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High-Strength and High-Ductility Nanostructured and Amorphous Metallic Materials Hongning Kou, Jian Lu,* and Ying Li
1. Introduction Traditional strengthening methods, such as including solid solution atoms, precipitates and dispersed particles, grain boundaries and phase tranformation, invariably suffer from undesirably high tensile strength accompanied by very limited tensile elongation, such as in nanostructured (NS) metals and alloys,[1,2] and it is even more difficult to generate deformation capacity in bulk metallic glasses (BMGs) although the strength is generally much higher. Low ductility has become the most crucial challenge in both NS materials and amorphous BMGs in spite of their ultrahigh strength. The possible solutions for this trade-off relationship rely on the strengthening features that can not only impede dislocation motion but are also able to accommodate plastic deformation.[3] In recent years, great Dr. H. N. Kou, Prof. J. Lu, Dr. Y. Li Department of Mechanical and Biomedical Engineering City University of Hong Kong Tae Chee Avenue Kowloon, Hong Kong, China E-mail: [email protected] Prof. J. Lu, Dr. Y. Li Centre for Advanced Structural Materials City University of Hong Kong Shenzhen Research Institute 8 Yuexing 1st Road Shenzhen Hi-Tech Industrial Park Nanshan District Shenzhen, China
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endeavors have been made and various approaches have been reported to improve ductility and toughness while preserving high strength, for example, the bimodal grain size,[4,5] nanoparticles,[6–8] artificial nanostructures[9] and composite BMGs.[10] Among these, nanotwinned structures have attracted considerable attention and are of the most widely pursued and studied approaches to improve tensile ductility, and have been successfully applied in extensive structural materials with diverse geometric dimensions, such as bulk stainless steel alloys,[11] pure Cu thin foils,[12] Cu nanopillars[13] and Au nanowires.[14] Nevertheless, it still remains challenging to synthesize engineering NS metals and alloys and BMGs with dual properties by using the existing methods due to the difficulties to embed toughening mechanisms. The main issue is how to delay the catastrophic propagation of initiated nanocracks in nanomaterials and embryonic shear bands in BMGs. In this Research News, we focus on recently developed routes and the underlying mechanisms, and present the breakthrough performances from the macro-/ microscale to nano-/atomic scale that can achieve high strength and high ductility in advanced bulk NS metals and alloys and amorphous metallic glasses. The specific processing strategies, from macroscopic to atomic level, have been developed by combining toughening mechanisms with new numerical simulations. Figure 1 shows some remarkable paradigms to achieve extraordinary high strength and high ductility with the associated mechanisms of nanomaterials processing and the supporting theoretical models. Some new promising strategies and developments are also illustrated to show the way for potential applications in the future.
The development of materials with dual properties of high strength and high ductility has been a constant challenge since the foundation of the materials science discipline. The rapid progress of nanotechnology in recent decades has further brought this challenge to a new era. This Research News highlights a few newly developed strategies to optimize advanced nanomaterials and metallic glasses with exceptional dual mechanical properties of high strength and high ductility. A general concept of strain non-localization is presented to describe the role of multiscale (i.e., macroscale, microscale, nanoscale, and atomic scale) heterogeneities in the ductility enhancement of materials reputed to be intrinsically brittle, such as nanostructured metallic materials and bulk metallic glasses. These nanomaterials clearly form a new group of materials that display an extraordinary relationship between yield strength and the uniform elongation with the same chemical composition. Several other examples of nanomaterials such as those reinforced by nanoprecipitates will also be described.
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2. Toughing Mechanism Embedded in the New Strategies The internal defects/boundaries that are used to block dislocation motion to achieve strengthening usually compromise the tensile elongation of nanocrystalline metals to only a few percentage[2,15] due to the lack of strain hardening.[16] To overcome this drawback, here we introduce several new methods originating from a unique mechanism of strain non-localization to achieve both high strength and high ductility. For convenience, the following mechanisms are discussed in terms of the scale
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RESEARCH NEWS Figure 1. Overall flowchart of selected outstanding mechanical properties of high strength and high ductility in NS materials and BMGs. The blue rectangles denote the representative toughening features by experiments; the pink rounded rectangles denote the corresponding numerical simulations; the yellow ovals denote the underlying mechanisms. The red rectangles indicate our new design (see Section 3).
variations of the strengthening features from macroscopic to atomic levels.
