Grain size dependence of the twin length fraction in nanocrystalline Cu thin films via transmission electron microscopy
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Andrew P. Warren and Kevin R. Coffey Department of Materials Science and Engineering, University of Central Florida, Orlando, Florida 32816, USA
Gregory S. Rohrer Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
Katayun Barmaka) Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA (Received 8 July 2014; accepted 1 December 2014)
Transmission electron microscopy (TEM) based orientation mapping has been used to measure the length fraction of coherent and incoherent R3 grain boundaries in a series of six nanocrystalline Cu thin films with thicknesses in the range of 26–111 nm and grain sizes from 51 to 315 nm. The films were annealed at the same temperature (600 °C) for the same length of time (30 min), have random texture, and vary only in grain size and film thickness. A strong grain size dependence of R3 (coherent and incoherent) and coherent R3 boundary fraction was observed. The experimental results are quantitatively compared with three physical models for the formation of annealing twins developed for microscale materials. The experimental results for the nanoscale Cu films are found to be in good agreement with the two microscale models that explain twin formation as a growth accident process.
I. INTRODUCTION 1,2
Properties of materials, such as electrical resistivity, corrosion resistance,3 and dielectric phenomena,4 are strongly influenced by the type of grain boundaries in the material and how they are connected. The concept of grain boundary design (now known as grain boundary engineering) was first proposed by Watanabe.5 Grain boundary engineering aims to increase the fraction of special grain boundaries with desirable properties.6 Desirable properties can often be associated with boundaries with simple structures and low energy. Such lowenergy structures are, in turn, associated with coincident site lattice (CSL) boundaries.7 Twin boundaries are a type of CSL boundary (noted as R3) and are commonly found in face centered cubic (fcc) materials with medium to low stacking fault energies, as is the case for Cu. Crystals separated by R3 twin boundaries are misoriented by a 60° rotation about a common ,111. axis. A R3 twin boundary is classified as coherent when the boundary is in the {111}
Contributing Editor: Robert C. Cammarata a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.393 528
J. Mater. Res., Vol. 30, No. 4, Feb 28, 2015
http://journals.cambridge.org
Downloaded: 03 Mar 2015
plane perpendicular to the misorientation axis and as incoherent otherwise. Properties of coherent and incoherent R3 grain boundaries can be significantly different, including the reflection coefficients for electron scattering.8 In recent years, extensive research has been focused on the effect of twin boundaries on the properties of Cu.9–13 For example, Shen et al.9 showed that a high density of twin boundaries in ultrafine grained Cu results in high tensile strength while re
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