Atomic mechanism of shear localization during indentation of a nanostructured metal F. Sansoz ⁎ , V. Dupont School of Engineering, University of Vermont, 33 Colchester Avenue, Burlington, VT 05405, USA Received 8 May 2006; received in revised form 17 July 2006; accepted 18 July 2006 Available online 17 August 2006 Abstract Shear localization is an important mode of deformation in nanocrystalline metals. However, it is very difficult to verify the existence of local shear planes in nanocrystalline metals experimentally. Sharp indentation techniques may provide novel opportunities to investigate the effect of shear localization at different length scales, but the relationship between indentation response and atomic-level shear band formation has not been fully addressed. This paper describes an effort to provide direct insight on the mechanism of shear localization during indentation of nanocrystalline metals from atomistic simulations. Molecular statics is performed with the quasi-continuum method to simulate the indentation of single crystal and nanocrystalline Al with a sharp cylindrical probe. In the nanocrystalline regime, two grain sizes are investigated, 5 nm and 10 nm. We find that the indentation of nanocrystalline metals is characterized by serrated plastic flow. This effect seems to be independent of the grain size. Serration in nanocrystalline metals is found to be associated with the formation of shear bands by sliding of aligned interfaces and intragranular slip, which results in deformation twinning. © 2006 Elsevier B.V. All rights reserved. Keywords: Indentation; Nanocrystalline metals; Plasticity; Quasi-continuum; Simulation 1. Introduction Nanostructured metals containing nanosized grains (b 50 nm) have drawn considerable interest in the last two decades due to the promise of achieving superior, unprecedented mechanical properties from intense grain refinement [1,2]. One inconve- nience limiting the application of nanocrystalline metals has long been the occurrence of plastic instability in the form of shear bands, which could lead to significant softening effects and rapid failure at large applied stress [3]. In many nanostructured systems such as hard biological tissues (nacre, tooth, bone, etc.), it is generally found that an appropriate control of the shear load transfer from one nanostructure constituent to the next can be an efficient way to retard this failure process [4]. By way of comparison, in metallic nanostructures, gaining fundamental understanding of the mechanisms of shear load transfer can also be considered crucial. The importance of shear localization in the deformation mechanisms of nanocrystalline metals has already been proposed in several models [5,6]. In particular, Lund and Schuh [5] have suggested using atomistic simulation that there could be some common aspects in the process of shear localization between bulk amorphous metals and nanocrystal- line metals as the grain size is small. In bulk metallic glasses, the propagation of shear bands has largely been observed during indentation, as shown by recent reports in the literature [7–9]. However the existence of local shear planes in nanocrystalline metals is very difficult to be verified experimentally, except when shear planes extend over the whole sample size as found during pure compression testing [10]. Alternatively, sharp indentation probes could be useful tools to investigate shear localization effects in nanocrystalline metals at different length scales. While significant progress has been made to characterize the mechanical behavior at the nanoscale using sharp probes [11–14], such as atomic force microscope and depth-sensing nanoindentation, major challenges in nano- crystalline materials remain due to the difficulties in interpreting the data from these contact studies. This paper describes an effort to provide direct insight on the mechanism of shear localization Materials Science and Engineering C 27 (2007) 1509 – 1513 www.elsevier.com/locate/msec ⁎ Corresponding author. Tel.: +1 802 656 3837; fax: +1 802 656 1929. E-mail address: frederic.sansoz@uvm.edu (F. Sansoz). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.07.019