“Materials under Extreme Dynamic Environments: The Case for Metals and Metallic Alloys under Shock Compression”
Presented by:
Cyril L. Williams, Ph.D., PE
Senior Research Engineer, U.S. Army Research Laboratory
Friday, May 20, 2016 – 11 a.m.
IMS Room 159
Sponsored by the Department of Materials Science and Engineering
Abstract: As received commercially, pure 1100-O aluminum was cold rolled (CR) to 30, 70, and 80 percent reduction in thickness respectively to study the effects of microstructural evolution on the spall response using plate impact experiments. The results show a sharp increase in pullback velocity for 1100-O aluminum with increase in peak shock stress between 4.0 and 8.3 GPa perhaps due to shock hardening, followed by a decrease for peak shock stresses up to 12.0 GPa possibly due to shock softening. This maximum was not observed for the 30% CR, which showed only an increase in pullback velocity over the shock stress range of 4.0 to 12.0 GPa due to hardening (net increase in dislocation density). For the 70% CR aluminum, no change was observed in the pullback velocity over the range tested (4.0 to 11.0 GPa) probably due to saturation in dislocation density. Similar observations were made for the 80% CR, that is, no change was observed in the spall response between 4.0 GPa and 11.0 GPa. However, large variations were observed in the spall response for the 80% CR and these variations are attributed to material inhomogeneity possibly caused by increased cold rolling beyond saturation. The results also show a significant increase in Hugoniot Elastic Limit (HEL) with increase in percent cold rolling. In addition, the effects of microstructure on the spall properties of two magnesium alloys fabricated via Equal-Channel Angular Extrusion (ECAE) and Spinning Water Atomization Process (SWAP) were also investigated. The Hugoniot Elastic Limit (HEL) for both AZ31B-4E and AMX602 magnesium alloys were found to be approximately 0.181±0.003 GPa and 0.187±0.012 GPa, respectively. The spall strengths extracted from the free surface velocity profiles were found to decrease by approximately 4% for AZ31B-4E between 1.7 GPa to 4.6 GPa shock stress. On the contrary, the spall strength for AMX602 was found to be random for the same shock stress range studied. Residual microstructures of the post-shocked magnesium alloys show that aluminum- manganese rich intermetallic inclusions in the AZ31B-4E magnesium were perhaps responsible for the reduction in spall strength as a function of shock stress. However, this reduction in spall strength was not observed for the AMX602 magnesium. In addition, the fracture surfaces of both materials were dominated by nanovoids and the AMX602 fracture surface was found to be striated. A more in-depth study is needed to better understand the spall behavior of both materials.
Bio: Dr. Williams is currently a Senior Research Engineer at the U.S. Army Research Laboratory. He is a fellow of the American Society of Mechanical Engineers (ASME), Fellow of the African Scientific Institute (ASI), Ronald McNair Fellow, Army Research Laboratory Distinguish Scholar, and Federal Engineer of the Year 2015 (Department of the Army). He earned his B.Sc. and M.Sc. in Mechanical Engineering (Fatigue and Fracture) from the University of Maryland Baltimore County, then M.Sc. and Ph.D. in Mechanical Engineering (Shock Compression Science) at The Johns Hopkins University. He is currently the Army’s Subject Matter Expert (SME) in ex-situ shock recovery experiments. He started his career as a design engineer with the American Bottlers Equipment Company (AMBEC) in 1992 and later joined DuPont Engineering Technology (DuET) in 2000 as a Consultant in Reliability and Mechanical Testing. Dr. Williams is a licensed Professional Engineer in Delaware (#13160) and Maryland (#44307). He is the executive head of ASME Government Relations (Delaware Section) and an active member of several research societies including the Institute of Shock Physics, Imperial College London. He has given numerous invited talks nationally and internationally including Cavendish Laboratory, University of Cambridge, California Institute of Technology, and Institute of Shock Physics, Imperial College London.
For more information, contact: Lorri Lafontaine/Department of Materials Science and Engineering at (860) 486-4620/lorri.lafontaine@uconn.edu.