Grain boundary strengthening in copper/niobium multilayered foils and fine-grained niobium
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C. Eberlb) Johns Hopkins University, Department of Mechanical Engineering, Baltimore, Maryland 21218
K.J. Hemker and T.P. Weihs Johns Hopkins University, Departments of Materials Science and Engineering, and Mechanical Engineering, Baltimore, Maryland 21218 (Received 31 May 2007; accepted 25 September 2007)
Uniaxial tensile tests were performed on Cu/Nb multilayered foils to investigate yield strength and grain boundary strengthening in the layered foils at room temperature and in fine-grained Nb at 600 °C. At room temperature, yielding in Cu/Nb multilayered foils is controlled by deformation in both layers, and grain boundary strengthening is observed with a Hall–Petch slope (kRT) of 198 ± 56 MPa·m1/2 at a strain rate of 10−4 s−1. At 600 °C, yielding in Cu/Nb multilayered foils is controlled by deformation in just the Nb layers. Hall–Petch strengthening is observed over a range of strain rates, but the Hall–Petch slope decreases from 197 ± 71 MPa·m1/2 for a strain rate of 10−4 s−1 to only 25 ± 40 MPa·m1/2 for a strain rate of 10−6 s−1. The significant drop in the Hall–Petch slope for Nb with decreasing strain rate indicates a change in the controlling deformation mechanism from dislocation glide to dislocation creep.
I. INTRODUCTION
Plastic deformation in thin films, multilayers, and finegrained materials has been studied extensively, and, in general, strength has been shown to vary inversely with both grain size and layer thickness at lower temperatures.1–10 However, plastic behavior in fine-grained systems is not well understood at elevated temperatures, in part because of the challenges of testing thin films and fine-grained samples at high temperatures. One of the major difficulties associated with studying plastic deformation in thin films on substrates is the inherent thermal stress that arises due to the difference in thermal expansion of the film and the substrate. Thermal stresses can drive deformation, but they cannot be controlled independently of temperature, and the total plastic strains that they impart to the film are small. Grain growth is another major difficulty associated with testing fine-grained systems at elevated temperatures. At tema)
Address all correspondence to this author. e-mail: [email protected] Present address: Naval Research Laboratory, Multifunctional Materials Branch, Washington, DC, 20375. b) Present address: University of Karlsruhe, Institut für Zuverlässigkeit von Bauteilen und Systemen, Karlsruhe, Germany. DOI: 10.1557/JMR.2008.0044 376
http://journals.cambridge.org
J. Mater. Res., Vol. 23, No. 2, Feb 2008 Downloaded: 14 Mar 2015
peratures high enough to generate measurable timedependent deformation, the kinetics of reducing grainboundary area via grain growth are significant, and grain coarsening is difficult to avoid. However, if a material’s microstructure can be stabilized by restricting the motion of grain boundaries and thereby eliminating grain growth, meaningful grain-size dependences of mechanical properties can be established at high temperatures.
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