High-tensile ductility in nanocrystalline copper
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High-tensile ductility in nanocrystalline copper L. Lu, L.B. Wang, B.Z. Ding, and K. Lua) State Key Laboratory for RSA, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, People’s Republic of China (Received 28 May 1999; accepted 24 November 1999)
In this work we report a high-tensile ductility in a fully dense bulk nanocrystalline (nc) pure copper sample prepared by electrodeposition. A tensile ductility with an elongation to fracture of 30% was obtained in the nc Cu specimen with an average grain size of 27 nm, which is comparable to that for the coarse-grained polycrystalline Cu. An enhanced yield stress (119 MPa) and a depressed strain hardening exponent (0.22) were observed in the nc Cu sample with respect to the conventional polycrystalline Cu. The high-tensile ductility was attributed to the minimized artifacts in the nc sample, and the grain-boundary sliding deformation mechanism resulted from the numerous amount small-angle grain boundaries and the low microstrain (dislocation density).
Following the grain-size dependence of mechanical properties in conventional polycrystalline materials, one may expect an enhancement in both the strength and the ductility when grains are refined down to the nanometer regime. In other words, nanocrystalline (nc) materials are expected to be harder and more ductile than their coarsegrained polycrystalline counterparts.1 However, experimental observations in a variety of nc materials in recent years are disappointing: most nc materials are extremely brittle. The measured ductility of single-phased nc materials in tension is very low, typically with an elongation of a few percent for the grain size less than 30 nm. Whether the coarse-grained materials are ductile or brittle, observed tensile ductility in nc specimens is very limited and decreases with decreasing grain size.2–4 For example, conventional coarse-grained annealed pure polycrystalline Cu is very ductile, normally with an elongation of about 60%. However, experimental measurements in nc Cu samples with grain sizes less than 30 nm indicated almost no ductility, with 艋5% elongation to fracture in tension. When the grain size increases, tensile ductility slightly increases, with ∼10% elongation for grain size larger than 100 nm.5 Compressional ductility in nc Cu specimens was found to be higher than the tensile ductility, with strains of 12–18% prior to failure,6 but still much less than that for the coarse grain form. The brittleness of nc materials has been attributed to the restricted dislocation activity and the artifacts (such as flaws, contamination, residual stress, etc.) induced in the processing procedures of nc samples. Most mechania)
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http://journals.cambridge.org
J. Mater. Res., Vol. 15, No. 2, Feb 2000 Downloaded: 25 Mar 2015
cal property studies of nc metals have been done on consolidated particulate samples in which large residual stress and flaws due to the im
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