Extra Strengthening Research Articles

Tuning microstructures and tensile properties of the CoNiV medium entropy alloy by minor alloying

The effects of alloying with only 1 at% Al or Nb on the microstructures and mechanical properties of a CoNiV medium entropy alloy (MEA) (i.e., (CoNiV)99M1) have been systematically investigated. Thermodynamic calculations suggest the σ-phase formation temperature is increased from 1143 K in the CoNiV MEA to 1186 K and 1195 K when alloyed with Al and Nb respectively. Similar to the parent CoNiV MEA, solid-solution and grain boundary strengthening contribute immensely to the yield strength of the alloys. In addition, the σ precipitates in the (CoNiV)99Nb1 MEA provide an extra strengthening effect which demonstrates the potential for secondary-phase strengthening of the CoNiV MEA. Our findings reveal that the microstructure tuning capabilities of CoNiV alloys can be harnessed by minor alloying strategies to affect their mechanical properties.

Mechanics of gradient nanostructured metals

The emergence of heterogeneous nanostructured materials (HNMs) offers exciting opportunities to achieve outstanding mechanical properties. Among these materials, gradient nanotwinned (GNT) Cu is a prominent class of HNMs, demonstrating superior strengths by gaining extra strengths compared to non-gradient counterparts. Its layered gradient structure provides a simplified quasi-one-dimensional model system for understanding the extra strengthening effects of structural gradients and resulting plastic strain gradients. This paper presents a comprehensive report for recent experimental and modeling studies on the mechanics of GNT Cu, covering advances in controlled material processing, back stress measurement, deformation field characterization, dislocation microstructure analysis, and strain gradient plasticity modeling. These studies unveil the spatiotemporal evolution of both plastic strain gradients and extra back stresses originating from structural gradients. Direct connections are established between the sample-level extra strength of GNT Cu and the synergistic strengthening effects induced by local nanotwin structures and their gradients. We emphasize the critical role of the size of the representative volume element in assessing the effects of plastic strain gradient and extra back stress. Moreover, lower-order strain gradient plasticity models are validated through experimental characterizations of GNT Cu, paving the way for future investigation into the mechanics of gradient nanostructured metals. Finally, we provide an outlook on research needs for understanding the mechanics of gradient nanostructured metals and, more broadly, HNMs, towards achieving exceptional mechanical properties.

Tailoring the microstructure of Al/Ti laminated composite through hot press sintering process to achieve superior mechanical properties

A novel method based on hot press sintering was proposed to fabricate the Al/Ti laminated composite, and the effects of sintering parameters on microstructure, interfacial structure, and mechanical properties were thoughtful examined. The results showed that insufficient sintering caused obvious voids and cracks at Al/Ti interface. With the increase of temperature or holding time, a good metallurgical bonding without defects was achieved, and both the recrystallization fraction and grain size of α phase in Ti layer increased, accompanied by the transformation of α to β phase and growth of intermetallic phase. The microstructure variation of Al layer is not evident with changing the sintering parameters. Due to the relatively low formation energy, the nanoscale TiAl3 phase with massive stacking faults formed at Al/Ti interface, and lots of dislocations existed at Al layer near the interface. The voids and cracks formed at Al/Ti interface led to the premature failure of laminate. On the basis of the metallurgical bonding and small TiAl3 phase, the high fraction and texture intensity of α phase were the dominated strengthening factors. Moreover, the back stress generated at Al/Ti interface contributes to an extra strengthening. The superior mechanical properties of Al/Ti laminate with a tensile strength of 670.9 MPa and a fracture strain of 0.33 were obtained with the sintering temperature of 600 °C and holding time of 2 h.

Elucidating the mechanical response and microstructure evolution of the constituent layers in gradient-structured Cu alloys

The design of gradient structure (GS) in metallic materials has been reported to obtain superior mechanical properties due to the interaction between the GS layer and the coarse-grained (CG) matrix. In this study, the mechanical response and microstructural evolution of the corresponding constituent layers of GS Cu-4.5Al alloy at different pre-strains were systematically investigated. The tensile results showed that the strain hardening capability of the GS layer was improved due to the constraint of the CG matrix. Meanwhile, an extra strengthening was observed at the interface between the GS layer and the CG matrix, resulting in a higher yield strength of the CG matrix constrained by the GS layer than that of the CG matrix without the GS layer constraints. Moreover, the extra strengthening at the interface of the GS and CG matrix was gradually reduced with increasing pre-strain due to the weakening of the interaction at the interface. Microstructural observations at the interface between the GS layer and the CG matrix revealed that more dislocations and twins were activated during pre-strain in the CG matrix having GS constraints than that of the CG matrix without GS constraints. The present work provides important insights on the interaction between constituent layers, in terms of the mechanical response and microstructure evolution.

