DEFORMATION MECHANISMS IN MULTILAYER MATERIALS AT SMALL LENGTH SCALES

Tarang Mungole

It is now well understood that mechanical behavior of materials at small length scales show some atypical characteristics. As a consequence, these materials show remarkable mechanical and chemical properties. These characteristics can be specified by transitions occurring in strength governing laws such as in the Hall-Petch (HP) relation, confined layer slip (CLS) and interface crossing mechanism in layered materials, and diffusion mediated flow accommodated by grain boundary sliding (GBS) mechanism. In this work, transitions in these mechanisms are further examined when the length scales are decreased from micrometer (µm) to nanometer (nm) regime. This is accomplished by fabricating multilayer materials consisting of alternating layers of different materials. Accumulative roll-bonding (ARB) and physical vapor deposition (PVD) processes are carried out to fabricate the materials at µm and nm length scales, respectively. At µm level, as the length scale is decreased, HP effect shows a transition from opaque interfaces to transparent interfaces to dislocation transmission depicting a decrease in interface energy. This transition is concurrent with the occurrence of bifurcation of deformation accommodation mechanisms in the two layers at ~ 40 µm. As the length scale is decreased further to nm level, the HP effect breaks down and the strength is governed by CLS mechanism down to ~ 7 nm, which is followed by interface crossing mechanism leading to softening at even smaller length scales. These effects were studied in multilayers of dissimilar thicknesses, where the dislocation dominant mechanisms occur in thin layers simultaneously with diffusion dominant processes in thick layers. Therefore, a bifurcation phenomenon is again observed leads to obtaining a configuration where maximum strain hardening rate is obtained due to CLS mechanism and high strain rate sensitivity is obtained due to diffusional flow. This enables us to design materials to show superior mechanical properties such as high strength and high ductility by altering the fabrication routine in multilayered materials. Characterization techniques such as high resolution electron microscopy, electron backscatter diffraction, transmission Kikuchi diffraction, atomic-force microscopy, X-ray diffraction and nanoindentation are implemented.

DEFORMATION MECHANISMS IN MULTILAYER MATERIALS AT SMALL LENGTH SCALES

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