Search

Many experiments have shown that plastic deformation of metals generates heat, leading to temperature rise of the material. However, little is known about the underlying factors that govern the heat generation, especially in polycrystalline metals involving high-rate plastic deformation. In this work, the work-to-heat conversion during high-rate plastic deformation of polycrystalline coppers with and without nanotwins is studied using molecular dynamics simulations. The results show that within a certain range of imposed deformation, smaller grain size results in higher plastic work-to-heat conversion, due to larger proportion of grain boundaries. For a given grain size of polycrystalline copper with twins, the portion of plastic work converted to heat increases as the twin-boundary spacing decreases within a certain deformation range. However, at large deformation the plastic work-to-heat conversion coefficient should be analyzed specifically for each individual sample due to the transformation of the deformation mechanism (i.e., the change of the energy storage). [ABSTRACT FROM AUTHOR]

The shock responses of crystalline Ni with two types of symmetric tilt grain boundaries (STGBs) are studied by large-scale molecular dynamics simulations, with special concerns on the effect of hydrogen segregated STGBs on them. The crystalline Ni with different kinds of H-free STGBs have significantly different plastic responses during the shock processes due to different mechanisms of dislocation emission from the STGBs. The segregation of hydrogen atoms at the STGBs significantly affects the dislocation emission from the H-segregated STGBs, and then heavily affects the shock plastic responses of crystalline Ni with H-segregated STGBs. On the one hand, dislocation emission from the H-segregated STGBs is delayed compared with that from the H-free ones at the low shock velocity, showing the so-called lagging effect. On the other hand, multi-mode dislocation emissions (or dislocation emission on the multi slip systems) are frequently activated from the H-segregated STGBs at the high shock velocity. In addition, the spallation behavior of the crystalline Ni under shock loading is also significantly affected by hydrogen segregation at the STGBs. In the targets with the H-free STGBs, spallation originates mainly from the interior of the grains due to strong interaction and reaction between dislocations there, however, it originates definitely from the H-segregated STGBs due to the reduced GB cohesive strength by hydrogen segregation. [ABSTRACT FROM AUTHOR]

Abstract: The uniaxial tension behavior of polycrystalline thin films, in which all grain boundaries (GBs) are penetrable by dislocations, is investigated by two-dimensional discrete dislocation dynamics (DDD) method with a penetrable dislocation-GB interaction model. In order to study thickness effect on the tensile strength of thin films with and without surface treatment, three types of thin films are comparatively considered, including the thin films without surface treatment, with surface passivation layers (SPLs) of nanometer thickness and with surface grain refinement zones (SGRZs) consisting of nano-sized grains. Our results show that thickness effects and their underlying dislocation mechanisms are quite distinct among different types of thin films. The thicker thin films without surface treatment are stronger than the thinner ones; however, opposite thickness effects are captured in the thin films with SPLs or SGRZs. Moreover, the underlying dislocation mechanisms of the same thickness effects of thin films with SPLs and SGRZs are different. In the thin films with SPLs, the thickness effect is caused by the sharp increase of dislocation density near the film-passivation interface, while it is mainly due to the sharp decrease of dislocation density within the refined surface grains of the thin films with SGRZs. No matter in what type of thin films, thickness effect gradually disappears when the number of grains in the thickness direction is large enough. Our analysis reveals that general mechanism of those thickness effects lies in the competition between the exterior surface-constraint and interior GB-constraint on gliding dislocations. [Copyright &y& Elsevier]

