations and differing compositions, incorporating both slip and twinning in terms of orientation, stacking fault energy, temperature, and strain rate. The threshold stress is calculated, considering the effect of shock heating. The constitutive description provides calculated thresholds in pure copper: 18 GPa for [0 0 1] and 25 GPa for [1 3 4]. The experimentally determined threshold twinning stress for pure copper in the [0 0 1] orientation is 25 GPa, whereas the one for the [1 3 4] orientation is between 40 and 60 GPa. The predictions are in agreement with experiments. It also provides thresholds for the Cu–Al alloys. The growth of voids in incipient spalling is modeled by dislocation loop emission from the void surface. The first dislocation-based void growth mechanism, proposed by Lubarda et al. [Acta Mat 2004; 52: 1397] is reviewed and extended to three dimensions. It has a generality rendering it applicable to a broad range of strain rates.

Atomistic study of dislocation loop emission from a crack tip.

We report the first atomistic calculation of the saddle-point configuration and activation energy for the nucleation of a 3D dislocation loop from a stressed crack tip in single crystal Cu. The transition state is found using reaction pathway sampling schemes, the nudged elastic band, and dimer methods. For the (111)[110] crack, loaded typically at 75% of the athermal critical strain energy release rate for spontaneous dislocation nucleation, the calculated activation energy is 1.1 eV, significantly higher than the continuum estimate. Implications concerning homogeneous dislocation nucleation in the presence of a crack-tip stress field are discussed.

Atomistic Simulations of Dislocation-Loop Glidings in Al(111) Nanoindentation

Molecular dynamics simulation of nanoindentation on Al(111) surface is presented. The simulation is performed using the Ercolessi-Adams glue potential and the Berendsen thermostat. Boundary conditions of 'pseudo' thin film are imposed in order to focus on the dislocation motion in ultra-thin film. Nucleation and development of defects underneath the indenter tip are visualized, and the gliding patterns of dislocation loops are investigated with particular emphasis on the effect of film thickness. Simulation results show that the early emission of dislocation loop is highly dependent on the film thickness.

Quantitative transmission electron microscopy analysis of the pressure of helium-filled cracks in implanted silicon.

The pressure of crack-shaped cavities formed in silicon upon implantation with helium and subsequent annealing is quantitatively determined from the measurement of diffraction contrast features visible in transmission electron micrographs taken under well-defined dynamical two-beam conditions. For this purpose, simulated images, based on the elastic displacements associated with a Griffith crack, are matched to experimental micrographs, thus yielding unambiguous quantitative data on the ratio p of the cavity pressure to the silicon matrix shear modulus. Experimental results demonstrate cavity radii of some 10 nm and p values up to 0.22, which may be regarded as sufficiently high for the emission of dislocation loops from the cracks.

Dislocation emission from a crack tip in MgO thin crystals

Dislocation emission from the tip of a crack in MgO thin crystals has been studied based on the geometry of crack tip dislocations observed using high voltage electron microscopy (HVEM). The effect of crack tip shielding due to the dislocations was also calculated using three-dimensional (3D) Bueckner–Rice weight function theory for the crack–dislocation interaction. Dislocation image analyses showed that dislocations ahead of the crack tip observed in the present study were almost right-handed (R-H) screw dislocations lying on the (0 1 ̄ 1) plane, while in the crack wake many of the dislocations were left-handed (L-H) on the (0 1 ̄ 1) plane. Formation of such dislocation configuration can be understood by the emission of dislocation loops from sources at a crack tip, where crack jogs, crack kinks and intersections of crack planes with free surfaces may act as significant sources for dislocation emission. 3D stress analysis exhibits that the largest component of crack tip stress intensities induced is mode I shielding type, and that mode II component is not negligible in the present case, suggesting the induction of the crack tip shielding for the mixed stress modes of I and II through the dislocation emission from the sources indicated above.

Dislocation loop evolution from a source in a field of randomly distributed obstacles with a discrete strength spectrum

We have simulated dislocation loop emission by a Frank-Read source, and its evolution in a field of randomly distributed point obstacles with a dicrete strength spectrum. The obstacles and their strengths are interpreted as the intersections of a dislocation loop with the base plane of the source. It is observed that behind the front of the expanding dislocation segment there remain islands of incomplete crystallographic slip that are hard-to-overcome regions of the glide plane that the dislocations bypass by the Orovan mechanism. We present the quantitative characteristics of the islands, and we classify them. We discuss the dynamical effects associated with pinching off a loop from a dislocation bounding the region of an expanding crystallographic glide. These effects must also be taken into account when describing the emission spectrum accompanying plastic deformation.

