Mechanical Behavior of Gradient-Structured Nano-Crystalline NiCoAl Alloy

Abstract

Nanostructured metallic materials are widely applied in various fields due to their excellent comprehensive properties. Enhancing mechanical properties through microstructure design has emerged as a novel strengthening strategy. In this contribution, the microscopic mechanical behavior of coarse-grained and gradient-structured nanocrystalline NiCoAl alloys during tensile deformation was investigated via molecular dynamics simulations. Based on the investigation of compositional effects, the Ni₆₀Co₃₀Al₁₀ alloy composition was selected, exhibiting a yield strength of 4.92 GPa. The results indicate that increasing Al content reduces the material’s strength, Young’s modulus, and work hardening effect. Furthermore, by introducing a gradient structure with grain sizes gradually varying from 1.8 nm to 6.5 nm into the alloy, the yield strength reaches 1.8 GPa and the flow stress reaches 3.35 GPa, demonstrating a significant improvement compared to the uniform coarse-grained structure. Upon introducing the gradient structure into the alloy, it was observed that geometrically necessary dislocations (GNDs) nucleate in the coarse-grained region during deformation and gradually extend towards the fine-grained region. The increased grain boundary density effectively impedes dislocation motion and enhances dislocation pinning capability, thereby inducing continuous strain hardening and improving plasticity. By promoting the accumulation and interaction of grain boundary dislocations, the gradient structure achieves further strengthening and strain hardening in the alloy, providing a theoretical basis and simulation foundation for designing high-performance advanced alloys.

1. Introduction

Nanostructured materials refer to those with structural units, such as grain sizes in polycrystalline materials, in the nanometer range. Their defining microstructural feature is the presence of numerous grain boundaries or other interfaces, leading to mechanical and physicochemical properties that differ markedly from conventional coarse-grained materials [1,2]. Such materials typically exhibit exceptionally high strength and hardness, reaching several times or even dozens of times that of their coarse-grained counterparts, thereby offering a novel pathway for developing high-performance materials. However, this significant enhancement in strength is accompanied by a notable reduction in plasticity and toughness, a deterioration in work hardening capacity, and diminished structural stability. These performance degradations critically limit the practical application and further development of nanostructured materials [3]. Many biological materials exhibit heterogeneous structures in terms of their local chemical composition or spatial structural characteristics, and the most common type is the gradient structure [4]. Gradient nanostructure refers to a structure in which the size of structural units (such as grain size or lamellar thickness) varies gradually in space. One part of the material is composed of a nanostructure, and the other part consists of a coarse-grained structure, with the size of the structural units transitioning continuously and gradually between these two parts [5]. The gradient structure imparts exceptional mechanical properties to materials while also enabling a range of additional functionalities. Inspired by naturally occurring gradient architectures, researchers have incorporated gradient designs into metallic materials and alloys [6]. It has been demonstrated that gradient nanocrystalline materials exhibit markedly enhanced mechanical performance, particularly in mitigating the long-standing trade-off between strength and ductility. By effectively addressing the strength–plasticity dilemma inherent in conventional materials, the gradient configuration has garnered significant research interest [7,8,9,10].

The formation of a large number of geometrically necessary dislocations (GNDs) in gradient-structured metals and alloys during plastic deformation can induce sustained strain hardening, accommodate the incompatibility of nanoalloy deformation processes, and enhance plasticity [11,12,13]. The unique gradient structure endows gradient nanostructured metals and alloys with outstanding mechanical properties, with extremely high strength and excellent crack resistance, making them suitable for safety-critical parts and load-bearing structural components, such as aerospace, automotive parts, and microelectronics industries. Gradient-structured metals and alloys can induce strain gradients under uniaxial tension, converting uniaxial stress into multi-axial stress. The mechanism is that the incoherent deformation of gradient grains diverts the uniaxial stress in all directions. This unusual additional strain hardening ensures the maximization of the mechanical properties of metals and alloys at a low cost. Constructing the non-uniform gradient structure of metals and alloys provides an important strategy for developing materials with high strength and toughness. In previous research, Wang et al. [14] designed a “bimodal structure” in the alloy, where smaller fine grains provided the strengthening effect, while larger coarse grains offered the ability to store dislocations, thereby achieving the simultaneous improvement in both the strength and plasticity of metals and alloys. Sun et al. [15] fabricated a gradient structure in H62 brass containing α and β phases via surface mechanical attrition treatment (SMAT). Tensile tests revealed that the specimen subjected to SMAT for 2 min exhibited a 1.77-fold increase in yield strength compared to the annealed sample while maintaining a uniform elongation of 45.8%, nearly equivalent to that of the annealed state, thereby achieving a desirable synergy between strength and ductility. This synergistic effect is attributed to the substantial generation of GNDs induced by deformation incompatibility at the interfaces between the gradient layer, coarse-grained matrix, and α/β phases. The significant accumulation of GNDs gives rise to pronounced hetero-deformation-induced (HDI) strengthening and strain hardening, enabling the simultaneous enhancement of strength and ductility. Jiang et al. [16] proposed a strategy for constructing a gradient-heterogeneous architecture via a continuous cold-drawing method, achieving, for the first time, the synergistic evolution of radially aligned gradient and heterogeneous structures in ultrafine single-crystalline copper wires. Through systematic analyses of microstructural scale transitions, texture evolution, spatially resolved recrystallization behavior, stored energy distribution, and the accumulation of GNDs, the study elucidated a composite strengthening mechanism driven by microstructural differentiation under highly non-uniform strain fields.