Abstract: The grain-size effect on the yield stress and the flow strength in micro-polycrystals relates closely to the penetrability of grain boundary (GB) to dislocations. To simulate the dislocation transmission across grain boundary, a dislocation–grain boundary penetration model is proposed and then integrated into the two-dimensional discrete dislocation dynamics (DDD) framework by Giessen and Needleman (1995). By this extended DDD technology, the Hall–Petch effect in micro-polycrystals and the strengthening mechanism are computationally studied, with the main focus on the significant influence of the dislocation transmission across grain boundary that is not fully considered formerly. Results indicate that the Hall–Petch type relation is still applicable, but depends strongly on the GB-penetrability to dislocations, especially for the flow strength at large offset strains. The fitting values of Hall–Petch grain-size sensitive exponents n for initial yield stress and flow stress basically agree with experimentally measured data in published literatures. [Copyright &y& Elsevier]

Abstract: A two-dimensional discrete dislocation dynamics (DDD) technology by Giessen and Needleman (1995), which has been extended by integrating a dislocation-grain boundary interaction model, is used to computationally analyze the micro-cyclic plastic response of polycrystals containing micron-sized grains, with special attentions to significant influence of dislocation-penetrable grain boundaries (GBs) on the micro-plastic cyclic responses of polycrystals and underlying dislocation mechanism. Toward this end, a typical polycrystalline rectangular specimen under simple tension-compression loading is considered. Results show that, with the increase of cycle accumulative strain, continual dislocation accumulation and enhanced dislocation-dislocation interactions induce the cyclic hardening behavior; however, when a dynamic balance among dislocation nucleation, penetration through GB and dislocation annihilation is approximately established, cyclic stress gradually tends to saturate. In addition, other factors, including the grain size, cyclic strain amplitude and its history, also have considerable influences on the cyclic hardening and saturation. [Copyright &y& Elsevier]

Abstract: Nanoindentation has been extensively used to measure the mechanical behavior of materials at the micro- and nanoscale. However, the material response of nano/microindentation and its intrinsic mechanisms are very complicated, especially when considering heterogeneous polycrystalline materials. In this contribution, nanoindentation in polycrystals, performed with a circular indenter, is studied by numerical modeling based on the two-dimensional discrete dislocation plasticity of Van der Giessen and Needleman [Van der Giessen E, Needleman A. Model Simul Mater Sci Eng 1995;3:689]. The dependence of indentation hardness is investigated with respect to four typical characteristic dimensions: indenter radius, grain size, indentation depth, and the distance between the grain boundary and the indenter. Results show that these characteristic dimensions have considerable influence on the nanoindentation hardness. Further investigation shows that their influence takes effect mainly in two ways, i.e. via strain hardening and the indentation size effect. Although both effects are size dependent, their underlying mechanisms are clearly different. For the present polycrystal case, the strain hardening effect is mainly associated with the constraints of grain boundaries and dislocation obstacles to the dislocation glide, while the indentation size effect is related to the average strain gradient beneath the circular indenter. [Copyright &y& Elsevier]

Highlights: [•] Crack tip stress field of a polycrystalline material has been modelled under cyclic loads using DDD. [•] Both dislocation climb and grain boundary penetration are considered in the model. [•] Strain ratchetting is found near the crack tip, to which dislocation climb is found to be mainly responsible. [•] Multiple slip systems are found close to the crack tip. [ABSTRACT FROM AUTHOR]

Abstract The dislocation-induced shielding effect on the crack-tip depends on not only the number of dislocations but also the distance of dislocations from the crack-tip. The former is closely related to the nucleation of dislocation but the latter to the grain size and the obstruction of grain boundaries (GBs) to dislocations. In this paper, a discrete dislocation dynamic framework was further developed with a crack-tip dislocation emission scheme especially introduced, and then a series of discrete dislocation dynamic simulations were performed to study the shielding effect of dislocations on the crack-tip in poly-crystals. The different roles of the dislocation nucleation from the Frank-Read sources (DNFR) and dislocation emission from the crack-tip (DECT) in the shielding effect were studied, which indicates the DECT mechanism plays more important role compared with the DNFR one for the cases considered here. The shielding effect of dislocation is dominated by the grain size as well as the energy barrier for dislocation transmission across GB. With the increase of grain size, the shielding effect of dislocations on the crack-tip increases significantly, showing strong grain size effect. Moreover, with the increase of the GB energy barrier for dislocation transmission, the shielding effect of dislocations on the crack-tip first increases and then reaches its peak and finally decreases, showing an interesting mountain-shaped dependence on the GB energy barrier. This mountain-shaped dependence is gradually flattened with the increases of grain size. These interesting results enrich our understanding of the dislocations-induced shielding effect on the crack-tip in the poly-crystals. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

