Nanoindentation Based Hardness Measurements Of ARB Processed Mo-Based Multilayers

Multiphase composites with sub-micrometer phase length scales have been used for structural and functional applications and have furthermore attracted research interest due to their potential for extreme properties. Recently, interest has shifted to develop bulk nanostructured materials using severe plastic deformation techniques such as accumulative roll bonding (ARB). ARB processing of arrays of elemental foils results in changes in microstructural length scale and in phase morphology. The phase morphology changes can range from codeformation to necking/rupturing of one of the elemental layers, depending on the differences in flow properties of the layers. A common practice is to use the initial hardness ratio of the layers to predict the deformation morphology. ^ It was suggested in the literature that the hardness ratio of the layers approaches an asymptotic limit of 2.5 with increasing ARB processing, irrespective of the material pairing. The key objective of the dissertation is to test the hypothesis that the hardness ratio converges to a value of about 2.5. The dissertation is based on ARB processing of copper-molybdenum and molybdenum-tantalum multilayers. The main idea of the technical approach was to prepare multilayers at a range of initial hardness ratios and determine the hardness changes in constituent layers in ARB processed multilayer using instrumented indentation technique.^ Two temper conditions of the molybdenum foil, annealed and unannealed, were used to induce initial hardness ratios (rh) between one and three. Experimental results of relative hardness ratio generally confirm the hypothesis; for the Cu-Mo system with a wide difference in flow properties, the hardness ratio approaches a value of about 2.5 at a true strain of about four. In Mo-Ta multilayers with initial hardness ratio of one, relative hardness ratio approaches a value of about 2 at similar strain of about four. Beyond this deformation strain of about four, the relative hardness ratio drops below two for both systems. This indicates that with increasing deformation, constituent phases develop comparable work hardening capabilities. Usefulness of relative hardness ratio of metallic multilayers is mainly in designing intermediate annealing in between cold rolling cycles thereby restoring the strain hardening capabilities whenever required, allowing finer refinements at minimum strain.