Making strong nanomaterials ductile with gradients K. Lu Science 345, 1455 (2014); DOI: 10.1126/science.1255940
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Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
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large-scale genomic studies conspicuously avoiding the complexities of plasmid structure. Genomic comparisons such as that described by Conlan et al. reveal how the dynamism in the structure and arrangement of resistance elements can only be realized by “closing” plasmid genomes with long-read sequencing (see the figure). Traditional Sanger sequencing is the gold standard for the analysis and assembly of complete plasmid sequences from antibiotic-resistant strains of bacteria. This approach may suffer from the need to isolate or subclone individual high molecular weight plasmids before sequencing (7), which is often technically difficult, timeconsuming, and costly, and may be intractable for multiple plasmids. Short-read sequencing technologies can affordably produce an assembly of a bacterial genome that contains nonrepetitive sequences typically in hundreds of “contigs” separated by “collapsed repeats” indicative of multiple copies of the same sequence located in several different locations within the genome. These repeats are often mobile elements such as insertion sequences that may be found in multiple copies on plasmids, thus making it difficult to assemble plasmid sequences. Cataloging the collection of antibioticresistance genes in any particular bacterium is relatively straightforward, but determining how these genes fit together within plasmids, which is critical for understanding how these elements spread in clinical settings, can be more difficult. By contrast, the genome sequences produced through long-read sequencing offer a complete picture of the plasmid content of a bacterium, including the number, position, and context within mobile elements of every acquired antibiotic-resistance gene. Long-read genome assembly offers clear advantages for the resolution of complete plasmid sequences that can discriminate plasmid diversity, antimicrobial-resistance gene context, and multiplicity. Such information will enhance our understanding of plasmid carriage, transfer, epidemiology, and evolution. REFERENCES
1. WHO, Antimicrobial resistance: Global report on surveillance 2014 (2014). 2. S. Conlan et al., Sci. Transl. Med. 254, 254ra126 (2014). 3. H. Ochman, J. G. Lawrence, E. A. Groisman, Nature 405, 299 (2000). 4. E. S. Snitkin et al. Sci. Transl. Med. 4, 148ra116 (2012). 5. R. M. Hall, C. M. Collis, Mol. Microbiol. 15, 593 (1995). 6. J. Mahillon, M. Chandler, Microbiol. Mol. Biol. Rev. 62, 725 (1998). 7. C. Venturini, S. A. Beatson, S. P. Djordjevic, M. J. Walker, FASEB J. 24, 1160 (2010). 10.1126/science.1260471
NANOMATERIALS
Making strong nanomaterials ductile with gradients Microstructures that increase metal crystallite size from nanoscale with surface depth are both strong and ductile nisms of the extremely fine grains that induces cracking. By applying surface plastic deformation onto a bulk coarse-grained metal, a distinctive microstructure is generated from the strain gradient: a nanograined layer (several tens of micrometers thick) covers the coarse-grained substrate with a graded variation of grain size in between (see the first figure). Tensile tests of the heterogeneously structured Cu cylinder (pulling the sample along the long axis) showed that the top
By K. Lu
S
teels can be made stronger, tougher, or more resistant to corrosion either by changing composition (adding in more carbon or other elements) or by modifying their microstructures. An extreme microstructural route for strengthening materials is to reduce the crystallite size from the micrometer scale (“coarse-grained”) to the nanoscale. Nanograined aluminum or copper (Cu) may become even harder than highstrength steels, but these materials 0 (m) can be very brittle and crack when pulled (deformed in tension), apparently because strain becomes localized and resists deformation. However, nanograined metals can be plastically deformed under 50 compression or rolling at ambient temperature, implying that moderate deformation can occur if the cracking process is suppressed. Tremendous efforts have been made to explore how to suppress strain lo100 calization in tensioned nanomaterials and make them ductile. Gradient microstructures, in which the grain size increases from nanoscale at the surface to coarse-grained in the core, were recently discovered to be an effective approach to improving 150 ductility (1–4). One advantage of metals in structural applications is that they “sigGradient nanograined structure. After a surface mechanical nal” their impending failure—they grinding treatment to copper, grain sizes are about 20 nm in the can deform and crack to some extopmost treated surface (outlined by dashed line) and increase tent