Dislocation loop evolution from a source in a field of randomly distributed uniform obstacles

We have created a computer simulation of dislocation loop emission by a Frank-Read source and its evolution in a uniform field of randomly distributed, weak, point obstacles. We discovered a threshold source length such that dislocation loops from shorter sources expand in a super-obstacle regime; otherwise they enlarge by thermal activation, and there is a relationship to the strength of the obstacle. We determine the range of stresses in which dislocation loops expand above the obstacle, shrink, or grow via thermal activation. In contrast with infinite linear dislocations, we do not observe stable evolution (i.e., velocity and other characteristics) of dislocation loops in all stress regimes.

Molecular-Dynamics Simulations Of Fracture: An Overview Of System Size And Other Effects

AbstractWe have studied brittle and ductile behavior and their dependence on system size and interaction potentials, using molecular-dynamics (MD) simulations. By carefully embedding a single sharp crack in two- and three-dimensional crystals, and using a variant of the efficient sound-absorbing reservoir of Holian and Ravelo [Phys. Rev. B 51, 11275 (1995)], we have been able to probe both the static and dynamic crack regimes. Our treatment of boundary and initial conditions allows us to elucidate early crack propagation mechanisms under delicate overloading, all the way up to the more extreme dynamic crack-propagation regime, for much longer times than has been possible heretofore (before unwanted boundary effects predominate). For example, we have used graphical display of atomic velocities, forces, and potential energies to expose the presence of localized phonon-like modes near the moving crack tip, just prior to dislocation emission and crack-branching events. We find that our careful MD method is able to reproduce the ZCT brittle-ductile criterion for short-range pair potentials [static lattice Green's function calculations of Zhou, Carlsson, and Thomson, Phys. Rev. Letters 72, 852 (1994)].We report on progress we have made in large-scale 3D simulations in samples that are thick enough to display realistic behavior at the crack tip, including emission of dislocation loops. Such. calculations, using our careful treatment of boundary and initial conditions - especially important in 3D - have the promise of opening up new vistas in fracture research.

Microplasticity of MMC. Experimental results and modelling

The microplastic behavior of several MMC is invesbgated by means of tension and compression tests. This behavior is assymetric: the proportionnal limit is higher in tension than in compression but the work hardening rate is higher in compression. These differences are analysed in terms of maxium of the Tresca's shear stress at the interface (proportionnal limit) and of the emission of dislocation loops during the cooling (work hardening rate). On another hand, a model is proposed to calculate the value of the yield stress, describing the composite as a material composed of three phases: inclusion, unaffected matrix and matrix surrounding the inclusion having a gradient in the density of the thermally induced dilocations

Crystal defects in Cd 1-xZn xS/GaAs heterostructures prepared by vapour phase chemical transport

X-ray diffraction techniques have been used for investigating the crystal perfection of Cd 1- x Zn x S/GaAs heteroepitaxial structures prepared by a vapour phase chemical transport method using the close-spaced geometry and H 2 as a transport agent. Composition gradients in Cd 1- x Zn x S epitaxial layers have been related to particle sizes of starting powders. Strong interdiffusion processes between layer and substrate have been evidenced. The defect configuration of the substrate-layer interface, as seen by X-ray scanning reflection topography, appeared to be controlled by precipitation phenomena of diffusing metal impurities around dislocations, followed by emission of dislocation loops.

In situ observation of dissolution of cementite in steel

SUMMARYThe mechanisms of dissolution of cementite particles in steel have been studied in situ by high voltage electron microscopy. This method permits direct, continuous observations of the process and provides new information which has not been obtained without this technique. During dissolution of cementite particles in austenite at about 770°C, dislocation helices, loops and complex dislocation configurations are formed from the interfaces of the particles. Dissolution of cementite in ferrite during decarburization at about 700°C also causes the formation and movement of dislocations. However, in contrast to the case of dissolution in austenite, the emission of dislocation loops and helices has not been observed. The migration of crystallographic and noncrystallographic interfaces has been observed during the ferrite to austenite transformation. The noncrystallographic interface is apparently attracted to the cementite particles. The migration rate of the noncrystallographic interfaces is five to ten times faster than that of the crystallographic interface.

Ostwald ripening of precipitates in solid matrices

In the aging of a precipitate at constant volume fraction of dispersed phase, the stage when large particles grow at the expense of smaller ones (Ostwald ripening) is driven by the interfacial free energy, γ. Hence, the kinetics of the coarsening may be used to determine γ if proper account is taken of the volume-transfer problem. It is shown that in the absence of dislocation loop emission, one of two kinds of coupling of diffusive fluxes occurs, as determined by the rate of entropy production. The effective diffusivity is calculated for an interstitial solid solution (αFe-C), a coherent precipitate system (Cu-Co), and oxide-in-metal type of system, and the modified Lifshitz-Wagner equation is applied to literature data. The thermodynamics of small precipitate particles subjected to hydrostatic pressure by the solid matrix is developed. Quantitative evaluation of γ for a coherent system is not possible because of the inability to evaluate the solute concentration for strained systems, although a qualitative understanding of the data for Cu(Co) is presented.