Previous studies have employed molecular dynamics simulations to investigate the Hall–Petch effect in NiCoAl alloys under the influence of nanotwins and elucidated the underlying strengthening mechanisms induced by nanotwins [17]. Furthermore, the introduction of carbon elements at the grain boundaries of nanocrystalline NiCoAl alloys led to the formation of interconnected carbon chain networks, resulting in a notable strengthening effect [18]. Additionally, it was found that variations in twin spacing and loading direction significantly influence the mechanical response of NiCoAl alloys [19]. However, the mechanical behavior of NiCoAl alloys with gradient structures remains to be further explored.

Nickel-based multi-component alloys exhibit strong corrosion resistance and high-temperature mechanical strength, demonstrating excellent mechanical properties at elevated temperatures. Consequently, they are widely utilized in high-end manufacturing sectors such as aerospace, offering broad application prospects and holding significant strategic importance. Therefore, in this contribution, the mechanical properties of the nickel-based superalloy are further enhanced by introducing a gradient nanostructure into it.

2. Computational Details

Before conducting the design and simulation calculation for the gradient-structured nanocrystalline NiCoAl alloy, the composition of the alloy must first be designed. During the composition design phase, the Ni content was set at 60% to maintain the base structure as the face-centered cubic (FCC) phase. Subsequently, Co and Al elements were added with a total content of 40%, though in varying proportions. A total of five alloy compositions were designed, including Ni₆₀Co₄₀Al₀, Ni₆₀Co₃₀Al₁₀, Ni₆₀Co₂₀Al₂₀, Ni₆₀Co₁₀Al₃₀, and Ni₆₀Co₀Al₄₀. Figure 1 illustrates the model and microstructure of the Ni₆₀Co₂₀Al₂₀ alloy composition. The simulation cell has three-dimensional dimensions of 150 × 150 × 150 ų, contains 10 grains, and exhibits an average grain size of 8.64 nm. Molecular dynamics tensile tests were conducted on the five alloy compositions to investigate the effect of compositional variations on material properties, leading to the determination of the optimal NiCoAl alloy composition.

Subsequently, a gradient microstructure was designed and introduced into the alloy. The conventional coarse-grained models and gradient nanostructure models of the nanopolycrystalline alloy were established with the classical software ATOMSK [20]. The size of the constructed model measured 100 × 100 × 300 ų and comprised 275,212 atoms. The coarse-grain model structure contains 20 grains, while the gradient structure alloy consists of 170 grains. A gradient is set in the Z-axis direction and the average grain size is controlled to increase from 1.8 nm at the surface to 6.5 nm at the bottom. After establishing a reliable and accurate alloy model, it is necessary to further select the correct simulation conditions [21]. Based on the molecular dynamics simulation software LAMMPS (3 March 2020) [22], a stretching simulation was conducted. The potential function adopted the Embedded Atom Method (EAM) classical approach for calculating the interatomic forces of metals and alloys, as established by Daw et al. [23]. To enhance the accuracy and reliability of the computational results, the conjugate gradient method and the steepest descent method were used to relax the initial structural model. Subsequently, relaxation was performed under the NPT ensemble with a Nose-Hoover thermostat at 300 K, thereby obtaining a stable alloy model that is close to the actual situation. The integration method employs the Velocity-Verlet scheme to simulate tensile deformation along the Z-axis under the NPT ensemble. The tensile rate and integration step size are set to 5 × 10⁸ s⁻¹ and 1 fs, respectively. The simulation runs for a total of 6 × 10⁵ steps, with computational results outputted. Subsequently, the simulation results were visualized using the open visualization tool OVITO [24], with modules such as Common Neighbor Analysis (CNA) employed to identify structural evolution and the Dislocation Extraction Algorithm (DXA) utilized for dislocation analysis and quantification.

