Nanocrystalline Alloys: Impervious to Fatigue Failure?
Fatigue failure is the process by which materials break during repetitive loading. In metals and alloys, fatigue cracks nucleate by an atomic-scale process called ‘persistent slip’. Transmission electron microscopy studies of conventional coarse grained metals show persistent slip bands as dislocation ladder structures with dimensions of several 100’s of nanometers to micrometers. However, in nanocrystalline alloys the grain size itself is less than 100 nanometers, thereby suppressing the formation of a persistent slip structure. As a result, these nanocrystalline alloys demonstrate substantial enhancement in fatigue resistance compared to conventional structural metals, even when scaled by the Hall-Petch strength enhancement. We have examined the fatigue behavior in three nanocrystalline nickel based alloys and compared behavior to their annealed coarse grained counterparts. In all three nanocrystalline nickel alloys, failure is preceded by fatigue-driven grain growth. Only when the grains are grown mechanically to several 100’s of nanometers, does crack nucleation occur. Synchrotron x-ray diffraction and in-situ TEM experiments are used to elucidate the kinetics of the fatigue-induced grain growth process while molecular dynamics and phase field simulations explore the atomic-scale and microstructural scale conditions that might give rise to this unexpected phenomenon.
Dr. Boyce is a Distinguished Member of the Technical Staff at Sandia National Laboratories. Dr. Boyce received the B.S. degree from Michigan Technological University in 1996 in Metallurgical Engineering and the M.S. and Ph.D. degrees in 1998 and 2001 from the University of California at Berkeley. Dr. Boyce joined the technical staff at Sandia in 2001 where his research interests lie in micromechanisms of deformation and failure. He was promoted to Principal Member of the Technical Staff in 2005, and received the Distinguished appointment in 2015. At Sandia, Dr. Boyce leads several team projects, totaling ~$4M/yr. He has ~100 peer reviewed archival publications (H-index=31) in areas such as microsystems reliability, nanoindentation, fracture in structural alloys, weld metallurgy, ocular tissue viscoelasticity, and fatigue mechanisms. Dr. Boyce has served as a Key Reader for Metallurgical and Materials Transacations and is currently on the editorial board of Fatigue and Fracture of Engineering Materials and Structures. He has also served as a guest editor for Thin Solid Films, Experimental Mechanics, International Journal of Fatigue, and International Journal of Fracture. He has organized 11 technical symposia at international conferences and has chaired the Rio Grande Symposium on Advanced Materials. Dr. Boyce is a past recipient of the Hertz Foundation fellowship, TMS Young Leader award, and ASM’s Marcus A. Grossman Young Author award.