2.1. Macroscopic and Microscopic Strain Non-Localization The general concept of strain non-localization was first proposed to enhance the ductility of nanomaterials by introducing three-layered stainless steel sheets toughening mechanisms using layered nanostructures produced via surface mechanical attrition treatment (SMAT)[18,19] and the co-rolling process to assemble NS stainless steel sheets.[17] The major factors that are responsible for the production of their excellent properties are essentially attributed to strain non-localization, which avoids the formation of one critical major crack and delays crack propagation by transferring the originally localized cracks to other positions and generating multiple necking failure modes as “necking/cracking multiplication” in the developed materials. Here the strain non-localization behavior is realized by the integration of three factors. The first is the introduction of a gradient grain size distribution to generate a crack blunting mechanism. The typical microstructure of the macrolaminates is characterized by a periodic distribution of nano-/microscale grain layers. Both the micrograined layer and the large-angle
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grain boundaries of ultrafine grains play key roles in elevating ductility.[20,21] The second embedded toughening factor is the compressive residual stresses produced during the process which promotes a crack closure mechanism. Since SMAT can introduce very high compressive residual stresses caused by the generated NS grains, the crack closure effect can be extremely high. In fact, the nanograins can lead to a higher level of residual stresses due to the higher level of yield strength compared to a basic coarse-grained (CG) alloy. The compressive residual stress can block the crack extension that is usually initiated in the top NS surface, and the subsequent co-rolling process further shields the crack propagation. The third reason is the interfaces between the NC surface layers of each bonded steel sheet that contribute to deflecting cracks against further penetration. Cracks usually propagate along the interface, and the temperature and area reduction of the co-rolling process also have a great effect on the interface strength. Therefore, by using appropriate co-rolling treatment to produce optimal interfaces, high dual mechanical properties can be achieved. We also further justify this strain non-localization mechanism from a damage mechanics point of view by introducing cohesive elements into finite element models to investigate crack initiation and propagation of the multilayered sheets processed by SMAT and co-rolling.[22] In this model, both the brittle
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NS interfaces and the ductile CG center parts are expected to follow the isotropic elastic-plastic constitutive relation. By adjusting the cohesive parameters, the simulated results can be compared with experimental results. For example, when setting the energy release rate of NS interfaces at a relatively low value, multiple cracks form easily, which tends to delay final fracture and leads to a considerable tensile elongation up to 50%. The simulated results thus validate the non-local failure as the primary toughening mode, demonstrating good agreement of crack nucleation and the propagation mechanism with microstructural observations and tensile testing when appropriate treatment is applied. This combined multilayered processing route may also be applied to other structural metals and alloys to achieve better performance.
2.2. Improving Ductility Through a Microscopic Hierarchical Structure Inspired by the biological concept,[23,24] a microscopic hierarchical multilayer structure without layer interfaces was fabricated.[25] The microstructure is composed (from the outer to the inner) of a nanograin (NG) layer, dual phase (martensite α’+austenite γ) layer, twinning layer, and CG layer. The unique feature of the hierarchical multilayer at the microscale is a distinct toughening mechanism. The four heterogeneous layers deform differently, i.e., the martensitic nanograins on the top surface provide ultrahigh strength but make it brittle; the dual phase layer acts as a transitional layer between the outer hard martensite (α′) and the inner soft austenite (γ) to stabilize the deformation; the twinning layer contributes not only to strengthening but also to preserving ductility; and the soft CG austenite layer in the center dissipates energy via plasticity. The microscopic hierarchical structure can delay final failure by generating multiple necking. Their unusual deformation behavior incorporates a dual gradient structure of grain size and phase constituents and together produce excellent mechanical properties, exhibiting superior advantages to homogeneous structures. One potential method of optimizing the constituent proportion of the beneficial layer (e.g., twinning layer) may further enhance this high strength and high ductility combination.
2.3. Strengthening and Toughening by the High Density of Nanotwins Coherent twin boundaries (TBs), whose atoms on one side are arranged symmetrically reflecting those on the other side with a twin plane, have been considered as effective as grain boundaries in strengthening materials.[26] Moreover, the special coherent lattice structure of TBs being superior to that of GBs, also enables them to accommodate dislocations. As a result, the strength increase caused by TBs is not compromised by a sharp reduction of ductility as in conventional strengthening approaches.[3] To theoretically relate the outstanding macroscopic mechanical properties of metals with nanotwin microstructures, we developed a mechanism-based constitutive model that demonstrated the grain size dependent twin spacing relationship to predict and optimize the strength and ductility
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of nanotwinned metals.[27] In virtue of the dislocation pile-up zones near TBs and twinning partial dislocations, this model provides a quantitative continuum method to demonstrate the relationship of strength, ductility and strain hardening with nanotwin spacing, and predicts a critical twin spacing in different grain sizes to achieve maximum strength. It is found that the mechanical properties of high strength, high ductility and sufficient strain hardening are decided by both twin spacing and grain size. Corresponding to the maximum dislocation density, the predicted yield stress peaked at a critical twin spacing of 13 nm, showing excellent agreement with the experimental result of 15 nm in nanotwinned Cu.[12] The assumed linear relationship of critical twin spacing with the grain size is also consistent with the large-scale molecular dynamic (MD) simulation results. On the other hand, we also fabricated highdensity of nanotwins in large-scale engineering stainless steel sheets.[11] After processed by the newly developed SMAT technique with a super-high strain rate up to ∼105 s–1,[28] the overall microstructure of the stainless steel sample is mainly characterized by high densities of nanotwins over almost the entire thickness, with twin spacing ranging from several nanometers in the subsurface layer to hundreds of nanometers in the core. These nanoscale twin/matrix (T/M) lamellae can create more paths for dislocation motion, which limit the effect of strain localization at the nanograin boundaries. The widespread distribution of nanotwins in grains at all depths gains much higher strength for the steel samples than those with only nanograins after undergoing SMAT at a low strain rate (about 10 s–1),[11] indicating a significant strain rate dependent strengthening effect of nanotwin spacing, as suggested by the modeling simulation results.[24] The optimization of nanograin size and nanotwin spacing provides a new way to further enhance yield strength to maximum value.
