Technical presentation on “Hierarchical and Gradient Microstructures for Controlling Mechanical Properties”

Dr. Ramathasan Thevamaran, Postdoctoral Research Associate, Department of Materials Science and Nano-Engineering, Rice University, Houston, TX. Delivered a technical presentation at the Faculty of Engineering. This talk was held at the Faculty of Engineering on 26th July 2017. This was on the cutting-edge technology on Science and Engineering field. Very interesting topic and presentation and many staff members and students participated.

Abstract: Recent advancements in material fabrication technologies have blurred the boundary between a material and a structure. This allows us to control the structure and geometry of micro- and nanoscale features, and their multiscale interactions to develop structured materials with superior bulk properties for desired applications. Successful development of materials with optimal bulk properties requires a thorough fundamental understanding of material behavior over multiple lengthscales—from nanometers to millimeters—across different response timescales—from nanoseconds to several minutes. I present two examples of structured materials having gradient functional properties—vertically aligned carbon nanotube (VACNT) foams and gradient-nano-grained (GNG) metals—that can find applications in extreme environments.

The macroscale VACNT foams have a hierarchical organization of carbon nanotubes. Using VACNT foams as a model for hierarchically-structured materials, I demonstrate their potential in developing lightweight energy-absorbing materials for protective applications. Subjected to striker impacts, the VACNT foams exhibit a nonlinear stress-strain response with hysteresis, and show the ability to recover near-completely from large compressive strains. By tailoring the nanostructure of the carbon nanotubes and their hierarchical organization during chemical vapor deposition synthesis, I show that the bulk mechanical properties and the energy absorption characteristics of VACNT foams can be improved significantly. The dynamic characterization of VACNT foams show that the energy absorption, for example, can be improved over two hundred times that of the commercial foams of similar densities

The GNG metals have spatial gradients in grain size that varies from nanometers to micrometers, typically from the surface to the interior. By using defect-free single-crystal silver microcubes as a model system and by using a laser induced projectile impact testing (LIPIT) technique, I have demonstrated that an extreme GNG structure can be created in metals via high-velocity (400 ms-1) impacts. The roles played by the intrinsic crystal symmetries and the extrinsic particle-shape symmetries in the creation of GNG structure provide insights into designing GNG structured metals in controlled ways. Creating GNG structure in metals can alleviate the brittle failure commonly found in strong nanocrystalline metals, and show promising pathways to developing ultra-strong metals that are also tough enough to resist failure.

Brief Bio of the resource person: Dr. Ramathasan Thevamaran obtained his B.Sc.Eng.(Hons.) in Civil Engineering from the University of Peradeniya (Sri Lanka) in 2008, and his M.S. and Ph.D. in Mechanical Engineering from the California Institute of Technology (USA) in 2010 and 2015. Starting from August 2017, he will be an Assistant Professor of Engineering Physics at the University of Wisconsin, Madison (USA). His research interests are in the structure-property relations of structured materials such as carbon nanotube foams, gradient-nano-grained metals, polymer nanocomposites, microlattices, and Parity-Time symmetric metamaterials.

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