UC Irvine

Nanocrystalline metals have been the focus of current literature due to their interesting mechanical properties. This is a result of having nanometer sized grains and high volume fraction of grain boundaries. While these materials have high strength, the large number of boundaries is also responsible for the limited ductility and thermal instability often observed for nanocrystalline systems. Despite the current efforts in the literature, these challenges still prevent widespread use of nanocrystalline metals in real engineering applications. In this thesis, we study these problems by focusing on tailoring the grain boundary structure and chemistry and propose a methodology that can be used to mitigate those challenges. First, we study the plastic flow and failure as a function of grain boundary volume fraction (i.e., grain size) using microcompression in a nanocrystalline Ni-W. Since grain boundary physics are extremely important here, we also study how the relaxation of nonequilibrium grain boundaries affects failure. We show that nanocrystalline metals with larger grain boundary volume fractions and relaxed boundary structures are stronger, but also more likely to fail prematurely through catastrophic shear banding. We also show that shear banding can create a dynamic microstructure leading to grain coarsening. A major take-away from this work is that disordered grain boundaries can actually be beneficial. Therefore, in the next study we introduce amorphous complexions, highly disordered grain boundaries, through grain boundary doping as an all-in-one solution to design against failure and thermal instability. We use nanocrystalline Cu with the addition of Zr as our model system to explore complexion engineering in these materials. High resolution transmission electron microscopy in conjunction with energy dispersive x-ray spectroscopy demonstrates segregation of Zr to the boundaries of Cu-Zr alloys created with mechanical alloying. This provided evidence for the formation of amorphous grain boundaries complexions under certain conditions. Microcompression and in-situ bending experiments are then used to quantify the effect of doping on mechanical behavior. Finally, our results show that strength, strain-to-failure, failure mode, and thermal stability can be controlled with boundary doping. The proposed methodology described here is rather general and can be applied to other material systems to make bulk nanocrystalline metals with improved mechanical properties.