Transmission electron microscopy study of the deformation behavior of Cu/Nb and Cu/Ni nanoscale multilayers during nanoi
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Nanoscale metallic multilayers, comprising two sets of materials—Cu/Nb and Cu/Ni—were deposited in two different layer thicknesses—nominally 20 and 5 nm. These multilayer samples were indented, and the microstructural changes under the indent tips were studied by extracting samples from underneath the indents using the focused ion beam (FIB) technique and by examining them under a transmission electron microscope (TEM). The deformation behavior underneath the indents, manifested in the bending of layers, reduction in layer thickness, shear band formation, dislocation crossing of interfaces, and orientation change of grains, has been characterized and interpreted in terms of the known deformation mechanisms of nanoscale multilayers. I. INTRODUCTION
Nanoscale metallic multilayers have been the subject of extensive study in the recent past because of their unique mechanical and physical properties.1–13 Multilayers consisting of a metal and an intermetallic material14 and a metal and a ceramic material15–17 have also been studied using nanoindentation and tensile tests. Whereas it is well known that diminishing grain sizes result in increasing yield strength of materials according to the Hall-Petch law,18,19 it has been shown that nanoscale multilayers cease to obey this hardening law with decreasing bilayer thickness below a certain thickness value.4–6,15 Different processes have been proposed for deformation in multilayers at different length scales such as confined layer slip of single dislocations and dislocation transmission across interfaces without assistance from stress concentration of a dislocation pile-up.4,6,20,21 Recent studies have also compared the deformation behavior of nanoscale metallic multilayers in compression and indentation.22 Although extensive research is being done on measuring the mechanical properties of nanoscale metallic multilayers, there are limited detailed experimental investigations of the deformation mechanisms, especially at individual layer thickness below 10 nm. Room temperature rolling of self-supported Cu-Nb multilayers sandwiched between austenitic 304 stainless steel sheets was used to study the deformation substructures at a layer thickness of 75 nm.23 For individual layer thickness of 5 nm, through-thickness shear cracks were observed in a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0147
the multilayer foils after the first rolling pass of 5% thickness reduction.24 Given the limited deformability of a Cu-Nb multilayer with 5 nm individual layer thickness rolled between stainless steel sheets, the nature and extent of deformation observed during nanoindentation may be questioned. The purpose of this investigation is to elucidate the deformation mechanisms underneath nanoindents, such as thinning versus shear fracture of layers, evolution of stored dislocation substructure, etc. This paper presents the results of transmission electron microscope (TEM) studies of the deformation structures under indents in two different syste
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