Understanding extra strengthening in gradient nanotwinned Cu using crystal plasticity model considering dislocation types and strain gradient effect

Gradient nanotwinned (GNT) Cu, composed of various homogeneous nanotwinned (HNT) components, successfully achieves the synergistic enhancement of strength and ductility due to the combined advantages of gradient and nanotwinned structures. However, the complicated dislocation-twin boundary (TB) interactions in each HNT component and extra strengthening mechanism in GNT Cu remain elusive. In this study, a crystal plasticity model, which takes into account the reaction characteristics of various dislocation types at the TB and the extra size dependent geometrically necessary dislocations (GNDs) induced by plastic strain gradients, is developed for understanding both HNT and GNT Cu. The developed model successfully captures the mechanical behavior and microstructure evolution of HNT Cu with different TB orientations and grain sizes (and twin thicknesses). The simulation results emphasize the importance of incorporating dislocation types when describing dislocation-TB interactions. Furthermore, the intrinsic mechanism for the extra strengthening in the GNT Cu is revealed through the analysis of deformation contours and microstructural evolutions. The results show that the HNT components with the lower original strength have the higher extra back stress increment with the increase of structural gradient. This study provides valuable insights into predicting and further optimizing the strength of the GNT Cu through manipulating the gradient microstructure.

Extra strengthening and Bauschinger effect in gradient high-entropy alloy: A molecular dynamics study

The gradient nano-grained (GNG) structures achieve a combination of high strength and ductility compared with homogeneous nano-grained structures. In order to study the mechanical behavior of GNG with extremely small grain size, the systematic tension, compression and cyclic deformation behavior of gradient and homogeneous nano-grained CoCrFeMnNi high-entropy alloy (G-HEA and H-HEA) are studied by the molecular dynamics (MD) simulation. The simulation results show that G-HEA and H-HEA have multiple plastic deformation mechanisms during uniaxial and cyclic loading, such as dislocation slip, deformation twinning, phase transformation from face-centered cubic (FCC) to hexagonal close-packed (HCP), and grain boundary plastic behavior. During uniaxial loading, the heterogeneous deformation makes the yield stress of G-HEA higher than that calculated by the rule of mixture (ROM), indicating an extra strengthening in GNG material. However, the extra strengthening of G-HEA is weakened with increased strain due to the dislocation without piled up in front of the grain boundary and relaxed heterogeneous deformation. During cyclic loading, the sessile dislocations promote the reverse movement of dislocations, resulting in the Bauschinger effect. However, the sessile dislocation density of G-HEA is close to that calculated by the ROM, resulting in the Bauschinger effect of G-HEA close to that calculated by the ROM. Therefore, as an alternative perspective, the MD simulation confirms that dislocation pinning in front of grain boundaries is an important source of extra strengthening for gradient structured materials as revealed by experiments. The results extend the understanding of the deformation mechanism of CoCrFeMnNi GNG HEA with extremely small grain size under tension, compression and cyclic loading.

Wavy interface enables extra strengthening in an additively manufactured high-entropy alloy with Mortise-Tenon architecture

Laminated structures have the potential to enhance the performance of high-entropy alloys (HEAs) by providing intriguing interface strengthening. However, laminated HEA designs is uncommon due to the challenges posed by the relatively poor machinability of HEAs. In this study, we successfully fabricate a CoCrNi-Fe50Mn30Co10Cr10 laminated HEA using additive manufacturing technique, incorporating a unique Mortise-Tenon architecture within the alloy. The laminated HEA introduces a novel wavy-shaped interface alongside the conventional flat-shaped interfaces. Tensile tests reveal that the laminated HEA exhibits an exceptional combination of strength and ductility. The measured yield strength exceeds the predicted value based on the rule-of-mixture principle by approximately 36.5 %. This enhancement is attributed to the extra strengthening resulting from the local chemical variation near the heterogeneous interfaces. Interestingly, the wavy-shaped interface has a larger local-chemical-variation zone than the flat-shaped interface, which can trigger more strong interface-dislocation interactions and therefore remarkable strengthening. The Mortise-Tenon architecture not only provides extra interface strengthening, surpassing the yield strength of the two monolithic HEAs, but also preserves the excellent work hardening ability in the early stage of deformation, enabling reasonable ductility. Remarkably, the interface strengthening is comparable in magnitude to dislocation strengthening, contributing to approximately 23 % of the overall yield strength. These findings highlight the potential of laminated HEA designs and the associated interface strengthening as promising strategies for enhancing mechanical performance.

