Issue 19

L. Kunz et alii, Frattura ed Integrità Strutturale, 19 (2012) 61-75; DOI: 10.3221/IGF-ESIS.19.06 71 These results on low-purity UFG copper evidence a better resistance to the crack propagation with respect to other UFG structures taken into consideration for comparison. Even, in stage II the present UFG Cu shows higher FCG resistance than the CG counterpart. On the other hand, the threshold regime and values of “extrapolated” threshold stress intensity factor  K th (details of the procedure are given in [39,40]) show that (i) the load ratio influences the threshold FCG regime, since higher is R , lower is  K th , as one can evince from Fig. 15, (ii) that the threshold K is higher than values found in literature for the same class of UFG copper, and (iii) if compared with the FCG behaviour of annealed and cold worked conventional Cu alloys, the present UFG microstructure shows higher threshold resistance for R -ratios 0.1 and 0.3, and lower when R -ratio increases, see Fig. 15; this result is partially in contrast with other investigations on UFG structures with different purity levels. Figure 15 : Threshold stress intensity factors of 8-ECAPed UFG copper. In order to explain the relatively high crack propagation resistance of the present UFG copper, its fatigue resistance was analyzed. As previously showed, the higher fatigue resistance has been justified demonstrating the stability of the bulk microstructure during cycling, due to the stable dislocations structure and to the presence of impurities. The grain structure within the plastic zone around the cracks was shown to differ from outside of the plastic zone: the grains were found markedly elongated, but their size was preserved. Also, in comparison with the CG structure, a small grain size can potentially result in more homogeneous deformation, which can retard crack nucleation by reducing stress concentrations and ultimately raise the fatigue limit of the UFG structure. This has been demonstrated by other studies on ECAPed copper structures on low and high cycle fatigue [41,42]. Thus, the interaction between a propagating crack and the GBs structure can produce retardation in the growth rate [43]. This phenomenon has been already noticed and theoretical models on the crack-boundaries interaction developed, with the support of experimental evidences [44,45]. The study of crack propagation in UFG Cu loaded at rotating bending and based on the observation of replicas, brought an evidence of crack growth retardation due to fine-grained structure [46]. The structurally induced retardation, however, was observed only for lowest loading amplitudes and in the region of very small crack propagation rates. The available knowledge on the propagation of fatigue cracks in UFG Cu is not sufficient for serious conclusions concerning the threshold values and details of the crack growth mechanism. It seems that the topological factors can be critical in the FCG behaviour of UFG metals, if one considers the huge number of GBs generated by the grain refinement process. C ONSIDERATIONS ON STABILITY AND IMPURITIES OF UFG STRUCTURES tability of a severely deformed structure is of utmost importance from the point of view of its fatigue properties [47]. The critical issue of a successful application of UFG materials is the long-term stability of microstructure, in services where cyclic loads, often with mean stress, are frequent. Also loading at elevated temperatures can be expected in engineering practice. Despite this, knowledge of the stability of UFG structure under dynamic and temperature loading is quite scarce. There are open questions concerning the mechanisms of the grain coarsening both under cyclic loading and temperature exposition. S