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This paper studies the stress field and crack nucleation behavior in a disclinated nanowire with a continuum model. The surface stress effects of the nanowire is accounted for with the Gurtin-Murdoch model. The Green's functions for the stress fields of a single wedge disclination and a single edge dislocation in a cylindrical nanowire are solved respectively with the complex variable method. To make the superposition principle valid, the stress field induced by the residual surface tension is properly handled in the Green's functions. After that, the distributed dislocation method is applied to simulate the crack nucleation behavior. The influences of the surface stress effects on the stress fields of the wedge disclination and edge dislocation as well as on the Griffith crack nucleation behavior are systematically discussed. [ABSTRACT FROM AUTHOR]

The mechanical behavior of a polycrystalline aluminum in tension was modeled using a climb-assisted discrete dislocation dynamics (DDD) technique. Special focus was on how dislocation climb influences the flow stress of the polycrystalline aluminum with regard to selected grain sizes at elevated temperature. A periodical representative cell (PRC) consisting of given number of grains was used in the simulations. Results showed that, at the high temperature considered, dislocation climb plays an important role in defining the mechanical behavior of the polycrystalline crystal. Specifically, dislocation climb decreases significantly the flow stress and hardening rate while increases the dislocation density by relieving the dislocation pile-ups against the grain boundaries (GBs). In addition, the grain size effect on the yield stress of polycrystalline aluminum is significantly weakened by dislocation climb, especially when the grain size falls in the range of submicron. Another interesting result is that, at high temperature, when both dislocation climb and glide are considered, the grain size effect seems to be insignificant with regard to the applied strain rate, although the strength of material increases with enhanced loading rate. [ABSTRACT FROM AUTHOR]

Abstract: The cyclic plastic response of a single crystalline thin beam subject to combined cyclic tension and bending is analyzed using two-dimensional discrete dislocation plasticity. In this contribution, special attention is paid to the difference in the inherent mechanism of the size effect for different cyclic loads. Results show that the cyclic plastic response has a strong size effect for both cyclic pure tension–compression and pure bending. However, the inherent mechanisms are different. The dislocation starvation mechanism dominates the cyclic tension–compression while the geometrically necessary dislocation dominates the cyclic pure bending. When the combined cyclic tension and bending are applied to the thin beam, the cyclic moment–rotation response shows strong size effect while the stress–strain response shows weak or even no size effect. In addition, it is also found that the cyclic loading paths have considerable influences on the shape of the cyclic stress–strain loops. [Copyright &y& Elsevier]

Abstract: Separation of the particle–matrix interface and breakage of the second-phase particle are two main void nucleation mechanisms, which are directly associated with the stress concentration factors (SCFs) at the interface and within the particle, respectively. This work investigates the coupled effects of particle size and particle shape on these stress concentrations by solving an infinite solid containing an oblate spheroidal particle under remote stress boundary condition. The phenomenological strain plasticity theory by Fleck–Hutchinson [Fleck, N.A., Hutchinson, J.W., 1997. Strain gradient plasticity. In: Hutchinson, J.W., Wu, T.Y. (Eds.), Advance in Applied Mechanics, vol. 33. Academic Press, New York, pp. 295–361] is adopted to capture the size effect, various particle aspect ratios are considered to depict the particle shape effect and an interfacial energy concept is introduced to settle the double-traction equilibrium problem at the matrix–particle interface. By using a Ritz procedure, solutions about the stress concentrations are numerically achieved and three main results are found. First, the interfacial normal stress near the particle pole, the interfacial shear stress and the particle opening stress are dramatically elevated and their distributions are significantly modified by decrease in the particle size. Second, this particle size effect is influenced by the remote effective strain, remote stress triaxiality and the interfacial energy to different extent. Finally, the particle shape effect is coupled with this particle size effect, and the more oblate the particle is, the more significant the size effect on SCF elevation is. These findings are helpful for us to understand deeply the void nucleation mechanism at the micron scale. [Copyright &y& Elsevier]

