PSI - Issue 65

Mikhail Sokovikov et al. / Procedia Structural Integrity 65 (2024) 269–274 Author name / Structural Integrity Procedia 00 (2024) 000–000

272

4

The temperature of the plastic strain localization area does not exceed ~155°С. This made it possible to conclude that thermal softening does not play a decisive role in the localized shear mechanism under these loading conditions. The investigation of the high-speed loading process with an infrared camera in real time mode shows that, for the АМg6 alloy tested under a specified load, there are no conditions for implementation of the thermoplastic instability mechanism. Prior to the metallographic studies, the skewed specimens were first pressed into conductive Bakelite on a Struers CitoPress-10 and then polished on a Struers Tegramin-30. Distilled water was used as a lubricant in rough cleaning modes. A lubricant with highly dispersed particles of silicon oxides was applied at the final stages. Etching was carried out with Keller’s reagent for 5-10 s at a temperature of 65-70°С. Metallographic examinations of the skewed specimens were performed before and after the dynamic tests on a split Hopkinson pressure bar. The structure analysis was carried out on the longitudinal (cut in the longitudinal direction) skewed specimens using the light and scanning electron microscopes. For light metallography, the Olympus GX 51microscope with a magnification range of 10-1 000x was used, and for electron metallography – the FEI PHENOM G2 ProX microscope at an accelerating voltage of 15 kV and with a magnification range of 2 000-15 000x. The results of the structural analysis of the specimen after testing with a split Hopkinson-Kolsky rod are shown in Figure 3, a-e. It has been found that the processes of plastic deformation occurring in the central region of stress concentration in the examined specimen lead to stretching and bending of both coarse grains and the ensemble of fine grains of the original structure. In the structural units (grains) where the conditions for implementation of large plastic strain were realized, the structural elements exhibiting the features of localized shear bands (arrows in Fig. 3 e) were formed. In the peripheral region, the formation of cracks was noted, at the top of which the formation of structural elements with characteristic signs of localized shear bands was also recorded. It is worth noting that the specimen also included a great number of structural elements in which the processes of structural transformation was accompanied by the formation of localized shear bands.

3. Results

The temperature measured in the strain localization zone does not confirm the widespread concept that the strain localization mechanism is induced by thermoplastic instability. The structural analysis performed in this study with the aid of electron and light microscopes revealed the correlated behavior of the ensemble of defect, which can be considered as a structural transition providing strain localization.

Conclusions

The temperature fields recorded for the AMg6 alloy tested at high strain rates on the split Hopkinson pressure bar by the infrared thermography technique and analysis of the structure of deformed specimens made it possible to suggest that, under the loading conditions at which the AMg6 alloy was investigated, one of the mechanisms of plastic strain localization is caused by structural transitions in the defect structure of the material, which are accompanied by the development of ordered ensembles of defects.