Deformation behavior and strain rate sensitivity of nanostructured materials at moderate temperatures
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Deformation behavior and strain rate sensitivity of nanostructured materials at moderate temperatures Cécilie Duhamel, Sandrine Guérin, Martin Hÿtch, Yannick Champion Centre d’Etude de Chimie Métallurgique (CECM-CNRS, UPR 2801) 15 rue Georges Urbain, 94407 Vitry-sur-Seine, France. ABSTRACT Strain-rate jump tests in compression are carried out on nanostructured copper (grain size = 90 nm) at moderate temperatures (353K – 393K). Strain-rate sensitivity m is measured as a function of temperature, T, and strain rate, ε& . Increasing temperature or decreasing strain rate induces an increase in the strain-rate sensitivity. For ( ε& , T) = (1×10-5 s-1, 393K), m is equal to 0.17 which is the highest value reported for nanocrystalline copper. These results of enhanced m are encouraging in terms of gain in ductility. The measurements emphasize the existence of a thermally activated mechanism different from the normal rate-controlling process observed for microcrystalline fcc metals. INTRODUCTION
Nanocrystalline (nc) and ultrafine grained (UFG) materials are of growing interest because of their mechanical behavior and unusual shaping abilities. Except for very small grain sizes where radically new stress relaxation effects occur, the yield strength of nanostructured metals, in accordance with the Hall-Petch relation, is much higher than the yield strength of their coarse-grained counterparts. However, one major drawback is the apparent low ductility in tension (at usual quasistatic strain rates) due to the absence of strain hardening [1, 2]. A better understanding of the mechanisms controlling plastic deformation in nanocrystalline materials is thus necessary in order to design new nano-architectures with enhanced ductility. The onset of non-homogeneous deformation is predicted by instability criteria. As fcc metals become strain-rate sensitive with decreasing grain size, Hart’s criterion 1 ∂σ −1+m≤0 (1) σ ∂ε ε& is applied to determine the onset of plastic instability [3]. Accordingly, the strain-rate sensitivity, m, is the rheological parameter that describes the delay in localized deformation and, consequently, material failure. Previous work conducted on nanostructured copper prepared by powder metallurgy (grain size: 90 nm) has revealed the near perfect elasto-plastic behavior in tension of this material [4]. Failure occurs after 12% deformation and no apparent necking is detected. In addition, compression tests with strain-rate jumps have revealed a maximal strainrate sensitivity, m, of 0.045 for ε& =1×10-5 s-1 at room temperature [5]. In the present study, the strain-rate sensitivity of the flow stress is measured from strain-rate jump tests in compression for different temperatures. The variation of m with strain-rate, temperature, grain size and its relation with ductility will be discussed.
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EXPERIMENTAL DETAILS Bulk nanostructured copper has been processed using powder metallurgy. Copper nanoparticles are synthesized using the cryo-melting technique where a metal vapor is instantaneously cond
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