Graduate Student Seminar

September 29, 2023

12:00 p.m. ET

Scaife Hall 105

Interface plasticity of grain boundaries and 2D heterostructures from bicrystallography

Crystal interfaces exist in diverse materials systems in the form of grain/phase boundaries (GBs) in three-
dimensional polycrystals and as heterointerfaces in two-dimensional (2D) heterostructures. Interfaces signifi-
cantly influence the mechanical response of a material as observed in phenomena such as superplasticity and creep in polycrystals and the much sought-after electromechanical coupling in 2D materials that is responsible for correlated electron physics. Recognizing interface dislocations as the primary carriers of interface plasticity, our overarching goal is to build a statistical mechanics model for heterointerfaces that connects the discreteatomic structure to interface continuum properties, such as energy and mobility.

Enumerating all line defects in arbitrary heterointerfaces is the first step towards building a statistical model. To this end, we will present a unified mathematical framework to enumerate all interface dislocations in 2D heterostructures, and disconnections (dislocation with a step) in rational GBs. Our framework is driven by the Smith normal form (SNF) for integer matrices which enables us to systematically explore the bicrystallography of interfaces. Analogous to the definition of a bulk dislocation, which relies on the translational symmetry of a 3D lattice, the framework characterizes a heterointerface’s translation symmetry, which is integral to the definition of interface dislocation. Central to our framework are two lattices — the coincident site lattice (CSL) and the displacement shift complete lattice (DSCL) — derived from the two lattices that constitute an interface. The constructive nature of the framework lends itself to an algorithmic implementation based exclusively on integer matrix algebra.

Next, we will demonstrate the calculation of nucleation barriers of the enumerated line defects in GBs and a large-twist (> 10◦) bilayer graphene. We conclude by showing how the resulting energetics of individual
line defects can be aggregated in the form of ensemble averages to arrive at thermodynamic and kinematic
properties of heterointerfaces.

Nikhil Admal, University of Illinois at Urbana-Champaign0929-admal.png

Admal is an Assistant Professor in the Department of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign (UIUC). Before joining UIUC, Dr. Admal was a Postdoctoral Research Scholar at the University of California Los Angeles (UCLA). He received his Ph.D. in Aerospace Engineering and Mechanics from the University of Minnesota in 2014. His research interests focus on developing materials models at various length and time scales ranging from the atomic to the continuum scales, with emphasis on modeling interface phenomena. His research currently focuses on modeling recovery, recrystallization, and grain growth in crystalline materials to establish the process-structure relationship and on the mechanics of two-dimensional heterointerfaces. He received the NSF Faculty Early Career Development Program (CAREER) Award in 2023. Dr. Admal is a recipient of the Institute for Digital Research and Education (IDRE) Postdoctoral Fellowship at UCLA and the Doctoral Dissertation Fellowship from the University of Minnesota. Dr. Admal has published in various international journals, including The Journal of Chemical Physics, Journal of Mechanics and Physics of Solids, International Journal of Plasticity, Materials Theory, and the Journal of Elasticity.

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