Application of focused ion beam milling and imaging for studying creep cavitation

Grain boundary creep cavitation has been observed in 316H austenitic stainless steel. Despite extensive studies on creep cavitation in the past few decades, aspects of the fundamental understanding of this phenomenon remain unresolved. This is especially true in the case of engineering alloys, where cavities are associated with inter-granular precipitates. In the present study, focused ion beam cross-section milling combined with ion channelling contrast imaging is used to identify unambiguously creep cavitation. The method has been applied to an ex-service 316H austenitic stainless steel thick section weldment of a header component operating at a temperature of ~550°C. The methodology is described in detail and the results compared with those obtained with conventional and widely adopted metallographic methods. The focused ion beam technique is shown to have unique advantages. A possible nucleation mechanism for inter-granular creep cavitation in Type 316H austenitic stainless steel weldment is discussed based on the present observations.