2.4. Strengthening by using Nanoscale Precipitations Precipitation strengthening is also a widely used strengthening approach in engineering alloys. Recently, a systematic investigation was performed in a low-carbon steel to study the Cu/Ni influence on its mechanical properties and the relationship with the traditional strengthening methods.[29] Compared with the ferritic base steels without Cu or Ni, the newly developed Cu/Ni steels more than doubled the strength at a sacrifice of limited elongation. Jiao et al. found that the effect of Ni on the strength increase was embodied by the enhancement of Cu-rich nanoprecipitates strengthening, which intrinsically resulted from the decreases of strain energy for nucleation, interfacial energy between the nucleus and the matrix, and the critical energy for the formation of Cu-rich nanoscale precipitates. These nanoprecipitates also act as multiple obstacles to avoid the motion of dislocations and stop the progression of strain localization. Thermodynamic calculation also indicates the benefit of Cu and Ni for grain refinement by lowering the austenite-to-ferrite transformation temperature. Nevertheless, both Cu and Ni are weak in relation to solid solution strengthening. The collaboration of Cu and Ni in regard to strengthening is also superior to the single effect of either Cu or Ni on the mechanical properties. Similar Cu/Ni/Al nanoprecipitates have also been found in
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2.5. Toughening Mechanisms in BMGs Distinct from the above crystalline counterparts, bulk metallic glasses (BMGs) with disordered atom arrangements have been extensively investigated, with the focus not only on their attractive properties and applications but also on their atomic structure and deformation mechanisms. In this section, the routes to achieve high strength and good plasticity in BMGs, as well as deformation behavior and embedded mechanisms, will be discussed.
2.5.1. Gaining Plasticity using a Structural Gradient Composite and Multiple Shear Bands A novel BMG composite with a gradient structure induced by SMAT has been developed.[31] Superior to the totally amorphous as-cast BMGs, this new hybrid material exhibits a compressive strain which is four times higher without sacrificing strength. Different from other BMG composites that have been reported as effective factors in relation to producing ductility,[10,32] the reasons that lead to the excellent mechanical properties of this SMATed BMG can be ascribed to the gradient microstructure and concomitant residual stress profiles. Processed under high vibration frequency and high strain rate of SMAT, nanocrystallines are formed in the fully amorphous matrix. The entire microstructure is characterized by three layers, i.e. partially crystallized layer, crystallization affected layer and the metallic glass matrix layer, from the top surface to the interior matrix. In contrast to the general rule that shear bands would like to occur after reaching the critical shear strength and further develop into cracks during plastic deformations, the crystallites produced by SMAT are able to limit shear band propagation and crack extension. The residual stress in the crystallization affected zones may also help to block the main shear crack.[33] Secondary shear bands are found to initiate due to the congregated crystallites, making a larger contribution to plasticity.[34] This ‘shear band multiplication’ resulting from the gradient microstructure deflects and delays the critical crack, producing global enhancement of plasticity and toughness in the present material.
2.5.2. Toughening BMGs at the Atomic Scale It is known that the occurrence of shear bands and localized deformation indicates the major variation in the atomic structure and deformation behavior of crystalline materials. In this regard, a particular constitutive theoretical model for shear bands in BMGs has been established.[35] Finite element modeling (FEM) was employed to analyze deformation behavior under dynamic loadings. According to the simulation results, more shear bands may lead to higher compressive ductility. On the other hand, BMGs also display extraordinary compressive
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ductility under quasi-static loading.[36] The reason is attributed to the blending of two phases that randomly embed a soft phase into the hard regions. The FEM modeling validated the experimental hypothesis with 20% volumetric variation of the two phases, resulting in the much more improved compressive ductility than their individual brittleness and in good agreement with the experiments. It is also found that a maximum ductility occurs when the two phases are mixed in equal volume. An example of a La-based BMG composite has successfully achieved 5% tensile ductility with the introduction of a soft second phase with 50% volume.[37,38] It can thus be concluded that the dual-phase mode can not only promote shear band multiplication but also carry more plastic strain to sustain deformation. On this basis, a mixed system composed of amorphous crystal-like clusters are believed to be ideal structures to achieve high strength and high ductility in BMGs.[39,40] In order to promote plasticity by synthesizing the ideal hybrid components, some preliminary work has been performed. Wang et al. revealed the mechanism for thermal stability against crystallization in a Zr-based amorphous BMG, which resulted from the pinning effect of a new kind of metastable icosahedra-like atomic cluster that appears after being processed by isothermal annealing.[39,40] Yang et al. characterized the deformation behavior of free-volume zones via dynamic micropillar tests.[41] The loosely bonded atomistic free-volume zones behave as a deformation feature similar to supercooled liquids. The atomistic ‘soft core-hard shell’ structure is considered to be related with both yield strength and plasticity in BMGs. The most recent result also showed remarkable tensile ductility up to 4% and unique strain hardening in Cu-Zr-/Zr-based BMGs by activating multiple shear banding and delaying shear cracking and cavitation.[42] After processed by SMAT, a gradient amorphous microstructure and a residual stress profile are both formed to enhance the tensile plasticity and enable extensive applications.
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a multi-component BMG alloy.[30] These findings help to understand the mechanism of achieving high strength and high ductility by using nanoscale precipitates.
