INITIATION OF FATIGUE DAMAGE IN ULTRA-FINE GRAINED THIN FILMS: SCHMID, TAYLOR OR HALL-PETCH?

摘要

The evolution of fatigue damage in metallic films is usually described by several subsequent stages. First, the dislocation slip induces formation of slip steps and slip bands. Application of further cyclic mechanical load leads to formation of extrusion/intrusion couples and, finally, crack propagation. Although this general description of the fatigue damage development is well established and can be generally applied to different materials, surprisingly little is known about the very early stage of fatigue damage initiation especially for ultra-fine grained (UFG) and nanocrystalline thin films. From the point of view of classical plasticity theory, the plastic slip should occur first within most favorably oriented grains with the highest resolved shear stress that corresponds to the lowest Taylor factor or the highest Schmid factor. The core question which this presentation will try to address can be formulated as following: is it possible to predict where the fatigue damage will be initiated for a given UFG thin film with a given microstructure?