Polycrystalline Model Research Articles

Reactive Force Field for Atomistic Understanding of Interfacial Processes in Sulfide Electrolytes

All solid-state lithium-metal batteries (ASSLMBs) using sulfide electrolytes offer tremendous promise due to their increased safety and high theoretical specific capacity. Lithium argyrodites have emerged as a lucrative class of solid-state electrolytes (SSEs) for ASSMLBs, owing to their high Li-ion conductivity (~10-4–10-3 S/cm), good elastic stiffness (~30 GPa), and low flammability. The Li-ion conduction pathways in single-crystalline compounds are typically examined using ab initio molecular dynamics (AIMD) simulations. However, the real-world materials have an added complexity of accounting for grain boundaries (GBs), which makes the AIMD simulations computationally expensive. To circumvent this limitation, we developed a novel ReaxFF interatomic potential, an empirical bond-order based force field, to investigate the effect of GBs on the Li-ion conductivity in argyrodite electrolytes. To carry out the fitting procedure, we used the fundamental static properties such as lattice parameters, heats of formation, elastic constants, surface energies, and equation of states in order to find the best set of ReaxFF parameters for LiPS system. All the static properties used in the fitting process were calculated using the periodic density functional theory. ReaxFF produced lithium diffusion maps of Li7PS6-LT and Li7PS6-HT clearly reproduces the formation of lithium diffusive cages around the sulfur atoms which validates the accuracy of the ReaxFF potential.At the atomic scale, we characterized the energetics, composition, and transport properties of three low-energy (Σ3 and Σ5) symmetric tilt GBs in lithium argyrodites. Our findings indicate that the presence of GBs impede the diffusion of Li ions. The activation energies for Li-ion conduction crossing the grain boundaries are consistently higher than that of the bulk crystal which confirms the significant grain boundary resistance in this material. We also formulate a polycrystalline model to quantify the atomistic effects of the grain boundaries. In this talk, we have explored these results to critically understand the role of grain boundaries on Li conductivity, and how altering the microstructure can be utilized to optimize new high-performance SSEs for emerging ASSLMBs.

Effect of Precipitation-Free Zone on Fatigue Properties in Al-7.02Mg-1.98Zn Alloys: Crystal Plasticity Finite Element Analysis.

Age-strengthened aluminum alloys, as important lightweight structural materials, have significantly lower fatigue properties compared to non-age-strengthened aluminum alloys. In this study, the polycrystalline models containing precipitation-free zones (PFZ) were constructed by secondary development of the traditional polycrystalline model by modifying the mesh file. Polycrystalline finite element simulations of peak age-treated Al-7.02Mg-1.98Zn alloys were carried out with this model. The results demonstrate that the PFZ's presence markedly reduces the alloy's yield strength and a substantial stress concentration occurs adjacent to the PFZ, generating significant compressive stresses at the PFZ. Under cyclic loading, the maximum strain energy dissipation in the model containing the PFZ far exceeds that observed in the conventional polycrystalline model, and the strain energy dissipation observed in the PFZ is significantly higher than that at other locations. This indicates that the PFZ is the main region for fatigue crack initiation. In addition, the introduction of a rotation factor to simulate the inhomogeneous rotation within the grain reveals that the additional stress concentration in the PFZ introduced by the aluminum alloy-forming process further increases the fatigue crack initiation driving force.

Deformation behavior study of SAC305 solder joints under shear and tensile loading by crystal plasticity finite element method

Micro solder joints experience various loads during operation, and the failure of a single micro solder joint can impact the overall reliability of the entire microsystem. This study utilized the Crystal Plasticity Finite Element Method (CPFEM) to create a polycrystalline model that accurately represents the shape of solder joints. By calibrating the crystal plasticity characteristics of SAC305 material solder joints using macroscopic stress-strain curves, the model successfully simulates the deformation mechanisms of micro-solder joints under both tensile and shear loads. This paper highlights that equivalent plastic strain serves as a key indicator of solder joint fracture behavior, revealing that strain components have varying effects on cracking: shear strain was found to cause crack initiation, while normal strain was found to promote crack extension. Moreover, Grain boundary features and orientation lead to uneven plastic strain by affecting the slip system and the Schmidt factor, especially small Schmidt factor grains at triple grain boundary intersections are more susceptible to crack initiation. Additionally, the study found that solder joints exhibit more pronounced mechanical responses under tensile loading compared to shear loading. These insights offer valuable guidance for the design and manufacturing of micro-solder joints.

