Fatigue behavior of nanocrystalline metals and alloys

In this work, the stress-life fatigue behavior and fatigue crack growth characteristics of pure Ni were studied as a function of grain size spanning a range of tens of nanometer to tens of micrometer. The fatigue response of electrodeposited, fully dense, nanocrystalline pure Ni, with average and total range of grain sizes well below 100 nm, was compared and contrasted with that of electrodeposited ultrafine-crystalline pure Ni with an average grain size of about 300 nm and conventional microcrystalline Ni with an average grain size in excess of 10 mu m.. It was found that grain refinement to the nanocrystalline regime generally leads to an increase in total life under stress-controlled fatigue whereas a deleterious effect was seen on the resistance to fatigue crack growth at low and high tensile load ratio levels. To explore the generality of the above trends, systematic experiments were also performed in Ultrafine-crystalline pure Ti produced by equal-channel angular pressing where a reduction in grain size was found to cause an increase in fatigue crack growth rates at different tensile load ratios. Grain refinement from the microcrystalline to the ultratime-crystalline regime by cryomilling of Al alloys also showed a similar response. Possible mechanistic origins of such trends are explored, and some general conclusions are extracted on strategies for improvements in the fatigue resistance of engineering structures by recourse to grain refinement down to the nanocrystalline regime. (c) 2005 Elsevier Ltd. All rights reserved.