Synergistic strengthening mechanism of gradient structure AZ31 magnesium alloy plate in hard plate rolling

Compared with homogeneous structural materials, gradient structural materials are easier to realize the strength-ductility optimization. Research on the strengthening mechanism of different gradient structure materials is a meaningful way to broaden the application of gradient structure materials. In this paper, the AZ31 magnesium alloy plate was rolled by hard plate rolling. The gradient structure was controlled by changing the initial plate thickness. When the initial plate thickness was 4 ​mm, AZ31 magnesium alloy shows excellent strength and ductility, the tensile strength was 282.2 ​MPa, and the ductility was 28.4 ​%. The change of the initial plate thickness changes the stress state which forming a gradient structure with different characteristic layers. The extra strengthening between different characteristic layers improves the properties of gradient structure, which makes the magnesium alloy change from a uniaxial stress state to a multi-axial stress state during the tensile process. The increase of the characteristic layer induces more geometrically necessary dislocation accumulation, improving the gradient structure's strength and ductility. This research provides a new design strategy for the forming and performance optimization of gradient structure magnesium alloy plate by hard plate rolling, which has good industrial production and application potential.

Interface strain gradient enabled high strength and hardening in laminated nanotwinned Cu

Interfaces play a crucial role in mechanical behaviors of laminated materials. In this study, a series of hard/soft nanotwinned Cu laminates with varying interface spacing from 200 to 33 μm are prepared by means of direct current electrodeposition. Simultaneous improvement of strength and work hardening with decreasing interface spacing is found in tensile tests. Extra strengthening and geometrically necessary dislocations (GNDs), but without any strain concentration, are found in the vicinity of interfaces. An obvious strain discrepancy appears across the interface and decreases with decreasing interface spacing. Most importantly, a highest plastic strain gradient appearing in the vicinity of interfaces, regardless of interface spacing, indicates a significant deformation compatibility across the interfaces. The spatially-distributed strengthening mediated by interface contributes for increasing strength and decreasing strain discrepancy by decreasing the interface spacing.

Grain Distribution for Optimal Strength in Homogeneous and Gradient Nano‐Grained Coppers

The distribution of grains plays a crucial role in determining the strength of polycrystalline copper when the grain size is constant. Herein, the mechanical properties of homogeneous nano‐grained (HNG) and gradient nano‐grained (GNG) coppers with different grain distributions are examined using molecular dynamics (MD) simulations. The HNG‐ordered structure has all triple junctions (TJs), whereas the random structure contains many quadruple and quintuple junctions. When grain size is below the critical size in the inverse Hall–Petch relationship, the high‐density TJs in the HNG‐ordered structure effectively inhibit grain boundary (GB) softening compared with the random structure, leading to higher strength. However, when grain size is above the critical size, subgrains are produced inside the large grains due to dislocation slip. In addition, disordered atoms in HNG‐random structure are stacked in the quadruple and quintuple junctions, resulting in thicker GBs. This triggers grain boundary migration, and forms more subgrains at GBs. Subsequently, grain boundary sliding and grain rotation of subgrains induce partial recrystallization in the structure. This consecutively triggered deformation mechanism leads to extra strengthening in the random structure. Further research indicates that combining small‐grained ordered and large‐grained random structures can be a new approach to effectively strengthen GNG materials.

Effect of solution annealing temperature on microstructural evolution and mechanical properties of NbC-reinforced CoCrFeNi based high entropy alloys

