Acta Materialia 51 (2003) 5159–5172 www.actamat-journals.com Some critical experiments on the strain-rate sensitivity of nanocrystalline nickel R. Schwaiger, B. Moser, M. Dao, N. Chollacoop, S. Suresh * Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA Received 7 May 2003; accepted 30 June 2003 Abstract Systematic experiments have been performed to investigate the rate sensitivity of deformation in fully dense nanocrys- talline Ni using two different experimental techniques: depth-sensing indentation and tensile testing. Results from both types of tests reveal that the strain-rate sensitivity is a strong function of grain size. Specifically microcrystalline and ultra-fine crystalline pure Ni, with grain size range of 1 μm and 100–1000 nm, respectively, exhibit essentially rate- independent plastic flow over the range 3 × 10 -4 to 3 × 10 -1 s -1 , whereas nanocrystalline pure Ni with a grain size of approximately 40 nm, exhibits marked rate sensitivity over the same range. A simple computational model, predicated on the premise that a rate-sensitive grain-boundary affected zone exists, is shown to explain the observed effect of grain size on the rate-dependent plastic response.  2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanoindentation; Tension test; Nickel; Nanocrystalline materials; Strain-rate dependence 1. Introduction Nanocrystalline materials, with grain size typi- cally smaller than 100 nm, are known to possess some attractive properties, such as high yield and fracture strengths [1–3], improved wear resistance [4,5], and superplasticity at relatively low tempera- tures [6,7], compared to their microcrystalline counterparts with grain size typically larger than 1 μm. Recently, there has been growing experi- * Corresponding author. Tel.: +1-617-253-3320; fax: +1-617-258-0390. E-mail addresses: ssuresh@mit.edu (S. Suresh); ruthFs@mit.edu (R. Schwaiger). 1359-6454/$30.00  2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S1359-6454(03)00365-3 mental evidence that nanocrystalline materials also exhibit highly strain-rate sensitive mechanical properties [8,9]. However, experimental data avail- able to date in the literature on the strain-rate sensi- tivity of nanocrystalline metals are very limited, and quantitative results are not conclusive. Lu et al. [8] studied the effects of strain rate on the tensile flow and fracture behavior of nanocrys- talline electrodeposited Cu specimens with a mean grain size of about 30 nm. The strain-to-failure was found to increase significantly with increasing strain rate, which is different from the behavior seen in conventional Cu where the fracture strain decreases slightly at higher strain rates. The flow stress was only mildly dependent on strain rate;