Passage Of Dislocations Research Articles

Characterization of fracture behavior of a nickel-based using Charpy instrumented in different conditions of heat treatment and evaluation temperatures

Since gas turbine blades work in acute conditions, the study of their mechanical behavior, including impact, is one of the most important tasks recommended. The aim of this study was to investigate the impact behavior of IN738 superalloy welded by pulsed laser under different heat treatment conditions and different temperatures. The results of this study showed that the lowest fracture toughness was related to the specimens that were welded under casting conditions. This was due to the formation of liquation cracks during welding and due to the reaction between the (Ti,Ta)C carbide, γ-γ′ eutectic, and γ′ phase with the superalloy matrix. As the test temperature increased from ambient temperature to 600 °C, the impact toughness increased. This is attributed to the free presence of dislocations, which causes ductile behavior in the alloy. As the temperature increased further to 770 °C, the fracture toughness increased due to the increased strength of the superalloy yield due to locking of dislocations. At temperatures above 770 °C, the toughness of the superalloy was reduced again due to the decrease in strength due to the passage of dislocations through the obstacles.

Kinetics of low-temperature plasticity of nanocrystalline titanium

The temperature dependences of the offset yield strength, strain-rate sensitivity of deformation stress, and the activation volume of plastic deformation in VT1-0 nanocrystalline titanium, have been determined under quasistatic tension in the temperature range 4.2–395 K. Thermal activation analysis of experimental data has been performed. The low-temperature plasticity of nanocrystalline titanium is shown to be controlled by the thermally activated passage of dislocations through local barriers formed by interstitial impurity atoms. The empirical values of the theoretical parameters of the dislocation–impurity interaction have been determined.

Strength of Graphene-Coated Ni Bi-Crystals: A Molecular Dynamics Nano-Indentation Study.

Nanoindentation simulations are performed for a Ni(111) bi-crystal, in which the grain boundary is coated by a graphene layer. We study both a weak and a strong interface, realized by a and a twist boundary, respectively, and compare our results for the composite also with those of an elemental Ni bi-crystal. We find hardening of the elemental Ni when a strong, i.e., low-energy, grain boundary is introduced, and softening for a weak grain boundary. For the strong grain boundary, the interface barrier strength felt by dislocations upon passing the interface is responsible for the hardening; for the weak grain boundary, confinement of the dislocations results in the weakening. For the Ni-graphene composite, we find in all cases a weakening influence that is caused by the graphene blocking the passage of dislocations and absorbing them. In addition, interface failure occurs when the indenter reaches the graphene, again weakening the composite structure.

Subsurface damage and material removal of Al–Si bilayers under high-speed grinding using molecular dynamics (MD) simulation

By performing three-dimensional molecular dynamics (MD) simulations, the effects of the tool radius, depth of cut and grinding speed are thoroughly studied in terms of the workpiece deformation, material removal, dislocation movement, atomic trajectory, grinding temperature and average grinding force. The strength of ductile/brittle (Al/Si) bilayers is largely enhanced, because the interface can hinder the passage of dislocations. The interface in brittle/ductile (Si/Al) bilayers contributes to its ductility by increasing the movability of dislocations when gliding on it. The brittle to ductile transition of bilayers, which strongly depends on the interface debond energy, has a key role in controlling the dislocation slipping mechanism. The investigation also reveals that a larger tool radius, higher grinding speed or deeper depth of cut results in more chipping volume and higher grinding temperature in both bilayers. At the same machining parameters, the above changes in brittle/ductile (Si/Al) bilayers are more apparent than that in ductile/brittle (Al/Si) bilayers, since Si is stiffer and has a higher yield strength than Al.

Hardening and Strain Localisation in Helium-Ion-Implanted Tungsten

Tungsten is the main candidate material for plasma-facing armour components in future fusion reactors. In service, bombardment with energetic neutrons will create collision cascades that leave behind lattice defects. Helium, injected from the plasma and produced by transmutation, strongly interacts with these defects, modifying their behaviour and retention. Helium-ion-implantation provides an effective tool for examining helium-defect interactions and their effect on the properties of tungsten armour. We use nano-indentation to probe the mechanical properties of the shallow helium-ion-implanted layer. Comparison of spherical indents in unimplanted and helium-implanted regions of the same single crystal shows a large increase in hardness and substantial pile-up in the implanted material. The complex lattice distortions beneath indents are probed non-destructively using 3D-resolved synchrotron X-ray micro-diffraction. Reduced lattice rotations and indentation-induced residual strains in the ion-implanted material indicate a more confined plastic zone. This is confirmed by HR-EBSD and TEM observations. Together our results suggest that dislocation motion is initially obstructed by helium-induced defects. The obstacle strength of these defects is reduced by the passage of dislocations, leading to a strain-softening and slip channel formation. A constitutive law for 3D crystal plasticity finite element simulations is developed based on this hypothesis. Simulations of the indentation process successfully capture the helium-implantation induced changes in deformation behaviour observed in experiments. Importantly, the effects we observe are markedly different from previous observations for self-ion-implanted tungsten, highlighting that the exact nature of the implantation damage plays a critical role in determining mechanical property change.

