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We examine the development of stable bimetal interfaces in nanolayered composites in severe plastic deformation. Copper-niobium multilayers of varying layer thicknesses from several micrometers to 10 nanometers (nm) were fabricated via accumulative roll bonding (ARB). Investigation of their 5-parameter character and atomic scale structure finds that when layer thicknesses refine well below one micrometer, the interfaces self-organize to a few interface orientation relationships. With atomic scale and crystal plasticity modeling, we identify that the two controlling factors that determine whether an interface is stable under high strain rolling are orientation stability of the bicrystal and interface formation energy. A figure-of-merit is introduced that not only predicts the development of the prevailing interfaces but also explains why other interfaces did not develop. Through a suite of nanomechanical and bulk test results, we show that ARB composites containing these stable interfaces are found to have exceptional hardness (∼4.5 GPa) and strength (∼2 GPa).
The high pressures generated at a contact during nanoindentation have a quantifiable effect on the measured indentation modulus. This effect can be accounted for by invoking a Murnaghan equation of state-based analysis where the measured indentation modulus is a function of the hydrostatic component of the stress state which is generated beneath the indenter tip. This approach has implications pertinent to a range of mechanical characterization techniques that include instrumented indentation and quantitative atomic force microscopy (AFM) since these techniques traditionally consider only zero-pressure modulus values during data interpretation. To demonstrate the validity of this approach, the indentation modulus of four materials (fused quartz, sapphire, rutile and silicon) is evaluated using a 1 μm radius conospherical diamond tip to maximum contact depths of 30 nm. The tip area function is independently determined via AFM while the unloading stiffness from the load-displacement data is determined using standard Oliver-Pharr analysis.
The mechanical properties of nanoscale volumes and their associated defect structure are key to many future applications in nanoengineered products. In this study, techniques of mechanical testing and microscopy have been applied to better understand the mechanical behavior of nanoscale volumes. Nanoindentation has been used to investigate important mechanical material parameters such as the elastic modulus and hardness for single nanoparticles. New sample preparation methods must be developed to allow the necessary TEM characterization of the inherent and induced defect structure of these nanoparticles. Issues of chemical homogeneity, crystallinity, and defect characteristics at the nanoscale are being addressed in this study. This integration of investigative methods will lead to a greater understanding of the mechanical behavior of nanostructured materials and insights into the nature of defects in materials at the nanoscale.
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