Materials Science and Engineering Seminar Series: Todd C. Hufnagel
Friday, April 29, 2011
1:00 p.m.-2:00 p.m.
Room 2108, Chemical and Nuclear Engineering Bldg.
Effect of Rapid Heating on Transient Phase Formation During Self-Propagating Reactions in Metallic Multilayers
Todd C. Hufnagel
Department of Materials Science and Engineering
Johns Hopkins University
Metastable solid phases can be produced by rapid quenching from the liquid state, which limits the time available for diffusion and nucleation and growth of competing phases. Similar effects can be expected due to rapid heating, but most studies of phase transformations involving interdiffusion of chemical species have been performed either isothermally or at moderate heating rates (~1 K/s). In this talk, we discuss the influence of rapid heating (~10^6 K/s) on phase formation during self-propagating reactions in metallic multilayers. Although the reaction front is <100 microns wide and moves at ~1 m/s, we can follow the sequence of phase formation in detail using newly developed time resolved synchrotron x-ray microdiffraction techniques. For example, we show that the intermediate phases formed during self-propagating reactions in Al/Ni multilayers are different from those formed at lower heating rates, even though the final phases are the same. We show that the observations from the in situ x-ray diffraction experiments are consistent with molecular dynamics simulations of the interdiffusion and nucleation processes in these multilayers. Finally, we discuss preliminary results from high-speed x-ray phase contrast imaging studies of the propagating reaction fronts, which show a transition in mode from step-wise to uniform front propagation with increasing reaction velocity.
About the Speaker
Todd Hufnagel received his B.S. degree in Metallurgical Engineering from Michigan Technological University, and M.S. and Ph.D. degrees in Materials Science and Engineering from Stanford University. His research emphasizes the use of advanced structural characterization techniques to study phase transformations and mechanical behavior in a variety of materials, particularly metals. He is the recipient of an NSF CAREER award and an Army Young Investigator award, as well as the Capers and Marion MacDonald Award for Excellence in Mentoring and Advising.