Recent studies have shown that NbC precipitates in high-entropy alloys (HEAs) are strong obstacles for dislocation motion, which can substantially contribute to the mechanical performance over a wide temperature range. However, there are limited reports discussing the exceptional adjustability of microstructures and mechanical properties using thermomechanical processing (TMP). Therefore, the microstructures and mechanical properties of NbC-reinforced CoCrFeNi based high entropy alloys (HEAs) tuned via TMP were systematically investigated. Homogenized HEAs with ∼0.3 at% C and Nb were solution annealed at 1100, 1200 and 1300 °C, followed by cold-rolling plus annealing. The microstructures and mechanical properties of the HEAs were systematically characterized using scanning electron microscopy, transmission electron microscopy and uniaxial tensile tests. The results indicated that NbC distribution evolved from eutectic-like primary NbC clusters surrounded by disorderly distributed secondary NbC to dispersive nano-sized secondary NbC as the solution annealing temperature increased above the NbC solvus temperature. Two kinds of microstructures were correspondingly produced: 1) samples solution annealed at 1100 and 1200 °C consisted of fine grain lamellae embedded with NbC precipitates and coarse grain lamellae without precipitates, which exhibited a combination of high strength (UTS ∼740 MPa) and good ductility (elongation ∼60%); and 2) samples solution annealed at 1300 °C obtained a homogeneous fine grain microstructure, which demonstrated excellent synergy of high strength (UTS ∼760 MPa) and exceptional ductility (elongation ∼63%). The enhanced strength of samples solution annealed at 1100 and 1200 °C originated primarily from the high back stress strengthening of lamellar microstructure, while precipitation strengthening caused by dispersed nanosized NbC precipitates and grain boundary strengthening provided extra strengthening for the sample annealed at 1300 °C. This work provides a new strategy for tailoring the microstructure and mechanical properties of NbC-reinforced HEAs via adjusting the solution annealing temperature of TMP.

Enhancing microstructure refinement and strengthening efficiency of TiBw/near α-Ti composites by combining solid-solution treatment with hot processing

Microstructure refinement can improve the strength of materials without trading off ductility. Plastic deformation is the most adopted method for microstructure refinement of metallic materials. However, excessive deformation can fracture reinforcements in metal matrix composites. To extensively refine the microstructure of TiBw/near α-Ti composites with limited deformation degree, solid-solution treatment above β transformation temperature was adopted before hot deformation in α+β region. Through the hot compression test at 870 °C–950 °C, the ideal deformation temperatures and strain rates for solid-solution treated composites were determined to be at 910 °C–940 °C, 0.01 s−1-0.001 s−1. Following this, large billets were prepared by isothermal hot pressing with and without solid-solution treatment, to testify the evolution of microstructure and mechanical properties. The microstructure of the composites deformed after solid solution treatment was more homogeneous and refined, attributed to the increase in the number of α/β interfaces and the decrease in the diffusion distance of β-stabilizer elements. An improvement of 10% in the yield strength was brought about by the solid-solution treatment. This extra strengthening was contributed by microstructure refinement and the dispersion of silicide particles, but weakened by the decrease in dislocation density. This work presents an effective method to refine the microstructure of TiBw/near α-Ti composites, and explains the underlying mechanisms behind the microstructure refinement and mechanical property improvement.

The Effect of Initial Texture on the Plastic Deformation of Gradient Aluminum.

The effect of specific processing-induced surface textures in gradient aluminum has not yet been investigated. A dislocation-based multi-scale framework is employed to explore the influence of various initial shearing textures and the depth from the surface of the region featuring each texture on the macroscopic behavior of gradient aluminum. By assigning different textures to the same grain size gradient aluminum sample, the initial texture was found to significantly affect the plastic deformation and macroscopic behavior of gradient aluminum. Specifically, the {111} texture can enhance the strength-ductility synergy, and this effect is dependent on the depth from the surface where the texture is located. This texture can lead to a slow stress/strain gradient in the assigned texture region and a sharp stress/strain gradient in the grain size gradient region connecting this region with the coarse grain region. Particularly, the sharp stress/strain gradient can result in extra strengthening by adjusting the stress/strain localization. These findings provide valuable insights for the design and optimization of surface textures in gradient aluminum.

Physical background of significant increase in mechanical properties and fatigue strength of groove-rolled Cu-Ni-Si alloy with discontinuous precipitates

Prolonged aging of Cu-Ni-Si alloy induced the formation of discontinuous precipitates (DPs) of fiber-shaped Ni2Si intermetallic compounds, rendering good conductivity and relatively inferior strength. To improve the strength of Cu-Ni-Si alloy with DPs, groove-rolling technique was utilized to plastically deform the aged Cu-6Ni-1.5Si specimen. As the rate of area reduction increased from 20% to 50% and 80%, the diameter and interspacing of Ni2Si fibers decreased. Both tensile strength and fatigue limit stress, which are essential for the design of electrical and electronic mechanical components, of Cu-6Ni-1.5Si specimen increased significantly with groove-rolling. It was suggested that an extra strengthening was achieved by Ni2Si fiber reinforcement in addition to work hardening. The increase in yield strength by fiber reinforcement was estimated by Orowan strengthening principle, assuming ideal fiber arrangement. The estimated yield strength based on Orowan mechanism was found to be much greater than the experimentally measured value. This was because actual morphology and arrangement of fibers were largely different from the ideal state, necessitating an error factor, k, which varied with the true strain, εt, due to groove-rolling. The k showed initial steep increase and subsequent mild increase to εt, followed by decreasing trend after εt ≒ 0.7. The physical background of enhanced fatigue strength of rolled specimen was discussed based on the detailed microstructural observation on crack initiation and propagation behavior.

