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I.

INTRODUCTION

STRENGTHENING the behavior of nanocrystalline materials is a topic of great interest. Many investigators are evaluating the limitations of dislocation-based strengthening mechanisms at the nanoscale. In several recent reviews,[1–3] the relationship of the strain-ratesensitivity exponent (m) with the strengthening of bulk nanocrystalline metals is explored as a function of the free volume derived from the grain size and intergranular structure. However, in general, there is a lack of experimental findings for nanocrystalline materials with grain size below 10 nm. For this reason, our objective is to effectively measure the rate-dependent strengthening in ultrafine nanocrystalline metals. The use of nanoscratch testing provides a tribological approach to determine the strength of nanocrystalline materials through hardness measurement. Surface defects and flaws internal to the bulk nanostructure are intrinsic to nanocrystalline alloys synthesized by methods such as pulsed electrodeposition.[4–6] Under uniaxial tension, these defects often lead to premature failure through stress concentration evidenced by highly localized deformation. The result can be a significant variation[7–9] in the measurement of tensile strength. Recently, similar nanocrystalline, gold-copper alloy specimens have been tensile tested and shown[6,8] to LUKE O. NYAKITI, Postdoctoral Fellow, and ALAN F. JANKOWSKI, Professor, are with the Mechanical Engineering Department, Whitacre College of Engineering, Texas Tech University, Lubbock, TX 79409-1021. Contact e-mail: [email protected] This article is based on a presentation given in the symposium entitled ‘‘Mechanical Behavior of Nanostructured Materials,’’ which occurred during the TMS Spring Meeting in San Francisco, CA, February 15–19, 2009, under the auspices of TMS, the TMS Electronic, Magnetic, and Photonic Materials Division, the TMS Materials Processing and Manufacturing Division, the TMS Structural Materials Division, the TMS Nanomechanical Materials Behavior Committee, the TMS Chemistry and Physics of Materials Committee, and the TMS/ASM Mechanical Behavior of Materials Committee. Article published online September 30, 2009 838—VOLUME 41A, APRIL 2010

exhibit ultrahigh strength. These specimens follow a Hall–Petch strengthening behavior with a reduction in grain size (dg) down to just a few nanometers. Testing of tensile specimens[7] at strain rates of 10
4 vs 10
2 s
1 appears to indicate that the nanocrystalline Au-Cu alloy system is sensitive to brittle failure. A general loss of localized plasticity is revealed in fracture cross sections when specimens are subjected to the greater strain rate. However, a microscratch test induces plastic flow at the surface to provide a measure of hardness, hence a parameter of plasticity. As such, this method is not as sensitive to the effects of defect structure that can induce premature brittle failure under tensile loading at higher strain rates. Thus, the strain-rate sensitivity of strength can be assessed in the grain size range of