Exploring multi-scale damage mechanisms in 8Cr4Mo4V alloy by molecular dynamics simulations and experiments

8Cr4Mo4V, as an ideal choice for aeroengines bearing materials, involves multi-scale damage mechanisms during the process of material failure. Herein, we propose a multi-scale analysis method to explore the crack initiation and propagation damage mechanisms of 8Cr4Mo4V alloy. We find that different crystal orientations produce distinctly different damage mechanisms. The second phase doping also has different strengthening effects on the matrix conformed to cut-off and Orowan mechanism. Meanwhile, the study of the polycrystalline model verified inverse Hall-Petch relation at the micro-scale, where material strength decreases with grain refinement. Phase transitions caused by dislocations lead to stress concentration at specific grain boundaries, becoming the main inducer for the nucleation of initial voids. To address micro-to-macroscale-crossing issues, we constructed a large-scale mesoscopic molecular dynamics simulation and proposed an analytical formula to predict the relationship between average grain size and yield stress. Finally, we combine macroscopic experiments, crystallographic analyses, and atomic-scale characterization, verifying the combined action of dislocations and stacking faults leads to the failure of the material.

Large magnetocrystalline anisotropic energy and its impact on magnetostriction of L10-FePt

Abstract The origin of magnetocrystalline anisotropic energy (MAE) guided by spin–orbit coupling in the L10-FePt alloy was analyzed. Correlation of MAE with the calculated magnetoelastic constants in the single and polycrystal model was determined by means of first-principles calculations using density functional theory (DFT). More specifically, a systematic analysis of the large MAE and magnetostriction coefficients (λ’s) were done by means of post-processing of eigenvalues (orbital energies) and eigenfunctions (orbital occupancies). Our numerical analysis includes the convolution of the projected wave-function (density of states) of each orbital of the Fe and Pt sub-lattices into their orbital energies, which in principle should be valid for any solid regardless of the strength of the MAE. The determined anisotropic magnetostriction coefficients are of the λ ∼ 10 − 4 –10–3 order for several crystallographic directions; which is in accord to the known experimental data. However, the polycrystalline model based on the uniform stress approximation leads to a decrease of the λ by two orders in the overall magnetostrictive behavior ( λ ∼ 10 − 5 ). Such a large difference is not unexpected as the magnetostriction for polycrystalline FePt thin films were recorded as very low, thus validating the robustness of the current theoretical proposition of analyzing MAE.

A Preisach Model Defining Correlation between Monotonic and Cyclic Response of Structural Mild Steel

This article delivers a new Preisach model representing the correlation between the elastoplastic behavior of structural mild steel under axial monotonic and cyclic loading with damage. The newly formed model is based on the experimentally defined correlation between axial monotonic and cyclic behavior of structural mild steel. To examine the monotonic and cyclic behavior of structural mild steel and find fitting material properties for the model, monotonic and cyclic axial tensile tests are performed. Tests are executed on coupons of the commonly used European structural steel S275. The model represents a mathematical description of modified single-crystal material behavior under monotonic loading. Two different approaches were used to describe damage in the multilinear mechanical model. The excellent agreement with experimental results is achieved by infinitely linking many single-crystal elements in parallel, forming the polycrystalline model. This model provides a good solution for everyday engineering practice due to its geometric representation in the form of the Preisach triangle and the lower costs of monotonic tests used to define material properties compared to cyclic tests.

Electroplasticity constitutive modeling of aluminum alloys based on dislocation density evolution

Electrical current can effectively improve the plasticity of metallic materials. The tensile deformation behavior of Al alloys under the pulsed electrical current assisted quasi-static unidirectional tension (EAT) has been investigated. Materials under the EAT exhibits periodic electro-softening and strain-hardening behaviors, i.e., a ratchet shape mechanical response. However, establishing a constitutive model to accurately predict the ratchet shape mechanical behavior, especially during the EAT interval, and accurately predicting the strain-hardening behavior of materials are critical issues that need to be solved urgently. In this study, based on the Taylor polycrystalline model, thermal activation theory and dislocation density evolution theory, a two-parameter dislocation density electroplasticity constitutive model with forward and reverse dislocation density evolution was developed to describe the periodic coupling effect of the electro-thermal-mechanical fields during EAT. The tensile deformation behaviors of AA 6061-T6 and AA 7075-T6 under the effect of a pulsed electrical current were quantitatively predicted using the proposed constitutive model. The results show that the correlation coefficient between the predicted and experimental results of the constitutive model can reach 0.84–0.99, implying that the proposed constitutive model can accurately predict the complex electroplasticity behavior of Al alloys during EAT.

