WE Colloquium: Dr. Ashwin Shahani, Tales of the Abnormal: Grain Growth in Particle-Containing Systems

Abstract

Nearly all classes of materials — from soap froths to metallic superalloys — are composed of small domains known as cells (or grains).  The sizes, orientations, and boundaries of the grains influence many of the solid’s properties.  During thermal processing, normal grain growth occurs, wherein some grains enlarge while others shrink and disappear.  A second possibility, termed abnormal grain growth, is that a very few grains grow at a much faster rate than the others and eventually consume the microstructure.  Unfortunately, the origins and mechanisms of abnormal grain growth have remained an enigma for at least the past seventy years.  One such long-standing mystery concerns abnormal grain growth in the presence of second-phase particles.  Our traditional understanding is that particles retard and eventually "pin" the grain boundaries.  Yet, and paradoxically so, abnormal grain growth is most frequently seen in particle-containing systems, such as transformer steels.  

Our goal is to shine new light on particle-assisted abnormal grain growth by making use of laboratory-based 4D (i.e., 3D space plus time) imaging.  Our work integrates two nondestructive imaging modalities — absorption-based and diffraction-based X-ray computed tomography — to understand particle-boundary interactions in all their complexity.  Following the in situ experiments, advances in data science methods and high performance computing open the doors to a wealth of information on the collection of grains undergoing abnormal growth.  For instance, the 4D data have been analyzed by Shahani’s team to extract grain boundary characteristics and velocities (via diffraction-based tomography) together with particle locations (via absorption-based tomography), as a function of time.  These metrics provide direct evidence on the kinetic pathways leading to the growth of abnormal grains.  They also serve as benchmark data that can be used to validate mesoscale simulations, e.g., phase field.

Bio

Prof. Ashwin J. Shahani is a third-year assistant professor in Materials Science and Engineering at the University of Michigan. He is a pioneer in the development and application of advanced characterization methods for the study of structural transformations in materials, from solidification to grain growth.  His past research focused on advancing the unique capabilities of synchrotron-based, X-ray absorption-, diffraction- and phase-contrast tomography and applied these techniques for the first time to watch crystal growth from a liquid phase.  Recently, he has won the AFOSR YIP (2017), ARO YIP (2018), and NSF CAREER (2019) awards in support of his research pursuits.  Shahani earned his B.S. degree from Cornell University and his Ph.D. from Northwestern University, both degrees in the field of Materials Science and Engineering.