The effect of helium implantation on the deformation behaviour of tungsten: X-ray micro-diffraction and nanoindentation

The effect of helium-implantation-induced defects on deformation behaviour is examined by comparing spherical nano-indents in unimplanted and helium-implanted regions of a tungsten single crystal. Helium-implantation increases hardness and causes large pileups. 3D-resolved X-ray micro-diffraction uniquely allows examination of the complex lattice distortions beneath specific indents. In the ion-implanted material we find reduced lattice rotations and residual strains due to indentation, indicating a more confined plastic zone. Together, our observations suggest that helium-induced defects initially act as efficient obstacles to dislocation motion, but are weakened by the subsequent passage of dislocations, causing a reduction in work hardening capacity.

Dislocations penetrating an Al/Si interface

We study indentation of a nanolayered material consisting of a Si top layer above an Al substrate, using molecular dynamics simulation. We focus on the activity of Si dislocations upon reaching the interface. We find that passage of the dislocations through the interface is possible, if the slip systems of the two crystals are aligned. Upon absorption at the interface, the Si dislocations generate slip which leads to 1-monolayer deep interface pits with well-defined steps; on the Al side dislocations and stacking fault planes are generated, which are pinned to the interface pit. For interfaces with not well aligned slip systems, the passage of dislocations is strongly suppressed. However, still interface pits, albeit with less well defined contours, and stacking fault planes aligned with the interface are created.

Experimental and numerical investigation of texture evolution and the effects of intragranular backstresses in aluminum alloys subjected to large strain cyclic deformation

Strain hardening in FCC metals and alloys can be accompanied by the formation of deformation induced dislocation microstructures. These microstructures tend to form dislocation cells or nets which are characterized by three-dimensional tangles of dislocations that serve as “cell walls” and less dense internal regions “cell interiors”. The presence of such dislocation microstructure induces intragranular backstresses due to blockage of dislocation passage leading to kinematic hardening at the macroscopic scale. The present work focuses on the effects of dislocation induced backstresses and the accompanying texture evolution on strain hardening behavior of AA 6063-T6 extruded aluminum alloy during cyclic forward-reverse-forward (FRF) simple shear deformation. The accompanying texture evolution is characterized using electron backscatter diffraction (EBSD) texture measurements. The development of the underlying dislocation substructure during the cyclic simple shear loading is determined using transmission electron microscopy (TEM). Furthermore, a new crystal plasticity backstress hardening model is proposed to incorporate the effects of intragranular backstresses due to the dislocation cells substructure. The hardening model is validated by comparing the numerically predicted results against the experimental cyclic simple shear flow response as well as texture evolution. An excellent agreement is found between the simulated and experimental results. In particular, the proposed model is able to predict the experimentally observed Bauschinger effect as well as the accompanying texture evolution during cyclic simple shear testing. The applicability of the proposed backstress hardening model is further validated by applying it to different cyclic strain paths (i.e. tension-compression-tension (TCT) and compression-tension-compression (CTC)) for different strain amplitudes. The numerically predicted results show good agreement to the corresponding experimental cyclic test results.

An atomistically informed kinetic Monte Carlo model of grain boundary motion coupled to shear deformation

The shear coupled motion of grain boundaries (GBs) is modelled by using two different atomistic simulation techniques: molecular dynamics (MD) and kinetic Monte Carlo (KMC). MD simulations are conducted to identify the elementary mechanisms that take place during the coupled motion of GBs. This process is described on the one hand, in terms of the geometrical approach of the dislocation content in the boundary; and on the other hand, by the thermodynamics of the dislocation passage, shown as a thermal activated process. Relevant MD output is extended into a KMC model that considers the GB migration as a result of a sequence of discrete rare events. The independent motion of each structural unit forming the boundary conforms a single event, having a rate per unit of time to move to the next stable position computed according to the transition state theory. The limited time scale of classical MD is overcome by KMC, that allows to impose realistic deformation velocities up to 10μm/s.

