tailieunhanh - Mechanical properties of nanocrystalline materials

A three gram oven-dried sample of holocellulose was placed in a 250 mL Erlenmeyer flask with a small watch glass cover. The flasks were placed into water bath that was maintained at 20 o C. The sample was then treated with 50 mL of percent NaOH and thoroughly mixed for one minute. After the specimen was allowed to react with the solution for 29 minutes, 50 mL of distilled water was added and mixed well for another minute. The reaction continued for five more minutes. The contents of the flask were filtered by aid of. | ELSEVIER Available online at Progress in Materials Science 51 2006 427-556 Progress in Mqterials Science locate pmatsci Mechanical properties of nanocrystalline materials . Meyers A. Mishra . 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 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 and possible 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 is already 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 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 discussed break-up of dislocation pile-ups core-and-mantle grain-boundary sliding grain-boundary dislocation .

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