Measurement of Three Dimensional Geometry of Creep Void and Grain Boundary With Combining 3D-EBSD Method and SEM Images

A-parameter, void area ratio and other methods about creep void are used to estimate creep damage resulting from creep voids. However, these methods are based on not three-dimensional but two-dimensional geometry, though creep voids are three-dimensional cavities. By combining the 3D-EBSD method with SEM images, we have observed the three-dimensional shape of creep voids and their geometrical relationship with grain boundaries at first. The method is applied to 1Cr-1Mo-0.25V turbine rotor steel subjected to a creep rupture test (580°C, 180MPa). Also, interrupted creep specimens are prepared to observe the progress of void growth. Forty sections with 0.5 μm interval and 100μm × 100μm area are measured by mechanical polishing in order to reconstruct the three-dimensional shapes. In the results, four types of creep void are observed. One is sphere type whose radius is approximately 1μm. It is observed in the specimen whose creep life fraction is 25%. In the specimens with 50% and 75% creep damage, prolate and oblate spheroid whose radius is approximately 2.5μm are observed. Finally, connected voids are located within ruptured specimen. As the creep damage is progressed, not only void growth but also void nucleation is observed. Especially, on prior austenite grain boundary which is three-dimensionally perpendicular to the stress direction, creep voids are nucleated and grow in a concentrated manner. However, such nucleated small voids do not affect the void volume fraction.