Atomistic investigation on the anisotropic elastic and plastic responses of nanotwinned metals

Introducing nanotwins into materials is one of the important strengthening methods to achieve the synergistic improvement of strength and ductility. The anisotropic mechanical behaviors of nanotwinned materials have been widely studied by experimental and computational methods. The dominant deformation mechanisms about dislocation slippages can be effectively switched among three modes, and controlled by twin spacing and the angle between twin boundaries (TBs) orientation and loading direction. Particularly, most of previous researches mainly focused on the deformation mechanisms during the plastic flow stage and researchers paid little attention on the anisotropic characteristics of TBs at the elastic stage which are also essential to manufacture the high-performance materials. Therefore, this study is aiming to systematically investigate the anisotropic effect of TBs both at the elastic and plastic stages within the single crystalline or polycrystalline systems by molecular dynamics (MD) simulations. It is revealed that, when the loading direction is parallel to twin planes, the introduction of TBs in single crystalline models will significantly affect the characteristics of atomic bond rotation and elongation dominated elastic deformation, which can alter the Poisson’s ratio of materials, generate elastic-softening behavior and inhomogeneous elastic deformation. At the plastic flow stage, the deformation mechanism transforms from trans-twin dislocation slippage into the coexistence of Hard Mode I and threading dislocation slippage (Hard Mode II) when the loading direction changes from parallel to perpendicular direction with respect to TBs. Moreover, the dislocation segments at the conjunction of trans-twin dislocation play a momentous role in enhancing material strength. The results for polycrystalline models are consistent with that of single crystalline ones. These findings are expected to be beneficial for the development of high-performance nanostructured materials for structural and functional applications by strain engineering and defect regulation.

Closed-Loop Analysis of Nanocomposite Magnets: Integrating Micromagnetic Simulation and Experimental Testing

This study presents a framework for simulation-experiment closed-loop analysis of nanocomposite magnets. Firstly, hysteresis loops of nanocomposite permanent magnets were calculated using micromagnetic simulations. Then, the experimental preparation was carried out by rapid quenching of the melt and spark plasma sintering (SPS). We analyzed the effect of different sintering temperatures on the physical phase composition and magnetic properties of the samples. Characterization results show that the samples sintered at 600 °C have the highest magnetic energy product. Using transmission electron microscopy data, the polycrystalline model was refined, and further micromagnetic simulations were performed. The close agreement between simulated and experimental data confirms the improved accuracy of our simulation model. This iterative process of experimental characterization and simulation improvement reduces the discrepancy between simulation predictions and actual sample performance. It could facilitate rapid iteration in magnet development and provide valuable insights for optimizing the preparation of nanocomposite permanent magnets.

Molecular dynamics simulation of the influence of nanocutting parameters on microstructure of AISI M2

Molecular dynamics simulation has become the main analysis method of ultra precision machining of various materials. In this research, molecular dynamics method was used to explore the influence of nanocutting parameters on amorphous atoms, atomic coordination numbers, atomic strain and dislocations of AISI M2 workpiece. LAMMPS was used to construct the molecular dynamics model for CBN tool cutting AISI M2. The polycrystalline model of the workpiece contained 12 grains and iron, chromium and tungsten atoms. The polycrystalline model of the tool contained 3 grains and boron and nitrogen atoms. Tersoff, EAM, Morse and L-J potential energy functions were used to characterize the interaction between atoms. The following results were obtained through 8 groups of simulation experiments: (1) Both cutting speed and cutting depth promoted the amorphous transformation of workpiece atoms. (2) Under the extrusion of the tool, atoms with high coordination numbers were produced in the contact area between the tool and the workpiece. (3) The cutting depth was positively correlated with the average strain of workpiece atoms. Large strain atoms mainly occurred in the cutting area and gradually extended into the workpiece. (4) 1/2 < 111> dislocation was the most common dislocation form in nanocutting. This research plays a rich role in molecular dynamics simulation of nanocutting polycrystalline alloys.

