STUDY OF DISLOCATION WALLS EVOLUTION DURING SPALL IN PURE ALUMINUM

The micro-structure of the spall plane of pure aluminum (99.9999%) was investigated in symmetric plate impact experiments. The aluminum targets were first heated at 450 degrees C for 22 h for grain growth. The impacted targets were softly caught and collected for metallurgical analysis. It was found that at weak impacts with partial spall, voids with average size of 50 +/- 10 mu m were developed along the grain boundaries. The grain sizes in the vicinity of the voids are 50-250 mu m, in comparison with grain sizes of 400-1000 mu m in other areas of the specimen, revealing grains splitting under dynamic tension. Transmission Electron Microscopy (TEM) and electron diffraction show parallel dislocation walls, that create a sub-grain micro-structure inside grains with (011) orientation to the beam. In grains with other orientations no dislocation walls or sub-grain structure were found. We also found out pile up of the dislocations by glide mechanism along < 011 > directions during the spall process.