Strategies for Improving Tensile Ductility of Bulk Nanostructured Materials

Abstract

Strength and ductility are two of the most important mechanical properties of structural materials. High strength is desired for structural components so that they can carry high loads. Good ductility is essential to avoid catastrophic failure in load-bearing applications and is also very important for many shaping and forming operations. The ductility of materials is defined as the extent to which a material can be plastically deformed. Usually, ductility is measured as the elongation to failure in uniaxial tension. It has been a long-standing goal for materials scientists to synthesize structural materials with balanced combinations of high strength and high ductility. However, strength and ductility usually exhibit an inverse relationship; i.e., increasing the strength sacrifices the ductility, and vice versa. This longstanding, strength-ductility empiric conundrum has been partially addressed, for example, by the development of solid solute and dispersion (2nd-phase particles) hardened alloys which have widespread engineering applications but have a lower ductility in comparison with their pure metallic matrix.Since 1980s, bulk nanostructured (NS) materials have emerged as a new class of materials with unusual physical and mechanical behavior and as a result, have attracted considerable attention in recent years. [1] The terminology ''nanostructured'' is generally defined as structural features smaller than 100 nm. [2] Bulk NS materials are single or multi-phase polycrystals with nanoscale grain/subgrain sizes, which are distinct from low-dimensional NS materials such as nanotubes, -wires, -films, and -clusters. Bulk NS materials are structurally characterized by a large volume fraction of grain boundaries (50% for 5 nm grains, 30% for 10 nm grains), which may significantly alter their physical, mechanical, and chemical properties in comparison with those of conventional coarse-grained (CG) materials. The relatively high strength of bulk NS materials, typically 5-10 times of conventional materials of similar composition, offers COMMUNICATIONThe low ductility that is consistently associated with bulk nanostructured (NS) materials has been identified as perhaps the single most critical issue that must be resolved before this novel class of materials can be used in a wide variety of applications. Not surprisingly, a number of published studies, published mostly after 2000, identify the issue of low ductility and describe strategies to improve ductility. Details of these strategies were discussed in review papers published by Ma in 2005 and 2006, respectively. [15,16] In view of continued efforts and recent results, in this paper we describe progress in attempting to address the low ductility of NS materials, after 2006. We first analyze the fundamental reasons for the observed low ductility of bulk NS materials, and summarize early (prior to 2006) attempts to enhance the ductility of bulk NS materials, which often sacrificed the strength. Then, we review recent progress in developing strategies for improving ...

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