• A novel hydrogen transport model is developed within the coupled framework of crystal plasticity and hydrogen balance, in which both the contributions of plastic strain and plastic strain gradient to hydrogen transport are considered. • The hydrogen transport flux driven by plastic strain gradient results in a significant dynamic hydrogen segregation at grain boundaries during deformation, which is expected to drive the hydrogen-induced intergranular cracking as observed in experiments. • The roles of grain refinement in hydrogen transport are clarified with the present model. Hydrogen transport by moving dislocations is one of the key mechanisms for hydrogen embrittlement, which is considered responsible for local hydrogen accumulation at preferential crack initiation sites. Although numerous experiments corroborate this mechanism, there are few theoretical models for it available. In the present work, a novel hydrogen transport model is developed within the coupled framework of crystal plasticity and hydrogen balance. The hydrogen transport flux in the present model is divided into two parts: the first-order hydrogen transport flux driven by plastic strain and the second-order hydrogen transport flux driven by plastic strain gradient. With the calibrated parameters by fitting experimental results, the first-order hydrogen transport flux is negligible even for nickel with a low hydrogen diffusivity. In polycrystals, due to the intrinsic plastic heterogeneity induced by grain orientation mismatch, the second-order hydrogen transport flux results in significant dynamic hydrogen segregation at grain boundaries during deformation, which is expected to drive the hydrogen-induced intergranular cracking as observed in experiments. This dynamic segregation behavior is related to the evolution rate of geometrically necessary dislocations, depending on the grain boundary characters. The roles of grain refinement in hydrogen transport are clarified with the present model. This study shows the vital role of plastic strain gradient induced by grain boundary constraint in hydrogen transport, which is essential in understanding hydrogen migration and accumulation in deformed polycrystalline metals. [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]

[Display omitted] • H segregation can inhibit grain boundary (GB) migration, ensuring crack propagation along the GB. • H segregation promotes dislocation emission from the GB via H-induced atomistic distortion on the GB. • H-enhanced dislocation emission from GB and H-enhanced GB decohesion can significantly reduce the critical stress intensity factor of the crack propagation along GB. • With H segregation, the process of crack propagation along GB is dominated by alternate GB dislocation emission and GB cleavage. Hydrogen ingression in metals generally causes catastrophic failure. The H-induced fracture surface mostly exhibits intergranular feature, suggesting that grain boundary (GB) is the dominant crack propagation path. In this paper, the grand canonical Monte Carlo method was first performed to obtain equilibrated H distribution around various symmetric tilt grain boundaries (STGBs) in nickel. Then, atomistic simulations were conducted to study the influence of H segregation on the Mode I crack propagation behavior. In the absence of hydrogen, significant crack tip blunting and transgranular fracture are observed, implying a tendency toward ductile transgranular fracture. However, in the case of H segregation, it is found that dislocation emission from the crack tip is enhanced but the crack tip still remains highly sharp. Moreover, crack propagation is dominated by alternate GB dislocation emission and GB cleavage, implying a tendency of ductile–brittle alternating intergranular fracture behavior. The H-inhibited GB migration and H-enhanced GB dislocation emission are believed to be the key mechanisms governing the H-assisted crack propagation along the GB. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two dislocation nucleation mechanisms, HDN and DDN, are observed during tensile deformation of bicrystal along different loading direction with and without H segregation. • When the DDN mechanism dominates, H segregation at the GB inhibits dislocation nucleation from GB. • When the HDN mechanism dominates, H segregation at the GB enhances or barely affects dislocation nucleation from GBs, depending on the loading direction. It is generally believed that the influence of hydrogen on plastic deformation of grain boundaries should be considered when analyzing hydrogen-induced intergranular fracture in polycrystalline metals. In this paper, the equilibrated H distribution around GBs was firstly investigated by employing the grand canonical Monte Carlo method. Then, MD simulations were performed to study the plastic response of GBs under uniaxial tensile loads in different directions, with various bulk H concentrations considered. The results indicate that the influence of H on dislocation nucleation from GB depends on both tensile directions and characteristics of GB structures. Specifically, two dislocation nucleation mechanisms, called dislocation dissociation nucleation (DDN) and heterogeneous dislocation nucleation (HDN), are identified. Careful analyses show that H segregation can increase the energy barrier of DDN, which results in H-inhibited dislocation nucleation. In contrast, the HDN mechanism involves H-enhanced or H-insensitive dislocation nucleation, which mainly depends on the influence of H on GB stress. [ABSTRACT FROM AUTHOR]

