Ciencia De Los Materiales

Progress in Materials Science 51 (2006) 427–556 www.elsevier.com/locate/pmatsci

Mechanical properties of nanocrystalline materials
M.A. Meyers *, A. Mishra, D.J. Benson
Department of Mechanical and Aerospace Engineering, Materials Science and Engineering Program, Mail Code 0411, University of California, San Diego La Jolla, CA 92093, United States Received 1 November 2004; revised 1 May2005; accepted for publication 1 August 2005

Abstract The mechanical properties of nanocrystalline materials are reviewed, with emphasis on their constitutive response and on the fundamental physical mechanisms. In a brief introduction, the most important synthesis methods are presented. A number of aspects of mechanical behavior are discussed, including the deviation from the Hall–Petch slope andpossible negative slope, the effect of porosity, the difference between tensile and compressive strength, the limited ductility, the tendency for shear localization, the fatigue and creep responses. The strain-rate sensitivity of FCC metals is increased due to the decrease in activation volume in the nanocrystalline regime; for BCC metals this trend is not observed, since the activation volume isalready low in the conventional polycrystalline regime. In fatigue, it seems that the S–N curves show improvement due to the increase in strength, whereas the da/dN curve shows increased growth velocity (possibly due to the smoother fracture requiring less energy to propagate). The creep results are conflicting: while some results indicate a decreased creep resistance consistent with the small grainsize, other experimental results show that the creep resistance is not negatively affected. Several mechanisms that quantitatively predict the strength of nanocrystalline metals in terms of basic defects (dislocations, stacking faults, etc.) are discussed: break-up of dislocation pile-ups, core-and-mantle, grain-boundary sliding, grain-boundary dislocation emission and annihilation, graincoalescence, and gradient approach. Although this classification is broad, it incorporates the major mechanisms proposed to this date. The increased tendency for twinning, a direct consequence of the increased separation between partial dislocations, is discussed. The fracture of nanocrystalline metals consists of a mixture of ductile dimples and shear regions; the dimple size, while much smaller than thatof conventional polycrystalline metals, is several times larger than the grain size. The shear regions are a direct consequence of the increased tendency of the nanocrystalline metals to undergo shear localization.

*

Corresponding author. Tel.: +1 858 534 4719; fax: +1 858 534 5698. E-mail address: mameyers@ucsd.edu (M.A. Meyers).

0079-6425/$ - see front matter Ó 2005 Published byElsevier Ltd. doi:10.1016/j.pmatsci.2005.08.003

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M.A. Meyers et al. / Progress in Materials Science 51 (2006) 427–556

The major computational approaches to the modeling of the mechanical processes in nanocrystalline metals are reviewed with emphasis on molecular dynamics simulations, which are revealing the emission of partial dislocations at grain boundaries and their annihilation aftercrossing them. Ó 2005 Published by Elsevier Ltd.

Contents 1. 2. 3. 4. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Inert gas condensation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Mechanical alloying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Electrodeposition . . . . . ....