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Abstract: Current and future applications in hypersonic flight, re-entry vehicles, propulsion, and power production create an insatiable demand for materials capable to perform in severe environments. Materials for these applications must possess a rare combination of properties, which include high specific strength, elevated melting temperature, high thermal conductivity, and low thermal expansion coefficient.

Micro-architecture materials, such as foams of refractory metals, are good candidates for some of these applications. Understanding the plastic behavior of these small-scale structures is of fundamental importance for designing resilient materials in severe environments. In the first part of this talk, I present discrete and continuum dislocation-based computational models of plasticity in crystalline materials at the microscale. Simulation results guide materials design by explaining phenomena such as the interplay between size and temperature effects in bcc metals subject to uniaxial loads, and the formation of geometrically-necessary dislocation structures under heterogeneous loading (e.g. indentation).

Ultra-High Temperature Ceramics are being considered for applications in extreme environments, especially when oxidation is a major concern. Currently, the factor limiting the use of UHTCs as structural materials is their low-temperature brittleness. The second part of the talk focuses on the plasticity and fracture mechanisms of the transition metal carbide TaC, one of the highest melting temperature materials known to mankind. In-situ mechanical testing and characterization reveal unexpected intrinsic ductility of TaC, which contrasts its well-known bulk brittleness. These findings unveil new properties of the material, such as a pronounced non-Schmid behavior and a remarkable temperature/orientation dependence of the yield strength. A variety of multiscale modeling techniques are employed to understand the small-scale behavior of TaC. Computer simulations shed light on the room-temperature brittleness of TaC by explaining the link between plastic deformation and fracture.

Biosketch: Giacomo is a Research Scientist and a Lecturer in the Mechanical and Aerospace Engineering Department at the University of California Los Angeles (UCLA). He received his Ph.D. in Mechanical Engineering from UCLA in 2011, under the supervision of Prof. Nasr Ghoniem. His research interest focuses on the mechanics of materials defects in metals and ceramics, with emphasis on discrete and continuum models of dislocation-based plasticity and fracture. Giacomo is the main developer of the Mechanics of Defects Evolution Library (MoDELib), a high-performance framework for dislocation-based plasticity used by researchers around the world. His work has been featured in international journals, including the Journal of the Mechanics and Physics of Solids, Physical Review Letters, the International Journal of Plasticity, and others.

Date(s) - Feb 22, 2018
11:00 am - 12:00 pm


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