Search

Transient phase transitions in ⟨ 100 ⟩ -oriented monocrystal coppers under ultrafast lasers induced shock compression are investigated using molecular dynamics simulation. Due to propagations and attenuations of compressive stress waves induced by ultrafast laser pulses, monocrystal coppers show distinct processes of structural phase transitions, i.e., first, face-centered cubic (FCC) → body-centered cubic (BCC), then BCC → FCC, then FCC → hexagonal close-packed (HCP), and finally some of HCP → FCC. The known Bain's phase transition path of FCC → BCC in copper is discussed in detail and the mechanisms are disclosed by using the modified Born stability criteria and the local minimum energy criterion. By considering the initiation of stacking faults, the mechanism of phase transition of FCC → HCP is well explained. Through the analysis of phonon spectra, the results show that both BCC and HCP phases are unstable phases of monocrystal coppers. [ABSTRACT FROM AUTHOR]

Shock compression behaviors of copper single crystals at room and elevated temperatures are investigated by molecular dynamics simulations. The results show that when increasing the initial temperature, the kinetic energy part of stress increases while the potential energy part of stress decreases; the shear strain and the density of defects decrease while the magnitude of temperature rise increases. To understand the effect of initial temperature on the shock response, the evolutions of dislocations under different initial temperatures are studied in detail. Finally, the effect of initial temperature on quasi-static compression behavior is also investigated and analyzed. [ABSTRACT FROM AUTHOR]

Highlights: [•] The SISF dissociation scheme is introduced into the present 3D-DDD framework. [•] The two typical stages of the APB-SISF transition are successfully captured. [•] The results considering SISF and APB dissociations are closer to experiments. [Copyright &y& Elsevier]

Abstract: To reveal the intrinsic strengthening mechanism in Ni-based single crystal superalloy, the interaction between matrix dislocations and interfacial misfit dislocation networks was modeled in this contribution via molecular dynamics (MD) method. Our results show that the role of interfacial dislocation networks is very complex. On the one hand, the interfacial dislocation networks can act as dislocation sinks to absorb/accommodate the matrix dislocations. During the accommodation process of matrix dislocation by the networks, both the interfacial Lomer–Cottrell locks and a[100] dislocation junctions are formed, which stabilize and strengthen the interfacial dislocation networks. On the other hand, the interfacial dislocation networks can provide dislocation pins to prevent the matrix dislocations from cutting into the γ′ precipitate. These matrix dislocation segments pinned at the phase interface can serve as Frank–Read sources with their length being about half of the dislocation network spacing, providing an explanation for the effect of dislocation network spacing on the creep strength of the Ni-based single crystal superalloy. [Copyright &y& Elsevier]

The interaction between a type-I blunt crack and a near-tip void were modeled by the two-dimensional discrete dislocation dynamics (2-D DDD), with special emphasis on the void growth, crack-tip deformation and their size effect. Different from the classical crystal plasticity-based FE modeling, the present 2-D DDD modeling clearly indicates that, even for the type-I opening crack, the horizontal slip bands connecting the crack tip and the void surface are easily formed, especially when the ligament between the crack-tip and void surface is not too long. Besides the horizontal slip bands, some asymmetric slip bands which are at an oblique angle to the horizontal x -axis can also be formed and intersect with the micro-void. On two coupled parallel slip bands, some oppositely signed dislocations, which can be regarded as equivalent prismatic dislocation loops, glide toward the void/crack-tip surface and thus drive the void growth and the crack tip further blunting. In addition, the size effect by ligament length and void radius on void growth and crack-void interaction has also been investigated carefully. It was found that the size effect is closely related to the formation of horizontal shear slip bands. There exists a critical void radius, under which the near-tip void tends to change into a flat shape and its area decreases gradually with increasing the applied stress intensity factor (SIF) K . These results might be helpful for a better understanding of the fracture and damage mechanisms in the crack-tip process zone. [ABSTRACT FROM AUTHOR]

In the nickel-based single crystal superalloys (NBSCSs) that usually works at high temperature, the vacancies diffusion-induced dislocation climb is an important creep/plasticity mechanism besides the dislocation glide. In order to uncover and capture the dislocation dynamics mechanisms behind primary creep and early plasticity of NBSCSs, the glide-only three-dimensional discrete dislocation dynamics (3D-DDD) simulation framework is extended by incorporating the vacancies diffusion-induced climb mechanism. By means of this extended 3D-DDD framework, the climb-assisted dislocation glide in the narrow γ matrix channels of NBSCSs is simulated to study the primary creep and early plasticity behaviors of NBSCSs serving at elevated temperature, with special attention on the important role of dislocation climb in them. The influences of some important factors, such as ambient temperature, applied stress and vacancy supersaturation, which can directly affect the dislocation climb velocity, and the sizes of the two-phase microstructure (i.e., the precipitate size and matrix channel width) on the primary creep of NBSCSs, are studied in detail. In addition, the important role that dislocation climb plays in the early plasticity behaviors of NBSCSs, including the strain rate effect and the tension–compression (T–C) asymmetry, is investigated and discussed carefully. Moreover, some dislocation climb-induced typical dislocation configurations and their dynamics evolutions, including the triangular dislocation loop wrapping around the corner of the precipitate and the dislocation junctions on the {0 0 1} γ/γ′ interface, are reproduced, in good agreement with the experimental observation and previous computational simulation published. [ABSTRACT FROM AUTHOR]

