Evolution Of Dislocation Network Research Articles

The effect of local chemical ordering on dislocation activity in multi-principle element alloys: A three-dimensional discrete dislocation dynamics study

The exceptional combination of strength and ductility in multi-component alloys is often attributed to the interaction of dislocations with the various solute atoms in the alloy. To study these effects on the mechanical properties of such alloys there is a need to develop a modeling framework capable of quantifying the effect of these solutes on the evolution of dislocation networks. Large scale three-dimensional (3D) Discrete dislocation dynamics (DDD) simulations can provide access to such studies but to date no relevant approaches are available that aim for a complete representation of real alloys with arbitrary chemical compositions. Here, we introduce a formulation of dislocation interaction with substitutional solute atoms in fcc alloys in 3D DDD simulations that accounts for solute strengthening induced by atomic misfit as well as fluctuations in the cross-slip activation energy. Using this model, we show that local fluctuations in the chemical composition of various CrFeCoNi-based multi-principal element alloys (MPEA) lead to sluggish dislocation motion, frequent cross-slip and alignment of dislocations with solute aggregation features, explaining experimental observations related to mechanical behavior and dislocation activity. It is also demonstrated, that this behavior observed for certain MPEAs cannot be reproduced by assuming a perfect solid solution. The developed method also provides a basis for further investigations of dislocation plasticity in any real arbitrary fcc alloy with substitutional solutes.

Formative and controlled mechanisms of nano-sized γ′ precipitates with local phase-transition within dislocation networks of nickel-based single crystal superalloys

The chemical and structural evolution within dislocation networks are investigated in a nickel-based single crystal superalloy crept under 1373K and 137MPa. The local region within dislocation networks can be divided to two separated parts: gridlines which compose the arrangements of dislocation networks and mesh regions as intervals between gridlines. With evolution of dislocation networks, precipitation of nano-sized γ′ phase (γ′n) are observed in mesh region, while gridlines always display the FCC feature of γ phase. The formative mechanism of γ′n is revealed as increased segregations of Cr, Co and Re along gridlines which enhance the contents of γ′-rich elements such as Al in mesh region to promote the local phase transition from FCC to highly ordered L12. However, growth of γ′n requires massive diffusion of γ′-rich elements that is confined by the chemical barrier along gridlines with strong segregations of Cr, Co and Re. The mesh region of γ′n and gridlines of γ reinforce each other forming potential γ/γ′ substructures to stabilize the dislocation networks during creep. Denser dislocation networks formed in dendrite core under larger local lattice misfit are accompanied with facilitated formation of γ/γ′ substructures which derive higher creep resistance than interdendritic region.

Role of hierarchical microstructure of additively manufactured AlSi10Mg on dynamic loading behavior

Microstructure of an additively manufactured AlSi10Mg through direct metal laser sintering (DMLS) process is studied using multi-scale characterization techniques including scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The microstructure of DMLS-AlSi10Mg consists of hierarchical characteristics, spanning three order of magnitude, where nanometer sized to sub-millimeter scaled features exist in the structure. These characteristics included grain and cell structures, nanoscale Si precipitates and pre-existing dislocation networks. Dynamic mechanical behavior of the material is studied using a Split Hopkinson Pressure Bar apparatus over a range of strain rates varying between 800 s−1 and 3200 s−1. Investigation of the deformed microstructures reveals the role of hierarchical microstructure on the dynamic behavior of the material. The high strain-rate deformation is accommodated by dynamic recovery (DRV) process, where low angle grain boundaries evolve due to the generation of dislocations, evolution of dislocation networks, and annihilation of dislocations. Both cell walls and Si precipitates contribute to impeding the dislocation motion and development of dislocation networks. At high strain rates, dislocation networks evolve in the nanoscale DRVed subgrains.

Dislocation based plasticity in the case of nanoindentation

Understanding plasticity initiation and evolution in nanoindentation tests is a fundamental issue. In this study, a continuum model is developed to analytically predict the force-depth curve by coupling the classical Hertzian solution for elastic field and the evolution of dislocation density. By considering multiple slip systems, the present model predicts the “pop-in” event well. Large-scale molecular dynamics simulations are performed to evaluate the mechanical behavior of bcc Fe under nanoindentation of a spherical indenter and to verify the continuum model. The comparison between molecular dynamics simulations and model predictions shows that the initiation and evolution of dislocation networks is strongly dependent on the loading orientation, which is associated with different deformation patterns. Applying the transition state theory, we investigate the slip-twinning transition as a function of loading rate, which supports the basic hypothesis in the continuum model that the activation of shear loop requires lower energy compared with twin nucleation, although twins can nucleate temporally and annihilate into shear loops finally. The present model provides a general framework to evaluate and rationalize the “pop-in” behavior of any materials with elastoplastic response.

