Accommodation of large plastic strains and defect accumulation in nanocrystalline Ni grains
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E. Mab) Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218 (Received 22 December 2006; accepted 12 April 2007)
A transmission electron microscopy (TEM) study has been carried out to uncover how dislocations and twins accommodate large plastic strains and accumulate in very small nanocrystalline Ni grains during low-temperature deformation. We illustrate dislocation patterns that suggest preferential deformation and nonuniform defect storage inside the nanocrystalline grain. Dislocations are present in individual and dipole configurations. Most dislocations are of the 60° type and pile up on (111) slip planes. Various deformation responses, in the forms of dislocations and twinning, may simultaneously occur inside a nanocrystalline grain. Evidence for twin boundary migration has been obtained. The rearrangement and organization of dislocations, sometimes interacting with the twins, lead to the formation of subgrain boundaries, subdividing the nanograin into mosaic domain structures. The observation of strain (deformation)-induced refinement contrasts with the recently reported stress-assisted grain growth in nanocrystalline metals and has implications for understanding the stability and deformation behavior of these highly nonequilibrium materials.
I. INTRODUCTION
In plastic deformation of polycrystalline materials, microscopic strain accommodation processes are required to maintain strain continuity across the grain boundaries (GBs). In conventional coarse-grained metals, compatible strains are realized through the activities and arrangements of dislocations, which are the predominant carriers of plastic deformation. It is known that a microscopic incompatibility region may exist in the vicinity of GB, forming a GB region micrometers wide containing high densities of dislocations.1,2 The GB region can, therefore, be more strained than the grain matrix and somewhat stronger than the matrix. The GBs may act as dislocation sources and sinks, and complex deformation structures, i.e., patterns of accumulated dislocations induced by plastic deformation, have been frequently observed in the GB region.1,2 Based on such a picture, Ashby3 and Kocks4 proposed a composite model of plastic deformation: the deformation of each grain may be separated into a uniform deformation region in the grain Address all correspondence to these authors. e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2007.0279 a)
J. Mater. Res., Vol. 22, No. 8, Aug 2007
http://journals.cambridge.org
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interior and a local, nonuniform deformation zone in the GB regions. A number of models invoke ideas along similar lines (e.g., Refs. 5–8), introducing concepts such as geometrically necessary dislocations near the boundaries and statistically stored dislocations in the interior. With the advent of nanocrystalline (NC) metals, a question naturally arises as to what would happen when the grains are extremely small. Would deformation also occur in an inhomogeneous manne
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