3. Designing Advanced Nanostructured Metals with Extremely High Strength and High Ductility by Hierarchical Nanotwins Motivated by the outstanding nanotwin strengthening effect, we propose a new concept of hierarchical nanotwins (HNTs) to further promote the mechanical properties. In contrast to the microscopic multilayered hierarchical structure (see Section 2.2), this HNT structure is a completely twinning system in different scales. As theoretically demonstrated by the dislocation-based model in pure copper, the HNTs are much more effective than monolithic nanotwins for strengthening and can reach a maximum value by optimizing the twin spacing (of two-order nanotwins) and grain size in each order.[43] Large-scale molecular dynamic (MD) simulations were also used to investigate the fracture characteristics of HNTs. In contrast to the cleavage mode of monolithic nanotwins, the crack resistance of primary twins is much higher. By crack blunting and dislocation storage of secondary twins with smaller twin spacing, overall crack behavior can be enhanced.[44] Very recently, we have successfully produced three-order HNTs in bulk NS TWIP (twinning induced
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Figure 2. A typical TEM (transmission electronic microscopy) morphology and a schematic diagram showing hierarchically twinned structure in the new SMATed TWIP steel samples before (b) and after (c) tensile testing. T1, T2 and T3 respectively denote the first-order, second-order, and third-order twins.
plasticity) steels,[45] achieving an unprecedented property that breaks the conventional milestone of yield strength for metallic materials over 2 GPa while still preserving a considerable uniform elongation of 15% for sufficient engineering formability. These newly developed HNTs on one hand inherit the advantages of monolithic nanotwins and on the other hand assemble simple nanotwins to constitute a new complex twinning system with a scale separation, as shown in Figure 2. Superior to general two-dimensional (2D) T/M lamellar nanotwins, this new hierarchically twinned system provides one extra dimension in space, exhibiting a three-dimensional (3D) structure (Figure 3) to further enhance the overall performance. Comparisons with conventional HSS[46] (high strength steels), AHSS (advanced high strength steels) and MS (martensitic steels) and other representative materials that have been reported indicate deviation from the general high strength high ductility metals and alloys,[4–6,8–14,29,47–50] revealing the unprecedented properties of the new TWIP steels (Figure 4). Both experimental results and numerical modeling show that the 3D HNTs play the primary role of inducing a sharp increase of tensile yield strength while maintaining sufficient elongation. After being experimentally shown in a steel alloy
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and theoretically demonstrated in pure copper, this hierarchically twinned structure is believed to be effective in diverse materials for global practice in the future.
4. Conclusions and Perspectives Unsustainable energy consumption and global warming are among the most significant problems for human beings worldwide. Increasing the fuel efficiency while maintaining sufficient formability pose a new challenge for manufacturers around the world to enhance the ductility in high-strength engineering materials. In this brief Research News article, we ascribe the new principle to a strain non-localization mechanism by introducing the above routes to achieve high strength and high ductility in NS and amorphous metallic materials. From the macroscopic multilaminates to nanotwins and atomic clusters, we have successfully realized simultaneous high strength and high ductility in diverse metallic materials by using multiplications of a series of strengthening factors (as summarized in Figure 1) to avoid strain/stress localization and delay the catastrophic failure. On this basis, further investigations will focus on the optimization
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Figure 3. Schematic of 3D three-ordered hierarchical twins illustrated by cutaway profiles (twins in the first, second, and third order are respectively denoted in blue, orange, and green) within a general grain, indicating a 3D cubic hierarchical twinned structure rather than the two-dimensional (2D) twin-matrix lamellae. The area in yellow dashed circles are cut away to show the generation sites of twins in the higher order; a lattice structure of the third twin is shown as an insert.
of mechanical properties of NS metals and BMGs under different conditions and rational analysis in terms of both experiments and theories. The new design of 3D HNTs opens up a new direction to fabricate “super metals and alloys” with extremely high yield strength and simultaneously high ductility, as suggested in Figure 4. The atomic crystal-like clusters also demonstrate the potential to produce tensile plasticity at ultrahigh stress levels of ∼2 Gpa. Quantitative correlations between the macroscopic performance and the complicated nanostructures are also challenging but very important to study. Systematic studies on 3D HNTs and atomic clusters/crystals in BMGs are necessary to be undertaken. Mechanical properties are usually controlled by intrinisic microstructures. We expect new developments of deformation mechanisms in relation to the promotion of high strength and high ductility in the future. Finally, additional research should also include the multiscale simulation methods, such as MD simulation and phase field modeling, to optimize the theoretical models for a better understanding of 3D hierachical twins and strain non-localizationmechanisms. The new progress of materials with extraordinary superhigh strength will benefit both light-weight construction and eco-friendly systems.
Figure 4. Comparison of yield strength-uniform elongation between the present new TWIP steel samples with other excellent metallic materials (solid circles for pure metals, and solid triangles for steel alloys) reported in literatures as examples with high strength and high ductility. The three oval regions respectively denote the ranges for conventional HSS (in light pink), AHSS (in light purple) and MS (in light blue) for comparison. The red triangles denote the new results of SMATed TWIP steel samples, which deviate obviously from all those with exceptional performances as divided by the red dashed line.