Abstract: There generally exist two void nucleation mechanisms in materials, i.e. the breakage of hard second-phase particle and the separation of particle–matrix interface. The role of particle shape in governing the void nucleation mechanism has already been investigated carefully in the literatures. In this study, the coupled effects of particle size and shape on the void nucleation mechanisms, which have not yet been carefully addressed, have been paid to special attention. To this end, a wide range of particle aspect ratios (but limited to the prolate spheroidal particle) is considered to reflect the shape effect; and the size effect is captured by the Fleck–Hutchinson phenomenological strain plasticity constitutive theory (Advance in Applied Mechanics, vol. 33, Academic Press, New York, 1997, p. 295). Detailed theoretical analyses and computations on an infinite block containing an isolated elastic prolate spheroidal particle are carried out to light the features of stress concentrations and their distributions at the matrix–particle interface and within the particle. Some results different from the scale-independent case are obtained as: (1) the maximum stress concentration factor (SCF) at the particle–matrix interface is dramatically increased by the size effect especially for the slender particle. This is likely to trigger the void nucleation at the matrix–particle interface by cleavage or atomic separation. (2) At a given overall effective strain, the particle size effect significantly elevates the stress level at the matrix–particle interface. This means that the size effect is likely to advance the interface separation at a smaller overall strain. (3) For scale-independent cases, the elongated particle fracture usually takes place before the interface debonding occurs. For scale-dependent cases, although the SCF within the particle is also accentuated by the particle size effect, the SCF at the interface rises at a much faster rate. It indicates that the probability of void nucleation by the interface separation would increase. [Copyright &y& Elsevier]

In a mass of engineering materials and structures, graded variations in yield stress is inherent. To investigate the macro-mechanical response and meso-mechanism of damage by void growth and coalescence in the materials with graded yield stress, detailed finite element computations of a representative cylindrical cell containing a spherical void are performed. By comparison with the responses of a homogeneous material cell model, significant effects of the matrix yield stress gradient (YSG) on the void growth and coalescence are revealed: (1) The evolution of voids in the homogeneous matrix materials has no close relations with the matrix yield stress level, however, the YSG distribution in matrix can lead to faster void growth rate and lower void coalescence strain. (2) In the homogeneous materials, the critical shapes of voids are independent on the yield stress of matrix materials, but the graded distribution of yield stress in the matrix materials, which leads to higher local plastic strain in the softer matrix layer, have an important influence on the critical void shapes. (3) The critical void volume fraction fc is insensitive to the yield stress of the homogeneous matrix, however the YSG in matrix materials has a stronger effect on fc, especially when the stress triaxiality level is lower. (4) Higher strain energy stored in the softer material layer encircling voids is internal driving force to faster void growth. The ratio of strain energy stored in the softer matrix layer surrounding voids to the whole energy provided by outside environment can give better description to the meso-mechanism of void evolution. [Copyright &y& Elsevier]

• The size-dependent microvoid growth in heterogeneous polycrystals is studied by local/non-local (MSG) CPFEM. • The first kind of void-growth size effect by void-grain size ratio is investigated. • The second kind of void-growth size effect by absolute microvoid size is studied. • Stress triaxiality T has a significant influence on the void-growth size effect, while Lode parameter L exhibits a negligible effect. • The void-growth size effect in polycrystalline environments should be understood from a statistical point of view. Microvoid growth involves a strong size effect, i.e., smaller microvoid presents a lower growth rate. In polycrystalline materials, the size ratio between microvoids and grains may also affect microvoid growth behavior. However, most previous studies treated material matrix surrounding microvoids as homogeneous. It turned out that such treatment cannot effectively depict the influence of the abovementioned void-grain size ratio on damage evolution. In the present study, both classical local and non-local strain-gradient crystal plasticity finite element simulations are performed to study size-dependent microvoid growth in heterogeneous polycrystals. The results indicate that both void-grain size ratio and absolute microvoid size influence microvoid growth significantly, referred to as first (induced by grain-scale heterogeneous deformation) and second kinds of (induced by plastic strain gradient) size effects, respectively. Besides, macroscopic stress triaxiality T has a significant influence on the size effect of microvoid growth, while Lode parameter L exhibits a negligible effect. Due to random grain-orientation distribution and grain-geometric characteristic, a smaller microvoid within polycrystalline environments may even grow faster than a larger one, implying that the size effect of microvoid growth should be understood from a statistical point of view in polycrystalline environments. The present study provides a fundamental understanding on the intrinsic mechanism of the size-dependent microvoid growth in heterogeneous polycrystals. [Display omitted] [ABSTRACT FROM AUTHOR]

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]

• 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]

