ABSTRACT
E-mail: llu@imr.ac.cn, lu@imr.ac.cnThis chapter will present an overview of mechanical behaviors and deformation mechanisms in face-centered-cubic (fcc) polycrystalline metals containing nanoscale twins. The twin-thickness (l) dependence on strength, ductility, work hardening, and strain-rate sensitivity of Cu samples will be included. For the ultrafine-grained (ufg) Cu with fixed grain size, the strengthening effect of the twin thickness is analogous to that of grain size and follows the Hall-Petch (H-P) relationship when l decreases down to several tens of nanometers. At l = 15 nm, a highest strength is observed, followed by a softening at smaller l and a significant enhancement in both strain hardening and tensile ductility. The enhanced strain-rate sensitivity is also observed with a decreasing twin thickness. A H-P-type relationship fitting on the experiment data of activation volume as a function of the twin thickness suggests a transition of the rate-controlling mechanism from the intratwin-to twin-boundary (TB)-mediated processes with decreasing l. The quantitative and mechanics-based models with the related transmission electron microscopy observations and molecular dynamics (MD) simulation are discussed with particular attention to the TB-mediated deformation processes. These findings provide insights into the possible routes for optimizing the strength and ductility of nanostructured metals by tailoring internal interfaces. Mechanical Properties of Nanocrystalline Materials Edited by James C. M. Li Copyright © 2011 Pan Stanford Publishing Pte. Ltd. www.panstanford.com
6.1 INTRODUCTIONThe strength, ductility, and many other mechanical characteristics of metals and alloys are strongly dependent upon their micro-or nanoscale structures. Traditional methodologies for strengthening materials are based on the generation of, and interactions among, various internal defects to resist the dislocation motions. The defects generally include the atomic vacancies and interstitials (point defects), dislocations (line defects), grain and interphase boundaries, and stacking faults (SFs) that introduce crystallographic disregistry between adjacent regions of the atomic lattice (planar defects), and dispersed reinforcement particles (volume defects) of a different material than the surrounding matrix.1 Several commonly used strengthening approaches for metallic metals and alloys, such as solid solution strengthening,2 second-phase strengthening, strain-hardening strengthening (or Taylor strengthening),3 and grain refinement strengthening, are summarized in Fig. 6.1a,b.4 However, these approaches invariably suffer from the undesirable consequence that increase in strength facilitated by dislocation interactions with internal barriers also causes reduced ductility and increased brittleness.
