Laser-induced shock compression of monocrystalline copper: characterization and analysis
TL;DR: In this paper, a method for estimating dislocation densities is proposed, based on nucleation of loops at the shock front and their extension due to residual shear stresses behind the front.
About: This article is published in Acta Materialia.The article was published on 2003-03-14. It has received 219 citations till now. The article focuses on the topics: Slip (materials science) & Crystal twinning.
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TL;DR: In this article, a femtosecond pulsed laser irradiation on oriented enstatite single crystals (En93Fs7) was used to investigate the deformation behavior and its orientation dependence.
Abstract: Space weathering by micrometeoroid bombardment is a cosmic phenomenon on atmosphere-free celestial bodies, a process that is expected to particularly overprint planetesimals and cosmic dust in debris discs. We reproduced micrometeoroid impact craters by femtosecond pulsed laser irradiation on oriented enstatite single crystals (En93Fs7) to investigate the deformation behavior and its orientation dependence. All microcraters show typical bowl shaped morphologies, a glass surface layer with splash like ejecta material and subsurface layering. Although we could reproduce melting and vaporization as typical space weathering effects in the enstatite experiments, there is no formation of agglutinate particles or metallic nanoparticles (npFe0). The shock effects in the deformation layer consist of planar structures like microfractures and cleavages, amorphous lamellae, stacking faults and clinoenstatite lamellae. Their activation and/or orientation depends on the shock direction. In special orientations we observe the activation of glide systems along specific low indexed crystallographic planes. Due to the short timescale and the high strain rates, the most prominent effect is the failure of enstatite by microfracturing along non-rational crystallographic planes. Common deformation mechanisms reported in meteorites like the formation of clinoenstatite lamellae via shearing along [001] (100) occur less frequently. Shear is apparently the dominant mechanism in the formation of the above-mentioned effects and causes also their modification by frictional heating. The wide-spread formation of amorphous lamellae is, for example, interpreted to be the result of this shear heating along planar structures. We interpret this unconventional deformation behavior as a consequence of the small spatial and temporal scale of the experiments, resulting in a short-lived spherical shock wave with high deviatoric stresses in contrast to a long pressure pulse and quasi-hydrostatic compression in large scale impacts that produce typical shock features.
5 citations
01 Jan 2020
TL;DR: In this article, the effect of powder size and shape on the particle velocity and resulting deposition is discussed, as well as the influence of the powder grain structure on particle distortion, dislocation generation, and recrystallization.
Abstract: Additive manufacturing is based on the concept of freeform structures built up using a consecutive layer-by-layer material deposition approach, enabling the production of complex and functional components in a single manufacturing step. It allows the creation of high complexity components with minimal time and cost, as opposed to traditional subtractive manufacturing techniques. Current metallic AM technologies include selective laser melting, directed energy deposition, laser engineered net shaping, and plasma spraying. Although used commercially, these processes all suffer from the detrimental effects of high temperature processing, generally resulting in component distortion, uncontrolled phase transformations, undesirable residual stresses, and non-uniform mechanical properties. The cold spray process has recently gained attention in the additive manufacturing field as it may mitigate the undesirable thermal effects of current freeform manufacturing techniques, as well as drastically increase the available deposition rates. In cold spray, feedstock particles are injected in a supersonic gas flow and accelerated to velocities as high as 1200 m/s prior to impact. This high impact velocity is responsible for the material consolidation in the cold spray process. The particle impact velocity is dictated by the particle/gas flow interaction, which can be altered through the modification of the gas stagnation properties and spray nozzle geometry. While the effect of the gas/particle interaction is typically the focus of most cold spray research, it has become apparent that the size, shape, microstructure and quality of the feedstock powder have a large influence on the process efficiency. Hence, the effect of powder properties needs to be properly explored, understood, and considered in the powder selection process. This chapter aims to provide a complete reference on the effect of the feedstock particles on deposition quality in an additive manufacturing framework. It should provide the reader with a comprehensive resource for powder selection, pre-treatment, and storage. The effect of powder morphology will be presented. A descriptive analysis of the manufacturing processes used to produce particles will be included. The broad effect of powder size and shape on the particle velocity and resulting deposition will be discussed. Furthermore, the influence of the powder grain structure on particle distortion, dislocation generation, and recrystallization will be described on the basis of high strain rate deformation processes. Beyond the expected properties of the feedstock materials, it is also apparent that the “quality” of the powder is of great importance. The powder quality is thoroughly described by oxygen content and oxide layer type and thickness. This quality has been shown to greatly influence the process efficiency for some materials, and best practices for handling and storage of the powders is discussed. Finally, the status of powder recycling methods in cold spray will be considered along with the advantages of reprocessing in the field of additive manufacturing.