before they completely fail. gradually to the microscale with depth. However, when a piece of fully nanograined copper is pulled, catastrophic nanograined layer and the coarse-grained failure occurs immediately when the load core can be elongated coherently by as much exceeds its yield strength (the point at as ~60% before failure—comparable to that which permanent deformation begins), just in conventional Cu, but the sample’s yield like most ceramics and other normal fragstrength is doubled (1). Almost no tensile ile materials. Such tensile brittleness is an elongation was observed in the nanograined Achilles’ heel of nanomaterials that hinders layer as it was removed from the substrate. their technological applications; for examEvidently, the observed extraordinary tensile ple, they cannot be strengthened by work Shenyang National Laboratory for Materials Science, Institute hardening. The microscopic origin appears of Metal Research, Chinese Academy of Sciences, Shenyang to be early necking (decrease in cross sec110016, China, and Herbert Gleiter Institute of Nanoscience, tion) induced by strain localization prior Nanjing University of Science & Technology, Nanjing 210094, to activation of plastic deformation mechaChina. E-mail: [email protected]
SCIENCE sciencemag.org
19 SEP TEMBER 2014 • VOL 345 ISSUE 6203
Published by AAAS
1455
INSIGHTS | P E R S P E C T I V E S
CG
Ductility
GNG
Homogeneous plastic deformation CG+NG NG 0
Grain refnement Strength
Strength-ductility synergy. The strength of a metal is increased at an expense of ductility for homogeneous plastic deformation of coarse-grained (CG) metals or homogeneous refinement to nanosized grains (NG), and follows a typical “banana-shaped” curve (blue line). Similar strength-ductility trade-offs occur for random mixtures of coarse grains with nanograins (CG+NG). However, strength-ductility synergy is achieved with gradient nanograined (GNG) structures (red line).
ductility of the nanograined skin resulted from the ideal confinement of the gradient microstructure. Comparable tensile ductility of gradient-structured nanomaterials with that of the coarse-grained counterparts was observed recently in a number of engineering alloys (2–4). When a homogeneous-grained material is under tension, the onset of plastic deformation in different grains occurs almost simultaneously. Because adjacent grains cannot deform in concert and displacements across grain boundaries are not matched, intergranular stress and strain localization may develop that create voids or cracks at the grain boundaries. For a material with a grain-size gradient, the onset of plastic deformation occurs initially in coarse grains and propagates gradually into smaller ones with increasing loads. The orderly plastic deformation releases intergranular stress between neighboring grains of different sizes so that strain localization is suppressed. At higher loads, such a strain delocalization process takes place progressively in finer and finer grains until it reaches the topmost nanograined layer. Effective suppression of strain localization and early necking enable the nanograined skin to elongate concurrently with other parts of the sample, and its plastic deformation mechanisms are activated. 1456
Deformation of the nanograined Cu is dominated by a mechanically driven grain boundary migration with concomitant grain coarsening and softening (1). Meanwhile, deformed coarse grains are hardened by dislocation slip and accumulations, providing work hardening of the global sample. Hence, both hardening and softening occurs simultaneously in the gradient microstructure, and the dominating deformation mechanisms change gradually from dislocation slip into grain boundary migration as grains become smaller. In a critical submicrosized region, neither hardening nor softening is induced as the two mechanisms are balanced, corresponding to the strain-induced saturation structures (5). The gradient microstructure allows various plastic deformation mechanisms of largely different microstructures to be activated concurrently. This balance does not exist in homogeneous nanograined structures, nor in random mixtures of nanograins and coarse grains. The extraordinary tensile ductility of the gradient nanograined surface layer, which is several times stronger than the coarsegrained structure, leads to a strength-ductility synergy, as opposed to the traditional trade-off between strength and ductility. In homogeneously deformed or homogeneous nanograined metals, or random mixtures of
