large-scale genomic studies conspicuously NANOMATERIALS avoiding the complexities of plasmid struc- ture.Genomic comparisons such as that described by Conlan et al.reveal how the Making strong nanomaterials dynamism in the structure and arrange- ment of resistance elements can only be realized by“closing”plasmid genomes with ductile with gradients long-read sequencing(see the figure). Traditional Sanger sequencing is the Microstructures that increase metal crystallite size from gold standard for the analysis and assem- nanoscale with surface depth are both strong and ductile bly of complete plasmid sequences from antibiotic-resistant strains of bacteria.This By K.Lu nisms of the extremely fine grains that in- approach may suffer from the need to iso- duces cracking.By applying surface plastic late or subclone individual high molecular teels can be made stronger,tougher deformation onto a bulk coarse-grained weight plasmids before sequencing (7). or more resistant to corrosion either metal,a distinctive microstructure is gener which is often technically difficult,time- by changing composition (adding in ated from the strain gradient:a nanograined consuming,and costly,and may be intrac- more carbon or other elements)or layer (several tens of micrometers thick) table for multiple plasmids.Short-read by modifying their microstructures. covers the coarse-grained substrate with a sequencing technologies can affordably An extreme microstructural route graded variation of grain size in between produce an assembly of a bacterial genome for strengthening materials is to reduce (see the first figure). that contains nonrepetitive sequences typi- the crystallite size from the micrometer Tensile tests of the heterogeneously cally in hundreds of "contigs"separated by scale ("coarse-grained")to the nanoscale. structured Cu cylinder (pulling the sample “collapsed repeats”indicative of multiple Nanograined aluminum or copper (Cu) along the long axis)showed that the top copies of the same sequence located in sev- may become even harder than high- eral different locations within the genome. strength steels,but these materials 0(m) These repeats are often mobile elements can be very brittle and crack when such as insertion sequences that may be pulled (deformed in tension),ap- found in multiple copies on plasmids,thus parently because strain becomes making it difficult to assemble plasmid localized and resists deformation. sequences. However,nanograined metals can Cataloging the collection of antibiotic- be plastically deformed under resistance genes in any particular bac- compression or rolling at ambient terium is relatively straightforward,but temperature,implying that moder- determining how these genes fit together ate deformation can occur if the within plasmids,which is critical for un- cracking process is suppressed.Tre- derstanding how these elements spread mendous efforts have been made to in clinical settings,can be more difficult. explore how to suppress strain lo- By contrast,the genome sequences pro- calization in tensioned nanomateri- duced through long-read sequencing offer als and make them ductile.Gradient a complete picture of the plasmid content microstructures,in which the grain of a bacterium,including the number,posi- size increases from nanoscale at tion,and context within mobile elements of the surface to coarse-grained in the every acquired antibiotic-resistance gene. core,were recently discovered to be Long-read genome assembly offers clear an effective approach to improving advantages for the resolution of complete ductility (-) plasmid sequences that can discriminate One advantage of metals in struc- plasmid diversity,antimicrobial-resistance tural applications is that they "sig- Gradient nanograined structure.After a surface mechanical gene context,and multiplicity.Such infor- nal"their impending failure-they grinding treatment to copper.grain sizes are about 20 nm in the mation will enhance our understanding of can deform and crack to some ex- topmost treated surface(outlined by dashed line)and increase plasmid carriage,transfer,epidemiology, tent before they completely fail. gradually to the microscale with depth. and evolution. However,when a piece of fully nanograined copper is pulled,catastrophic nanograined layer and the coarse-grained REFERENCES failure occurs immediately when the load core can be elongated coherently by as much 1.WHO.Antimicrobial resistance:Global report on surveil- exceeds its yield strength (the point at as ~60%before failure-comparable to that lance2014(2014). which permanent deformation begins),just in conventional Cu,but the sample's yield 2.S.Conlanetal..Sci.Transl.Med.254.254ral26(2014). 