In situ transmission electron microscopy observations of toughening mechanisms in ultra-fine grained columnar aluminum t
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J. Han and M.T.A. Saif Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, Illinois 61801
I.M. Robertsona) Department of Materials Science and Engineering, University of Illinois, Urbana, Illinois 61801 (Received 28 December 2004; accepted 15 March 2005)
A unique straining device, fabricated using microlithographic techniques, has been developed to permit real-time investigation in the transmission electron microscope (TEM) of the deformation and failure mechanisms in ultrafine-grained aluminum. The tensile specimen is a freestanding thin film with a columnar microstructure that has a uniform cross-section (100 × 0.125 m) and a gauge length of 300 m. In situ TEM straining experiments show the fracture mode is intergranular with no accompanying general plasticity. Propagating cracks were halted at large grains, and crack blunting occurred through grain-boundary-mediated processes. The blunting process was accompanied by dislocation emission and deformation twinning in the grain responsible for arresting the crack. Voids or microcracks nucleated and grew on grain boundaries ahead of the arrested crack, and crack advance occurred through linkage of the microcracks and the primary crack.
I. INTRODUCTION
Nanocrystalline materials have been found to exhibit a higher yield and tensile strength and a lower ductility than their large-grained counterparts. However, the fundamental deformation processes responsible for these macroscopic properties are not well understood with much of the insight coming from molecular-dynamics computer simulation.1–10 From these investigations, it has been suggested that in high stacking-fault energy materials, the deformation mode changes from slip involving perfect dislocations (grain sizes greater than 18 nm in Al) to slip involving Shockley partial dislocations (grain sizes less than 18 nm in Al), to grain boundary deformation processes at the smallest grain size.4 The transition in deformation mode is associated with the competition between the grain size and the separation distance between partial dislocations. A similar transition a)
Address all correspondence to this author. e-mail: [email protected] This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http:// www.mrs.org/publications/jmr/policy.html. DOI: 10.1557/JMR.2005.0233 J. Mater. Res., Vol. 20, No. 7, Jul 2005
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is predicted to occur in low stacking-fault energy materials, although only partial dislocations are involved.9 Comparing the results from large-scale molecular dynamics simulations reveals that there is controversy over the deformation mechanisms10 and the ability of the simulations to predict the rate-limiting process.6,11 The failure mechanism of nanograined metals has also been evaluated using molecular-dynamics computer simulations. Qi et al.8 modeled columnar nanograined Ag and determined that the f
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