nanograins and coarse grains (6), the overall strength gain comes at a loss of ductility leading to a “banana-shaped” curve, as shown in the second figure. Gradient nanostructuring avoids this ductility loss, and the use of even smaller nanograins or thicker gradient skin (7) may further upbow the strength-ductility line. Exceptionally superior strengthductility combinations were discovered in a number of gradient nanograined or gradient nanotwinned materials (2–4). The enhanced ductility in gradient nanograined interstitialfree steel sheets was alternatively explained by an extra strain hardening induced by a macroscopic strain gradient and a change in stress states (2). The strain delocalization of gradient microstructures also greatly enhances fatigue resistance after cyclic loading and unloading in several gradient nanograined materials (8). In homogeneous nanograined or submicrograined materials, resistance to fatigue crack growth is reduced relative to that in coarse grains, and the low-cycle, strain-controlled fatigue properties become even worse. A gradient nanostructured skin covering a coarse-grained substrate is actually optimal for enhancing fatigue resistance. Fatigue crack initiation would be suppressed by the hard-and-deformable gradient nanograined skin while the coarse-grained interior is effective in arresting the crack propagation. The highly deformable gradient nanograined surface layer eliminates the deformation-induced surface roughening that is frequently seen in tension or drawing of metals, which suppresses surface cracking and facilitates subsequent mechanical processing (1). Quantifying correlations between gradient microstructures and properties is vital for optimizing global properties of the hierarchical nanostructured materials. The development of processing techniques for stabilizing nanostructures via proper alloying (9), grain boundary modifications, or both to enlarge the microstructure gradient is challenging and critical for exploration of more properties and functionalities. ■ REF ERENCES AND NOTES
1. T. H. Fang, W. L. Li, N. R. Tao, K. Lu, Science 331, 1587 (2011). 2. X. Wu, P. Jiang, L. Chen, F. Yuan, Y. T. Zhu, Proc. Natl. Acad. Sci. U.S.A. 111, 7197 (2014). 3. Y. Wei et al., Nat. Commun. 5, 3580 (2014). 4. H. Kou, J. Lu, Y. Li, Adv. Mater. 26, 5518 (2014). 5. T. H. Fang, N. R. Tao, K. Lu, Scr. Mater. 77, 17 (2014). 6. Y. S. Li, Y. Zhang, N. Tao, K. Lu, Scr. Mater. 59, 475 (2008). 7. J. Li, A. K. Soh, Model. Simul. Mater. Sci. Eng. 20, 085002 (2012). 8. H. W. Huang, Z. B. Wang, X. P. Yong, K. Lu, Mater. Sci. Technol. 29, 1200 (2013). 9. D. A. Hughes, N. Hansen, Phys. Rev. Lett. 112, 135504 (2014). ACKNOWL EDGMENTS
Supported by Ministry of Science & Technology of China grant 2012CB932201 and National Natural Science Foundation of China grants 51231006 and 5126113009. 10.1126/science.1255940
sciencemag.org SCIENCE
19 SEP TEMBER 2014 • VOL 345 ISSUE 6203
Published by AAAS
This copy is for your personal, non-commercial use only.
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
The following resources related to this article are available online at www.sciencemag.org (this information is current as of September 26, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/345/6203/1455.full.html This article cites 9 articles, 2 of which can be accessed free: http://www.sciencemag.org/content/345/6203/1455.full.html#ref-list-1 This article appears in the following subject collections: Materials Science http://www.sciencemag.org/cgi/collection/mat_sci
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on September 26, 2014
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here.
large-scale genomic studies conspicuously avoiding the complexities of plasmid structure. Genomic comparisons such as that described by Conlan et al. reveal how the dynamism in the structure and arrangement of resistance elements can only be realized by “closing” plasmid genomes with long-read sequencing (see the figure). Traditional Sanger sequencing is the gold standard for the analysis and assembly of complete plasmid sequences from antibiotic-resistant strains of bacteria. This approach may suffer from the need to isolate or subclone individual high molecular weight plasmids before sequencing (7), which is often technically difficult, timeconsuming, and costly, and may be intractable for multiple plasmids. Short-read sequencing technologies can affordably produce an assembly of a bacterial genome that contains nonrepetitive sequences typically in hundreds of “contigs” separated by “collapsed repeats” indicative of multiple copies of the same sequence located in several different locations within the genome. These repeats are often mobile elements such as insertion sequences that may be found in multiple copies on plasmids, thus making it difficult to assemble plasmid sequences. Cataloging the collection of antibioticresistance genes in any particular bacterium is relatively straightforward, but determining how these genes fit together within plasmids, which is critical for understanding how these elements spread in clinical settings, can be more difficult. By contrast, the genome sequences produced through long-read sequencing offer a complete picture of the plasmid content of a bacterium, including the number, position, and context within mobile elements of every acquired antibiotic-resistance gene. Long-read genome assembly offers clear advantages for the resolution of complete plasmid sequences that can discriminate plasmid diversity, antimicrobial-resistance gene context, and multiplicity. Such information will enhance our understanding of plasmid carriage, transfer, epidemiology, and evolution. REFERENCES
