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 AAASINSIGHTS | 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 engineer￾ing alloys ( 2– 4). When a homogeneous-grained material is under tension, the onset of plastic de￾formation in different grains occurs almost simultaneously. Because adjacent grains cannot deform in concert and displace￾ments 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 on￾set 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 local￾ization 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 defor￾mation mechanisms are activated. Deformation of the nanograined Cu is dominated by a mechanically driven grain boundary migration with concomitant grain coarsening and softening ( 1). Mean￾while, deformed coarse grains are hard￾ened by dislocation slip and accumulations, providing work hardening of the global sample. Hence, both hardening and soften￾ing occurs simultaneously in the gradient microstructure, and the dominating defor￾mation mechanisms change gradually from dislocation slip into grain boundary migra￾tion as grains become smaller. In a critical submicrosized region, neither hardening nor softening is induced as the two mecha￾nisms 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 coarse￾grained structure, leads to a strength-duc￾tility 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 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￾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 resistance after cyclic loading and unloading in several gradient nanograined materials ( 8). In homogeneous nanograined or submi￾crograined materials, resistance to fatigue crack growth is reduced relative to that in coarse grains, and the low-cycle, strain-con￾trolled 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 ef￾fective in arresting the crack propagation. The highly deformable gradient nanograined surface layer eliminates the deformation-in￾duced 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 gradi￾ent microstructures and properties is vital for optimizing global properties of the hi￾erarchical nanostructured materials. The development of processing techniques for stabilizing nanostructures via proper alloy￾ing ( 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