Screw Dislocation Emission Research Articles

Spallation of polycrystalline tungsten using laser-induced shock at ultrahigh strain rates

Laser-induced shock technology was used to achieve the shock loading of polycrystalline tungsten (W) at an ultrahigh strain rate. The shock response and fracture behavior of W were investigated at different tensile strain rates. Unlike the intergranular fracture mode under quasistatic conditions, the transgranular–intergranular-mixed fracture mode of W was detected at high strain rates and the proportion of transgranular cleavage mode increased. The spall strength of W was positively correlated to the strain rate, reaching 5.86 GPa when the tensile strain rate reached 106 s−1 in the spall plane. The mobility of screw dislocations increased at high strain rates, and the simultaneous emission of screw dislocations improved the spall strength while accelerating the expansion of crack nucleation positions, inducing numerous transgranular fractures.

On the influence of crack front curvature on the fracture behavior of nanoscale cracks

Atomic-scale fracture processes are traditionally investigated in quasi-2D models of straight, infinite cracks. This approach neglects crack front curvature effects, which might be particularly important for nanoscale crack nuclei in semi-brittle materials. Here we use 3D atomistic simulations to study penny-shaped cracks in body-centered cubic metals. Our results show extensive crack tip plasticity initiated by deformation twinning and followed by emission of screw dislocations which cross-slip along the crack front. Together with the interactions of dislocations and/or twins that are nucleated at differently oriented parts of the crack, these processes determine the fracture behavior of highly curved nanoscale cracks.

Molecular dynamics study of deformation and fracture in a tantalum nano-crystalline thin film

We present results from molecular dynamics simulations of two nano-crystalline tantalum thin films that illuminate the variety of atomic-scale mechanisms of incipient plasticity. Sample 1 contains approximately 500 K atoms and 3 grains, chosen to facilitate study at 105 s−1 strain rate; sample 2 has 4.6 M atoms and 30 grains. The samples are loaded in uniaxial tension at deformation rates of 105–109 s−1, and display phenomena including emission of perfect 1/2〈1 1 1〉-type dislocations and the formation and migration of twin boundaries. It was found that screw dislocation emission is the first deformation mechanism activated at strain rates below 108 s−1. Deformation twins emerge as a deformation mechanism at higher strains, with twins observed to cross grain boundaries as larger strains are reached. At high strain rates atoms are displaced with the characteristic twin vector at a ratio of 3 : 1 (108 s−1) or 4 : 1 (109 s−1) to characteristic dislocation vectors. Fracture is nucleated through a nano-void growth process. Grain boundary sliding does not scale with increasing strain rate. Detailed analysis of nano-scale deformation using these tools enhances our understanding of deformation mechanisms in tantalum.

Shielding Effect and Emission Criterion of a Screw Dislocation Near an Interfacial Crisscross Crack

Shielding effect and emission criterion of a screw dislocation near an interfacial crisscross crack are dealt with in this paper. Utilizing the conformal mapping technique, the closed-form solutions are derived for complex potentials and stress fields due to a screw dislocation located near the interfacial crisscross crack. The stress intensity factor on the crack tips and the critical stress intensity factor for dislocation emission are also calculated. The influence of the orientation of the dislocation and the morphology of the crisscross crack as well as the material elastic dissimilarity on the shielding effect and the emission criterion is discussed in detail. The results show that positive screw dislocations can reduce the stress intensity factor of the interfacial crisscross crack tip (shielding effect). The shielding effect increases with the increase of the shear modulus of the lower half-plane, but it decreases with the increase of the dislocation azimuth angle and the distance between the dislocation and the crack tip. The critical loads at infinity for dislocation emission increases with the increase of emission angle and the vertical length of the crisscross crack, and the most probable angle for screw dislocation emission is zero. The present solutions contain previous results as special cases.

Shielding effect and emission condition of a screw dislocation near a blunt crack in elliptical inhomogeneity

The shielding effect and emission condition of a screw dislocation near a blunt crack in elastic elliptical inhomogeneity is dealt with. Utilizing the Muskhelishvili complex variable method, the explicit series form solutions of the complex potentials in the matrix and the inclusion regions are derived. The stress intensity factor and critical stress intensity factor for dislocation emission are also calculated. The influences of the orientation of the dislocation and morphology of the blunt crack as well as the material elastic dissimilarity upon the shielding effect and emission criterion are discussed in detail. As a result, numerical analysis and discussion show that the positive screw dislocation can reduce the stress intensity factor of the crack tip (shielding effect) only when it is located in the certain region. The shielding effect increases with the increase of the shear modulus of the matrix and the curvature radius of the blunt crack tip, but decreases with the increase of dislocation azimuth angle. The critical loads at infinity for dislocation emission increases with the increment of the emission angle and the curvature radius of the blunt crack tip, and the most probable angle for screw dislocation emission is zero. The present solutions contain previous results as the special cases.