1. WHO, Antimicrobial resistance: Global report on surveillance 2014 (2014). 2. S. Conlan et al., Sci. Transl. Med. 254, 254ra126 (2014). 3. H. Ochman, J. G. Lawrence, E. A. Groisman, Nature 405, 299 (2000). 4. E. S. Snitkin et al. Sci. Transl. Med. 4, 148ra116 (2012). 5. R. M. Hall, C. M. Collis, Mol. Microbiol. 15, 593 (1995). 6. J. Mahillon, M. Chandler, Microbiol. Mol. Biol. Rev. 62, 725 (1998). 7. C. Venturini, S. A. Beatson, S. P. Djordjevic, M. J. Walker, FASEB J. 24, 1160 (2010). 10.1126/science.1260471
NANOMATERIALS
Making strong nanomaterials ductile with gradients Microstructures that increase metal crystallite size from nanoscale with surface depth are both strong and ductile nisms of the extremely fine grains that induces cracking. By applying surface plastic deformation onto a bulk coarse-grained metal, a distinctive microstructure is generated from the strain gradient: a nanograined layer (several tens of micrometers thick) covers the coarse-grained substrate with a graded variation of grain size in between (see the first figure). Tensile tests of the heterogeneously structured Cu cylinder (pulling the sample along the long axis) showed that the top
By K. Lu
S
teels can be made stronger, tougher, or more resistant to corrosion either by changing composition (adding in more carbon or other elements) or by modifying their microstructures. An extreme microstructural route for strengthening materials is to reduce the crystallite size from the micrometer scale (“coarse-grained”) to the nanoscale. Nanograined aluminum or copper (Cu) may become even harder than highstrength steels, but these materials 0 (m) can be very brittle and crack when pulled (deformed in tension), apparently because strain becomes localized and resists deformation. However, nanograined metals can be plastically deformed under 50 compression or rolling at ambient temperature, implying that moderate deformation can occur if the cracking process is suppressed. Tremendous efforts have been made to explore how to suppress strain lo100 calization in tensioned nanomaterials and make them ductile. Gradient microstructures, in which the grain size increases from nanoscale at the surface to coarse-grained in the core, were recently discovered to be an effective approach to improving 150 ductility (1–4). One advantage of metals in structural applications is that they “sigGradient nanograined structure. After a surface mechanical nal” their impending failure—they grinding treatment to copper, grain sizes are about 20 nm in the can deform and crack to some extopmost treated surface (outlined by dashed line) and increase tent before they completely fail. gradually to the microscale with depth. However, when a piece of fully nanograined copper is pulled, catastrophic nanograined layer and the coarse-grained failure occurs immediately when the load core can be elongated coherently by as much exceeds its yield strength (the point at as ~60% before failure—comparable to that which permanent deformation begins), just in conventional Cu, but the sample’s yield like most ceramics and other normal fragstrength is doubled (1). Almost no tensile ile materials. Such tensile brittleness is an elongation was observed in the nanograined Achilles’ heel of nanomaterials that hinders layer as it was removed from the substrate. their technological applications; for examEvidently, the observed extraordinary tensile ple, they cannot be strengthened by work Shenyang National Laboratory for Materials Science, Institute hardening. The microscopic origin appears of Metal Research, Chinese Academy of Sciences, Shenyang to be early necking (decrease in cross sec110016, China, and Herbert Gleiter Institute of Nanoscience, tion) induced by strain localization prior Nanjing University of Science & Technology, Nanjing 210094, to activation of plastic deformation mechaChina. E-mail: [email protected]
SCIENCE sciencemag.org
19 SEP TEMBER 2014 • VOL 345 ISSUE 6203
Published by AAAS
1455
INSIGHTS | P E R S P E C T I V E S
CG
Ductility
GNG
Homogeneous plastic deformation CG+NG NG 0
Grain refnement Strength
Strength-ductility synergy. The strength of a metal is increased at an expense of ductility for homogeneous plastic deformation of coarse-grained (CG) metals or homogeneous refinement to nanosized grains (NG), and follows a typical “banana-shaped” curve (blue line). Similar strength-ductility trade-offs occur for random mixtures of coarse grains with nanograins (CG+NG). However, strength-ductility synergy is achieved with gradient nanograined (GNG) structures (red line).