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Acknowledgements The authors acknowledge financial supports provided by the MOST through the National Basic Research Program of China (973 Program) under the grant 2012CB932203 and Research Grants Council for the Hong Kong Special Administrative Region of China under grants CityU8/ CRF/08, GRF/CityU51911, the Croucher Foundation CityU9500006, CityU Strategic Research Grant GRF/ECS (SRG-Fd) /CityU7002913. Received: April 9, 2014 Revised: May 24, 2014 Published online: June 27, 2014
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High-Strength and High-Ductility Nanostructured and Amorphous Metallic Materials Hongning Kou, Jian Lu,* and Ying Li
1. Introduction Traditional strengthening methods, such as including solid solution atoms, precipitates and dispersed particles, grain boundaries and phase tranformation, invariably suffer from undesirably high tensile strength accompanied by very limited tensile elongation, such as in nanostructured (NS) metals and alloys,[1,2] and it is even more difficult to generate deformation capacity in bulk metallic glasses (BMGs) although the strength is generally much higher. Low ductility has become the most crucial challenge in both NS materials and amorphous BMGs in spite of their ultrahigh strength. The possible solutions for this trade-off relationship rely on the strengthening features that can not only impede dislocation motion but are also able to accommodate plastic deformation.[3] In recent years, great Dr. H. N. Kou, Prof. J. Lu, Dr. Y. Li Department of Mechanical and Biomedical Engineering City University of Hong Kong Tae Chee Avenue Kowloon, Hong Kong, China E-mail: [email protected] Prof. J. Lu, Dr. Y. Li Centre for Advanced Structural Materials City University of Hong Kong Shenzhen Research Institute 8 Yuexing 1st Road Shenzhen Hi-Tech Industrial Park Nanshan District Shenzhen, China
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endeavors have been made and various approaches have been reported to improve ductility and toughness while preserving high strength, for example, the bimodal grain size,[4,5] nanoparticles,[6–8] artificial nanostructures[9] and composite BMGs.[10] Among these, nanotwinned structures have attracted considerable attention and are of the most widely pursued and studied approaches to improve tensile ductility, and have been successfully applied in extensive structural materials with diverse geometric dimensions, such as bulk stainless steel alloys,[11] pure Cu thin foils,[12] Cu nanopillars[13] and Au nanowires.[14] Nevertheless, it still remains challenging to synthesize engineering NS metals and alloys and BMGs with dual properties by using the existing methods due to the difficulties to embed toughening mechanisms. The main issue is how to delay the catastrophic propagation of initiated nanocracks in nanomaterials and embryonic shear bands in BMGs. In this Research News, we focus on recently developed routes and the underlying mechanisms, and present the breakthrough performances from the macro-/ microscale to nano-/atomic scale that can achieve high strength and high ductility in advanced bulk NS metals and alloys and amorphous metallic glasses. The specific processing strategies, from macroscopic to atomic level, have been developed by combining toughening mechanisms with new numerical simulations. Figure 1 shows some remarkable paradigms to achieve extraordinary high strength and high ductility with the associated mechanisms of nanomaterials processing and the supporting theoretical models. Some new promising strategies and developments are also illustrated to show the way for potential applications in the future.
The development of materials with dual properties of high strength and high ductility has been a constant challenge since the foundation of the materials science discipline. The rapid progress of nanotechnology in recent decades has further brought this challenge to a new era. This Research News highlights a few newly developed strategies to optimize advanced nanomaterials and metallic glasses with exceptional dual mechanical properties of high strength and high ductility. A general concept of strain non-localization is presented to describe the role of multiscale (i.e., macroscale, microscale, nanoscale, and atomic scale) heterogeneities in the ductility enhancement of materials reputed to be intrinsically brittle, such as nanostructured metallic materials and bulk metallic glasses. These nanomaterials clearly form a new group of materials that display an extraordinary relationship between yield strength and the uniform elongation with the same chemical composition. Several other examples of nanomaterials such as those reinforced by nanoprecipitates will also be described.
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2. Toughing Mechanism Embedded in the New Strategies The internal defects/boundaries that are used to block dislocation motion to achieve strengthening usually compromise the tensile elongation of nanocrystalline metals to only a few percentage[2,15] due to the lack of strain hardening.[16] To overcome this drawback, here we introduce several new methods originating from a unique mechanism of strain non-localization to achieve both high strength and high ductility. For convenience, the following mechanisms are discussed in terms of the scale
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RESEARCH NEWS Figure 1. Overall flowchart of selected outstanding mechanical properties of high strength and high ductility in NS materials and BMGs. The blue rectangles denote the representative toughening features by experiments; the pink rounded rectangles denote the corresponding numerical simulations; the yellow ovals denote the underlying mechanisms. The red rectangles indicate our new design (see Section 3).
variations of the strengthening features from macroscopic to atomic levels.
