Nanostructure stabilization and mechanical behavior of binary nanocrystalline alloys

Abstract

The unique mechanical behavior of nanocrystalline metals has become of great interest in recent years, owing to both their remarkable strength and the emergence of new deformation physics at the nanoscale. Of particular interest has been the breakdown in Hall-Petch strength scaling, which is frequently attributed by atomistic simulations to a mechanistic shift to interface dominated plasticity. Experimental validation has been less abundant, primarily due to the processing challenges associated with achieving homogeneous nanocrystalline samples suitable for mechanical testing. Alloying has been proposed as a potential route to high-quality nanocrystalline metals, although choice of an appropriate alloy system, based on available thermodynamic data, remains elusive. In this thesis, we propose a thermodynamic model for nanostructure stabilization that derives from the energetic state variables characteristic of binary alloys. These modeling results motivate the study of Ni-W alloys in particular, which may be synthesized via aqueous electrodeposition, accessing grain sizes across the entire Hall-Petch breakdown regime as characterized by x-ray diffraction and transmission electron microscopy. Ambient temperature nanoindentation testing is employed to evaluate the mechanical behavior of the as-deposited alloys, assessing the nature of flow, the rate sensitivity, and pressure sensitivity of deformation, with emphasis on property inflections required to bridge the behavior of nanocrystalline metals to amorphous solids. The rate sensitivity, in particular, demonstrates an inherent dependence on nanocrystalline grain size, exhibiting a maximum in the vicinity of the Hall-Petch breakdown as a consequence of a shift to glass-like shear localization. In light of this finding, we study the Hall-Petch breakdown at high strain rates, and show that an "inverse Hall-Petch" weakening regime emerges at high rates. Additional effects from structural relaxation are investigated, and illustrated to strongly influence the strength scaling behavior and shift to inhomogeneous flow. Relaxed samples are also subjected to elevated temperature indentation tests, and the results discussed in the context of thermally-activated plasticity, thus providing a more quantitative analysis of the nanoscale deformation mechanisms.