High temperature strain hardening behavior in double forged and potassium doped tungsten

The strain-hardening behavior of two recently developed double forged and K-doped tungsten grades in the 300–2000°C range was analyzed applying a phenomenological model describing the evolution of the flow stress as a function of the dislocation density. The applied model allowed establishing a correlation between the strain hardening curvature and the size of microstructural features controlling the dislocation multiplication. The obtained results demonstrated that plastic deformation was controlled by the resistance of the low angle grain boundaries below 1000°C and the high angle grain boundaries at 1500°C and above. The experimental results obtained at different loading rates showed that thermal activation was essential for the passage of dislocations through grain boundary interfaces at 1000°C and above. The limitations of the applied model and need for further development of the physical model accounting for stress- and temperature-induced grain growth are discussed.

Criterion for the softening effect in crystalline materials under their doping

A dislocation model of the effect of softening (i.e., decrease in the microhardness or yield stress of the doped material as compared to the initial material) is developed. The motion of dislocations in metals with the body-centered cubic structure, semiconductors, and other materials in accordance with the kink mechanism is described taking into account the effect of impurity atoms creating the centers for benign generation of kink pairs and obstacles for propagation of kinks. The conditions under which the velocity of dislocations increases after the introduction of impurities are determined using the equation describing the kinetics of passage of dislocations through the Peierls barrier in the doped material.

A new analysis of yielding and work hardening in AA1100 and AA5754 at low temperatures

Microstructural evolution determined by mechanical testing is usually modelled from perceived mechanisms of work hardening during dislocation passage and storage in complex arrays. The predicted stress–strain behaviour is then compared with the measured one. Recently a new functional constitutive relation was derived by Saimoto and Van Houtte based on Taylor slip analysis that is optimally fitted to replicate the measured data. The curve fitting parameters arise from the incremental strain Δγ producing a mean slip distance λ and the retained dislocation density ρ assessed from the flow stress τ. A method has been derived to precisely determine λ from the measured data. Moreover, the yield stress determined from the optimum curve fitting is derived from strains beyond the yield point and, hence, has the capacity to separate the solid solution and grain size effects from that of the yield phenomenon. Typically two fit curves are required, the intersection of which defines τ3, γ3 and λ3, akin to the stage II to stage III transition in single crystals. A tabulation of these parameters permits reproduction of the mechanical curves. Furthermore, a plot of flow stress τ vs. 1/λ results in a description of work hardening evolution onto which the inverse cell size 1/dC can be superimposed. Using the results from standard materials testing practice the temperature dependence and evolution of the pertinent parameters show that point defect drag effects are prominent below 273K for AA1100 but not for AA5754.

A concurrent scheme for passing dislocations from atomistic to continuum domains

This paper presents a concurrent atomistic–continuum (CAC) methodology for three-dimensional dynamic simulation of dislocation nucleation, migration and interaction. The method is based on a new continuum field formulation of balance laws with relevant atomistic information (the arrangements and interactions of atoms) considered. In this work, we show that the new CAC method allows the smooth passage of dislocations through sharp interfaces between the atomistic and the coarse-grained finite element domains without unphysical reflection of dislocations or the need for heuristic rules; meanwhile, complex dislocation phenomena such as dislocation nucleation, dynamic strain bursts associated with nucleation and migration avalanches, formations of Lomer–Cottrell locks, dislocation–rigid boundary interactions, formation of intrinsic and extrinsic stacking faults, deformation twinning, and curved dislocation loops can be reproduced by the CAC method. All of the CAC simulations are directly compared with the corresponding atomic-level molecular dynamics (MD) simulations. The efficiency, accuracy and potential applications of the method are discussed along with necessary additional development of criteria for coarse graining.

The backstress effect of evolving deformation boundaries in FCC polycrystals

Shape change of metal systems generates deformed microstructures of dislocation arrays that are comprised of walls of high density separating low density cells. The flow stresses of these composite structures are equilibrated by an evolving internal stress due to the blockage of dislocation passage resulting in kinematic hardening in the meso-scale. The method of intra-granular backstress of Eshelby using Kröner based approach in closed form formulae can easily be incorporated into a crystal–plasticity-based model to predict the kinematic hardening. We have previously developed finite element analyses based on the rate dependent crystal plasticity theory, which can incorporate electron backscatter diffraction (EBSD) maps. We will use this model with inclusion of the calculated backstress to investigate the effect of changes in strain paths.