• Under shock compression lower than HEL of copper, plastic deformation occurs due to the existence of a cylindrical void. • The mechanism of plastic deformation caused by low-speed shock compressions is that the shock wave is reflected from on the free surface of a cylindrical void and then results in change of Schmid factor. • The plastic deformation caused by shock compression is significantly different from that caused by quasi-static uniaxial compression. The main reason is that the activated slip system under shock compression is totally different from that under uniaxial compression. • The significant difference of stacking fault structures caused by shock and quasi-static uniaxial compressions is mainly attributed to the large resolved shear stress gradient formed on the surface of void by shock compression. • The size of void has a slight influence on the velocity of shock wave. The plastic deformation decreases with the decrease of void diameter. This phenomenon can be well explained using the concept of geometrically necessary dislocations. Molecular dynamics simulations are performed to investigate the response to low-speed shock compression of single-crystal copper with cylindrical voids. The results show that plastic deformation occurs under shock compression that is lower than the Hugoniot elastic limit for copper due to the existence of cylindrical voids. An analysis of the interaction between shock waves and the free surface of voids is conducted to determine the mechanism of plastic deformation. Quasi-static uniaxial compression is also investigated. It is found that the resulting plastic deformation is significantly different from that caused by shock compression. Possible reasons for these differences are systematically analyzed. In addition, the effects of the shock velocity, shock direction and axial direction of cylindrical voids on the plastic deformation response are investigated and the relevant mechanisms are analyzed. Finally, the size effect of cylindrical voids is investigated and the underlying mechanism is analyzed based on the concept of geometrically necessary dislocations. [ABSTRACT FROM AUTHOR]

Abstract An XFEM-based DDD scheme is developed to study multiple-dislocation emission from the crack tip and crack propagation in the ductile fracture of a single crystal. A dislocation emission model based on Rice-Thomson theory is incorporated to capture the dynamic dislocation emission from the crack-tip and a normal traction-separation distance model to depict the propagation of mode I cohesive crack. The boundary value problem containing complex surfaces/interfaces and discrete dislocations can be solved by the present XFEM-based DDD scheme directly in a unified framework with satisfactory accuracy. A careful examination shows that the local stress field at the crack tip induced by emitted dislocations can be exactly depicted by this XFEM-based DDD scheme. This local stress field can not only shield the crack from the applied load but also inhibit the emission of subsequent dislocations from the crack-tip; therefore, it plays a crucial role in the ductile-to-brittle competition during crack propagation. After careful verification, this XFEM-based DDD scheme is used to simulate dynamic propagation of mode I cohesive crack. The ductile-to-brittle competition in the crack-tip process zone is investigated with a special attention. Both the dislocation emission-crack propagation process within the crack-tip zone and the discrete dislocation dynamics details behind it are captured and show good agreements with the previous MD simulations conducted by other researchers. Highlights • A XFEM-based 2D-DDD scheme for crack propagation was developed. • The stress fields due to the emitted dislocation was captured accurately enough. • A crack-tip dislocation emission scheme was incorporated into the XFEM-based DDD scheme. • The dislocation dynamics mechanism of ductile-to-brittle transition was captured. • The competition between crack-tip dislocation emission and crack propagation was studied. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

The glide and climb of dislocations are two important plastic deformation mechanisms of the metallic crystals at elevated temperatures. In this work, a new dislocation density-based constitutive model for single crystal plasticity with explicit consideration of both dislocation glide and climb is presented. Three contributions of dislocation climb to the plastic deformation are involved: the kinematics, the climb-enhanced dislocation mobility and the climb-induced dislocation annihilation. A fully implicit time-integration scheme for this model is given and implemented by a user material subroutine in software ABAQUS. Then, the compression tests of 〈110〉 single crystalline aluminum at high temperature and low strain rate are simulated, showing good agreements with the experimental results. Moreover, this model is used to predict the creep deformation of single crystalline aluminum at different temperatures and applied stress levels. The results show that the present model can capture the power law creep behavior of single crystalline aluminum and the predicted creep exponent falls within the range suggested by earlier research. [ABSTRACT FROM AUTHOR]

