Issue 35

A. Kowalski et alii, Frattura ed Integrità Strutturale, 35 (2015) 449-455; DOI: 10.3221/IGF-ESIS.35.51 453 quasi-cleavage type (Fig. 7), demonstrating mainly flat and smooth surfaces of jogs in the planes of cleavages, practically without any traces of plastic deformation of the surface. Evident is also the local occurrence initiation of brittle intercrystalline cracking (Fig. 8). The observed details of fractures caused by fatigue suggest in this case a interlocking mechanisms of fracture, which is characteristic for processes of plastic deformation of cold-rolled metals and alloys. A somewhat different microfractography was detected in samples of the investigated Al-Zn-Mg alloy subjected to varying bending loads in the case of the minimum value of the amplitude of stresses (σ a ) amounting to about 160 MPa in the boundary zone of fatigue. In the surface zone of the tested samples symmetric local points of the fracture have been noticed (Fig. 9) with smooth primary jogs in the character of transcrystalline cleavage, planes and numerous tongue-steps. Evident are also local overstrains of the material and intercrystalline cracks (Fig. 10). A detailed microfractographic analysis of the tested samples of the alloy with the FCC lattice permitted to see that the observed transcrystalline quasi- cleavage fracture (Fig. 11) indicates distinctly a larger share of plastic deformation on the surface of the steps as well as traces of such a deformation on potentially stochastic surfaces of cleavage (Fig. 12). The grain boundaries of the Al-Zn- Mg alloy with a differing orientation probably impede the propagation of cleavage cracking due to various systems of steps, and also by the absorption of energy in the course of cracking. Thus, in the investigated conditions of fatigue the degree of brittleness can be decreased and the plasticity of the alloy improved. Figure 7 : Transcrystalline quasi-cleavage fracture with flat and smooth surfaces of the AlZn6Mg0.8Zr alloy Figure 8 : Transcrystalline cleavage fracture with a local initiation of brittle cracking of the intercrystalline type of the investigated alloy Figure 9 : Symmetric focal points of a fracture in the surface zone with primary jogs of the investigated alloy. Figure 10 : Temporary zone of a fracture, local cracking of the intercrystalline character of the AlZn6Mg0.8Zr alloy.