Laser powder bed fusion of Fex(CoCrMnNi)100-x medium-entropy ferrous alloys: Processability, microstructure and dynamic deformation mechanism

In this work, Fex(CoCrMnNi)100-x (x = 40, 60 and 80 at.%) medium-entropy ferrous alloys (MEFAs) were fabricated through laser powder bed fusion (LPBF) of mixed powders composed of FeCoCrNiMn and Fe powders, and the dynamic compressive properties and deformation mechanisms were studied with combination of experiment and molecular dynamics simulation. The results demonstrate that all MEFAs exhibit good processability.The Fe40 and Fe60 samples were predominantly composed of FCC phase with coarse grains, while the Fe80 mainly consisted of BCC phase with a smaller grain size. When subjected to quasi-static and dynamic compression at 3000/s, the yield strengths of Fe40, Fe60 and Fe80 increases by 131, 220 and 256 MPa, respectively, illustrating the positive strain rate sensitivity and influence of Fe on the mechanical properties of MEFAs. Further, both the single-crystal and the polycrystalline models suggested that the incompatibility between the BCC and FCC phases is responsible for the enhancement of strength. Therefore, it is more likely to produce deformation twins, dislocations and thus stress concentration around the BCC phase, which is consistent with the TEM observations.

Insights into fatigue crack propagation behaviour of WAAM produced SS316 stainless steel: Molecular dynamics simulation and experimental analysis

The current work focuses on the crack growth mechanism / behavior of wire arc additively manufactured SS316L. Beside the experimental investigation, a molecular dynamics based polycrystalline model is developed to simulate the crack propagation under cyclic loading. The purpose of this work is to understand the crack growth behavior of wire arc additively manufactured SS316L, particularly the directional dependence of crack propagation. The experiments and molecular dynamics analysis are performed for the crack propagation along the build and traverse direction for the SS316L deposit. The experimental procedure involves fatigue crack growth rate tests, while the molecular dynamics simulations model the crack propagation in polycrystalline structures of different grain sizes. The ultimate tensile strength in build and traverse direction were 535±2 MPa and 495 ± 2 MPa, respectively. The experimental results reveal faster crack propagation along build direction in comparison to traverse direction. The values for Paris law material constants i.e. C and m are 7x10-15 and 6.78 along build direction while 8x10-14 and 6.02 along traverse direction, respectively. The crack growth along the build direction is primarily intergranular, which leads to high crack growth rate. The molecular dynamics simulations reveal the initiation and closure of secondary cracks for the traverse direction, which leads to slower crack growth rate along the traverse direction. The fractography of the fractured surfaces from fatigue crack growth rate test also indicate the formation of secondary cracks for the case of traverse direction. The molecular dynamics simulations performed with three grain sizes and thickness variation indicate the insensitivity of predicted crack growth mechanisms with respect to grain sizes used for simulations.

Effect of grain orientation distribution on the mechanical properties of Al-7.02Mg-1.78Zn alloys

PurposeDuring the forming process, aluminum alloy sheets develop various types of textures and are subjected to cyclic loading as structural components, resulting in fatigue damage. This study aims to develop polycrystalline models with different orientation distributions and incorporate suitable fatigue indicator parameters to investigate the effect of orientation distribution on the mechanical properties of Al-7.02Mg-1.78Zn alloys under cyclic loading.Design/methodology/approachIn this study, a two-dimensional polycrystalline model with 150 equiaxed grains was constructed based on optical microscope images. Subsequently, six different orientation distributions were assigned to this model. The fatigue indicator parameter of strain energy dissipation is utilized to analyze the stress response and fatigue crack driving force in polycrystalline models with different orientation distributions subjected to cyclic loading.FindingsThe study found that orientation distribution significantly influences fatigue crack initiation. Orientation distributions with a larger average Schmid factor exhibit reduced stress response and lower fatigue indicator parameters. Locations with a larger average Schmid factor experience greater plastic deformation and present a higher risk for fatigue crack initiation. RVE with a single orientation undergoes more rotation to reach cyclic steady state under cyclic loading due to the ease of deformation transfer.Originality/valueCurrently, there are no reports in the literature on the calculation of fatigue crack initiation for Al-Mg-Zn alloys using the crystal plasticity finite element method. This study presents a novel strategy for simulating the response of Al-7.02Mg-1.78Zn materials with different orientation distributions under symmetric strain cyclic loading, providing valuable references for future research.

