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 May 2005; accepted for publication 1 August 2005 Abstract The mechanical properties of nanocrystalline materials are reviewed, with emphasis on their con- stitutive 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 dis- cussed, including the deviation from the Hall–Petch slope and possible negative slope, the effect of porosity, the difference between tensile and compressive strength, the limited ductility, the ten- dency for shear localization, the fatigue and creep responses. The strain-rate sensitivity of FCC met- als 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 is already low in the conventional poly- crystalline 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 indi- cate a decreased creep resistance consistent with the small grain size, 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 dis- cussed: break-up of dislocation pile-ups, core-and-mantle, grain-boundary sliding, grain-boundary dislocation emission and annihilation, grain coalescence, and gradient approach. Although this clas- sification is broad, it incorporates the major mechanisms proposed to this date. The increased ten- dency 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 that of conventional polycrystalline metals, is sev- eral times larger than the grain size. The shear regions are a direct consequence of the increased ten- dency of the nanocrystalline metals to undergo shear localization. 0079-6425/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.pmatsci.2005.08.003 * Corresponding author. Tel.: +1 858 534 4719; fax: +1 858 534 5698. E-mail address: [email protected] (M.A. Meyers). Progress in Materials Science 51 (2006) 427–556 www.elsevier.com/locate/pmatsci
Mechanical properties of nanocrystalline materials
Jun 17, 2023
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