Making strong nanomaterials ductile with gradients

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Science  19 Sep 2014:
Vol. 345, Issue 6203, pp. 1455-1456
DOI: 10.1126/science.1255940

Steels 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 high-strength 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 (14).

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 mechanisms 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 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 (24).

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.

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).

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 strength-ductility combinations were discovered in a number of gradient nanograined or gradient nanotwinned materials (24). 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 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. Acknowledgments: Supported by Ministry of Science & Technology of China grant 2012CB932201 and National Natural Science Foundation of China grants 51231006 and 5126113009.
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