3. Results and Discussion

3.1. Effect of Alloying Elements on NiCoAl Alloys

To investigate the effect of elemental composition ratios on the mechanical properties of nickel-based alloys, this study designed an alloy system with a fixed Ni content of 60% and a combined Co and Al content of 40%, comprising five compositions: Ni₆₀Co₄₀Al₀, Ni₆₀Co₃₀Al₁₀, Ni₆₀Co₂₀Al₂₀, Ni₆₀Co₁₀Al₃₀, and Ni₆₀Co₀Al₄₀. Figure 2a presents the stress–strain curves obtained from tensile simulations of polycrystalline NiCoAl alloy nanostructures with different compositional ratios. The results indicate that the Co-to-Al ratio significantly influences the mechanical properties of the alloy. All five compositions sequentially undergo elastic deformation, yielding, and subsequent plastic deformation, characterized by zigzag oscillations in the stress–strain curves. As the Al content increases, the alloy strength progressively decreases, accompanied by a reduction in work-hardening capability, manifested as diminished oscillation amplitudes in the stress–strain curves (Figure 2a). To quantitatively analyze the effect of compositional variations on the strength of NiCoAl alloys, Figure 2b illustrates the evolution of yield strength and Young’s modulus with changing alloy composition. When the Al content increases from 0% to 40%, the yield strength decreases from 5.89 GPa to 2.70 GPa. Young’s modulus, a physical quantity describing the resistance of solid materials to deformation, decreases from 119 GPa to 88.44 GPa with increasing Al content, indicating reduced material stiffness and diminished deformation resistance. Furthermore, previous studies have shown that in nickel-based alloys, the optimal mechanical properties are achieved when the Co content is 30% [25]. Therefore, the Ni₆₀Co₃₀Al₁₀ alloy composition was selected for subsequent investigations into gradient structures. The radial distribution function describes the probability of finding other atoms around a given atom and serves as a crucial tool for analyzing the crystal structures of metals and alloys [26]. Figure 2c,d presents the radial distribution functions of the alloys with compositions of Ni₆₀Co₄₀Al₀ and Ni₆₀Co₃₀Al₁₀. It can be noticed that the addition of the solute element Al alters the crystal structure of the alloy. For the former, the first peak of the radial distribution function reached a maximum of 8.5. When 10% Al was added to the alloy, the first peak of the radial distribution function decreased to 8.1, indicating that the addition of Al reduced the occurrence probability of Ni–Ni potential function pairs.

To further investigate the effect of Al addition on the mechanical behavior of nanocrystalline nickel-based alloys, Figure 3 illustrates the structural evolution of nanocrystalline Ni₆₀Co₃₀Al₁₀ and Ni₆₀Co₄₀Al₀ with increasing strain. In the figure, blue atoms represent the FCC structure, gray–white atoms denote grain boundaries or amorphized atoms (Other), red atoms indicate the hexagonal close-packed (HCP) structure, and purple atoms represent the body-centered cubic (BCC) structure. In the nickel-based alloy without Al, the model structure after relaxation is relatively regular. However, as the degree of deformation continues to increase, most grain boundaries begin to fracture, causing some small grains to be engulfed by larger grains. Throughout this process, grain boundary migration and grain coalescence occur. When 10% Al is added to the nickel-based alloy, the grain boundaries begin to widen, which is primarily reflected in the relaxed model structure. This thickening of grain boundaries enhances structural stability. With increasing strain, no large-scale fracture or migration of grain boundaries is observed. Furthermore, by comparing the structural change processes of nano-crystalline Ni₆₀Co₃₀Al₁₀ and Ni₆₀Co₄₀Al₀ during the process of strain increase, it is clearly observed that a significant phase transformation occurred in the alloy with 10% Al content. Particularly, the extensive occurrence of HCP-phase dislocation structures indicates a deterioration in material performance, suggesting a high defect density within the alloy.