3.H.Ochman,J.G.Lawrence,E.A.Groisman,Nature 405. like most ceramics and other normal frag- strength is doubled (D).Almost no tensile 299(2000). ile materials.Such tensile brittleness is an elongation was observed in the nanograined 4.E.S.Snitkin etal.Sci.Transl.Med.4.148ral16(2012) Achilles'heel of nanomaterials that hinders layer as it was removed from the substrate 5.R.M.Hall,C.M.Collis.Mol.Microbiol.15.593(1995). their technological applications;for exam- Evidently,the observed extraordinary tensile 6.J.Mahillon,M.Chandler,Microbiol.Mol.Biol.Rev.62.725 (1998) ple,they cannot be strengthened by work 7.C.Venturini.S.A.Beatson,S.P.Djordjevic.M.J.Walker. hardening.The microscopic origin appears Shenyang National Laboratory for Materials Science.Institute FASEB124.1160(2010) to be early necking (decrease in cross sec- of Metal Research,Chinese Academy of Sciences,Shenyang 110016.China,and Herbert Gleiter Institute of Nanoscience tion)induced by strain localization prior Nanjing University of Science Technology.Nanjing 210094. 101126/science.1260471 to activation of plastic deformation mecha- China.E-mail:lu@imr.ac.cn SCIENCE sciencemag.org 19 SEPTEMBER 2014.VOL 345 ISSUE 6203 1455 Published by AAAS
SCIENCE sciencemag.org 19 SEPTEMBER 2014 • VOL 345 ISSUE 6203 1455 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). 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 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 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 localization 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 ductility ( 1– 4). One advantage of metals in structural applications is that they “signal” their impending failure—they can deform and crack to some extent before they completely fail. However, when a piece of fully nanograined copper is pulled, catastrophic failure occurs immediately when the load exceeds its yield strength (the point at which permanent deformation begins), just like most ceramics and other normal fragile materials. Such tensile brittleness is an Achilles’ heel of nanomaterials that hinders their technological applications; for example, they cannot be strengthened by work hardening. The microscopic origin appears to be early necking (decrease in cross section) induced by strain localization prior to activation of plastic deformation mechaBy K. Lu Microstructures that increase metal crystallite size from nanoscale with surface depth are both strong and ductile NANOMATERIALS Making strong nanomaterials ductile with gradients 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 nanograined layer and the coarse-grained core can be elongated coherently by as much as ~60% before failure—comparable to that in conventional Cu, but the sample’s yield strength is doubled (1). Almost no tensile elongation was observed in the nanograined layer as it was removed from the substrate. Evidently, the observed extraordinary tensile 0 (m) 50 100 150 Gradient nanograined structure. After a surface mechanical grinding treatment to copper, grain sizes are about 20 nm in the topmost treated surface (outlined by dashed line) and increase gradually to the microscale with depth. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China, and Herbert Gleiter Institute of Nanoscience, Nanjing University of Science & Technology, Nanjing 210094, 10.1126/science.1260471 China. E-mail: lu@imr.ac.cn Published byAAAS
INSIGHTS PERSPECTIVES nanograins and coarse grains (6),the overall strength gain comes at a loss of ductility lead- ing 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-ductil- ity line.Exceptionally superior strength- GNG ductility combinations were discovered in a number of gradient nanograined or gradient nanotwinned materials(2-4).The enhanced ductility in gradient nanograined interstitial- free 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 mi- crostructures also greatly enhances fatigue Homogeneous plastic deformation resistance after cyclic loading and unloading in several gradient nanograined materials CG+NG (8).In homogeneous nanograined or submi- NG crograined materials,resistance to fatigue crack growth is reduced relative to that in 0 Grain refinement coarse grains,and the low-cycle,strain-con- trolled fatigue properties become even worse. Strength A gradient nanostructured skin covering a Strength-ductility synergy.The strength of a metal is increased at an expense of ductility for homogeneous plastic coarse-grained substrate is actually optimal deformation of coarse-grained(CG)metals or homogeneous refinement to nanosized grains(NG).and follows a for enhancing fatigue resistance.Fatigue typical"banana-shaped"curve(blue line).Similar strength-ductility trade-offs occur for random mixtures of coarse crack initiation would be suppressed by the grains with nanograins(CG+NG).However.strength-ductility synergy is achieved with gradient nanograined(GNG) hard-and-deformable