• A polycrystal model containing > 100 grains with dislocation-penetrable grain boundaries was constructed within a discrete dislocation dynamics framework. • The effects of dislocation source parameters and grain size on the yield stress of ultrafine-grained polycrystals were systematically studied. • In the initial, grain boundary dominated regime, the change of dislocation density is proportional to plastic strain and independent of the initial dislocation density. • The combined effects of source length, grain size, and initial dislocation density on flow stress can be captured by a single equation. In this study, the effects of grain size and dislocation source properties on the yield stress of ultrafine-grained (UFG) polycrystals were examined using three-dimensional multiscale discrete dislocation dynamics (DDD). A polycrystal model containing multiple grains with randomly distributed orientations was constructed within a multiscale DDD framework. Grain boundaries (GBs) were assumed to be penetrable by dislocations, with two dislocation-GB interaction mechanisms, i.e., dislocation absorption at GBs and dislocation emission from GBs, being considered. The simulation investigated the dislocation source effect and demonstrated a non-monotonic dependency of flow stress on dislocation source length, where the lowest flow stress corresponds to a Frank-Read (FR) source length of d /4 where d is the grain size. When the length of a FR source in the polycrystalline sample exceeds this value, the simulated yield stress increases owing to the constraining effect of grain boundaries on dislocation movement. The grain size dependence of the yield stress shows deviations from the classical Hall–Petch relationship as the exponent in the Hall–Petch type relation ranges from about 0.91 to about 0.98, depending on the initial dislocation density in the samples. Detailed analysis indicates that the grain size dependence of the yield stress is mainly controlled by the effect of grain boundary constraints on dislocation activation. A secondary effect arises from grain size dependent dislocation accumulation and the resulting Taylor hardening. The activation and operation of FR sources were quantitatively examined to further understand the origins of source length and grain size effects. A theoretical model is proposed to account simultaneously for the effects of source length, grain size, and initial dislocation density on the yield stress of polycrystals in the UFG regime. [ABSTRACT FROM AUTHOR]

To study macroscopic mechanical response and microscopic deformation mechanism of nano-polycrystalline (NPC) Ta under shock compression, massive-scale non-equilibrium molecular dynamics (NEMD) simulations were systematically performed. The effects of grain size, shock strength on the wave structure, plasticity mechanism and flow stress were unveiled. Under the weak shock compression, as the grain size increases, a transition of plastic wave structure from single to double and then to single one was found, driven by the transition of dominant deformation mechanism from grain boundary (GB)-mediated plasticity to its coexistence with twinning and slipping and then to twinning and slipping, respectively. A transition from twinning-to slipping-preferred plastic deformation as the grain size exceeds ~30 nm was captured in the simulations and explained by a simple theoretical model proposed in this work. Under the strong and ultra-strong shock compressions, twinning-detwinning and amorphization-recrystallization were revealed as the dominant deformation mechanisms, respectively, which show weak grain size dependences. The flow stresses at the Hugoniot state were calculated, which follow the Hall-Petch relation under the weak and strong shocks but show complexity under the ultra-strong shock. These results can help understand intrinsic grain size and extrinsic shock strength effects on the mechanical and microstructural responses of Ta, as a key structural and typical BCC metal. [Display omitted] • Grain size dependent plastic wave structure is observed on shock profiles. • Transition of GB sliding-twinning-slipping as grain size varies is captured by atomistic and theoretical analyses. • Grain size effects on flow strength and underlying mechanisms are revealed by MD simulations. [ABSTRACT FROM AUTHOR]

