Thermal stability of twins and strengthening mechanisms in differently oriented epitaxial nanotwinned Ag films
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Haiyan Wanga) Department of Electrical Engineering, Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843-3128
Xinghang Zhangb) Department of Mechanical Engineering, Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843-3123 (Received 25 September 2012; accepted 6 February 2013)
Sputter-deposited epitaxial (111) and (110) Ag films have high-density nanotwins with respective twin boundary orientations perpendicular and angled to the growth direction. Twin density in as-deposited (111) Ag films is much greater than in (110) films, leading to higher hardness in the (111) films. Annealing up to 800 °C (homologous temperature of 0.85 Tm) leads to increased twin thickness, although the average twin thickness remains ,100 nm in both systems. Twinned volume fraction falls dramatically in annealed (110) films but remains constant at ;50% in (111) films. The mechanisms leading to the elimination of nanotwins in (110) films and their remarkable stability in (111) films at elevated temperatures are discussed. Coarsening and elimination of twins result in hardness reduction after annealing. The variety of microstructures achieved via annealing allows for the introduction of a strengthening model considering both twin and grain boundaries.
I. INTRODUCTION
Nanotwinned (nt) face-centered cubic (fcc) metals have gained intense interest recently due to their unique assortment of desirable properties, including low electrical resistivity, high strain rate sensitivity,1–3 improved mechanical stability under cyclic loading,4,5 and increased mechanical strength.3,6–11 Different nt materials fabricated by pulsed electrodeposition7 and magnetron sputtering12,13 have various twinned microstructures, including randomly oriented polycrystalline grains filled with dense twin boundaries and, in certain cases, single crystal-like epitaxial structures (in sputtered Cu and Ag) with extremely high densities of twin boundaries exclusively oriented normal to the growth direction.6,13,14 The contributions of nanotwins to high mechanical strength have garnered special attention.7 Numerous studies demonstrated that twin boundaries act similarly to grain boundaries when blocking transmission of dislocations, and nt metals exhibit a)
This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.50 J. Mater. Res., Vol. 28, No. 13, Jul 14, 2013
strength similar to their nanocrystalline (nc) counterparts.2,3 However, given that twin and grain boundaries in most cases are significantly different in length scale, various microstructures must be carefully incorporated into strengthening models. Gu et al.15 proposed a strengthening model that considered simultaneously the influence of twin and grain boundaries. Meanwhile, twin
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