Observations of Dislocations in Cu/Nb Nanolayer Composites After Deformation
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Observations of dislocations in CuyyNb nanolayer composites after deformation Y-C. Lu, H. Kung, A. J. Griffin, Jr., M. A. Nastasi, and T. E. Mitchell Center for Materials Science, Materials Science Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 8 July 1996; accepted 19 February 1997)
Dislocations have been observed in deformed CuyNb nanolayer composites of wavelength 17 and 7 nm. The dislocations thread through the CuyNb interfaces even though there is a change of Burgers vector. Conventional and high resolution transmission electron microscopy studies show that the in-plane bowing direction of these dislocations in the Cu layers is opposite to that in the Nb layers, so that the dislocations appear to zig-zag. These observations are explained by the presence of residual tensile stresses in Cu and residual compressive stresses in Nb, which make dislocations bow in opposite directions in the alternating layers.
Copper-niobium microcomposites have been studied because of their exceptional high strength and excellent thermal and electrical conductivity for a variety of applications.1–3 However, much of the work on this system has focused on manufacturing and evaluating wire drawing or rolling of Cu–Nb alloys,4,5 which results in a microstructure consisting of Nb filaments or platelets in a Cu matrix due to the insolubility of Nb in Cu. Recently, Cu–Nb nanolayer composites with a laminate structure have been prepared by sputtering, which show interesting properties.6 Mechanical property measurements show that decreasing the compositional wavelength of the Cu–Nb multilayer spacing increases the hardness. Enhancement in hardness as a result of decreasing length scale has been studied extensively. In general, two strengthening mechanisms have been most often proposed: (i) Hall–Petch strengthening7,8 where the strength increases in inverse proportion to the square root of the layer thickness, which is a result of dislocation pileups at boundaries, and (ii) Koehler modulus strengthening9 where the strength increases in inverse proportion to the layer thickness, which has been explained by the concept of image forces on dislocations at boundaries due to modulus difference. However, at small dimension Orowan bowing of dislocations could also be important in a similar way to the mechanism of strain release by dislocation motion in strained layer superlattices,10–12 which would also yield an inverse first power relationship for the yield strength and the layer thickness. Recently, Embury and Hirth13 have shown that at fine length scales single dislocations are in motion rather than arrays of dislocations; therefore, the flow stress should be controlled by the Orowan-type mechanism. Mechanical properties of nanolayer composites are directly related to how they respond to mechanical J. Mater. Res., Vol. 12, No. 8, Aug 1997
stresses. So far, only few physical observations have been reported about how nanolayer composites or nanocrystalline phases defo
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