Microstructure-Enabled Plasticity in Nano-To Microscale Materials

Abstract

Microstructure-governed damage resistance in materials enables a variety of functional applications, such as durable biomedical implants and robust product packaging. For example, the refined phase compatibility qualifies NiTi for artery stents, while carbon fiber reinforced polymers improve structural strength in aerospace engineering. As the overall size of industrial applications continue to decrease, it has become increasingly apparent that when a material's external structural size and internal microstructural size become comparable, its mechanical behavior starts to deviate from that of bulk, such as the smaller-is-stronger size-effect in metals. This elucidation necessitates the characterization of materials at lengthscales relevant to their internal microstructure to guarantee accuracy in the design of real-world applications.

This thesis aims at deciphering the microstructure-mechanics relationship for materials at lengthscales bridging the gap between 1nm and 1µm, with shape memory ceramics, scorpion shells, and jellyfish biogel as sample systems. We use electron and x-ray diffraction to characterize microstructures such as twinning, defects, and fiber organization, while revealing strength, toughness, and other deformation mechanisms through in-situnanomechanical experiments. We show improved shape recovery in an otherwise brittle ceramic by tuning its phase compatibility at the nanoscale and reveal unprecedented smaller-is-stronger size-dependence for its twinning-induced plasticity. We then unveil competing fiber orientations in Scorpion shells that follow fiber-mechanics principles and demonstrate a combined poroelasticity/viscoelasticity constitutive relation in jellyfish that explains their self-healing behavior. The correlation between microstructure and mechanical behavior unveils unique damage mitigation and energy dissipation techniques in both brittle ceramics and natural biomaterials at each order of lengthscale, paving the road to designing macroscopic materials with hierarchical mechanical behavior and improved plasticity.