Influence of precipitates on low-cycle fatigue and crack growth behavior in an ultrafine-grained aluminum alloy

Abstract The strain-controlled fatigue and near-threshold fatigue crack growth behavior of an ultrafine-grained (UFG), age-hardening aluminum alloy after severe plastic deformation by equal-channel angular pressing (ECAP) are discussed. The main question addressed is how different precipitate morphologies affect low-cycle fatigue (LCF) and fatigue crack growth. An AlMg0.5Si0.4 alloy is subjected to two and eight passes of ECAP to obtain different degrees of grain refinement and fragmentation of the initially semi-coherent precipitates. Furthermore, a thermally recovered condition with newly formed, small coherent precipitates, which is obtained by aging after two ECAP passes, is considered. Strain-controlled fatigue tests and Δ K -controlled crack growth measurements are conducted and microstructural evolution during cycling and fracture surfaces are carefully analyzed using scanning and transmission electron microscopy. Most importantly, the results of this study show that newly formed, coherent precipitates in the thermally recovered condition directly contribute to a more planar slip behavior, to slip localization and to early failure during LCF loading. It is clearly demonstrated that precipitate morphology also affects fatigue crack propagation, and that this is closely related to slip reversibility, even in the UFG regime.