A Peierls perspective on mechanisms of atomicfriction

Stick–slip behavior observed from nanoscale asperity friction experiments are often modeled by the one-degree-of-freedom Tomlinson model, which is unable to explain the effects of lattice structure and interface defects, or by molecular simulations which suffer temporal limitations in modeling the velocity- and temperature-dependent frictional behavior. A Peierls-type model developed in this work views the atomic frictional process as the initiation and gliding passage of dislocations with diffused cores on the interface. As a consequence of loss-of-ellipticity instability, the occurrence of stick–slip behavior relies on the interaction among interface slip field, contact stress fields, and existing defects. The friction stress for commensurate interface under large contact area can be approximated from the Rice model of screw-dislocation nucleation from a planar crack tip. The spatially inhomogeneous nature of rate-limiting processes and the coupling effects between contact size and interface incommensurability are successfully determined, which cannot otherwise be tackled in the Tomlinsonmodel.

Influence of the stress field due to pressurized nanometric He bubbles on the mobility of an edge dislocation in iron

Voids and He bubbles are strong obstacles to dislocation, which induce hardening and loss of ductility. In Fe, molecular dynamics simulation is used to investigate the basic mechanisms of the interaction between a moving edge dislocation and a void or He bubble, as a function of its He content, temperature, interatomic potentials and interaction geometry. Different interatomic potentials for Fe–Fe and Fe–He interactions are used. It appears that temperature eases the dislocation release, due to the increased mobility of the screw segments appearing on the dislocation line upon bowing from the void or He bubble. The mobility includes the cross-slipping of these segments, which leads to the formation of a jog. It appears that the He bubble induces an inhomogeneous stress field in its surroundings, which strongly influences the dislocation passage depending on the geometry of the interaction.

Dislocation–void interaction in Fe: A comparison between molecular dynamics and dislocation dynamics

Multiscale modeling, including molecular dynamics (MD) and discrete dislocation dynamics (DDD) methods, appears as a significant tool for the description of plasticity and mechanical properties of materials. This research concerns the influence of irradiation on the plasticity of pure Fe and focuses on the interaction of a single dislocation with a spherical void at various geometries. MD simulation shows that the void strengthening is proportional to the interaction area. Stress field around the void influences also the dislocation passage. DDD calculation coupled to finite element method (FEM) is used to simulate the interaction of an edge dislocation with a void. DDD calculations present a good match to the MD simulation results at the near-atomic scale. They highlight the impact of image forces on the dislocation due to the free internal surface of the void and the one of its surrounding stress field.

Discrete Dislocation Dynamics Simulation on Strengths of Dislocation Network Stacks in Multilayered Structures

Strengths of multilayered structures have been investigated using three-dimensional discrete dislocation dynamics (DDD) simulation. The multilayered structure was modeled as a stack of misfit dislocation networks which must exist at an interface between adjoining crystals having different lattice constants. Passages of a single mobile dislocation through several kinds of network stacks were simulated. The critical stress required for the dislocation passage depended on the dislocation spacing of the network, the number of network sheet and the spacing between network sheets.

Flow stress anisotropy and hardening parameters

The flow stress anisotropy of cold rolled metal sheets has been modelled based on the deformation induced extended dislocation boundaries – geometrically necessary boundaries (GNBs) being the main cause of the anisotropy. The GNBs are modelled as barriers to glide dislocations, yielding a resistance to dislocation passage proportional to their angle of misorientation and inversely proportional to the square root of the boundary spacing. Good agreement between model and experiment is demonstrated for the flow stress anisotropy and the absolute flow stress of commercial purity aluminium prestrained by rolling to three different reductions in thickness.

A unifying analysis of the microplastic kinetics with the operative plastic potential

The studies on the existence of a plastic potential in metal systems pre-dates by decades those of thermally activated flow as elucidated by the seminal work of Basinski in 1959. However, to this date a direct correlation between the microplastic kinetic events which integrally manifests itself as the macroscopically determinable plastic potential has not been made. In this work a trial notion of using the activation volume as the fundamental internal variable results in re-expressing the macroscopic rate of doing work in terms of the work of activation necessary to overcome an internal discrete obstacle to dislocation passage. The subsequent differentiation results in a rate of power dissipation relation which under strict conditions of constant-strain-rate sensitivity is only dependent on the square of the strain rate and a linear work-hardening coefficient. On the other hand, if the time dependences of the internal parameters are considered, the power dissipation rate becomes a function of not only the strain rate but also the microstructural evolutionary parameters. The examinations of these predictions suggest means of materials characterization to assess which alloy heat treatment will enhance greater ductility.