Observing early cavitation damage evolution from repeated laser-induced single bubbles on aluminium, austenitic steel and nickel-aluminium bronze

Abstract

Cavitation is the formation and collapse of gas bubbles in fluids due to local pressure changes. The implosion of these bubbles in the immediate vicinity of a surface can lead to shock waves and microjets, inducing plastic deformation and erosion of the material. Reproducible single bubbles with a defined diameter can be induced by a focused laser pulse in liquid, causing local evaporation. Combined with in-situ microscopy, this allows a direct correlation of single cavitation bubbles with the damage caused.Here, this method was applied for a detailed investigation of the damage development on pure aluminium (Al99.999%) as a model material, as well as on the technical alloys austenitic steel (316LVM; 1.4441) and NiAl bronze (NAB; CuAl10Ni5Fe5), especially regarding a comparison of the deformation behaviour. The damage evolution on these materials was monitored in-situ with a light microscope. Additionally, the damage was examined ex-situ using light microscopy, confocal microscopy and scanning electron microscopy.The investigated materials show clear differences in their deformation behaviour under cavitation load. Pure aluminium and 316LVM show a high degree of plastic deformation due to the high ductility and multiple slip planes in their fcc lattice structure. With increasing number of bubbles, strain accumulates at grain boundaries, where material removal initiates. This effect is more pronounced for 316LVM and appears in the presence of slip lines in the material. In contrast to this, NiAl bronze shows a different deformation behaviour, mainly due to the more complex microstructure. The soft fcc copper matrix deforms easily, whereas the hard intermetallic precipitates (containing Fe, Al, and Ni) act as an impediment for plastic flow. This leads to a more localized damage than in the other materials.