Understanding the Mechanistic Role of Grain Boundaries on the Strength and Deformation of Nanocrystalline Metals using Atomistic Simulations

Abstract

Nanocrystalline (NC) materials, defined structurally by having average grain sizes less than 100nm, exhibit a number of enhanced mechanical properties such as ultrahigh strength, improved wear resistance and greater resistance to fatigue crack initiation compared to coarser grained polycrystalline (PC) materials. NC materials exhibit these improved properties, in part, due to the increased grain boundary (GB) volume fraction. NC materials strength increases with decreasing grain size, known as the Hall-Petch (HP) effect often resulting in a peak strength between 10-20nm. Studies have shown that NC materials strength decreases due to the shift from dislocation-dominant to GB-dominant deformation mechanisms in the plastic flow regime as average grain size decreases below 10-20nm. While the potential improved properties are of interest, the application of NC materials are hindered due to microstructural instability i.e., grain growth to reduce the total energy of the system, thus degrading desired mechanical properties. Numerous studies have looked at avenues to stabilize NC microstructure, namely through thermodynamics and kinetics, alloying has been one significant strategy used to stabilize NC materials. As these processes are used to stabilize NC microstructures to thermally-induce grain growth, they add additional uncertainty as the deformation and GB behavior of pure NC materials are still not fully understood. Experimental work on NC materials is difficult due to the length scale being investigated as it is difficult to manufacture and can be time consuming to analyze with current technology. Atomistic simulations have shown the potential to investigate fundamental behavior at the nanoscale and provide important insight in the mechanisms that drive the mechanical behavior of NC materials. This thesis will use atomistic simulations to study the structure-property relationship of face-centered-cubic (fcc) metals by focusing on GBs to investigate the strength of NC nickel. During the course of this thesis, four aspects that govern NC behavior will be studied, yielding, plasticity, thermal effects, and GB disorder to elucidate deeper insight into the underlying deformation mechanisms that control the strength of FCC NC metals i.e., flow stress, in the grain size regime 6 to 20nm.%%%%Ph.D., Mechanical Engineering and Mechanics – Drexel University, 2019