Xian Chen is awarded the Lawrence E. Goodman Fellowship
Ms. Xian Chen was awarded the Lawrence E. Goodman Fellowship in Theoretical and Applied Mechanics in the Department of Aerospace Engineering and Mechanics. This award honors a graduate student with an outstanding academic record who demonstrates a special commitment to graduate studies.
Chen is pursuing a Ph.D. in Aerospace Engineering and Mechanics and is advised by Professor Richard James. She is studying solid mechanics with application to materials that undergo phase transformations, especially a fascinating family of transformations called martensitic phase transformations. When heated or cooled through its phase transformation temperature, one of these materials suddenly changes crystal structure, but remains solid. Usually, on cooling, the lattice undergoes a distortion from a symmetric crystal structure, often cubic, to a lower symmetry structure. These materials are some of the most interesting in all of science, because the two crystal structures can have very different properties. For example, one of the materials that Chen discovered (together with another collaborator from James’ group) is a strong magnet in the high temperature phase, but nonmagnetic in the low temperature phase. Such materials have enormous potential for technology, in energy conversion and storage devices, microelectronics, solid state refrigeration, information storage, actuators, sensors and optics.
All of these applications require that the material goes back and forth through the phase transformation many, many times, but this rarely happens. After a few cycles, the transformation temperature migrates, and the materials may fatigue or crack. Chen’s main discovery is a strategy for making these transformations highly reversible, so that thousands or millions of cycles are possible, despite the disruptive change of crystal structure. She does this by “tuning” the lattice parameters (i.e., the geometric parameters that describe the size and shape of the unit cell) of the two phases so that the two phases fit together in many ways. As Chen explains, “It is like a jigsaw puzzle. Imagine a bizarre jigsaw puzzle in which the pieces can change shape. Under the conditions on lattice parameters I have found, the pieces still fit together perfectly!”
Figure 1. The perfect fitting together of two phases, despite distortion, in an alloy satisfying conditions on lattice parameters studied by Xian Chen.
James notes that “It turns out that these special conditions on lattice parameters have a profound influence on the reversibility of martensitic transformations. In collaboration with other members of my group, we have achieved these conditions by carefully synthesizing alloys, measuring the lattice parameters, and marching closer and closer to these conditions. The resulting alloys show hysteresis less than 1 degree, and unprecedented reversibility.” James reflects, “Xian is using modern versions of some of the most important classical ideas in theoretical and applied mechanics to discover important new materials.”
James recruited Chen during a visit to China to visit a former postdoctoral fellow, Jian Li, who is now Chair of the Department of Materials Science at Huazhong University of Science and Technology in Wuhan. Jian Li, who directs a huge group of students, postdocs and assistant professors in diverse areas of energy science, recommended Chen as his top student. Chen said of James’ lecture series there, “As a student in materials science, I could not believe that one could use sophisticated ideas in mathematics to understand materials.”
Chen’s research has been sponsored by two MURI projects (managed by the Army Research Office and the Air Force Office of Scientific Research).
The fellowship is named in honor of Lawrence E. Goodman, who was hired as a professor in the Department of Mechanics and Materials at the University of Minnesota in 1953. He taught graduate classes in continuum mechanics, the theory of elasticity, the theory of mechanical vibrations and undergraduate classes in Newtonian mechanics and stress analysis. He was appointed as a member of the graduate faculty in the Department of Aeronautical Engineering in 1961 During his tenure at the University of Minneapolis, he had played an active role in graduate education and served on several graduate committees for the Physical Sciences. Professor Goodman left the department and was a Professor of Civil Engineering at the University of Minnesota and Chair of the Civil and Mineral Engineering Department from 1965 to 1972. Professor Goodman primarily conducted research in the field of stress analysis, particularly the stress-optical properties of AgCl crystals in the elasto-plastic range, the damping of turbine blade vibrations, and the effects of contact stresses which arise at the interface of rounded solids pressed together. He also served as a senior fellow of the National Science Foundation. Apart from his academic duties at the University of Minnesota, he occasionally served as consultant to industry, notably to the Gyro Design Section, Aeronautical Division, Minneapolis Honeywell Regulator Company, the General Electric Corporation and Electric Machinery Manufacturing Company. He was the holder of domestic and foreign patents on gyroscope improvements and telemetering devices. In addition, he has co-authored several books with Professor Warner on Statics and on Dynamics.