Shielding effect and emission criterion of a screw dislocation near an interfacial blunt crack

Shielding effect and emission criterion of a screw dislocation near an interfacial blunt crack are dealt with in this paper. Utilizing the conformal mapping technique, the closed-form solutions are derived for complex potentials and stress fields due to a screw dislocation located near the interfacial blunt crack. The stress intensity factor on the crack tips and the critical stress intensity factor for dislocation emission are also calculated. The influence of the orientation of the dislocation and the morphology of the blunt crack as well as the material elastic dissimilarity on the shielding effect and the emission criterion is discussed in detail. The results show that positive screw dislocations can reduce the stress intensity factor of the interfacial blunt crack tip (shielding effect). The shielding effect increases with the increase of the shear modulus of the lower half-plane, but it decreases with the increase of the dislocation azimuth angle. The critical loads at infinity for dislocation emission increases with the increase of emission angle and curvature radius of blunt crack tip, and the most probable angle for screw dislocation emission is zero. The present solutions contain previous results as special cases.

Atomic-Scale Analysis of Strain Relaxation Mechanisms in Ultra-Thin Metallic Films

AbstractA comprehensive computational analysis is presented of the atomistic mechanisms of strain relaxation over a wide range of applied biaxial tensile strain in free-standing Cu thin films. The analysis is based on large-scale isothermal-isostrain MD simulations using slab supercells with cylindrical voids normal to the film plane and extending throughout the film thickness. Our analysis has revealed various regimes in the film's mechanical response as the applied strain level increases. Following an elastic response at low strain (≶ 2%), plastic deformation occurs accompanied by emission of screw dislocations from the void surface and threading dislocations from the film surfaces, in parallel with generation of vacancies due to slip of jogged dislocations. At the lower strain range following the elastic-to-plastic deformation transition (⋚ 6%), void growth is the major strain relaxation mechanism, while at higher levels of applied strain (≥ 8%), a subsequent transition leads to a new plastic deformation regime where void growth plays a negligible role in the film strain relaxation.

Emission of screw dislocations from the surface of a thin defect into an isotropic medium

By using the method of jump functions, we investigate the quasistatic stress field and stress concentration near a thin strip at the interface between materials during emission of screw dislocations from it into the medium. We construct analytic solutions in the cases of finite and semiinfinite cracks and perfectly rigid films.

Dynamic emission of screw dislocations from a propagating crack tip

The dynamic emission of screw dislocations from a propagating crack tip has been investigated. The crack velocity and velocity of each dislocation are assumed to be proportional to the third power of the stress intensity factor and the effective stress, respectively. The stress intensity factor for the ith dislocation emission is derived according to the spontaneous emission criterion. It is found that the stress intensity factor for dislocation emission is independent of crack mobility but the number of dislocations emitted decreases with increasing crack mobility. The number of emitted dislocations and the size of the plastic zone decrease but the maximum stress intensity factor for dislocation emission increases with increasing initial crack length. The size of the dislocation-free zone increases with increasing initial crack length for steady state. The dislocation–crack system reaches the steady state when the dislocations and crack tip move at the same velocity.

The effect of cracks on the superconducting transport current in thin films: The analogy with two-dimensional elasticity and plasticity

Simulations of arrays of resistively shunted Josephson junctions containing a crack of uncoupled junctions indicate that the crack can distort the supercurrent flow and provide a nucleation site at the crack tip for the formation of superconducting vortices at applied currents below the critical current of the homogeneous material. An analogy is established between the supercurrent distribution in two dimensions and the stress field distribution around the crack for antiplane mechanical loading. The analogy is used to show that the supercurrent distribution can be described analytically in terms of a Westergaard function used in elasticity theory. In addition, using a correspondence between the forces acting on a vortex and a crystal dislocation, models for screw dislocation emission from a crack tip are transposed to describe vortex emission from a crack tip. These lead to predictions for the pinning force required to prevent dissipation by vortex emission from the crack tip, as well as for the size of a vortex zone ahead of the crack for different values of the ratio of the applied current to the pinning force.

Dynamic and optical properties of screw dislocations introduced by plastic deformation of CdS crystals at 77-4.2 K

The dislocation loop shapes, dislocation motion and piezospectroscopy of screw dislocation emission are studied using a spatially resolved photoluminescence at T = 77 to 4.2 K. The symmetry of dislocation centers in the screw dislocation core is found. Optical properties of the screw dislocations are discussed.

