Nanotwinned microstructures from low stacking fault energy brass by high-rate severe plastic deformation J. Cai, S. Shekhar, J. Wang and M. Ravi Shankar * Department of Industrial Engineering, 3700 O’Hara Street, University of Pittsburgh, Pittsburgh, PA 15261, USA Received 23 October 2008; revised 14 November 2008; accepted 8 December 2008 High-rate severe plastic deformation by machining is explored in a prototypical low stacking fault energy material, brass, as a route for generating a high density of nanometer-scale twins amidst dislocation structures. This nanotwinned system shows a greatly improved strength compared to the bulk microcrystalline state. The twin densities are found to be retained even after modest ther- mal agitation, which is also seen to engender a small ‘‘hardening by annealing” effect in the nanotwinned material. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Severe plastic deformation; Twinning; Nanostructure Nanostructured materials possess a very high tensile strength when compared to the strength of the same materials with microcrystalline grains [1,2]. This extraordinary strengthening has led to an intense focus on development of nanomaterials for structural applica- tions. The preeminent methodology for the creation of metallic nanostructured systems has revolved around the use of severe plastic deformation (SPD). SPD has been found to offer a grain refinement route that is di- rectly applicable to most extant coarse-grained alloys. Repetitive SPD protocols using equal angular channel processing (ECAP) have been studied for a range of low- to medium-strength materials to impose high levels of plastic strain at relatively small strain rates [3,4]. Re- cently, chip formation by machining in a configuration that is illustrated in Table 1 has emerged as an alterna- tive SPD scheme wherein deformation strains in the range of 1–10 can be imposed in a single deformation pass in a range of material systems, including moderate- to high-strength materials [5–12]. The magnitude of strains during machining can be modulated by an appropriate choice of the rake angle of the cutting tool (a), and the strain rates are determined by the velocity of tool advance or the machining speed (V) [9]. Despite the potential for achieving very high strain rates, the focus on achieving highly grain refined structures by suppressing any in-process thermal agitation had restricted the choice of the machining speeds and strain rates to small levels [10]. Combining this aspect with the capability for imposing high strain levels had ensured the achievement of structures as fine as 100 nm in chips produced by machining [5,6,11]. In nanostructured materials the high density of inter- faces greatly limits dislocation mobility, leading to the high strengths. When such nanomaterials are ‘‘interface engineered” to possess a dense dispersion of nanotwins, they can achieve several improved multifunctional char- acteristics including substantial mechanical strengthen- ing [13–17]. To create such nanotwinned systems, Lu et al. performed high-rate severe deformation on copper at cryogenic temperatures to suppress dislocation mobil- ity and enhance deformation twinning [16,17]. We recognized that creating nanotwinned materials by dynamic deformation approaches can be particularly effective in low stacking fault energy (SFE) materials, such as the face-centered cubic 70%Cu–30%Zn brass considered here (SFE 15 mJ m –2 ) [18,19]. Under suit- able strain and strain-rate conditions, it may be possible to induce dense deformation twins even at ambient and supra-ambient conditions, without the need to suppress dislocation mobility using cumbersome cryogenic condi- tions. To explore this possibility a microcrystalline brass bar was annealed at 600° C for 10 h to ensure a coarse and equiaxed microstructure. Severely deformed sheet- like morphologies, hereafter referred to as as-deformed samples, several millimeters in width, were generated using large-strain machining by imposing simulta- neously high levels of strains and strain rates. Unlike 1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.12.024 * Corresponding author. E-mail: shankarr@engr.pitt.edu Available online at www.sciencedirect.com Scripta Materialia xxx (2008) xxx–xxx www.elsevier.com/locate/scriptamat ARTICLE IN PRESS Please cite this article in press as: J. Cai et al., Scripta Mater. (2008), doi:10.1016/j.scriptamat.2008.12.024