MSE Seminar: Probing Nanoscale Damage Gradients in Irradiated Metals using Nano-mechanical Test Tech
Speaker: Siddhartha Pathak, Assistant Professor, Chemical and Materials Engineering, Unviersity of Nevada, Reno
Title: Probing Nanoscale Damage Gradients in Irradiated Metals using Nano-mechanical Test Techniques
Materials with modified surfaces – either as a consequence of a graded microstructure, or due to an intentional alteration of the surface – are of increasing interest for a variety of applications ranging from enhanced wear and corrosion resistance, superior thermal and biomedical properties, higher fracture toughness, and reduced stress intensity factors etc. Quantifying the resulting property gradations poses a significant challenge, especially when the changes occur over small (sub-micrometer) depths, such as during ion-irradiation. The first half of this presentation will focus on a novel indentation approach which, together with the corresponding local structure information obtained from electron backscatter diffraction (EBSD), allows us to probe nanoscale surface modifications in solid materials and quantify the resulting changes in its mechanical response. Using tungsten as a specific example we discuss the capabilities of spherical nanoindentation stress-strain curves, extracted from the measured load-displacement dataset, in characterizing the elastic response, elasto-plastic transition, and onset of plasticity in ion-irradiated tungsten under indentation, and compare their relative mechanical behavior to the unirradiated state. Time permitting we will also use a series of examples to show the capabilities of our nanoindentation techniques in (a) characterizing the local indentation yield in individual grains of deformed polycrystalline metallic samples and relating them to increases in the local slip resistances, (b) correlating the stored energy differences of individual grains to their Taylor factors as a function of imposed cold work, and (c) understanding the role of interfaces (grain boundaries) in the deformation of a polycrystalline sample.
The second part of this presentation will focus on microscale testing under extreme conditions of pressure, temperature and strain rate. In particular we will look at the response of body centered cubic (bcc) Mg, stabilized at ambient pressures in a Mg/Nb multilayer nanocomposite. We investigated the structure of the hitherto-unknown bcc Mg phase in the nanocomposite under high pressures in a diamond anvil cell experiment using synchrotron radiation x-ray diffraction. Additionally, Pathak's team performed high temperature micro-pillar compression tests and strain rate jump tests on Mg (bcc)/Nb 5nm/5nm and Mg (hcp)/Nb 50nm/50nm nanolaminates to compare the responses of hcp vs. bcc Mg. Results from these tests were analyzed in terms of the measured activation energies and activation volumes from sub-micrometer sized Mg/Nb multilayer nanocomposites.
Siddhartha "Sid" Pathak is an assistant professor in the Chemical and Materials Engineering department at the University of Nevada, Reno (UNR) since Fall 2015. Before joining UNR he was a Director’s Postdoctoral Fellow 2012-2015 at Los Alamos National Laboratory (LANL), and a Keck Institute Postdoctoral Fellow at Caltech (2010-2012). His research interests are in the (i) mechanistic design of multi-layered composites, for both biological, structural and functional applications, and (ii) quantitative understanding of their local mechanical structural changes under extremes of temperature, strain rate and damage (irradiation). He has co-authored >55 peer reviewed articles and 2 book chapters in various scientific journals. He has received numerous scientific awards based on his work including the 2017 TMS Young Leaders Professional Development Award, one of the 2019 Top UNR Researchers for his university, as well as the 2019 DARPA Young Faculty Award and the 2020 NSF EPSCoR Research Fellowship.