Microstructural Damage Mechanisms in Metals exposed to Cavitation
Stefanie Hanke, University of Duisburg-Essen, Germany
Cavitation is the formation and collapse of gas or vapour bubbles in liquids, induced by changes in pressure. It can be used e.g. for cleaning of surfaces or the controlled destruction of kidney stones in the human body. In many technical applications, e.g. pumps, propellers or valves, cavitation is an undesired phenomenon. Here, it reduces efficiency and causes damage to the components in the vicinity. Damage to solid surfaces is referred to as cavitation erosion, and has been widely studied in the past. Still, to date the damage mechanisms induced by the violent collapse of single bubbles or structures composed of many bubbles close to a solid surface are not fully understood. This is because the collapse takes place very fast (ms) and acts on very small surface areas at high repetition rate, making it hard to observe or analyse experimentally. An exact description of the load as well as material properties in the surface under high loading speeds are typically not available, making it difficult to formulate predictive models.
In the presented work the damage appearances resulting from cavitation impact on different metals and alloys are observed and analysed using different microscopy techniques. This approach allows to understand the material reaction to the loading, and draw conclusions on the specific loads causing it. Bronze, different steels and aluminium have been exposed to cavitation in a standard test-rig using ultrasound, as well as to single bubbles created by short laser pulses. On all metals studied initially small pits form, which are commonly attributed to liquid jets hitting the surface during bubble collapse. For materials of low strength plastic deformation accumulates, eventually leading to material detachment. With increasing material strength higher numbers of collapses are needed before damage is observable, as fatigue-type internal strain accumulation dominates. Larger damage structures (e.g. slip steps) are observed for some materials. It can be concluded that not only direct impacts on the surface contribute to the damage, but possibly also pressure waves propagating through the material volume. The overview on the various material reactions given in this work shows that in order to model and predict cavitation erosion precisely, models must be adjusted to each material’s specific progression of damage mechanisms.