Dislocations, as two-dimensional defects, play a critical role in understanding the mechanical behavior of metals and alloys, and investigating their motion is essential for revealing the underlying microscopic mechanisms [27,28,29]. Figure 4 illustrates the evolution of dislocations in nanocrystalline Ni₆₀Co₃₀Al₁₀ and Ni₆₀Co₄₀Al₀ alloys with increasing strain. During the deformation process, dislocations in both alloys exhibit distinct patterns of multiplication and propagation, depending on their initial structural characteristics. The emergence of these dislocation defects leads to a gradual degradation of the mechanical properties of the alloys. In the initial relaxed state, the alloy containing 10% Al exhibits a relatively low dislocation density, which may be attributed to grain boundary thickening. As strain increases, the dislocation density begins to rise, with dislocations nucleating at grain boundaries and gradually gliding into the grain interiors. This results in the formation of numerous stacking faults, as shown in Figure 3f,g. Eventually, a high density of dislocation defects develops within the alloy. In contrast, the Al-free alloy shows a higher dislocation density in the initial model, with dislocations predominantly concentrated at grain boundaries. With continued deformation, grain boundary sliding and fracture occur, causing dislocations at the boundaries to slip and annihilate, ultimately leading to a lower dislocation density compared to the Al-containing alloy. This is because the critical stress required for the dislocation slip is relatively high in the Al-free alloy, resulting in fewer dislocations and stacking faults within the grains during the later stages of deformation.

By investigating the mechanical property variations of nanocrystalline Ni₆₀Co₃₀Al₁₀ and Ni₆₀Co₄₀Al₀ alloys during tensile deformation, it was found that by increasing the Al content, the strength of the nanocrystalline alloy decreases and its work hardening capacity is correspondingly reduced. Consequently, in the following section, the Ni₆₀Co₃₀Al₁₀ composition was selected for the further design of a gradient structure within the nanocrystalline alloy to deeply explore its mechanical response and underlying strengthening mechanisms.

3.2. Tensile Mechanical Properties and Microscopic Deformation Mechanisms of Gradient-Structured Ni₆₀Co₃₀Al₁₀ Alloys

To observe the influence of the introduction of the gradient structure on the mechanical properties of the nano-polycrystalline NiCoAl alloy, a model with a size of 100 × 100 × 300 ų was established, which contained a total of 20 grains. The gradient structured nanoalloy model comprises 170 grains, with a gradient established along the Z-axis and controlled such that the average grain size increases from 1.8 nm at the surface to 6.5 nm at the bottom. Subsequently, simulation of tensile deformation is conducted using LAMMPS. Figure 5 illustrates the evolution of mechanical properties during tensile deformation for both uniformly grained alloys and gradient-structured nanopolycrystalline alloys. Among them, Figure 5a shows the stress–strain curves of the gradient-grain alloy and the coarse-grain alloy during the stretching deformation process. It can be noticed that as the strain increases, the alloy first enters the elastic stage, during which the alloy undergoes recoverable deformation. As the strain continues to increase, the alloy exhibits a yield behavior and enters the plastic deformation stage, appearing as a sawtooth pattern on the stress–strain curve. Figure 5b presents the yield strength and average flow stress for coarse-grained and gradient-structured alloys. It is evident that the strength of gradient-structured nanopolycrystalline alloys is significantly higher than that of coarse-grained alloys. When a gradient structure is introduced into the alloy, the mechanical properties of the nano-granular alloy are significantly improved. The yield strength and average flow stress increase from 1.20 GPa and 2.49 GPa to 1.80 GPa and 3.35 GPa, respectively. Compared with the coarse-grained alloy, the gradient-structured alloy exhibits higher strength and greater resistance to plastic deformation. Figure 5c,d plot the radial distribution function curves for coarse-grained alloys and gradient-structured alloys. It can be seen that the peak values and quantities of the radial distribution function curves for the coarse-grained alloys and the gradient alloys have not undergone significant changes, indicating that the introduction of the gradient structure has not altered the crystal structure of the alloys.

