Amorphous Intergranular Film Design Criteria and Application as Damage Tolerant Features in Nanocrystalline Alloys

The large grain boundary volume fraction of nanocrystalline metals can bestow desirable mechanical behaviors, but the same grain boundaries that impart these beneficial properties challenge their application. Excess surface energy due to the substantial interfacial volume drives grain growth, and thus a loss of the benefits gained by nanocrystalline grain sizes. Also, not all grain boundaries are equal, where material behaviors can fluctuate dramatically depending on the grain boundary structure and composition. New pathways to fundamentally alter the grain boundary structure are needed to stabilize the nanocrystalline grain size and leverage the full potential of nanocrystalline metals. “Complexion” is a term used to describe the phase-like behavior of grain boundaries, where grain boundaries, similar to bulk phases, can undergo discrete transitions in structure and composition based on external factors such as temperature. Amorphous intergranular films, a type of complexion, are of particular interest due to their ability to enhance both mechanical and radiation damage tolerance due to the excess free volume present in these structures. In this thesis, we seek to expand the current materials toolbox of alloys that can form amorphous intergranular films, and then investigate the impact of these unique damage tolerant features in applications where they can be leveraged. First, we propose a set of materials selection rules aimed at predicting the formation of amorphous intergranular films, and then apply these rules to discover new alloys that can form these features. In doing this, we also discover a counterintuitive, ultra-high temperature grain size stabilization regime driven by the formation of amorphous intergranular films. Next, we investigate the impact of these features in applications where damage tolerance is critical, particularly fatigue and radiation. We find that incorporation of amorphous intergranular films throughout the grain boundary network of nanocrystalline alloys increases plasticity preceding a fatigue crack, and dramatically improves radiation tolerance. In summary, through an array of sputtered, electroplated and ball-milled Cu and Ni-based alloys, specialized heat treatments, and electron microscopy techniques, we find that amorphous intergranular films can potentially be found in a large number of alloys, and can dramatically improve nanocrystalline alloy behaviors