A method to predict fatigue crack initiation in metals using dislocation dynamics

AbstractA theory is proposed to predict the initiation of fatigue cracks using cyclic dislocation dynamics (DD) simulations. The evolution of dislocation networks in a grain is simulated over several cycles. It is shown that the dislocation density and the energy stored in the dislocation networks increase with the number of cycles. The results of the DD simulations are used to construct an energy balance expression for crack initiation. A hypothetical crack is inserted into the grain, and the Gibbs energy consisting of the energy of the dislocation structure, the surface energy of the hypothetical crack, and the reduction in continuum energy is evaluated. Once the Gibbs energy attains a maximum, the dislocation structure becomes unstable, and it becomes energetically more favorable to form a real crack. The proposed method is applied to oxygen-free high conductivity copper, and the results are compared against experiments. Finally, it is shown how the method can be amended to account for environmental effects.

Radiation damage buildup and dislocation evolution in Ni and equiatomic multicomponent Ni-based alloys

Single-phase multicomponent alloys of equal atomic concentrations (“equiatomic”) have proven to exhibit promising mechanical and corrosion resistance properties, that are sought after in materials intended for use in hazardous environments like next-generation nuclear reactors. In this article, we investigate the damage production and dislocation mobility by simulating irradiation of elemental Ni and the alloys NiCo, NiCoCr, NiCoFe and NiFe, to assess the effect of elemental composition. We compare the defect production and the evolution of dislocation networks in the simulation cells of two different sizes, for all five studied materials. We find that the trends in defect evolution are in good agreement between the different cell sizes. The damage is generally reduced with increased alloy complexity, and the dislocation evolution is specific to each material, depending on its complexity. We show that increasing complexity of the alloys does not always lead to decreased susceptibility to damage accumulation under irradiation. We show that, for instance, the NiCo alloy behaves very similarly to Ni, while presence of Fe or Cr in the alloy even as a third component reduces the saturated level of damage substantially. Moreover, we linked the defect evolution with the dislocation transformations in the alloys. Sudden drops in defect number and large defect fluctuations from the continuous irradiation can be explained from the dislocation activity.

Multiscale strength (MS) models: their foundation, their successes, and their challenges

Multiscale strength (MS) models are constructed to capture a natural hierarchy in the deformation of metals such as V and Ta starting with atomic bonding and extending up through the mobility of individual dislocations, the evolution of dislocation networks and so on until the ultimate material response at the scale of an experiment. In practice, the hierarchy is described by quantum mechanics, molecular dynamics, dislocation dynamics, and so on, ultimately parameterizing a continuum constitutive model. We review the basic models and describe how they operate at extremely high pressures and strain rates, such as in Rayleigh-Taylor plastic flow experiments. The models use dislocation density as a state variable, and describe time-dependent, as well as rate-dependent, plasticity. They make interesting and testable predictions about transients in plastic flow. There are also clear challenges, however. The current MS models do not include a variety of mechanisms known to be important at low rates. Still, MS models provide compelling insight into plastic deformation of metals under extreme pressures and strain rates.

On the nature of γ′ phase cutting and its effect on high temperature and low stress creep anisotropy of Ni-base single crystal superalloys

The creep anisotropy of the single crystal superalloy LEK 94 deformed in tension along [001] and [110] directions at 1293K and 160MPa was investigated. Elementary microstructural processes which are responsible for a higher increase in creep rates with strain during [110] as compared to [001] tensile loading were identified. [110] tensile creep is associated with a higher number of γ′ phase cutting events, where two dislocations with equal Burgers vectors of type 〈110〉 jointly shear the γ′ phase. The resulting 〈220〉-type superdislocation can move by glide. In contrast, during [001] tensile loading, two dislocations with different 〈110〉-type Burgers vectors must combine for γ′ phase cutting. The resulting 〈200〉-type superdislocations can only move by a combination of glide and climb. The evolution of dislocation networks during creep determines the nature of the γ′ phase cutting events. The higher [110] creep rates at strains exceeding 2% result from a combination of a higher number of cutting events (density of mobile dislocations in γ′) and a higher superdislocation mobility (〈220〉 glide) in the γ′ phase.

Thermodynamics of Dislocation Pattern Formation at the Mesoscale

We introduce a mesoscopic framework that is capable of simulating the evolution of dislocation networks and, at the same time, spatial variations of the stress, strain and displacement fields throughout the body. Within this model, dislocations are viewed as sources of incompatibility of strains. The free energy of a deformed solid is represented by the elastic strain energy that can be augmented by gradient terms to reproduce dispersive nature of acoustic phonons and thus set the length scale of the problem. The elastic strain field that is due to a known dislocation network is obtained by minimizing the strain energy subject to the corresponding field of incompatibility constraints. These stresses impose Peach-Koehler forces on all dislocations and thus drive the evolution of the dislocation network.

Evolution of interphase misfit dislocation networks in Ni-based single-crystal superalloy

The molecular dynamics simulations are used to study the characteristics of misfit dislocation networks in the γ/γ′ phase interface of a Ni-based single-crystal superalloy. From the analyses of the formation of interphase dislocations, the reaction of dislocations, and the evolution of dislocation networks, we find that the shape of the dislocation network finally evolves into a regular hexahedron from an original tetradedron.