• A gradient nano-grained polycrystal model with dislocation-penetrable grain boundaries was constructed within a discrete dislocation dynamics framework. • The mechanisms of synergetic strengthening in gradient nano-grained aluminum were systematically investigated from the view of dislocation dynamics. • The distribution of geometrically necessary dislocation is non-uniform along the gradient direction in the gradient nano-grained sample. • The gradient nano-grained sample has a stronger Bauschinger effect compared to its homogenous counterparts. Gradient nano-grained (GNG) metals have shown a better strength-ductility combination compared to their homogeneous counterparts. In this paper, the mechanical properties and the related deformation mechanisms of GNG aluminum were investigated using three-dimensional multiscale discrete dislocation dynamics (DDD). GNG polycrystalline models and uniform nano-grained (UNG) counterparts were constructed within a multiscale DDD framework. A dislocation-penetrable grain boundary model based on a coarse-graining approach was adopted to handle the interaction between dislocations and grain boundaries. The simulation results show that the yield stress and strain hardening of the GNG sample is larger than the value calculated by the rule of mixtures, indicating a synergetic strengthening induced by the gradient structure. The associated microstructure evolution demonstrates that dislocations initially activate and glide in the larger grains and then gradually propagate into the smaller grains in the GNG sample, this sequential yielding of layers with different grain sizes generates stress and strain gradients, which is accommodated by geometrically necessary dislocations (GNDs). Moreover, we found that the Bauschinger effect in GNG sample is stronger than those in component UNG samples, suggesting a significant back stress strengthening in the GNG sample during plastic deformation. Finally, a theoretical model is established which successfully describes the Bauschinger effect of GNG and corresponding UNG samples according to the features of dislocation evolution upon unloading. The present study provides insights into the outstanding mechanical property of GNG metals from the view of dislocation dynamics at the submicron scale and offers theoretical guidance for designing strong-and-ductile metals. [Display omitted] [ABSTRACT FROM AUTHOR]

The correlation between mechanical behavior and heterogeneous microstructure is still unclear for laser powder bed fusion AlSi10Mg. The present work investigates strain hardening and damage mechanisms with different build platform temperatures (35 °C and 200 °C) and loading directions (horizontal and vertical, building direction being vertical) to probe the effects of microstructure size and melt pool border orientation. The 35 °C microstructure involves load direction independent properties, with ductility determined by damage initiation occurring extremely close to final failure. In contrast, the 200 °C microstructure exhibits anisotropic mechanical behavior, as revealed by higher strain hardening capacity, earlier damage initiation and more dispersed damage distribution for the vertical loading direction. Although the vertical samples generally involve strain localization and crack propagation along the melt pool border, their ductility is not compromised compared to the horizontal counterparts. We attempt to explain these hardening and damage behaviors with stress partition and trade-off between phase stress and strain gradient. [ABSTRACT FROM AUTHOR]

The <110>{111} superdislocations play a key role in the mechanical behaviors of L1 2 Ni 3 Al. Through the combination of density functional theory (DFT) calculations and Peierls-Nabarro (P–N) model, the physical properties of Ni 3 Al and the characters of superdislocations are systematically investigated under different uniaxial tensile/compressive loads normal to {111} slip planes. The influence of applied loads on ideal stress-strain relation, ideal strength, shear modulus along slip direction, anti-phase boundary (APB) energy, γ surface, splitting width of superdislocation (i.e. , APB width), core width of super partial dislocations and Peierls stress is thoroughly discussed. The results show that the effects of uniaxial normal tensile/compressive loads on above properties are remarkable for both nonmagnetic (NM) and ferromagnetic (FM) states. The compressive loads along the <111> direction tend to expand the splitting width of superdislocations and the core width of super partial dislocations, which further induce higher Peierls stress and yield strength of Ni 3 Al, while the tensile loads lead to opposite effects. In addition, it is found that the stability zone of Ni 3 Al is in the normal applied strain range of −0.14–0.14 rather than −0.18–0.22 as suggested by previous works. For the case free of applied strain (i.e. ε = 0), both the predicted APB width and Peierls stress of the NM state are about 10% percent larger than those of the FM state. Obviously, since the physical properties of Ni 3 Al and characters of superdislocations are asymmetric under tensile/compressive normal loads, the tension/compression asymmetry of the mechanical behaviors for Ni 3 Al is inherent. [Display omitted] • The T/C asymmetric mechanical responses are inherent for intrinsic asymmetry in physical parameters and core structures. • The values of ideal strength, APB energy, APB width, partial core width and Peierls stress are all T/C asymmetric. • The stability zone of Ni 3 Al should fall into the range of − 0.14 ≤ ε ≤ 0.14 , rather than − 0.14 ≤ ε ≤ 0.22 as reported previously. • The compressive loads tend to expand the splitting width of superdislocations and the super partial core and vice versa. • The compressive loads tend to increases the Peierls stress and yield strength of Ni3Al in the <111> loading direction. [ABSTRACT FROM AUTHOR]