Transient emission of screw dislocations from a dynamically-loaded interface crack

Exact transient solutions for the motion and arrest of a screw dislocation at an interface crack subjected to the diffraction of incident plane SH-waves are obtained, and then examined in light of a dislocation force-based emission criterion. It is found that, unless deceleration is allowed, dislocation arrest may be temporary. Expressions for the time of emission and the arrest distance are derived in terms of material properties such as the interface lattice friction, and a dimensionless variable related to the size of a zone of attraction at the crack edge. For both step-stress and sinusoidal incident SH-waves, the expressions show that emission will initiate in a minimum time at zero speed, and that emission at a critical speed can occur only after an infinite time. The magnitudes of the expressions indicate that the emission process is micromechanical in scale, and that incident wave frequencies may have to be in the MHz range in order to trigger emission. Study of the dynamic stress intensity factor shows that emission-generated crack edge shielding may not necessarily occur, and that emission will generally preclude purely brittle debonding.

Incident Wave and Dislocation Acceleration Effects in Dislocation Emission From Cracks

Some previous transient analyses of dislocation emission from dynamically loaded cracks have treated glide at constant speeds, and have invoked an emissions criterion that allows the dislocation force to exceed the yield stress level during glide. In the context of nonstrain-hardening plasticity, this analysis requires that the force remain at the yield stress level. For an exact solution to the problem of screw dislocation emission from a crack subjected to plane SH-wave diffraction, the result is an equation of motion for the dislocation, which can be integrated exactly. The dislocation is found to accelerate during glide to a high but subcritical speed, before decelerating to its arrest position. A general incident SH-wave is considered, but is then specialized to the cases of a step-stress and a sinusoidal wave. Calculations on the basis of material and wave parameters show that purely brittle fracture can be difficult to achieve, and that wavelength/frequency cutoffs exist, beyond which emission cannot occur.

Transient studies of screw and edge dislocation generation at crack edges

Exact transient analyses of the generation of screw and edge dislocations at the edges of stationary cracks subjected to the diffraction of, respectively, plane SH- and SV-waves, and their subsequent arrest are performed. The solutions are examined in light of a dislocation emission criterion which is based, simultaneously, on standard dislocation force concepts and quasi-static emission studies. This examination allows expressions for the times of emission and arrest, the distances traveled by the dislocations, and the dynamic stress intensity factors to be derived in terms of parameters such as dislocation speed and yield stress. These expressions exhibit distinctive dynamic effects and reveal several features of the generation process: In particular, the times and distances are, while on a micromechanical scale, not necessarily insignificant, and imply that purely brittle fracture may not easily occur. Then, edge dislocation emission would appear to occur at a preferred speed, while screw dislocation emission apparently prefers to take place quasi-statically. Examination of a general incident waveform class shows that a continuous wave could cause dislocation generation to occur before a step-stress wave can. Moreover, the emission process depends upon a weighted time history of the incident wave stress, not its instantaneous value. This, in turn, implies that dislocation emission does not necessarily shield the crack edge by lowering the dynamic stress intensity factor. Finally, unless the dislocations are allowed to decelerate to zero speed upon arrest, a repetitious process of start-stop motion can in principle take place.

An Exact Transient Study of Dislocation Emission and its Effects on Dynamic Fracture Initiation

Closed-form transient solutions for the micromechanical process of screw dislocation emission from a stationary crack which is subjected to SH-wave diffraction are presented. The dislocations are allowed to leave the crack edge in arbitrary directions, either singly or in pairs. Imposition of an emission criterion that is both based on the dislocation force concept and is similar to criteria applied to quasi-static emission studies allows expressions for the instants of dislocation emission and arrest and the distance traveled by the dislocation to be obtained. These expressions are studied for their dependence on parameters such as emission direction and speed, and several distinctive dynamic effects are observed. A standard fracture criterion is then imposed, and conditions for determining whether fracture will precede or follow emission are established in terms of real time. Finally, some estimates for the orders of magnitudes of the parameters involved in this micromechanical process are given.

Transient analysis of dislocation emission from a growing crack

A n exact transient solution for screw dislocation emission in arbitrary directions from a Mode III crack growing due to SH-wave diffraction is performed. An emission criterion based on the dislocation force is then imposed to obtain the instant of emission, the distance traveled by the emitted dislocation prior to its arrest, and the instant of arrest. The formulae for these quantities are examined for dependence on parameters such as emission angle, material yield stress and crack and dislocation speeds. Dynamic effects are clear, and order-of-magnitude evaluations suggest that the quantities are on a micromechanical scale, but not insignificant. In particular, the emission instant behavior indicates that purely brittle fracture is difficult and, moreover, that the instant itself might be incorporated in the emission criterion. Indeed, general suggestions for rendering the criterion more precise are noted. The solution and formulae are then modified to study multiple emission, post-arrest behavior, emission in the crack plane and the response of the dynamic stress intensity factor. These studies indicate that dislocation shielding is enhanced in a transient analysis but decreases after dislocation arrest occurs, and that dislocation arrest can be momentary.