“Order-Disorder-Property Relationships in Structural Materials: Guided by Randomness”
Presented by Professor Daniel S. Gianola
Materials Department, University of California Santa Barbara
Emerging classes of structural materials that have been developed to meet the demands of aggressive applications requiring structural integrity in extreme environments, such as those encountered in the aerospace and power generation sectors, all share a common theme: complexity across a large range of length scales. A common denominator is the intentional design of randomness – or disorder – in these new materials. In load-bearing materials, this disorder can manifest as topological disorder (uncertainty in atomic positions) as found in interface-dominated or glassy materials, or chemical disorder (uncertainty in elemental occupancy) as found in compositionally-concentrated alloys. This begs a question that underpins new materials-by-design strategies: should the conventional wisdom of searching for structure-property relationships give way to those that focus on disorder-property ones? This talk will show two examples that lend credence to the notion of embracing the role of disorder in materials.
The first will highlight the novel materials design paradigm of multi-principal element (MPE) alloying that has shown great success, yet opportunities to advance refractory-based high temperature body centered cubic (BCC) alloys for high temperature structural applications must confront fundamentally different avenues for the accommodation of plastic deformation. We show a unique combination of homogeneous plastic deformability and strength at low temperature in the BCC MPE alloy MoNbTi, enabled by the rugged atomic environment through which dislocations must navigate. In situ observations of dislocation motion and atomistic calculations unveil the unexpected dominance of non-screw character dislocations and numerous equiprobable slip planes for dislocation glide. This remarkable behavior reconciles theories explaining the exceptional high temperature strength of similar alloys. Our results, when paired with a material density lower than that of state-of-the-art superalloys, provide sharp focus to alloy design strategies for materials capable of performance across the temperature spectrum.
The second example will demonstrate novel synthesis and processing routes for controlling disorder in nanocrystalline materials – and as a consequence, the mechanical properties. We study relaxation processes at grain boundaries in nanocrystalline materials that facilitate atomic reconfigurations toward a lower energy state such as low temperature annealing, which enhance mechanical strength while promoting shear localization. A particular focus in this talk will be on strategies for rejuvenation at grain boundaries with the goal of suppressing shear localization and endowing damage tolerance. Parallels between our results and rejuvenation processes in glasses, as well as the interplay between grain boundary structure and chemistry through segregation engineering, will be discussed in the context of controlling metastable structural configurations.
Daniel S. Gianola is a Professor of Materials at the University of California Santa Barbara and can be reached at: gianola@engr.ucsb.edu. He is currently the faculty director of the Microscopy and Microanalysis Facility at UCSB, which is a central shared facility with over 400 active users. Dr. Gianola joined the Materials Department at UCSB in early 2016 after holding the positions of Associate Professor and Skirkanich Assistant Professor, all in the Department of Materials Science and Engineering at the University of Pennsylvania. He received a BS degree from the University of Wisconsin-Madison and his PhD degree from Johns Hopkins University. Prior to joining the University of Pennsylvania, Gianola was an Alexander von Humboldt Postdoctoral Fellow at the Forschungszentrum Karlsruhe (now Karlsruhe Institute of Technology) in Germany. Dr. Gianola is the recipient of the National Science Foundation CAREER, Department of Energy Early Career, and TMS Early Career Faculty Fellow awards. His research group at UCSB specializes in research dealing with deformation at the micro- and nanoscale, particularly using in situ nanomechanical testing techniques.