Nucleation of cracks from shear-induced cavities in an α/β titanium alloy in fatigue, room-temperature creep and dwell-fatigue

In titanium alloys, dwell periods during room-temperature stress-controlled fatigue tests are responsible for substantial reductions in lifetime compared to pure fatigue loading. The mechanisms of such a creep-fatigue interaction have been investigated for alloy Ti-6242. Scanning and transmission electron microscopy observations revealed crack initiation by coalescence of shear-induced cavities nucleated at alpha/beta interfaces in large colonies of alpha laths nearly parallel to the loading axis. The density and average size of cavities were larger in dwell-fatigue and creep than in fatigue. A qualitative micromechanical model of cavity nucleation based on discrete dislocation dynamics was developed. The number of cycles for cavity nucleation was computed as a function of the applied stress range. A finite threshold, dependent on the size of alpha laths colonies with similar orientation, was found. The simulations predict earlier cavity nucleation in creep or dwell-fatigue than in pure fatigue, which is consistent with the performed experiments. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.