On June 7th, Prof. David Srolovitz was invited to speak on the Unified Theory of Interface Dynamics in Crystalline Materials for the 187th edition of the Luojia Lecture series. He is the author of over 450 papers and an established expert on topics such as grain boundaries, dislocations, microstructure evolution, dislocations, and film growth. He is a member of the National Academy of Engineering, Fellow of MRS, TMS, ASM, Institute of Physics and is the winner of the 2013 MRS Materials Theory Award. He received his PhD from the University of Pennsylvania and served as a professor at the University of Michigan, Princeton University and the University of Pennsylvania, where he is currently the Joseph Bordogna Professor of Engineering and Applied Science and Director of the Penn Institute for Computational Science.
Prof. David Srolovit
He expressed his delight over this opportunity to visit Wuhan University and meet some of the mathematicians he had been working with. Earlier this year, Q. Zhao, W. Jiang, W. Bao, and Prof. David J. Srolovitz published a paper on triple junction drag effects in the journal Acta Materialia. He commenced his lecture with a quick introduction to crystals and its composition, “Each of the individual crystals are formed by grains. The grains meet each other at flat interfaces. Although they are made up of the same material, there is a discontinuity in crystal structure.” He clarified that grain boundaries are not liquid films and went on to illustrate them with details, “Each bubble can have different gas pressure inside and the liquid films can migrate. The network of bubble gets closer and closer. As they get closer, the average bubble size and number of bubbles increases.”
He talked about what we know on curvature flow and grain boundaries in networks, and charting out mathematical relations. In all those generalizations, he suggested that topological changes be taken into consideration and also gave the reason behind it, “As the grain shrinks, its number and size changes.” He touched down on several of the topics on crystallography, lattice dislocations, disconnections and intrinsic grain boundary dynamics.
He demonstrated properties of grain boundaries in relation to crystal structure, “If I don’t apply stress, it doesn’t move, and interestingly, by applying force in the opposite direction, the grain boundary moves the other way. That doesn’t happen in curvature flow. That has to do with crystal structure.” He added further, “If I look at all the possible shifts that I can make, I end up tracing out the position of where the atoms overlap.”
Basically a crystal is a periodic arrangement of atoms and a translation vectors helps to map out the atoms. Based on his findings, he said, “From all the possibilities, we found that the system has no correlation between shear and translation.” He stated that shear coupling means stress develops and the two driving forces may cancel. In unconstrained dynamics, grain boundary migrates at constant velocity but in constrained motion grain migration stops after some time. Shear couples with one mode and switches back and forth. Multiple coupling mode is more complicated and under applied stress, and disconnections nucleate propagate to the vertices.
Topologically, dislocation is translational invariance. In terms of grade boundary, he stated, “motion of grain boundary is motion of disconnection.” He stated that disconnections couple Burgers vectors and steps, and in support, added, “these individual steps along the grain boundary hit each other and annihilate, that moves the grain boundary.”
Before drawing the conclusions of his lecture, he graphically demonstrated triple junctions. He asserted that when disconnection reactions at junctions are very slow or stationary, junctions don’t move and grain boundary is curved; but when disconnection reactions are very fast, grain boundary is flat. In the end, he casted some light on the fact that disconnections and their dynamics are key to understanding a wide range of grain boundary dynamical phenomena and also briefly stated the challenges in crystallography. He humbly answered questions posed by attending students and professors, and the lecture drew to a close with a rapturous applause.
Edited by Li Yunzhen, Edmund Wai Man Lai & Hu Sijia
Photo By University Of Pennsylvania School Of Engineering And Applied Science
