Grain refinement at the nanoscale via mechanical twinning and dislocation interaction in a nickel-based alloy
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X.L. Wu State Key Laboratory of Nonlinear Mechanics, Institute of Mechanism, Chinese Academy of Sciences, Beijing 100080, China
M.L. Sui Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
J. Lu LASMIS, University of Technology of Troyes, 10000 Troyes, France
K. Lua) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China (Received 21 July 2003; accepted 2 February 2004)
A nanostructured surface layer was formed on an Inconel 600 plate by subjecting it to surface mechanical attrition treatment at room temperature. Transmission electron microscopy and high-resolution transmission electron microscopy of the treated surface layer were carried out to reveal the underlying grain refinement mechanism. Experimental observations showed that the strain-induced nanocrystallization in the current sample occurred via formation of mechanical microtwins and subsequent interaction of the microtwins with dislocations in the surface layer. The development of high-density dislocation arrays inside the twin-matrix lamellae provides precursors for grain boundaries that subdivide the nanometer-thick lamellae into equiaxed, nanometer-sized grains with random orientations.
I. INTRODUCTION
Due to increasing interest in nanostructured materials in recent years, various synthesis techniques for nanostructured or ultrafine-grained materials have been developed based on the plastic deformation of coarsegrained polycrystals, such as ball milling,1,2 cold rolling,3,4 equal-channel angular pressing,5,6 high-pressure torsion,7,8 and surface mechanical attrition.9,10 Therefore, understanding the underlying mechanisms of grain refinement induced by plastic straining is crucial, especially at the nanometer scale. On one hand, understanding the mechanism of strain-induced grain refinement is crucial for development of synthesis techniques that are oriented toward practical applications for engineering materials. On the other hand, the grain refinement process at the nanometer scale is of particular academic interest as the plastic deformation takes place at extreme conditions (very large strains and/or high strain rates). a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0227 J. Mater. Res., Vol. 19, No. 6, Jun 2004
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Many previous investigations showed that the grain refinement process during plastic deformation originates from dislocation activity in some metals and alloys such as Fe,11 Cu,12,13 Ni,14,15 an Al-based alloy,16 and low-carbon steels.17 Plastic straining induces generation of a high density of lattice dislocations in original grains. These dislocations arrange themselves into various configurations depending on the crystal structure of the material, such as dense dislocation walls on specific slip planes, dislocation tangles, and dislocation cells. Dislocation interactions lead to
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