DOI: 10.1002/adma.200800214 High Tensile Ductility and Strength in Bulk Nanostructured Nickel** By Yonghao Zhao, Troy Topping, John F. Bingert, Jeremy J. Thornton, Andrea M. Dangelewicz, Ying Li, Wei Liu, Yuntian Zhu, Yizhang Zhou, and Enrique J. Lavernia* Bulk nanostructured (NS) materials have relatively high strength but disappointingly low tensile ductility (elongation to failure) at ambient temperatures. [1–4] The limited ductility of NS materials has emerged as a particularly challenging issue in the study and application of this novel class of materials. Recently, a variety of strategies aimed at improving the poor ductility of NS materials have been reported; the results reveal varying degrees of success. [5,6] Despite some encouraging reports, the improve- ments in ductility remain quite limited, usually below 15% for most of the strategies, except possibly for bi-modal Cu (with a ductility of 65%) and ultrafine grained (UFG) Fe-Cr-Ni-Mn steel (ca. 30%). [7,8] In this Communication, we use cryomilling and subsequently quasi-isostatic (QI) forging processes (formerly known as Ceracon forging), to prepare bulk dense multimodal and bimodal NS Ni with tensile ductility of 42% and 49%, and yield strengths of 457 and 312 MPa, respectively. This combination of strength and ductility is much superior to those of the NS Ni prepared by electro-deposition (ED), [9–16] cryorol- ling, [17] equal-channel angular pressing (ECAP) and high pressure torsion (HPT) methods, [18] and cold drawing. [19] Microstructural analyses suggest that significantly reduced extrinsic processing artifacts, the presence of equilibrium high-angle grain boundaries (including twin boundaries), and multi-/bimodal grain size distributions are responsible for the measured high ductility. The high strength is argued to originate from several sources, including a high density of dislocations, UFGs, and from solid solution strengthening. Compared with other synthesis methods, the synthesis methodology described in the present work has no scale or material limitations, and therefore has important implications in terms of its potential for the large-scale fabrication of bulk NS metals, alloys, and composites that can be used in applications requiring both high ductility and strength. Bulk NS materials are usually synthesized by either a two-step approach involving the synthesis and consolidation of nanoparticles (e.g., via inert-gas condensation) [2,3] or nano- crystalline powders (e.g., via ball milling or cryomilling), [20] or a one-step approach such as severe plastic deformation (SPD). [21] In the case of NS materials prepared by the two-step approach, powder handling can yield extrinsic processing artifacts (such as porosity, incomplete bonding, impurities, and others). It is now well-established that these artifacts, when present, will cause premature failure under tensile stresses, sometimes even before the onset of yielding. [3] In a recent study, the material was consolidated in situ via an approach involving cryomilling followed by room-temperature milling. [22] In this case, the NS Cu prepared by this novel method was reported to have a uniform tensile elongation (strain before necking) of 14% and a high yield strength of 790 MPa. Despite these encouraging results, this approach has material and scale limitations: the NS spheres are limited to a size comparable to that of the milling media, and it is only applicable to metals with low melting points that can be welded into dense spheres at room temperature. Moreover, the structural features responsible for this excellent strength and ductility behavior are not fully understood, and this has hindered the search for procedures that may be used effectively to improve the ductility of NS materials. The one-step SPD approaches can be used to synthesize flaw-free NS materials with higher ductility than those synthesized by the two-step approach. However, even these NS materials often exhibit a very low or near-zero uniform tensile elongation owing to their low strain hardening (dislocation storage capacity). Several different strategies have recently been developed to improve the poor ductility of NS materials; these include approaches such as the introduc- tion of a bimodal grain size distribution, [7,20] or pre-existing nanoscale twins, [23,24] using dispersions of nanoparticle- s/-precipitates, [25,26] preparing a mixture of two or multiple phases, [27,28] transformation-/twinning-induced plasticity, [8,29] and changing the deformation conditions. [30,31] Despite varying degrees of success, most of the strategies have inherent processing and/or material limitations. The objective of the present study was two-fold: First, to explore a generic synthesis COMMUNICATION [*] Prof. E. J. Lavernia, Dr. Y. H. Zhao, T. Topping, Dr. Y. Li, W. Liu, Dr. Y. Z. Zhou Department of Chemical Engineering and Materials Science University of California Davis, CA 95616 (USA) E-mail: lavernia@ucdavis.edu Dr. J. F. Bingert, J. J. Thornton, A. M. Dangelewicz, Prof. Y. T. Zhu Los Alamos National Laboratory Los Alamos, NM 87545 (USA) Prof. Y. T. Zhu Department of Materials Science & Engineering North Carolina State University Raleigh, NC 27695-7919 (USA) [**] This project is supported by the Office of Naval Research (Grant number N00014-04-1-0370) with Dr. Lawrence Kabacoff as program officer. Supporting Information is available online from Wiley InterScience or from the authors. 3028 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20, 3028–3033