2.1. Macroscopic and Microscopic Strain Non-Localization The general concept of strain non-localization was first proposed to enhance the ductility of nanomaterials by introducing three-layered stainless steel sheets toughening mechanisms using layered nanostructures produced via surface mechanical attrition treatment (SMAT)[18,19] and the co-rolling process to assemble NS stainless steel sheets.[17] The major factors that are responsible for the production of their excellent properties are essentially attributed to strain non-localization, which avoids the formation of one critical major crack and delays crack propagation by transferring the originally localized cracks to other positions and generating multiple necking failure modes as “necking/cracking multiplication” in the developed materials. Here the strain non-localization behavior is realized by the integration of three factors. The first is the introduction of a gradient grain size distribution to generate a crack blunting mechanism. The typical microstructure of the macrolaminates is characterized by a periodic distribution of nano-/microscale grain layers. Both the micrograined layer and the large-angle
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grain boundaries of ultrafine grains play key roles in elevating ductility.[20,21] The second embedded toughening factor is the compressive residual stresses produced during the process which promotes a crack closure mechanism. Since SMAT can introduce very high compressive residual stresses caused by the generated NS grains, the crack closure effect can be extremely high. In fact, the nanograins can lead to a higher level of residual stresses due to the higher level of yield strength compared to a basic coarse-grained (CG) alloy. The compressive residual stress can block the crack extension that is usually initiated in the top NS surface, and the subsequent co-rolling process further shields the crack propagation. The third reason is the interfaces between the NC surface layers of each bonded steel sheet that contribute to deflecting cracks against further penetration. Cracks usually propagate along the interface, and the temperature and area reduction of the co-rolling process also have a great effect on the interface strength. Therefore, by using appropriate co-rolling treatment to produce optimal interfaces, high dual mechanical properties can be achieved. We also further justify this strain non-localization mechanism from a damage mechanics point of view by introducing cohesive elements into finite element models to investigate crack initiation and propagation of the multilayered sheets processed by SMAT and co-rolling.[22] In this model, both the brittle
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NS interfaces and the ductile CG center parts are expected to follow the isotropic elastic-plastic constitutive relation. By adjusting the cohesive parameters, the simulated results can be compared with experimental results. For example, when setting the energy release rate of NS interfaces at a relatively low value, multiple cracks form easily, which tends to delay final fracture and leads to a considerable tensile elongation up to 50%. The simulated results thus validate the non-local failure as the primary toughening mode, demonstrating good agreement of crack nucleation and the propagation mechanism with microstructural observations and tensile testing when appropriate treatment is applied. This combined multilayered processing route may also be applied to other structural metals and alloys to achieve better performance.
2.2. Improving Ductility Through a Microscopic Hierarchical Structure Inspired by the biological concept,[23,24] a microscopic hierarchical multilayer structure without layer interfaces was fabricated.[25] The microstructure is composed (from the outer to the inner) of a nanograin (NG) layer, dual phase (martensite α’+austenite γ) layer, twinning layer, and CG layer. The unique feature of the hierarchical multilayer at the microscale is a distinct toughening mechanism. The four heterogeneous layers deform differently, i.e., the martensitic nanograins on the top surface provide ultrahigh strength but make it brittle; the dual phase layer acts as a transitional layer between the outer hard martensite (α′) and the inner soft austenite (γ) to stabilize the deformation; the twinning layer contributes not only to strengthening but also to preserving ductility; and the soft CG austenite layer in the center dissipates energy via plasticity. The microscopic hierarchical structure can delay final failure by generating multiple necking. Their unusual deformation behavior incorporates a dual gradient structure of grain size and phase constituents and together produce excellent mechanical properties, exhibiting superior advantages to homogeneous structures. One potential method of optimizing the constituent proportion of the beneficial layer (e.g., twinning layer) may further enhance this high strength and high ductility combination.
2.3. Strengthening and Toughening by the High Density of Nanotwins Coherent twin boundaries (TBs), whose atoms on one side are arranged symmetrically reflecting those on the other side with a twin plane, have been considered as effective as grain boundaries in strengthening materials.[26] Moreover, the special coherent lattice structure of TBs being superior to that of GBs, also enables them to accommodate dislocations. As a result, the strength increase caused by TBs is not compromised by a sharp reduction of ductility as in conventional strengthening approaches.[3] To theoretically relate the outstanding macroscopic mechanical properties of metals with nanotwin microstructures, we developed a mechanism-based constitutive model that demonstrated the grain size dependent twin spacing relationship to predict and optimize the strength and ductility
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of nanotwinned metals.[27] In virtue of the dislocation pile-up zones near TBs and twinning partial dislocations, this model provides a quantitative continuum method to demonstrate the relationship of strength, ductility and strain hardening with nanotwin spacing, and predicts a critical twin spacing in different grain sizes to achieve maximum strength. It is found that the mechanical properties of high strength, high ductility and sufficient strain hardening are decided by both twin spacing and grain size. Corresponding to the maximum dislocation density, the predicted yield stress peaked at a critical twin spacing of 13 nm, showing excellent agreement with the experimental result of 15 nm in nanotwinned Cu.[12] The assumed linear relationship of critical twin spacing with the grain size is also consistent with the large-scale molecular dynamic (MD) simulation results. On the other hand, we also fabricated highdensity of nanotwins in large-scale engineering stainless steel sheets.[11] After processed by the newly developed SMAT technique with a super-high strain rate up to ∼105 s–1,[28] the overall microstructure of the stainless steel sample is mainly characterized by high densities of nanotwins over almost the entire thickness, with twin spacing ranging from several nanometers in the subsurface layer to hundreds of nanometers in the core. These nanoscale twin/matrix (T/M) lamellae can create more paths for dislocation motion, which limit the effect of strain localization at the nanograin boundaries. The widespread distribution of nanotwins in grains at all depths gains much higher strength for the steel samples than those with only nanograins after undergoing SMAT at a low strain rate (about 10 s–1),[11] indicating a significant strain rate dependent strengthening effect of nanotwin spacing, as suggested by the modeling simulation results.[24] The optimization of nanograin size and nanotwin spacing provides a new way to further enhance yield strength to maximum value.