• The growth of both intragranular and intergranular voids and their competition are first investigated systematically. • The void-growth mechanisms for single crystals (resp. bi-crystals) are not preserved in intragranular (resp. intergranular) cases. • The strong orientation effects observed in single crystals are weakened in polycrystals. • The statistical void growth distribution is closely related to the void-GB relative position. In polycrystalline materials, void growth is strongly dependent on its surrounding microstructural environment such as grain morphologies, crystallographic orientations and void-grain boundary (GB) relative position. In this study, two kinds of void-GB relative positions are considered to investigate the orientation effect on void growth in FCC polycrystals: intragranular type, i.e., void located in interior of a grain; and intergranular type, i.e., void located at boundary of two adjacent grains. The analysis of orientation effect in intragranular (resp. intergranular) cases is conducted through a comparative study with single-crystal (resp. bi-crystal) counterparts. Moreover, the competition between the intragranular and intergranular void growth behaviors is statistically investigated for the first time. To this end, the representative volume element (RVE) of polycrystalline aggregates containing either intragranular or intergranular void is created by 3D Voronoi tessellation. Multiple realizations with different grain morphologies and crystallographic orientation permutations are implemented using crystal plasticity finite element (CPFE) simulations. We show that the mechanism for orientation effect on void growth in single crystals (resp. bi-crystals) is not preserved in intragranular (resp. intergranular) cases. According to the statistical analysis, both the intragranular and intergranular void growth follows a Gaussian distribution, whose profile is affected by both macroscopic stress triaxiality and Lode parameter. It is revealed that the statistical void growth distribution induced by random grain morphologies and crystallographic orientations is closely associated with the void-GB relative position (intragranular or intergranular). The present study may provide much richer insights into the mechanism of damage evolution in polycrystalline metals. [ABSTRACT FROM AUTHOR]

• New peridynamic model for dislocation-based elastoplastic deformation and fracture. • Model allows autonomous interactions between dislocations and crack growth. • Simulates elastoplastic fracture without a preset cracking path or a cohesive zone model. • Obtained crack path for Mode I elastoplastic fracture in single crystal is smooth and undulating, as observed in experiments. • Ductile-to-brittle transition happens naturally in the model. In Part II of this work we extend the method introduced in Part I to consider dislocation dynamic evolution through dislocations nucleation, glide, pile-up, and annihilation. The SP DDD-PD scheme is employed to investigate uniaxial tension in a single crystal and a polycrystal and verify its accuracy. The model is then used to simulate elastoplastic fracture by considering interactions between dislocations and crack growth. For Mode I elastoplastic fracture in a single crystal, we observe that the crack path is "attracted" towards regions of high density of gliding dislocations, leading to an undulating crack paths, as observed in experiments but never replicated by continuum-level computational models before. Tests on different sample sizes show how the proximity of constraints to the crack tip can lead to plastic hardening. Ductile-to-brittle transition happens naturally in this model when the crack, under Mode I displacement-controlled loading, approaches a free edge. A new way to calibrate the critical bond strain based on the material toughness or fracture energy is proposed. The present SP DDD-PD scheme can be used to investigate complicated elastoplastic fracture problems in which the interaction between dislocation motion and damage is critical. [Display omitted] [ABSTRACT FROM AUTHOR]

Dynamic fracture of silicon multi-crystal merits a systematic investigation given the complexity induced by microstructure defects, in particular grain boundaries. How crack selects its path in the presence of substantial grain boundaries is an issue still under debate in literature. In this work, fracture behavior of multi-crystalline silicon is carefully examined through bending tests, bringing to light new crack propagation scenarios. In the case of multi-cracking, massive branching events emerge, and crack paths are heavily affected by burst waves. When a single crack propagates across grain boundaries, orientation mismatch between cleavage planes generates an important constraint effect, leading the crack to eventually propagate along (112) plane, i.e., a path scarcely seen in single crystal. Perfect crack propagation along coherent Σ 3 twin boundary can hardly be achieved, presumably due to the crack inertia effect. Moreover, crack is found to incessantly deviate at the two sides of high order grain boundaries, resulting in a zigzag crack path. The observations suggest that grain boundaries are not prone to break up but significantly affect crack path in silicon multi-crystals. • Cascading waves and branching break up the monopoly of (111) cleavage in silicon. • Fracture along coherent TB cannot be achieved presumably due to crack inertia effect. • Misorientation plays a vital role in fracture path selection upon crossing GB. • High order GB is revealed to be unfavorable crack path in silicon multicrystal. [ABSTRACT FROM AUTHOR]

• Effects of coating and confinement layer on LSP are investigated by MD simulations. • Confinement layer significantly improves strengthening effect of LSP. • Ablation coating has little effect on LSP and causes contamination of the target metal. We study laser shock peenings of copper single crystals using molecular dynamics simulations based on the ultrafast thermomechanical coupling model. We reveal the effects of ablation coating and confinement layer on response induced by laser shock peening and discuss the potential mechanisms. The results show that the confinement layer significantly enhances the mechanical response of laser shock peening and increases densities of defects caused by laser shock peening, whilst ablation coating has relatively little influence on mechanical response and defect density. Ablation coating inevitably causes the target metal to be contaminated by impurities. Our theoretical study can provide simplifications for improving laser shock peening technology. [ABSTRACT FROM AUTHOR]