The Department of Mechanical, Materials, and Aerospace Engineering presents its MMAE 593 seminar series featuring guest speaker Nikhil Chandra Admal, assistant professor in the Department of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign. Admal will present “Equilibrium Statistical Mechanics of Crystal Interfaces.” This event is open to the public and will take place on Wednesday, January 14, from 12:45–1:45 p.m. in Room 104 of the Stuart Building.

Abstract

Crystalline interfaces—grain boundaries (GBs) and phase boundaries—control many of the degradation modes of polycrystalline materials, such as intergranular fracture caused by dislocation pile-up, creep, corrosion, and swelling due to point defect segregation. Traditionally, GB constitutive properties—free energy, mobility, defect absorption capacity—have been parameterized using only the macroscopic degrees of freedom (DOFs): misorientation and boundary-plane inclination. However, experiments and simulations overwhelmingly demonstrate that these macroscopic DOFs are insufficient to describe the energetics of interfaces interacting with point defects and chemical species. Interfaces with identical macroscopic DOFs can have profoundly different atomic structures, segregation tendencies, and irradiation responses. There is a growing consensus that the atomic-scale configuration of a GB, known as a microstate, has a significant impact on GBmediated phenomena. Moreover, GBs undergo microstate transformations, which reinforces the importance of exploring these configurations. In this talk, we will present a statistical mechanics framework specifically designed for GBs. We will focus on how to enumerate all microstates in a GB ensemble, which is defined by imposing constraints on the macroscopic degrees of DOFs. Since a microstate represents an atomic-scale description of a GB, the microstate space is vast, making it a computationally intensive task to navigate. Given the universal impact of bicrystallography on GB characteristics, we will show that, in stark contrast to the multitude of microstate DOFs, there is a much smaller set of bicrystallography-informed mesoscale DOFs, termed mesostates, that dictate the intrinsic properties of GBs. We will detail the enumeration of GB mesostates and the calculation of internal energy, free energy, and entropy of GBs as statistical averages over these mesostates, along with predictions of GB phase transitions observed in experiments and molecular dynamics simulations.

Biography

Nikhil Admal is an assistant professor in the Department of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign (UIUC). Prior to his appointment at UIUC, Admal served as a postdoctoral research scholar at the University of California, Los Angeles (UCLA). He earned his Ph.D. in aerospace engineering and mechanics from the University of Minnesota in 2014. Admal’s research interests lie at the intersection of mechanics, materials science, and mathematics, with a particular focus on multiscale modeling of interface mechanics in polycrystalline materials and 2D heterostructures. Currently, his group is investigating grain boundary-mediated plasticity, recrystallization, and nanoscale friction phenomena. He received the NSF Faculty Early Career Development Program (CAREER) Award in 2023.