2.4. Strengthening by using Nanoscale Precipitations Precipitation strengthening is also a widely used strengthening approach in engineering alloys. Recently, a systematic investigation was performed in a low-carbon steel to study the Cu/Ni influence on its mechanical properties and the relationship with the traditional strengthening methods.[29] Compared with the ferritic base steels without Cu or Ni, the newly developed Cu/Ni steels more than doubled the strength at a sacrifice of limited elongation. Jiao et al. found that the effect of Ni on the strength increase was embodied by the enhancement of Cu-rich nanoprecipitates strengthening, which intrinsically resulted from the decreases of strain energy for nucleation, interfacial energy between the nucleus and the matrix, and the critical energy for the formation of Cu-rich nanoscale precipitates. These nanoprecipitates also act as multiple obstacles to avoid the motion of dislocations and stop the progression of strain localization. Thermodynamic calculation also indicates the benefit of Cu and Ni for grain refinement by lowering the austenite-to-ferrite transformation temperature. Nevertheless, both Cu and Ni are weak in relation to solid solution strengthening. The collaboration of Cu and Ni in regard to strengthening is also superior to the single effect of either Cu or Ni on the mechanical properties. Similar Cu/Ni/Al nanoprecipitates have also been found in
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2.5. Toughening Mechanisms in BMGs Distinct from the above crystalline counterparts, bulk metallic glasses (BMGs) with disordered atom arrangements have been extensively investigated, with the focus not only on their attractive properties and applications but also on their atomic structure and deformation mechanisms. In this section, the routes to achieve high strength and good plasticity in BMGs, as well as deformation behavior and embedded mechanisms, will be discussed.
2.5.1. Gaining Plasticity using a Structural Gradient Composite and Multiple Shear Bands A novel BMG composite with a gradient structure induced by SMAT has been developed.[31] Superior to the totally amorphous as-cast BMGs, this new hybrid material exhibits a compressive strain which is four times higher without sacrificing strength. Different from other BMG composites that have been reported as effective factors in relation to producing ductility,[10,32] the reasons that lead to the excellent mechanical properties of this SMATed BMG can be ascribed to the gradient microstructure and concomitant residual stress profiles. Processed under high vibration frequency and high strain rate of SMAT, nanocrystallines are formed in the fully amorphous matrix. The entire microstructure is characterized by three layers, i.e. partially crystallized layer, crystallization affected layer and the metallic glass matrix layer, from the top surface to the interior matrix. In contrast to the general rule that shear bands would like to occur after reaching the critical shear strength and further develop into cracks during plastic deformations, the crystallites produced by SMAT are able to limit shear band propagation and crack extension. The residual stress in the crystallization affected zones may also help to block the main shear crack.[33] Secondary shear bands are found to initiate due to the congregated crystallites, making a larger contribution to plasticity.[34] This ‘shear band multiplication’ resulting from the gradient microstructure deflects and delays the critical crack, producing global enhancement of plasticity and toughness in the present material.
2.5.2. Toughening BMGs at the Atomic Scale It is known that the occurrence of shear bands and localized deformation indicates the major variation in the atomic structure and deformation behavior of crystalline materials. In this regard, a particular constitutive theoretical model for shear bands in BMGs has been established.[35] Finite element modeling (FEM) was employed to analyze deformation behavior under dynamic loadings. According to the simulation results, more shear bands may lead to higher compressive ductility. On the other hand, BMGs also display extraordinary compressive
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ductility under quasi-static loading.[36] The reason is attributed to the blending of two phases that randomly embed a soft phase into the hard regions. The FEM modeling validated the experimental hypothesis with 20% volumetric variation of the two phases, resulting in the much more improved compressive ductility than their individual brittleness and in good agreement with the experiments. It is also found that a maximum ductility occurs when the two phases are mixed in equal volume. An example of a La-based BMG composite has successfully achieved 5% tensile ductility with the introduction of a soft second phase with 50% volume.[37,38] It can thus be concluded that the dual-phase mode can not only promote shear band multiplication but also carry more plastic strain to sustain deformation. On this basis, a mixed system composed of amorphous crystal-like clusters are believed to be ideal structures to achieve high strength and high ductility in BMGs.[39,40] In order to promote plasticity by synthesizing the ideal hybrid components, some preliminary work has been performed. Wang et al. revealed the mechanism for thermal stability against crystallization in a Zr-based amorphous BMG, which resulted from the pinning effect of a new kind of metastable icosahedra-like atomic cluster that appears after being processed by isothermal annealing.[39,40] Yang et al. characterized the deformation behavior of free-volume zones via dynamic micropillar tests.[41] The loosely bonded atomistic free-volume zones behave as a deformation feature similar to supercooled liquids. The atomistic ‘soft core-hard shell’ structure is considered to be related with both yield strength and plasticity in BMGs. The most recent result also showed remarkable tensile ductility up to 4% and unique strain hardening in Cu-Zr-/Zr-based BMGs by activating multiple shear banding and delaying shear cracking and cavitation.[42] After processed by SMAT, a gradient amorphous microstructure and a residual stress profile are both formed to enhance the tensile plasticity and enable extensive applications.
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a multi-component BMG alloy.[30] These findings help to understand the mechanism of achieving high strength and high ductility by using nanoscale precipitates.
