Nanostructured Ni-Co alloys with tailorable grain size and twin density
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I. INTRODUCTION
BULK nanocrystalline materials are characterized by a large volume fraction of intercrystalline component, i.e, grain boundaries and triple junctions, and have been produced from various pure metals, alloys, and ceramics over the past two decades.[1–4] One of the superior properties these materials exhibit with respect to their microcrystalline counterparts is their extremely high strength or hardness. This high hardness is often explained by the Hall–Petch relationship,[5,6] which states that the hardness of a material increases with the reciprocal square root of grain size. However, it has also been observed that when the grain size is reduced below a critical value, materials no longer deform via the same mechanisms, and their strength or hardness no longer follows the Hall–Petch relationship.[7–12] From molecular dynamics (MD) simulations, several authors have found that below a critical grain size dislocation behavior begins to change character, giving way to processes dominated by grain boundary activity, including grain boundary shearing.[8,13–22] An interesting aspect of this transition is that near or below the critical grain size, deformation may become controlled by the nucleation and motion of partial dislocations, which leads to the formation of stacking faults and, potentially, deformation twinning. The first suggestion that stacking faults may be important in the deformation of nanocrystalline metals was provided by MD simulations, in which partial dislocations were observed nucleating from the grain boundaries of nanocrystalline fcc metals.[8,13,16,21] For example, Yamakov and co-workers[15,16,23] as well as Van Swygenhoven and co-workers[24] have explicitly discussed the effect of stacking fault energy on the nucleation and propagation of partial dislocations in simulated nanocrystalline solids. Froseth and co-workers[25] have studied B.Y.C. WU, Graduate Student, and C.A. SCHUH, Salapatas Assistant Professor of Metallurgy, are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Contact e-mail: [email protected] P.J. FERREIRA, Assistant Professor, is with the Materials Science and Engineering Program, University of Texas at Austin, Austin, TX 78712. Manuscript submitted September 24, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
the effect of pre-existing twins upon the deformation mechanism of nanocrystalline Al. They reported that in defect-free nanocrystalline Al twinning was not a dominant deformation mechanism, but in the presence of pre-existing twins, the deformation mechanism could change substantially and twin boundary migration could emerge as the controlling mechanism. Some experimental work has also addressed the issue of twins and stacking faults in nanocrystalline materials. For example, Lu and co-workers[26,27] observed that fine-grained samples of copper with a high density of nanoscale growth twin boundaries showed a unique combination of strength, strain hardening, and ductility. Chen et al.,[28] as we
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