Dislocations with edge components in nanocrystalline bcc Mo
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Y.F. Zhang and P.C. Millett Fuels Modeling and Simulations, Idaho National Lab, Idaho Falls, Idaho 83415 (Received 16 October 2012; accepted 19 November 2012)
We report high-resolution transmission electron microscopy (HRTEM) observation of a high density of dislocations with edge components (;1016 m2) in nanocrystalline (NC) body-centered cubic (bcc) Mo prepared by high-pressure torsion. We also observed for the first time of the ½,111. and ,001. pure edge dislocations in NC Mo. Crystallographic analysis and image simulations reveal that the best way using HRTEM to study dislocations with edge components in bcc systems is to take images along ,110. zone axis, from which it is possible to identify ½,111. pure edge dislocations, and edge components of ½,111. and ,001. mixed dislocations. The ,001. pure edge dislocations can only be identified from ,100. zone axis. The high density of dislocations with edge components is believed to play a major role in the reduction of strain rate sensitivity in NC bcc metals and alloys.
I. INTRODUCTION
Body-centered cubic (bcc) metals and alloys are one of the most widely used engineering materials. For example, ferrite steels are extensively applied in machinery, land transportation vehicles, and infrastructures. Recently, ferrite steels have been proposed to be the main candidates for the in-core and out-of-core structures in several envisioned generation IV nuclear power reactors.1,2 This has raised more interest in the study of bcc metals and alloys in recent years. Dislocation slip and interactions of dislocations are the most important deformation mechanism for metals and alloys.3,4 Therefore, understanding the characteristics and behavior of dislocations in metals or alloys will help to understand their mechanical behavior and properties. Despite of the importance of bcc metals and alloys, their dislocation structures are much less studied than those of face-centered cubic (fcc) metals and alloys. Recently, there have been increased research activities on the dislocation structures of bcc metals.5–14 However, most studies are computer simulations; only a few are experimental observations. In bcc systems, the shortest Burgers vector of the perfect dislocation is of the type ½,111., which is along the close-packed directions.15 ½,111. screw dislocations are well studied experimentally and theoretically in coarse-grained (CG) bcc metals and alloys9–13,16–19, but ½,111. edge dislocations have been rarely studied. Another less common perfect dislocation is proposed to a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.403 1820
J. Mater. Res., Vol. 28, No. 13, Jul 14, 2013
http://journals.cambridge.org
Downloaded: 01 Apr 2015
be ,100., which is believed formed by the dislocation reaction: ½[111] 1 ½[111] ! [100].15 This reaction is energetically favorable in theory, but the magnitude of the ,100. Burgers vector is ;15% larger than that of ½,111., which makes it less stable than the ½,111. dislocations. To our knowledge, pure edge di
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