Issue 35

A. Pawełek et alii, Frattura ed Integrità Strutturale, 35 (2016) 21-30; DOI: 10.3221/IGF-ESIS.35.03 26 boundaries. We suppose that these dislocation tangles can be the result of the dislocation pile-up formation which, however, undergo scattering at elevated temperature due to e.g. dislocation climb and/or recovery processes. On the other hand, in the context of plastic deformation, the TEM pictures suggest also that twinning may possibly contribute to the plastic instability, whereas, in turn, the microtwins intersection may also play a role in microcracking occurrence. (a) (b) Figure 6 : The TEM pictures of Mg4Li5Al alloy after tensile test at 150ºC: (a) inclusions, microcracks and dislocation tangles at grain boundaries, (b) microtwins intersection and inclusions at twin boundaries. (a) (b) Figure 7 : The TEM pictures of Mg4Li5Al alloy after tensile test at 200ºC: (a) inclusions and microcracks at the region of grain boundary, (b) microtwins and inclusions at twin boundaries. Mg4Li5Al – compression tests The relation between the plastic instability and the fracture was investigated first for the Mg4Li5Al compressed at room temperature. Fig.8a shows the AE behaviour to the break up of the sample and the corresponding SEM image of its fracture (Fig.8c). The high AE peak is accompanied with the sample disruption. In this case the fracture of cubic sample occurred along the diagonal surface of the cube (the normal of which lies in the ND-ED plane, see Fig.1). This diagonal plane is generally parallel to the plane of maximal stresses leading to the strain localization and shear band formation, that will be more detail discussed further, in the context of optical images presented in Fig.9. The essence of idea of the next experiment (Fig.8b) was based on a precise compression of another sample up to the moment just before the previous sample had broken. The fracture surface for the SEM observation was obtained in this case by manual breakdown of the sample. The corresponding SEM image of the fracture (Fig.8d) shows, that the length of crack path is visibly shorter than in the case observed in Fig.8a. It means that the final break of the sample is a result of successive growth of the length of crack path. On the other hand Fig.9a shows a fully developed shear band, which is observed on the side wall (parallel to ND-ED plane) of cubic sample compressed up to adequate external loading. Additionally, Fig.9b shows that the fully formed shear bands may be realized by the development of slip bands and next microshear bands which cut cross many grains, what means that the final fracture in this case is of transcrystalline character.