Correlations of Microstructure and TEM Observations of Plasticity in Metallic Nanolaminates
- PDF / 2,200,040 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 88 Downloads / 225 Views
Correlations of Microstructure and TEM Observations of Plasticity in Metallic Nanolaminates Donald E. Kramer and Tim Foecke Metallurgy Division, National Institute of Standards and Technology Technology Administration, US Department of Commerce Gaithersburg, Maryland 20899-8553 ABSTRACT Nanolaminate materials exhibit increases in hardness and yield strength beyond those expected according to rule of mixtures calculations. Several models have been proposed to explain this enhancement of strength, but conclusive experimental verification is hindered by the complex interaction between ingrown defects, in-plane microstructure and compositional modulation. In this study, mechanisms of plastic deformation in nanolaminates are investigated by in situ TEM straining of epitaxial Cu/Ni nanolaminates grown on Cu (001) single crystal substrates. Two distinct types of deformation are observed. Initial plastic deformation is accommodated by motion of “Orowan” and threading dislocations in a uniform and random fashion. As the stress levels increase, fracture occurs creating a mixed mode crack. Subsequent observations suggest that intense plastic deformation occurs over many bilayers in the direction of crack growth, but is contained to within one or two bilayers in a direction normal to the crack faces. INTRODUCTION The capability of microstructural control on the nanoscale has lead to the development of materials exhibiting significant enhancements in hardness [1], tensile strength [2], and wear properties [3] compared to “bulk” materials fabricated through traditional processing routes. Nanolaminated composites consist of alternating layers of at least two different materials, with each layer being up to tens of nanometers in thickness. Nanometer level modulation in composition may also introduce nanometer scale modulation in elastic modulus, lattice spacing and crystal structure. These effects in turn lead to image forces, interfacial misfit dislocations, alternating residual stresses and variations in Burgers vector, all of which have been proposed as potential interfacial strengthening mechanisms in nanolaminates [1, 4-7]. The lack in understanding of the fundamental deformation mechanisms in nanolaminates has made it difficult to predict, and thereby optimize, their mechanical properties. This is due is part to the paucity of observations of deformation induced structures in these materials. Difficulty in sample preparation and microstructural complexity have made characterization studies difficult to perform. Lu et al. have observed threading dislocations crossing multiple interfaces in Cu/Nb nanolaminates deformed under a knife edge contact [8]. Early in-situ straining experiments on Cu/Ni systems have suggested that threading segments deposited at the interfaces can bow into adjacent layers under high stresses [9-11]. One drawback to those studies was that the sample geometry resulted in localization of deformation in a narrow region of the nanolaminate. In this study, in situ TEM straining experiments are performed on Cu
Data Loading...
