UC Santa Barbara

Nanocrystalline metals exhibit exceptional mechanical properties, such as high strength and wear resistance, and are actively being investigated for use in high performance structural applications. The ubiquitous use of nanocrystalline alloys in engineering applications is currently limited by two intrinsic material instabilities: rampant, thermally induced grain growth; and catastrophic shear localization during plastic deformation. These instabilities, which originate in the high concentration of grain boundaries and corresponding structural disorder present in nanocrystalline metals, prevent the widespread use of these alloys, especially in mission- or safety-critical applications. However, outstanding questions remain regarding the precise role of grain boundary structure on these instabilities, presenting an exciting opportunity to enable their use.

Using both targeted processing and interface aware alloying strategies, the role of grain boundary structure and chemistry on these instabilities will be explored. Experiments suggest that ultrafast (fs) laser processing provides tunability of the mechanical behavior of nanocrystalline metals by selectively modifying grain boundaries. The degree of tunability is sensitive both to local chemistry and initial relaxation state, suggesting that laser processing increases the energy of the grain boundaries, akin to rejuvenation processing in amorphous metals.

A nanocrystalline aluminum alloy, doped with nickel and cerium, was developed to investigate the chemical and structural effects of multicomponent alloying. The Al-Ni-Ce alloy exhibits extremely desirable mechanical properties, including high hardness, tunable shear localization behavior, and strength retention at elevated temperature; as well as excellent microstructural stability. The origin of these desirable attributes is explored using ex- and in-situ diffraction experiments, which suggest the topological disordering of grain boundary regions during processing imparts thermal stability and strength at temperature. These results collectively underscore that grain boundary structure plays a deterministic role in the instabilities associated with nanocrystalline metals, providing novel strategies for alloy design and processing to circumvent these behaviors.