Cyclic deformation and fatigue properties of very fine-grained metals and alloys

Bulk ultrafine-grained (UFG) metals and alloys prepared by techniques of severe plastic deformation (SPD), in particular by equal channel angular pressing (ECAP), exhibit exceptional mechanical properties. With respect to potential applications of this new class of very fine-grained bulk materials, the cyclic deformation and fatigue behaviour, relative to that of conventional grain size (CG) materials, is of crucial importance. In the present review, the research performed on the fatigue properties of very fine-grained bulk materials during the last decade will be discussed. Mainly bulk UFG materials prepared by ECAP will be considered and, to a smaller extent, also bulk truly nanocrystalline materials. The discussion will focus on simple materials and on more complex alloys and structural materials. Presentation of fatigue performance in total strain fatigue life diagrams has been found to be particularly suitable to compare the strong UFG and the more ductile CG materials in both the high cycle fatigue (HCF) and the low cycle fatigue (LCF) ranges. In general, the fatigue strength of UFG materials is enhanced considerably by grain refinement, in particular in the HCF regime. However, at the same time, the LCF performance is impaired by microstructural instabilities of the strongly hardened but less ductile UFG materials, as manifested in cyclic softening, fatigue-induced grain coarsening (by dynamic recrystallization at rather low homologous temperature!) and massive shear banding. These effects are discussed critically, also with respect to the effects of the route of ECAP-processing employed, the purity of the material – and the mode of fatigue testing. Remedies by mild annealing treatments which have been partially successful in improving the LCF strength by enhancing the ductility at the expense of a moderate loss of strength will be discussed. Examples of modelling of the cyclic deformation and fatigue properties will be presented, and some directions of future research will be outlined.