Bourns Hall

Colloquium

 

Department of Mechanical Engineering PRESENTS Professor Enrique J. Lavernia University of California, Davis Synthesis and Behavior of Nanostructured Alloys Abstract Inspection of the scientific literature shows that there are many techniques that can be used to produce nanostructured materials, including inert gas condensation or chemical vapor condensation, pulse electron deposition, plasma synthesis, crystallization of amorphous solids, severe plastic deformation, mechanical alloying or cryomilling. However, only a few of these techniques, such as equal-channel angular pressing (grain sizes 200 - 1000 nm), electro-deposition and cryomilling (grain sizes 30 - 500 nm), generate nanostructures with sufficient thermal stability to permit the fabrication of bulk materials. In the present work, nanocrystalline materials were produced by mechanical attrition under liquid nitrogen (i.e., cryomilling). The grain refinement process was dominated by the total microstrain introduced by the deformation process. The microstructures were investigated in detail using transmission electron microscopy and high- resolution electron microscopy. Three nanostructures with different grain size ranges and shapes were observed and the deformation mechanisms in these structures were found to be different. High densities of dislocations were found in large crystallites, implying that dislocation slip is the dominant deformation mechanism. The dislocations rearranged to form small angle sub-boundaries upon further deformation, resulting in the formation of medium-sized crystallites with diameters of 10 - 30 nm. In very small crystallites with dimensions less than 10 nm, twining becomes an important deformation mechanism. Some defects, such as twin boundaries, and small- and large-angle grain boundaries were investigated in detail. Both non-equilibrium and equilibrium grain boundaries were found to exist in the cryogenic ball milled powders. The grain growth kinetics in the nanocrystalline Al and Al-Mg exhibits extremely high resistance against grain growth at elevated temperatures. Tensile behavior of bulk nanostructured Al alloys consolidated by cryomilled powders was characterized by high strength, high ductility and low strain hardening. The present lecture will also address the hypothesis that one can promote dislocation activity, without a significant loss in strength, by attaining a microstructure that contains multiple length scales, that is from tens of nanometers to hundreds of nanometers. Biosketch In September 2002, Enrique J. Lavernia was appointed Dean of the College of Engineering at UC Davis. Dean Lavernia received his M.S. and Ph.D. degrees from MIT (1984 and 1986 respectively) and his bachelor's degree from Brown University (1982). After completing a postdoctoral research program at MIT, he joined the Chemical Engineering and Materials Science Department at UC Irvine in 1987. He served as the department’s chair prior to joining UC Davis. He has held prestigious fellowships from the Ford Foundation, Alexander von Humboldt Foundation, Iketani Science and Technology Foundation of Tokyo, and Rockwell International. Dean Lavernia has co-authored over 320 journal papers, 180 conference papers and one book, Spray Atomization and Deposition. He is currently the principal editor of the international journal, Materials Science and Engineering A. Dean Lavernia’s research interests include spray atomization, reaction, and deposition techniques. His more recent work has focused on nanostructured materials (NMs). DATE: May 26, 2004 TIME: 10:10—11:00 a.m. PLACE: Surge 284 Refreshments are provided