To further investigate the effect of introducing a gradient structure on the mechanical behavior of nanocrystalline nickel-based alloys, Figure 6 illustrates the microstructural evolution of coarse-grained and gradient-structured alloys during stress application, with the same atomic coloring scheme as in Figure 3. It can be observed that as the strain increases, grain boundaries within the alloy structure undergo fracture and phase transformation occurs. When the strain exceeds 20%, a substantial number of stacking faults begin to emerge in the system. With further increases in strain, most grain boundaries start to fracture, leading to the engulfment of some small grains by larger ones. Consequently, grain boundary migration and grain coalescence take place throughout this process. A comparative analysis of the two alloy structures reveals that the number of stacking faults in the gradient-structured alloy is significantly lower than that in the coarse-grained alloy, indicating fewer defects and superior mechanical properties in the gradient-structured alloy. Furthermore, it is observed that defects in the gradient structure initially appear in coarse grains. As the strain continues to increase, stacking fault defects begin to propagate into fine grains, demonstrating the progressive nature of deformation in gradient-structured alloys. Defects and stress concentration first emerge in the coarse grains of the gradient alloy, after which the interaction between normal stress and back stress alters the deformation mechanism of the gradient alloy, resulting in a strengthening effect.

To observe the phase transformation behavior and the evolution of various dislocation lengths during deformation more clearly and precisely, we quantitatively calculated and statistically analyzed the number of atoms in different structural phases and the dislocation line lengths in the two alloy samples. As shown in Figure 7a,b, in the homogeneous coarse-grained alloy, the number of perfect FCC atoms gradually decreases, transforming into stacking fault atoms classified as Other and HCP phases. This phenomenon occurs because the coarse-grained structure undergoes extensive dislocation slip during deformation, which subsequently generates a higher density of stacking faults, leading to an increase in HCP atoms. Concurrently, this process is accompanied by structural amorphization and disordering. The increase in both Other and HCP atoms ultimately results in a significant decrease in the proportion of FCC-structured atoms. In contrast, for the gradient-structured nanocrystalline alloy, as the strain increases, the number of perfect FCC atoms gradually rises, while only a small number of HCP phase structures emerge. This is attributed to the fact that the grain boundary volume fraction in the gradient structure is substantially higher than that in the uniform coarse-grained structure. During deformation, grain boundary sliding occurs in the smaller grains, leading to grain coalescence and growth, which increases the count of FCC-structured atoms. Furthermore, the presence of fine grains restricts dislocation slip, thereby limiting the formation of HCP-structured atoms. For both alloy types, the number of BCC-structured atoms remains negligible and can be effectively ignored. As can be seen from Figure 7c,d, during the deformation process, the number of Shockley dislocations is the highest, which indicates that Shockley dislocations are the dominant factor in the plastic deformation mechanism of the entire deformation process. Moreover, by comparing the changes in dislocation lengths in the two alloys, it can be observed that the gradient-structured alloy has a higher number of dislocations. This indicates that the existence of GNDs plays a role in dislocation strengthening, significantly enhancing the mechanical strength of the alloy.

Figure 8 displays the evolution of dislocations in coarse-grained alloys and gradient-structured alloys during deformation. As can be seen in the figure, after the relaxation of the model structure, most of the dislocations are located at the grain boundaries. Because the grain boundaries are composed of a large number of disordered atoms, and the lattice mismatch of these disordered atoms is relatively large, many dislocation defects will be generated [30,31]. As the strain gradually increases, dislocations at the grain boundaries begin to multiply and diffuse into the grain interiors. Meanwhile, certain vacancy defects emerge within the grains, serving as nucleation sites for dislocations. The rising strain causes dislocations inside the grains to progressively spread to surrounding areas, leading to a substantial increase in the total dislocation density within the system. Furthermore, a comparison of the evolution in the number of dislocation clusters between the two alloys clearly reveals that when the strain reaches 0.2, the number of dislocation clusters in the gradient-structured alloy begins to rise sharply. This occurs because, within the gradient structure, dislocations are initially predominantly located in the larger grains. As strain is applied, the dislocation density in these large grains increases, after which dislocations gradually transition toward the smaller grains. During this process, most dislocations become impeded by the numerous grain boundaries present in the fine-grained regions. Simultaneously, these GNDs induce gradual deformation in the alloy to accommodate the inhomogeneity between coarse and fine grains.

Figure 9 presents the stress distribution contour plots for the two alloys during deformation. It can be observed that as strain increases, stress concentration initially occurs at the grain boundaries, where most atoms deviate from their original lattice positions. Consequently, these atoms appear as disordered structures following visualization. Upon gradual application of external stress to the system, stress concentration first emerges at the grain boundaries. A comparison of the stress distribution contour plots between the coarse-grained and gradient-structured alloys reveals that when the strain reaches 0.3, the coarse-grained alloy exhibits stress concentration, with a transverse crack pattern indicated by the white box, suggesting that fracture would initiate at this location when the alloy can no longer sustain further strain. In contrast, for the gradient-structured alloy, the regions of stress concentration are predominantly localized to isolated points or small segments.

