Issue 19

L. Kunz et alii, Frattura ed Integrità Strutturale, 19 (2012) 61-75; DOI: 10.3221/IGF-ESIS.19.06 72 Figure 16 : Microstructure after 10 10 cycles. Figure 17 : Microstructure after fatigue loading exhibiting “shaken down” features. ECAP results in structures that are in metastable state. There is a natural tendency for recovery and recrystallization driven by a decrease of high stored energy. Hence, substantial changes of microstructure can be expected in course of fatigue. Really, the total strain-controlled tests of UFG Cu showed a marked heterogeneity of dislocation structure after fatigue loading, which resulted in failure of specimens after 10 4 cycles. Three types of structures were described: a) subgrain/cell structure, which resembles the well-known structure from LCF tests of CG Cu; b) a fine-grained lamellar structure as observed in Cu after ECAP; c) areas with large grains with primary dipolar dislocation walls. The first two types of structure were found to be the majority. The intensity of grain coarsening decreases with decreasing plastic strain amplitude in a strain-controlled test [48]. Pronounced local coarsening of microstructure when compared to the initial state was found after cycling with  ap = 1 x 10 -4 . Observation by TEM revealed very pronounced fatigue-induced grain coarsening that occurred in some areas by dynamic recrystallization. This process takes place at a low homologous temperature of about 0.2 of the melting temperature [26]. The structure is described as “bimodal”. Dislocation patterns, characteristic for fatigue deformation of CG Cu, have developed in the coarser recrystallized grains. It is believed that this grain coarsening is closely related to the strain localisation [49]. On the other hand, it is interesting to note that after fatigue at the plastic strain amplitude of 10 -3 the grain coarsening was not observed. Examination of the microstructure of failed specimens, which were used for the determination of the S-N curve of UFG Cu in Fig. 6, brought no evidence of structural changes, even for the highest stress amplitudes in LCF region. The average grain size is 300 nm with the scatter usual for determination of the grain size in as ECAPed material. Cyclic softening is a characteristic feature for the whole lifetime. This means that the cyclic softening is not directly related to the grain coarsening. This finding is in agreement with the observations in [30], where is noticed that the decrease in hardness of UFG Cu after fatigue does not scale with the cell size d cell according to a well-known relationship between the saturation stress,  a,sat , and d cell of the type  a,sat ~ (d cell ) -1/2 . This suggests that the mechanism of softening is related to the decrease of defect density and changes of boundary misorientation rather than to the gain size. The highest number of cycles applied by the determination of the S-N curve, Fig. 6, was of the order of 10 10 and was reached by ultrasonic loading at 20 kHz. Fig. 16 shows the TEM image of a structure of a specimen, which failed after 1.34 x 10 10 cycles. Comparison with Fig. 2 implies no grain size changes after stress-controlled loading in gigacycle region. The characteristic cyclic stress-strain response in a very high-cycle region is continuous cyclic hardening; i.e., qualitatively different from that under cycling with high stress amplitudes. Detailed analysis of many TEM micrographs tempts to believe that the fatigue loading with constant stress amplitude in the interval form 320 to 120 MPa, Fig. 6, does not result in the grain growth. The only observed structural change is a weak tendency to develop more “shaken down” dislocation structures [24]. An example is presented in Fig. 17. In the case of UFG Cu significant differences in the structure stability were observed depending on the mode of fatigue testing. Generally, low stability of UFG structure was reported for plastic strain-controlled tests. The characteristic effect is formation of bimodal structure and shear banding [25]. Due to the obvious high sensitivity of fatigue behaviour of UFG