[Display omitted] Hydrogen-affected dislocation motion and hydrogen-induced intergranular fracture play key roles in hydrogen embrittlement. The quantitative characterization of H-affected interaction between dislocation and grain boundary (GB) is of great significance to understand the underlying physics of hydrogen embrittlement, which is systematically studied here by hybrid Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The structural features of GBs are depicted through the structural unit model (SUM) and grain boundary dislocation model (GBDM). The mechanisms of interactions between dislocation and H-free/H-segregated GB are thoroughly investigated and classified. For the cases without H segregation at GB, dislocation nucleation from GB and dislocation gliding on GB are the fundamental mechanisms governing the dislocation-GB interaction. In contrast, for the cases with H segregation, the dislocation-GB interaction mechanism is changed owing to H-inhibited GB dislocation emission, dislocation transmission across GB and dislocation gliding on GB. Due to dislocation pile-up promoted by H segregation, crack initiation is facilitated at the H-segregated GB. These results can provide essential information not only for understanding H-induced intergranular fracture but also for developing an up-scaled discrete dislocation dynamics (DDD) simulation framework. [ABSTRACT FROM AUTHOR]

A new efficient framework coupling the two dimensional (2D) discrete dislocation dynamics (DDD) and the extended finite element method (XFEM) is developed for modeling the dynamic evolution of multiple dislocations in crystalline materials with abundant interfaces/surfaces. By a specially introduced stress calculation scheme with cut-off radius and virtual elements, the present algorithm can capture interaction between neighboring dislocations accurately although no time-consuming dislocation-core enrichment is employed. Since it is not sensitive to the FE mesh size and large XFEM time step can be used, its efficiency is evidently improved compared with other DDD frameworks. Due to the merits of XFEM itself, the present DDD-XFEM scheme can treat intractable surface/interface problems conveniently, with no additional image stress calculation as in the DDD-FEM superposition framework and special plastic-strain distribution near the surfaces/interfaces as in the discrete-continuous method (DCM) needed. For these, it can be used to model plastic behaviors of crystals with abundant interfaces/surfaces (i.e. grain/phase boundaries and crack/void surfaces). To show the ability of present DDD-XFEM scheme, it is used to simulate the mechanical responses of polycrystalline aluminum with assumed rigid and penetrable grain boundaries. A three-stage model for dislocation penetrating through grain boundary (DPTGB) is suggested. The results show that the present DDD-XFEM scheme can capture the formation of dislocation pile-ups and thus the Hall-Petch effect successfully. The DPTGB can decrease the flow stress and hardening rate evidently; moreover, it can weaken the grain size effect and may enhance the cyclic stress relaxation of polycrystals greatly. Image 1 • A more efficient and accurate XFEM-DDD framework for simulating the micro-plasticity of materials is developed. • The newly developed XFEM-DDD framework can well deal with various complex discontinuities in materials. • The present XFEM-DDD framework can simulate efficiently the problems on much longer/larger temporal/spatial scale. • A three-stage model for dislocation penetrating through GB (DPTGB) is suggested. • The grain size effect on strength and the cyclic stress relaxation are captured efficiently by the present scheme. [ABSTRACT FROM AUTHOR]