Therefore, when stress concentration occurs at the grain boundaries of large grains, the stress gradually transitions to the small grain regions, resulting in a gradual transition of the stress continuity and gradient. More GNDs can be stored in the material, inducing the emergence of dislocation strengthening effects in the alloy and thereby enhancing the strength of the alloy. Furthermore, the continuous and gradual deformation provides the alloy with sufficient ductility [32]; thus, the introduction of the gradient structure can result in advanced alloys with a synergistic effect of high plasticity.

Figure 10 presents two alloy structure models and a schematic diagram of dislocation movement. Figure 10a,b show schematic diagrams of coarse-grained and gradient models, respectively. It can be observed in Figure 10c,d that in the traditional coarse-grained structure model, the nucleation of dislocations is relatively easier. When the strain reaches the dislocation nucleation condition of the weakest grain among all the coarse grains, dislocations first form in the weakest coarse grain, causing the alloy to undergo the yielding phenomenon. Subsequently, the dislocations start to rapidly diffuse from the nucleation point to the surrounding grains. This process involves the accumulation and release of dislocations, which manifests as sawtooth-like fluctuations in the stress–strain curve. The earliest formation of dislocation nuclei and the rapid proliferation and diffusion of dislocations make the yield of traditional coarse-grained alloys very easy, thus resulting in relatively low strength. In gradient-structured alloys, to accommodate the heterogeneous structural deformation caused by grains of varying sizes, the dislocation forms within the alloy transform into GNDs. At this stage, dislocations generated in the weakest coarse grains begin to propagate and spread into the fine-grained regions. The increased grain boundary density enhances the alloy’s resistance to dislocation motion, suppresses the progressive movement of GNDs, and induces continuous strain in the alloy, thereby triggering strain hardening and improving the resistance to plastic deformation. The increased number of grain boundaries significantly enhances their capacity to accommodate GNDs. After nucleating in coarse grains, dislocations migrate slowly toward fine-grained regions. The effectiveness of grain boundaries in pinning dislocations far exceeds that of other alloy models, leading to a further enhancement of induced alloy strength beyond that achieved through gradient structure strengthening [12,33].

4. Conclusions

In summary, molecular dynamics simulations were employed to investigate the tensile deformation process of coarse-grained and gradient NiCoAl alloys. The microstructural mechanisms underlying different strengthening stages during deformation were observed at the atomic and molecular scales. The deformation mechanism of this strengthening strategy was revealed from the perspectives of the nucleation conditions of GNDs and the motion process of dislocations, providing theoretical guidance and a data basis for the design of advanced alloys with excellent performance.

(1)

Before proceeding with the design and simulation calculations of gradient-structured nanopolycrystalline NiCoAl alloys, the composition design is carried out first. The content of the Ni element is set to occupy 60% of the total model while maintaining the base structure as the FCC phase. Subsequently, Co and Al elements are added to the matrix with a combined content of 40%. Tensile calculations are performed via molecular dynamics simulations to investigate the impact of compositional variations on mechanical properties. It was found that increasing the Al content in the alloy reduces the material’s stiffness, significantly decreases the Young’s modulus, and leads to a gradual decline in the alloy’s deformation resistance, a substantial reduction in strength, and a decrease in work hardening capacity.

(2)

After introducing a gradient structure into the NiCoAl alloy, to accommodate the heterogeneous structural deformation caused by different grain sizes, the dislocation form in the alloy will transform into GND. First, dislocations will form in the weakest coarse grains and gradually diffuse towards the fine grain regions. The increase in grain boundary density enhances the alloy’s ability to resist dislocations. The progressive movement of GNDs is inhibited, leading to continuous strain in the alloy and triggering its strain hardening, thereby enhancing its resistance to plastic deformation. The increased number of grain boundaries significantly enhances their capacity to accommodate GNDs. After nucleating in coarse grains, dislocations migrate slowly toward fine-grained regions. The effectiveness of grain boundaries in pinning dislocations far exceeds that of other alloy models, leading to a further enhancement of induced alloy strength beyond that achieved through gradient structure strengthening.