Mastering the complex time-scale interaction during Stress Corrosion Cracking phenomena through an advanced coupling scheme

Stress corrosion cracking (SCC) failure is a multi-physics phenomena and is usually modelled at the microstructural level, which includes mechanical, chemical, and electrochemical contributions. In aggressive corrosive environments, materials prone to localized corrosion, can suffer heavy pitting corrosion. Subsequently, pit-to-crack transition events might be initiated. Respective mechanical assisted pitting corrosion propagation processes are determined by mechanical, chemical, and electrochemical properties as well as transport properties of ions at grain boundaries. Variations in the electrolyte exposure conditions (including the kinetics at the solid–liquid interface), different mechanical loading scenarios, grain-specific crystal anisotropies, and other local factors combine to produce a highly complex SCC-type interaction scenario at various time and length scales. Since corrosion is a slow process whereas brittle fracture is a rapid failure mechanism, the individual domain discretization-based solvers need to use distinct time steps and appropriate solver settings to become capable to accurately simulate such coupled events. In order to address the issues that arise while accounting for scaling effects in both time and space, and retaining coupled mechanistic interplay and including the aspects specified earlier, a sophisticated modelling method is necessary for the analysis of structural failure by SCC. In this work, a partitioned multi-physics computational approach is presented, using two separate single physics solvers coupled by the open-source coupling library preCICE. In the proposed computational setup, two separate software environments are used, with dedicated solver settings and different time steps, to simulate the mechanical fracture and dissolution-driven pitting corrosion for various loading and corrosion conditions, while also taking into account the effects of microstructural anisotropy. For a 2D polycrystalline model representing face-centered-cubic (fcc) material systems, selected numerical experiments are conducted that predict the evolution of fracture and crack path resulting from SCC. The framework has been further extended to simulate 3D problems as well. The corresponding results are evaluated to show the applicability of the proposed methodology.

Extending intergranular normal-stress distributions using symmetries of linear-elastic polycrystalline materials

Intergranular normal stresses (INS) are critical in the initiation and evolution of grain boundary damage in polycrystalline materials. To model the effects of such microstructural damage on a macroscopic scale, knowledge of INS is usually required statistically at each representative volume element subjected to various loading conditions. However, calculating INS distributions for different stress states can be cumbersome and time-consuming. This study proposes a new method to extend the existing INS distributions to arbitrary loading conditions using the symmetries of a polycrystalline material composed of randomly oriented linear-elastic grains with arbitrary lattice symmetry. The method relies on a fact that INS distributions can be accurately reproduced from the first (typically) ten statistical moments, which depend trivially on just three stress invariants and a few material invariants due to assumed isotropy and material linearity of the polycrystalline model. While these material invariants are complex averages, they can be extracted numerically from a few existing INS distributions and tabulated for later use. Practically, only three such INS distributions at properly selected loadings are required to provide all relevant material invariants for the first 11 statistical moments, which can then be used to reconstruct the INS distribution for arbitrary loading conditions. The proposed approach is demonstrated to be accurate and feasible for an arbitrarily selected linear-elastic material under various loading conditions.

Deformation mechanisms based on the multiscale molecular dynamics of a gradient TA1 titanium alloy.

The heterogeneous gradient TA1 titanium alloy holds great potential for a wide range of industrial applications. Considering the influence of the gradient structure on the plastic deformation behavior of the material, the TA1 gradient polycrystalline model under uniaxial compression is established. The deformation behavior of TA1 gradient polycrystals under uniaxial compression is investigated by molecular dynamics simulation. The simulation shows that there is significant transmission during the plastic deformation of TA1 gradient polycrystals. The transmissibility of plastic deformation is specified by the alternating appearance of twinning and grain refinement. Besides, the uniaxial compression process is accompanied by active dislocation motions. Moreover, the movement of dislocations is a dynamic cyclic process. In the same uniaxial compression environment, the triggering of the plasticity mechanism in the gradient polycrystalline model is closely related to grain size. The smaller grain size crystals hardly produce plastic deformation. Grain boundary migration of medium grain size crystals dominates in plastic deformation. The proliferation of dislocations under compressive stress is the primary trigger mechanism in larger grain size crystals. In addition, the stress concentration phenomenon in regions with medium grain sizes is more significant than that in regions with larger and small grain sizes.

