1988 Volume 74 Issue 10 Pages 2009-2016
For the purpose of evaluating creep damage, systematic study of creep damage was performed, particularly for grain boundary creep damages by SEM observation and density change measurements. Three different kinds of grain boundary creep damages leading to creep fracture were observed. They are cracks formed at triple points at higher loading stress, grain boundary cavities at medium stress and cracks at sigma/austenite interface at lower stress. Both the grain boundary cavities and the cracks at sigma/austenite interface comence to form at comparatively low creep strain (0.1-0.6%) at the steady-state creep stage (t/tr:0.2-0.4).The increasing feature of cavity volume with creep time corresponds to the creep curves. Precipitation of AIN associated with sigma phase on grain boundary leads to early formation of the cracks at their interface. In this case, the volume of the cracks increases considerably in the accelerating creep stage and constitutes a main component of the creep strain. The rapid increase in the volume causes the transition from the steady-state to the accelerating creep stage.