Breaks in the Hall–Petch Relationship after Severe Plastic Deformation of Magnesium, Aluminum, Copper, and Iron

Strengthening by grain refinement via the Hall–Petch mechanism and softening by nanograin formation via the inverse Hall–Petch mechanism have been the subject of argument for decades, particularly for ultrafine-grained materials. In this study, the Hall–Petch relationship is examined for ultrafine-grained magnesium, aluminum, copper, and iron produced by severe plastic deformation in the literature. Magnesium, aluminum, copper, and their alloys follow the Hall–Petch relationship with a low slope, but an up-break appears when the grain sizes are reduced below 500–1000 nm. This extra strengthening, which is mainly due to the enhanced contribution of dislocations, is followed by a down-break for grain sizes smaller than 70–150 nm due to the diminution of the dislocation contribution and an enhancement of thermally-activated phenomena. For pure iron with a lower dislocation mobility, the Hall–Petch breaks are not evident, but the strength at the nanometer grain size range is lower than the expected Hall–Petch trend in the submicrometer range. The strength of nanograined iron can be increased to the expected trend by stabilizing grain boundaries via impurity atoms. Detailed analyses of the data confirm that grain refinement to the nanometer level is not necessarily a solution to achieve extra strengthening, but other strategies such as microstructural stabilization by segregation or precipitation are required.

Dual enhancement in strength and ductility of Ti-V-Zr medium entropy alloy by fracture mode transformation via a heterogeneous structure

The formation of a homogeneous body center cubic (BCC) solid solution and short-range clustering (SRC) allow refractory multi-principal elements alloys to reach incredibly high yield strengths, but their uses are severely constrained by their inherent brittleness. In this study, Ti-V-Zr medium entropy alloys with SRC are designed and prepared, and the heterogeneous grain structures (HGS) with multiphase are formed. By tunning microstructure, movable dislocations provide deformation ability, since shear beads formation is depressed. The strength and ductility of Ti40Zr60-xVx alloys are enhanced by the increasing proportion of V. Ti40Zr15V45 alloy achieves a high yield tension strength of 1067 MPa and ductility of >7 % at room temperature. With a 1793 MPa compressive strength, it outperforms other BCC alloys with a homogeneous structure. Thermodynamic stability of phases indexed by parameters tends to form heterogeneous microstructure in grain scale, which realizes dual enhancement of strength and ductility by altering the ratio of phases. Dislocation reactions, which are also visible in SRC, result in shear band fractures with dimples on the cleavage plane surfaces. The <001> immovable dislocation is formed in the SRC structure and provides extra strengthening. The formation process model has been developed and validated. The model for the formation process has been built and proven.

New insight into tailorable eutectic high entropy alloys with remarkable strength–ductility synergy and ample shaping freedom fabricated using laser powder bed fusion

Eutectic high entropy alloys (EHEAs) are typical heterostructured materials comprising ductile and hard phases that can be easily tailored to fabricate components with desirable structures and properties. Herein, an AlCoCrFeNi 2.1 EHEA with dual-phase nanolamellar structure and high strength and ductility (yield strength ( σ YS ) = 1210 MPa, ultimate tensile strength ( σ UTS ) = 1414 MPa, and elongation ( ε ) = 16%) was produced using laser powder bed fusion (LPBF). The AlCoCrFeNi 2.1 EHEA presented fine-grain structure with nanoprecipitates (L1 2 and BCC phases) embedded within alternating FCC and B2 nano-scale lamellae. We demonstrated that the high strength of the EHEA originated primarily from the high back stress strengthening of fine grains and high-density heterophase interfaces, and high dislocation density caused by rapid solidification as well as dispersed nanoparticles provided extra strengthening. Furthermore, the cellular structure comprising nearly square FCC cells and surrounding B2 phases was observed, which was induced by the presence of oxides on the surface of the feedstock powder. AlCoCrFeNi 2.1 EHEA with two-hierarchical dual-phase structure, that is, a mixture of lamellar and cellular structures, was obtained. Such heterogeneous structure presented outstanding strength–ductility synergy ( σ YS = 1042 MPa, σ UTS = 1303 MPa, and ε = 26%). These results promote the development of high-performance materials with manufacturing flexibility using additive manufacturing approaches to tailor their multiscale microstructure.

