Reduced hardening of nanocrystalline nickel under multiaxial indentation loading
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Iain Brooks Integran Technologies Inc., Mississauga, Ontario L4V 1H7, Canada
Uwe Erb Department of Material Science and Engineering, University of Toronto, Toronto, Ontario M5S 3E4, Canada (Received 5 May 2015; accepted 6 October 2015)
The work hardening behavior of electrodeposited nanocrystalline nickel (29 and 19 nm) was investigated under multiaxial loading and compared with coarse-grained nickel. Plastic strain gradients were introduced into the materials using large Rockwell D hardness indentations, and measured through cross-sectional hardness profiles. The results showed that the coarse-grained material exhibited substantial hardening up to twice the hardness of the deformation-free area due to dislocation mediated deformation, while the nanocrystalline materials displayed small hardness variations along the strain gradient, indicative of considerably reduced dislocation interactions. Moreover, the grain structure analysis (cumulative volume fraction and size distribution) for the nanocrystalline materials suggested the operation of both dislocation mediated and grain boundary controlled deformation mechanisms, the latter becoming more significant with increasing cumulative sample volume of very small grains. The plastic deformation zone sizes under Rockwell indentation of the 29 nm Ni are similar to those conventional materials with reduced strain hardening. Microhardness-indentation size effects were negligible in both the nanocrystalline and coarse-grained materials.
I. INTRODUCTION
Contributing Editor: Andrea Maria Hodge a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.321
after cold rolling up to 76% of thickness reduction at room temperature for nanocrystalline Ni with an initial grain size of 30–40 nm.12 While low strain hardening was reported in most studies on nanocrystalline metals as per tensile testing, it was found in an investigation that deformation by rolling of nanocrystalline Ni (20 nm grain size) at cryogenic temperature induced significant strain hardening.13 This discrepancy in the reported strain hardening behavior manifests the complexity of plastic deformation mechanisms for nanocrystalline metals in general, whereby multiple factors including average grain size, grain size distribution, temperature, and loading conditions can play significant roles during deformation. Despite the substantial progress through many experimental and simulation studies in recent years, the issue of deformation mechanisms as well as strain hardening is not fully understood and still a subject of great interest for nanocrystalline metals (e.g., Refs. 7 and 14). In this study, the strain hardening behavior was investigated through a multiaxial indentation loading, a condition that is different from those widely used in previous studies on nanocrystalline materials, e.g., uniaxial tensile loading and compressional rolling. Examination was performed on nanocrystalline Ni electrodeposits with average grain sizes less than 30 nm, a size range where multiple d
