Fracture Kinetics and Mechanisms of Ultrafine-Grained Materials during Fatigue Tests in the Low-Cycle Fatigue Region

In this paper, the fracture kinetics and mechanisms in the low-cycle fatigue region were analyzed for different ultrafine-grained (UFG) materials with body-centered cubic (bcc), hexagonal close-packed (hcp) and face-centered cubic (fcc) lattices. Three-point bending principle fatigue tests were performed. The tests show that the UFG structure formation in the investigated materials has an ambiguous effect on the total number of cycles to failure (life) of the samples. The number of cycles to fatigue crack initiation (Nin) is about 20% of the total life of the samples, irrespective of the material state and the crystal lattice type. At the same value of the stress intensity coefficient range (∆K), for the majority of the investigated UFG materials, the fatigue crack propagation rate (dl/dN) is close to or lower than that of the initial coarse-grained (CG) materials. For the UFG materials, the coefficient n in the Paris equation is, in most cases, lower than that for the CG materials, which indicates that the UFG materials are less sensitive to cyclic overload. The fatigue fracture mechanisms of the investigated CG and UFG materials are rather similar, although the fracture of the UFG materials is accompanied by the formation of many secondary cracks, irrespective of the crystal lattice type.