5 citations
TL;DR: In this paper, a transient analysis of a dislocation accelerating through the shear wave speed barrier is presented, where the stress on the forming Mach front is analyzed, as well as the effect of the acceleration on the curvature of the front from which, inversely, information about the motion may be inferred.
5 citations
TL;DR: In this article, the effect of random populations of dislocations on a material is examined through stochastic integration of a random cloud of disllocations lying at some distance away from a material point.
Abstract: The effect that random populations of dislocations have on a material is examined through stochastic integration of a random cloud of dislocations lying at some distance away from a material point. The problem is studied in one, two, and three dimensions. In 1D, the cloud consists of individual edge dislocations placed along the real line; in 2D, of edge dislocations and edge dipoles on the plane; in 3D, of dislocation loops. In all cases, the dislocation cloud is randomly distributed in space, associated to which several relevant physical parameters, including the material’s slip geometry, the dislocation’s sign, and its relative orientation, are also stochastically treated. A fully disordered population, i.e., one where the dislocation’s signatures and orientations are entirely random, is first studied. It is shown that such disordered systems entail a strong indeterminacy in the collective stress fields, which here is solved by enforcing mass conservation locally. In 2D, this is achieved by modelling a cloud of edge dipoles instead of individual dislocations; in 3D, this is naturally guaranteed by the modelling of closed dislocation loops. The long-range fields of the dipoles in 2D and of the loops in 3D is modelled via their multipolar force expansions, which greatly simplifies the analytical treatment of the problem. The cloud’s effect is then studied by performing the stochastic integration of the multipolar fields via Campbell’s theorem. The local order, but not the magnitude of the dislocation density, is shown to be critical in contributing to the plastic relaxation of the material: fully disordered systems are shown to self-attenuate, leading to plastic neutrality; ordered and partially ordered systems, achieved when dislocation signatures are aligned, display a direct relationship between the dislocation density and the average stress shielding the material. We establish and generalise the conditions that a system of dislocations must fulfil to display Taylor’s equation and the Hall–Petch relation, and offer adequate scaling laws related to this.
5 citations
TL;DR: In this paper, a kinetic equation for the density of dislocations, which reflects the main stages of the formation of dislocation structures of different types in a shock wave, has been formulated based on the analysis of the interaction of two kinetic processes described by reaction-diffusion type equations for densities of mobile disllocations and dislocation forming immobile dipoles, respectively.
Abstract: A kinetic equation for the density of dislocations, which reflects the main stages of the formation of dislocation structures of different types in a shock wave, has been formulated based on the analysis of the interaction of two kinetic processes described by reaction-diffusion type equations for densities of mobile dislocations and dislocations forming immobile dipoles, respectively. It has been shown that an inhomogeneous (cellular) dislocation structure is formed at relatively low pressures behind the front of a shock wave, whereas a uniform distribution of the dislocation density with stacking faults appears at high pressures. The transition from a cellular dislocation density distribution to a uniformly distributed dislocations with stacking faults depends on the stacking fault energy γ
D
of the metal: the lower is the stacking fault energy, the lower is the pressure in the shock wave σ
c
at which the cellular dislocation structure transforms into the structure with a uniform dislocation density distribution. It has been found that the dependence of the critical pressure on the stacking fault energy γ
D
is described by the law σ
c
∼ (γ
D
/μb)2/3 (where μ is the shear modulus and b is the Burgers vector), which is confirmed in the experiment.
5 citations
References
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Book•
27 Sep 1994
TL;DR: In this paper, the authors present a method to produce dynamic deformation at high strain rates by using Shear Bands (Thermoplastic Shear Instabilities) and dynamic fracture.
