Description:

Abstract: 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. The fatigue cracking process of nanocrystalline metals involves room-temperature grain growth. Only when the grains are grown mechanically to several 100’s of nanometers, does crack nucleation occur. This new mechanism has been confirmed by synchrotron x-ray diffraction and in-situ TEM experiments as well as molecular dynamics simulations. To mitigate this new failure mechanism, binary nanocrystalline alloys have been formulated with improved thermodynamic resistance to grain growth. In a first study on these alloys, we find that there are no signs of fatigue damage after 10 billion cycles at cyclic strain amplitudes up to 0.8% corresponding to stress amplitudes in excess of 1 GPa. This same alloy has also been shown to exhibit remarkable resistance to wear, with a wear rate superior to sapphire and comparable to diamond-like carbon. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. 

Bio: Dr. Boyce is a Senior Scientist 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 has published over 175 peer reviewed articles and holds 6 U.S. patents on topics such as microsystems reliability, nanoindentation, fracture in structural alloys, weld metallurgy, and fatigue mechanisms. Dr. Boyce is a past recipient of the Hertz Foundation fellowship, the J. Keith Brimacombe Medal, and the Marcus A. Grossman Young Author award. He served as the 2023 President of TMS, The Minerals Metals and Materials Society.