Quantitative analysis of hetero-deformation induced strengthening in heterogeneous grain structure

A single-phase bimodal grain structure is considered to develop a physical model to quantify hetero-deformation induced (HDI) strengthening at the yield point, which cannot be simply predicted by the conventional rule-of-mixtures using the Hall-Petch equation. Based on the classic theory of single-ended continuum dislocation pileup, the modified model parameterizes the effective width of hetero-boundary affected region (Hbar) as well as the contribution of HDI stress to 0.2% proof stress. To further verify the model equations, the equimolar CoCrNi medium entropy alloy was selected as a model material. The heterogeneous grain structure (HGS) was introduced via thermal-mechanical treatment, and statistical analysis of microstructure was performed by means of electron backscattered diffraction. By substituting derived parameters, our model can predict theoretical values of the yield stress and the width of Hbar, both comparable to the experimental value from tensile testing, as well as previous experimental observations. The reasonable agreements can not only prove the validity of the current modified model, but also bring out physical explanations for the extra strengthening in heterostructured materials.

High-resolution mapping of the strain heterogeneity in heterogeneous and homogeneous structured Mg 13Gd alloy

The hetero-deformation induced (HDI) stress can effectively promote extra strengthening and strain hardening behavior, which originates from the heterogenous deformation between soft and hard domains. How to quantitatively evaluate the strain heterogeneity at the micron and sub-micron scales remains a great challenge during deformation incompatibility. In this study, the strain evolution at grain level of heterogeneous (HES) and homogeneous-structured (HOS) Mg 13Gd (wt%) alloy was characterized by high-resolution digital image correlation. Both HES and HOS samples show strain heterogeneity related to the physical slip bands inside grains, and the magnitude of local deformation in grain level is much higher than the macroscopic strain, especially at grain boundaries. The HOS sample can also generate HDI stress due to the remarkable difference in Schmid factor and lower geometrical compatibility factor between adjacent grains. The formation of dispersed strain bands in the coarse grains with hard orientation of the HES sample contributes to strain delocalization behavior and the improvement of strain hardening capacity. These results provide us with a new insight for heterostructured design of Mg alloys on a more microscopic scale. • The strain evolution at grain level of heterogeneous and homogeneous-structured Mg 13Gd (wt%) alloy was characterized by high-resolution digital image correlation. • The heterogeneous and homogeneous-structured sample showed strain heterogeneity, and the magnitude of local deformation in grain level was much higher than the macroscopic strain, especially at grain boundaries. • The homogeneous-structured sample can also generate the hetero-deformation induced stress due to the remarkable difference in Schmid factor and lower geometrical compatibility factor between adjacent grains. • The formation of dispersed strain bands in the coarse grains of the heterogeneous-structured sample contributed to strain delocalization behavior as well as the improvement of strain hardening capacity.

Effect of volume fractions of gradient transition layer on mechanical behaviors of nanotwinned Cu

Heterogeneous nanostructured (HNS) materials, such as gradient and laminated nanostructured metals, possess superior mechanical properties with respect to their homogeneous counterparts. The additional strengthening mechanism of HNS is mainly attributed to the non-uniform plastic deformation induced by gradient transition layers (GTLs) between components. Here, we designed three gradient nanotwinned (GNT) Cu samples with different volume fractions of GTLs (fg) of 10%, 50% and 100% while the rule-of-mixture strength and overall structural gradient are constant to quantitatively reveal its effect on the extra strengthening behaviors. As fg increases, the yield strength is improved and the elastic to plastic stage is prolonged at small strain. Moreover, larger fg suppresses the strain localization and reduces nucleation of cracks at larger strain, accompanied by more widely distributed geometrically necessary dislocations (GNDs) and more bundles of concentrated dislocations (BCDs), thus improving the elongation. Stress partitioning analysis and numerical simulations show that the more widely-distributed GNDs at larger fg result in higher overall back stresses but hardly affect effective stresses, indicative of the origin of the improved strengthening. Such GND-distribution dominated strengthening mechanism paves a fundamental way for developing higher-performance HNS materials.