Abstract: Dynamic Deformation and Waves. Elastic Waves. Plastic Waves. Shock Waves. Shock Waves: Equations of State. Differential Form of Conservation Equations and Numerical Solutions to More Complex Problems. Shock Wave Attenuation, Interaction, and Reflection. Shock Wave-Induced Phase Transformations and Chemical Changes. Explosive-Material Interactions. Detonation. Experimental Techniques: Diagnostic Tools. Experimental Techniques: Methods to Produce Dynamic Deformation. Plastic Deformation at High Strain Rates. Plastic Deformation in Shock Waves. Shear Bands (Thermoplastic Shear Instabilities). Dynamic Fracture. Applications. Indexes.
2,609 citations
"Laser-induced shock compression of ..." refers background or methods in this paper
...[22]....
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...The dislocation density can be expressed as a function of pressure, P, through one of the equations obtained directly from the Rankine–Hugoniot equations and the equation of state [22]:...
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...In a similar manner, the residual temperature, TR, can be obtained from [22]:...
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TL;DR: An improved description of copper and ironcylinder impact (Taylor) test results has been obtained through the use of dislocation-mechanics-based constitutive relations in the Lagrangian material dynamics computer program EPIC•2.
Abstract: An improved description of copper‐ and iron‐cylinder impact (Taylor) test results has been obtained through the use of dislocation‐mechanics‐based constitutive relations in the Lagrangian material dynamics computer program EPIC‐2. The effects of strain hardening, strain‐rate hardening, and thermal softening based on thermal activation analysis have been incorporated into a reasonably accurate constitutive relation for copper. The relation has a relatively simple expression and should be applicable to a wide range of fcc materials. The effect of grain size is included. A relation for iron is also presented. It also has a simple expression and is applicable to other bcc materials but is presently incomplete, since the important effect of deformation twinning in bcc materials is not included. A possible method of acounting for twinning is discussed and will be reported on more fully in future work. A main point made here is that each material structure type (fcc, bcc, hcp) will have its own constitutive beha...
1,718 citations
TL;DR: In this article, a constitutive expression for the twinning stress in BCC metals is developed using dislocation emission from a source and the formation of pile-ups, as rate-controlling mechanism.
1,366 citations
TL;DR: In this paper, a periodic relation between shear stress and atomic shear displacement is assumed to hold along the most highly stressed slip plane emanating from a crack tip, which allows some small slip displacement to occur near the tip in response to small applied loading and, with increase in loading, the incipient dislocation configuration becomes unstable and leads to a fully formed dislocation which is driven away from the crack.
Abstract: Dislocation nucleation from a stressed crack tip is analyzed based on the Peierls concept. A periodic relation between shear stress and atomic shear displacement is assumed to hold along the most highly stressed slip plane emanating from a crack tip. This allows some small slip displacement to occur near the tip in response to small applied loading and, with increase in loading, the incipient dislocation configuration becomes unstable and leads to a fully formed dislocation which is driven away from the crack. An exact solution for the loading at that nucleation instability is developed via the J -integral for the case when the crack and slip planes coincide, and an approximate solution is given when they do not. Solutions are also given for emission of dissociated dislocations, especially partial dislocation pairs in fcc crystals. The level of applied stress intensity factors required for dislocation nucleation is shown to be proportional to √γ us , where γ us , the unstable stacking energy, is a new solid state parameter identified by the analysis. It is the maximum energy encountered in the block-like sliding along a slip plane, in the Burgers vector direction, of one half of a crystal relative to the other. Approximate estimates of γ us are summarized and the results are used to evaluate brittle vs ductile response in fcc and bcc metals in terms of the competition between dislocation nucleation and Griffith cleavage at a crack tip. The predictions seem compatible with known behavior and also show that in many cases solids which are predicted to first cleave under pure mode I loading should instead first emit dislocations when that loading includes very small amounts of mode II and III shear. The analysis in this paper also reveals a feature of the near-tip slip distribution corresponding to the saddle point energy configuration for cracks that are loaded below the nucleation threshold, as is of interest for thermal activation.
1,320 citations
01 Jan 1940
TL;DR: In this paper, the size of a dislocation and critical shear stress for its motion were calculated for a single dislocation with respect to the size and motion of the dislocation.
Abstract: Calculations are made of the size of a dislocation and of the critical shear stress for its motion.
1,226 citations