Microscopic study of cavitation erosion behaviour in copper and Cu-5.7wt.%Al single crystals

Several crystallographic orientations of pure copper and Cu-7.9wt.%Al single crystals have been subjected to vortex cavitation collapse. Specimens damaged by single impacts, multiple impacts and severe erosion have been studied using scanning, transmission and high resolution electron microscopy as well as microhardness measurements. The substructural features at various distances from the eroded surface along a normal cross-sectional plane have been observed. For single impacts, dislocations occur in tangles and microtwins contribute to deformation only under strong impacts causing a rupture in the surface. In eroded copper specimens deformation extends to a depth of about 1.2 mm whereas several mechanisms contribute to hardening. At a depth of 20 μm, twinning is the dominant mechanism which gives rise to cell structures beyond 30 μm. The intersection of these induced twins is accommodated by the formation of secondary microtwins in the crossing area. The cell size of 0.3–0.4 μm near the surface increases to 10–12 μm at a depth of 700 μm and to 40 μm near the end of the hardened layers. The misorientation between cells varies randomly and reaches a maximum of 15°. At intermediate strain regions, corresponding to depths of 100–300 μm, well-defined parallel wall dislocation microbands 0.5 μm wide were superimposed on the cell structures. Transition between twinned regions into cell structures occurs at depths of 20–30 μm by dislocation tangles and incipient cells. Dynamic recrystallization occurs within a depth of 0–120 μm, as shown by the appearance of twin orientation grains. The erosion rate was not significantly affected by crystallographic orientation.