ductility of the nanograined skin resulted from the ideal confinement of the gradient microstructure. Comparable tensile ductility of gradient-structured nanomaterials with that of the coarse-grained counterparts was observed recently in a number of engineering alloys (2–4). When a homogeneous-grained material is under tension, the onset of plastic deformation in different grains occurs almost simultaneously. Because adjacent grains cannot deform in concert and displacements across grain boundaries are not matched, intergranular stress and strain localization may develop that create voids or cracks at the grain boundaries. For a material with a grain-size gradient, the onset of plastic deformation occurs initially in coarse grains and propagates gradually into smaller ones with increasing loads. The orderly plastic deformation releases intergranular stress between neighboring grains of different sizes so that strain localization is suppressed. At higher loads, such a strain delocalization process takes place progressively in finer and finer grains until it reaches the topmost nanograined layer. Effective suppression of strain localization and early necking enable the nanograined skin to elongate concurrently with other parts of the sample, and its plastic deformation mechanisms are activated. 1456
Deformation of the nanograined Cu is dominated by a mechanically driven grain boundary migration with concomitant grain coarsening and softening (1). Meanwhile, deformed coarse grains are hardened by dislocation slip and accumulations, providing work hardening of the global sample. Hence, both hardening and softening occurs simultaneously in the gradient microstructure, and the dominating deformation mechanisms change gradually from dislocation slip into grain boundary migration as grains become smaller. In a critical submicrosized region, neither hardening nor softening is induced as the two mechanisms are balanced, corresponding to the strain-induced saturation structures (5). The gradient microstructure allows various plastic deformation mechanisms of largely different microstructures to be activated concurrently. This balance does not exist in homogeneous nanograined structures, nor in random mixtures of nanograins and coarse grains. The extraordinary tensile ductility of the gradient nanograined surface layer, which is several times stronger than the coarsegrained structure, leads to a strength-ductility synergy, as opposed to the traditional trade-off between strength and ductility. In homogeneously deformed or homogeneous nanograined metals, or random mixtures of
nanograins and coarse grains (6), the overall strength gain comes at a loss of ductility leading to a “banana-shaped” curve, as shown in the second figure. Gradient nanostructuring avoids this ductility loss, and the use of even smaller nanograins or thicker gradient skin (7) may further upbow the strength-ductility line. Exceptionally superior strengthductility combinations were discovered in a number of gradient nanograined or gradient nanotwinned materials (2–4). The enhanced ductility in gradient nanograined interstitialfree steel sheets was alternatively explained by an extra strain hardening induced by a macroscopic strain gradient and a change in stress states (2). The strain delocalization of gradient microstructures also greatly enhances fatigue resistance after cyclic loading and unloading in several gradient nanograined materials (8). In homogeneous nanograined or submicrograined materials, resistance to fatigue crack growth is reduced relative to that in coarse grains, and the low-cycle, strain-controlled fatigue properties become even worse. A gradient nanostructured skin covering a coarse-grained substrate is actually optimal for enhancing fatigue resistance. Fatigue crack initiation would be suppressed by the hard-and-deformable gradient nanograined skin while the coarse-grained interior is effective in arresting the crack propagation. The highly deformable gradient nanograined surface layer eliminates the deformation-induced surface roughening that is frequently seen in tension or drawing of metals, which suppresses surface cracking and facilitates subsequent mechanical processing (1). Quantifying correlations between gradient microstructures and properties is vital for optimizing global properties of the hierarchical nanostructured materials. The development of processing techniques for stabilizing nanostructures via proper alloying (9), grain boundary modifications, or both to enlarge the microstructure gradient is challenging and critical for exploration of more properties and functionalities. ■ REF ERENCES AND NOTES
1. T. H. Fang, W. L. Li, N. R. Tao, K. Lu, Science 331, 1587 (2011). 2. X. Wu, P. Jiang, L. Chen, F. Yuan, Y. T. Zhu, Proc. Natl. Acad. Sci. U.S.A. 111, 7197 (2014). 3. Y. Wei et al., Nat. Commun. 5, 3580 (2014). 4. H. Kou, J. Lu, Y. Li, Adv. Mater. 26, 5518 (2014). 5. T. H. Fang, N. R. Tao, K. Lu, Scr. Mater. 77, 17 (2014). 6. Y. S. Li, Y. Zhang, N. Tao, K. Lu, Scr. Mater. 59, 475 (2008). 7. J. Li, A. K. Soh, Model. Simul. Mater. Sci. Eng. 20, 085002 (2012). 8. H. W. Huang, Z. B. Wang, X. P. Yong, K. Lu, Mater. Sci. Technol. 29, 1200 (2013). 9. D. A. Hughes, N. Hansen, Phys. Rev. Lett. 112, 135504 (2014). ACKNOWL EDGMENTS
Supported by Ministry of Science & Technology of China grant 2012CB932201 and National Natural Science Foundation of China grants 51231006 and 5126113009. 10.1126/science.1255940
sciencemag.org SCIENCE
19 SEP TEMBER 2014 • VOL 345 ISSUE 6203
Published by AAAS
