Creep damage occurs in metals and alloys after prolonged exposure to stress at elevated temperatures. It is usually associated with the tertiary stage of creep, and brings about the onset of creep failure. It can, however, initiate at the relatively early stages of creep, and develop gradually throughout creep life. Creep damage is manifested by the formation and growth of creep voids or cavities within the microstructure of the material.
At low stresses, the cavities develop preferentially on grain boundaries oriented normal to the principal stress direction. During creep, the cavities increase in number and size. The continuous nucleation of cavities is generally considered to be strain dependent and often associated with precipitates or particles on grain boundaries. Growth of the intergranular cavities at low stresses is normally by stress-directed vacancy diffusion, although the rate of growth is often limited or constrained by local creep strain. The cavities eventually coalesce to form grain boundary cracks which, in turn interlink to form multiple boundary length cracks which subsequently propagate to cause failure.
At high stresses, the cavities can nucleate both at the grain boundaries and also within the grains, again normally at particles or inclusions. Cavity growth, under these circumstances, can occur by a process of grain boundary sliding or by plastic growth at particles within the grains. Final failure subsequently occurs by a plastic tearing process.
Cavity sizes typically range from around 0.1 microns to 10 microns. Whilst electron microscopy is therefore required to study the cavity nucleation stage, optical microscopy is normally adequate to follow the development of creep damage. Conventional NDT methods, e.g. magnetic particle inspection or ultrasonic inspection techniques, are not able to detect creep damage prior to the formation of a creep crack. The most commonly accepted technique for detecting creep damage is metallographic replication. This normally involves the partial dissolution of acetate replica film on to the polished and etched surface of the material. The replica is subsequently peeled off, sputter-coated with a heavy metal, and then viewed under the microscope. ASTM standard E1351 [1] gives guidance on the replication process.
The replication method is however restricted to the detection of creep damage at the outer surface. There are no proven methods for reliably detecting the early stages of damage if it occurs sub-surface. Under the latter circumstances, detection of the early stages of cracking by means of high resolution NDT methods such as ultrasonic time-of-flight diffraction (TOFD) is the only recourse.
References
- ASTM E1351-96, Standard practice for production and evaluation of metallographic replicas, ASTM 1996.
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