Detecting the Onset of the Bulk Crystal Plasticity Transition in Face Centered Cubic Metals Using Nanoindentation

The present thesis showcases the investigation of nanoindentation at ultra-low depths (<500nm), for the study of the origin and initiation of crystal plasticity in a number of Face Centered Cubic (FCC) metals: Nickel (Ni), Copper (Cu), and Aluminum (Al) in single crystalline or polycrystalline form. Nanoindentation was carried out using a Berkovich tip, and indentations were carried out in the nano-regime of polycrystalline FCC metals and single crystalline Cu and Al. Two custom-built four-point bending apparatuses were used to apply in-plane tension on all samples during indentation. Samples were indented using both displacement and load controls, in order to both carefully estimate the hardness and plasticity noise as a function of depth and applied in-plane tension. The noisy character of crystal plasticity manifests itself through nanoindentation pop-in events, which are abrupt and stochastic plastic displacements during the initial stage of plasticity. I discuss and demonstrate indentation size effects induced and/or promoted by the in-plane stress in FCC materials. The hardness analysis shows a statistically inverse dependence on in-plane stress for small depth indentation and I discuss the consistency of the effects in both the hardness and pop-in events among the various FCC crystals studied. Finally, I discuss a novel approach to detect the onset of pop-in noise in a realistic sample without special treatment and demonstrate its consistency for Cu single crystalline samples.