3. Designing Advanced Nanostructured Metals with Extremely High Strength and High Ductility by Hierarchical Nanotwins Motivated by the outstanding nanotwin strengthening effect, we propose a new concept of hierarchical nanotwins (HNTs) to further promote the mechanical properties. In contrast to the microscopic multilayered hierarchical structure (see Section 2.2), this HNT structure is a completely twinning system in different scales. As theoretically demonstrated by the dislocation-based model in pure copper, the HNTs are much more effective than monolithic nanotwins for strengthening and can reach a maximum value by optimizing the twin spacing (of two-order nanotwins) and grain size in each order.[43] Large-scale molecular dynamic (MD) simulations were also used to investigate the fracture characteristics of HNTs. In contrast to the cleavage mode of monolithic nanotwins, the crack resistance of primary twins is much higher. By crack blunting and dislocation storage of secondary twins with smaller twin spacing, overall crack behavior can be enhanced.[44] Very recently, we have successfully produced three-order HNTs in bulk NS TWIP (twinning induced
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Figure 2. A typical TEM (transmission electronic microscopy) morphology and a schematic diagram showing hierarchically twinned structure in the new SMATed TWIP steel samples before (b) and after (c) tensile testing. T1, T2 and T3 respectively denote the first-order, second-order, and third-order twins.
plasticity) steels,[45] achieving an unprecedented property that breaks the conventional milestone of yield strength for metallic materials over 2 GPa while still preserving a considerable uniform elongation of 15% for sufficient engineering formability. These newly developed HNTs on one hand inherit the advantages of monolithic nanotwins and on the other hand assemble simple nanotwins to constitute a new complex twinning system with a scale separation, as shown in Figure 2. Superior to general two-dimensional (2D) T/M lamellar nanotwins, this new hierarchically twinned system provides one extra dimension in space, exhibiting a three-dimensional (3D) structure (Figure 3) to further enhance the overall performance. Comparisons with conventional HSS[46] (high strength steels), AHSS (advanced high strength steels) and MS (martensitic steels) and other representative materials that have been reported indicate deviation from the general high strength high ductility metals and alloys,[4–6,8–14,29,47–50] revealing the unprecedented properties of the new TWIP steels (Figure 4). Both experimental results and numerical modeling show that the 3D HNTs play the primary role of inducing a sharp increase of tensile yield strength while maintaining sufficient elongation. After being experimentally shown in a steel alloy
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and theoretically demonstrated in pure copper, this hierarchically twinned structure is believed to be effective in diverse materials for global practice in the future.
4. Conclusions and Perspectives Unsustainable energy consumption and global warming are among the most significant problems for human beings worldwide. Increasing the fuel efficiency while maintaining sufficient formability pose a new challenge for manufacturers around the world to enhance the ductility in high-strength engineering materials. In this brief Research News article, we ascribe the new principle to a strain non-localization mechanism by introducing the above routes to achieve high strength and high ductility in NS and amorphous metallic materials. From the macroscopic multilaminates to nanotwins and atomic clusters, we have successfully realized simultaneous high strength and high ductility in diverse metallic materials by using multiplications of a series of strengthening factors (as summarized in Figure 1) to avoid strain/stress localization and delay the catastrophic failure. On this basis, further investigations will focus on the optimization
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Figure 3. Schematic of 3D three-ordered hierarchical twins illustrated by cutaway profiles (twins in the first, second, and third order are respectively denoted in blue, orange, and green) within a general grain, indicating a 3D cubic hierarchical twinned structure rather than the two-dimensional (2D) twin-matrix lamellae. The area in yellow dashed circles are cut away to show the generation sites of twins in the higher order; a lattice structure of the third twin is shown as an insert.
of mechanical properties of NS metals and BMGs under different conditions and rational analysis in terms of both experiments and theories. The new design of 3D HNTs opens up a new direction to fabricate “super metals and alloys” with extremely high yield strength and simultaneously high ductility, as suggested in Figure 4. The atomic crystal-like clusters also demonstrate the potential to produce tensile plasticity at ultrahigh stress levels of ∼2 Gpa. Quantitative correlations between the macroscopic performance and the complicated nanostructures are also challenging but very important to study. Systematic studies on 3D HNTs and atomic clusters/crystals in BMGs are necessary to be undertaken. Mechanical properties are usually controlled by intrinisic microstructures. We expect new developments of deformation mechanisms in relation to the promotion of high strength and high ductility in the future. Finally, additional research should also include the multiscale simulation methods, such as MD simulation and phase field modeling, to optimize the theoretical models for a better understanding of 3D hierachical twins and strain non-localizationmechanisms. The new progress of materials with extraordinary superhigh strength will benefit both light-weight construction and eco-friendly systems.
Figure 4. Comparison of yield strength-uniform elongation between the present new TWIP steel samples with other excellent metallic materials (solid circles for pure metals, and solid triangles for steel alloys) reported in literatures as examples with high strength and high ductility. The three oval regions respectively denote the ranges for conventional HSS (in light pink), AHSS (in light purple) and MS (in light blue) for comparison. The red triangles denote the new results of SMATed TWIP steel samples, which deviate obviously from all those with exceptional performances as divided by the red dashed line.
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Acknowledgements The authors acknowledge financial supports provided by the MOST through the National Basic Research Program of China (973 Program) under the grant 2012CB932203 and Research Grants Council for the Hong Kong Special Administrative Region of China under grants CityU8/ CRF/08, GRF/CityU51911, the Croucher Foundation CityU9500006, CityU Strategic Research Grant GRF/ECS (SRG-Fd) /CityU7002913. Received: April 9, 2014 Revised: May 24, 2014 Published online: June 27, 2014
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