When 10:30 AM - 11:30 AM Nov 18, 2016
Where 1571 G.G. Brown
Add event to calendar vCal

Nanocrystalline Alloys: Impervious to Fatigue Failure?

Brad L. Boyce
Materials Science and Engineering Center, Sandia National Laboratories

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.