Atomic simulation study of the factors affecting nucleation in deformation-induced martensitic transformation in grains and at grain boundaries in pure iron

Molecular dynamics simulations were performed to investigate the dominant factors affecting the nucleation in deformation-induced martensitic transformation in grains and at grain boundaries in pure iron, and the effectiveness of microstructure designs incorporating the obtained dominant factors was verified. First, the local nucleation stress required for grain-boundary nucleation was lower than that for in-grain nucleation, indicating that the grain boundary is an effective nucleation site for martensitic transformation. Next, we investigated the dominant factors affecting the in-grain nucleation and the grain-boundary nucleation, and found that the dominant factor of the in-grain nucleation is the magnitude of the loading-direction component of the transformation strain induced by lattice deformation (Bain distortion), while the dominant factors of grain-boundary nucleation are the grain-boundary misorientation angle and grain-boundary free volume. Finally, the possibility of controlling the nucleation timing by designing the microstructures based on the obtained nucleation dominant factors was demonstrated by comparing two polycrystalline models with the same texture but different distributions of grain-boundary misorientation angles.

Nanoindentation behaviour simulation of a polycrystalline NiTi shape memory alloy using the crystal plastic finite element method

Polycrystalline NiTi shape memory alloys (SMAs) are inevitably involved in plastic deformation under the action of an external force, thus significantly affecting the mechanical behaviour of materials. To deeply understand the plastic deformation mechanism of the NiTi SMA at 400 ℃, the representative volume element (RVE) model combined with the crystal plastic finite element method and the improved Voronoi algorithm were used to construct a three-dimensional polycrystalline spherical nanoindentation model for studying the contribution of different slip systems to the plastic deformation. The calculation results show that the constructed polycrystalline model can well match the experimental curve of the macroscopic stressstrain response curve of the polycrystalline NiTi SMA; the nanoindentation curve also confirms this result. Based on the constructed 3D polycrystalline nanoindentation model, three slip systems ({110}<100>, {010}<100>and {110}<111>) were introduced at the same time. According to the average cumulative shear strain calculated during the indentation process, the results show that the {110}<111> slip system provides the largest contribution to plastic deformation, and the {010}<100>slip mode provides the smallest contribution. The elastic modulus of the model was calculated using the Oliver-Pharr method, and the calculated results are consistent with the experimental results, which proves the validity of the model.

Investigation of the influence of macrozones in titanium alloys on the propagation and scattering of ultrasound

The presence of macrozones (or micro-textured regions) in Ti-6Al-4V (Ti-64) was shown to be a potential cause to the onset of cold dwell fatigue which reduces fatigue life significantly. Past research has demonstrated the potential of using ultrasonic testing for macrozone characterization, with the variation of ultrasound attenuation, backscatter and velocity in the presence of macrozones. However, due to the complexity of the microstructure, some physical phenomena that were observed are still not well understood. In this study, we propose the use of finite-element polycrystalline models to provide us with a means to systematically study the wave-macrozone interaction. Through this investigation performed using two-dimensional models, we are able to identify important correlations between macrozone characteristics (size, shape and texture) and ultrasound responses (attenuation, backscatter and velocity). The observed behaviours are then validated experimentally, and we also highlight how this understanding can potentially aid with the characterization of macrozones in Ti-64 samples.

Effect of residual stress in gradient-grained metals: Dislocation dynamics simulations

A three-dimensional discrete dislocation dynamics (DDD) method was used to investigate the effect of residual stress on the stress-strain response of gradient nano-grained (GNG) metals. The GNG polycrystalline model was constructed, and different residual stress distributions were introduced in the DDD framework. The simulation shows that the distribution of residual stress has a significant influence on the tensile stress-strain curve. For a GNG sample containing both compressive and tensile residual stresses, the flow stress is firstly lower but ultimately higher than that of the sample without residual stress, which is caused by the combined effects of residual stress and gradient-grained structure. The evolution of dislocation microstructures indicates that the initial tensile residual stress in the sub-surface of the sample can promote the dislocation activation and thus lead to the decrease of overall yield stress. Whereas the compressive residual stress in the topmost surface of the sample can suppress the dislocation activation and multiplication in other regions, resulting in finally higher flow stress in comparison to that of the sample without residual stress. In addition, it is demonstrated that the reduction in the elastic modulus for GNG samples with residual stress observed in some experiments is due to the earlier plastic deformation that occurred in the region with initial tensile residual stress. The present study provides insights into the individual and combined effects of residual stress and gradient-grained structure in GNG metals.