gradient nanograined structures(red line). skin while the coarse-grained interior is ef- fective in arresting the crack propagation. ductility of the nanograined skin resulted Deformation of the nanograined Cu is The highly deformable gradient nanograined from the ideal confinement of the gradient dominated by a mechanically driven grain surface layer eliminates the deformation-in- microstructure.Comparable tensile ductility boundary migration with concomitant duced surface roughening that is frequently of gradient-structured nanomaterials with grain coarsening and softening ()Mean- seen in tension or drawing of metals,which that of the coarse-grained counterparts was while,deformed coarse grains are hard- suppresses surface cracking and facilitates observed recently in a number of engineer- ened by dislocation slip and accumulations, subsequent mechanical processing (1). ing alloys (2-4). providing work hardening of the global Quantifying correlations between gradi- When a homogeneous-grained material sample.Hence,both hardening and soften- ent microstructures and properties is vital is under tension,the onset of plastic de- ing occurs simultaneously in the gradient for optimizing global properties of the hi- formation in different grains occurs almost microstructure,and the dominating defor- erarchical nanostructured materials.The simultaneously.Because adjacent grains mation mechanisms change gradually from development of processing techniques for cannot deform in concert and displace- dislocation slip into grain boundary migra- stabilizing nanostructures via proper alloy- ments across grain boundaries are not tion as grains become smaller.In a critical ing (9),grain boundary modifications,or matched,intergranular stress and strain submicrosized region,neither hardening both to enlarge the microstructure gradient localization may develop that create voids nor softening is induced as the two mecha- is challenging and critical for exploration of or cracks at the grain boundaries.For a nisms are balanced,corresponding to the more properties and functionalities. material with a grain-size gradient,the on- strain-induced saturation structures (5) REFERENCES AND NOTES set of plastic deformation occurs initially The gradient microstructure allows various in coarse grains and propagates gradually plastic deformation mechanisms of largely T.H.Fang.W.LLi.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 into smaller ones with increasing loads. different microstructures to be activated US.A1117197(2014). The orderly plastic deformation releases concurrently.This balance does not exist Y.Weietal..Nat Commun.5.3580(2014). intergranular stress between neighboring in homogeneous nanograined structures, 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). grains of different sizes so that strain local- nor in random mixtures of nanograins and 6 Y.S.Li.Y.Zhang.N.Tao.K.Lu.Scr.Mater.59.475(2008). ization is suppressed.At higher loads,such coarse grains. J.Li.A.K.Soh.Model.Simul.Mater.Sci Eng.20.085002 a strain delocalization process takes place The extraordinary tensile ductility of the (2012). 8 H.W.Huang.Z.B.Wang.X_P.Yong.K.Lu.Mater.Sci.Technol. progressively in finer and finer grains until gradient nanograined surface layer,which 29.1200(2013). it reaches the topmost nanograined layer. is several times stronger than the coarse- 9.D.A.Hughes.N.Hansen,Phys.Rev.Lett 112.135504(2014) Effective suppression of strain localization grained structure,leads to a strength-duc- ACKNOWLEDGMENTS and early necking enable the nanograined tility synergy,as opposed to the traditional Supported by Ministry of Science Technology of China grant skin to elongate concurrently with other trade-off between strength and ductility.In 2012CB932201and National Natural Science Foundation of parts of the sample,and its plastic defor- homogeneously deformed or homogeneous China grants 51231006 and 5126113009. mation mechanisms are activated. nanograined metals,or random mixtures of 10.1126/science.1255940 1456 19 SEPTEMBER 2014.VOL 345 ISSUE 6203 sciencemag.org SCIENCE Published by AAAS
INSIGHTS | PERSPECTIVES 1456 19 SEPTEMBER 2014 • VOL 345 ISSUE 6203 sciencemag.org SCIENCE 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. 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. ■ REFERENCES AND NOTES 1. T. H. Fang, W. L. Li, N. R. Tao, K. Lu, Science331, 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). ACKNOWLEDGMENTS Supported by Ministry of Science & Technology of China grant 2012CB932201 and National Natural Science Foundation of China grants 51231006 and 5126113009. CG NG CG+NG 0 Homogeneous plastic deformation Grain refnement Strength Ductility GNG 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). 10.1126/science.1255940 Published byAAAS
